Laboratory Test Methods for Foamed Asphalt Mix Resilient Modulus

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Laboratory Test Methods for Foamed Asphalt MixResilient ModulusPengcheng Fu a , David Jones a , John T. Harvey a & Syed A. Bukhari ba University of California Pavement Research Center, Department of Civil andEnvironmental Engineering University of California, Davis, One Shields Ave., Davis, CA,95616, USA E-mail:b University of California Pavement Research Center Institute of Transportation StudiesUniversity of California, Berkeley, 1353 S. 46th St, Bldg. 452, Richmond, CA, 94804, USAE-mail:Version of record first published: 19 Sep 2011.

To cite this article: Pengcheng Fu , David Jones , John T. Harvey & Syed A. Bukhari (2009): Laboratory Test Methods forFoamed Asphalt Mix Resilient Modulus, Road Materials and Pavement Design, 10:1, 188-212

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Road Materials and Pavement Design. Volume 10 – No. 1/2009, pages 187 to 212

Laboratory Test Methods for FoamedAsphalt Mix Resilient Modulus

Pengcheng Fu* — David Jones* — John T. Harvey*Syed A. Bukhari**

* University of California Pavement Research CenterDepartment of Civil and Environmental EngineeringUniversity of California, DavisOne Shields Ave.Davis, CA 95616, USA{pfu; djjones; jtharvey} @ucdavis.edu

** University of California Pavement Research CenterInstitute of Transportation StudiesUniversity of California, Berkeley1353 S. 46th St, Bldg. 452Richmond, CA 94804, [email protected]

ABSTRACT. This paper investigates laboratory test methods for resilient modulus of foamedasphalt mixes. By comparing test results from different laboratory test methods, the effects ofvarious stress states are identified. The indirect tensile resilient modulus test and the free-freeresonant column test were found to yield stress states irrelevant to pavement structures, andto greatly overestimate resilient modulus. Triaxial resilient modulus test results showed thatthe role of foamed asphalt treatment is to transform the material behavior from that of typicalunbound granular materials to that of partially asphalt-bound materials. Flexural beam testsindicated that when subjected to tension, the resilient modulus of soaked foamed asphaltmixes can be very low. Since triaxial and flexural beam tests each characterize one of the twomost important stress states relevant to pavement design, test results from these two methodsneed to be combined to be used in design.KEYWORDS: Foamed Asphalt, Resilient Modulus, Mechanistic-Empirical Design, LaboratoryTest.

DOI:10.3166/RMPD.10.187-212 © 2009 Lavoisier, Paris

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188 Road Materials and Pavement Design. Volume 10 – No. 1/2009

1. Introduction and literature review

1.1. Resilient modulus of foamed asphalt mixes

Full Depth Reclamation (FDR) of cracked asphalt pavements with foamedasphalt is a promising flexible pavement rehabilitation technique. In the past twodecades, successful application has been reported in numerous countries (e.g.Mohammad et al., 2003; Saleh, 2004; Loizos, 2007; Ramanujam and Jones, 2007).The prevailing mix design methods as well as structural design methods are largelyempirical (e.g. Muthen, 1998; the South Africa TG2, 2002). In recent years,considerable effort has been devoted to achieving better knowledge of the behaviorof this material and to developing mechanistic-empirical (M-E) design methods (e.g.Long and Theyse, 2004; Twagira et al., 2006; He and Wong, 2007). Regardless ofthe specific formats of the M-E design schemes that have been proposed or will beestablished in the future, resilient modulus is an important input parameter forcalculating pavement structural responses.

Resilient modulus characterizes the resistance of pavement materials to resilientdeformation under applied loads. It is defined as the ratio of the amplitude of theapplied stress to the amplitude of the resultant recoverable strain. Although thedefinition prefers measuring “recoverable” deformation under cyclic loading, theinitial elastic modulus measured in monotonically loaded tests is also often used asresilient modulus. In a typical FDR pavement structure, resilient modulus of thefoamed asphalt layer greatly influences bending deformation and fatigue life of thesurface asphalt concrete layer, and also influences how well the foamed asphalt baselayer can distribute the traffic load to reduce stresses in the underlying layers.

1.2. Microscopic structure and general material behavior

Foamed asphalt mixes have characteristics different from those of Hot MixAsphalt (HMA) and granular base materials, for which extensive research has beencarried out and considerable engineering experience has been accumulated over theyears. It is believed that the microscopic structure of a foamed asphalt mix featurespartially coated large aggregates that are ‘spot welded’ with fines mortar (Jenkins,2000). Fine aggregate particles can only be partially coated by asphalt binder duringcold foaming to form the asphalt mastic phase, while a considerable portion of thevoids in the aggregate skeleton are filled by fine mineral particles (referred tohereafter as “mineral fillers”) without asphalt coating. Portland cement is frequentlyadded to foamed asphalt mixes. If the portland cement content is relatively low (0 to1.5% by mass) and the foamed asphalt content is moderate to high (2 to 3.5% bymass), which is the case for most mixes reported in the literature, the foamed asphaltmix can be regarded as a weakly asphalt-bound material. It is well known that thestrength, resilient modulus and permanent deformation resistance of foamed asphaltmixes are dependent on its stress state (e.g. Ebels and Jenkins, 2006; Fu and Harvey,

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Laboratory Tests Methods for Foamed Asphalt 189

2007; Jenkins et al., 2007), which is a typical behavior of unbound or weakly boundgranular materials. On the other hand, foamed asphalt mixes can bear tensile orbending deformation, and even show some fatigue resistance, which is a typicalcharacteristic of bound materials. This has been demonstrated by indirect tensile(IDT) strength tests (e.g. Nataatmadja, 2001), monotonic flexural beam tests (Longand Theyse, 2002; Long and Ventura, 2004) and cyclic flexural beam tests(Ramanujam and Jones, 2007; Twagira et al., 2006).

1.3. Laboratory test stress states vs. field stress states

Laboratory resilient modulus test methods and procedures that are used forassessing foamed asphalt mixes were all originally developed for other pavementmaterials. For instance, the indirect tensile resilient modulus (referred to in thispaper as “IDT RM”) test (AASHTO TP31, ASTM D41231 and LTPP P07) and thecyclic flexural beam test for dynamic modulus and fatigue (AASHTO T321) wereboth originally developed for HMA materials. The triaxial resilient modulus(referred to in this paper as “Tx RM”) test (AASHTO T307) is a conventional testmethod for unbound granular materials. This test and the frequency sweep withcyclic flexural beam test were specifically designed to measure resilient modulus,whereas resilient modulus is a “byproduct” of some other tests, such as the triaxialpermanent deformation (Tx PD) test and the cyclic flexural beam test for fatigue.

Although these tests all quantify stiffnesses of materials, the boundary conditionsapplied and the resultant stress states are significantly different. The flexural beamtest to some degree simulates the stress state of the asphalt concrete layer subjectedto loading of a tire, with tensile stress at the bottom and compressive stress at the topof a beam specimen, but with no horizontal confinement stresses. In contrast, the TxRM test applies various combinations of compressive confining stresses anddeviator stresses, whereas no tensile stress can be induced within the specimen intypical test setups. The stress state within a specimen subjected to the IDT resilientmodulus test is more complicated. According to elastic theories for a homogenouscontinuum, horizontal tensile strain and stress are induced within the cylindricalspecimen subjected to narrow vertical strip loads. However, the applicability of suchtheories to foamed asphalt mixes, which present characteristics of typical granularmaterials, is questionable.

The stress state in a foamed asphalt base layer subjected to traffic loading cannotbe represented by any one of these laboratory tests alone. The stress state at certainlocations in the foamed asphalt treated base layer is similar to that of a triaxial test;at some other locations, for instance at the bottom of the foamed asphalt layer,tensile strain is induced which is similar to the stress state at the bottom of a flexuralbeam specimen. Therefore, laboratory test results should be interpreted with caution 1. AASHTO TP31 was deleted in 2002 and ASTM D4123 was withdrawn withoutreplacement in 2003.

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and should not be assumed to be completely representative of the properties in thepavement structure for design.

The IDT RM test was the most widely used test method for foamed asphaltmixes in the literature (Nataatmadja, 2001; Chiu and Lewis, 2003; Marquis et al.,2003; Collings et al., 2004; Ramanujam and Jones, 2007; Khweir, 2007), mainlydue to the ready availability of the equipment. However, unrealistically highresilient modulus values (higher than 5,000 MPa) were reported by most of theabove researchers. On the other hand, researchers who used other test methods,namely the Tx RM test (Jenkins et al. 2002; Jenkins et al. 2004; Fu and Harvey2007; Jenkins et al., 2007), the Tx PD test (Long and Theyse, 2002; Long andVentura 2004; Jenkins et al. 2007), the monotonic flexural beam test (Long andTheyse, 2002; Long and Ventura, 2004), the cyclic flexural beam fatigue test(Twagira et al. 2006; Ramanujam and Jones, 2007), and the temperature-frequencysweep with cyclic flexural beam test (Twarira et al., 2006) generally reported valueswithin a range of between 500 and 3,000 MPa, which is consistent with the back-calculation results from field deflection measurements, including Falling WeightDeflectometer (FWD) tests (Lane and Kazmierowski, 2003; Ramanujam and Jones,2007) and multi-depth deflectometer (MDD) tests (Long and Theyse, 2004). Thisdiscrepancy between resilient modulus determined using IDT RM tests and othertest methods is evident in studies where multiple test methods were carried out forthe same materials. These studies have shown that the IDT test yields much higherresilient modulus values than other test methods (Ramanujam and Jones, 2007),while triaxial tests, beam tests and field deflection back-calculation all yield valueswithin a similar range (Long and Theyse, 2002; Long and Ventura, 2004; Long andTheyse, 2004).

A preliminary observation from the literature reveals that the IDT RM test mightoverestimate the resilient modulus of foamed asphalt mixes and thus should not beused in mix design and structural design. On the other hand, resilient moduli testedwith triaxial type or beam type tests are more credible indicators and their testconditions are more relevant to field stress states. Two potential reasons aresuggested below, but further investigation with theories and models capable ofcapturing the semi-granular nature of foamed asphalt mixes, such as the discreteelement method (Ullidtz, 2001) is needed to better understand the IDT test, which isbeyond the scope of this paper.

– The calculation of stress in the IDT test more heavily relies on the assumptionsof continuum mechanics than triaxial or beam tests. In IDT tests, loads are appliedvertically through two narrow loading strips, and calculation of horizontal tensilestress relies on the continuum mechanics, whose applicability to foamed asphaltmixes is questionable. In triaxial tests, confining stress and deviator stress areapplied uniformly and in a global sense, the calculation of resultant stresses onlyrelies on the assumption that the internal stress should balance the applied externalload. The situation for the internal stress of bending beam specimens is similarbecause on any transversal cross section, the normal stress has to balance the applied

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bending moment. This fundamental difference between the IDT test and the othertwo types of tests is evident noting that the Poisson’s ratio is used in calculatingstress of the IDT test while no material-specific constant is involved in the stresscalculation for the other two test types.

– In IDT tests, the width of the loading strips (13 mm) and the distance (25 mm)between the two gages measuring deformation is smaller than or close to thedimension of large aggregate particles. The specimen sizes for triaxial tests andflexural beam tests are much larger and stress distribution is more uniform.

1.4. Water conditioning

The effect of water conditioning on foamed asphalt mix behavior is an importantissue in foamed asphalt mix- and structural design. Compared to HMA materials,aggregate particles in foamed asphalt mixes are only partially coated (Jenkins, 2000)and the voids ratio and permeability are much higher, which makes the materialproperties highly sensitive to moisture conditioning. It was found that the propertiesof foamed asphalt mixes under soaked conditions are critical to their fieldperformance (Fu et al., 2008b).

Resilient modulus measurements of water soaked foamed asphalt mixes wereoccasionally reported by Australian researchers (Nataatmadja, 2001; Ramanujamand Jones, 2007), but they were all carried out with the IDT RM test. Literaturesearches for resilient modulus measurements for soaked foamed asphalt mixes withtriaxial or beam type tests were unsuccessful. Resilient modulus tests for watersoaked foamed asphalt mixes with Tx RM tests and monotonic flexural beam testsare a main focus of this paper.

1.5. Temperature and loading rate

Because of the presence of asphalt (in the form of both newly introduced foamedasphalt and partially oxidized asphalt from the original HMA), resilient modulus offoamed asphalt mixes shows temperature and loading rate dependency. Fu andHarvey (2007) studied temperature dependency of foamed asphalt mix resilientmodulus as well as its interaction with stress dependency under triaxial testboundary conditions. Temperature sensitivity coefficients (a dimensionlessparameter) from 0.0065 to 0.013 were measured. Limited FWD back-calculationresults also showed similar temperature sensitivity coefficient values.

Frequency sweep from cyclic flexural beam tests was reported by Twagira et al.(2006). The materials tested contained between 2.4 and 3.6% foamed asphalt and 0to 1.0% portland cement. It was found that generally a 10 fold increase in loadingfrequency increases the measured resilient modulus by approximately 25%.

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Compared to the effects of water conditioning and stress states, the effects oftemperature and loading rates are of a less complicated and more predictable nature.

1.6. Scope of this study

This paper discusses laboratory test methods for resilient modulus of foamedasphalt mixes, emphasizing the effects of water conditioning and different testboundary conditions on test results. In (Fu et al., 2009), an bilinear anisotropicconstitutive model is presented to predict equivalent resilient modulus of foamedasphalt mixes in the field stress state by combining the laboratory test results of TxRM tests with monotonic flexural beam test results. The temperature and moisturedependencies of foamed asphalt mix resilient modulus observed in field FallingWeight Deflectometer (FWD) tests are also discussed in (Fu et al., 2009).Guidelines for using resilient modulus values in project level design are proposed in(Fu et al., 2009), based on the conclusions of the two papers.

2. Laboratory testing program

This paper is based on work completed as part of an ongoing comprehensiveresearch program (UCPRC, 2005; Jones, et al., 2008) on FDR with foamed asphalt,being undertaken by the University of California Pavement Research Center(UCPRC) for the California Department of Transportation (Caltrans). This researchincludes extensive laboratory testing and field monitoring. The laboratory testingprogram aims to identify the roles of constituents of foamed asphalt mixes and toselect suitable laboratory practices and appropriate test methods, both for advancedresearch and routine project-level tests. Field monitoring on FDR-foamed asphaltprojects built by Caltrans is being performed by the UCPRC with the objective ofrelating laboratory tested material properties to field tested properties and fieldperformance. The test results presented in this paper are from a phase of the studythat investigated parent materials of foamed asphalt mixes (asphalt cements,recycled asphalt pavement and aggregate base) and their influence on mixproperties, and compared various laboratory specimen preparation procedures andtest methods.

2.1. Materials

The granular materials treated with foamed asphalt in this study primarily consistof recycled asphalt pavement (RAP) which is in accordance with California FDRpractice. California has been using conventional asphalt overlays as the main asphaltpavement rehabilitation and maintenance strategy for more than 50 years. InCalifornia, even low traffic volume state roads typically have multiple cracked

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asphalt concrete layers 150 to 600 mm thick. In this study RAP materials werecollected from two California highways: State Route 88 (SR88) in Amador Countyand SR33 in Ventura County. The pavements were pulverized by recyclingmachines commonly used in FDR projects in California to depths of about 200 mm,but without the addition of any stabilizing agent. Both materials contained more than75% pulverized asphalt concrete and less than 25% granular base and otherpavement layers by mass. Three gradations (denoted as Gradation A, B and C) wereconstituted from each source by sieving the RAP materials into four fractions andrecombining them as shown in Figure 1. This ensured that consistent materials wereused throughout the study. RAP 33-A and 88-A represented the average gradationsas pulverized on each road, containing 8% and 10% fines passing the 0.075 mmsieve by mass, respectively; RAP 33-B and 88-B were coarser gradations with 6.5%fines passing the 0.075 mm sieve; RAP 33-C and 88-C were produced by addingbaghouse dust (collected from a local aggregate processing plant) to RAP 33-B and88-B to produce materials that had 20% passing the 0.075 mm sieve.

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Sieve Size (mm)

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88-A 33-A88-B 33-B88-C 33-C

Figure 1. Gradation of the RAP used in the test program (the curves for 33-B and88-B overlap and those for 33-C and 88-C overlap. Zone definition follows TG2,2002)

Detailed quantitative morphological analyses were not carried out for the RAPmaterials collected. A visual inspection found that the aggregate angularities of theRAP from SR33 and SR88 were not significantly different as illustrated in Table 1.However, many more aggregate particles of SR33 RAP were coated by oxidizedasphalt binder compared to those from SR88, and coated aggregate particles hadrougher surface texture than those of the uncoated particles. The surface texture oftypical RAP particles is shown in Figure 2.

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Two grades of asphalt (PG 64-16 and PG 64-10) from a local California refinerywere used in the study. The asphalt was foamed with a Wirtgen WLB10 laboratoryplant at 165°C with 4% foaming water by mass added. These two asphalts hadsimilar foaming characteristics with an average expansion ratio of 23 and an averagehalf-life of 21 seconds. Foamed asphalt was injected directly onto the RAP in acustom built laboratory-scale twin-shaft pug-mill mixer. The aggregate temperaturewas controlled between 25°C and 30°C. The foamed asphalt content was 3% (bymass of aggregate). For each mix type, one batch of loose mix (65 kg total) wasprepared to fabricate different types of specimens for laboratory testing, includingIndirect Tensile Strength (ITS), Unconfined Compressive Strength (UCS), flexuralbeam, and Tx RM tests. No portland cement or other active fillers were added. Theireffects will be studied in a later phase of the research program.

Table 1. Aggregate particles from the two RAP sources

Particles passing 19mm sieve andretained on 9.5mm sieve

Particles passing 9.5mm sieve andretained on 4.75mm sieve

SR33

SR88

It was found that in terms of the properties concerned in this study, the effects ofasphalt binder grade were less significant that those of RAP type. Therefore, the testresults presented in later sections for each RAP type are the average of the twoasphalt types. Two replicate batches of each combination of RAP type and asphalttype were typically prepared. The values shown in Tables 2 to 5 are the average ofreplicate batches and specimens, while results for replicate batches and specimensare plotted separately in Figures 4 to 7.

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(a) (b)

Figure 2. Surface texture of typical RAP particles. (a) SR33 RAP;(b) SR88 RAP.(The diameters of both particles are approximately 5 mm)

2.2. Triaxial resilient modulus test

Cylindrical specimens with a diameter of 152 mm and a height of 305 mm wereprepared for Tx RM tests. The compaction procedure is based on the modifiedProctor compaction method (T180) with some additional modifications. Eachcylindrical specimen was compacted with 12 lifts of 25 mm thick layers, and themass of the mix for each layer was calculated based on 100% modified AASHTOdensity. It should be noted that AASHTO T307 suggests that specimens shall becompacted with a vibratory impact hammer without kneading action.

The Tx RM test procedure adopted in this study was modified from theAASHTO T307 test protocol. Resilient moduli at various confining stress levels,deviator stress levels and loading rates were tested. The confining stress anddeviator stress levels adopted were the same as those of AASHTO T307. At eachcombination of confining stress and deviator stress, haversine load pulses at fourdifferent loading rates: 0.05 s pulse width with 0.45 s relaxation, 0.1 s pulse widthwith 0.4 s relaxation, 0.2 s pulse width with 0.8 s relaxation and 0.4 s pulse widthwith 0.6 s relaxation were applied. Since the Tx RM test undertaken in this studywas largely nondestructive, each specimen was first tested for resilient modulus afterdry curing (unsoaked), and then tested for resilient modulus after soaking.

All triaxial tests as well as flexural beam tests were preformed at 20°C.

2.3. Flexural beam specimen and test

A new monotonic flexural beam test procedure was developed at the UCPRC forthis study (see Figure 3). The nominal dimensions of the beam specimens are

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560 mm x 152 mm x 80 mm. The amount of moist material needed to fabricate abeam was calculated based on the 100% modified AASHTO density. The materialwas then compacted in a steel mold to the target volume by alternately applying twosteel compaction heads (one flat and one curved, both with dimensions of 150 mm x150 mm). The compaction heads were driven by a Hilti® TE 76P Combihammerwith a vibration force. Specimens were tested as compacted, with no cutting to finaldimensions.

The flexural beam test configuration was similar to that of AASHTO T97, butthe beam thickness was 80 mm instead of 150 mm, and loading was displacementrate controlled rather than stress rate controlled. The span length was 450 mm andloads were applied monotonically at the two third-points with a constantdisplacement rate of 25 mm/min. Two metal plates were glued at the mid span of thebeam and one linear variable displacement transducer (LVDT) was attached to eachmedal plate to measure the deflection during testing.

2.4. Free-free resonant column test

The free-free resonant column (FFRC) test was carried out on triaxial and beamspecimens. The test setup was similar to that reported by Nazarian et al. (2003) andHilbrich and Scullion (2007). This test normally utilizes cylindrical specimens witha length to diameter ratio of 2:1, while the ratio for beam specimens adopted was4.5:1, which should be more preferable for wave velocity measurement. Since thistest is nondestructive, all cylindrical and beam specimens were subjected to this testbefore the triaxial and flexural beam tests. The specimens were only tested under theunsoaked condition, as it was not possible to mount the accelerometer on soakedspecimens.

150 mm 150 mm

150 mm 150 mm10 mm

10 mm

R = 108 mm Connector to a Hilti Combihammer

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Steel compaction mould

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Target beam surface

80 mm

Apply vibration force

450 mm

152 mm 80 m

m150 mm

Displacement controlloading

Deflection measurementat mid-span

LVDT

Metal plateglued to beam

Figure 3. The beam specimen compaction and test setup. (a) The curved compactionhead and the flat compaction head; (b) beam specimen compaction by applyingvibration force on compaction heads; (c)beam test setup

2.5. Curing and water conditioning

It is a well known fact that the strength development mechanism of a foamedasphalt mix during curing is closely related to the loss of moisture, especially formixes that do not contain portland cement. Bowering (1970) found that foamedasphalt specimens do not develop full strength until most of the mixing moisture hasevaporated. In this study, all compacted specimens were cured in a forced draft ovenat 40°C for 7 days. Specimens subjected to water conditioning were soaked in awater bath at 20°C for 72 hours with the water level 100 mm above the surface ofthe specimen. The prolonged drying and soaking durations were designed to

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represent extreme and critical field conditions and to minimize the effects ofdifferent specimen sizes. Both the foamed asphalt treated mixes and untreatedcontrol mixes were subjected to the same curing and water conditioning conditionsbefore testing.

2.6. Fracture face image analysis

A fracture face image analysis technique providing insight into the internalmicroscopic structure characteristics of foamed asphalt mixes was developed for thisresearch program (Fu et al., 2008a). In this technique, the internal asphalt masticphase distribution in foamed asphalt mixes is inferred from the visible distribution ofasphalt mastic spots on the fracture faces of tested ITS or flexural beam specimens.

(a) (b) (c)Figure 4. Example of fracture face image analysis. (a) a tested ITS specimen;(b)two fracture faces; (c) asphalt mastic spots identified on one fracture face

To quantify the characteristics of the asphalt mastic phase visible on fracturefaces, a new concept, Fracture Face Asphalt Coverage (FFAC) was defined as theratio of the area of the mastic phase visible on a fracture face to the total area of thefracture face. Figure 4 shows a tested ITS specimen, two fracture faces of thisspecimen, and the asphalt mastic phase visible on the fracture faces identified bydigital image processing techniques. The FFAC value for this fracture face is 16%.FFAC is primarily a function of the asphalt mastic dispersion in the mix. Highvalues of FFAC generally indicate better asphalt dispersion. As a research tool,FFAC combined with other test results provides useful information for investigatingthe stabilization mechanism of foamed asphalt in the mix and for understanding howother variables in mix design affect mix properties, as will be demonstrated inSection 3.3.

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3. Test results

3.1. ITS test results

Although the main focus of this paper is the resilient modulus characteristics offoamed asphalt materials, ITS values of the material tested in this study are listed inTable 2 as a reference. ITS test results are the most abundant quantitativecharacteristics for foamed asphalt mixes available in the literature, and thus a goodreference property characterizing some basic properties of the materials (Fu et al.,2008b).

Table 2. ITS test results on 100 mm briquettes

ITS (kPa)

33-A 33-B 33-C 88-A 88-B 88-C

Unsoaked 724 805 301 314 413 221UntreatedRAP Soaked 80 74 9 66 52 NR*

Unsoaked 898 792 587 512 569 5133% foamedasphalt Soaked 166 174 86 196 191 125

NR*: No result, specimens disintegrated during water conditioning.

3.2. Free-free resonant column test (FFRC)

Several observations can be made regarding the FFRC test results.

The repeatability of this test is satisfactory. Figure 5 shows the test resultcomparison for two replicate beams made of the same batch of mix. The relativedifference is generally within 5%.

There is a high correlation (with a Pearson correlation coefficient of 0.97)between FFRC resilient modulus values for beam specimens and those for triaxialspecimens made of the same batch of mix (Figure 6). The FFRC resilient modulusvalues for triaxial specimens were consistently lower (by 13% on average) thanthose of the beam specimens. One important reason for this difference might be theaggregate particle orientation induced by compaction. In FFRC tests, the wavepropagation direction in a triaxial specimen is the same as the direction of thecompaction action whereas it is perpendicular to the direction of compaction actionin a beam specimen.

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y=0.95xy=1.05x

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33-A33-B33-C

88-A88-B88-C

Figure 5. Repeatability of FFRC tests (Each combination of specimen A and B arethe two beam specimens made of the same batch of foamed asphalt mix.)

y= 0.87x

R2 = 0.90

33-A33-B33-C

88-A88-B88-C

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0 2,000 4,000 6,000 8,000 10,000 12,000

Figure 6. Correlation of FFRC resilient modulus values between beam specimensand triaxial specimens made of the same batches of mixes

FFRC tests apparently overestimate resilient modulus of foamed asphalt mixes.As will be shown in Section 3.3 and 3.4, resilient modulus values determined fromtriaxial and flexural beam tests for the same mixes were generally lower than2,000 MPa, while typical values of 4,000 to 12,000 MPa were produced by FFRCtests. As mentioned before, resilient modulus of foamed asphalt mixes is stress andloading rate dependent. The stress induced in FFRC tests is of a very smallamplitude and high frequency. For instance, the natural frequency of triaxial

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specimens is as high as several thousand Hz. This kind of stress state has minimalrelevance to the stress state induced by traffic loading to pavement structures.Similar observations were made by Hochuli et al. (2001) on resilient modulusbehavior of HMA. In their study, HMA rods were tested by wave propagationmethods. Because the amplitudes of the waves used were very small and frequencieswere very high, the measured modulus values were as high as 16,000 to 22,000 MPaat ambient temperature (22ºC). The study concluded that “the resulting moduli aredifferent from those commonly used for pavement design and cannot be useddirectly for that purpose”.

0

2,000

4,000

6,000

8,000

10,000

12,000

0 500 1,000 1,500 2,000

Modulus of Rupture - Beam(kPa)

Mr-

FFR

C-B

eam

(MPa

)

33-A

33-B33-C

88-A

88-B88-CR

2 = 0.82

Figure 7. Correlation between FFRC resilient modulus values of beam specimensand modulus of rupture of the same beam

The FFRC modulus values for unsoaked specimens appeared to be verydependent on RAP type. All specimens made of RAP 33-A and 33-B hadsignificantly higher modulus than did other RAP type. The same phenomenon canbe observed from ITS test results in Table 2, and from the correlation betweenFFRC resilient modulus and modulus of rupture (or stress-at-break as termed inSouth Africa) results of monotonic flexural beam tests as shown in Figure 7.Although no X-Ray diffraction tests were carried out on any of the materials, someweak chemical bonding, attributed to the chemical composition of the fines,appeared to be evident in the unsoaked state in the material sampled from SR33.Results of pH tests on this material indicated slight alkalinity (pH of 8.2 usingAASHTO T289), compared to slight acidity (pH of 6.7) on the material sampledfrom SR88, which did not show the same signs of chemical bonding. These pH

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ranges are realistic for the natural materials and not indicative of earlier modificationwith lime or cement. This weak chemical bonding exhibited brittle, but stiff,properties especially at low stress levels, in the unsoaked state, but did not influenceperformance of the material in the soaked state. This bonding appeared to play adominant role in FFRC testing. Fifteen percent inert baghouse dust was added toRAP 33-C, which diluted the semi-reactive filler and impeded filler particles fromcontacting each other. Unsoaked strength and stiffness values of 33-C were thereforeclose to that of RAP 88-C, and also closer to that of 88-A and 88-B than to 33-A and33-B. In the following discussion, RAP 33-A and 33-B are classified into onecategory, and the other four types are classified into another category to avoidcomplicating the discussion.

As an overall conclusion, although the FFRC test is easy and inexpensive tocarry out with high repeatability, the testing stress state is very different from theworking stress state of foamed asphalt mixes in pavement structures, and hence theresults are of questionable value for pavement design with these materials.

3.3. Triaxial resilient modulus test

All triaxial specimens were subjected to resilient modulus tests under unsoakedand soaked conditions consecutively. Combinations of various load pulse durations,confining stresses and deviator stresses were applied to each test. Equation [1],which is modified from Uzan’s (1985) general model by the addition ofconsideration of loading pulse durations, was used to fit the Tx RM test data. Anaverage R2 value of 0.983 was achieved.

2 3

1 0.1 second

Tk kk

octr a

a a

TM k pp p

τθ =

[1]

where pa= atmospheric pressure used to nondimensionalize stresses; T= duration ofthe haversine load pulses; σ0 = confining stress; σd = deviator stress; θ = 3σ0+σd =bulk stress; τoct =octahedral shear stress, and in the triaxial stress state τoct= σd/3;kT, k1, k2, and k3 are material related constants. Model fitting results are presented inTables 3 and 4.

In triaxial test stress states, the resilient modulus of foamed asphalt mix isprimarily a function of the confining stress (σ0), the deviator stress (σd) and theloading rate (characterized by the half-sine load pulse duration T), i.e. Mr = Mr (σ0,σd, T). Based on the fitting results, resilient modulus values at two reference stressstates, Mr1= Mr(20.7 kPa, 62.1 kPa, 0.1 second) and Mr2= Mr(137.9 kPa, 103.4 kPa,0.1 second) were calculated as shown in Tables 3 and 4. The resilient modulus atlow confining pressure and relatively high deviator stress levels is represented byMr1, while Mr2 represents the resilient modulus at high confining stress and relativelylow deviator stress levels. Both stress states were used in the testing sequence of

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AASHTO T307, but the values shown were calculated on the basis of model fittingresults. Resilient modulus retained (RMR) values are provided as a generalindication of the moisture sensitivity of the resilient modulus of each material and iscalculated as the average of the moisture sensitivity at the two reference stress states.Table 4 shows the Tx RM test results for control mixes, which were made of thesame aggregate with the same procedure, but did not contain foamed asphalt.

The following general observations can be made by comparing the tests resultsof the control mixes and the foamed asphalt treated mixes in both soaked andunsoaked states.

1) Soaked control mixes of RAP 33-A and 33-B had significantly higher resilientmoduli than specimens of 88-A and 88-B. According to the parallel ITS test results(Table 2), although weak cementation attributed to chemical bonding appeared todevelop for mixes 33-A and 33-B, they were largely damaged after waterconditioning. The soaked ITS values for 33-A and 33-B RAP were similar to thosefor 88-A and 88-B RAP. The weak cementation/bonding probably did notsignificantly contribute to the measured high soaked resilient modulus values forthese two RAP types although it might have contributed to the higher resilientmodulus of the corresponding unsoaked specimens. Apart from cementation and/orbonding, resilient modulus of granular materials is also affected by aggregategradation, water content, density, aggregate size, and morphological characteristics(Pan et al., 2006). Since the gradation, moisture content and density were allcontrolled in the tests, morphological properties probably had some influence on theresults. As mentioned in Section 2.1, RAP from the two sources had similarangularity, but the SR33 RAP had coarser surface texture, to which the highersoaked resilient modulus values of the untreated control mixes of 33-A and 33-Bwas attributed.

2) Adding 3 percent foamed asphalt generally did not increase resilient modulusvalues in the unsoaked state, except for 33-C. The moduli of 33-C increased slightly(by approximately 10%) at both stress levels, but remained essentially unchangedfor all the other RAP types, or even decreased slightly when foamed asphalt wasadded.

3) Adding 3 percent foamed asphalt substantially increased the soaked resilientmoduli for 88-A and 88-B RAP, especially at the low confining stress level. Soakeduntreated specimens of 33-C and 88-C softened significantly after waterconditioning and eventually collapsed in the water bath with minor disturbance.Therefore, the foamed asphalt treatment increased their stiffnesses from nearlyimmeasurably low values to acceptable values.

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Table 3. Tx RM test results for foamed asphalt mixes, Mr values are in MPa

Unsoaked Soakedk1 kT k2 k3 Mr1 Mr2 K1 kT k2 k3 Mr1 Mr2 RMR*

33-A 10,433 -0.06 0.19 -0.03 1131 1467 7,406 -0.09 0.17 -0.06 833 1026 72%

33-B 9,794 -0.04 0.16 -0.01 1038 1298 8,153 -0.11 0.15 -0.06 916 1106 87%

33-C 9,450 -0.04 0.15 -0.03 1015 1235 5,469 -0.09 0.27 -0.10 664 920 70%

88-A 8,467 -0.04 0.18 -0.05 941 1188 7,864 -0.09 0.21 -0.05 881 1163 96%

88-B 8,560 -0.05 0.19 -0.03 938 1205 6,672 -0.09 0.22 -0.06 763 1006 82%

88-C 7,528 -0.03 0.19 -0.05 842 1078 4,600 -0.08 0.31 -0.10 564 837 72%

* RMR = Resilient Modulus Retained. In this case, 1 2

1 2

12

soaked soaked

dry dry

Mr MrRMRMr Mr

− −

− −

= +

Table 4. Tx RM test results for untreated control mixes, Mr values are in MPa

Unsoaked Soakedk1 kT k2 k3 Mr1 Mr2 K1 kT k2 k3 Mr1 Mr2 RMR

33-A 10,901 -0.03 0.16 -0.05 1211 1484 8,004 -0.06 0.24 -0.05 908 1239 79%

33-B 9,240 -0.04 0.24 -0.04 1031 1420 6,953 -0.07 0.25 -0.10 833 1131 80%

33-C 6,469 -0.01 0.29 -0.19 880 1199 Specimens disintegrated during water conditioning.

88-A 8,369 -0.04 0.25 -0.07 967 1332 3,693 -0.05 0.40 -0.16 495 807 56%

88-B 9,278 -0.03 0.26 -0.10 1116 1548 3,553 -0.06 0.45 -0.17 487 845 49%

88-C 8,447 -0.03 0.23 -0.08 983 1322 Specimens disintegrated during water conditioning.

204 Road M

aterials and Pavement D

esign. Volum

e 10 – No. 1/2009

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4) The differences in soaked resilient moduli between SR33 (A and B) RAP andSR88 (A and B) RAP were much less significant for the foamed asphalt treatedmixes than for the untreated control mixes. In the control mixes, the characteristicsof the aggregate (e.g. surface texture) dominated the resilient modulus behavior. Onthe other hand, in the foamed asphalt mixes, the foamed asphalt played a dominantrole

The effects of the dispersed asphalt on the foamed asphalt mix resilient modulusvalues were also observed by tracking the change of material constants (kT, k1, k2 andk3) in Equation [1] with the change of asphalt dispersion. In Figure 8, “FFAC-ITS150mm-soaked” denotes the fracture face asphalt coverage (Fu et al., 2008a) ofthe soaked 150 mm ITS specimens, which were made of the same batches of mixand compacted following a similar procedure as the triaxial specimens. Figure 8shows the correlations between the FFAC values and all four material constants inEquation [1] for soaked specimens. Data points with FFAC = 0 correspond to thevalues for the soaked untreated (control) materials. It should be noted that theuntreated 33-C and 88-C specimens collapsed during soaking and thus resilientmodulus values were not available.

The constants kT, k2 and k3 represent the sensitivity of the foamed asphalt mixresilient modulus to loading rates (or load pulse durations), bulk stresses, anddeviator stresses, respectively. Parameter k1 is a scalar term: if all the otherparameters are the same, the higher the k1 value, the higher the resilient modulus atlow confining stress levels.

It can be seen that as the FFAC value increased (which generally representsbetter asphalt dispersion in the mix), the resilient modulus at low confining stresslevels also increased (Figure 8a). The resilient modulus was more sensitive toloading rates (Figure 8b), but less sensitive to bulk stress values (Figure 8c) anddeviator stress values (Figure 8d). This effect was more significant for RAP fromSR88 and RAP 33-C, than for RAP from 33-A and 33-B.

As an overall conclusion, Tx RM test results showed that foamed asphalttreatment did not always increase the absolute values of resilient modulus, undereither unsoaked or soaked conditions. The main role of foamed asphalt was totransform the material behavior from that of typical unbound granular materials tothat of partially asphalt-bound materials, with resilient modulus behavior that wasmore loading rate dependent but less stress dependent. The significance of thistransforming effect was also influenced by some characteristics of the RAP materialitself. For example, RAP materials with coarser surface texture appeared to be lessaffected by foamed asphalt stabilization in triaxial stress states, during whichaggregate particle interlocking and frictional sliding play significant roles in additionto the cohesion provided by the foamed asphalt.

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3,000

4,000

5,000

6,000

7,000

8,000

9,000

10,000

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35

FFAC-ITS150mm-soaked

k 1-s

oake

d

33-A 33-B33-C 88-A88-B 88-C

(a)

-0.12

-0.10

-0.08

-0.06

-0.040.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35


k T-s

oake

d

33-A 33-B33-C 88-A88-B 88-C

(b)

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0.1

0.2

0.3

0.4

0.5

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35


k 2-s

oake

d

33-A 33-B33-C 88-A88-B 88-C

(c)

-0.18

-0.15

-0.12

-0.09

-0.06

-0.03

0.000.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35


k 3-s

oake

d

33-A 33-B33-C 88-A88-B 88-C

(d)

Figure 8. Correlation between FFAC and material constants for soaked resilientmodulus. (a) FFAC vs. k1-soaked; (b) FFAC vs. kT-soaked;(c) FFAC vs. k2-soaked;(d) FFAC vs. k3-soaked

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3.4. Flexural beam test

The monotonic flexural beam test results for both unsoaked and soakedspecimens are shown in Table 5. Ebend is the equivalent tangential Young’s modulusfor bending determined from the stress-strain curves. Strain-at-break (εb) was thecalculated tensile strain at the bottom of the beam at the mid-span, computed basedon the measured beam deflection when the deflection-load curve reached its peak.All calculations were based on Euler-Bernoulli beam theories. Values listed in Table5 are the averages of pooled values for mixes treated with the PG64-16 and PG64-10binders and replicate specimens. Many of the metal plates detached after soaking,thus only a small number (1 to 2) of successful tests were available for several of theRAP types. In these instances, test results should be interpreted carefully since thevariance could be large.

Table 5. Monotonic flexural beam test results

Unsoaked SoakedRAP bend

unsoakedE(MPa)

εb N*bendsoakedE

(MPa)εb N

bendsoaked

bendunsoaked

EE 1

bendsoaked

r soaked

EM −

33-A 1689 2632 3 117 4230 1 7% 14%33-B 1381 2632 2 249 2444 1 18% 27%33-C 1673 2444 2 70 3760 1 4% 11%88-A 855 2181 5 98 4230 5 11% 11%88-B 1073 2820 2 82 4512 2 8% 11%88-C 873 2444 3 50 4606 2 6% 9%

* N = number of specimens that were tested with successful deflection measurement.

In the unsoaked state, beams made of SR33 RAP had higher bending stiffnessthan those made of SR88 RAP, while the difference in strain-at-break was small.Interestingly, the magnitude of the equivalent Young’s modulus for bending (Ebend)for unsoaked specimens was similar to that of the Tx RM. However, when thebeams were soaked, they lost 82% to 94% of their stiffnesses, while the strain-at-break values had a moderate increase. On the other hand, triaxial specimens onlylost 21% of their stiffnesses on average when soaked.

The large discrepancy between the Tx RM test results and the monotonicflexural beam test results in terms of moisture sensitivity can be attributed to thedifferent stress states associated with these two tests. Foamed asphalt materialswithout portland cement resist applied loading primarily by three mechanisms: 1)interlocking and frictional sliding of aggregate particles, 2) bonding of foamedasphalt, and 3) bonding (i.e. weak chemical cementation and suction of residualwater) in the mineral filler phase. These three mechanisms are 1) insensitive to waterconditioning, 2) moderately sensitive to water conditioning and 3) highly sensitive

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to water conditioning (Fu et al., 2008b), respectively. The first mechanism can resistcompression and shearing forces under confinement in triaxial stress states, so it hasa dominant role in soaked triaxial specimens when the other two mechanisms areimpaired. Consequently water conditioning only reduces Tx RM slightly ormoderately. For unsoaked beam specimens, the third mechanism contributes most ofthe deformation resistance, which is relatively strong but brittle. When beams aresoaked, foamed asphalt bonding becomes the only available mechanism resistingtensile deformation, so the overall stiffness of beam specimens is highly sensitive tomoisture damage. At the same time, because asphalt bonding is more ductile thanthe bonding in the mineral filler phase, the strain-at-break is moderately increasedfor soaked beams.

4. Summary and conclusions

Previous research programs have often not sufficiently considered that theresilient modulus of foamed asphalt mixes is highly dependent on the stress state,and that the available laboratory test methods cannot fully simulate field stressstates. This paper investigates these laboratory test methods, emphasizing theimportance of stress states associated with different test boundary conditions,especially under soaked conditions.

In a laboratory testing program performed at the UCPRC, foamed asphalt mixesmade of RAP from different parent materials and different asphalt grades weretested with three methods, namely the free-free resonant column (FFRC) test, thetriaxial resilient modulus (Tx RM) test, and the monotonic flexural beam test.

The FFRC test yields stress states that are very different from those in thepavement. It greatly overestimates the resilient modulus values of foamed asphaltmixes, and thus presents problems for its use in pavement design.

Tx RM tests were carried out for both unsoaked and soaked specimens. It wasfound that foamed asphalt transformed the material behavior from that of typicalunbound granular materials to that of partially asphalt-bound materials, withoutsignificantly increasing the resilient modulus values.

Flexural beam tests were also carried out for both unsoaked and soakedspecimens. The range of values of tangential Young’s modulus for bending wassimilar to the resilient modulus determined from Tx RM tests in the unsoaked state.However, the modulus reduction due to water conditioning of beam tests was 85%to 95%, while that of triaxial tests was only approximately 5% to 30%.

Tensile stress due to bending and shear/compressive stress with lateralconfinement both exist in pavement structures under traffic loading. The Tx RM testor the flexural beam test alone can only partially represent the field stress state. Thetest results from both tests should be combined for calculating an equivalent resilientmodulus value characterizing the overall deformation resistance of foamed asphalt

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materials in field stress states. This issue is discussed in a subsequent paper by Fuet al. (2009). It should be emphasized that simply using Tx RM test results couldresult in an unsafe design because of the dramatic reduction of resilient modulus intensile stress states that are not characterized by the triaxial test.

Acknowledgements

The work presented in this paper was sponsored by the California Department ofTransportation, Division of Research and Innovation, for which the authors aregrateful. The authors also wish to thank their collaborators in the CaliforniaDepartment of Transportation, and at the UCPRC. The results presented in thispaper do not represent any standard or specification of the California Department ofTransportation, and the opinions expressed are those of the authors alone.

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Received: 21 January 2008Accepted: 13 July 2008

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Documents

Laboratory Test Methods for Foamed Asphalt Mix Resilient Modulus