Foamed Asphalt Mix

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FOAMED ASPHALT MIX

PRESENTED BY

ARINDAM DEY & PRADIPTA KUNDU


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HISTORY OF FOAMED ASPHALT

In 1889, Nebraska, USA, in full depth repairs, bitumen was added to basecourse materials to improve the bearing capacity

In 1928, August Jacobi from Darmstadt, Germany, produced andpatented the first hot bitumen foaming system

In 1957, Prof. Ladis Casanyi of Iowa State University, demonstrated theaddition of foamed bitumen to marginal quality aggregates

In 1971, Mobil Oil Corporation patented their foaming system in Australia

In 1991, new foaming systems were developed worldwide

In 1994, Scandinavian system, Nesotec OY was developed by NestorSalminen; followed by Salvaco Sweden and other Home-made systems

Recent researches are going on in the countries of South Africa,Australia, Canada, Mexico, Europe, Middle East and the Scandinavian

countries


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WHAT IS FOAMED ASPHALT MIX

Foamed asphalt mix refers to a mixtureof pavement construction aggregatesand foamed bitumen

WHAT IS FOAMED BITUMEN

Produced by a process in which water(typically 2 %) is injected into the hotbitumen, resulting in spontaneousfoaming and temporary alteration ofphysical properties of bitumen

Water, on contact with hot bi tumen isturned into vapour, which is trapped inthousands of tiny bitumen bubbles

Foam dissipates in a very short time inless than a minute and the originalproperties of bi tumen are regained

Incorporating foamed bitumen into theaggregates produces foamed asphalt mix


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TECHNIQUES OF FOAMED BITUMEN PRODUCTION

Steam foaming system

Process of injecting steam intohot bitumen

Convenient for asphalt plantswhere steam is readily

available Impractical for in-situ foaming

operations, as it requires

special equipments as steamboilers etc.

Cold foaming system

Patented by Mobil OilCorporation, Australia

Addition of cold water into hot

bitumen Practical and economical


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HOW FOAMING OF BITUMEN TAKES PLACE

The moment a cold water droplet (atambient temperature) makes contact withthe following chain of events occur:

The bitumen exchanges energy with thesurface of the water droplet, heating it toa temp. of 100C and cooling the bitumen

This transferred exceeds the latentenergy of steam resulting in explosiveexpansion and generation of steam, inthe expansion chamber

Encapsulated steam from the nozzleexpands until a thin film of bitumen

holds it intact through surface tensionSurface tension of bi tumen fi lmcounteracts the everdiminishing steampressure, until a stable equilibrium isreached

Due to low thermal conductivity of

bitumen, bubble remain stable over ameasurable time


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APPROPRIATE SITUATIONS OF USING FOAMED ASPHALT

Situations that should trigger the consideration of the use of foamed bitumentechnology include the following ::

A pavement has been repeatedly patched to the extent that pavement repairsare no longer cost effective

A weak granular base overlies a reasonably strong subgradeGranular base too thin to consider using cementit ious binders

Conventional reseals or thin asphalt overlays can no longer correct flushingproblems

An alternative to full depth asphalt in moderate to high traffic roadsThe unfavourable wet cyclic conditions unsuitable for granular construction

Situations where an overlay is not possible due to site constraints e.g..entries to adjacent properties and flood prone areas

A requirement to complete the rehabilitation quickly to prevent disruption tobusiness or residents


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OBJECTIVE OF FOAMED ASPHALT MIX DESIGN

Select mix proportions in order to achieve

Optimum values for laboratory measured properties

Structural and functional requirements of the in-service mix

Retention of the relevant engineering properties at in-service

conditions of temperature, moisture and loading conditions

DESIGN CONSIDERATIONS

BITUMEN PROPERTIES

AGGREGATE PROPERTIES

MOISTURE CONDITIONS

CURING CONDITIONS

TEMPARATURE CONDITIONS ENGINEERING PROPERTIES


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BITUMEN PROPERTIES

Foamed bitumen is

characterised by ::

Expansion ratio (ER) Ratiobetween the maximum volume

achieved in the foam state andthe final volume of the binderonce the foam has dissipated

Half life (1/2) Time, in seconds,between the moment the foamachieves maximum volume andthe time it dissipates to half ofthe maximum volume

DECAY CURVE OF FOAMED BITUMEN


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FOAMED BITUMEN DECAY

DECAY CURVE

Decay curve defines the rate at which a foam collapsesIt gives an indication of the time available for mixing

Area under the decay curve gives the FOAM INDEX of the particularfoamed bitumen

FACTORS INFLUENCING DECAYReduction in the temperature of the steam due to contact of the bitumenfilms with ambient air (and vessel) at lower temperature

This occurs with the bubbles at the frontier of the colloid mass

Larger bubbles experience higher rate of reduction of temperaturedue to greater exposed surface area, and thus collapses earlier

Exceedance of the elongation limit of the bitumen film

Polydiverse colloidal mass consisting of bitumen bubbles of various

sizes


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FOAMING POTENTIAL

An important consideration during the mixing stage of foamed asphalt production

Maximised expansion ratios and half lives promote binder dispersion within the mix

Bitumen, irrespective of grade or origin, can be foamed with an appropriate combination ofnozzle type, water, air and bitumen injection pressure (Castedo Franco & Wood, 1983)

According to Abel (1978)

Bitumen with silicones have reduced foaming abilities

Bitumen with lower viscosity foams more readily and has higher foam ratios and halflives than bitumen with higher viscosi ty

Anti-stripping agents intensifies the foaming ability of bitumen

Above 149C, acceptable foaming is achieved

Half-li fe and expansion ratio of the foam produced is affected by the volume of the foam,

quantity of water used and temperature at which the foam is produced (Brennen et al, 1983)Cohesion and compressive strength are significantly greater for high expansion (15:1)foamed bitumen (Bowering & Martin, 1976)

Certain surface active additives produce highly expanded and stable foamed bitumen withER>15 and 1/2>60 sec and resulted in improved aggregate coating (Maccarrone, 1994)


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GRADE OF BITUMEN

No appreciable differences between themeasured properties of foamed asphalt mixes

with different grades of bitumen (Lee, 1981)

Load-rate and temperature dependent behaviourare indicative of visco-elastic binder activity

Needs further investigation


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FOAMED BITUMEN CONTENT

Optimum bitumen cannot beclearly determined

Upper range of binder

content is governed by theloss in stability of the mixand lower range by the watersusceptibility

Mix stabili ty is governed bythe (binder content):(finescontent) i.e. the viscosity ofthe binder-fines mortar


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AGGREGATE PROPERTY

Wide range of aggregates may beused with foamed bitumen

Certain soils may require lime-treatment and grading adjustments

Fines content of the aggregateshould be above 5% (Ruckel et al,1982)

Resultant f iller (mix of bi tumen andfines) acts as a mortar between thecoarse aggregate and increase thestrength of the mix

Excess bitumen tends to act as alubricant, resulting in loss ofstrength and stability


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MOISTURE CONDITION

Moisture softens and breakdowns agglomerations in the aggregates to

aid bitumen dispersion

Insufficient water reduces the workability of the mix & results ininadequate dispersion of the binder

Excess water lengthens the curing time, reduces the strength anddensity of the compacted mix

OMC depends on the optimisation of the mix properties (strength,density, water absorption & swelling)

OMC lies at the fluff point of the aggregate i.e., the MC at which theaggregates have maximum loose bulk volume (Mobil Oil, Australia)

Best compactive moisture condition occurs when the total fluidcontent (moisture + bitumen) OMC (Castedo Franco & Wood, 1983)

Higher the bitumen content, lower the compaction moisture content


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CURING CONDITIONS

Curing is the process whereby the foamed asphalt gradually gainstrength over time accompanied by a reduction in MC

MC during curing period affects the ultimate strength of the mix

(Ruckel et al, 1982)Curing of foamed asphalt mixes in the field occurs over severalmonths, hence an accelerated laboratory curing procedure is requiredto correlate the field behaviour

Lab tests required 3-days oven curing at 60C, resulting in moisturecontent stabilisation at about (0-4)%, which represents the driest stateachievable in the field

Represents the in-service state approximately a year after

construction


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TEMPERATURE CONDITIONS

Optimum mixing temperature of the aggregates lies in the range of (13-23)C, depending on thetype of aggregate

Temperature below this range result in poor quality mixes (Bowering & Martin, 1976)

ENGINEERING PROPERTIES

Most common method to select the design binder content is to optimise the Marshall stabil ity &minimise the loss in stability under soaked moisture condition

The different engineering properties which affect the foamed asphalt mix design are ::

MOISTURE SUSCEPTIBILITY TEMPERATURE SUSCEPTIBLITY UNCONFINED COMPRESSIVE STRENGTH

TENSILE STRENGTH

STIFFNESS RESILIENT MODULUS

ABRASION RESISTANCE

DENSITY AND VOLUMETRICS FATIGUE RESISTANCE


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MOISTURE SUSCEPTIBILITY

Strength characteristics of foamed asphalt are highly moisturedependant, due to relatively low binder and high cement contents

Additives such as lime, cement etc reduce the moisture susceptibil ity ofthe mix (Castedo Franco et al, 1983)

Higher bitumen content reduce moisture susceptibility as higherdensities are achievable, leading to lower permeabil ities, lower void

contents, and increased coating of the moisture sensitive fines withbinder

TEMPERATURE SUSCEPTIBILITY

Both the tensile strength and modulus decrease with the temperature

Coarse aggregates are not affected by higher temperatures

Stability and viscosity of the bitumen fines decrease with increasingtemperatures, resulting in loss of strength


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UCS & TENSILE STRENGTH

Mixes used as a base course under thin surface treatments/seals have UCS criteria as 0.5MPa (4day soaked) & 0.7MPa (3 day cured at 60C) (Bowering, 1970)

UCS of foamed asphalt lies between (1.8 - 5.4) MPa and estimated tensile strength lie between (0.2 0.55) Mpa (Bowering & Mart in, 1976)

Cured foamed asphalt samples should have minimum ITS 100 kPa & 200 kPa for soaked and dryconditions resp. (Macarrone, 1998)

STIFFNESS RESILIENT MODULUSDepends on loading rate, stress level and temperature

Stiffness increase with increase in fines content

Can have high stiffness with added advantage of f lexibi lity & fatigue resistance (Fernando &Ramanujam, 1997)

ABRASION RESISTANCELack resistance to abrasion and ravelling

Not suitable for friction / wearing courses applications


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DENSITY AND VOLUMETRICS

Density increases and void ratio decreases with the gradual increase ofbinder content

Strength of the mix depends largely on the density of the compacted mix

FATIGUE RESISTANCE

Governs structural capacity of foamed asphalt pavement layersMechanical characteristics of foamed bitumen fall between a cementedand a granular structure

Controversy exist over the fatigue properties of foamed asphalt

Fatigue property will be inferior to to that HMA (Bissada, 1987)

Fatigue property will be superior to that of HMA (Little et al, 1983)

Fatigue property is similar to those of HMA (Macarrone et al, 1993)

Needs further investigation


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MIX DESIGN PROCEDURE

GENERAL

NO STANDARDISED MIX DESIGN PROCEDURE IS AVAILABLE

The most common mix-design method is based on Marshall stabili ties

and densitiesMarshall stabili ty of foamed asphalt mixes tends to increase to amaximum as the binder content is increased

Optimum Binder content (OBC) is determined when the ratio between

wet and dry stabilities is at a maximum i.e., bitumen content at whichthe mix retains most of its strength when soaked

OBC is selected based on highest resilient modulus value, obtainedfrom Dynamic Creep Test & Indirect Tensile Test (Lancaster et al,

1994; Lewis, 1998)


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BASIC STEPS IN MIX DESIGN

The basic steps in foamed asphalt mix design are

Binder characterisation and preparation

Aggregate characterisation and preparation

Binder content for trial mixes Moisture content

Mixing and compaction

Curing, testing and design binder content determination


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BINDER CHARACTERISATION AND PREPARATION

Foaming characteristics of bitumen

needs to be optimised for producingfoamed asphalt mixes

Optimisation can be achieved withtrial tests (generally 5) by measuringthe 1/2 & ER, using various

percentages of waterTemperature of the bitumen beforefoaming should be in range of (180-200)C

By recommendation, for foamed

bitumen 1/2 > 12 sec

ER > 10:1

Addit ives may be used to catalysethe foaming, but has a significantcost implication


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AGGREGATE CHARACTERISATION AND PREPARATION

Aggregates are characterised by thegrading and the PI

Grading is adjusted, if required, byadding fine or coarse materials sothat the conforms to the standard

grading envelope

Materials with PI>12 should betreated with lime to reduce the PI

Addit ion of 1-2% of cement to the mix

aids bitumen adhesion (Lewis, 1998)The oven dried aggregate are riffledinto 5 batches of 10kg each


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BINDER CONTENT FOR TRIAL MIXES

Appropriate range of foamedbitumen contents is selectedfor the trial mixes using the

Table (Ruckel et al , 1982)Five batches of trial mixes areprepared at binder contentsdiffering by 1%


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MOISTURE CONTENT

A crucial mix design parameter

Recommended MC for mixing and compaction is the greater of (OMC BC) &the fluff point of the aggregate

MIXING AND COMPACTION

Each 10 kg sample and reqd mass of foamed bitumen are mixed in themechanical mixer at prescribed MC

Foamed asphalt is stored in sealed containers to prevent moisture loss

Duplicate samples from each batch are tested for determination of MC & BC

From each batch, 6 samples are prepared forIndirect Tensile Test & 2 for Volumetric Evaluation

Specimens are prepared for standard Marshall Test


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CURING,TESTING AND DESIGNBINDER CONTENT DETERMINETION

Samples are subjected for accelerated curing procedure

Indirect Tensile Strength Test is conducted to determine the ultimatestrengths of both dry and soaked samples

Recommended values of ITS for dry and soaked samples are greater

than 200kPa & 100 kPa resp. (Macarrone, 1997)Design BC is selected at maximum soaked ITS

For resilient modulus testing, loading time of 50ms (25C) isrecommended, the acceptance criteria being at least 1500MPa &

6000MPa for soaked and dry samples resp. (Lancaster et al, 1994)Dynamic creep testing evaluates the permanent deformationcharacteristics of the mix, with a minimum dynamic creep modulus of20MPa


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ADVANTAGES OF FOAMED ASPHALT MIX

Foamed binder increases strength and reduces moisture susceptibilityof granular materials

Flexible and fatigue resistant

Foam treatment can be used with wider range of aggregates

Reduces binder and transportation costs

Time savingConserves energy

No environmental side effects

No risk of binder runoff or leaching from stockpilesCan be constructed even in some adverse weather conditions

Easy to apply

Rapid strength gain


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DISADVANTAGES OF FOAMED ASPHALT MIX

Cost Relatively more expensive as

compared to other forms ofstabilisation

Sealing Work Seal design requiresspecial attention due to the pertinentstripping problems

Bitumen Temperature The processrequires hot bitumen (180C) for thesuccessful foaming action, thusinitiating a risk of burning

Grading Very sensitive to the

grading of the host material,requires imported material to mixwith the exist ing material to achievethe grading requirement

Purpose built foamed bitumen

stabilising equipment is required

RELATIVE COST OF STABILISTAION(KENDALL ET AL, 2000)

TREATMENT COST ($/m2)

2-3% LIME / FLYASH(200 mm)

$6 - $9

BITUMEN 2%EMULSION / CEMENT

2% (200mm)

$12 - $14

Ad BASE 4 / CEMENT(175 mm)

$12 - $14

FOAMED BITUMEN

(250 mm)

$13 - $15


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FOAMED BITUMEN TESTING MACHINE

Laboratory unit for

producing foamedbitumen, capable ofvarying differentparameters such as forexample the bitumentemperature, water contentor air pressure during theinjection process. Seriesof measurementsdesigned to determine thefoamed bitumen propertiescan be easily carried outwith the aid of this system.


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CONCLUSION

Foamed asphalt mixes is gaining popularity owing to their good

performance, ease of construction and compatibili ty with a wide range ofaggregate types

Mix design can be accomplished by simple test procedures and byadhering to certain restrictions with respect to the materials used

The mix design is carried out to optimize the mix strength characteristicsat the worst-case operating environment i.e., under soaked condition

Simple tests such as ITS & Marshall Test can be conveniently used todetermine the optimum binder content

Other tests such as Resilient Modulus, Dynamic Creep & Mix Volumetricscan be conducted to ensure the adequate performance of the selectedoptimum mix

More development and research is still required


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Foamed Asphalt Mix