Laboratory Investigation on Foamed Bitumen Bound Mixtures Made With Steel Slag, Foundry Sand

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Laboratory investigation on foamed

bitumen bound mixtures made with

steel slag, foundry sand, bottom ash

and reclaimed asphalt pavementMarco Pasetto a& Nicola Baldo b

aDepartment of Civil, Environmental and Architectural

Engineering, University of Padua, Via Marzolo 9, Padova 35131,

ItalybDepartment of Chemistry, Physics and Environment, University of

Udine, Via del Cotonificio, 114, Udine 33100, Italy

Version of record first published: 27 Nov 2012.

To cite this article:Marco Pasetto & Nicola Baldo (2012): Laboratory investigation on foamedbitumen bound mixtures made with steel slag, foundry sand, bottom ash and reclaimed asphalt

pavement, Road Materials and Pavement Design, 13:4, 691-712

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Road Materials and Pavement Design

Vol. 13, No. 4, December 2012, 691712

Laboratory investigation on foamed bitumen bound mixtures made withsteel slag, foundry sand, bottom ash and reclaimed asphalt pavement

Marco Pasettoa* and Nicola Baldob

aDepartment of Civil, Environmental and Architectural Engineering, University of Padua, Via Marzolo 9,

Padova 35131, Italy; bDepartment of Chemistry, Physics and Environment, University of Udine, Via delCotonificio, 114, Udine 33100, Italy

The paper describes an experimental investigation on foamed bitumen bound mixtures forroad pavements, with the aggregate matrix entirely composed of industrial by-products, suchas steel slags, foundry sand, bottom ash and reclaimed asphalt pavement. The study consistedof a laboratory analysis of the physical and leaching properties of the single recycled granularmaterials considered, followed by mechanical characterisation of the foamed bitumen boundmixtures, in terms of indirect tensile strength, stiffness modulus and permanent deformationtests. The mechanical tests were also performed in wet conditions, in order to investigate themoisture sensitivity of the mixes.

Keywords: steel slag; bottom ash; foundry sand; reclaimed asphalt pavement; foamedbitumenmixtures

1. Introduction

The recycling of industrial wastes in road infrastructures represents a possible solution to two

important and delicate questions: the growing problem of disposing of these industrial by-products

and the need to find alternative sources to the traditional quarried stone materials used for road

construction. In the last 10 years, the use of recycled materials in the Northeastern Italian area, for

the building of both strategic and ordinary roads, has noticeably grown (the 36 km Mestre-Venice

motorway by-pass is one of the most recent and relevant examples of this trend). Nowadays

there is a strong demand for succedaneum materials to be used in the road construction field,

by private companies, highway agencies and public administrations. Therefore, more research

is needed in order to provide solutions to this growing demand, which have to be technically

feasible as well as economically and environmentally sustainable. In this sense, a cold recycling

of marginal materials, based on the use of cement and foamed bitumen, provides some advantages:

the binder can be used when the aggregates are cold and damp; the foamed bitumen is applicable

to a wider variety of aggregates; the optimal binder content is generally lower than that for the

other types of mix. Furthermore, emissions into the atmosphere are almost absent and the use

of solvents is unnecessary (a health hazard to the workers) to reduce the viscosity of the mixes.

Lastly, it is possible to quickly reopen the road to traffic after compaction, thanks to the adequate

resistance offered by the mixture, and there is no insurgence of phenomena of water infiltration

if it should rain immediately after the recycled layer is applied (a problem when emulsions are

used instead). The main disadvantages of foamed mixes are the following: the need for trained

*Corresponding author. Email: [email protected]

ISSN 1468-0629 print/ISSN 2164-7402 online

2012 Taylor & Francis

http://dx.doi.org/10.1080/14680629.2012.742629

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692 M. Pasetto and N. Baldo

technical personnel, both during the mix formation and mixing stage during the laying; a correct

cost-benefit analysis of the pavements produced using this technique; specific equipment, usually

integrated in the foam generator, required to heat the bituminous binder to the required foaming

temperature. Moreover, the moisture resistance of foamed mixes is usually poor, as compared to

other materials, such as cold mixes with emulsion (Cazacliu et al., 2008); the temporal evolution

of mechanical properties (curing) makes it difficult to estimate the in-situ behaviour (Eckmann,Delfosse, Chevalier, & Pouteau, 2008); finally, the low cohesion of foamed mixes makes it

difficult to evaluate the laboratory fatigue resistance, even if the field behaviour could be excellent

(Eckmann et al., 2008). The absence of a reliable fatigue law for these materials makes it difficult

to attain a safe pavement design.

The present study was aimed at investigating cold recycled mixtures, made with non-

conventional aggregates, cement and foamed bitumen. Leaching and physical-mechanical testing

was performed on some waste materials in order to evaluate their technical suitability as a recycled

aggregate in foamed mixtures, for road pavements. Mixtures of this type are typically used in

road foundation layers and, from the technological point of view, can be slotted within the ambits

of stabilisation.The granular materials considered were what are known as marginal aggregates (Pasetto

& Baldo, 2006, 2008a, 2008b, 2010a, 2010b, 2011, 2012); more specifically, they are bottom

ash from municipal solid waste (MSW) incineration, waste foundry sand, electric arc furnace

(EAF) steel slag and reclaimed asphalt pavement (RAP). The analysis investigated the marginal

materials individually, plus mixtures of these components in different proportions.

2. Materials and methods

The study was divided into three main phases. The first was relative to the toxicological and

physicomechanical characterisation of the aggregates. The second was regarding the mix designof the foamed mixtures: two characterised by a predominant steel slag content and two with equal

percentages of steel slag and RAP. The performance of the foamed mixtures was studied during

the third phase.

2.1. Materials used

Exclusively manufactured and recycled aggregates (EN 13242 and 13043) were used for the

composition of the foamed mixtures: EAF slag, bottom ash, foundry sand and RAP.

Slag from the steel industry comes from the rapid cooling of the oxidised and superficial

liquid phase present in EAFs, from circa 1300C, to ambient temperature (Jones, Bowman, &

Lefrank, 1998; Preston, 1991). The blocks are solidified in the open air, sometimes acceleratingthe process by jets of water. In this way, a quantity of free calcium oxide may remain inside

the blocks, potentially subject to hydration or carbonation. These alterations can be the cause of

uneven expansion and disintegration of the material. For this reason, slag from electric furnaces is

seasoned, exposing the material to the weather for a period of at least 2 months, during which the

unbound fraction of calcium oxide is stabilised naturally. It is also reported that another reaction

related to volumetric expansion involves the dicalcium silicate (C2S) phase. The C2S phase is

commonly present in all types of steel slags, even if it is typically abundant as the main phase in

ladle slags. C2S exists in four well-defined polymorphs: ,, and. At temperatures below

500C,-C2S starts transforming into -C2S. This transformation produces volumetric expansion

up to 10%. If the steel slag cooling process is slow, crystals break, resulting in a significant amountof dust. This phase conversion and the associated dusting are typical for ladle slags (Shi, 2004;

Yildirim & Prezzi, 2011), different from those used in the research described here.


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Road Materials and Pavement Design 693

At the end of seasoning process, the material may still contain some metal, represented by steel

inclusions, which must be extracted, usually by a magnet and/or by crushing. Lastly, the raw slag

is crushed, generally done first with a jaw breaker and then finished off with a cone crusher in

order to obtain the desired grading fractions.

The US steelmaking industries produce 1015 million tons of steel slag, every year. In

2006, about 5070% of the total steel slag generated in the USA was recycled as aggregatefor road constructions. About 1540% of the steel slag that was not reutilised was stock-

piled in the steel plants and, eventually, sent to slag disposal sites (Yildirim & Prezzi, 2009).

Nearly 12 million tons of steel slag are produced annually in Europe and about 65% is used in

different applications, mainly as aggregate in road constructions (Mladenovic, Kokot, Cotic,

Bianco, & Said, 2009). In Italy, the production of steel slags in 2009 was estimated about

three million tons (Sorlini, Sanzeni, & Rondi, 2012), the largest amount of which has been

dumped.

The incineration of MSWs provides by-products that can mainly be classified as bottom ash and

fly ash (Fortezza, Far, Segu, & Cerd, 2004). The former, corresponding to 25% on the weight of

the wastes to be incinerated, therefore the principal product of the process, is mainly composedof silica (Si), iron (Fe), calcium (Ca), aluminium (Al), sodium (Na) and potassium (K) oxides,

so presenting a composition similar to that of different natural materials. Its metallic fraction can

be recovered; the remainder is usually stocked in dumps or, less frequently, recycled. Bottom

ash can be re-utilised in the construction of new infrastructure, especially in bitumen and cement

bound mixtures, because of the inert properties of the binder that covers the grains with a thin

film, which prevents the release of heavy metals. Fly ash is not usually used in road construction

because of the high content of heavy metals.

The Italian production of bottom ash was equal to 755,000 tons in 2003 (Crillensen & Skaarup,

2006), with a recycling rate of 20%; the largest amount was therefore dumped. In France

and in Germany, the production amounted to 2,995,000 and 3,140,000 tons respectively; the

corresponding recycling rates in road and civil construction were 79 and 65%.

Waste foundry sands are a by-product of the permanent smelting process (Guney, Aydilel,

& Demirkan, 2006; Javed & Lovell, 1995; Pasetto & Bortolini, 2007; Siddique, Kaur, & Rajor,

2010). This process involves the use of consumable sand moulds, formed by reusable models and

hardened by compaction and the addition of binders. The molten metal is poured into the moulds,

which determine the external shape, while internal cavities are obtained by positioning nuclei

of sand, prior to pouring the metal. As the sand-covered binder is exposed to high temperatures

when the metal is poured, its physical and chemical properties deteriorate over time. New natural

sands are usually added in foundries, on a daily basis, to renew the exhausted sand. The sands

pass through many reuse cycles, before being finally dismissed. Many foundries (especially in

Europe and Japan) practice the recovery and recycling of the sand as a way to limit costs, thusreducing the volume of both new and exhausted sands. In the above processes, the molten metal

is poured into a mould made of sand that has been modelled and hardened to resist the pressure

and heat of the metal. After the metal has been cast, the sand is separated and recycled. Although

foundries tend to recycle as much sand as possible, some sands must be eliminated at each

cycle, because of the physical and chemical collapse of the aggregate, as well as the need to

use virgin sand for every part of the mould. In the metallurgical industry, exhausted foundry

sand is defined as a sand that has completely lost its usefulness in the foundry process as it has

already been recycled many times and must therefore eventually be removed to maintain the

conditions for high-quality smelting. Many types of virgin sand are used for smelting moulds;

they can be divided into four groups (silica, olivine, chromite and zircon). The most commonlyused one is silica. Each group has many sub-groups based on the different characteristics of the

aggregate.


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Figure 1. EAF steel slags.

The annual production of foundry sand is approximately equal to 9 million and 12 million tons

in Europe and the USA, respectively; this sand is mainly disposed in waste containment facilities

(Guney et al., 2006), while only the 3032% is currently reused in construction.

The RAP material comes from the recycling of degraded bituminous pavements. The use of

recycling in the construction of road pavements in Italy goes back to the end of the 1970s when,

due to the first big oil crisis, it became necessary to find new working methods that could limit the

use of oil and its derivatives (bitumen). Under those circumstances, the first changes were made

to off-site plants for the production of bituminous concrete and the technique of in-situ recycling

was set up. Since then, two techniques of recycling have become established, named hot (with

material heated to above 150C) and cold (without heating), in both off-site plants and in-situ.

The amount of milled material, namely RAP material, that can be re-utilised in the production of

new mixtures varies depending on technique: for hot recycling, between 10% and 50% is usually

reused, even if some Authors have investigated very high recycling rates (up to 90%) in hot mix

and warm mix asphalts (Alvarez, Bonneau, Dupriet, Le Noan, & Olard, 2008); for cold recycling,

it can potentially reach the 100% (Pasetto, Bortolini, Scabbio, & Carta, 2004).

The Italian production of RAP was equal to 11 million tons in the 2010 (Ravaioli, 2011); a 20%recycling rate was stated for new asphalt concretes, while the remaining amount was dumped.

In the same period, production in France and Germany was about 7 million and 14 million tons,

respectively, but with recycling rates of 40 and 82%. In the USA, where a production of about 66

million tons and a recycling rate of 84% are reported, the situation appears very different.

Concerning the research described here, all the materials have been supplied by an Italian private

company, located in the province of Padua (Northeastern area of Italy, Veneto Region), specialised

in the recovery and valorisation of industrial by-products, for road construction. Figures 14

represent samples of EAF steel slags, RAP, bottom ash and foundry sand, investigated in the

present study.

The recycling feasibility of waste materials in road infrastructures is strongly dependent onthe evaluation of toxic compounds in the non-conventional aggregates, because of the poten-

tial heavy metals leaching phenomenon. Table 1 summarises the toxicological characteristics of


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Figure 2. Reclaimed asphalt pavement.

Figure 3. Bottom ash.

the industrial wastes used, in terms of the release of heavy metals by leaching, measured with

the toxic characteristic leachability procedure (TCLP) standard. This is a commonly used test

aimed at evaluating the leaching characteristics of hazardous waste materials, performed on the

aggregates investigated, before any other physical-mechanical testing, in order to establish their

environmental compatibility.

The TCLP requires end-over-end agitation, a 20:1 liquid-to-solid ratio and an equilibrium time

equal to 18 hours. A sample of at least 100 g is extracted with a proper leaching solution. After

agitating it for 18 h, the extracts are separated from the solids using a glass fiber filter (Siddiqueet al., 2010). The extracts are subsequently analysed in a laboratory, in our case by inductively

coupled plasmaatomic emission spectrometer (ICP-AES) methodology.


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Figure 4. Foundry sand.

Table 1. Major heavy metal leaching concentration of the aggregates used from industrialwastes.

TCLP leaching concentration

Element EAF slag Foundry sand Bottom ash Legal thresholds

Copper (Cu) 0.004 m g/l


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Table 2. Marginal materials grading curves.

Passing percentage (%)

Sieve size (mm) EAF 0/20 EAF 20/60 Bottom ash Foundry sand RAP

71 100.0 100.0 100.0 100.0 100.0060 100.0 98.41 100.0 100.0 96.3850 100.0 96.61 100.0 100.0 96.3840 100.0 90.36 100.0 100.0 92.7530 100.0 74.00 100.0 94.37 87.7425 100.0 51.99 100.0 94.37 82.7020 98.16 30.26 100.0 91.39 76.0215 95.85 2.25 100.0 87.56 67.4110 84.49 1.42 96.35 82.27 53.135 54.17 1.31 76.55 69.83 29.112 27.24 1.28 58.79 57.72 14.240.40 4.78 0.80 28.06 21.03 1.330.18 2.24 0.36 11.48 2.33 0.26

0.075 0.61 0.15 4.18 0.46 0.04

Table 3. Physical and mechanical characteristics of the aggregates.

Property Standard EAF 0/20 EAF 20/60 Bottom ash Foundry sand RAP

Los Angeles coefficient (%) CNR 34/73 15 14 27Equivalent in sand (%) CNR 27/72 83 50 27 82Shape index (%) CNR 95/84 10 9 15 10Flakiness index (%) CNR 95/84 12 19 5 7Grain bulk density (g/cm3) CNR 64/78 4.14 4.14 2.59 2.76 2.53

Grain dry density (g/cm3) CNR 63/78 3.92 3.88 2.53 2.64 2.36

Porosity of the grains (%) CNR 65/78 5.31 6.28 2.32 4.35 Particle voids content (%) CNR 65/78 48.72 58.51 65.22 49.24

(ER is usually inversely proportional to HL: if the HL is too short the bitumen tends to return to a

liquid state, with irregular-sized lumps dispersed in the mass; if the HL is too long, there is a net

reduction in the expansion and a consequent unsuccessful covering of the grains of aggregate).

The bituminous binder used was a soft bitumen, 80/100 penetration grade, because, as has

often been verified, softer bitumen should maximise the foam properties, i.e. the ER and the HL.

Portland cement CEM II/B LL 32.5R was used as active filler, for all the mixtures tested in

the trial. The added water was transparent and without harmful concentrations of acids, alkalis,

salts, glucose or other chemical or organic substances, as required by the regulations.

2.2. Mix design principles

The key steps in the mix design of foamed bitumen stabilised granular materials (FBSGMs), once

the type of material for the lithic skeleton has been established, are the grading curve definition,

followed by the mixing moisture and foamed bitumen content (FBC) evaluation. When lime

or cement is added to the mixture, its dosage must also be optimised. The investigation of the

foaming properties of the binder and optimum content of foaming water is also necessary.

In our case, the lithic matrix was exclusively composed of the marginal aggregates analysedin the study, in order to satisfy the specific demand of the producer, for a full recycled aggregate

skeleton, in view of some relevant road construction works for the highway network in the


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Table 4. Aggregate type and composition of the mixtures.

Mixture EAF 0/20 (%) EAF 20/60 (%) RAP (%) Foundry sand (%) Bottom ash (%)

Mix 1 30 20 10 10 30Mix 2 30 20 10 30 10

Mix 3 10 20 30 10 30Mix 4 10 20 30 30 10

Figure 5. Foamed bitumen mixtures: grading curves and reference envelope.

Northeastern Italian area (motorways and highways between Padua and Trieste). Having decided

to rework the material coming from the production plant as little as possible, the design grading

curves assumed were those from the combination, in different proportions, of the raw materials,

limited to the fraction of interest, i.e. 0/25 mm. The composition of the mixtures is reported in

Table 4 and the corresponding grading curves are presented in Figure 5.The proportions of mixtures components represent the possible aggregate combinations during

construction, with respect to the real available amount of each one, in the producer factory, and in

relation to the company production plan, for the current and the intermediate future necessities.

Higher percentages have been fixed for the materials, which also give higher economic profit

for the producer in consequence of recycling, any penalisation being avoided in technical terms.

Currently, from the producers economical point of view, the more advantageous materials are

the bottom ash and the foundry sand, followed by the steel slags and RAP.

The use at the mixing stage of the basic materials in their original conditions (each with its

specific grading assortment, without screening for the single particle-sizes) being fundamental

for the practicability of the foamed mixtures, the study of the design grading curves was con-ducted through tests of compatibility of the curves resulting from the integration of the marginal

aggregates with the ANAS (Italian National Road Authority) reference envelope (Figure 5).


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Optimisation of the binding phase, which obviously regards the FBC, does not always involve

the cement that, being the active filler, is most frequently used at a constant rate for all the mixtures

investigated (Ebels & Jenkins, 2007a).

For the optimisation of the percentage of foamed bitumen, Jenkins suggests a two-step approach

(Ebels & Jenkins, 2007b). The first is based on tests of compressive strength and indirect tensile

strength (ITS), for the initial identification of the grading curve and FBC. The mix design is thenrefined in the second phase, involving triaxial tests (monotonic or cyclic), for the verification of

the performance of the mixtures in terms of shear strength (cohesion and friction angle) (Ebels

& Jenkins, 2006) and stiffness (resilient modulus, tangent modulus, secant modulus) (Ebels &

Jenkins, 2007a). The studies conducted by Jenkins and his collaborators (Twagira, Jenkins, &

Ebels, 2006) revealed a more critical aspect of the mechanical performance of the mixtures

stabilised with foamed bitumen (at least for the ones they analysed) in terms of the accumulation

of permanent deformations, rather than cracking (investigated with the four-point bending test).

Jenkins therefore suggests paying more attention to permanent deformations, to be investigated

with cyclic triaxial tests (Ebels & Jenkins, 2007b).

Kim and Lee emphasise the advisability of adopting mix design procedures based on thestandard equipment available in road laboratories (Kim & Lee, 2006). They thus considered both

the indirect tensile strength (ITS) test and the Marshall test, verifying the better sensitivity of the

former in the identification of the optimal content of foamed bitumen. Kim and Lee therefore

propose an optimisation based on the verification of the ITS in dry and wet conditions (Kim &

Lee, 2007), and a successive performance analysis in terms of dynamic modulus and repeated

load axial test (RLAT) (Kim & Lee, 2009).

He and Wong (2007) consider the optimal percentage of foamed bitumen to be that at which the

ITS of specimens immersed for 24 hours in water at 25C is highest. He and Wong (2008) then

focused on the performance of the optimised mixtures, with regard to permanent deformations

(also in soaked conditions), by means of dynamic creep tests.

The cited examples, while not being exhaustive of the many studies reported in the literature

(Hodgkinson & Visser, 2004; Mallick & Hendrix, 2004; Muthen, 1998; Nataatmadja, 2001;

Sunarjono, 2007; Zulakmal, Nafisah, Yazip, & Zin, 2009), introduce the basic concepts of the

mix design procedure followed in the present research. From the previous cited references, the key

laboratory test for the evaluation of the FBC, used in all the previous international investigations,

appears to be the ITS test, in dry and wet conditions; therefore it has been assumed as a primary

optimisation test in the present study.

2.3. Mix design procedure

The investigation regarded four different lithic skeletons (Mix 1, Mix 2, Mix 3, Mix 4) combined

with three percentages of cement (1%, 2%, 3%) and three of foamed bitumen (2%, 3%, 4%). It

was not considered advisable to consider contents of more than 3% of cement to avoid potential

problems of fragility of the mixtures, while with more than 4% of bitumen the stabilisation would

no longer be economical.

Prior to the mix design, the compactability of the granular mixtures without foamed bitumen

was investigated by means of Proctor compaction tests (EN 13286-2), in order to determine the

maximum dry density and corresponding optimum moisture content (OMC); this value can be

used to determine the mixing moisture content, according to the following equation recommended

by Wirtgen GmbH (2004):

Wadd =1 + (0.5WOMC Wmoist), (1)


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where Wadd is the water content to be added to the specimen (% by mass), WOMC is the opti-

mum moisture content (% by mass), Wmoist is the moisture content in the specimen dried in air

(% by mass). The mixing moisture content values, computed using Equation (1), result as basi-

cally within the range between 65% and 85% of the OMC, as suggested in previous investigations

(Bissada, 1987; Lee, 1981).

For each of the 36 different mixtures studied, a batch size of 10 kg of marginal aggregates(previously dried in an oven at 105C and then allowed to cool at a room temperature) was

prepared, with respect to the proportions reported in Table 4. After the proper percentage of

cement was added to the dried and cooled aggregates, each batch was pre-mixed with the specific

respective OMC, by means of a Hobart mixer with a spiral dough hook agitator, at a room

temperature, for 1 min. The foamed bitumen, produced using Wirtgen foaming equipment WLB

S10, was subsequently directly mixed with the pre-wet aggregates for 1 min, using the Hobart

mixer. The foamed materials were then immediately compacted using the Marshall hammer,

following the specifications in Standard EN 12697-34, with the exception of the number of blows

of the Marshall standard tamper utilised for the compaction of the bituminous mixtures, which

was chosen, as sometimes suggested, as 75 for each side of the specimen (Wirtgen GmbH, 2004),instead of the 50 given in the Standard; the value adopted is also in line with those referred to in

the principal investigations performed on the foamed bitumen mixtures (Kim & Lee, 2006, 2007;

Sunarjono, 2007). As outlined by some authors, the timing of adding and mixing components

and subsequent compaction should be similar in the laboratory and field procedures (Mallick &

Hendrix, 2004).

All compacted samples were allowed to cure for 1 day in the mould, at room temperature, before

being demoulded by means of an extrusion jack (He & Wong, 2008; Hodgkinson & Visser, 2004;

Sunarjono, Zoorob, & Thom, 2007). The specimens were subsequently aged for 3 days at 40C,

in a forced draft controlled temperature oven (He & Wong, 2007, 2008; Kim & Lee, 2006, 2007;

Sunarjono et al., 2007).

The water content necessary in order to optimise the above-mentioned foaming properties of

the bitumen, was determined by means of foaming tests under different conditions, using Wirtgen

GmbH (2004) foaming equipment WLB S10 and following the laboratory procedures suggested

in the Wirtgen Cold Recycling Manual. Since a variation of the foaming water content (FWC)

produces opposite effects on the ER and HL (an increase of the FWC causes an increase of the ER

accompanied by a reduction of the HL), the optimal value has to be a compromise. The Wirtgen

methodology is still widely used (He & Wong, 2007, 2008; Sunarjono, 2007), despite refinements

having been proposed based on the foam index concept (Jenkins et al., 2000), which put into play

the area subtended by the curve of decline of the ER over time. However, more recent research has

demonstrated some limits of the optimisation based on the foam index (He & Wong, 2005; Saleh,

2006; Sunarjono et al., 2007), in particular, for bitumens that present a substantially constantHL below a certain value of the FWC, rendering the identification of the optimal water content

almost impossible. It was therefore decided to maintain the original Wirtgen protocol, in which

the optimal value of foaming water is defined as the average value among those associated to the

lowest ER and HL.

2.4. Mix design testing methods

It was decided to test the ITS of a series of cylindrical specimens for each of the 36 mixtures

considered, in order to identify the optimal binder contents. After the specimens had been curedand subsequently conditioned at the required test temperature, in an environmental conditioning

cabinet, their ITS was determined at 25C, following the specifications in Standard EN 12697-23.


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The ITS was determined using the well-known equation:

ITS =2Pmax

dh, (2)

wherePmaxrepresents the rupture load [N] of the specimens under diametral compression, dandh are average values of the diameter [mm] and height [m] of the Marshall specimens, respectively.

In order to render the mix design procedure more robust and reliable, indirect tensile stiffness

modulus (ITSM) tests (EN 12697-26, Annex C) were also performed on Marshall specimens,

produced in the same way as those tested for ITS. The stiffness was evaluated at 25C, with a rise

time of 125 ms. According to Annex C of the European Standard, calculation of the Modulus is

based on the average of five load pulses, for each of which the Modulus is determined with the

equation:

ITSM =F( +0.27)

z h, (3)

where ITSM is the stiffness modulus [MPa], F represents the peak value of the applied verticalload [N], z the amplitude of the horizontal deformation obtained during the load cycle [mm], h the

mean thickness of the cylindrical specimen [mm] and the Poissons ratio [].

Moreover, the ITSM, as well as the ITS, were determined on both dry and soaked specimens,

in order to evaluate the moisture susceptibility of the foamed mixtures. The soaked specimens

were immersed in water at 25C for 24 hours before testing (He & Wong, 2007; Hodgkinson &

Visser, 2004; Kim & Lee, 2006; Mallick & Hendrix, 2004; Sunarjono, 2007).

The optimal FBC and the design cement content (CC) for each mixture were determined in

correspondence to the maximum ITS and maximum ITSM, at both dry and soaked conditions.

2.5. Performance test programme

The resistance to permanent deformation has been investigated by means of the RLAT with free

lateral expansion, following the specifications of BS DD 226 Standard. The British Standard

prescribes 1800 pulses of a 100 kPa stress, with loading and unloading times fixed at 1 s, at

a temperature of 30C. This temperature is acceptable in the present study, given the climatic

conditions in Northern Italy and the depth of the foundation layers (for which a higher temperature

would not be appropriate). With respect to the Standard protocol, which provides for determination

of the cumulative axial strain after 1800 loading cycles, the RLAT data were elaborated to compute

the creep rate (CR) and corresponding secant creep modulus (SCM) in the linear portion of the

creep curves. In fact, it is possible to identify an initial stretch in the cyclic creep curves at a

decreasing CR (Phase 1) and a successive stretch at constant CR (Phase 2). Phase 1 correspondsto the initial densification of the mixtures, and is notably influenced by the compaction method,

while Phase 2, definable between 600 and 1800 cycles, reflects the real tendency of the mixtures to

develop permanent deformations under repeated loads. This approach, also followed recently by

He and Wong (2007, 2008), therefore deducts the contribution given by the initial densification,

not exclusively correlated with the intrinsic properties of the mixtures, from the analysis on the

resistance to permanent deformations.

Therefore, the SCM, calculated as the ratio between the constant stress applied and the cumu-

lative axial strain of the specimen between 600 and 1800 cycles, and the CR, i.e. the ratio between

the cumulative axial strain in the above-defined interval of load applications and the width of that

interval, were both computed.The RLAT investigation was performed on both dry and soaked Marshall specimens in order

to further investigate the moisture sensitivity of the mixes and to validate the mix design phase.


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3. Results and discussion

3.1. Materials characterisation

By Italian Law, the recycled aggregates considered are non-hazardous, special non-toxic and

non-noxious refuse. They are solid material and odour-free. The EAF slags and bottom ash are

greyish in colour, whereas the foundry sand as well as the RAP appear basically black. The pH is12.6 for the bottom ash, 11.1 for the EAF slags and 8.6 for the foundry sand.

Table 5 summarises the toxicological characteristics of the EAF slag and foundry sand, in terms

of initial concentrations of heavy metals, measured with the ICP-AES methodology. The initial

concentration of heavy metals in the two metallurgical wastes differs, with that of the foundry

sand being higher in terms of copper and nickel. The EAF slag contains more zinc and chromium,

with the content of the latter being highest, but anyhow less than 0.4% of the total volume.

The results of Table 1 demonstrate that, for the marginal materials analysed, the release of

heavy metals by leaching is within the limits of the environmental regulations in force in Italy

(Legislative Decree no. 152/2006). Therefore, given that the constituents of the mixtures do not

present toxicological problems, it was considered unnecessary to proceed with leaching tests on

specimens with foamed bitumen and cement added.

The EAF slags, in terms of oxides (analysed with X-ray fluorescence), contain a prevalence of

FeO (30.4%) and CaO (27.7%), as well as SiO2 (17.5%), MgO (6.6%) and Al2O3 (4.8%). The

SiO2/CaO ratio characterises the EAF slags as substantially alkaline aggregates and therefore

suitable to guarantee the necessary adhesion with the weakly acid bitumen.

The marginal materials resulted as practically unaffected by the action of water, presenting non-

determinable Atterberg Limits (Liquid Limit and Plastic Limit according to CNR-UNI 10014/64),

and this, in relation to the values for passing through sieves of 2, 0.4, 0.075 mm (Table 2), has led

to the wastes being classified, based on the HRB AASHTO methodology, as A1 soils, which

can be used in road construction (foundry sand and bottom ash can be assimilated to an A1-b soil,

while steel slags and RAP to an A1-a soil).According to the data in Table 3, the steel slag, as well as the bottom ash and RAP, are

characterised by a cleaning level (equivalent in sand) above the minimum acceptance requisite

for materials to be used in foamed mixtures for road foundations, set at 30% in the ANAS

specification. Instead, for the foundry sand, the equivalent in sand is below the threshold (10%).

Table 5. Major heavy metal content of the EAF slagand the foundry sand.

Initial concentration (mg/kg)

Element EAF slags Foundry sand

Copper (Cu) 221 580Cadmium (Cd)


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Table 6. OMC and dry densities of the mixtures.

Propriety Mix 1 Mix 2 Mix 3 Mix 4

OMC (%) 6.00 5.50 6.50 6.00Dry density (g/cm3) 2.50 2.55 2.32 2.36

The cubical shape of the EAF slags grains, as well as that of the foundry sand particles and

RAP (Shape and Flakening Indexes characterised by low values), enable a strong interlocking

effect within the foamed mixtures.

The Los Angeles test displayed a good resistance of both fractions of EAF slags to abrasion and

friction, with a Los Angeles coefficient lower than the acceptance requisite of 30% in the ANAS

specification. A borderline value, with respect to the ANAS acceptance requisite, was recorded

for the RAP.

The bulk and dry density of both the bottom ash and foundry sand, as well as that of the RAP, are

decidedly lower than that of the steel slags. The volumetric properties of the grains (particle voids

and porosity) resulted in being quite similar for the metallurgical wastes; the data recorded for the

bottom ash were consistently different. It has to be noticed that the relatively high porosity of the

steel slags could require a high amount of binder for conventional hot bituminous mixtures, but

the difference is often really low (Pasetto & Baldo, 2010a, 2011, 2012). For the foamed bitumen

mixtures, since the binder does not cover the larger aggregate particles, an anomalous absorption

of bitumen by the steel slags grains is not expected.

Cold extraction (by centrifugation) of the bitumen of the RAP revealed a binder percentage of

4.9% (CNR 38/73) on the weight of the aggregate. The extracted bitumen showed a penetration

of 9 dmm at 25C (CNR 24/71) and a softening point, with the Ring and Ball Method, of 75C

(CNR 35/73); it is therefore a decidedly aged and particularly hard binder.The foamed bitumen used showed a penetration value of 82 dmm at 25C (CNR 24/71); in

relation to the optimal water content of 3% and a foaming temperature of 180 C, the ER and

the HL resulted in 18 and 87 s, respectively. These are much higher than the minimum values

usually indicated of 8 s for ER and 16 s for HL (Wirtgen GmbH, 2004). The high value of HL

was particularly advantageous in allowing an adequate amount of time for the mixing operations

with the aggregates.

The OMC was determined through the study of the compaction of the granular mixtures, with

the modified Proctor test, conducted according to the operative procedures reported in the EN

13286-2 Standard, which provided the results reported in Table 6. As expected, the granular

mixtures with higher steel slag and foundry sand contents, were characterised by the higher drydensities.

3.2. Grading and composition of the mixes

The grading curves reported in Table 2 show that both the finest fractions, namely the foundry sand

and the bottom ash, have basically a continuous grading. The steel slags are instead characterised

by a discontinuous and a continuous grading for the 20/60 and 0/20 mm fractions, respectively.

For the RAP, Table 2 reports the black curve, i.e. the grading curve of the RAP material, which

is substantially continuous and similar to that of the EAF 0/20. With the exception of the bottom

ash, all the materials have an extremely low percentage that passes through a sieve of 0.075 mm,being always less than 1%; it is therefore clearly impossible to achieve a grading assortment with

sufficient filler.


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Table 4 reports the composition of the foamed mixtures, while the corresponding grading curves

are presented in Figure 5. The EAF 20/60 mm fraction was fixed at a constant contribution (20%),

while different amounts of EAF 0/20 mm, RAP materials, bottom ash and foundry sand were

integrated in the skeleton. This was done to guarantee a minimum percentage of coarse aggregate

in all the mixes, constituted by the EAF 20/60, to then alternately combine the RAP and EAF 0/20

at a ratio of 3:1, as with the foundry sand and bottom ash, i.e. the pairs of materials characterisedby a relatively similar grading assortment. This was in order to verify the feasibility of using the

various materials in different proportions, in relation to their availability in the production factory,

as explicitly requested by the producer. A key point for a successful reuse of these materials in

the transport infrastructures field is represented by the possibility of guaranteeing, during the full

road construction period, the necessary amount of the mixture, even in the case of a relatively

unexpected reduced availability of some components. In this sense Mix 1 and Mix 2 present a

greater amount of EAF 0/20 than RAP, because slag availability is currently higher than RAP

one. Mix 3 and Mix 4 represent the alternatives, in view of a possible inversion in the trend, as

considered by the company in its production plan scenarios, where an increased RAP availability

balances a hypothetical lower quantity of EAF 0/20. Similar considerations have been assumedwith respect to the foundry sand and the bottom ash proportions in the mixes, the former currently

being more available than the latter.

With respect to ANAS envelope (Figure 5), the grading curves of the four mixes present a

critical low content of fines. This grading characteristic, even if not surprising in consequence

of the particular wastes analysed, cannot be ignored, since the need of a minimum fine content

(approximately 5%) is well known for an efficacious dispersion of the foamed bitumen in the mix

and therefore for adequate mechanical properties of the product. In similar cases, the integration

of natural filler could be hypothesised (Sunarjono, 2007), but in this research the aim was to verify

the possibility of using exclusively marginal materials in their original conditions, using cement

as active filler, up to a maximum of 3% on the weight of the aggregate, and studying the best mix

design of the binding phase later.

3.3. Optimisation of the mixtures

Tables 7 and 8 present the results of the ITS tests for the mixtures. For each combination of bitumen

and cement, the top line represents the ITS in dry conditions (dry ITS [MPa]) and the middle one

the resistance after immersion in water (soaked ITS [MPa]). Each ITS value is calculated as the

average of the values recorded on three specimens. The bottom line reports the tensile strength

ratio (TSR). The TSR is computed as the ratio between the ITS of the specimens treated in water

(soaked) and untreated (dry).

For all the mixtures the ITS had an increasing trend with the CC, for each of the percentages offoamed bitumen, in both dry and soaked conditions, confirming what has been widely reported

in the literature regarding the beneficial effect of the active filler on the increase of resistance in

foamed mixtures (Ebels & Jenkins, 2007b; Hodgkinson & Visser, 2004; Khweir, 2007; Wirtgen

GmbH, 2004; Zulakmal et al., 2009), due to the formation of pozzolanic bonds. Considering

instead each single percentage of active filler, for all four lithic skeletons, with the varying of the

FBC, there was a peak of resistance at 3% in dry conditions, while after the treatment in water the

ITS increased as the percentage of bituminous binder rose. The peaks of ITS in dry and soaked

conditions therefore do not coincide. In the analysis of mix design it is thus considered useful and

necessary to integrate the ITS data with those of ITSM, prior to formulating a final judgment.

With reference to the acceptance requisites of the Italian specifications, expressed only in termsof ITS in dry conditions, all the mixtures, with any combination of cement and foamed bitumen,

exceeded the minimum value of 0.32 MPa. Also the minimum requisites suggested by Bowering


16/23


Table 7. Indirect tensile test results: Mix 1 and Mix 2.

Mix 1 CC (%) Mix 2 CC (%)

FBC (%) 1 2 3 1 2 3

2 0.44 0.54 0.66 0.40 0.50 0.630.15 0.33 0.42 0.11 0.29 0.3934 61 64 28 58 62

3 0.60 0.60 0.73 0.53 0.56 0.690.22 0.38 0.47 0.18 0.32 0.4437 64 65 34 57 64

4 0.47 0.55 0.60 0.44 0.51 0.590.25 0.41 0.49 0.21 0.35 0.4154 75 82 48 69 70

Note: For each FBC, dry ITS (MPa), soaked ITS (MPa) and TSR (%)are respectively quoted.

Table 8. Indirect tensile test results: Mix 3 and Mix 4.


FBC (%) 1 2 3 1 2 3

2 0.39 0.45 0.51 0.37 0.43 0.440.1 0.24 0.31 0.08 0.2 0.2426 53 61 22 47 55

3 0.47 0.54 0.55 0.44 0.51 0.520.15 0.29 0.34 0.11 0.26 0.3132 54 62 25 51 60

4 0.36 0.5 0.52 0.36 0.41 0.480.17 0.33 0.35 0.13 0.27 0.3247 66 68 37 65 67

Note: For each FBC, dry ITS (MPa), soaked ITS (MPa) and TSR (%)are respectively quoted.

and Martin (1976), of 0.2 MPa and 0.1 MPa, in dry and soaked conditions, respectively, were

amply met. Vice versa, not all the mixtures reach the Bowering and Martin requisite minimum

TSR value of 50%. Mixes 1, 2 and 3 meet this requisite with 2% of cement, independently of

the bitumen content, while Mix 4 needs 3% of cement. With 4% of bitumen, Mix 1 satisfies the

minimum requisite of TSR with just 1% of cement and, for the higher percentages of active fillerand bituminous binder, it reaches 82% of ITS.

Comparing the mixtures with the same percentages of bitumen and cement, a reduction in

resistance with the increase in the percentage of RAP can be noted. Mix 1, with 50% of EAF slags

and 30% of foundry sand, gave the highest values of ITS, in both dry and soaked conditions, as

well as the highest value of TSR.

The EAF slags have high surface roughness, which favours a firm adhesion of the bitumen

on the grains (Pasetto & Baldo, 2010a, 2011, 2012), while the bottom ash, in comparison to the

foundry sand, as well as having almost double the equivalent in sand, has a grading assortment

with more filler, which favours more homogeneous dispersion of the bubbles of foamed bitumen

in the mixing phase, benefitting the cohesion of the mix.For all the mixes investigated, there were no problems, during the making, compaction or

seasoning of the specimens, caused by uneven expansion or phenomena of disgregation, ascribable


17/23


Table 9. Stiffness modulus test results: Mix 1 and Mix 2.


FBC (%) 1 2 3 1 2 3

2 3804 4066 4833 3676 3793 43672548 3050 3915 2242 2542 3362

67 75 81 61 67 773 4735 5172 5217 4483 4908 5056

3362 4086 4538 2914 3533 424771 79 87 65 72 84

4 4603 5070 5196 4479 4542 50103544 4259 4728 3225 3633 4409

77 84 91 72 80 88

Note: For each FBC, dry ITSM (MPa), soaked ITSM (MPa) and SMR(%) are respectively quoted.

Table 10. Stiffness modulus test results: Mix 3 and Mix 4.


FBC (%) 1 2 3 1 2 3

2 3053 3454 3544 2923 3443 35401679 2176 2587 1491 2066 2478

55 63 73 51 60 703 3599 3843 3973 3297 3548 3783

2123 2613 3258 1813 2235 291359 68 82 55 63 77

4 3332 3354 3720 3095 3348 33822199 2482 3162 1857 2343 2706

66 74 85 60 70 80

Note: For each FBC, dry ITSM (MPa), soaked ITSM (MPa) and SMR(%) are respectively quoted.

to the presence of free lime potentially subject to hydration or carbonation in the EAF slags

particles, as has instead been reported (Pasetto & Baldo, 2010a, 2011, 2012) for some types of

steel slags not or insufficiently seasoned.

Tables 9 and 10 report the results of the ITSM tests conducted on all the mixes analysed in the

phase of mix design. For each combination of bitumen and cement, the top line represents thestiffness in dry conditions (dry ITSM [MPa]) and the middle one the modulus after immersion

in water (soaked ITSM [MPa]). Each modulus value represents the average calculated on the

values of three different specimens. The bottom line reports the stiffness modulus ratio (SMR

[%]). Similarly to the TSR, the SMR is evaluated as the ratio between the Stiffness Modulus of

the specimens treated in water (soaked) and untreated (dry).

The stiffness recorded in dry conditions ranges between 2900 and 5200 MPa, in relation to the

type of lithic skeleton and combination of bitumen and cement. The moduli are therefore of great

significance, especially given the particularity of the studied mixtures, and comparable with those

of hot mix asphalts investigated by the authors in the same working conditions (Pasetto & Baldo,

2010a, 2011, 2012).There is also an analogy with the behaviour observed in the ITS test, with the varying of the

percentages of cement and foamed bitumen. Higher CCs always correspond to higher modulus


18/23


Figure 6. ITSMvs. ITS (at 25C).

values in both dry and soaked conditions. Vice versa, with the same active filler content, the

mixes show greater stiffness at 3% of bituminous binder, but only in dry conditions, because after

immersion in water, a higher percentage of foamed bitumen leads to a further increase in the

modulus values.

As mentioned in the introduction, in foamed mixtures the bituminous binder only partially

covers the large grains, while it closely adheres to the fine grains; the cohesion of the mix is

guaranteed by the spot welding of the bitumen-filler mastic, which provides the necessary inter-

granular adhesion. Therefore, in dry conditions, a lower bitumen content (2%) produces a mastic

with too little bituminous binder to guarantee a sufficient inter-granular adhesion, which leads to

a lower stiffness of the mixtures. The increase of bitumen, due to its lubricant effect, favours the

reciprocal convergence of the grains and a better densification condition to be reached, from which

a better interlocking effect derives. In addition, the higher bitumen content, increasing the welding

spots available in the mix, leads to a further increase of the inter-granular adhesion. The double

effect translates into an increase in stiffness of the mixtures, with a peak at 3% (of binder) in dry

conditions. A further increase of bitumen to 4%, if on the one hand it increases the inter-granular

adhesion, on the other it produces a more deformable mastic. The results of the ITSM tests leadto the conclusion that in dry conditions, at 4% of bitumen, the deformability of the bituminous

mastic has a greater effect than the inter-granular adhesion, with a consequent reduction in stiffness

of the mixtures. In soaked conditions, vice versa, the inter-granular adhesion has a predominant

role and therefore a progressive increase of bitumen translates into a corresponding increase in

stiffness of the mixtures, which reaches the maximum (modulus) value at 4% of bitumen.

The study of the stiffness thus confirms the ITS tests; the correlation between the ITSM and

ITS data in dry conditions, represented in Figure 6, is reasonably satisfactory. The ratio between

the two parameters was expressed with a power function, characterised by an appreciable value

of the coefficient of determination R2 (0.66). The outlined correlation is very important in order to

develop a reliable relationship on the fundamental and engineering properties of foamed bitumenmixtures (Sunarjono, 2007).

In comparative terms, a higher concentration of EAF slags and bottom ash is always preferable.


19/23


Table 11. CR results.

Mix 1 CC (%) Mix 2 CC (%) Mix 3 CC (%) Mix 4 CC (%)

FBC (%) 2 3 2 3 2 3 2 3

3 0.2017 0.1525 0.2125 0.2058 0.3208 0.2592 0.3650 0.32000.2658 0.1692 0.2975 0.2483 0.4817 0.3650 0.6317 0.47331.3182 1.1093 1.4000 1.2065 1.5013 1.4084 1.7306 1.4792

4 0.3450 0.2983 0.5258 0.4192 0.6508 0.5158 0.6742 0.62080.5217 0.3942 0.8475 0.5775 1.1267 0.7933 1.2467 1.01831.5121 1.3212 1.6117 1.3777 1.7311 1.5380 1.8492 1.6403

Note: For each FBC, dry CR (microstrain/cycle), soaked CR (microstrain/cycle) and retained CR (%) arerespectively quoted.

Table 12. SCM results.

Mix 1 CC (%) Mix 2 CC (%) Mix 3 CC (%) Mix 4 CC (%)FBC (%) 2 3 2 3 2 3 2 3

3 413 546 392 405 260 322 228 260313 493 280 336 173 228 132 176

76 90 71 83 67 71 58 684 242 279 158 199 128 162 124 132

160 211 98 144 74 105 67 8266 76 62 73 58 65 54 62

Note: For each FBC, dry SCM (MPa), soaked SCM (MPa) and retained SCM (%) are respectivelyquoted.

The results from the mix design allow all four lithic skeletons to be considered favourably, and

the contents of 2% and 3% for cement and 3% and 4% for foamed bitumen to be identified as

preferable. Nevertheless, the definition of a precise percentage of bitumen and cement for each of

the four mixtures requires further study, based on the resistance to the accumulation of permanent

deformations.

In terms of SMR, even if in the literature there is as yet no well-established reference threshold,

it is possible to confirm the best performance of the mixes with 3% of cement and 4% of foamed

bitumen and, in absolute terms, the superiority of Mix 1. It can finally be observed that the SMR

always assumes higher values than the corresponding values of TSR. The greater severity of the

ITS test, already observed for hot mix asphalts (Pasetto & Baldo, 2010a, 2011, 2012), wouldtherefore seem to be confirmed also for foamed mixtures in the study of moisture sensitivity.

3.4. Permanent deformation test

Tables 11 and 12 report, in terms of CR and SCM, the results of the RLATs conducted on the

mixtures identified as potentially optimal in the phase of mix design, i.e. on a reduced number

of cement-foamed bitumen combinations. For each combination of bitumen and cement, the top

line represents the CR [microstrain/cycle] and the SCM [MPa] in dry conditions and the middle

line the CR and SCM after immersion in water. Each value of the RLATs is calculated as the

average of the values obtained with three specimens. The bottom line reports the retained CR(Table 11) and retained SCM (Table 12). Both the parameters are computed as the ratio between

the CR/SCM of the specimens treated in water (soaked) and untreated (dry).


20/23


Figure 7. Retained SCMvs. retained CR (at 30C).

The results, in terms of both CR and SCM, in dry conditions are congruent with those recorded in

the ITS and stiffness modulus tests. A higher percentage of cement always limits the deformability

of the mixtures and translates into a reduction of the CR and an increase of the SCM. With the same

hydraulic binder content, a diminution of stiffness is recorded, already observed in the ITSM tests,

which is indicated by a net reduction of the SCM passing from 3% to 4% of bitumen; in parallel,

the CR increases markedly. The results from the CR and SCM appear therefore congruent and

represent a useful tool for the interpretation of the deformation behaviour of the foamed mixtures.

Figure 7 shows the excellent correlation between the data of retained SCM and retained CR,

computed as the ratio between the values of SCM and CR respectively after and before the immer-

sion of the specimens in water. The interpolation of the data, expressed with a power function,

gives a high value of the coefficient of determination R2 (approximately 1) and demonstrates the

full congruence between SCM and CR data. From the experimental results, it can be deduced that

CR decreases with an increase of SCM. Therefore if the SCM of a foamed bitumen mixture is

being improved, its susceptibility to permanent deformation will be consequently reduced.

Regarding the moisture sensitivity of the mixtures, the results from the RLATs in soaked

conditions differ from those observed in the ITS tests after treatment in water. The accumulationof permanent deformations in soaked conditions is higher with 4% of bitumen, although the

welding spots are more numerous, because the deformation behaviour in the RLATs is different

from the resistance mechanism in the ITS tests. In the ITS, the greater interlocking leads to a

higher rupture load, while in the RLAT a mastic with more bitumen favours viscous creep; this

effect, amplified by the treatment in water, appears with a higher accumulation of permanent

deformations.

The RLATs provide information on the mechanical behaviour of the foamed mixtures that is

fundamental for a correct mix design. The study of permanent deformations, in both dry and

soaked conditions, is therefore more than a mere performance check of the mixtures optimised

with ITS and ITSM tests, and should be considered an integral part of the mix design process. Inthis study, for example, if on the one hand the RLATs confirm 3% of cement as preferable, on the

other they suggest 3% as optimal bitumen percentage, not 4% as evinced from the ITS and ITSM


21/23


tests. Indeed, for 3% of cement and bitumen, the TSR ranges between 60% and 65%, in relation

to the type of lithic skeleton, and is therefore above the minimum threshold of 50% given as the

requisite of ITS by Bowering and Martin.

As regards the lithic composition of the mixtures, the RLATs support the results of the ITS

and ITSM tests, evidencing a greater resistance to permanent deformations of the mixtures

with more EAF slags (Mix 1 and Mix 2). Both the high density and mechanical strength, aswell as the polyhedron shape, high angularity and rough texture of the steel slags, are funda-

mental characteristics in order to ensure resistance to permanent deformations under repeated

loading.

4. Conclusions

The marginal materials studied, namely EAF steel slags, foundry sand, bottom ash and RAP,

possess physical and mechanical properties that are comparable to the characteristics of natural

aggregate usually used in transportation infrastructure, complete chemical suitability with the

bitumen used for the foamed mixtures and full environmental compatibility.

The mix design procedure, based on ITS and stiffness modulus tests, completed by RLATs,

has identified mixtures made exclusively with marginal materials, in variable proportions, thus

being interesting in terms of both the environment and economics, which satisfy the requisites

for acceptance in the road technical standards for cold mixtures; the best-suited binder contents

resulted as being equal to 3%, for both cement and foamed bitumen.

The experimental results are satisfactory, especially in the case of foamed bitumen mixtures

with the higher EAF slag content (50%), with good values of dry ITS, up to 0.73 MPa (mixture

integrated with 30% of bottom ash and 10% of RAP) and 0.69 MPa (mixture combined with 30%

of foundry sand and 10% of RAP), depending on the binder dosages.

The foamed mixtures made with EAF slags, foundry sand, bottom ash and RAP, are charac-terised by low moisture damage in terms of tensile strength and SMR, as well as retained CR and

SCM, so demonstrating a satisfactory durability.

After a comparison between foundry sand and bottom ash, we can conclude that the latter

gives higher strength, stiffness and resistance to permanent deformation to the foamed bitumen

mixtures, irrespective of the binder contents.

In the investigation on the permanent deformation resistance of the foamed mixtures, the CR

and the SCM resulted as consistent parameters, which provide a reliable analysis of the creep

behavior of the mixes under repeated loading.

In conclusion, for the mix design and testing procedure, according to the results obtained in the

present research, it is strongly recommended to complete the classic ITS test-based mix design,with subsequent RLATs, in order to identify the optimal binder percentages. The stiffness modulus,

even if useful in order to confirm the ITS results, does not allow the optimisation process to be

properly concluded, without the RLAT investigation.

Finally, we remark, on the basis of experimental considerations, that the missing fulfilment

of conventional grading requirements does not necessarily imply the impossibility of designing

mixtures satisfying the acceptance requisites related to mechanical behaviour and to durability

performance.

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Documents

Laboratory Investigation on Foamed Bitumen Bound Mixtures Made With Steel Slag, Foundry Sand