Pervious pavement systems and materials State-of-the-Art

RESEARCH REPORT VTT-R-08222-13

Pervious pavement systems and materials State-of-the-Art

Authors: Hannele Kuosa, Emma Niemeläinen and Kalle Loimula

Confidentiality: Public

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Preface

This is a WP2 State-of-the-Art report in the Finnish CLASS-project (Climate Adaptive Surfaces, 2012–14). This project develops surfacing materials and pavement structures to mitigate impacts of climate change in urban environments. The new materials are surfacing layers of porous concrete, porous asphalt and interlocking modular paving stones together with subbase structures of aggregate, pipes, geotextiles and water storage tanks and other systems. The CLASS-project is funded by TEKES (Finnish Funding Agency for Technology and Innovation) together with VTT, Finnish cities, companies and organizations.

Participants of the steering group in the CLASS-project are: Pirjo Sirén (chairperson), Espoon kaupunki, tekninen keskus Markus Sunela, FCG Suunnittelu ja tekniikka Oy Osmo Torvinen, Helsingin kaupunki, Rakennusvirasto Tommi Fred, Helsingin seudun ympäristöpalvelut – kuntayhtymä (HSY) Olli Böök, Kaitos Oy Pekka Jauhiainen, Kiviteollisuusliitto ry Lars Forstén, Lemminkäinen Infra Oy Pasi Heikkilä, Oulun kaupunki Mika Ervasti, Pipelife Finland Oy Tomi Tahvonen, Puutarha Tahvoset Oy Juha Forsman, Ramboll Finland Oy Tiina Suonio, RTT Betoniteollisuus Kimmo Puolakka, Rudus Oy Ab Kati Alakoski, Saint Gobain Weber Oy Ab Ismo Häkkinen, SITO Antti Auvinen, Vantaan kaupunki Angelica Roschier, TEKES Eila Lehmus, VTT

Espoo, December 2013 Hannele Kuosa, Emma Niemeläinen and Kalle Loimula


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Summary

In the Finnish CLASS-project (Climate Adaptive Surfaces, 2012–14) new pervious surfacing materials and pervious pavement structures are developed to mitigate climate change associated with increased rain intensities and amounts. These pervious structures can decrease flooding for instance in cities with large areas of impervious surfaces. They can be a part of the overall stormwater system decreasing the need of conventional drainage systems.

This CLASS-project State-of-the-Art Report reviews published research results and field experiences on pervious pavements, especially with regards to the materials and products needed in these structures. This report is a part of WP2 where pervious pavement materials are studied also experimentally. Other more specific CLASS-project State-of-the-Art Reports are sited in this report. These other reports are concentrated on the city demands with respect to pervious structures, laboratory and field testing methods for pervious pavements and pavement materials, winter performance of pervious pavements, impact of pervious pavement on water quality as well as pervious pavement dimensioning and hydrological permeable pavement models and their parameter needs.

All the CLASS-project State-of-the-Art Reports were made to serve as the basis for the Finnish guidelines on the construction and maintenance of pervious pavements. For this the winter performance and durability of pervious pavements in the climates types close to the Finnish climate, including also hard winter periods, are reviewed especially.

With respect to the surfacing material, the main pavement types included are pervious concrete pavement (PCP), permeable interlocking concrete pavement (PICP) together with permeable natural stone pavement (PNSP) and porous asphalt pavement (PAP). Besides, examples on some other permeable surfacing types as green solutions are included shortly. Detailed material specific information, such as mix design and durability information or other essential information on the surfacing type is reviewed to gain State-of-the-Art knowledge prior to the production of the surfacing materials and pervious pavement structures later on in the project.

As the whole pavement structure is more than essential for the function of a pervious pavement, also the materials and products needed to build the substructures are included. These materials include aggregates used in the different pavement layers (bedding, base, subbase) below the surfacing layer, as well as geotextiles, impervious liners and water draining and collection systems.

Clogging is an inherent property of all the pervious pavements, and proper maintenance actions are therefore also essential. This report reviews also research results and experiences on these both. Also some studies on how pervious pavement can serve as so called cool pavement, to mitigate the heat island effect, are reviewed shortly. Besides, some points of views are presented on the costs and service life of pervious pavements.

The choice of what kind of pervious pavement to use is influenced by site-specific design factors, and the intended future use of the permeable surface. The major design goal of permeable pavement is to maximize runoff reduction and nutrient removal. Designers may choose to use a baseline permeable pavement design or an enhanced design that maximizes nutrient and runoff reduction. In all there are several functional demands for pervious pavements to fill, as presented in this report. Below is a list to consider:

· surface infiltration capacity, drainage, surface layer permeability · water storage capacity (by aggregate base and subbase, by other draining and

collection systems) · water quality enhancement capacity · bearing capacity


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· durability, service life · winter performance · costs (construction, maintenance) · service life · suitability for reuse or recycling.

Yhteenveto

Suomalaisessa CLASS projektissa (CLimate Adaptive SurfaceS, 2012–14) kehitetään uusia vettä läpäiseviä ympäristörakenteiden pinnoitteita sekä niihin oleellisesti liittyviä alusraken-teita, jotka ovat myös vettä läpäiseviä, mutta toimivat ennen kaikkea vettä varastoivina ker-roksina. Koko rakenteen toiminnan kannalta oleellista on myös se, että rakenteen kantavuus ja muut ominaisuudet ovat käyttökohteen asettamien vaatimusten mukaisia. Erityisesti pro-jektissa kiinnitetään huomiota siihen, että tällaiset rakenteet soveltuvat Suomen ilmasto- ja muihin olosuhteisiin. Kylmässä ilmastossa on otettava huomioon sekä routa että jäätymis-sulamissyklien vaikutukset.

Vettä läpäisevät päällysteet käsittävät yleensä pintakerroksen ja alapuolisen rakennekerrok-sen, joka koostuu kiviaineksesta sekä pohjalla olevasta suodatinkerroksesta tai -kankaasta. Lisäksi on olemassa monenlaisia muunnelmia, joissa voi olla esimerkiksi erilaisia läpäiseviä materiaaleja sekä putkirakenteita, säiliöitä ja muita systeemejä, erityisesti pohjamaan ollessa vettä läpäisemätöntä. Avoimessa systeemissä vesi virtaa suoraan rakennekerrosten läpi. Suljetussa systeemeissä vesi ei siirry alapuoliseen maaperään vaan vesi johdetaan salaoja-putkilla pois. Tämä voi olla tarpeen, jos pohjamaa on huonosti vettä läpäisevää tai veden ei haluta siirtyvän suoraan pohjavedeksi. Tällöin vesi voidaan varastoida esimerkiksi muovi- tai betoniholveihin tai ns. hulevesikasettijärjestelmiin.

Läpäisevillä rakenteilla voidaan vähentää ilmastonmuutoksen eli lisääntyvien ja voimistuvien sateiden haitallisia vaikutuksia kuten tulvimista ja tavanomaisen sadevesiverkoston ylikuor-mittumista. Läpäisevillä rakenteilla voidaan vaikuttaa myös hulevesien kemialliseen kuormit-tavuuteen. Erityisesti ne soveltuvat kaupunkeihin ja muille alueille, joissa erilaisten vettä läpäisemättömien pintojen määrä on suuri. Muutoin ne soveltuvat ominaisuuksiensa puolesta parhaiten kevyen liikenteen väylille ja alueille ja sekä muille suhteellisen vähäisen kuormituk-sen alueille kuten henkilöliikenteen pysäköintialueille sekä kävelyalueille ja väylille. Pinta-materiaalin valinnalla voidaan vaikuttaa useisiin tekijöihin kuten toimivuuteen vedenläpäise-vyyden ja sen pysyvyyden osalta, säilyvyysominaisuuksiin Suomen olosuhteissa sekä es-teettisyyteen.

Tämä State-of-the-Art raportti (nykytilakatsaus), joka on tehty CLASS-projetin WP2:ssa (Material and Products Development) keskittyy lähinnä läpäisevissä rakenteissa käytettäviin materiaaleihin ja tuotteisiin sekä niiden toimintaan. Raportti perustuu julkaistuun tietoon, tut-kimustuloksiin ja julkaistuihin käytännön kokemuksiin. Muissa projektin State-of-the-Art raporteissa, joihin tässä raportissa viitataan, käsitellään mm. kaupunkien tarpeita vettä läpäi-sevien rakenteiden osalta, rakenteiden ja niissä käytettävien materiaalien tutkimuksessa, testauksessa ja laadunvalvonnassa käytettävissä olevia standardoituja ja muita menetelmiä, rakenteiden talvikäyttäytymistä routimisen ja läpäisevyyden kannalta, rakenteiden vaikutusta vesilaatuun, rakenteiden mitoitusta sekä kantavuuden että hydrologisen toiminnan osalta sekä eri laajuuden hydrologisen toiminnan malleja ja niissä tarvittavia parametreja.

Sekä tämä että muut CLASS-projektin State-of-the-Art raportit palvelevat projektin Suomeen ja sen olosuhteisiin soveltuvan ohjeistuksen laadinnassa. Tämä ohjeistus tulee sisältämään sekä rakentamisen että ominaisuuksien kuten erityisesti riittävän läpäisevyyden ylläpidon. Erityistä huomiota joudutaan tällöin kiinnittämään Suomen talviolosuhteiden kuten routasy-vyyden sekä toistuvien jäädytys-sulatussyklein kautta tuleviin vaatimuksiin.


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Vettä läpäisevät päällysteet voidaan pintamateriaalista riippuen luokitella huokoiseksi tai läpäiseväksi ja ne voivat olla joko monoliittisia (yhtä kappaletta olevia) tai modulaarisia (toi-siinsa liitettävistä kappaleista koostuvia). Huokoisen pintamateriaalin tapauksessa vesi läpäi-see koko pinta-alan; läpäisevät päällysteet sen sijaan koostuvat yleensä vettä läpäisemättö-mistä kivistä tai laatoista, joiden välissä on kiviainesta, joka päästää veden lävitseen. Tässä raportissa lähemmin käsiteltävät läpäisevän rakenteen pintamateriaalit ovat läpäisevä betoni (pervious concrete pavement, PCP), läpäisevä betonikivipinnoite (permeable interlocking concrete pavement, PICP) sekä läpäisevä luonnonkivipinnoite (permeable natural stone pavement, PNSP) ja avoin asfaltti (porous asphalt pavement, PAP). Lisäksi käsitellään lyhy-esti joitakin muita läpäiseviä pinnoitteita kuten nk. vihreitä ratkaisuja.

Läpäisevän betonin huokoisuus, läpäisevyys ja lujuus ovat suhteessa toisiinsa. Esimerkiksi avointa huokoisuutta 15–20% voi vastata 20–25 MPa lujuus ja noin 1–2 mm/s vedenläpäi-sevyys. Todelliset arvot ovat kuitenkin aina tapauskohtaisia ja niihin vaikuttaa paitsi betoni-massan koostumus myös valettavan laatan tiivistystekniikka ja sen teho.

Läpäisevän betonin avoin rakenne päästää veden materiaalin sisälle, mikä aiheuttaa haas-teita kylmässä ilmastossa, kuten Suomessa. Ensisijaisesti läpäisevissä rakenteissa pyritään siihen, että pinta-osa ei pääse kyllästymään pitkäaikaisesti vedellä. Tämän ehkäisemisessä alapuoliset vettä johtavat kerrokset ovat oleellisessa asemassa. Lisäksi on kuitenkin havaittu, että läpäisevän betonin tulee olla erityisesti kaikkein vaikeimmissa olosuhteissa myös hyvin jäädytys-sulatusrasitusta kestävää. Tämä asettaa omat vaatimuk-sensa sen koostumukselle ja mikrorakenteelle. Tutkimustulosten mukaan läpäisevän betonin valmistuksessa tulee käyttää huokostavaa lisäainetta, jotta myös sen sideaineeseen voi massan sekoitusvaihees-sa muodostua pieniä pakkasvaurioitumiselta suojaavia suojahuokosia. Pelkkä avoin vettä läpäisevä makrotason huokosrakenne ei kykene suojaamaan sementtipastaa kauttaaltaan.

Avoin asfaltti on toinen vettä läpäisevä päällystemateriaali. Avoimen asfaltin huokoisuus on yleensä 15–20%. Toiminnallisesti avoin asfaltti vastaa muita vettä läpäiseviä päällystemate-riaaleja. Avoimia asfaltteja on käytetty 70-luvulta alkaen mm. Pohjois-Amerikassa. Euroo-passa niitä käytetään mm. Sveitsissä ja Hollannissa. Suomen Ásfalttinormeissa´ avointa asfalttia suositellaan käytettäväksi kevyesti liikennöidyillä asuinalueilla, kentillä ja pihoilla.

Avoin asfaltti on yleensä halvempi kuin muut vettä läpäisevät päällysteet. Asianmukaisella rakentamisella ja säännöllisellä ylläpidolla avoin asfaltti on pitkäikäinen ja käyttökelpoinen päällystemateriaali erityisesti paikoitusalueilla ja vähän liikennöidyillä teillä. Avoimen asfaltin huonot puolet ovat huokosten tukkeutuminen ja heikompi kantavuus kuin tavallisella asfaltil-la. Kun huokoset tukkeutuvat hiekoituksen ja huleveden mukana kulkeutuvan hienoaineksen johdosta, vedenläpäisevyys huononee kuten kaikilla muillakin vettä läpäisevillä pinnoiterat-kaisuilla. Tukkeutuminen on potentiaalinen ongelma, mutta riittävän usein toistuva tehokas puhdistus on havaittu hyväksi keinoksi riittävän läpäisevyyden ylläpitoon. Tyypillisiä puhdis-tusmenetelmiä ovat alipaineimu, pesu ja lakaisu.

Kolmas vettä läpäisevä päällystetyyppi koostuu betoni- tai luonnonkivistä tai betoni- tai luon-nonkivilaatoista. Myös itse betonikivet tai laatat voivat olla vettä läpäiseviä, mutta yleensä saumat tai aukkokohdat läpäisevät veden. Sauma/aukkomateriaalin eli yleensä kiviaineksen tulee olla tähän tarkoitukseen rakeisuudeltaan sopivaa. Rakeisuus, jossa kivet voivat pak-kautuvat tiivisti tai suuri hienoaineksen määrä eivät ole läpäisevyyden kannalta hyväksi. Lisäksi saumojen tai aukkojen määrän tulee olla riittävän suuri, yleensä 5–15 % pinta-alasta. Pääkriteeri on kuitenkin riittävä koko pinnoiteratkaisun vedenläpäisevyys.

Muitakin vettä läpäiseviä pintaratkaisuja on olemassa kuten reikälaattoja ja -kiviä sekä nurmi- ja sorakennostoja. Erilaisten ratkaisujen läpäisevyydessä on suuria eroja ja aukkojen täyttö-materiaali vaikuttaa siihen oleellisesti. Koko rakenteen hydrologisen toiminnan kannalta oleellista on myös se, kuinka hyvin valittavat alusrakenteet läpäisevät vettä.


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Lähemmin käsiteltävien pintamateriaalien osalta niiden koostumuksesta ja ominaisuuksista olevaa tietoa on käyty läpi niin tarkoin, että sitä voidaan hyödyntää CLASS-projektin jatkos-sa, kun materiaaleja valmistetaan laboratoriossa ja käytännössä. Tällainen tietous liittyy mm. massakoostumuksiin (mix design), eri käyttötarkoituksen kiviaineksiin ja niiden rakeisuuteen (läpäisevä betoni, avoin asfaltti, erilliskivien asennus- ja saumahiekka), materiaalien huokoi-suuteen, läpäisevyyteen, lujuuteen ja säilyvyyteen kuten pakkasen-kestoon ja pakkas-suola-kestoon (tiesuolaus).

Läpäisevän rakenteen toiminnan kannalta pintamateriaalin ja sen läpäisevyyden lisäksi myös koko rakenteen eli kaikkien sen kerrosten ominaisuudet ovat erittäin oleellisia. Tämä raportti sisältää myös alusrakenteissa käytettävät materiaalit ja tuotteet. Näitä ovat erityisesti kanta-van ja jakavan kerroksen kiviainekset sekä geotekstiilit ja -membraanit sekä veden johtami-sessa käytettävät tuotteet ja varastoinnissa käytettävät tuotteet kuten säiliöt ja kasettisys-teemit.

Läpäisevän rakenteen luontainen ominaisuus on sen ajan kuluessa tapahtuva tukkeutumi-nen. Tukkeutuminen on pitkälti suhteessa rakenteen sijaintiin ja ympäristön tukkeutumiseen vaikuttaviin tekijöihin sekä erityisesti läpäisevän rakenteen huoltoon ja puhdistukseen. Yleensä kunnollinen huolto ja puhdistus ovat välttämättömiä pitkäaikaisen toiminnan takaa-miseksi. Raportissa käydään läpi läpäisevyydestä, tukkeutumisesta ja läpäisevyyden palauttamisesta saatuja tutkimustuloksia ja käytännön kokemuksia.

Vettä läpäisevä rakenne voi toimia myös viilentävänä eli nk. ´Heat island effectíä vähentä-västi (ćool pavement´). Tästä esitetään joitakin tuloksia. Raportissa käsitellään myös teki-jöitä, jotka tulee ottaa huomioon läpäisevän rakenteen rakentamis- ja käyttökustannuksia sekä käyttöikää arvioitaessa.

Läpäisevän rakenteen tyyppiä valittaessa tulee kustannusten ja käyttöiän lisäksi ottaa huo-mioon kaikki vaikuttavat paikalliset tekijät ja olosuhteet. Näitä ovat mm. alueen ja päällysteen käyttö ja sen asettamat vaatimukset sekä hydrologian että kantavuuden ja kestävyyden osalta. Arkkitehtoniset ja esteettiset tekijät vaikuttavat myös oleellisesti pintamateriaalin valinnassa. Ulkonäöltään erilaisia ja erilaisiin kohteisiin soveltuvia pintamateriaaleja onkin paljon. Raportissa on esitetty joitakin sekä ulko- että kotimaisia esimerkkejä. Erityisesti Suo-malaiselle materiaali- ja tuotekehitykselle on kuitenkin vielä paljon sijaa.

Läpäisevien rakenteiden oleellisia suunnittelutavoitteita ovat sekä mahdollisimman suuri pintavalumien pienentäminen että niiden mukana ympäristöön ja vesistöihin kulkeutuvien haitallisten aineiden määrän pienentäminen. Suunnittelussa voidaan joko valita tietty nor-maalitason perusratkaisu tai ratkaisu, joka maksimoi läpäisevän rakenteen toiminnan sekä hydrologisen että vettä puhdistavan toiminnan osalta. Kaikkiaan läpäisevälle rakenteelle voi-daan asettaa useita funktionaalisia vaatimuksia. Raportissa on myös esitetty tyypillisiä vaa-timuksia ja soveltuvin osin myös numeerisina arvoina. Alla on lueteltu oleellisimmat tekijät, joille voidaan asettaa funktionaalinen vaatimus:

· pinnan tai rakenteen vedenläpäisevyys · veden varastointikapasiteetti (kiviaineskerrokset tai muut vedenjohto- ja/tai varas-

tointituotteet) · vesilaadun parantamiskapasiteetti · kantavuus · kestävyys-, säilyvyysominaisuudet · käyttöikä · huollettavuus, läpäisevyyden palautettavuus · talvikäyttäytyminen (aurattavuus, liukkaus, kitka, routivuus, ym.) · kustannukset (rakentaminen, huolto, peruskorjaus) · materiaalien kierrätettävyys, uusiokäyttö.


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Contents

Preface ................................................................................................................................... 2

Summary ................................................................................................................................ 3

Yhteenveto ............................................................................................................................. 4

Contents ................................................................................................................................. 7

Abbreviations ......................................................................................................................... 8

1. Introduction ....................................................................................................................... 9

2. Comparison with other stormwater solutions ..................................................................... 9

3. Surface layer materials ................................................................................................... 11 3.1 General ................................................................................................................... 11

3.1.1 General ....................................................................................................... 11 3.1.2 Mix design, mixing, compaction and curing ................................................. 13 3.1.3 Porosity, pore sizes, infiltration/percolation rate, hydraulic conductivity,

permeability ................................................................................................ 22 3.1.4 Strength and mechanical performance ........................................................ 25 3.1.5 Freeze-thaw durability – with/without chlorides ........................................... 28 3.1.6 Abrasion and raveling ................................................................................. 33 3.1.7 Noise reduction ........................................................................................... 34

3.2 Porous asphalt ........................................................................................................ 35 3.2.1 General ....................................................................................................... 35 3.2.2 Porous asphalt mix design .......................................................................... 37 3.2.3 Porosity, permeability and drainage ............................................................ 39 3.2.4 Strength and mechanical performance ........................................................ 40 3.2.5 Winter durability, freeze-thaw durability – with/without chlorides ................. 41

3.3 Permeable interlocking concrete and natural stone pavement ................................ 42 3.3.1 General ....................................................................................................... 42 3.3.2 Pavers ........................................................................................................ 44 3.3.3 Joints and joint material .............................................................................. 47 3.3.4 Permeability, surface infiltration .................................................................. 49 3.3.5 Strength and mechanical performance ........................................................ 50 3.3.6 Freeze-thaw durability – with/without chlorides ........................................... 51 3.3.7 Abrasion resistance .................................................................................... 51

3.4 Other pervious solutions ......................................................................................... 51

4. Subbase systems ........................................................................................................... 54 4.1 General ................................................................................................................... 54 4.2 Aggregates ............................................................................................................. 55 4.3 Geotextiles, filter layer, geosynthetic barriers, impervious liners ............................. 57 4.4 Water draining and collection systems .................................................................... 62 4.5 Modifying soil for handling water inflow ................................................................... 67

5. Dimensioning .................................................................................................................. 68

6. Laboratory and field testing – standards and methods .................................................... 69

7. Performance ................................................................................................................... 69 7.1 Clogging and maintenance ..................................................................................... 69 7.2 Winter performance ................................................................................................ 78 7.3 Cool pavement ....................................................................................................... 79 7.4 Costs and service life .............................................................................................. 80

8. Water quality ................................................................................................................... 82

9. Choice of pavement type and functional demands .......................................................... 83

References ........................................................................................................................... 85


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Abbreviations

µ Dynamic viscosity of the fluid, kg/(m×s) a/c aggregate-cement ratio b Experimental constant BMP Best management practice CBR California Bearing Ratio EOS End of service life g Acceleration due to gravity, m/s2 HPPC High performance pervious concrete ICPI Interlocking concrete pavement institute ITZ Internal transition zones K Hydraulic conductivity, m/s LA Los Angeles (abrasion) NRMCA National ready mixed concrete association (U.S.) OGFC Open-graded friction course PA Porous asphalt PANK Päällystealan neuvottelukunta (The Finnish pavement technology advisory

council) PAP Porous asphalt pavement PC Pervious concrete PCP Pervious concrete pavement PIBP Permeable interlocking block pavement PICP Permeable interlocking concrete pavement PNSP Permeable natural stone pavement RA Recycled aggregate RCA Recycled concrete aggregate SAP Superabsorbent polymer SBR Styrene butadiene rubber SF Silica fume UHI Urban Heat Island V Void content, % VMA Viscosity modifying admixtures w/b water-binding material ratio w/c water-cement ratio WWA Wide wheel abrasion (test) ρ Density of the fluid, kg/m3 σ Compressive strength,MPa σo Compressive strength of paste at zero void, MPa к Permeability, m2


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1. Introduction

All pervious pavements have normally a somewhat similar structure, consisting of a surface pavement layer, an underlying reservoir layer composed normally of stone aggregates, and usually also a filter layer or fabric installed on the bottom. Besides there are several modifications which can include for instance different kind of pervious subbase materials, and also water collection pipes, tanks or other systems in connection with more or less impervious layers. Pervious pavement materials and structures need to be selected and dimensioned for each case taking into consideration all local demands and circumstances.

Pervious pavement can be defined as porous or permeable pavement based on the surface type, and it can be either monolithic or modular. Porous pavements are constructed with pervious material where water can infiltrate through the entire surface area. However, for permeable pavements, the paver material is made out of impervious blocks while the spaces between the paver blocks are typically filled with coarse grained materials, normally stone aggregate, which allow water to pass through. [Ferguson 2005, Zhang 2006]

There are also other paver options, such as concrete grid pavers and reinforced turf pavers. These solutions are not widely covered by this review. Open void fill media may be aggregate, topsoil and grass. These structures function in the same general manner as permeable pavement. Open void fill media can be aggregate, topsoil and grass. [Virginia DCR 2011]

This report is based mainly on published research results, experiences and guidelines on pervious concrete, porous asphalt and permeable interlocking concrete pavements. Besides preliminary Finnish company information on available materials to be used in these structures is included. The main emphasis is in material properties such as permeability, strength and durability, and on the performance of the pavements. Information on the pavement surface layer materials and materials for the subbase system are included.

Besides this report there are separate CLASS-project State-of-the-Art Reports (VTT Research Reports; see References) on the laboratory and field testing methods for pervious pavements and pavement materials, winter performance of pervious pavements, impact of pervious pavement on water quality as well as pervious pavement dimensioning and hydrological permeable pavement models and their parameter needs.

2. Comparison with other stormwater solutions

Pervious pavements have the potential to be an effective ultra-urban Best Management Practice (BMP). Conventional pavement results in increased rates and volumes of surface runoff, PPs, when properly constructed and maintained, allow stormwater to percolate naturally through the pavement and enter the soil below, or can be forwarded to the selected water collection system. At the same time they can provide the structural and functional features needed for the materials they replace in conventional streets, parking lots, sidewalks and other covers. [FHA 2013]

PP is not generally suited for areas with high traffic volumes or loads. However composite designs that use conventional solutions in high-traffic areas adjacent to PPs have been designed. [FHA 2013]

Wahlgren & Kling (2013) have examined background drivers for developing pervious surfacing materials and solutions for urban storm-water management. They have collected views of Finnish cities and water authorities on the needs and potentials of pervious pavement structures, and listed their expectations from the Finnish CLASS-project (Climate Adaptive Surfaces, 2013–2014) [CLASS 2012]. Suitable areas in Finland for new permeable


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materials and structures and technical solutions were found to be streets, walking and bicycling lanes, middle areas, plazas, sport fields and combined areas. [Wahlgren & Kling 2013]

Besides PP, there are several traditional solutions for urban stormwater management such as wet and dry ponds and vegetative practices. Pervious pavements have a potential to supplement these especially in highly urbanized, highly impervious ultra-urban areas.

FHA (2013) provides a planning-level review of the applicability and use of new and more traditional BMPs in ultra-urban areas. In many cases, controlling stormwater discharges in ultra-urban areas addresses multiple objectives and concerns as protection from flooding generated by highly impervious surfaces, protection of sensitive downstream conditions such as stream physical stability, or maintenance of instream water quality. To address these concerns comprehensively through the development of effective stormwater management alternatives, both structural and nonstructural practices may be considered. Structural BMPs control runoff and improve water quality through storage, flow attenuation, infiltration, filtration, and biological degradation processes. Their use in ultra-urban environment, however, generally requires deviations from standard designs to meet space limitations and other site restrictions. [FHA 2013]

In a study by Bäckström and Viklander (2000) presented in [Westerlund 2007] three general indicators (runoff control, pollution control and level of integration) and a fourth indicator specific for cold climate (winter performance) were defined and estimated for a chosen set of stormwater BMPs. The study was based on a literature review of stormwater BMPs and the score for each indicator ranged between low (--) to very high potential (++) effect. Table 1 presents the results of this survey. The general indicators were then plotted against the winter performance as seen in Figure 1. From this figure it was then possible to determine which BMPs were more or less suitable in a cold climate. This information is based upon literature reviews and on the perception of the authors: it should not be seen as a definite answer or solution to the best BMP to choose in a cold climate, but should instead be used as a base for discussion.

Table 1. Positive (+) and negative (-) effects of different stormwater components. [Westerlund 2007]


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Figure 1. Winter performance vs. general performance indicators (Runoff control + Pollution control + Level of Integration) for different BMPs for stormwater source control in cold climate conditions.[Westerlund 2007]

3. Surface layer materials

3.1 General

The main pavement surface layers and surface layer materials covered in this Chapter are · pervious concrete (PC) · porous asphalt (PA) and · permeable interlocking concrete pavement (PICP) with solid blocks, and permeable

natural stone pavement (PNSP).

Other solutions as green solutions are presented shortly.

This Chapter is concentrated mainly on the mix design, strength, porosity, permeability and durability properties of these surfacing materials and systems. Some information on the whole structure is included but mainly the material specific data in this Chapter is completed with the information in Chapters 4 (Subbase systems) and 7 (Performance), to cover the whole pavement structure. Besides there are separate CLASS-project State-of-the-Art Reports (VTT Research Reports; see the reference list) on the laboratory and field testing methods for pervious pavements and pavement materials, winter performance of pervious pavements, impact of pervious pavement on water quality as well as pervious pavement dimensioning and hydrological permeable pavement models and their parameter needs.

3.1.1 General

Pervious (porous, permeable, enhanced porosity) concrete (PC) was developed as an environmentally friendly material in Japan in the 1980s. Since then it has been widely used in various applications in Japan, USA and Europe because of its multiple environmental benefits: controlling stormwater runoff, restoring groundwater supplies, and reducing water and soil pollution. [Bhutta et al.2012] Because of its ability to significantly reduce stormwater runoff, it is today considered in the United States as stormwater Best Management Practice (BMP).

PC is a concrete with interconnected pores (typically 1–8 mm, and with total volume 15–30%) which are intentionally incorporated into concrete. Its physical characteristics differ greatly from those of normal concrete. Pervious concrete allows for high water and air


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permeability. On the other hand, the compressive, tensile, and flexural strength of pervious concrete tend to be lower than conventional concrete due to the high void ratio and lack of fine aggregate. The single-diameter aggregate forms the concrete framework. The cement paste or mortar binds the aggregate together. Load is transferred through the cement paste between aggregates. (Figure 2) The unit weight of PC is approximately 70% of conventional concrete, typically from 1600 to 2000 kg/m3. PC shrinks less and has higher thermal insulating values than conventional concrete.

Figure 2. a) A schematic model of pervious concrete [Yang & Jiang 2003]; A pervious concrete specimen [Delatte & Cleary 2006]; c) Traditional and pervious concretes during melting [Kevern 2010].

The typical thickness of a PC layer ranges from 100 mm to 200 mm, in addition to the granular base or subbase layer.

Also paving blocks made of some kind of pervious concrete type are produced [Beeldens et al. 2008, PTV 122 2009, EBEMA 2012] to fulfil similar performance. In [PTV 2009], an example of the permeability demand for pervious concrete paving block is 5.4 ×10-5 m/s (average value determined according to the specification).

There are also other benefits associated with PC pavements and other possible applications for PC. Some of these are the same as for the other porous or permeable pavement types (see Chapters 3.3 (Porous asphalt) and 3.4 (Permeable interlocking concrete and natural stone pavements). [Putman & Neptune 2011]:

· good acoustic absorption properties - quieter pavements. • Studies have shown that PC has generally produced a quieter-than-normal

concrete, with noise levels from 3% to 10% lower than those of normal concrete. [Schafer et al. 2006]

· allows air to percolate through the matrix into the subsoil beneath, · performs the role of a filter by the degradation and entrapment of contaminants (e.g.,

oils and debris) – See Chapter 8 (Water quality) and [Loimula & Kuosa 2013] · potential capability of lowering the urban heat island effect as the open structure of

pervious concrete allows air to flow through it, · roots of plants and trees adjacent to these pavements experience improved watering

and also aeration, · reduces road spray, · improves skid resistance.

In addition to pavements, there are several other applications of PC, such as [ACI 522R-10]: · rigid drainage layers, permeable drain tiles, · greenhouse floors – as thermal storage systems and to keep water from bonding, and

to eliminate the growth of weeds, · lightweight noise barriers and building walls to reduce noise, and to increase thermal

insulation, · floors with enhanced acoustic absorption characteristics, · surface course for parks and tennis courts,


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· artificial reefs.

Typically, unreinforced PC is used because of the high risk of reinforcement corrosion in the open PC pore structure. [ACI 522R-10]

3.1.2 Mix design, mixing, compaction and curing

3.1.2.1 Mix design It is essential to have proper mix design and suitable method of mixing and compaction to produce good porous concrete with the desired strength and durability at the designed void ratio and permeability. Workability properties are also essential. The correct quantitative proportions for a specific project depends on the character of the locally available aggregate and other variable conditions. Mixture proportions for PC are usually less forgiving than conventional concrete mixtures and tight controls are usually required in order to provide the required characteristics. [Mata 2008, Ferguson 2005]

Mix design is often based on trial-and-error efforts. Typically PC mix design includes: · 270–415 kg/m3 cement/binding material and · 1190–1480 kg/m3 aggregate. · Aggregate-cement ratio (A/C) is normally 4–4.5:1 and · water-cement ratio (w/c) 0.27–0.34. · Some fine aggregate may be added, e.g. 7% of all aggregate, to increase for instance

durability properties, make air entrainment possible. · Chemical admixtures are used in pervious concrete mainly for the same reasons as

in conventional concrete. · Coarse aggregate is the principal load bearing component.

Aggregates

Fine aggregate is normally not used. If used, fine-coarse aggregate ratio may be from 0 to 1:1. Addition of fine aggregate will typically increases strength but decreases the void content. When fine aggregate is used, the paste volume should be reduced to maintain the volume of voids (1–2% for each 10% of fine aggregate). [ACI 522R-10] However, in places with severe freeze-thaw exposure, research has shown up to 6% fines in the mix can provide added durability without losing porosity (see Chapter 3.2.6 (Freeze-thaw durability – with/without chlorides).

Porosity and the size of interconnected pores will be affected by the gradation and type of the aggregate, as well as on the amount and properties of the paste fraction (water + cement). It is recommended to use a single sized clean (free of coatings) coarse aggregate (size 5–25 mm) or aggregate grading between about 9.5 mm and 19 mm. Rounded and crushed aggregates, both normal and lightweight have been used. The fine aggregate portion is limited so as not to compromise the pore system connectedness. Flaky or elongated particles should be avoided. [ACI 522R-10]

Typically ´single-sized´ coarse aggregate is used. The maximum aggregate size is between 4.8 and 25 mm. Aggregate can also be for instance:

· from 9.5 to 12 mm, · from 4.8 to 9.5 mm, · from 4.5 mm to 19.0 mm, · from 2.4 mm to 9.5 mm or · from 1.2 mm to 9.5 mm.

Figure 3 and Figure 4 present examples on aggregate gradations used for PC. The main thing is that the aggregate packing should remain low enough to leave open pores in the concrete for the desired infiltration rate. A relatively uniform large aggregate size is


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preferable for maximum infiltration rate. For high filtration rates, 6–13 mm large aggregates have been used. A small amount of fine aggregate (<2.4 mm) has been found to be beneficial for strength and durability. [ACI 522R-10, Huang et al. 2010a, Kevern 2008]

Figure 3. Optimal area for aggregate gradation in the research by Kevern (2008).

Figure 4. Aggregate gradations for porous concrete in [Huang et al. 2010a].

Both rounded and angular aggregates are used. (Figure 5) Rounded aggregates can pack tight in the placement process, limiting some of the permeability. Using angular aggregates can make it a little harder to get the mix out of the trucks and they also increase water demand slightly.

Figure 5. Examples of round, semi angular, and angular aggregates for PC. [Kevern 2008]

The size of the aggregate will have an effect on aesthetics and the top size of the “holes” in the surface. [CPG 2013]

How wet or dry the large aggregates are can make a huge difference in the consistency and slump of the concrete. Saturated surface dry aggregate is preferable. Very dry or moist


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aggregate may cause problems as lack of workability or draining of paste and clogging of the void structure. [ACI 522R-10] It is wise to keep aggregates wet and cool in the hot summer time. [CPG 2013]

Aggregates should be good quality, clean and freeze-thaw durable. Dust and debris can add to clogging the wet pervious matrix.

The amount of aggregate relative to the amount of cement is another important feature. The more cement paste available for compaction the higher the compressive strength. Again this will clog the pores and is detrimental to the function of the pervious concrete.

Rizvi et al. (2009) studied if recycled concrete aggregate (RCA) is suitable to be used in pervious concrete. The purpose of a research study was to incorporate RCA into pervious concrete to create a very sustainable concrete product for paving. The research methodology involved substituting the coarse aggregate in the PC with 15%, 30%, 50%, and 100% RCA. Based on the specific mix design and RCA quality used, the recommendation was that the optimum percentage of RCA in PC be 15% direct replacement of virgin coarse aggregate.

Binding material, paste content, water-cement ratio

The optimum cementitious material content is strongly dependent on the aggregate size and gradation. It must be selected so that porosity will reach the designed value. An insufficient cementitious material content can result in reduced paste coating of the aggregate particles, and reduced compressive strength. An excessive paste amount may result in filled void structure and reduced porosity. [ACI 522-R-10]

Figure 6 presents the needed paste content (% of volume) for porous concrete with different void contents when aggregate is 2.36–4.75 mm. For a 20% void content the needed paste volume is 15–22%, depending on the compaction (well – lightly).

Figure 6. Relationship between paste and void content for porous concrete when aggregate is 2.36–4.75 mm. [ACI 522R-10]

The thickness of the paste layer surrounding aggregates can have a significant impact on the strength of PC.

An appropriate designed void ratio and paste flow is essential for good porous concrete. There will be slightly less voids in the top portion followed by the middle and the bottom portions. If the paste content is too low, or if paste flow is not sufficient, there will be too many voids. [Chindaprasirt et al. 2008] Performing a ´binder drainage test´ can help to find the optimum paste content. [Nelson & Phillis 1994, ACI 522R-10]


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The water-cement ratio (w/c) or water-binding material ratio (w/b) is also an essential parameter. The type of the binding materials will also affect the properties of the paste and porous concrete. Fly as (FA) and blast furnace slag are often used. In addition to cement paste, fine aggregates or powder materials can also be included. In this case porous concrete is composed of fine mortar phase, aggregates and interconnected pores.

Cement paste with relatively low w/c (0.26–0.40) but sufficiently high workability is needed. W/c can also be as low as 0.20. In the study by Copra et al. (2007), water was added during mixing to the aggregate and cement until a sheen was developed throughout the mix, and w/c was calculated afterwards. In practice, the water content should be controlled strictly as mix properties are very sensitive to it.

Visual inspection may be used to check if the water content and paste properties are right. Insufficient water content produces a lustreless, dull-appearing paste. Too much water causes the paste to flow visibly from aggregate. The quantity of cement paste is considered sufficient when “it coats all coarse aggregate with a shining film giving a metallic gleam”. [Ferguson 2005]

Admixtures, additives, fibres

The rheological and other fresh paste properties should be adjusted so that they are suitable for making porous concrete. In addition to normal paste workability measurement (e.g. Flow cone method with impacts), measurement of rheological properties of the paste can give valuable information, such as yield stress and plastic viscosity. [Chindaprasirt et al. 2008]

Especially paste or mortar fresh properties such as viscosity and stability can be adjusted by the use of admixtures. Normally the use of water reducing admixture or superplasticizer is necessary. They can assist porous concrete installations during placement, because wet porous concrete is by nature stiff and zero slump concrete. Too much plasticizer will cause paste flow from aggregate.

Retarding admixtures delay setting of concrete and keep the concrete workable during placement. Hydration stabilizers (hydration controlling admixtures, stabiliser + activator) are often used to improve handling and in-place performance characteristics. This admixture is even considered critical to the success of the mix. It has been suggested that every delivery of pervious should have a water reducer and/or the hydration stabilizer available at the jobsite for redosing as needed. Ambient temperatures affect pervious concrete even more than conventional concrete because it is at a lower water cement ratio, and thus admixtures play a key role in achieving optimal fresh mixture properties. [CPG 2013]

In cold weather a set-accelerator admixture can be used with PC. [Finnsementti 2013]

Cement paste properties can also be adjusted by the use of other concrete admixtures, as by viscosity modifying admixtures (VMA). The use of VMAs has resulted in better flow, quicker discharge time from a truck, and easier placement and compaction. Furthermore, VMAs prevent paste drain down, and may increase both compressive and flexural strength of pervious concrete. It should be noted that not all VMAs are made with pervious concrete in mind, and therefore, care should be taken when choosing the right VMA for pervious installation. There may be too little mix water in PC to get VMA properly dispersed throughout the mix. VMA’s have made the mix sometimes “gummy” or “sticky”. [CPG 2013, Delatte et al. 2007, Bury et al. 2006]

Cement paste, or especially in this case mortar phase, can also include entrained air pores. Distribution and average spacing of the air pores (0.020–0.800 mm) in hardened concrete is essential for freeze-thaw resistance. For this air-entraining admixture are used. Especially when paste thickness coating aggregate particles is more than 0.20 mm, air entrainment is a


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way to increase freeze-thaw resistance. Freeze-thaw durability is discussed more closely in Chapter 3.4.6 (Freeze-thaw durability – with/without chlorides). [Wang et al. 2006]

New admixtures for pervious concrete are appearing “every day”. For instance, there are admixtures including water reducer, hydration stabilizer and viscosity modifying admixture in one product. [CPG 2013] In Sweden HeidelbergCement AG has lately developed a pervious concrete especially to be used in railway tunnels between the tracks. This concrete includes a special admixture (ETONIS® 260) by Wacker. By using this admixture, a polymer modified pervious concrete with enhanced fresh and hardened properties can be produced. Fresh concrete is easier to process and compact. Thanks to an increase in viscosity, it is less prone to run through during concrete compaction. Additionally, the new polymer improves the non-sag properties and extends the fresh concrete’s processing window. Hardened concrete flexural and tensile strength increases and resistance to frost and road salts is improved. (Figure 7) [Wacker 2013, Riffel 2012]

Also according to Gerhardt (1999) the use of a second binder in PC, a polymer emulsion, in addition to cement can improve the mechanical stability, the resistance to deformation, meaning no rutting. This is especially true for more trafficked areas such as roads and motorways. [Gerhardt 1999]

Figure 7. Pervious (´drainage´) concrete modified with a new polymer admixture is spread between and beside tracks and thus provides rescue vehicles and fire engines with fast access to railway tunnels. (photo: Wacker Chemie AG, courtesy of Deutsche Bahn AG). [Wacker 2013]

Schaefer et al. (2006) have studied the use of silica fume (SF) at 5 % binder replacement to improve PC strength and bonding characteristics of paste to aggregate. They used also styrene butadiene rubber (SBR) latex to improve the cement-aggregate bond and the freeze-thaw durability of PC. [Schaefer et al. 2006]

In some cases macro fibres have also been used in pervious concrete. For instance, in the greater Kansas City area, polypropylene or cellulose fibres, <38 mm, fibrillated or micro fibre type, have been included in all locally placed pervious projects. The fibre dosage should be a minimum. Fibre benefits include preventing excess raveling, preventing too dry of a mix and preventing over compaction from too wet mix.

Bhutta et al. (2012) developed and evaluated the properties of a high performance pervious concrete (HPPC) in the laboratory. This PC required no special vibration and showed good cohesiveness. (Figure 8) The optimum mixture proportions were used containing three sizes of coarse aggregates with appropriate amount of high water-reducing and thickening (cohesive) agents. Strength improvement was possible due to the strong bond between cement paste and aggregate produced by the use of superplasticizer and thickening agent. With the same porosity, the compressive strength for the HPPC was somewhat higher than for normal PC.


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Figure 8. Slump and slump-flow of conventional pervious concrete (CPC) and high performance pervious concrete (HPPC). [Phutta et al. 2012]

3.1.2.2 Mixing For PC proper mixing is very essential. As PC water to cement ratio is very low, an improvement in the texture and property of the paste is obtained with sufficient mixing time and efficient mixing. Bond is also created between cement paste and aggregate within mixing. In addition, as several admixtures are typically used in PC, efficient mixing will ensure their full functionality. Especially air-entrainment, i.e. creation of small air pores in the PC paste, demands powerful mixing. On the other hand, if mixing is for some reason extended too much, or there is too long of a remixing state on site, the PC workability decreases, and the required compaction energy increases. Different mixing operations have been used to get good PC mixes. [Chindaprasirt et al. 2008, Wang et al. 2006, Kevern 2008]

Kevern (2008) made PC mixing with a rotating-drum mixer, using the following mixing procedure:

· aggregate was first introduced, · 2/3 of the water and the air entraining agent (AEA) was added and mixed until foam

was observed, then · the cement and 1/3 of the water with high-range water reducer were added, · the concrete was mixed for three minutes, · covered and allowed to rest for three minutes, and finally · mixed for an additional two minutes before casting. [Kevern 2008]

To improve the bond between the cement paste and the aggregate, Schaefer et al. (2006) used a special mixing method:

· first dry mixing a small amount of cement (<5% by mass) with the aggregate until completely coated (about one minute),

· add remaining cement and water (with or without high-range water reducer), · mix the concrete for three minutes, · rest for three minutes, · mixed for an additional two minutes before casting. [Schaefer et al. 2006]

3.1.2.3 Placement, compaction and curing The amount of compaction can have considerable effects on the function of pervious concrete. It will have an effect on most of the PC properties, as especially permeability, strength and durability. Normal laboratory concrete compaction methods (vibration methods) will not represent field compaction methods properly.

For instance, in laboratory trials top surface compaction has often been used. It is simple and can be applied directly to field construction. The compaction energy should be controlled. One lift casting can be used since it simulates actual placing condition in the field.

In [Schaefer et al. 2006], specimens were placed by rodding 25 times in three layers while applying a vibration for five seconds after rodding each layer.


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Before vibration and compaction, the paste covering coarse aggregate touches each other very lightly (small contact area). With the process of vibration, porous concrete is compacted starting from top to bottom. The top portion undergoes compaction more than the other parts. With sufficient vibration a large number of adjacent aggregate at the top of concrete specimen starts to get closer to each other, and also to touch each other. The excess paste fills the voids and also moves downwards. At this point, compaction should be completed.

Also in the field PC requires alternative placement methods to those used for conventional concrete placement. PC must be compacted properly and quickly protected. If PC is not compacted well enough, or is placed too dry, the aggregates will not bond well and the pavement will be susceptible to raveling. If, on the other hand, the concrete is placed too wet or is overcompacted, the surface will be sealed and the pavement will not be permeable. [Delatte et al. 2007]

PC should be consolidated normally by static and vibrating rollers and screeds. Rolling compaction can be achieved for instance by using a motorized or hydraulically actuated, rotating, weighted, tube screed that spans the width of the section placed and exerts a minimum vertical pressure of ca. 70 kPa on the concrete. (Figure 9a) In all, with the wide variety of placement techniques in use (plate compactor, vibratory screed, roller, high density paver), an attempt to standardize the equipment used is important. The goal is to seat the aggregate and enhance the bond between aggregates. Excessive compaction effort shall not be used. Too heavy compaction will cause the voids to collapse thus reducing the porosity. [Brown 2006, Putman & Neptune 2011, Delatte et al. 2007]

Cross rolling shall be performed using a roller specifically designed to smooth and compact pervious concrete. Figure 9b shows an appropriate roller for pervious concrete pavement. Lawn rollers are not allowed. [CRMCA 2009]

Henderson (2012) compared five field sites in Canada. When there was in one site no compaction effort applied to the surface of PC, the highest amount of raveling of any of the sites was detected. In this location placing was with the Bidwell Bridgedeck paver. The other sites were all compacted during construction and generally performed adequately in terms of permeability.

Joints

Once the pavement has been placed and compacted, joints may be installed with a jointing tool. Alternatively, the joints may be saw cut later. According to a field study by Henderston (2012), both saw cutting and forming the joints had similar long term performance. Formed joints have been normally the preferred method because saw cut joints may cause raveling. A joint roller, often called a pizza cutter, quickly and easily forms PCP joints, as shown in Figure 9c. When joints were not included, cracks developed. Since PC shrinks less than standard concrete, joint spacing larger than ńormal´ 3.7 m has been used, e.g. 6 m. In the field study by Henderson (2012), the cracks were not observed to have a large impact on pavement serviceability. [Delatte et al. 2007, Scaefer et al. 2006, Henderson 2012]


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Figure 9. a) Roller Screeding Fresh Pervious Concrete; b) Cross Rolling Pervious Concrete; c) Roller used to make joints in pervious concrete. [CRMCA 2009, Schaefer et al. 2006]

Putman & Neptune (2011) collected some information on how properties of laboratory specimens compacted by different methods have correlated with the field core properties:

· Cylinder (Ø 150mm; h 300 mm) compacted in two layers, for each layer ten blows of a standard Proctor hammer (2.5 kg):

o none of the samples were within the range of the void content for the field cores, but the void contents were within the generally anticipated range of values for pervious concrete.

· Rodding in accordance with ASTM C192: o compressive strength values were significantly higher and porosity values

significantly lower than for the field cores. · A custom built pneumatic press that applied a compactive effort of 70 kPa uniformly

over the 100 mm diameter cylinders: o statistically similar properties (compressive strength, permeability, and

porosity) as for the pavement cores.

In the comparison of test specimen preparation techniques for pervious concrete pavements by Putman & Neptune (2011), the aim was to produce specimens having properties similar to in-place pervious concrete pavement. In the field a hydraulic roller screed was used. It spun in the opposite direction than it was pulled. Following the screed, the fresh concrete was cross-rolled using a 1 m long roller having a 150 mm diameter. Preparation of cylinders (Ø 150 mm; h 300 mm) were by different consolidation techniques:

· by the use of a 15.9 mm diameter steel rod (standard tamping rod) · by a standard Proctor hammer (2.50 kg) and · by dropping a mould on a concrete surface from a height of 50 mm.

In the field slabs with the same thickness as the pavement (150 mm), and by the same consolidation method as the pavement itself were prepared.

The slabs (600 mm) were most consistent with the in-place pavement density and porosity. Of the cylinder consolidation procedures tested, the standard Proctor hammer provided the least variability of results and yielded properties similar to the in-place pavement. Normal concrete compaction methods (vibration methods) will not represent field compaction methods properly.

The Modified Proctor compaction test (hammer weight 10 lb = 4.54 kg) was used in the study by Chopra et al. (2007). Compaction for a specimen (Ø 101 mm; h 203 mm) was in 5 layers with 25 blows/layer, dropping height was 0.457 m. This compaction provided a high level of energy but it was concluded that even higher levels should be tested and compared.

Compaction energy in the Standard and Modified Proctor Compaction can be calculated as follows, where the “Volume” is specimen volume [Chopra et al. 2007]:


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3.1.3 Porosity, pore sizes, infiltration/percolation rate, hydraulic conductivity, permeability

In the complex microstructure of normal concrete, the pores can be present from the nano-scale to the macro-scale, normally from 1 nm to 1 mm. Some bigger than 1 mm compaction pores may also be included. All these pores are included in cement paste (gel pores, capillary pores and entrained air voids or compaction pores). Aggregate include also some pores but good quality aggregate porosity is very small.

The structure of PC differs from normal concrete as it includes also a significant amount of relatively big interconnected air pores between coarse aggregate particles. Actually the majority of pores in PC are formed by the spaces left between coarse aggregates. It is important to distinguish between different porosities defined or determined for PC:

· effective porosity (interconnected voids that make the concrete permeable) · total porosity (includes all cement paste nano- and microporosity (gel+capillary)) and

also traditional concrete air content in the mortar phase · air content in the mortar phase (% of mortar phase or % of pervious concrete) · porosity values determined based on different testing methods (see Chapter 6

(Laboratory and field testing - standards and methods).

Because of the unique structure of PC, all the normal standard methods for normal concrete to measure porosity, and for instance entrained air pore content by pressure or volumetric method, are not all applicable. (See Chapter 6 (Laboratory and field testing - standards and methods) [Kevern et al. 2009b]

Porosity of PC is normally 20–30% [ACI 522R-10]. Porosity may also be as low as 11% or as high as 35%. In general, higher porosity means lower strength and normally also higher permeability. (Figure 15).

The size range of pores, the pore structure and pore connectivity, are all major factors influencing PC’s properties. For large-sized pores, larger aggregate sizes are recommended. A range of porosities can be obtained by blending aggregates of two different sizes. When using aggregate blends, the ratio of the smallest aggregate size to the largest aggregate size may not be too high (should be less than 2.5) so that the smaller aggregates will not fill the voids between the larger aggregates.

Compaction will also have a great effect on the porosity and permeability. The unit weight of PC can be e.g. 1680–1920 kg/m3 depending on the compaction level. With a constant paste content, the void content is reported to be a function of compaction effort, aggregate particle shape and texture, and aggregate uniformity coefficient. Smoother, more rounded aggregates produce lower effective void contents at the same compaction effort. [Crouch et al. 2006].

Given the value of hydraulic conductivity (m/s) for a system, the permeability (m2) can be calculated as:


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In-situ drainability or infiltration rate (mm/s) is naturally an important property of porous concrete. In general it will increase when porosity is higher and when there are more interconnected air pores.

Different infiltration rate values can be found for different studies and mix designs. Below is some information on porosity and infiltration rate/hydraulic conductivity [ACI 522R-10, Neithalath et al. 2006, Huang et al. 2010a]:

· A minimum porosity for any significant infiltration rate is ca. 15%. · Typical flow water through pervious concrete are from 2 to 5.5 mm/s. [Mata 2008] · For a PC porosity of 20% to 25% the infiltration rate is ca. 2–4 mm/s, or even

15 mm/s. o For an accessible porosity of 20–29%, the infiltration rate is around 10 mm/s.

· For PC porosity of 30%, the infiltration rate is ca. 5–20 mm/s. · For gravel and coarse sand the infiltration rate is ca. 1 mm/s. · For fine sand the infiltration rate is 1–0.01 mm/s. · A drainage rate of 100 to 750 l/min/m2 has been reported for several pervious

concretes [Tennis et al. 2004]. · Intrinsic permeability of 1 x 10-10 m2 to 5 x 10-10 m2 has been reported for pervious

concretes with porosity ranging from 17% to 28% (Neithalath et al. 2006).

According to Beeldens et al. (2009), in the case a pervious lean concrete is used as a base layer, a minimum compressive strength of 13 MPa (measured on cores with a surface of 100 cm²) has to be reached as well as a permeability coefficient of 0.4 mm/s (4×10-4 m/s). [Beeldens et al. 2009]

After all, the efficiency of pervious concrete to drain stormwater is also highly dependent on the clogging rate with time. (see Chapter 7.1 (Clogging))

Figure 10 presents some examples on how aggregate sizes, pore sizes, porosity, and intrinsic permeability are related. All the three specimens have similar values of porosity but the specimen with 3/8” aggregate (9.5–12.5 mm) shows a remarkably higher permeability, possibly due to its larger pore sizes (a = 4.76 mm). It can be seen that the specimen that exhibits the highest permeability neither has the highest porosity, nor the largest pore size. Permeability generally increases with an increase in porosity but there is no definitive relationship between these parameters (Figure 11). Porosity is a volumetric property but permeability is a parameter that defines the flow properties through the material. For this also the distribution of the pore volume and the pores’ connectivity are important.

Figure 10. Relationship between aggregate sizes (#8 = 2.36–4.75 mm; #4 = 4.75–9.5 mm and 3/8” = 9.5–12.5 mm), pore sizes (a [mm]), porosity (Ø), and intrinsic permeability [m2], (a) single sized aggregate mixtures, (b) blended aggregate mixtures. [Neithalath et al. 2006]


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Figure 11. Porosity-Permeability relationship for porous concrete mixtures. [Neithalath et al. 2006]

There is no straightforward methodology to measure the pore tortuosity and connectivity.

There are several methods and standards for the determination of porous concrete porosity and permeability (see Chapter 6 (Laboratory and field testing – standards and methods)).

Hydraulic conductivity will decrease if porous concrete is not homogenous, i.e. there is low porosity on the bottom or surface portion of the porous concrete layer. That is why it is useful to also study the void distribution with height. This can be done for instance by cutting a specimen into three equal portions, and by determining void ratio for each portion. Visual examination of in half cut specimens can also give good information on the mix and the effect of compaction.

Chindaprasirt et al. (2008) studied voids and state at the bottom surface of cylinders and compared it with compressive strength. The strengths with designed void ratios of 15%, 20% and 25% were between 38–44 MPa, 29–35 MPa and 15–22 MPa, respectively. At full compaction, void patterns at the bottom surface of concrete cylinders are different depending on the designed void ratios and flow values of paste. (Figure 12) The low voids contents at the bottom of the concrete are caused by relatively high flow of paste, high paste content, and high compaction energy. If paste has a tendency to flow downward it may fill the voids resulting in non-porous concrete. In the case where the designed void ratio was high, high paste flow value was favourable. In this case strength was higher than with lower paste flow value. (Figure 12)

Figure 12. State at the bottom of porous concrete and compressive strength. [Chindaprasirt et al. 2008]


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In [Putman & Neptune 20011], the infiltration rate of pervious concrete pavement was studied. The results indicate that the infiltration rate varied widely throughout the pavement. According to [Puttman & Neptune 2011] this is fairly typical. The potential sources for this are variability during construction (variation in mix properties from truck to truck, mix delivery delays or queues, equipment malfunction, weather). Anyway the number of locations having too low infiltration values is limited and they are surrounded by areas having adequate infiltration where the water can drain to and infiltrate through the surface and into the underlying infiltration bed. The average infiltration rates for the 3 different projects included in the study were 5 mm/s, 6 mm/s and 9 mm/s.

3.1.4 Strength and mechanical performance

For PC, strength is as important as its permeability characteristics. The strength of the system not only relies on the compressive strength of the pervious concrete but also on the strength of the soil beneath it for support. The needed pavement thickness is dependent on the compressive strength of the concrete, the quality of the subgrade beneath the pavement, as well as vehicle volumes and loadings. [Chopra et al. 2007]

In this Chapter only the strength of the PC itself is reviewed. Dimensioning is discussed more in Chapter 5 (Dimensioning).

In porous concrete the cement paste is limited and the aggregate rely on the contact surfaces between one another for strength. Therefore harder aggregate, such as granite or quartz, will yield higher compression strength than a softer aggregate like limestone. [Chopra et al. 2007]

Strength and permeability tend to counteract one another. In a study by Chopra et al. (2007) experimental studies on the compressive strength of PC as it relates to water-cement ratio (w/c), aggregate-cement ratio (a/c), aggregate size, and compaction were made. The aim was to find a balance between water, aggregate, and cement contents. In this study, the compressive strengths of acceptable mixtures were less than 12 MPa. On the other hand extremely high permeability rates were always achieved. Based on the high permeability rates obtained in this study, it was concluded that a/c ratios to less than 5:1 should be used.

When pore volume increases, strength typically decreases. For PC, strength of paste is also an important factor because strength of aggregate is generally higher than that of paste. (Figure 13)

The equation of strength and void ratio of porous brittle material is useful for estimating strength of porous concrete:

σ = σoexp(-bV), (2)

where σ is compressive strength (MPa) σo is compressive strength of paste at zero void [MPa] V is void content [%] and b is experimental constant.

In [Chindaprasirt et al. 2008] the strength and air void of pastes were found to be 135 MPa and 1.0%. The fitting of the curve as shown in Figure 13 results in the value of σo = 152 MPa, b = 0,084 (R2 = 0.96). Porous concrete with dripping of paste gave slightly lower strength than that obtained from the equation.


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Figure 13. Porous concrete compressive strength as a function of total void ratio. [Chindaprasirt et al. 2008]

Too little water or too stiff paste results in no aggregate bonding, and too much water or too low paste workability will settle the paste at the base of the pavement and clog the pores.

Strength can be increased by using selected aggregates, fine mineral admixtures as silica fume, by using admixtures and by adjusting the concrete mix proportion. Fly ash has been used to increase workability. Latex modification is also possible to increase mechanical properties. For instance in Europe many highways have being constructed with using an overlay of latex modified PC. Polymer-modified porous concretes have exhibited better fatigue behaviour than those without polymer. [Yang & Jiang 2003, ACI 522R-10, Pindado et al. 1999]

Increase in paste volume fractions and larger aggregate sizes resulted in increased compressive strengths in the research by Deo & Neithalath (2010). All the mixtures were designed to achieve a volumetric porosity of approximately 20 ±2%. Figure 14 presents compressive strength as a function of paste content for two mix designs with different aggregate contents (aggregate/cement (a/c) ca. 2.75 or 5). Higher paste content resulted in an increase in the thickness of the cement paste layer around the aggregates and thus better bonding and increased peak strength. The reduced compressive strength of the smaller sized aggregate mixtures was attributed to the larger number of pores engaged in the damage process. [Deo & Neithalath 2010]

Deo & Neithalath (2010) found also that an increase in pore volume fraction by approximately 10% resulted in a reduction in the compressive strength by about 50%. The pore structure features other than porosity value alone were also responsible for the compressive strength. They found that compressive strength was influenced by the pore sizes, their distributions and spacing. [Deo & Neithalath 2010]


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Figure 14. Influence of paste volume on the compressive strengths of pervious concrete specimens. [Deo & Neithalath 2010]

Schaefer et al. (2006) found that the PC made with single-sized coarse aggregates generally had high permeability but not adequate strength. Addition of a small amount of fine sand (approximate 7% by weight of total aggregate) to the mixes significantly improved the concrete strength.

PC compressive strength is normally 3.5–28 MPa, and a normal value is 17 MPa. It has been typical to have PC with strength less than 17 MPa when used in low traffic areas. For more demanding cases the strength should be higher, for instance in the range of 24–28 MPa. [Schaefer et al. 2006, Huang et al 2010a]

Figure 15 presents some collected values for normal porous concrete strength as a function of effective porosity. [Yang & Jiang 2003, Huang et al. 2010a, Deo & Neithalath 2001, Chopra et al. 2007, Delatte et al. 2007]

Figure 15. Some collected values for porous concrete strength as a function of effective porosity. [Yang & Jiang 2003, Huang et al. 2010a, Deo & Neithalath 2010, Chopra et al. 2007, Delatte et al. 2007]

Flexural strength and dynamic modulus of elasticity are important for the structural behaviour of pavements. Flexural strength is influenced by many factors, especially the degree of compaction and porosity.

Flexural strength is not normally tested but it is typically from 1.0 MPa to 3.8 MPa. Many factors have been shown to influence the flexural strength, particularly the degree of compaction, porosity, and the aggregate to cement ratio. [Tennis et al. 2004] An increase in aggregate size results in a reduction in the flexural and compressive strengths of PC, due to


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the increased total porosity and pore size. Flexural strength of 3 MPa has been reported with 25% porosity and aggregate size of 6 to 10 mm. A small amount of sand (ca. 5% of volume) can possibly increase the flexural strength. Polymer addition has also increased flexural strength [Huang et al. 2010b].

In [Wang & Wang 2011] the elastic modulus of the pervious concrete was chosen to be 13.8 GPa which was considered to be a conservative value comparing to some reported laboratory testing data.

An estimation and statistical analysis on pervious concrete pavement design inputs (static modulus of elasticity, split tensile strength and flexural strength) as a function of compressive strength and effective void content is presented in [Crouch et al. 2008]. According to them the ACI 318 (Building code requirements for structural concrete) equation appeared to be a promising but conservative method of estimating static modulus from PC compressive strength and unit weight. The Áhmad and Shah´ equation (1985) appeared to be a promising method of estimating flexural strength of PC compressive strength.

PC shrinkage is much smaller than normal concrete shrinkage. [Huang et al. 2010a]

Nano-modified cementitious materials show the greatest potential of improving internal transition zones (ITZ) characteristics in conventional concrete and may prove even more beneficial for use in pervious concrete. By controlling the nanoscale behavior, macrolevel properties can be improved. [Kevern 2008]

3.1.5 Freeze-thaw durability – with/without chlorides

When assessing the durability of pervious concrete pavements in cold climates, there are two aspects that should be considered: • durability of the pervious concrete material itself and • durability and winter performance of the whole system.

Winter performance of pervious pavements as the whole structure is reviewed more closely in a Finnish CLASS-project State-of-the-Art Report concentrated on the winter performance of pervious pavements [Kuosa & Niemeläinen 2013]

In all there are several testing methods for assessing conventional concrete freeze-thaw resistance with de-icing salt solution. In these methods specimens are in contact with salt solution during the testing. There is solution layer on or below the specimen surface, or the specimens are immersed in the salt solution. Solution contact is needed for scaling. Most typically a 3% salt (NaCl) solution is used as it normally is the most critical for the surface scaling degree. Specimen surface scaling (g/m2) or volume change (vol.-%) is normally measured. Internal deterioration caused by cracking can also be determined, e.g. the change of relative dynamic modulus of elasticity.

For instance in the European testing specification for concrete [CEN/TS 12390-9:2006(E)] there are three testing methods for concrete freeze-thaw scaling with de-icing salt: Slab-Test, CDF-method and Cube-test. The Slab-Test is normally used in Finland (Figure 16b). This method is based on the earlier Swedish standard SS 13 72 44 (so called “Borås-method”). In this test there is one freeze-thaw cycle per one day. For structural concrete normally 56 cycles are needed. In the testing of paving units the demand (in Finland) is presented based on the scaling degree after 28 cycles (average <1 kg/m2). The minimum temperature is -20±2 °C, maximum temperature is +20±4 °C, and cooling is from +0 °C to -20 °C in 12 hours (1.7 °C/h).

For the estimation of the freeze-thaw resistance of structural concrete with regard to internal structural damage, there are three different methods in the European testing standard


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[CEN/TR 15177 (2006)]. No single test method is established as a reference test method as these methods produce relatively consistent results. These methods are:

· Slab-Test, · CIF-method and · Beam test.

In Finland mostly Slab-Test is used. Besides SFS 5447 (1988) is still in use (Freeze-thaw durability; normally with beams 500×100×100 mm3). This method essentially includes freezing in air (-20 °C) and thawing in water (water temperature less than 40 °C, and end of thawing cycle temperature is +20°C). SFS 5447 is a very loosely defined method. Besides, thawing by the relatively warm water (< 40 °C) is a harsh method especially for relatively brittle high strength concrete, and at the same time for low w/c paste. Thus it is also a harsh method for PC with typically very low w/c (< 0.25…0.30).

In [By 50 2012] this method (SFS 5447) is an optional quality control method in addition to CEN/TR 15177 Slab-Test and air pore analysis (Spacing factor). Based on the demanded design service life and exposure class, the amount of freeze-thaw cycles is 100 or 300. The acceptance criterion is based on either RDM as measured by UPTT (≥75%) or relative flexural or splitting tensile strength (≥67%). [By 50. 2012]

In North America the most commonly used testing standard for freeze-thaw resistance is ASTM C666 /C666M-03(2008). This standard includes two methods:

o Rapid Freezing and Thawing in Water (Procedure A) and o Rapid Freezing in Air and Thawing in Water (Procedure B).

The open structure of PC allows free ingress of water into the specimen. Subsequent freezing of saturated PC could cause rapid deterioration. Anyway, freeze-thaw damage has been reported to develop in pervious concrete primarily as paste deterioration of lower porosity pervious concrete. [Mata 2008]

The general recommendation for pervious concrete systems in freeze-thaw environments is to install a layer of aggregate base below the PC pavement to store stormwater in order to avoid saturation of the pervious concrete during freeze-thaw events. According to Kevern et al. (2008) there are no documented cases of freeze-thaw failures of existing installations when these recommendations are followed. However, there is still some potential for saturation of the pervious concrete layer and it is therefore necessary to design pervious concrete mixtures to be freeze-thaw resistant in case the pervious concrete does become saturated during freeze-thaw events.

Testing of concrete by freezing and thawing while submerged in water is a harsh performance test method. This is because cement paste water saturation will get high during the testing. However, a PC mixture that passes this kind of testing will have a high probability of performing well in the field. If the pavement system drains well enough to keep the PC from being saturated, then the harsh conditions represented by for instance the ASTM C 666 (Procedure A) test do not apply. This is, therefore, the goal of the system design. [Delatte et al. 2007]

According to [Mata 2008] the de-icer salt test described in ASTM C672 [ASTM C672/C672M-12] may be better suited for evaluation of durability of concrete in paving applications because it involves slower, more realistic freezing cycles in the presence of a de-icing salt solution (4 % NaCl) and attempts to more realistically simulate frost exposure conditions in a pavement. In this test minimum temperature is -18 ±2 °C, maximum temperature is +23 ±2 °C and there is one freeze-thaw cycle per one day.

In his work Mata (2008) used a modification of the ASTM C672 method. (Figure 16a) Evaluation was based on mass loss and changes in dynamic modulus of 25 mm thick concrete disks with 100 mm diameter. The results of this study indicated that small disks,


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tested at realistic freezing rates, could be successfully used to evaluate frost resistance of paving mixtures. To get information on the effect of PC microstructure on the frost resistance, Mata (2008) used one additional disk specimen to assess qualitatively the entrained air void system by stereoscopic microscope. The entrained air void system could be interpreted as “inadequate,” “acceptable,” or “excellent” by an experience analyst.

Mata (2008) concluded that the cement paste alone, i.e. without any fine sand, is not sufficient to develop the air entrained air voids during the mixing phase, required to protect the concrete to water expansion on freezing. The results of the freezing and thawing tests confirmed that the addition of 7% sand by weight as replacement for coarse aggregate increases the frost resistance of pervious concrete significantly and, when used with an adequate amount of air entraining agent can provided adequate frost resistance as measured by the modification of ASTM C672 method used in this study. According to Mata (2008) additional research is still needed to determine the percentage of entrained air voids related to air entraining admixture and sand content in a low w/c PC mixture.

For concrete surface scaling the binding material and the also the microstructural changes caused by carbonation and drying in it (long term ageing) are known to be essential, in addition to water-cement ratio and amount (vol.-%) and spacing (mm) of small (<0.3 mm) entrained air pores. [Kuosa et al. 2012]

a) b)

Figure 16. a) ASTM C670 modified pervious concrete freeze-thaw testing with salt solution [Mata 2008]; b) CEN/TS 12390-9:2006(E) Slab-Test for the determination of surface scaling and internal deterioration caused by freeze-thaw with de-icing salt solution.

Partially saturated pervious concrete in air has demonstrated substantially higher durability than those subjected to freezing and thawing under water. This means that a good way to increase freeze-thaw durability is to take care of good drainage, i.e. bv using a good subbase system. Caution should be exercised when using pervious concrete in a situation where complete saturation may occur. [ACI 522R-10]

The NRMCA (National Ready Mixed Concrete Association) in the U.S.A. has defined four exposure climate categories based on moisture (wet or dry) and temperature (freeze or hard freeze). Below is some basic information on the expected performance of PC in these exposure climates. [Delatte et al. 2007]

Dry Freeze and Hard Dry Freeze · As there is little precipitation during the winter (´dry´) PC is unlikely to be fully

saturated in these environments. No special precaution is necessary for successful performance of pervious concrete but a 100 mm to 200 mm thick layer of clean aggregate base below the PC is recommended.

Wet freeze


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· Since the ground does not stay frozen for long periods in ´wet freeze´ it is unlikely that the PC will be fully saturated, because of drainage. No special precaution is necessary for successful performance of pervious concrete but a 100 mm to 200 mm thick layer of clean aggregate base below the PC is recommended.

Hard Wet Freeze · These areas may have situations where the pervious concrete becomes fully

saturated. The following precautions organized in the order of preference, are recommended to enhance the freeze-thaw resistance of PC:

o Use an 200 mm–600 mm thick layer of clean aggregate base below the PC. o Attempt to protect the paste by incorporating air-entraining admixture in the

PC mixture. o Place a perforated PVC pipe in the aggregate base to capture all the water

and let it drain.

Laboratory study by Yang et al. (2006) give detailed information on the effects of moisture conditions on the damage development in PC during cyclic freeze-thaw. The degree of saturation of the paste played an important role in the damage development in PC. Both the fundamental transverse resonant frequency and the mass of each specimen were monitored during freeze-thaw testing. In the beginning of the freeze-thaw testing, mass change included possible water absorption (if freezing was in water). Vacuum saturation was used in some cases to get all the pores water saturated, including capillary and air pores in the paste fraction of PC. It was found that:

· PC specimens that were vacuum-saturated and then frozen and thawed in water exhibited the lowest freeze-thaw durability.

· The vacuum saturated specimens that were sealed and frozen and thawed in air showed a relatively higher freeze-thaw resistance. (Figure 17 a)

· The highest freeze-thaw resistance occurred on the PC specimens without vacuum saturation, which were frozen and thawed in air under sealed conditions.

· When PC specimens without vacuum saturation were frozen and thawed in water, a drastic decrease in the freeze-thaw resistance was observed.

· Vacuum-saturated and sealed conventional concrete exhibited very low resistance to freezing and thawing cycles (completely failed at about 13 cycles), whereas PC with the same pre-treatment and test conditions showed higher freeze-thaw durability (lasted for 100 cycles). (Figure 17 a)

o The slow damage development in PC can be attributed to the lower internal pressure generated during freezing in a thin layer of paste in PC.

· Instead, when PC was partially saturated and exposed to a wet environment, it was found to deteriorate much more rapidly than conventional concrete.

o For PC a drastic increase in weight occurred during the first few cycles. This was because of the rapid water uptake that occurred at the beginning of the test. (Figure 17 b)

o PC specimens with air-entraining admixture could resist more than 300 cycles implying that air entrainment had significant impact on the freeze-thaw durability in partially saturated PC. (Figure 17 b)

o Conventional concrete (with 5.5% air) was able to resist more than 2000 freeze-thaw cycles. The primary reason for the relatively poor PC freeze-thaw resistance in water (for PC 300 cycles, and for conventional concrete 2000 cycles) was the fast saturation of air void system in the thin PC paste layer. (Figure 17 c)

o Partially saturated (water-cured specimens, no vacuum saturation) air entrained PC was found to be very durable when freezing and thawing was in air. (Figure 17 d)

It should be noted that the rapid freezing and thawing (ASTM C666 - 6 to 7 cycles per day) was used in the study by Yang et al. (2005). This is significantly differed from field conditions.


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Higher freezing rate typically leads to faster damage development. According to [Yang et al. 2006] and [Mata 2008] using a fast freezing rate may not be suitable for evaluating the freeze-thaw durability of field PC.

a) b)

c) d)

Figure 17. a) Comparison on freeze-thaw damage development between PC and conventional concretes with vacuum saturated and sealed specimen (no water uptake); b) Variation of specimen weight (water uptake + volume loss) as a function of freeze and thaw cycles for PC with water cured specimen (no vacuum saturation) subject to freeze-thaw in water; c) Typical slow freeze-thaw damage development in water for conventional concrete; d) PC specimen weight as a function of freeze-thaw cycles with water cured specimen (no vacuum saturation) subject to freeze-thaw as sealed (no water uptake) [Yang et al. 2006]

Schaefer et al. (2006) provide considerable information on developing durable PC mixtures. His tests indicated that entraining air in the cement paste improved resistance to freeze-thaw. He also found that addition of a small amount of fine sand (approximate 7% by weight of total aggregate) to the mixes significantly improved the concrete strength and freezing-thawing resistance while maintaining adequate water permeability. Shu et al. (2011) found also that by air entrainment the freeze-thaw durability of PC can be essentially improved, and besides Mata (2008) found that some sand must be included in the mixture for good freeze-thaw resistance. Mata (2008) used realistic freezing rates to study freeze-thaw resistance of PC. [Mata 2008, Schaefer et al. 2006, Shu et al. 2011]

According to Wang et al. (2006) well designed PC can meet freeze-thaw durability requirements for cold weather climates. In their laboratory study a PC including river gravel (4.75–9.5 mm,) 7% sand (90% < 2.36 mm), and air entrainment, showed the best freeze-thaw durability. For this PC there was only 2% mass loss after 300 cycles (ASTM C 666,


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Method A, freeze-thaw in water). Addition of a small amount of fibres (0.03 or 0.10% by volume) to the mixes increased strength, air void content and freeze-thaw resistance. Fibres also increase toughness. [Schaefer et al. 2006, Wang et al. 2006]

Henderson (2012) studied the effects of winter maintenance on PC slabs. There were different exposures besides freeze-thaw. These exposures were moderate or heavy sand loading with moderate or heavy precipitation. In addition, one group of slaps was with heavy salt loading and heavy precipitation. Freeze-thaw cycling of the slabs involved moving them in and out of a walk-in freezer. Freeze-thaw cycling and moisture without salt exposure were found to alter the internal structure of pervious concrete. The internal structure was altered when the paste in the PC deteriorated due to the freeze-thaw cycling. This deterioration created small particles which were able to move throughout the pervious concrete and close some of the previously available drainage paths. In all, the extent to which this occurs was anticipated to be highly dependent on the characteristics of the paste in the PC. The surface distresses that developed on the slab were generally limited in this study, and included only a small amount of raveling, some paste loss and fracturing of very few aggregates. However, one group of slabs showed more surface condition deterioration than the others. These slabs were those with heavy salt loading and heavy precipitation. These slabs had also cracks developed throughout the slabs, and substantial portions of some of the slabs fell finally off after 255 cycles, the equivalent of five years of freeze-thaw exposure in Toronto. No information on the cement type in these PC mixes, or information on the hardened paste air content and quality (e.g. size of the pores) was given in [Henderson 2012]. The true reasons for the deterioration of the PC slabs with freeze-thaw exposure with salt remain unclear. Based on the low durability with freeze-thaw exposure with salt it was concluded that PC pavement winter maintenance in Canada should preferably not include salt or salt solutions. [Henderson 2012]

According to [Mata 2008], sedimentation and frost resistance may be directly related in wet cold climates. If clogging of either the PC or the PC system occurs, water may accumulate in the pavement and freeze. Based on field observations of PC pavements located in cold weather climates, with an average of 5 years of service, it appears that the technology to protect pervious concrete itself from the effects of freezing and de-icing salts already exists but additional research is needed to determine the percentage of entrained air voids related to air entraining admixture and sand content in a low w/c pervious concrete mixture. Instead, combined effects of clogging on potential saturation, reduced infiltration of the subsoil, changes in the depth of freezing and seasonal differences in storm intensity, including snow melt have not been fully resolved.

3.1.6 Abrasion and raveling

Because of the rougher surface texture and open structure of pervious concrete, abrasion and raveling of aggregate particles can be a problem. Especially when pervious concrete is applied to pavements in areas which undergo freeze-thaw, durability also refers to the surface abrasion resistance against snow clearing operations. If pervious concrete is to progress from parking lot applications to low-volume and potentially high-volume applications, the pavement must be resistant also to all aspects of cold weather maintenance. [Tennis et al. 2004, Kevern et al. 2009a]

Henderson (2012) found that the surface distress of the slabs subjected to freeze-thaw showed a common trend. This trend was the initial presence of paste loss and at a later point the presence of raveling. This tendency suggests that raveling maybe a result of the paste characteristics. The integrity of the paste to withstand exposure to freeze-thaw cycling may have a large effect on the amount of raveling experienced by a pervious concrete pavement. [Henderson 2012]

Kevern et al. (2009) presents results of combinations of four different PC mixtures cured using six common curing methods. The surface abrasion of the concrete was tested using a


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rotary cutter device (ASTM C944). The results show that the concrete abrasion resistance was improved with a majority of surface-applied curing compounds; however the surfaces covered with plastic sheets produced the lowest abrasion levels. Of the surface-applied curing compounds, the best abrasion resistance and highest strength concrete was that applied with soybean oil. The best abrasion resistance and highest strength overall was the mixture containing fly ash and cured under plastic for 28 days. [Kevern et al. 2009a]

The addition of fibres has the potential to reduce surface abrasion and increase tensile strength while potentially increasing at the same time porosity and permeability. Kevern et al. (2009b) found that the “birds nest effect” caused by the fibres increased the porosity by 7.9% and yet produced a tensile increase of 21% over the control without significantly impacting surface abrasion. [Kevern et al. 2009b]

For the laboratory testing methods for abrasion see [Kuosa & Niemeläinen 2013] (Chapter 2.1.7 Strength and resistance to degradation).

Figure 18. Surface raveling (observed at Charter School, Gary, Indiana). [Delatte et al. 2007]

3.1.7 Noise reduction

Significant increases in noise is a problem in urban areas. Noise is typically generated from many sources, such as various types of vehicles on the road, airplanes, factories, and construction sites. Normal concrete or thick glass panels can be thought of as ‘sound shielding materials’. Instead, porous concrete and porous asphalt can be used as a sound absorbing construction materials in urban areas. Many experimental studies have been conducted in the past to develop efficient sound absorbing porous concrete or asphalt. [Kim & Lee 2010, Danish Road Directorate 2012]

The open structure of the porous pavement causes a difference in arrival time between direct and reflected sound waves, as shown in Figure 19. This difference decreases the noise level intensity, causing porous pavements to absorb the sound. Olek et al. (2003) found that the pore volume and pore sizes had a significant influence on acoustic absorption. The tortuosity of the pore network, which forces the waves to travel longer, and the frictional losses in the pore walls were the main mechanisms responsible for energy loss. [Olek et al. 2003, Schaefer et al. 2006]


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a) b)

Figure 19. Reflection of sound waves resulting from moving vehicle: a) Wave reflection from a dense surface; b) Wave reflection from a porous surface. [Olek et al. 2003]

Kajio et al. (1998) [Schaefer et al. 2006] compared the noise levels produced from pervious concrete and dense asphalt pavements at different vehicle speeds. Table 2 Kajio et al. (1998) showed that, for both sizes of PC aggregate studied, the noise level was reduced using pervious concrete. Small-size aggregate generally produced a quieter response, ranging from a 3% to 10% lower noise level, with a maximum difference of 8 decibels (dB). [Schaefer et al. 2006]

Table 2. Results of measurement of noise from pervious concrete slabs by Kajio et al. (1998), as presented in [Schaefer et al. 2006]

3.2 Porous asphalt

3.2.1 General

Porous asphalt (PA) is one type of permeable pavement material. It is often called also open-graded friction course (OGFC). Porosity of the asphalt mastic is usually between 15 and 20 percent. The subbase layers may include water storage systems or normal pavement structure as used also under impermeable surfaces (overlay structures). Functionality of the porous asphalt pavement is the same as that of other pervious pavements. Storm water infiltrates to soil thorough pavement layer and it reduces ponding and need of sewer systems (Figure 20). Effects on friction and hydrological system are positive [Ferguson 2005]. Additional benefits are also noise reduction, which is desirable feature in residential or high trafficked areas [Bendtsen et al. 2005] and better visibility when there is hydroplaning, spraying and light reflection reductions [Ferguson 2005, Hamzah et al. 2012A].


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Figure 20. The porous asphalt in the foreground offers a dry surface while a conventional dense-graded asphalt (background) remains wet. [Hansen 2008]

Porous asphalts have been studied and used already for many years, for example in Northern America. The very first porous asphalt structures were constructed in the 1970s. The oldest functioning permeable parking areas may be over 30 years old. One example of such a structure is a parking lot in Walden Pond State Reservation in Concord, Massachusetts. [Ferguson 2005] Today research and use of porous asphalts have spread around the world. In Europe porous asphalts are widely used in Switzerland [Poulikakos et al. 2006A] and the Netherland [Bendtsen 2011]. User experiences and research results are widely available. The Finnish The Finnish Pavement Technology Advisory Council (PANK) specification [PANK 2011] recommends using porous asphalts in fields, yards and other residential and light trafficked areas.

Choosing porous asphalt instead of other permeable pavements depends usually on economic factors. Porous asphalts are usually less expensive than pervious concretes or interlocking structures [Virginia DCR 2011]. Compared to impermeable asphalt structures including individual sewer system, fibre-reinforced porous asphalt is economically competitive [Virginia DCR 2011Also the structural demands such as bearing capacity will have an effect when choosing the pavement material. With proper construction and regular maintenance, porous asphalt is a long-life and functional pavement material especially for parking areas and low-trafficked roads.

Disadvantages of porous asphalts are void clogging and lower bearing capacity compared to conventional impermeable asphalts. When the voids of asphalts clog due to for instance sanding or debris in storm water, it reduces the infiltration rate. In cold climates (such as Finland), where sanding and the use of dust releasing studded tyres is common, the clogging potential is high. A draindown phenomenon in which the asphalt binder creeps and solidifies onto lower layer in the asphalt overlay (or in asphalt mixture while production or transporting) is not as common of a clogging occurrence in colder environments [Hamzah et al. 2012B]. However, clogging has to be taken seriously and that is why the regular maintenance, e.g. sweeping and washing of the surface, is needed.

When designing porous asphalt structures it is taken account location, hydrology, and road structure [Hansen 2008]. Location planning includes general weather and hydrology, underneath soil, and surrounding surface structures of the planned area. In cold climate areas (e. g. Finland) it is necessary to pay attention to freeze-thawing and permafrost.

The asphalt mixture design includes proportioning the aggregate mixture and the binder recipe. Thicknesses of different layers and general layer structure depend on e.g. traffic loads, frost depth and hydrology. Several design guides and specifications for porous


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asphalts are available. A porous asphalt pavement structure is presented in Figure 21. This structure is suitable for areas with poor quality soils and high groundwater. [Hansen 2008]

Figure 21. A porous asphalt pavement structure suitable for areas with poor quality soils and high groundwater. [Hansen 2008]

3.2.2 Porous asphalt mix design

Asphalt mixture consists of aggregate, binder and optional additives [PANK 2011]. The most important features of the porous asphalts are permeability and hydraulic conductivity. The other features are adhesion of binder to aggregate, stiffness, resistance to permanent deformation, fatigue and abrasion, skid resistance, noise absorption and general durability against ageing, weathering, chemicals, raveling, stripping etc. [EN 13108-7: 2006].

Aggregate proportioning and binder content haves significant effects on properties of porous asphalts [Suresha et al. 2010]. The Finnish PANK specification (2011) determines also qualities of e.g. the composition, grading, binder and additive contents, homogeneity, drainage capacity, water sensitivity, particle loss, binder drainage, temperature of the mixture and durability of the porous asphalt mixture. If the asphalt mixture is produced according to the standards and is properly tested (quality controlled), it receives the official CE marking.

There are several ways to design the PA mixtures. This depends on the circumstances, regulations and project-specific demands. In Finland designers of porous asphalt mixtures have to follow the standard SFS-EN 13108-7 + AC (Bituminous mixtures. Material specifications. Part 7: Porous Asphalt) which determines the required properties [EN 13108-7:2006]. PANK has more accurate specification guide for proportioning porous asphalts. Alvarez et al. (2006) and Dooley et al. (2009) present wide comparisons on the regularities and specifications concerning porous asphalts in different states worldwide.

General guideline for designing and proportioning the functional porous asphalt is to make the mixture sufficiently permeable and durable at the same time [Suresha et al. 2010]. Permeability can be achieved using coarser aggregates and less binder than in conventional, dense asphalts. Air voids in the mixture are relatively big and are well connected to each other. On the other hand, high porosity and minor amount of binder may cause the mixture to be less durable. Additional fibres, e.g. polymer, mineral or cellulose fibres, increase durability and decrease the drain-down phenomenon [Hansen 2008]. Proportioning can be empirical or performance based design, depending on how demanding the project is [PANK 2011].

Air void content is usually at least 16 vol.-% [Hansen 2008]. In the Finnish PANK specification (2011) the tested void content average is 17–25 vol.-%. A too high void content decreases durability. However, it is important to take note that the void content does not


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necessarily correlate directly to permeability; the connections between air voids makes the mixture permeable [Suresha et al. 2010, Qiu & Guan 2011] (Figure 22).

Figure 22. Sample slices of two porous asphalts for image analysis. Dmax is the maximum aggregate size. [Qiu & Guan 2011]

Thickness of the asphalt overlay depends on the traffic loads and can be between 76 and 180 mm. [Virginia DCR 2011]. In some cases the demand for the thickness depends also on the other factors such as frost depth and noise matters. The minimum is 38 mm, as thinner layers cannot bear any loads [Ferguson 2005]. The National Cooperative Highway Research Program (NCHRP) in the USA recommends a thickness of 19–25 mm with + 6 mm tolerance [Dooley et al. 2009]. Noise can also be decreased by a twin-layer structure, which consist two separate asphalt layers with different void contents [Masondo et al. 2002].

Aggregate is the skeleton of the porous asphalt. The aggregate amount is 60–90 w.-% of the porous asphalt [Ferguson 2005]. A high proportion of coarse aggregates provides the sufficient stone-to-stone contact [Dooley et al. 2009]. Aggregate consist mostly of stones or in some cases recycled stones of asphalt. Grains have to be angular and clean [Dooley et al. 2009]. Durability and stability of the aggregates in porous asphalt must meet the same standards as in conventional, impermeable asphalt, as resistance to polishing, stripping and raveling [Ferguson 2005, Dooley et al. 2009]. In colder climates such as in Finland, the freeze-thaw resistance and durability against studded tyres is important [PANK 2011]. Porous asphalts do not hold water, so the risk of moisture damages is dimished [Hansen 2008]. The aggregate mixture should contain fines as little as possible, because they fill the void space [Ferguson 2005]. The aggregate grain size is usually 9.5 mm or larger [Hansen 2008]. In a twin layer structures, the grain size of the base layer is larger and in the top layer it is finer for maintaining the permeability [Hansen 2008]. Aggregate proportioning is project specific, and depends on the needed strength, porosity and durability, and the available rock type.

Binder (bitumen, asphalt cement) is the glue between aggregate grains and it makes the asphalt mixture viscous and flexible [Ferguson 2005]. Binder makes a thin film around the aggregate grains. Binder consistency is measured with a penetration test. In colder climates it is recommended to use softer binders to avoid brittleness [Ferguson 2005]. The drain-down phenomenon happens when the binder trickles deeper in the asphalt layer and releases the surface aggregate grains. Warm temperature contributes this [Hamzah et al. 2012B], so risks are lower in northern, colder circumstances. In PA, the binder content is usually 5.75% of the whole weight of the mixture if the aggregate size is 9.5 mm [Hansen 2008]. On the other hand, Suresha et al. (2010) have investigated that the maximum binder content is 5.0 % for maintaining the sufficient void content and permeability. According to Alvarez et al. (2006) in the European mixtures the binder content is smaller than 6%. Penetration of binder varies between 60/70 and 100 depending on the project specific needs and used additives.


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Additional material can be added to stabilize and enhance strength of the asphalt binder. The most important properties of these additives are increasing the durability and reducing drain-down [Dooley 2009]. The more the strength of the binder is, the less the needed amount of it is. Therefore the void content increases as the binder strength increases. These additives are mostly fibres as cellulose, as well as minerals and modifiers such as polymer and rubber [Dooley 2009, Alvarez et al. 2011]. Other additives can be natural asphalt [Alvarez et al. 2006], anti-stripping agents such as lime or other fillers. Additives have also many other favourable properties, because the film around the aggregate particles can be increased by them [Wu et al. 2006]. Modifiers make the asphalt less viscose in warm temperatures and flexible in cold temperatures [Watson et al. 1998]. Wu et al. (2006) have illustrated the cellulose fibre having e.g. better resistance to abrasion and moisture sensitivity than polyester fibres. There are many encouraging empirical experiences using the additives [Dooley et al. 2009, Alvarez et al. 2011].

The Finnish proportioning specification for porous asphalt is described in Table 3 [PANK 2011].

Table 3. Porous asphalt mixture specifications according to the Finnish PANK (2011) specification.

Porous asphalt type

(AA = avoin asfaltti)

Aggregate, passing-% (sieve size, mm)

Bitumen (KB = kumibitumi

= rubber bitumen)

Binder content

[%]

Addition (optional)

The mass per unit kg/m2

(constant thickness

area)

0.063 0.5 2 8

AA 5 2…4 4…10 18…30 100 35/50…70/100 KB65 or KB75 5.0…6.0 50…75

AA 8 2…4 5…10 12…23 90 35/50…70/100 KB65 or KB75 5.0…5.8

Cellulose fibre or Natural asphalt

60…100

AA 11 2…4 5…9 10…19 33…53 35/50…70/100 KB65 or KB75 5.0…5.5 75…100

AA 16 2…4 4…10 9…19 27…43 35/50…70/100 KB65 or KB75 4.7…5.3 100…125

3.2.3 Porosity, permeability and drainage

Permeability is the key characteristic of porous asphalt. It makes the surface of the pavement drainable in wet conditions [Alvarez et al. 2011]. Porous pavement allows water not only to infiltrate into lower layers, but also evaporate to air from water storage layers [Ferguson 2005]. This means that the effects to the general hydrology are positive, and there is no vast need for stormwater collection systems. Permeability consists of air voids in the asphalt and connections between the voids and pores. When the liquid reaches the surface of the porous material, it flows thorough the below layers in some speed. Flow speed, alias hydraulic conductivity, K, depends on the asphalt mix properties and its construction and maintenance.

Porosity of the pavement and other structures must meet the site specific demands, e.g. water amount of local rainfall, runoff, and flood occasions in some time range (i.e. for the design storm). The porous top layer asphalt is less permeable than the reservoir layer. The underlying soil is usually the most critical layer with regards permeability, whereas the soil permeability is dependent on the soil material [Stenmark 1995]. The asphalt surface may be distributed layers of different void contents [Masondo et al. 2002, Bendtsen et al. 2005]. The subbase layers (chocker course, water storage layer, geotextiles, and underlying soil) are also with different permeabilities.


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If the porous asphalt is laid on the top of a conventional, impermeable asphalt, storm water drains off to the shoulder of the road [Bäckström & Bergström 2000]. The draining rate is depended on the porosity of the asphalt, runoff coefficient and road slopes [Ferguson 2005].

Porosity can be easily disturbed by clogging, which makes the porous asphalt voids smaller and more impermeable. Clogging is a cumulative phenomenon, which is caused by e.g. debris in the runoff water from adjacent areas, dust in air and winter sanding. Small particles penetrate into the voids on the surface layer in time. The lifespan of the porous asphalt may shorten quickly because of clogging in the case of inadequate maintenance. Other factors reducing permeability are drain-down, compacting and wheel friction, which drags the plastic asphalt sealing the pores [Ferguson 2005]. The drain-down or binder creep phenomenon is more apparent in warmer temperatures [Hamzah et al. 2012B].

Clogging of pervious pavements is reviewed more closely in Chapter 7.1 (Clogging and maintenance).


Mechanical performance of porous asphalts may not be as good as mechanical performance of conventional dense asphalt. A higher void content makes the pavement more permeable, but also more susceptible to damages by aging, air and moisture [Poulikakos & Partl 2009]. Porous asphalt surface cannot bear as heavy of loads as conventional asphalts, and the lifetime is supposed to be shorter. On the other hand, skid resistance and noise reduction are better for porous asphalt than for dense asphalts [Poulikakos et al. 2006b]. Research of porous asphalts is focused on permeability properties; the mechanical performance has been studied less. The mechanical subjects studied are e.g. strength and bearing capacity, durability, skid resistance, and wearing as raveling, rutting and shoving.

The service life of PA depends on e.g. the binder content, aggregate proportion, traffic and climate circumstances [Poulikakos et al. 2006B]. Open structure makes material oxidized by air and damaged by water and moisture faster than in dense asphalt. One of the most critical component of asphalt against wearing is binder and binder recipe [Alvarez et al. 2006]. Flexible binder contributes to the asphalt’s resistance to abrasion. Aged binder hardens and brittles especially in colder weather when susceptibility to moisture increases and adhesion may be reduced. It may also lead to the bond between aggregate and binder being lost [Poulikakos & Partl 2009]. Also the bitumen interlayers and binder mortar itself may suffer fatigue and loss of cohesion [Mo et al. 2008].

The main fatigue problems of PA are raveling and stripping [Alvarez et al. 2006, Hamzah 2012A, Roseen et al. 2012, Poulikakos & Partl 2009]. Raveling begins when the first aggregate stones are removed from the surface of loosened asphalt layer and the aggregates around are not supported with each other. The removal of stones expands like a domino effect and the surface begins to strip [Hamzah 2012A]. Binder hardening caused by aging can begin 6 - 9 years after installation and raveling progress of the entire surface may last only for some months [Ferguson 2005]. Other reason for raveling can be in some cases softening of the binder by oil and fuel dripping [Alvarez 2006]. Drain-down phenomenon can also contribute the loosening of aggregates [Ferguson 2005]. Clogging and excessive compaction may decrease permeability causing the susceptibility for moisture to increase [Poulikakos et al. 2006B].

Inadequate compaction makes the porous asphalt less durable and susceptible for raveling [Alvarez et al. 2006]. Control of the proper compaction rate is important because it ensures the stone-on-stone contact and the balance between mixture durability and functionality [Alvarez et al. 2006]. Also the improvement the adhesion between binder and aggregate stones makes the asphalt more durable [Mo et al. 2008]. Addition of polymers or other anti-stripping agents may enhance performance of the binder mixture.


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Other, minor damage problems are rutting and shoving [Ferguson 2005]. Rutting is a phenomenon which makes longitudinal tracks on the road surface. Respectively shoving makes crosswise tracks, so that the surface will wave. Both are caused by wheel pushing, tension, distortion and compaction [Ferguson 2005, Poulikakos et al. 2006B]. Moriyoshi (2013) have studied that even if the rutting depth is smaller than 2 mm at high temperature, this causes damages to lower layers. Weakness of the lower layers and subgrade can cause settlement and potholes upward into surface. Oxidized binder is susceptible to form cracks [Roseen et al. 2012]. Materials can spall around the cracks [Ferguson 2005]. Studded tyres may also abrade the surface of porous asphalt.

When estimating the bearing value of the whole road structure, the subgrade bearing capacity must be taken into account because it is ordinarily the weakest part of the pavement structure. Other important factors are thickness of the whole structure and traffic loads. Usually porous asphalt is used at low trafficked passenger car roads of parking lots, so the loads are relatively low. When the thickness of the structure is sufficient, the load spreads evenly and widely to the subgrade. Thickness design depends on the bearing ratio of different layers of the structure. In addition, the possible frost protection demand must be taken onto account. The weaker the subgrade, the thicker will be needed for the structure. Road edges may need special support to maintain the resistance to deformation. Porous asphalt can also be used over conventional road structures meant for heavy loads (overlay asphalt). [Ferguson 2005.]

The European standards require some mechanical properties for porous asphalts. These are e.g. water sensitivity, particle loss, binder drainage and horizontal and vertical permeability [Poulikakos 2006a]. There can also be some national Annexes. In Finland, EN 13108-7 + AC: (Bituminous mixtures. Material Specifications. Part 7: Porous asphalt), and the National Annex Finnish Asphalt specifications 2011 (PANK 2011) are in use. In this Annex the porous asphalt is maintained to be suitable for fields, yards and passenger car and other light trafficked roads.

3.2.5 Winter durability, freeze-thaw durability – with/without chlorides

Porous asphalt pavements can be used also in colder climates, where temperature is under zero in winter. The skid resistance remains good and spring melting is faster than with conventional asphalt [Houle 2008].

Designing the porous asphalt to resist the frost damages, a sufficient void content of the surface asphalt must be considered, as well as porosity of the reservoir layer, and thickness of the whole road structure. Voids allow the meltwater to infiltrate. The asphalt mix must be also sufficiently soft to avoid winter brittleness [Ferguson 2005].

There are experiences on porous asphalt winter performance from many countries, e.g. from Denmark, Switzerland, Netherlands, and Sweden and from the northern parts of USA. Permeability of the porous asphalt in winter conditions consists of infiltration capacity and e.g. water (ice) content, air temperature and heat properties of the asphalt. [Bendtsen 2011, Stenmark, 1995, Roseen et al. 2012]

Frost and frost heaving must be taken into account while designing the permeable structure. Also the maintenance operations differ from the conventional pavement structures. Winter sanding clogs the pores of porous asphalts, but on the other hand the surface is usually not as slippery as the surface of a conventional asphalt because water does not pond on the top layer. The amount of snow is also usually lower on porous asphalt compared to conventional asphalt [Houle 2008].

Winter performance of pervious pavement structures including porous asphalt pavements is reviewed more closely in a separate Finnish CLASS-project State-of-the-Art-Report [Kuosa & Niemeläinen 2013b].


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3.3 Permeable interlocking concrete and natural stone pavement

3.3.1 General

Permeable interlocking block pavement (PIBP) can be permeable interlocking concrete pavement (PICP) surfaced with manufactured concrete units, or in principal also permeable natural stone pavement (PNSP) surfaced with setts or slabs of natural stone. Figure 23 presents typical PICP components, and Figure 24 some types of paving units. [Smith 2011]

A lot of technical information on PICP is provided by the Interlocking Concrete Pavement Institute (ICPI, http://www.icpi.org/). It was founded in 1993, and it is the North American trade association representing the interlocking concrete paving industry. Figures 25–27 presents the three main PICP types with regard to hydrological functioning. [Smith & Burak 2004]

Figure 23. Typical PICP components. [Smith & Hunt 2010, ICPI 2013]

An open-graded bedding course is typically 50 mm thick and consists of small-sized, open-graded aggregate. This layer provides a level bed for the pavers.

An open-graded base reservoir layer is typically from 75 to 100 mm thick. It is normally made of crushed stones from 20 mm down to 5 mm. This high infiltration rate layer provides a transition between the bedding and subbase layers. [Smith & Hunt 2010]

In an open-graded subbase reservoir, the stone sizes are larger than the base, typically from 65 mm down to 20 mm. The thickness of this layer depends on water storage requirements and traffic loads and may not be required in pedestrian or residential driveway applications. In such instances, the base layer thickness is increased to provide water storage and support. The base and subbase layers should have a minimum void ratio of 32% for water storage.


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Figure 24. Some types of paving units for permeable pavements. [Smith 2011, EBEMA 2012, Beeldens et al. 2008]

Figure 25. A typical design for full exfiltration: the system infiltrates most of the water into the soil. [Smith & Burak 2004]

Arabianranta Yhteispiha 12 Photo by Tiina Suonio


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Figure 26. Partial exfiltration with perforated pipes located at the bottom of the base. [Smith & Burak 2004]

Figure 27. No exfiltration of water from the base, the water is completely detained within an impermeable liner and released slowly to storm sewer or stream. [Smith & Burak 2004]

An underdrain is optional but in sites where PIBP is installed over low infiltration soils, underdrains facilitate water removal from the base and subbase. The underdrains are perforated pipes that connect to an outlet structure. Supplemental storage can be achieved by using a system of pipes in the aggregate layers. The pipes are typically perforated and provide some additional storage volume beyond the stone base. Significant amounts of runoff can be stored under or adjacent to PIBP using plastic or concrete vaults or plastic crates.

Using a geotextile is an optional and can be used to separate the subbase from the subgrade and prevent the migration of soil into the aggregate subbase or base. (see Chapter 4.3 (Geotextiles, filter layer, impervious liners))

The infiltration capacity of the subgrade determines how much water can exfiltrate from the aggregate into the underlying soils. The subgrade soil is generally not compacted.

3.3.2 Pavers

For instance in the U.S.A, concrete pavers should conform to ASTM C936/C936M - 13 (Standard Specification for Solid Concrete Interlocking Paving Units). These pavers are typically 80 mm or greater in thickness for handling load of vehicles. Pedestrian areas may


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use 60 mm thick units. The bedding layer is no greater than 50 mm thick. According to [Virginia 2011], the compressive strength of the pavers should be over/ca. 55 MPa.

In the U.S.A., concrete pavers are manufactured in a range of shapes and colours. They can also be produced with light coloured surfaces to satisfy a minimum solar reflectance index. The top surface of units may be coated with photocatalytic cement materials to reduce nitrous oxide air pollutants. [Beeldens 2008, Smith & Hunt 2010, Swan & Smith 2009] Interlocking concrete pavers are also produced in Europe. Some examples from [EBEMA 2012] are presented in Figure 24.

In Europe, there are no harmonized standards for concrete interlocking paving units to be used in pervious pavements. Harmonized standards for concrete tiles and concrete paving flags are partially applicable also to interlocking paving units to be used in pervious pavements:

· EN 1338: 2003. Concrete paving blocks. Requirements and test methods · EN 1339: 2003. Concrete paving flags. Requirements and test methods.

In Finland there is also a national application standard: · SFS 7017. 2009. Betonista tai luonnonkivestä tehdyille ulkotilojen päällystekiville, -

laatoille ja reunakiville eri käyttökohteissa vaaditut ominaisuudet ja niille asetetut vaatimustasot.

For setts and slabs of natural stone there are harmonized standards: · EN 1342:2012. Setts of natural stone for external paving. Requirements and test

methods. · EN 1341:2012. Slabs of natural stone for external paving. Requirements and test

methods.

In Belgium there is technical specification [PTV 122] including requirements for the raw materials used, the production and the finished products. PTV 122 (2009) is based on the above standards for normal concrete paving blocks and flags but gives additional requirements specific for blocks for pervious concrete surfaces. It does not apply to grass paving clinkers. For pervious concrete blocks the demand for average permeability is ≥5.4 * 10-5 m/s, and in the case of pavement blocks with enlarged joints or drainage holes the demand for an open surface area is ≥10%.

Figure 28 presents some concrete paving blocks with enlarged joints by Rudus Oy in Finland. These blocks are today mainly used for green surfaces with, i.e. with turf (see also Chapter 5.3 (Other pervious solutions)). The jointing material can also be open graded gravel, and the pavement can be combined with more or less pervious sub-structures. [Rudus 2013] Anyway, when any kinds of blocks are used in pervious pavements, a large enough open area and suitability with the jointing material should be considered to enable the designed drainage (see Chapter 3.4.3 Joints and joint material).

In Finland there are also concrete pavers including some kind of holes. These pavers are today used for instance in green solutions and for embankment stabilization (see Chapter 3.5 (Other pervious solutions). They can also be filled with open graded gravel in combination with more or less pervious sub-structures. [Rudus 2013, HB 2013]


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Figure 28. “Hulelaatta”, “Hulekivi”, “Nurmikivi”, ”Akvakivi”, ”Golfkiv”, ”Vihernappula”, ”Louhi-kivi” [Rudus 2013]


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Natural stone is also used as a permeable paver because of its durability and aesthetic appeal. Stone such as Porphyry, a natural granite, is used today to design driveways, streets, walkways and parking lots. [Chaffee 2010, Monarch Stone International 2013] (Figure 29)

Figure 29. Examples of reclaimed cobblestone pavers set on a porous or permeable base of sand, and with permeable sand joints. [Monarch Stone International 2013]

3.3.3 Joints and joint material

PIBP includes joints and/or openings that allow stormwater to enter a crushed stone, open graded aggregate bedding course. The joints typically comprise 5% to 15% of the paver surface to provide sufficient drainage. Joints are filled with highly permeable, small-sized aggregates, e.g. ASTM No. 8, 89 or 9 stone (Figure 30). [Smith 2011, Borst et al. 2010]

Figure 31 presents an example on the gradation of the joint filling material. [Beeldens & Herrier 2006] The pavers are either pervious or not pervious concrete blocks. For pervious blocks, the joint filling material can be normal, i.e. as used in normal non-pervious concrete block pavements.

Borgwardt (2006) found that there is a correlation between infiltration performance and permeability of the aggregates of joint fillings. Aggregates with a coarse particle size exhibit a higher infiltration than those with fine grained aggregates. Still joint material aggregate sizes, if big enough, did not have especially notable effect on the ability of PIBP to take water in. Borgwardt (2006) noted also that sand provided the lowest infiltration. That is also why sand is not recommended, e.g. by IPIC, to be used as joint filling material. One reason for this is also the high clogging potential of sand.

Instead of open area or joint material properties, surface infiltration rates are a better way of defining permeable pavements (see Chapter 3.4.4 (Permeability, surface infiltration)). Figure 32 presents some field infiltration rates for both new and aged pavements with different joint filling materials. [Smith 2011, Borst et al. 2010, Borgwardt 2006] The effects of clogging are discussed more in Chapter 7.1 (Clogging and maintenance).


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Figure 30. Grain size limits for ASTM No. 8, 89 and 9 aggregates used as joint material in PIBP.

Figure 31. Gradation of the joint filling material in case of porous pavement blocks (sand 0/2) and of pavement blocks with widened joints and drainage holes (Porphyry 2/4). [Beeldens & Herrier 2006]

Figure 32. Infiltration performance in relation to different aggregates for joint fillings. Results for both new pavements and old pavements with clogging. [Borgwardt 2006]


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Table 4 presents laboratory results in a Newcastle University infiltration tests where permeability (liters/second/hectare) is presented for different joint widths with different joint filling materials and different joint filling material minimum particle size. [Knapton et al. 2002]

Table 4. Permeability results with different joint filling materials and joint widths. [Knapton et al. 2002]

3.3.4 Permeability, surface infiltration

The initial surface infiltration rate is typically very high for PIBP, and depends mainly on the infiltration rates of joint filling material, bedding layer, and base/subbase materials. Instead, based on research and experience, the key factor affecting the surface infiltration is the with time decreasing infiltration performance. This means that sediment deposition from traffic, soil eroding on the surface, and spills of topsoil or mulch are the most important factors for PIP infiltration. The sources and amounts of sediments will vary from site to site. [Smith 2011, Borst et al. 2010]

There are numerous studies on the short and long term surface infiltration rates of PIBPs. e.g. [Smith 2011, Borst et al. 2010]:

· Borst et al. (2010) outlines the methods and results of the surface infiltration monitoring of the permeable parking surfaces during the first six months of operation. For PIBP the measured infiltration rate was from 6.9 to 9.7 mm/s. The measurement was with ASTM C1701, a single-ring infiltrometer test method (see Chapter 6.2 (Field testing and quality control)).

· average 5.6 mm/s for nine parking lots in Maryland, U.S., · for continuously unmaintained PIBPs (age of 8–10 years) 0.0013–0.104 mm/s (0.5–

37.5 cm/hour), i.e. still some infiltration, · Results from Germany demonstrated that PIBP surfaces lose 75% to 90% of their

surface infiltration rate in 7–8 years due to sedimentation, and after that level off.

Reductions of initial surface infiltration of 75% to 90% still yields rates that will take practically all storms. For instance, if PICP has an initial infiltration rate of 3.5 mm/s, a 90% reduction over several years yields an infiltration rate 0.35 mm/s. For design purposes, a conservative lowest surface infiltration for maintained PICP is 0.07 mm/s. [Borgward 2006, Smith 2011]

More information on long term performance, clogging and maintenance of permeable pavements is presented in Chapter 7 (Performance).


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Borgwardt (2006) found limited correlation between the infiltration rate and the percentage of paver surface open area. Instead, infiltration rate has been found to be highly dependent on the joint filling material. The permeability of ASTM No. 8 stone can exceed 14 mm/s, and 89 and 9 stone often exceeds 3.5 mm/s. Gradation of these stone materials is presented in Figure 30 (above in Chapter 3.3.3 (Joint and joint material)). [Borgwardt 2006, Smith 2011]

Initial infiltration rates of open graded base and subbase reservoirs are very high (´thousands of inches or cm per hour´). They are not considered as obstacle for water moving vertically. Instead, a key design parameter is the lifetime infiltration of soil subgrade. There can be short term variations from a saturated soil subgrade, and long-term infiltration reductions caused by deposition of sediments.

In Belgium guidelines for hydraulic design for permeable pavements are developed. They include standard structures related to the traffic intensity and soil characteristics. Hydraulically speaking, water-permeable pavements were designed for a ´30 years frequency rainfall event of ten minutes´. Belgium statistics indicate this is a rainfall with an intensity of 270 liters/s/hectare. An initial permeability of 0,054 mm/s (5.4×10-5 m/s) is demanded. A safety factor 2 is included to compensate for the fact that the permeability of the structure may be reduced by air enclosures or by clogging at the surface. [Beeldens et al. 2009]


The strength demands for normal concrete paving blocks and flags are presented in EN 1338 (2003) and EN 1339 (2003). In principle these demands are applicable also to the pavers which are to be used in pervious pavements. Only materials with suitability established in terms of their properties and performance shall be used in the manufacture of concrete paving blocks.

In Belgium there is technical specification [PTV 122] for the paving blocks to be used in pervious pavements. With respect to material requirements, the provisions of EN 1338 (2003) for concrete paving blocks, and the provisions of EN 1339 (2003) for concrete tiles apply also in this specification.

According to EN 1338 ”The characteristic tensile splitting strength T shall not be less than 3.6 MPa. None of the individual results shall be less than 2.9 MPa, nor have a failure load less than 250 N/mm of splitting length”.

For setts and slabs of natural stone, there are harmonized standards EN 1342 (2012) and EN 1341 (2012) where the requirements for strength are included.

For instance in EN 1341 informative Annex A, there is guidance for determining the appropriate thicknesses for natural stone paving slabs for different classes of use. As pervious pavement structures are not included, this standard can only be used as indicative. Overall, a number of structural calculation methods are available for determining the thickness of paving slabs for specific situations and loadings. EN 1341 informative Annex gives a simple method which can be used as a part of the selection method. In this method the thickness of a slab is determined by calculation from the minimum required breaking load P (in kN), where P is the breaking load for the expected use of the paving. Guidance is also given on the expected breaking loads for different uses (Table 5). [EN 1341]


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Table 5. Guidance on the expected breaking loads for different uses for the estimation of the appropriate thicknesses for natural stone paving slabs. [EN 1341]

In Finland there is a national application standard for concrete paving stones and flags and setts and slabs of natural stone [SFS 7017: 2009]: Characteristics and requirement levels of sets, slabs and kerbs made of concrete or natural stone in different outdoor applications. It gives recommendations on the properties to be declared for the CE-marking according to the relevant EN-standard. Specific paving products for pervious pavements are not included.

3.3.6 Freeze-thaw durability – with/without chlorides

In Finland concrete paving blocks and flags must be resist freeze-thaw exposure with salt due to their use outdoors [SFS 7017: 2009]. Testing of the scaling degree with salt is done according to EN 1338, EN 1339 and EN 1340 Annex H (“Slab test”). After 28 freeze-thaw cycles the average scaling value must be ≤ 1.0 kg/m2, with no individual value > 1.5 kg/m2.

In Finland the methods and demands for passing for slabs of natural stone for external paving, as well as for setts of natural stone for external paving, are described in [SFS 7017: 2009]. In Finland, according to the test method in EN 12371 (2010), and by using 1 w.-% NaCl in the testing, passing is if after 48 cycles the breaking load decrease is not more than 20%. In the case of freeze-thaw with no de-icing salts, the demand for passing is the same but the testing is without salt, i.e. with water. Testing according to EN 12371 (2010) consists of cycles of freezing in air and thawing in water.

3.3.7 Abrasion resistance

According to EN 1338 for concrete paving blocks, the abrasion resistance is determined by the Wide Wheel Abrasion test (WWA) or as an alternative by the Böhme test. The Wide Wheel Abrasion test is the reference test. According to EN 1341 for slabs of natural stones, the abrasion resistance shall be determined according to EN 14157 (2004). In this standard the WWA test is the reference method.

In the case of light traffic, there are no demands on abrasion resistance for any paving product (concrete, natural stone) in Finland [SFS 7017: 2009]. But in the case of vehicle traffic there are demands which are presented in SFS 7017.

3.4 Other pervious solutions

There are also several paver options, such as concrete grid pavers and turf pavers as well as modular plastic paving grids which function in the same general manner as pervious pavements. [Virginia DCR 2011] Besides the pavement type, the sub-structure will decide the whole function and capacity with regard to water infiltration and retention.


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Concrete grid pavers and slabs have been used for a long time especially for erosion control, to stabilize embankments, in ditch liners and fire lanes but also in green paving areas and to reinforce grassy areas subjected to wheeled traffic that would otherwise become so compacted as to inhibit the permeability of the soil that is necessary for the grass to survive [Virginia DCR 201] They have typically an open void content of 20–50%. The openings of the grids can be filled with grass plugs, topsoil or aggregates. Some examples are presented in Figure 33. [Rudus 2013]

Figure 33. Concrete grid slabs (“Reikälaatta”) and pavers (“Reikäkivi”). [Rudus 2013]

Figure 34. Turfstone Grid Pavers. [Angelus 2013]

One green product example is “Drivable Grass®” which is a permeable, flexible and plantable concrete pavement system. This product is made of wet cast, low moisture absorption air entrained concrete and is thus also freeze-thaw resistant. The design eliminates sharp edges. Holes in it allow for infiltration and root penetration. There is a cast-inside engineered polymer grid which allows it to flex and conform to irregular ground surface contours. It also can move with frost heave in a freeze thaw cycle with no cracking. The product’s interconnecting grid is made from an engineered polymer that remains flexible even in low temperature conditions. It can also be filled with sand and gravel or other suitable granular materials. Figure 35). [Soil Retention 2013]


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Figure 35. Draivable pervious, flexible and plantable concrete grass/pavement system. [Soil Retention 2013]

There are also modular plastic grid pavers which can be used for ground reinforcement in the same way as those made of concrete. Open void fill media may be aggregate, topsoil and grass. Some examples are presented in Figure 36 and Figure 37.

Figure 36. RITTER Nurmikennosto and Sorakennosto GroundGrid. [Kaitos 2013]


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Figure 37. BODPAVE® 85 permeable pavers can be installed with either a grass or gravel filled surface depending on the application required. [TYPAR 2013]

Figure 38. TRUCKPAVE’s open plastic grid cell structure can be filled with either a grass seed/topsoil or gravel. [TERRAM 2012]

4. Subbase systems

4.1 General

Early pervious pavements were built on freely draining sandy soils, so that the water could flow straight through the pavement and into the soil. If the soil does not drain well, an open graded crushed stone reservoir base is placed under the pavement to retain water. This technique is also used to keep the pervious concrete dry where there is a risk of freeze-thaw damage.

Installations, where the water flows directly downward through the pavement layers, are referred to as open systems. Instead, in closed systems an impermeable membrane is placed under the subbase to direct water to pipes. [Delatte et al. 2007, Delatte & Cleary 2006]

As presented for instance in Beeldens & Herrier (2006), carefully chosen materials are a prerequisite for a permeable paving structure to work well. The foundation can be made of unbound materials. It is important to find a good agreement between permeability and stability. The lack of fine material will hamper the compaction of the layer to a large extend. The use of different compaction equipment, such as a vibration plate can lead to good results where a drainable structure is obtained with a good mechanical resistance. [Beeldens & Harrier 2006]


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Another important factor is the possibility to compact the material in-situ. It is also possible to use cement-bounded materials in the base layer – for example porous lean concrete, which combines high porosity with relatively high strength. To ensure the stability of the structure, contamination between layers must be prevented, if necessary by separating them with a geotextile. However, according to Beeldens & Herrier (2006), this should be avoided as much as possible, to prevent one layer from sliding over another under the action of traffic. [Beeldens & Herrier 2006]

4.2 Aggregates

Open-graded bases have no fines (≤0.075 mm) and are typically with 1520–1920 kg/m3 density. While in dense graded aggregate all void spaces are small, in open-graded aggregate the voids between the aggregate particles enlarge as particle size increases. According to one rule of thumb, the diameter of the voids is up to 1/5 of the diameter of the particles. [Ferguson 2005]

The total porosity does not vary measurably with particle size, and it is 30–40%. The exact value depends on the gradation of the aggregate, particle shape, and degree of compaction. If the particle sizes are very uniform, porosity is very high, i.e. 33–45%. The highest value is for rigorously angular particles, and the lowest for rounded gravels. Internal pores in aggregates will increase porosity. [Ferguson 2005]

The permeability of aggregates comes from its total porosity, and from the size of the individual voids. Open graded, and especially course open graded, aggregates have very high permeabilities. Table 6 presents approximate permeabilities of aggregate materials. [Ferguson 2005]

Table 6. Approximate permeabilities of aggregate materials. [Ferguson 2005]

Gradation Permeability, cm/hour (mm/s)

25.5 mm aggregate (uniform size) 63500 (176)



Coarse aggregate 127 (0,35)

Dense-graded sand and gravel 0,635 (0,002)

The aggregate storage layer may be comprised of more than one layer of aggregate, each layer having a different size aggregate and void content.

Before installing a surface course on top of a base course, it is normally necessary to smooth and stabilize the top of the base course. This is done by a setting bed or “chocking” layer of finer open-graded material. Chocking particles must be smaller than those of base course but not so small that they would fall through the voids. A combination of two layers must provide permeability through the whole material while making voids small enough to prevent small particles from penetrating. Table 7 presents filter criteria for three aggregate layers, i.e. bedding, base and subbase layer. Dx is the particle size at which x percent of the particles are finer, based on the sieve analysis result. [Ferguson 2005, Smith 2011]


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Table 7. Filter criteria for bedding, base and subbase aggregates. Dx is the particle size at which x percent of the particles are finer. [Ferguson 2005, Smith 2011]

According to [CRMCA 2009], PC base course aggregate should be uniformly graded, coarse aggregate per ASTM No 57, or approved equal, with a loss by wash of no more than 1.0%. Based on U.S. experience ASTM No. 8 bedding stone chokes well into ASTM No 57 base, and this ASTM No 57 base material chokes well in ASTM No. 2 subbase material. These materials also provide high permeability when chocked into each other. [Smith 2011] Standard gradations for ASTM no 8, No 57 and No 2 aggregates are presented in Table 8.

Table 8. Standard gradations for ASTM no 8, No 57 and No 2 aggregates.

The factors that contribute to structural stability and bearing strength are different from those filling the filter criteria. They include using crushed stone, hard aggregate and interlock of particles with each other as well as appropriate thicknesses and compaction (see Chapter 5 Dimensioning). For the evaluation of mechanical strength of road subgrades and base courses (bearing resistance), there are geotechnical testing methods (see Chapter 6 Laboratory and field testing - standards and methods).

Ferguson (2005) presents that jointing, bedding, base and subbase aggregates used in vehicular PIP applications should be crushed with minimum 90% fractured surfaces. Minimum Los Angeles (LA) abrasion should be <40 (see Chapter 6.1.7 Strength and resistance to degradation; ASTM C131, ASTM C535 and EN 1097-3). For base/subbase material, he recommends a minimum porosity of 32% and a California Bearing Ratio (CBR) of at least 80% (see Chapter 6.2.3 Bearing strength).

Lightweight aggregates

Lightweight aggregates, e.g. Leca-gravel, have been used in pavement structures. Leca-products have also been used in e.g. green roofs where the ability to retain water and behave as an underdrain has been exploited. In courtyard construction, lightweight

Permeability D15 Base/D15 Bedding layer >5

D50 Base/D50 Bedding layer <25

D15 Base/D85 Bedding layer <5

Permeability D15 Subbase/D15 Base >5

D50 Subbase/D50 Base <25

D15 Subbase/D85 Base <5

Choke

Choke

No. 8 (bedding)

No. 57 (base)

No. 2 (subbase)

75 10063 90 - 10050 35 - 7037 100 0 - 1525 95 - 10019 0 - 5

12,5 100 25 - 609.5 85 - 100

4,75 10 - 30 0 - 102,36 0 - 10 0 - 51,19 0 - 5

Passing [%]Sieve size

[mm]


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aggregates such as Leca-gravel are already used to for instance prevent or diminish sagging, to prevent frost damage and in drainage and drying. Besides Leca-gravel can be used to produce ´Leca-concrete´, which can be used for instance in pavement subbase structure. Leca-products can be considered to be a potential product also in pervious pavements especially when lightweight and frost protection is needed. [RT-kortti 2005, Maxit 2005a, Maxit 2005b]

Leca-gravel has also been used as a water purification media. This product is called Filtralite® filter media. For several years the main Weber laboratory at Lillestrøm, Norway has carried out testing of Filtralite filter media. The number of tests and characterisation procedures are continuously increasing, according to the increase in requirements for good quality filter media. [Weber 2013]

Recycled aggregates (as base/subbase material)

Aggregate is granular material used in construction. Aggregate may be natural, manufactured or re-cycled. Recycled aggregate (RA) is aggregate resulting from the processing of inorganic material previously used in construction. For instance it can be recycled concrete aggregate (RCA). Manufactured aggregate is aggregate of mineral origin resulting from an industrial process involving thermal or other modification.

Using RCA offers several environmental advantages. But like any other aggregates also RCA when used in permeable pavements, must be specified and evaluated for gradation and durability to meet all the project objectives. [Ferguson 2005]

Crushed concrete aggregate may be strong and angular but must be sorted for size to produce courses with high enough porosity and permeability. Without sorting it may include a high amount of very fine material. Especially fine material is capable of leaching out alkalinity which is not good for nearby plants and trees. Recycled concrete aggregate has also a potential to re-cement to solid and low permeable mass. Both re-cementing and leaching can be minimized, but not totally. Coarse, single sized material with low surface area exposed to water is beneficial. [Ferguson 2005]

Beeldens et al. (2009) made also testing on recycled aggregates. Some good results towards permeability were obtained, but the analysis of the aggregate distribution indicated that during placement and compaction, fines were formed. A mixed recycled aggregate (concrete and masonry) 10/40 (10 to 40 mm) was tested. The fraction 0/2 (<2 mm) raised to almost 10%, only due to friction on the sieves. The risk to create fines due to the loading by traffic during service life is high, what can result in a decreased permeability. [Beeldens 2009] In the case of using structures with impervious liners and pipes, this is not a problem as water does not enter the surrounding soil.

4.3 Geotextiles, filter layer, geosynthetic barriers, impervious liners

Geotextiles, filter layer

Geotextile materials are a common element within permeable pavement and subbase designs. EN 13249:2000/A1:2005 (Geotextiles and geotextile-related products - Required characteristics for use in the construction of roads and other trafficked areas) presents the requirements for geotextiles for use in the construction of roads and other trafficked areas.

Review of the testing methods related to pervious pavements, including the standard testing methods for geotextiles, is included in a separate CLASS-project State-of-the-Art Report [Kuosa & Niemeläinen 2013] (Pervious pavement testing methods).


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The European geotextile product standards do not specify minimum requirements, as these are related to the construction, in which the geotextile is used. The information the manufacturer provides should mention a nominal value and a tolerance value, corresponding to the 95% confidence interval. [Foubert 2009]

In general, geotextiles used in any applications fulfil one or more of the functions presented in Figure 39. [Foubert 2009]

Figure 39. Geotextile functions [Foubert 2009].

Table 9 presents the properties of geotextiles in different functions provided by the geotextile harmonization. [Tammirinne et al. 2004]

Table 9. The properties of geotextiles in different functions provided by the geotextile harmonization. [Tammirinne et al. 2004]

Within permeable pavements, geotextiles can be used mainly in two locations where they should function as a filter. (Figure 40) An optional upper geotextile can be included at the laying course/coarse graded aggregate interface, and a geotextile can be used to protect the bottom of the reservoir layer from intrusion by underlying soils. If a geotextile is not used, the design of the aggregate layers and their gradation optimization is another option. [Interpave 2007, Virginia DCR 2011]


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Figure 40. Location of geotextiles. [Interpave 2007]

According to [Interpave 2007], the geotextile can be either a monofilament woven, non-woven firmly bonded or needle punched non-woven fabric. It should be manufactured from a suitable polyethylene or polypropylene filament able to withstand naturally occurring chemical and microbial effects. Only products with a CE-mark should be used.

Adjacent rolls of the geotextile should be overlapped by at least 300 mm. All vehicles should be prevented from trafficking directly over the material. The material should be protected from ultraviolet light. [Interpave 2007]

According to the PC Handbook [CPG 2013], filter fabric, i.e. a 4 oz non-woven geotextile, shall be placed on the subgrade/soil prior to placing the base material. The filter fabric should continue up the sides to the surface to keep fines from migrating into the storage layer. It is also advised to continue the filter fabric up and over the surrounding soils ca. 0.6–0.9 m (Figure 41). This will “hold” the soils under the fabric acting as erosion control until the project is complete and landscaping is ready to secure the soils next to the pervious pavement. When the soils are secured the filter fabric can be cut to the edge of the pervious pavement.

Figure 41. The filter fabric should continue up the sides to the surface to keep fines from migrating into the storage layer, and also up and over the surrounding soils when needed for erosion control. [CPG 2013]

Some practitioners recommend avoiding the use of filter fabric since it may become a future plane of clogging within the system. Permeable filter fabric is still recommended to protect the excavated sides of the reservoir layer, in order to prevent soil piping. [Virginia DCR 2011]

Also, for instance, according to the design guide [UNHSC 2009] filter fabrics or geotextile liners are not recommended for use on the bottom of the porous asphalt system (at the base of the stone reservoir subbase) if designing for infiltration. This is because filter fabric usage in stormwater filtration has caused premature clogging. Graded stone filter blankets are recommended instead [UNHSC 2009].


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The function of a geotextile filter is to retain the soil while allowing the liquid to flow as freely as possible. In order to achieve this objective, a geotextile filter needs to meet [Landfilldesign.com 2013]:

· Retention criterion: the filter opening size must be sufficiently small to retain soil particles,

· Permeability criterion: the filter must be sufficiently permeable to ensure that the liquid flow is as free as possible, and

· Porosity criterion: the filter should remain a high porosity so the probability for clogging is small.

Giroud (2010) has studied mainly theoretically criterions for geotextiles and granular filters. According to him there is also a thickness criterion, i.e. minimum thickness, for geotextiles. He also showed that porosity criterion and thickness criterion are always met by granular filters, and therefore, are needed only for geotextile filters. [Giroud 2010]

According to [Smith 2011], the geotextile filter criteria should be checked if it is used between the subbase and subgrade. For instance the AASHTO M-288 (Standard Specification for Geotextile Specification for Highway Applications) requirement is that permeability should exceed that of the soil. The standard practice is that the geotextile permeability is ten times that of the soil being filtered.

The requirements in [Smith 2011] for the geotextiles to be used in PICPs are presented in Table 10 and Table 11. These values are from the U.S. ´M 288-09´ (Geotextile Specifications for Highways Applications), used by permission in [Smith 2011].

According to [Smith 2011], geotextile strength properties should be the highest (Ćlass 1´) if exposed to severe installation conditions with greater potential to damage. Lower strength properties (Ćlass 2´) are typically used the installation conditions are less severe.

Table 10. Geotextile strength property requirements (e.g. application in PICP). [Smith 2011]

Table 11. Subsurface drainage geotextile requirements (e.g. application in PICP). [Smith 2011]


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Virginia stormwater design specification [Virginia DCR 2011] describes general material specifications for the component structures installed beneath the permeable pavement. Specifications for optional filter fabric are also included. A needled, non-woven, polypropylene geotextile should be used. The specification includes e.g. a value flow rate determined according to ASTM D4491 (Standard Test Methods for Water Permeability of Geotextiles by Permittivity). The demand is >306 m3/h m2 (125 gpm/sq. ft). Other important geotextile properties are so called ´Grab Tensile Strength´ and ´Mullen Burst Strength´, as well as so called Apparent Opening Size (AOS).

Durability of geotextiles is linked to a number of parameters [Foubert 2009]: · Duration of exposure to sunlight on site · Soil conditions (pH, temperature, contamination) · Expected lifetime of the construction · Composition and structure of the geotextile.

Tota-Maharaj et al. (2012) examined the effectiveness of permeable pavements in treating concentrated urban runoff for water reuse and recycling, assessing the presence of geotextile membranes within the permeable pavement structures in terms of its efficiencies for removing water pollutants. The inflow and outflow water quality were measured from the three experimental pavement rigs on a weekly basis. Water quality analysis indicates that the infiltration and absorption capabilities of the geotextile membrane provide higher removal efficiency for typical contaminants in urban runoff when compared to permeable pavements without the geosynthetic layer. A more detailed review on the specific studies and experiences on the effect of pervious pavement on water quality is presented in a separate CLASS-project State-of-the-Art Report [Loimula & Kuosa 2013] (The impact of pervious pavements on water quality).

Geosynthetic barriers, impervious liners, geomembranes

Closed systems, where an impermeable membrane is placed under the subbase to direct water to pipes, may be preferred in some pervious pavement cases. The use of impervious liners (geosynthetic barriers, geomembranes), represents a conservative approach if there are concerns about water quality in the soil or about increasing moisture levels under adjacent pavements. [Delatte et al. 2007, Delatte & Cleary 2006]

EN 15382 (2013) (Geosynthetic barriers. Characteristics required for use in transportation infrastructure) presents the requirements for geosynthetic barriers used as fluid barriers in infrastructure works, e.g. roads, railroads, runways of airports, and the appropriate test methods to determine these characteristics. The intended use of these products is to control the pathway of liquids through the construction and to limit any contamination, e.g. by de-icing products, of groundwater or water sources.

There are also harmonized European product standards presenting the characteristics requires for geomembranes for different use, as for the use in the construction of reservoirs and dams, use in the construction of canals, use as fluid barrier in the construction of tunnels and underground structures, use in construction of liquid waste disposal sites, solid waste storage and disposal sites, and sites for the storage and disposal of hazardous solid materials.

Product standards for Geosynthetic barriers, as e.g. EN 15382, presents the testing methods and standards for three different product types, i.e. polymeric (GBR-P), bituminous (GBR-B) and clay (GBR-C, i.e. bentonite carbets) geosynthetic barriers. Review of the testing methods related to pervious pavements, including the standard testing methods for geosynthetic barriers, is included in a separate CLASS-project State-of-the-Art Report [Kuosa & Niemeläinen 2013a] (Pervious pavement testing methods).


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According to Bowers (2013) no infiltration design with a geomembrane is typically used in the following conditions:

· The soil has very low permeability, low strength, or is expansive · High depth to a water table or bedrock · To protect adjacent structures and foundations from water · When pollutant loads are expected to exceed the capacity of the soil subgrade to

treat them.

A no infiltration retention design may be used also as a part of a water harvesting and reuse on site system. [Bowers 2013]

Geomembranes have different engineering properties depending on polymer type, thickness and manufacturing process. Typically the nominal thickness, density, tensile strength, tear resistance, dimensional stability and puncture resistance are provided in manufacturers' literature and referenced in product specifications. [Bowers 2013]

The thickness of the geotextile is typically selected based on the materials placed next to the geomembrane and the importance of preventing punctures of the geomembrane. When designing a no infiltration pervious pavement system, there are many factors that must be considered in selecting the geomembrane and protection materials. Bowers (2013) recommends consultation with an engineer familiar with the design of a geomembrane.

Geomembranes can be manufactured from a range of polymers including polyvinyl chloride (PVC), chlorosulfonated polyethylene (CSPE), chlorinated polyethylene (CPE), or, more recently, polypropylene (PP), ethylene propylene diene monomer (EPDM), high-density polyethylene (HDPE) and linear lower density polyethylene (LLDPE), very flexible polyethylene (VFPE). Each of these polymers is unique and provides varying levels of resistance to acids, alkalis or petrochemicals. Some geomembrane polymers can also function in extreme heat or cold. Normally, the surface of a geomembrane is smooth, but some sloped applications can benefit from a textured surface that provides greater friction with the adjacent geotextiles or soil. [Bowers 2013]

Tammirinne et al. (2007) studied the life time design and product acceptance of ground water protection systems for landfills. No tests were made to find out the design parameters for different materials but general guidelines how to make the comparisons was presented. Usability criteria for different materials as linings were also given, partly for test use in the described acceptance procedure.

4.4 Water draining and collection systems

Under-drains (see Figure 23 Typical PICP components in Chapter 3.3, and Figure 42) are perforated pipes that connect to an outlet structure. Supplemental storage can be achieved by using a system of pipes in the aggregate layers. The pipes are typically perforated and provide some additional storage volume beyond the stone base. Sub-drain pipes can exit to drainage ditches, storm sewers and natural drainage features such as ponds or streams. [Smith & Hunt 2010, Swan & Smith 2009]

For additional storage volume, the aggregate layer(s) may extend beneath adjacent impervious pavements on the site, and may include pipes, under-drains, chambers, cisterns, vaults, tanks or other receptacles, as necessary to economically accommodate the design storm-water storage volume. [CRMCA 2009, Interpave 2007]


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Figure 42. Perforated pipes will be used at the bottom of the base for projects over slower draining silt and clay soils. [Smith & Burak 2004]

There are a number of permeable subbase replacement systems on the market. Some examples on the subbase replacement systems are presented in Figure 43. These enable the use of pervious pavement in areas with low soil permeability. Significant amounts of runoff can be stored under or adjacent to pervious pavements using plastic or concrete vaults or plastic crates. They usually consist of a series of lattice plastic, cellular units, connected together to form a raft structure that replaces some or all of the permeable subbase, depending upon the anticipated traffic loading. They may be manufactured also using recycled plastic. [Interpave 2007, StormTech 2011]

According to the CRMCA (2009) guide, the maximum draw-down time shall be five days; any combination of subgrade soil infiltration, evaporation, and positive outlets may be used to achieve draw-down. Perforated pipes are normally used at the bottom of the base for projects over slower draining silt and clay soils. (Figure 44)

Soil conditions do not constrain the use of permeable pavement, although they do determine whether an under-drain/sub-drains is needed. Under-drains prevent over-saturation of the pavement during high depth rain events. To accomplish this, sub-drain pipes are typically placed above the soil subgrade. They will be filled only after a substantial portion of the base material under them has become saturated. [Swan & Smith 2009]

The use of under-drains is recommended when there is a reasonable potential for infiltration rates to decrease over time, when underlying soils have a low infiltration rate, e.g. less than 0.035 mm/s, or when soils must be compacted to achieve a desired Proctor density. High infiltration rate soils will generally not require under drains in the subbase while some silts and most clay soils will require under-drains to remove excess water. [Virginia DCR 2011, Smith & Hunt 2010]

Under-drains can also be used to manage extreme storm events to keep detained storm-water from backing up into the permeable pavement. [Virginia DCR 2011]

According to [Virginia DCR 2011], 100 to 150 mm diameter perforated PVC [AASHTO M 252: 2012] pipe, with 9.5 mm (3/8 in) perforations at 150 mm on center; each underdrain installed at a minimum 0.5 % slope located 6 m or less from the next pipe (or equivalent corrugated HDPE may be used for smaller load-bearing applications). Perforated pipe installed for the full length of the permeable pavement cell, and non-perforated pipe, as needed, is used to connect with the storm drain system. T’s and Y’s are installed as needed, depending on the underdrain configuration. Cleanout pipes should be extended to the surface with vented caps at the Ts and Ys. [Virginia DCR 2011]


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Figure 43. Subbase replacement systems. [Interpave 2007, Pipelife 2013a, StormTech 2011, Pouta 2010]


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An impervious liner may also be used to store larger quantities of storm water for re-use. Barriers can also be used to keep storm water from entering contaminated soils. Figure 44 presents the principle of this kind of closed pavement system. [CRMCA 2009]

Figure 44. PC pavement as a closed system. [Delatte & Cleary 2006]

According to Pouta (2010) it is important to follow in long term the functioning of the stormwater cassette systems. Also it is important to enforce the separation of harmful substances from stormwater in the use of stormwater cassettes. [Pouta 2010]

The water storage capacity of a pervious subbase replacement systems is higher than that of a conventional granular systems. Typically 30–40% of the depth of a granular permeable subbase pavement is needed for the hydraulic design of the pavement. This can lead to a shallower excavation and reduced material disposal to landfill which, in turn, makes them particularly economical for ‘brown field’ and contaminated sites. [Interpave 2007]

Permeable subbase replacement system design is specialised and advice should be sought from the suppliers/manufacturers of these systems. For instance, Pipelife Oy has its own dimensioning program including statistical climate data. [Pipelife. 2013a]

The manufacture provides also information on the mechanical performance and guidance on the use of the products especially in trafficked areas. The bearing capacity of the cassettes is small and it is essential to consider the capacity of the ground layers above them in the dimensioning. Table 12 presents basic properties for some subbase replacement systems. [Pouta 2010]

There may be also restrictions with regard to the ground water level, and ways to keep the systems stable against buoyancy. [Pipelife 2013a, Stormtech 2004]

Subbase replacement system should be inspected after every half year and the accumulated sludge should be cleaned at least after every 3 years. [Pouta 2010]

Subbase replacement systems may also be useful to form inlets or outlets to and from the permeable subbase as they can be placed at a much shallower depth below trafficked areas than most pipes.


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Table 12. Basic properties for some subbase replacement systems/cassettes. [Pouta 2010]

Water that penetrates the pervious pavement can also be drained and collected or harvested in high volume places where it is detained before infiltration, reuse or drainage to sewer or natural water systems. Figure 45 presents some examples. It is important to select appropriate materials, especially when ground water must be protected. In connection with plastic materials (HDPE, LDPE, PVC, PP), also bentonite blankets may be useful. [Pouta 2010] There are also rainwater management solutions which include catching the water on roofs or streets and getting the water both cleaned, stored and/or reused. [Pipelife 2013b]

Row of big pipes.

Underground stormwater basins made of concrete

HDPE-membrane sealed stormwater casettes

Figure 45. Possibilities for high volume stormwater collection. [Pouta 2010]

Rainwater harvesting is a system where rainwater from roofs and hard surfaces is collected and used in or around buildings for instance for watering gardens. The runoff used for harvesting needs to be of reasonable quality and free of debris and sediments. Permeable pavements will provide filtration to achieve this. It is important to note that the storage volume for reuse should be separated from that for rainfall attenuation. Figure 46 presents an example layout of rainwater harvesting system. [Interpave 2007]


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Figure 46. Example layout of a rainwater harvesting system below permeable pavement. [Interpave 2007]

4.5 Modifying soil for handling water inflow

Subgrade is the soil below the paving and the subbase. The subgrade upon which paving and subbase is constructed is critical to the design and performance of a system with detention. Infiltration rate of the subgrade soil will affect the design of the stormwater storage layer. In most cases, porous paving is designed to encourage water to saturate the subgrade below the paving.

Structural and drainage capacity of the subgrade must be known before specifying a compaction range. Pervious pavement subgrades and cuts necessary to establish proper subgrade level are compacted less or not at all, and are not subjected to excessive construction equipment traffic prior to coarse aggregate bed placement. When fill is needed to meet proper subgrade level, some compaction may be necessary. Field testing of the subgrade after compaction is also important to confirm that they still conform to both structural and hydraulic calculations used for the site. [ACI 522R-10, CRMCA 2009]

According to [Zhang 2006], the pavement design will depend on physical properties such as permeability and California Bearing Ratio (CBR) of the sub-grade soil. Recommendation by Knapton et al. (2002) is that soil tests such as soil classification, moisture content (as %) and soaked California Bearing Ratio (CBR) of the sub-grade soil should be carried out before the construction of the pavement. They pointed out that the optimal permeability should exceed 0.01 mm/s and CBR values should exceed 5 % for a successfully performing permeable pavement. Additional aggregate layers should be added between the subbase and sub-grade if the sub-grade cannot reach the above standards.

Once storm water runoff is captured by the system, runoff will be stored until it exfiltrates the system. The exfiltration rate can be defined as the rate at which runoff leaves the permeable pavement and enters the underlying soil. The time it takes for the storm water to exfiltrate the system will depend on the infiltration rate of the underlying soil. It has been recommend that the permeable pavement to be designed to drain fully in no more than five days. [Kevern 2008, Leming et al. 2007]

Soil infiltration capacity can be measured through on-site testing with a double-ring infiltrometer (ASTM D 3385:2009 or ASTM D 5093:02(2008). For more details, see the separate Finnish CLASS-project report on pervious pavement testing methods [Kuosa & Niemeläinen 2013a].

According to Beeldens et al. (2009), soil that mostly consisted of sand with a permeability ranging from 0.02 mm/s to 0.045 mm/s was very suitable for infiltration in a project in


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Belgium. For sandy soils, the infiltration rate may be 0.2–0.4 mm/s but for silty to clayey soils only 0.05–0.005 cm/s.

According to Smith & Hunt (2010), permeable pavements should be located at least 30 m from drinking water wells and with a minimum of 0.6 m of soil above the seasonally high ground water table. With an impermeable liner, there should be at least 0.3 m between the bottom of the liner and the seasonal high groundwater table.

In the UK, a permeable pavement is required to absorb 180 litres/second/hectare, but most UK subgrades would only absorb a fraction of this. The remainder has to be retained in the pavement to gradually percolate into the subgrade or to be directed to a sub-surface drainage system. [Knapton et al. 2002]

In order to ensure complete drainage of water between rain events and reduce the potential for freezing during winter, consideration should be given to requiring underdrains with adjustable flow restrictors to be installed in facilities located on fine-textured soils with percolation rates less than 15 mm/hour (0.004 mm/s). [Toronto and Region Conservation 2009]

While guidelines in some jurisdictions discourage the application of infiltration practices on sites with fine textured soils containing greater than 20% clay, recent studies have shown that substantial volumes of stormwater can be infiltrated in tight soils beneath permeable pavement installations. [Toronto and Region Conservation 2009]

Concerns about the effectiveness of infiltration practices in cold climates and on fine-textured soils have been topics addressed in several recent studies on stormwater infiltration technologies. Permeable pavements have been observed to function well in cold climates during winter months, even with frost in the ground, albeit at lower efficiencies than during warm weather. [Toronto and Region Conservation 2009] More comprehensive review on the winter performance of pervious pavements is presented in a separate Finnish CLASS-project State-of-the-Art Report. [Kuosa & Niemeläinen 2013b]

5. Dimensioning

Pervious pavement differs from conventional pavement in that both structural and hydraulic requirements must be met. Hydrological analysis determines if the volume of water from user-selected rainfall events can be stored and released by the pavement base. Designer-selected parameters determine how much water infiltrates the soil subgrade and/or is carried away by subdrains. In many cases the hydrological requirements will require a thicker base than required for supporting traffic. [Smith 2011, Swan & Smith 2009, Smith & Hunt 2010, Korkealaakso et al. 2013]

Review of pervious pavement hydrological and structural dimensioning is included in a CLASS-project report by Korkealaakso et al. (2013). This report include also a comprehensive review of available computational models that are able to integrate permeable pavement systems into the overall urban drainage modelling, and can help in designing and sizing the permeable pavement structures.

A complete design of a permeable pavement must consider many factors, including: · geotechnical – support value and permeability of the soil · transportation – traffic weights, volumes, geometrics · pavement structural design – layer thickness, load carrying capacity, fatigue life · hydraulics and hydrology – the amount of water, where it comes from, and where it

goes · environmental – water quality, pollutant capture within the pavement structure


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· durability and service life – resistance to freeze-thaw cycles, de-icing chemicals, abrasion

· costs and project management. [Korkealaakso et al. 2013]

6. Laboratory and field testing – standards and methods

There are several testing methods needed in the development and quality control of pervious pavement materials and structures. Both laboratory and field testing methods are needed. For instance field and laboratory methods for hydraulic conductivity and water infiltration are essential. In material studies, besides all the traditional methods for strength and bearing strength, also methods for durability as methods or freeze-thaw resistance are important.

All the suitable methods and testing standards are typically material specific. For instance there are different standards for porous asphalt and pervious concrete. For porous asphalt pavements, also some EN-standards are already available. In many cases testing methods for pervious pavement materials and structures in Europe must be adapted today from the testing methods and standards for non-pervious structures, and also from some other international methods (such as USA-based ASTM standards).

Information on the testing methods related to pervious pavements is included in a separate CLASS-project State-of-the-Art Report [Kuosa & Niemeläinen 2013a] (Pervious pavement testing methods).

7. Performance

7.1 Clogging and maintenance

Infiltration performance is a design criterion for drainage and sewer systems. That is why also clogging and maintenance of permeable pavements and constructions are essential factors during their whole service life. Attention must be paid to the construction phase and to correct, regular maintenance. [Borgwardt 2006, Hansen 2008]

Basically, permeable structures are filters, and filters remove particles from fluids. The flow rate is reduced when particles are removed, and maintenance is required to restore the flow rate. The rate of clogging of a filter is based on the initial permeability and pore size, type and amount of material to be filtered, rate of the fluid carrying the material, and the level of service requiring regeneration of the filter. [Kevern 2010]

Clogging is defined as the processes of reducing porosity and permeability (and hence decreasing the infiltration rate of the system) due to physical, biological and chemical processes. In storm water systems, clogging occurs primarily due to the deposition of sediments. Pavement clogging is a key issue associated with porous pavements. Porous pavements when ‘‘new’’ often have infiltration capacities >4500 mm/h (1.3 mm/s). While some systems with 15–20 years of operation still provide infiltration rates far above the design storm requirements of 100–1000 mm/h (0.03–0.3 mm/s), many have reported clogging, with infiltration rates reduced to unacceptable levels within the same period. [Siriwardene et al. 2007, Yong et al. 2013]

The rate of sedimentation depends on the amount of traffic and other sources that wash sediment to the joints or pores, base and soil. Clogging may occur on the surface due to debris and fines. This can be readily observed from the surface. Clogging can also occur from the bottom (in the base) due to penetration of fines into the drainable base. Subsurface clogging is generally addressed through filter fabrics, and the condition of these cannot be inspected without removing part of the pavement. [Delatte et al. 2007, Smith 2011]


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Permeable pavement is typically designed to treat storm water that falls on the actual pavement surface area, but it may also be used to accept run-on from small adjacent impervious areas, such as impermeable driving lanes or rooftops. However, careful sediment control is needed for any run-on areas to avoid clogging of the down-gradient permeable pavement. Permeable pavement is not intended to treat sites with high sediment or trash/debris loads. Many sites that have become clogged have become so from large amounts of nearby unstabilized soil running onto the pavement during construction. [Virginia DCR 2011, Kevern 2010]

The fact of a decreasing infiltration performance means that both accurate design, application and maintenance instructions are needed to achieve a long lasting infiltration performance on a high level. The property owner should clearly understand the unique maintenance responsibilities inherent with permeable pavement, particularly for parking lot applications. The owner should be capable of performing routine and long-term actions (e.g., vacuum sweeping) to maintain the pavement’s hydrologic functions, and avoid future practices (e.g. winter sanding, seal coating or repaving) that diminish or eliminate them. [Borgwardt 2006, Virginia DCR 2011]

There are several studies on the clogging of PICP, and also on the effect of different maintenance actions. As clogging is often considered the main concern with regards PICPs, several research or field results and conclusions based on these are presented below, to give a truthful and realistic understanding on this subject.

As there are some differences in the clogging scenario and maintenance of pervious interlocking pavement (PICP) over pervious monolithic pavements, i.e. pervious concrete pavement (PCP) and porous asphalt pavement (PAP), these are reviewed below separately.

PICP

The enrichment of fines in the joint material over the years will have a significant influence on the PICP water infiltration rate. According to Borgwardt (2006), an overall trend of the infiltration performance during the service life of a permeable pavement allows constructing the hypothesis of a decrease to 10 to 25% of its original output power.

PICP must be properly maintained to prevent the surface from becoming clogged, which reduces permeability. Most PICP sites function well without regular maintenance if protected from sand. Waniliesta & Chopra (2007) investigated field sites which had a service life from 6 to 20 years, and were with no notable maintenance. Before rehabilitation the average infiltration was from 0.09 cm/min to 3.17 cm/min (0.015–0.5 mm/s), including zero rates for those pavements not properly installed. The results by Waniliesta & Chopra (2007) on the effectiveness of vacuum sweeping and pressure washing indicated that pressure washing, vacuum sweeping and the combination of the two methods could restore infiltration rates of a clogged pervious concrete surface on a magnitude of 100%, 90% and 200% respectively (Figure 47). As a general rule of thumb, one or a combination of these techniques should be performed on an annual basis. However, it was noted that pressure washing may dislodge pollutants that cannot be captured before entering receiving waters, thus in these situations, vacuum sweeping may be the preferred method. They also recommend that Émbedded infiltrometer´ should be used to annually test the system infiltration capability, and if it is less than acceptable, one of the recommended remediation techniques should be performed. [Waniliesta & Chopra 2007]


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a) b)

Figure 47. Comparison of original clogged and a) vacuum swept infiltration rates, and b) pressure washed and vacuum swept infiltration rates. The pervious pavement at sites D2 and SC2 were not installed properly and exhibited the density and zero infiltration characteristics common to regular concrete. [Waniliesta & Chopra 2007]

According to Smith (2011), when monitoring PICP the surface infiltration can be detected by two methods:

· by observing drainage immediately after a heavy rainstorm for standing water or · by conducting surface infiltration test (ASTM C1701:2009, see [Kuosa & Niemeläinen

2013a]

Cleaning is recommended if the surface infiltration rate falls below 25 cm/h (0.07 mm/s). [Smith 2011]

Borgwardt (2006) found that the long-term in-situ infiltration of PICP performance and its observed decrease depend on the grain size of the aggregates used for joint filling. It has also been detected that much of the sediment will be trapped in the first 12–25 mm of the jointing aggregates in PICP. A maintenance advantage of PICP over pervious monolithic pavement is the ability to restore also heavily clogged joints only by the removal and replacement of the jointing material by vacuum machine adjusted to that. [Borgwardt 2006, Smith 2011]

Figure 48a presents a comparison of a joint material from the upper 20 mm of a joint with unaffected material from below. The fines (particles < 0.063 mm) increase from original 3 mass-% in the average up to 26 mass-%. The difference is highly significant and relate to a drop of permeability from 2.4 to 0.2 cm/min (0.4–0.03 mm/s), as presented in Figure 48b. [Borgwardt 2006]


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a)

b)

Figure 48. Effects of PICP joint clogging: a) Particle-size distribution for the joint material from the upper 20 mm of a joint, and for the unaffected joint material from the below of the joint; b) Relation between permeability and particle fraction < 0.063 mm in the joint material. [Borgwardt 2006]

As much of the sediment is trapped within the joints and bedding aggregates at the surface, and removal of this sediment is possible, this also helps slow down the deposition of sediment onto the soil subgrade. However, deposition rates on the soil subgrade are almost impossible to predict. A conservative approach should be taken with regards the soil subgrade infiltration rate over the long term. For instance, ICPI recommends applying a safety factor of 2 for hydrologic design. Even a higher safety factor should be used e.g. for sites with highly variable soil filtration rates. [Smith 2011]

Also Beeldens et al. (2009) reported a decrease in permeability of different pervious pavements with time. These pavements included pavements with pervious pavement blocks. The results indicated, especially in the case of pavement blocks with enlarged joints and drainage holes, a very large permeability just after construction which however diminishes with time and stabilized around 0.02–0.05 mm/s. The permeability remained sufficient to


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pass the rain of 270 l/s/ha, i.e. 0.027 mm/s, and the original demand of 0.057 mm/s was still reached after 10 years in service.

Lucke & Beecham (2011) presented results from an investigation of a PICP system that had been in service for over eight years. They found that the majority of the sediment was retained in the 2–5 mm aggregate bedding layer, irrespective of the pavement blockage conditions. A maximum of only 8.3% of the total sediment mass was retained in the geofabric layers which were located below the bedding aggregate. Over 90% of the sediments were trapped in the paving and bedding aggregate layers. Figure 49 presents accumulation of sediments between the pavers. The conclusion by Lucke & Beecham (2011) was that he beneficial role of geofabric in filtering out sediments and protecting the integrity of the underlying base course may not be significant enough to warrant its inclusion in permeable pavement installations below the bedding layer. The overall infiltration performance of the PICP system was still satisfactory after eight years of continuous service in spite that no maintenance was performed on the PICP system during these eight years in service. However, it was not possible to quantify future maintenance requirements based on this 8 year result. [Lucke & Beecham 2011]

Figure 49. Accumulation of sediments between the pavers. [Lucke & Beecham 2011]

If not necessary, sand should not be applied on the PICP as it will accelerate clogging. If needed, the joint material type should be used (ASTM 8. 89 or 9 stone or similar (see Figure 30), and vacuuming after the winter period should be performed. [Smith 2011]

PCP

For a pervious concrete pavement (PCP) the main controlling aspects in clogging are: · the initial permeability and pore structure of the pavement, · the amount of additional surrounding stormwater designed to infiltrate through the

surface, · the amount of soil in the stormwater, and · the slope of the pavement.

The maintenance required for a permeable pavement is very site dependent. [Kevern 2010]

Sediment can fill the voids in pervious concrete or the stone base and form layers on the surface or along the bottom of the PCP. The most critical aspect of sedimentation may be the formation of a layer of fine-entrained material on the bottom of the PCP structure. [Kevern 2008]

However, as a particle enters the pervious concrete system, the torturous path causes particles to become caught near the surface. As more and more particles become filtered out there is a progressive failure of permeability from the top. This causes the top layer to clog, protecting the middle and bottom of the concrete from clogging. The progressive clogging at


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the surface is highly desirable because surface cleaning is both relatively easy and effective at remediating lost permeability.

Leming et al. (2007) estimated conservatively (depositions were estimated to be 1125 kg/ha/year or higher) that fine grained sediments deposited in the PC pavement will most likely occupy less than 12 mm of the depth of the aggregate base in 20 years of service, resulting in only a few percent loss in storage capacity. An extra 25 mm of aggregate base was estimated to be adequate to supply sufficient storage capacity.

Instead, according to Leming et al. (2007) the effects of sedimentation on PCP permeability may be more significant. Research has shown that sand-sized particles are more likely to be retained on the surface. Instead silt and clay sized particles are more likely to become deposited at the bottom of the aggregate layer. [Kevern 2010] Sedimentation of larger particles (sands) may be concentrated at the typically denser surface such that flow into the pervious concrete is reduced. This kind of sedimentation may be largely restored by routine maintenance operations. The most significant effect on hydrological behaviour of the PCP is likely to be the introduction of another element, a layer of material (sediment) which could affect the exfiltration of stormwater runoff from the PCP into the underlying soil. Sediment accumulation at the base of the PCP could reduce the exfiltration of the system if the layer of sediment is fundamentally different from the underlying soil. [Leming et al. 2007]

Mata (2008) examined the sedimentation rates of pervious concrete with 20% porosity with three different soil types: sand, clayey silt, and clayey silty sand. Storage capacity was minimally affected by sediment. Instead, he found that exfiltration rate can be affected in some situations. A simple and economical test for estimating exfiltration rates of the system in these situations was developed. The results of the study were used to develop design guidelines that complement the hydrological design of PCP considering the effects of sedimentation of the system at end of service.

Mata (2008) found that recovery of PCP permeability is never complete with sediments containing fine grained particles. This observation is consistent with blocking of small diameter voids that may be connecting much larger voids. Additional studies of the relationships between void size distribution, porosity and sediment characteristics was recommended.

The specific effects of sedimentation will depend on the volume of sediment. Precise estimation of the quantity of sediment is very difficult, if not impossible. The concentration of pollutants found in urban runoff is directly related to the land use. Some estimates are presented in Table 13. [Mata 2008, US EPA 1999]

Table 13. Typical Pollutants Loadings from Runoff by Urban Land Use. [Mata 2008]

The main conclusions and recommendations in Mata (2008) are: · PCP subjected to typical loads of clayey silt or sandy sediments will most likely not be

affected either in the storage capacity or the exfiltration rate at the End of Service Life (EOS). The addition of 25 mm of the base layer should be sufficient in most cases to overcome storage capacity losses.


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· A simple test was developed to estimate the exfiltration value of the PCP at EOS conditions including the effects of sedimentation.

· Analysis did not include the effects of significant quantities of organic material that could be washed onto the PCP from the application of mulch on additional top soil to adjacent areas. Runoff in these conditions must be strictly controlled until these ground covers have stabilized.

· Some dust will inevitably be deposited on all surfaces. This fine grained material must be considered in design and analysis if the site will be exposed to unusually large quantities of dust, such as at or near an industrial site with limited dust abatement controls.

· The substantial loss in permeability found with severe cases of sedimentation (multiple applications of sediments and washing) indicates internal clogging. Permeability between 40% and 50% can be lost. With this type of sediment, maintenance by sweeping vacuuming will not be effective since the sediment trapped within the PCP will be very difficult, if not impossible, to be recovered. In most practical cases, the permeability of the PC is so much greater than that of the subgrade, a reduction of even 100% would have little meaningful effect on hydrological behaviour.

o It is not known if this clogging will be different in pervious asphalt pavements or if warmer temperatures in service resulting in viscous flow of the asphalt, in conjunction with the clogging make pervious asphalt more susceptible to clogging. Additional research to address this possibility was recommended.

Many factors control how often maintenance must be performed on PCP pavements. Generally, if the site is infiltrating large amounts of water or there are substantial amounts of fine soil from the surrounding areas, maintenance activities will be more frequent than if the pavement experiences lower hydraulic and solid loading. The chance of clogging is highest during and just after construction, and the site must be protected by an erosion control fence until vegetation has been established on the adjacent ground. [Schaefer et al. 2006]

In a field survey of PCP by Delatte et al. (2007), the sites visited were less than four years old. Both vacuuming and pressure washing had worked well to restore infiltration capability. Some of the pavements had very poor infiltration capability due to improper installation. Too aggressive pressure washing, however, may damage the surface of the pavement. [Delatte et al. 2007]

Henderson (2012) found in his permeability renewal maintenance methods evaluation that: · the initial permeability of the pervious concrete pavement can influence future

performance, · power washing using personal sized equipment can push debris deeper into voids

and decrease permeability rather than improve it, · sweeping of the surface can be effective in removing debris off the surface but not

from deeper voids, therefore not necessarily improving permeability, · washing the surface with a large diameter hose can dislodge debris deep in voids and

renew permeability, in some cases, to near initial permeability values, · intense rain events may increase permeability. [Henderson 2012]

In the study by Henderson (2012), the application of sand as the winter maintenance method decreased the permeability but not to an unacceptable level. When a salt solution was used the permeability also decreased. However, the decreased permeability was not as large as compared to when sand was applied. Instead the surface conditions of the slabs with salt exposure were worse than without salt, and finally the slabs deteriorated to a point where the original size and shape of the slabs was not apparent.

Studies by Borgwardt (2006) on the long term surface permeability demonstrated high infiltration rates initially, a decrease, and then a levelling off with time typically within 5 to 7


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years. With initial infiltration rates of hundreds of centimetres per hour, the long term infiltration capacity remained high even with clogging. When substantially clogged, surface infiltration rates still usually well exceeded 2.5 cm/h (0.007 mm/s). [Borgwardt 2006]

Measures should be taken to protect permeable pavements from high sediment loads, particularly fine sediment during and after construction. This is especially critical during and after construction while adjacent vegetation is growing. [Smith & Hunt 2010]

According to [Virginia DCR 2011], the property owner should clearly understand the maintenance responsibilities inherent with permeable pavement, particularly for parking lot applications. The owner should be capable of performing routine and long term actions to maintain its hydrologic function (such as vacuum sweeping). He should also avoid practices such as winter sanding, seal coating or repaving. Permeable pavement is not intended to treat sites with high sediment or trash/debris loads.

Reported best practices for maintaining pervious concrete includes sweeping with a streets weeper that uses water in conjunction with brushes, to agitate the debris in the voids, and using a vacuum to clean debris from the surface. If the extent of clogging is too severe to be effectively treated by vacuuming, washing the porous pavement with low pressure water and vacuuming the surface after washing has been recommended. [CRMCA 2009, Wang & Wang 2011 Cahill Associates 2009, Danish Road Institute 2002, Henderson & Tighe (2011]

Table 14 presents the typical maintenance activities for porous pavement according to ´Georgia Stormwater Management Manual 2002´. [Shirke et al. 2009]

Table 14. Routine Maintenances for Porous Pavement Data from ´Georgia Stormwater Management Manual 2002´ [Shirke et al. 2009]

Activity Schedule Initial inspection Ensure that the porous paver surface is free of sediment Ensure that the contributing and adjacent area is stabilized and mowed, with clippings removed Vacuum sweep porous concrete surface followed by high pressure hosing to keep pores free of sediment Inspect the surface for deterioration and spalling Check to make sure that the system dewaters between storms Spot clogging can be handled by drilling ca. 13 mm holes through the pavement every few 30 cm Rehabilitation of the porous concrete system, including the top and base course as needed

Monthly for three months after installation Monthly As needed, based on inspection Four times a year Annually Upon failure

Shirke et al. (2009) studied by laboratory experiments a potential process of removing particles trapped in the pores of the pavement by flushing water from the bottom to the top of the pavement. This method was called “reverse flush process”. There were four variables included to determine the effects on particle removal, i.e. water pressure, clogging material, pavement porosity and number of flushes. Results indicated that the reverse flush process was effective on both types of clogging material (sand) evaluated, and was independent of the pavement porosity. The highest pressure of 21 kPa and the next highest of 14 kPa removed particles equally well (with no statistical difference) at about 80% and 73%, respectively. Although statistically different than these pressure levels, percent removal at 3.5 kPa of about 66% was also considered encouraging. The conclusion in [Shirke et al. 2009] was that reverse-flushing of porous pavements with water at relatively low pressure levels may be an effective process for maintaining porosity. Sand mixed with clay or pure


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clay might be more difficult to remove, and was not included in this study. According to Shirke et al. (2009) a full-scale experimental trial should be conducted to determine how this technology could be implemented.

PA

Also the clogging and permeability loss of porous asphalt begins usually soon after construction. The change of permeability may be significant, and the reduction can be up to 90% [Ferguson 2005]. On the other hand, if the initial void rate is sufficiently big, even this relatively high reduction does not always disturb the permeability too much [Stenmark 1995].

Figure 50 shows the effect of aggregate size on the clogging rate. The porous asphalt including coarser aggregate and therefore a larger void content maintains permeability better than asphalt with finer aggregate. The infiltration rate declined rapidly in the first few months, but later on the clogging rate leveled out. After 22 months the permeability had declined to less than 50% of the initial rate. (Croney and Croney 1998) [Ferguson 2005, Bendtsen et al. 2002]

Figure 50. Decline in infiltration in porous asphalt overlays. 50 in/hour is 0.35 mm/s. (Croney and Croney 1998) [Ferguson 2005]

Also in the case of porous asphalt pavements, the most important ways to maintain pavements are vacuuming, washing and sweeping [Ferguson 2005]. Cleaning frequency is depended on the site-specific clogging rate. In motorways there may be less need for cleaning compared to low-trafficked roads, because the tires of cars push/sucks the water rinsing in the pores and reduce clogging rate [Yildrim et al. 2007, Roseen et al. 2012]. However, the cleaning is most effective when it is done before it is too late and the surface is fully clogged. Regular clogging control is important. If cleaning and other maintenance procedures are unhelpful, the most damaged layer can be removed and it is possible to construct a new porous layer onto the old structures [Ferguson 2005]. Clogging concentrates mostly on the uppermost part of the porous asphalt; underlying courses usually stay clean and permeable.

Figure 51 presents a comparison of clogged and washed asphalt in Issaquah city, Washington, USA. The city streets were sanded every winter and the asphalt pores were filled by fine sand and moss within five years. The first attempt to clean the asphalt with a regular vacuum sweeper and a heavy-duty straight suction truck was ineffective. After a 21 MPa power washer was used with success, and the infiltration rate returned close to the initial value. When the water was no more ponding on the surface, the sanding need was also reduced. [Ross 2012]


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Figure 51. a) Clogged asphalt. b) Asphalt surface after washing. [Ross 2012]

Al-Rubaei et al. (2013) studied how high pressure washing and vacuum cleaning would restore old, clogged porous asphalt roads at two different sites in northern Sweden. The Luleå site was 18 years old and was sanded two to four times each winter with 2/4 mm gravel, swept every spring, washed and vacuumed regularly. Over the last 5 or 6 years before cleaning study the washing and vacuuming was interrupted. The Haparanda site was 24 years old and was sanded 5 to 10 times each winter with 0/6 mm sand mixed with 2% salt and only swept at spring. At this site a soil piles had been stored on the porous asphalt while nearby construction work was performed, with no maintenance or cleaning actions afterwards. Porosities and infiltration rates initially, before and after cleaning are presented in Table 15. The oorosities had not significantly changed, but the infiltration rates changed remarkably over the years. Although the infiltration capacity was only a few percentages of the initial rates at the Luleå site, there was still some functionality left before cleaning. The cleaning procedure was also partly successful at the Luleå site; infiltration capacity increased. At the Haparanda site there was no effect. This study shows the importance of regular cleaning procedures and moderate winter sanding with coarser sand for maintaining the infiltration capacity of porous asphalts. [Al-Rubaei et al. 2013.]

Table 15. Effects of high pressure washing and vacuum cleaning (VC) at northern Sweden [Al-Rubaei et al. 2012].

Initial rates Before VC (mean) After VC (mean)

Porosity [%]

Infiltration [mm/min] Porosity Infiltration Porosity Infiltration

Luleå 18 290 17,2 0,5 16,03 3,48 Haparanda 18 470 15,7 0,22 17,9 0,12

7.2 Winter performance

The usage of permeable pavements in cold climates has many challenges, most of which relate to the extreme cold and frost penetration into the porous media. Permeable pavement winter performance consists of mainly:

· surfacing material performance in freeze-thaw, including the effects of possible de-icing chemicals,

· overall structural performance in freeze-thaw, i.e. the effects of frost heave caused by subgrade performance and pavement reservoir performance,

· water infiltration during wintertime and snowmelt periods, · effect of winter on the chemical purification properties of pervious pavement systems, · performance in winter maintenance:


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o clogging caused by sanding, o effects of the possible use of de-icing chemicals, widely including also the

effects on subsoil and groundwater, o mechanical effects of snow removal as plowing.

Freeze-thaw durability of the porous pavement surfacing layer materials (PC, PA and pavers in pervious pavements) is reviewed in Chapter 3 (Surface layer materials). Clogging caused by sanding is included in Chapter 7.1 (Clogging and maintenance).

Ice formation may also clog the pores of porous asphalt [Stenmark 1995]. In this case clogging is not a permanent phenomenon, but when the temperature is around zero, the melting water may not be able to infiltrate into the lower layers. In northern climates the porous asphalt could be very beneficial to even out flow peaks of melting water in spring, so the ice clogging effect must be taken into account e.g. in the dimensioning of the reservoir layer, and by regular maintenance (snow plowing). Then the water does not pond and freeze on the surface, and sun can melt possible ice formations quickly. Usually the infiltration rate reduces a little during winter time while temperature decreases, but the infiltration capacity remains still sufficient [Roseen et al. 2012]. Additional information on the specific studies and experiences on the winter performance of the whole pervious pavement system is included in a separate CLASS-project State-of-the-Art Report [Kuosa & Niemeläinen 2013b] (Pervious pavement winter performance).

Effect of winter on the chemical purification properties of pervious pavement systems is included in a separate CLASS-project State-of-the-Art Report [Loimula & Kuosa 2013] (The impact of pervious pavements on water quality).

7.3 Cool pavement

Urban Heat Island (UHI) effect means the occurrence of higher air and surface temperatures occurring in medium and large sized urban centres due to the retention and emittance of mainly solar heat from roads, buildings and other structures, than in surrounding rural areas. [Gilbert 2013]

Cool Pavements mean materials and construction techniques which act to reduce the absorption, retention and emittance of solar heat. Porous materials allow for convective cooling because air can flow through the pavement voids and also allow for evaporative cooling because water can also enter the pavement voids in a rain event. The use of high reflective and porous materials can significantly reduce the heat gain of pavements by the sun. (Figure 52) [Gilbert 2013, Cambridge Systematics 2005]

The combination of high albedo (reflection ratio) and pervious pavements are especially well suited for relatively light traffic flow areas such as driveways and parking lots while helping to mitigate the heat island effect. [Gilbert 2013]


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a) b)

Figure 52. a) Heat-Related characteristics and processes in a pavement b) Porous pavement promoting cooling through evaporation. [Cambridge Systematics 2005, Gilbert 2013]

Higher pavement temperatures can heat stormwater runoff. Higher water temperatures can, in turn, affect metabolism and reproduction of aquatic species. Permeable cool pavements can help water quality through reduced heating of runoff. Laboratory tests with permeable pavers have shown reductions in runoff temperatures of 2–4 °C, in comparison to conventional asphalt paving. [Cambridge Systematics 2005]

Flower et al. (2010) addressed the need to present results quantifying the impact of pervious concrete on surface temperatures in semi-arid (semi-dry) urban environments. They monitored surface and internal temperatures at a new pervious concrete site, at an adjacent traditional concrete site, and at a traditional asphalt pavement site. The results from the summer of 2009 showed a significant reduction of surface temperature at the PC site compared to the asphalt site. Interestingly, as the monitoring moved into June the traditional concrete site became shaded, providing a comparison between pervious concrete and shaded traditional concrete. The surface temperatures were very similar, leading to the conclusion that pervious concrete may serve as an Urban Heat Island (UHI) effect mitigation measure equivalent to shading of traditional concrete. [Flower et al. 2010]

Compared to asphalt, grassed grid pavements can reduce surface air temperatures by 1° to 2 °C and radiometric temperatures by 2° to 4 °C. [Angelus 2013]

According to [CPG 2013], pervious concrete pavements are able to lose 19–25 mm of water a day in the summer time to evaporation. This means that a 76 mm rain would be gone in 4 days just due to evaporation.

Pervious concrete like regular concrete is light reflective, especially when there is blast furnace slag included in the mix, and therefore cooler because of the colour. This helps also in the fight against the UHI effect. [CPG 2013]

7.4 Costs and service life

In comparison to traditional drainage systems, stormwater retention and infiltration is often considered to be sustainable and cost effective process, which is suitable for urban areas. [Scholz & Grabowiecki 2007] In this Chapter only some general or limited experiences on permeable pavement costs and service life are reviewed. A more detailed analysis will be done in 2014, based on Finnish solutions generated in the CLASS project.

The effective life of a pervious pavement is defined as the number of years it is in service, until which the hydraulic performance drops to a level where the drainage ´design storm


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event´ is unmanageable and remedial works are required. One barrier to use of porous pavements more widely has been susceptibility to clogging. Along with careful design and construction, proper maintenance of a pervious pavement is a good method to ensure a long term life-span. At the same time, an essential maintenance cost for porous pavements is caused due to the measures which prevent clogging of the void spaces within the pavement. [Yong et al. 2013, Zhang 2006, Waniliesta & Chopra 2007]

Overall project costs and impacts must always be considered. For instance pervious concrete pavement is likely to cost more than conventional asphalt and concrete pavement. However, the additional cost may be offset by even greater savings in drainage and water treatment systems. [Delatte & Cleary 2006]

According to [FHA 2013], installation costs for porous asphalt are approximately 10–5% higher than those for regular asphalt. Porous concrete pavement is about 25 % more expensive than regular concrete pavement. Requirements for site preparation or the use of specialized equipment may also increase these costs. The use of modular paving stones can be up to four times as expensive as either regular asphalt or concrete.

The higher costs of installation of porous pavements can be offset to some extent by the elimination of curbs, gutters, and storm drains. In some cases this may lower the overall cost for a project [Field et al. 1982]. The final economics associated with a particular site are also affected by site-specific conditions, such in-situ permeability, and the cost and proximity of gravel supplies.

Also according to [Hein et al. 2010] the cost of a permeable pavement section is typically higher than the cost of a conventional pavement primarily due to the fact that the permeable pavement is thicker to allow sufficient water storage and to provide sufficient structural capacity to accommodate vehicle loading. However, cost comparison of the entire permeable pavement system compared to a conventional pavement shows a reasonable cost comparison when taking into account the reduction or elimination of catchbasins, underground piping, drainage ditches and stormwater management ponds required for conventional designs. There are also other advantages such as reduced downstream erosion, reduced pollutant loads, and less impact on existing stormwater management infrastructure.

The lifetime of porous asphalt has been studied at laboratory and also in-situ. Alvarez et al. (2006) claims the service life of open-graded friction courses to be 7–10 years. Roseen et al. (2012) says that the typical life span of porous asphalts is about 15 years in northern climates. Huber (2000) and Bendtsen (2011) have collected information about the lifespan of open-graded asphalt mixtures in North America and Europe. These results are presented in Table 16. The main finding is that a typical porous asphalt is less durable in every country/state than dense asphalts. Use of fibres in binder will strengthen the porous asphalt so that the lifespan may be longer. Construction costs are usually higher than for conventional asphalt, so the shorter lifespan may raise the price even more. Results of Swiss research and experiences show that with proper design and maintenance, porous asphalt can obtain up to 15 years lifetime with good mechanical, permeability, and acoustical behaviour [Poulikakos et al. 2006a].


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Table 16. Comparison of different asphalts lifetime in years. PA = porous asphalt. [Huber 2000, Bendtsen 2011]

Lifetime (years)

Mixture Arizona Georgia California Wyoming Germany Switzerland Netherlands

Neat/normal PA (1 layer) 7 8 3 - 5 15 7 - 10 8 - 12 11 - 171)

PA with modified binder 13 12 2-layered PA 9 - 131)

Dense asphalt 11 - 20 10 5 - 10 10 - 241) 16 + 15 12 - 18 1) Depends on road type.

Based on information from the U.S. National Ready Mixed Concrete Association, Henderson (2012) presents pervious concrete pavements to have the potential to exhibit the same low life cycle costs as conventional concrete pavements. The life cycle cost of conventional concrete pavement is low because concrete has a longer life time than other paving materials and can require less maintenance throughout the life cycle. Pervious concrete that is well designed and constructed should also exhibit similar life cycle performance.

From the results generated during the monitoring period of the field sites and the laboratory testing in [Henderson 2012], it was anticipated that pervious concrete pavement in Canada can achieve a design life of 15 years, when used in a suitable application. Based on the experiences, it was expected, that similar to any paving material, opportunities for advancements will always be present.

As pervious pavement maintains the natural water cycle, it can offer economic benefits to individuals using it on their own property. Since pervious pavement is a low impact development it ensures that surrounding vegetation, such as gardens and lawns, receive higher amounts of natural moisture. This limits expenses for the home owner related to watering. In the case of both private and commercial properties, pervious pavement can be used in a water harvesting system which reduces the public water demand. [Henderson 2012]

The use of pervious pavement can also give economic benefits as it eliminates the need for land and infrastructure to support other stormwater management systems. Additional land is often required for e.g. stormwater retention ponds. Stormwater management systems also require infrastructure such as pipe networks. [Henderson 2012]

8. Water quality

Urban surfaces are being covered with impermeable materials at increasing speed. This leads to increase in surface runoff as the ground is not able to take in all the rainwater at a sufficient rate. Surface runoff can transfer pollutants like heavy metals and hydrocarbons into sewer systems and natural watercourses.

Pervious pavements offer a solution for the problem of increased stormwater runoff and decreased stream water quality. These pavements are designed to take in a sufficient amount of water causing practically no surface runoff during normal storm events. Permeable pavements can also act as pollution sinks because of their particle retention capacity. Impurities such as heavy metals, hydrocarbons and organic compounds are absorbed onto suspended solids and trapped inside the permeable pavement structure.

Effluent quality from pervious pavements has been studied to be significantly better than typically monitored from impermeable sources in similar residential areas.


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A more detailed review on the effect of pervious pavement on water quality is presented in a separate CLASS-project State-of-the-Art Report [Loimula & Kuosa 2013] (The impact of pervious pavements on water quality).

9. Choice of pavement type and functional demands

The choice of what kind of pervious pavement to use is influenced by site-specific design factors, and the intended future use of the permeable surface. As an example Table 17 presents one general comparison of the engineering properties of the three major permeable pavement types, basically as presented in [Virginia DCR 2011].

Table 17. Comparative Properties of the three major pervious pavement types. Modified from [Virginia DCR 2011].

Design factor Pervious Concrete (PC) Porous Asphalt (PA) Interlocking Pavers (IP)

Scale of application Small and large scale paving applications

Small and large scale paving applications

Micro, small and large scale paving applications

Pavement thickness 1) 127 - 203 mm 76 – 102 mm 76 mm1) 8)

Bedding layer 1) 8) None 51 mm “No. 57 stone” (2.4 – 37.5 mm)

51 mm “No. 8 stone” (1.2 --12.5 mm)

Reservoir layer 2) 8) “No. 57 stone” (2.4 – 37.5 mm)

“No. 2 stone” (19 - 75 mm)

“No. 2 stone” (19 - 75 mm) 76 - 102 mm of “No.57 stone”

(2.4 – 37.5 mm)

Construction properties 3)

Cast in place, seven day cure, must be covered

Cast in place, 24 hour cure

No cure period; manual or mechanical installation of pre-

manufactured units Design permeability 4) 0.035 mm/s 0.021 mm/s 0.007 mm/s Relative construction

cost 5) 2 – 6.5 0.5 - 1 5 - 10

Min. batch size 46 m2 NA Longevity 6) 20 to 30 years 15 to 20 years 20 to 30 years

Overflow Drop inlet or overflow edge Drop inlet or overflow edge

Surface, drop inlet or overflow edge

Temperature reduction

Cooling in the reservoir layer

Cooling in the reservoir layer

Cooling at the pavement surface & reservoir layer

Colors/texture Limited range of colours and textures

Black or dark grey colour (if with no painting)

Wide range of colours, textures, and patterns

Traffic bearing capacity 7) Can handle all traffic loads, with appropriate bedding layer design.

Surface clogging Replace paved areas or install drop inlet

Replace paved areas or install drop inlet

Replace permeable stone jointing materials

Other issues Avoid seal coating Snowplow damage

Design reference American Concrete Institute (ACI)

U.S. National Asphalt Pavement Association

(NAPA) (Jackson 2007)

U.S. Interlocking Concrete Pavement Institution (ICPI)

(Smith 2006) 1 Individual designs may depart from these typical cross-sections, due to site, traffic and design conditions. 2 Reservoir storage may be augmented by corrugated metal pipes, plastic arch pipe, or plastic lattice blocks. 3 ICPI (2008) 4 NVRA (2008) 5 WERF 2005 as updated by NVRA (2008) 6 Based on pavement being maintained properly, Resurfacing or rehabilitation may be needed after the indicated period. 7 Depends primarily on on-site geotechnical considerations and structural design computations. 8 Stone sizes correspond to ASTM D 448: Standard Classification for Sizes of Aggregate for Road and Bridge Construction.

The major design goal of permeable pavement is to maximize runoff reduction and nutrient removal. Designers may choose to use a baseline permeable pavement design or an enhanced design that maximizes nutrient and runoff reduction. [Virginia DCR 2011]


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In all there are several functional demands for pervious pavements to fill, as presented in this report:

· surface infiltration capacity, drainage, surface layer permeability · water storage capacity (by aggregate base and subbase, by other draining and

collection systems) · water quality enhancement capacity · bearing capacity · durability, service life · capacity for restoration of the infiltration capacity · all winter performance properties (easiness for ploughing, slipperiness, friction,

degree of frost resistance) · costs (construction, maintenance) · service life · suitability for reuse or recycling.


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