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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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THANK YOU