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Laboratory evaluation of moisture damage inasphalt
Rafiqul A. Tarefder and Seyed S. Yousefi
Abstract: Traditionally, moisture damage in asphalt is determined by laboratory testing of liquid asphalt binder, loose mix,and (or) solid asphalt concrete samples under wet and dry conditions. Yet some asphalt mixes pass such laboratory moisturedamage tests but show poor moisture damage potential in the field. To this end, this study presents asphalt constituents suchas mastic and matrix testing for true evaluation of field moisture damage in asphalt concrete. Three mixes included in thisstudy have almost identical laboratory moisture damage potential, but coarse mix (SP-III) has higher field moisture damagepotential than two fine mixes (SP-B, SP-C). Related to these mixes, three mastics namely natural fines (NF), crushed fines(CF), and combined natural and crushed fines (NF+CF) are tested using a dynamic shear rheometer (DSR) at varying tem-perature and frequency. Also, matrix materials passing #4 sieve is tested in DSR, tension, pull-off, and direct shear loadings.It is shown that mastic and matrix test results can better identify field moisture damage of asphalt concrete. Fine mastic andmatrix have low moisture damage potential than the coarse matrix and mastic. At high temperature (above 25 °C), shearmodulus converges to a small number irrespective of mastic type. Due to moisture conditioning, matrix ultimate stress de-creases in tension and pull-off tests but not in shear test.
Key words: asphalt mastic, matrix, moisture damage, dynamic shear rheometer, tension, compression, shear modulus, binder,mixture.
Résumé : Les dommages causés à l’asphalte par l’humidité sont traditionnellement déterminés en laboratoire, par des essaissur des échantillons de liants routiers liquides, de mélange en vrac et/ou de béton bitumineux solide sous des conditions hu-mides et sèches. Malgré cela, certains mélanges d’asphalte passent avec succès de tels essais de résistance à l’endommage-ment par l’humidité mais montrent une piètre résistance à l’endommagement par l’humidité une fois sur le terrain. Ainsi,cette étude présente les composants de l’asphalte, tels que le mastic, et les essais matriciels pour une évaluation réelle desdommages causés par l’humidité au béton bitumineux sur le terrain. Trois mélanges inclus dans cette étude présentaient unpotentiel de résistance à l’endommagement par l’humidité presque identique lors des essais en laboratoire, mais le mélangegrossier (SP-III) présentait sur le terrain un potentiel de résistance à l’endommagement par l’humidité supérieur à celui desdeux mélanges fins (SP-B et SP-C). Trois mastics, dont les fines naturelles (FN), les fines concassées (FC) et une combinai-son de fines naturelles et concassées (FN+FC), ont été mis à l’épreuve utilisant un rhéomètre de cisaillement dynamique(RCD) à différentes températures et fréquences. De plus, tous les matériaux matriciels passant le tamis #4 ont été mis à l’é-preuve en tension, en arrachement, en cisaillement direct et en utilisant le DSR. Il est démontré que les résultats des essaissur les mastics et les produits matriciels peuvent mieux identifier les dommages causés au béton bitumineux par l’humiditésur le terrain. Les mastics et les produits matriciels fins présentent un potentiel de résistance à l’endommagement par l’hu-midité inférieur à celui de la matrice grossière et du mastic. À une température élevée (au-dessus de 25 °C), le module decisaillement converge vers un petit nombre, peu importe le type de mastic. En raison du conditionnement contre l’humidité,la contrainte à la rupture de la matrice diminue dans les essais en tension et en arrachement mais non dans les essais en ci-saillement.
Mots‐clés : mastic d’asphalte, matrice, dommage par l’humidité, rhéomètre de cisaillement dynamique, tension, compres-sion, module de cisaillement, liant, mélange.
[Traduit par la Rédaction]
1. Introduction
To date, numerous test methods have been developed andused to predict moisture-induced damage in asphalt concrete(Solaimanian et al. 1993; Cheng et al. 2002; Hicks et al.2004; Kanitpong and Bahia 2005; Masad et al. 2006; Dela-
porte et al. 2008). The historical development of moisturedamage tests unfolds as follows: boil test (1930s, 1980s), ab-sorption effects (1940s), sonic test (1950s), immersion com-pression (1950s), aggregate characteristics (1950s), waterpressure – pore size effect (1950s), thermally induced pore
Received 6 October 2010. Revision accepted 20 October 2011. Published at www.nrcresearchpress.com/cjce on 6 January 2012.
R.A. Tarefder and S.S. Yousefi. Department of Civil Engineering, University of New Mexico, MSC01 1070, Albuquerque, NM 87113,USA.
Corresponding author: Rafiqul A. Tarefder (e-mail: [email protected]).
Written discussion of this article is welcomed and will be received by the Editor until 31 May 2012.
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Can. J. Civ. Eng. 39: 104–115 (2012) doi:10.1139/L11-114 Published by NRC Research Press
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pressure test (1960s), surface reaction test (1970s), pore pres-sure – double punch (1970s), cyclic water pressure strength(1970s), desorption by solvent extraction (1970s), inversegas-liquid chromatography (1980s), freeze–thaw pedestal(1980s), freeze–thaw conditioning with strength test (1970s,1980s), original Lottman test or retained strength test (70sand 80s), Hamburg wheel tracking device (1970s, 1990s), as-phalt pavement analyzer (1990s), pneumatic pull-off (1990s),net adsorption (1990s), environmental conditioning system(1990s), ultrasonic test (2000s), asphalt chemistry (2000s),cyclic pressure/suction (2000s), modified Lottman test or in-direct tensile ratio test (1990s, 2000s), moisture inducedstress tester (2000s), AASHTO T-283 (2000s), the Hamburgwheel track test (2000s), and environmental conditioning sys-tem, and simple performance test (2002s) (Hicks et al. 2004).These highlights are only a glimpse into a body of researchon the general subject of moisture damage. However, mostof these tests do not address moisture damage in mastic ormatrix, and so as a result the moisture damage is still one ofthe most common and complex problems (Tarefder and Za-man 2010; Khattak and Kyatham 2008). Asphalt concretebeing a multi-phase material, derives its strength mainlyfrom matrix defined by asphalt coated fine aggregate passing#4 sieves, in mastic defined by asphalt coated fines passing#200 sieve, and (or) in asphalt binder. Moisture damage testson mastic and matrix material can provide useful insightsinto the understanding of the causes and factors of moisturedamage and the assessment of testing methods capable of ex-amining moisture damage in asphalt concrete, which is donein this study.Asphalt mastic and matrix materials are generally weak
compared to the coarse aggregate phase. Identifying theweak points within an asphalt concrete can aid engineers inthe design of asphalt concrete that is more resistant to mois-ture damage and cracking (Tarefder et al. 2009). Recently,several researchers have done some component (binder andmastic) testing for generating inputs of micromechanicalmodels (Buttlar et al. 1999; Shashidhar and Shenoy 2002;Birgisson et al. 2004; Kim et al. 2006; Liu and You 2009).Several researchers have studied the amount of load carriedby asphalt binder within the asphalt concrete samples usingfinite element and X-ray image analysis (Abbas et al. 2005;Masad 2004; Kose et al. 2000). Liquid asphalt binders havebeen tested mainly using a dynamic shear rheometer (Khattakand Kyatham 2008; Muniandy et al. 2008; Rowe et al. 2009).Kim and Little (2004) and Kim et al. (2008) used DSR tocharacterize linear viscoelastic properties of asphalt cementand sand mastic. Buttlar et al. (1999) studied the shear mod-ulus ratio of asphalt mastic to binder for validation of micro-mechanical model. In this study, for the first time, an attemptis made to characterize mastic and matrix behavior as forunderstanding moisture damage in asphalt concrete.
2. Objective and scopeThe main objective of this study is to
• Compare moisture damage of mastic and matrix to that ofasphalt concrete. Damage in mastic and matrix is definedby comparing complex shear modulus of wet to dry sam-ples tested in dynamic shear rheometer and through me-chanical tests, whereas damage in asphalt concrete is
defined by the ratio of the indirect tension strength of wetto dry samples.
• Determine the amount of moisture damage in asphalt mas-tic and matrix by comparing complex shear modulus ofwet to dry samples tested in dynamic shear rheometer. Ex-amine whether mastic type (fine, coarse, mixed) and binderPG affect the moisture damage characteristics of mastic.The scope of this study is to measure complex shear mod-
ulus of solid mastic and matrix samples using a DSR. Tradi-tionally, a DSR has been used to measure the rheologicalresponse of asphalt binders, not for mastic or matrix testing.A DSR may be suitable for testing soft samples such asbinder, mastic; however, it may not be suitable for testingmatrix or asphalt concrete which requires large loading ca-pacity and aggregate or sample size. Therefore, an attempt ismade in this study to find an appropriate mechanical testingmethod such direct tension, pull-off, direct shear test insteadof a DSR test on matrix samples.
3. Materials description
3.1. AsphaltThree mixes known to have similar moisture damage per-
formance but different field moisture damage performanceare selected. One of the mixes is a coarse mix (SP-III) andthe other two are fine mixes (SP-B, SP-C). The coarse mixhas shown some moisture damage failure in the field;whereas the fine mixes do not have such problems (Kias2008). Table 1 presents mix gradation and binder informa-tion. Mix SP-C has a maximum aggregate size of 3/4 in.whereas mixes SP-B and SP-III have a maximum aggregatesize of 1 in. If these mixes are plotted in Superpave 0.45power chart, SP-B and SP-C plots pass above their respectivemaximum density lines and mix SP-III plot passes below itsrespective density line. Therefore, mix SP-III is a coarse mixand mixes SP-B and SP-C are fine mixes. Of the fine mixes,SP-B is coarser than SP-C. As shown in Table 1, liquid as-phalt binders and fine aggregate materials corresponding tothese three mixes are also collected. Binder includes two pol-ymer modified performance grade (PG) binders: PG 70-22and PG 76-28. Fine aggregate includes crushed fines (CF)and natural fines (NF).
3.2. MasticThe CF and NF are mixed proportionally for reaching SP-
B and SP-C gradations, however SP-III contains only crushedfines. These fines are then sieved to separate the fines thatpass only #200 sieves. Fillers are mixed with PG 70-22 andPG 76-28 binders to produce mastic materials. One of themastics was prepared using only crushed fines (CF) to repre-sent the mastics of SP-III mix, and the other was preparedusing both crushed and natural fines (CF+NF) to representthe mastics of SP-B or SP-C mix. Additional mastic was pre-pared using natural fines (NF) to examine how NF mastic’sbehavior differs from CF mastic’s behavior. To keep the testmatrix simple, only CF+NF mastic representing SP-B mixproportion (CF:NF) is included in this study.
3.3. MatrixMatrix is obtained by sieving loose asphalt mixes, but not
by mixing asphalt binder with virgin aggregates. Mixes are
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made loose by heating for one hour at 160 °C (321°F) andseparating the grains by hand and spatula using a kneadingaction as the mixes cooled. Next mixes are sieved on #4sieve (0.187 in. or 4.76 mm opening), and materials passing#4 sieve are collected as matrix.
4. Test matrix and moisture condiitoningTable 2 displays the type and number of samples prepared
and tested in this study. It can be seen that DSR test is con-ducted on both mastic (i.e., NF, CF, NF+CF) and matrix(i.e., SP-B, SP-C, SP-III) samples. Triplicate samples aretested in DSR, of which one set is tested under dry condi-tions and the other set tested under wet conditions. Thus 18samples are tested for each of the mastic and matrix materi-als. Matrix is also tested in direct tension, pull-off, and directshear modes. Due to sample and testing preparation diffi-culty, only replicate samples are considered for mechanicaltests under both wet and dry conditions. A total of 12 matrixsamples are tested in tension, pull-off, and direct shear. Aver-age of two test results is reported herein. Indirect tension testwas performed on three replicate asphalt concrete samples ofSP-B, SP-C, and SP-III. A total of 18 samples are tested inindirect tension mode considering both dry and wet condi-tioning.For wet conditioning, solid samples were placed in water
under 50 kPa vacuum pressure to replace air in the samplewith water. The samples were then wrapped tight in a plasticbag and placed in a fridge at 0 °C for 24 h. After the freezingcycle, the samples were removed from the fridge, unwrapped,and placed in a water bath at 40 °C for 12 h. Next samplesare brought to room temperature in a 21 °C water bath for2 h before testing.
5. Results and discussions
5.1. Indirect tension test results on asphalt concreteIn an indirect tension test, cylindrical asphalt concrete
(AC) samples are subjected to loading across its vertical dia-metric plane with a loading rate of 50 mm/min (ASTM D4123-82). Indirect tension strength is calculated from eq. [1]
½1� St ¼ 2P
pDt
where St is the indirect tensile strength (psi), P is the maxi-mum load (lb.), t is the core length (in.), and D is the corediameter (in.). The indirect tensile strength of wet and drySP-B, SP-C, and SP-III samples are shown in Fig. 1. The ra-tio of wet to dry tensile strengths, also known as tensilestrength ratio (TSR), for all three mixes are in the range of0.93 to 0.94. This confirms that all three mixes show verysimilar moisture damage behavior in the laboratory. It can benoted that most state DOTs accept mixes having a TSR valueabove 0.80 to ensure low to no moisture damage. In that re-spect, it can be said the mixes included in this study are lesslikely to be damaged by moisture. It is also evident that theSP-C mix shows low tensile strength value compared to SP-Band SP-III mixes. Specifically, tensile strength of dry SP-Cmix is around 100 psi, whereas tensile strength of dry SP-Bor SP-III is around 130 psi. During sample splitting, probablythe crack path is less resisted by the mastic or matrix or inter-face of fine mix (SP-C) than by those of coarse mixes (SP-B,SP-III). Large size aggregates in SP-B and SP-III mix mighthave contributed to more resistance to the crack path.
5.2. DSR test results on masticMastic samples are prepared in a rectangular prism assem-
bly of bending beam rheometer mold shown in Fig. 2a. Thedimensions of the rectangular mastic sample is 50 mm(1.97 in.) by 12 mm (0.47 in.) by 6 mm (0.24 in.). Mastic sam-ples are tested in torsion in a research grade Anton-Paar Phys-ica MCR 301 dynamic shear rheometer with CTD 450temperature control unit shown in Fig. 2b. In this unit, test tem-perature is controlled to ±0.1 °C and complex shear modulus(G*) is measured as a function of temperature and frequency.There is no standard procedure for DSR testing of solid
samples; however the value of complex shear modulus canvary with the magnitude of the applied shear strain. To deter-mine a true complex shear modulus, materials have to betested in a linear viscoelastic region, where modulus is rela-tively independent of shear strain. In this study, the value ofsuch strain for mastic is determined by trials. Sixteen differ-ent strain rates are applied and at each strain rate ten datapoints were collected to determine a specific strain rate thatwould make no damage to the sample. Figure 3 shows theresults of sixteen strain amplitudes from 0.001% to 0.2% onan SP-B mastic sample. The modulus of matrix materials de-creases with increasing shear strain. It can be seen that athigh strain rates (from 0.009% to 0.2%) G* value decreases,indicating some damages might have occurred in the sample.At low strain rate, the G* data at all ten points become thesame. It became evident that at 0.007% strain rate the com-plex shear modulus G* value becomes constant. Therefore,this strain rate is selected for DSR testing on solid (both mas-tic and matrix) samples. All mastic samples are tested at ninetemperatures: –10, –5, 0, 5, 10, 15, 20, 25, and 30 °C with afrequency sweep ranging from 0.01 Hz to 16 Hz.
5.2.1. Wet vs. dry mastics of PG 70-22Figure 4 compares complex shear modulus of wet and dry
mastics with PG 70-22 at room temperature (23 °C). Overall,wet samples have smaller G* values than the dry samples athigh frequencies. The gap between the wet and dry modulus
Table 1. Aggregate gradation and mix information.
Sieve size (mm)
% Passing
SP-B mix SP-C mix SP-III mix25 100 10019 94 100 9612.5 85 92 759.5 77 89 694.75 — 65 —2.36 41 46 291.18 32 35 210.6 24 26 160.3 14 16 110.075 5.6 6.5 4.8% Binder 4.5 5.0 4.5Binder grade PG 70-22 PG 70-22 PG 76-28% Crushed fines (CF) 40 53 40% Natural fines (NF) 16 15 —
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is highest in the NF sample indicating that the highest mois-ture damage occurred in the natural fine samples. As seen inFig. 4b, shear modulus of NF sample is the smallest amongthe three mastic types (Fig. 4a and Fig. 4c). This is becausethe rounded NF particles have very little interlocking com-pared to the angular crushed fines. Interestingly shear modu-lus of CF+NF sample is shown to be greater than the shearmodulus of CF sample. It indicates that angularity is notenough to ensure high shear properties. Rather, gradationplays a role in contributing to high shear modulus. This maybe the case in the CF+NF sample. Probably, gaps within thenatural fine particles are filled by angular crushed fines and
thereby producing mastic materials that are very strong inshear. From this plot, the difference in shear modulus be-tween the wet and dry samples is not evident at low frequen-cies. A time–temperature master curve can capture all thefrequency behavior.
5.2.2. Wet vs. dry mastics of PG 76-28The complex shear modulus values of wet and dry mastics
of PG 76-28 binder are plotted in a log frequency plot, alsoknown as time–temperature master curve shown in Fig. 5.Shear modulus at different temperatures are shifted to 23 °Cto plot this master curve. Clearly, the NF mastic shows the
Table 2. Test matrix.
Asphaltconstituents Tests Materials Samples (wet and dry)Mastic DSR tests Crushed fines (CF), natural fines (NF),
CF+NF mastics3 replicates × 2 wet-dry × 3 mastics = 18
Matrix DSR tests SP-B, SP-C, SP-III matrix 3 replicates × 2 wet-dry × 3 matrix = 18Direct tension test SP-B matrix 2 replicate × 2 wet-dry = 4Pull-off test SP-B matrix 2 replicates × 2 wet-dry = 4Direct shear test SP-B matrix 2 replicates × 2 wet-dry = 4
Mix Indirect tension test SP-B, SP-C, SP-III mixes 3 replicates × 2 wet-dry × 3 mixes = 18
Fig. 1. Wet versus dry tensile strength of mix.
Fig. 2. Test setup for dynamic shear rheometer (DSR) test.
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lowest value of shear modulus. When comparing wet vs. drymodulus, both CF and CF+NF mastics have shown slightdrops in shear modulus due to sample wetting. However, wetNF mastic does not show any drop in shear modulus. Rather,there is an increase in shear modulus of the wet samples atlow frequency. Ideally, low frequency test results are similarto test results at high temperatures. The PG 76-28 binderbeing stiffer than the PG 70-22 binder at a common temper-
ature, low frequency tests may not show any moisture damagein PG 76-28 mastics. This is opposite of what has been seenpreviously in the case of NF mastic made of PG 70-22 binder.
5.2.3. Effects of mastic type, temperature, and frequency onwet and dry moduliFigures 6a to 6d are G* vs. temperature plots at four dif-
ferent frequencies for mastic of PG 70-22. Clearly, mastic
Fig. 3. Sixteen strain amplitudes applied to SP-B matrix materials passing #4 sieve.
Fig. 4. Wet versus dry mastic (PG 70-22) complex shear modulus at 23 °C.
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type (CF vs. NF vs. CF+NF) affects the G* value at lowtemperature. At high temperature, G* values are similar irre-spective of mastics types. Overall, it can be seen that the dif-ference in G* values between wet and dry samples isprominent at low temperature. At high temperature shearmodulus values of a wet and a dry sample are almost equal.It is possible that the damage that occurs due to high temper-ature may exceed the damage that occurred during wet condi-tioning. It is clear that at high temperature (i.e., 25 °C) all thesamples irrespective of wet and dry reaches to the same valueof shear modulus for all mastic. At low frequency, the differ-ence between the modulus of wet and dry samples is not soclear, rather such difference is clearly observed at high fre-quencies. Based on this, it can be said that mastic samplesshould be tested at low temperature and high frequency forconsistent results.Table 3 shows air void data of individual sample measure-
ments, mean, and standard deviations. An analysis of var-iance is also presented to assess the variability of the airvoids in the tested samples. Mastic samples were compactedwith a target air void of 4%. Based on the F-statistics, it isshown that the variance of air voids in three different samplesdoes not have any significant effects on the test results, that
is, G* values. Therefore, average value of shear modulusfrom three samples is used to interpret results in this study.
5.2.4. Wet vs. dry shear modulus ratioThe ratio of the wet to dry shear modulus is plotted
against frequencies in Fig. 7. For all mastics, except NF withPG 70-28, show shear modulus ratios of less than one, whichmeans damage has occurred in these samples at high fre-quencies. At low frequencies, this ratio is more than 1.0 inNF mastics. Clearly, modulus ratio is constant with respectto frequency. Usually, the effect of temperature on modulusis inversely proportional to the effect of frequency. Therefore,it can be said that wet to dry modulus ratio is not a functionof testing parameter such as frequency and temperature. Asmentioned previously, CF+NF with PG 70-22 mastic repre-sents the mastics of SP-B mix, and mastic of CF withPG 76-28 represents the mastics of SP-III mix. From Fig. 7a,the average shear modulus ratio of CF+NF mastic is 0.96,which is close to the SP-B mix indirect tensile strength ratioof 0.94. This indicates that the laboratory mastic shear mod-ulus ratio matches with laboratory mix indirect tensilestrength ratio for mix SP-B. However, from Fig. 7b, the aver-age shear modulus ratio of CF mastic is 0.81, which is well
Fig. 5. Wet versus dry mastic (PG 76-28) complex shear modulus at 23 °C.
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below the SP-III mix indirect tensile strength ratio of 0.94.This indicates that the SP-III mix should fail based on labo-ratory mastic shear modulus ratio (below or close to 0.80).On the other hand, mix should not fail based on the labora-tory mix indirect tensile strength ratio (well above 0.80). Thefact that SP-III mix fails in the field though it passes mixmoisture damage test, it can be said that shear modulus ratioof wet to dry mastic can be a good indicator of field moisturedamage behavior of asphalt mixes.
5.3. DSR test results on matrixThe configuration and size of the matrix samples from
DSR testing are the same as those described for mastic sam-ples previously. The applied strain amplitude is 0.007%.However, all matrix samples are tested at one temperature(i.e., 23 °C) and at four frequencies: 1 Hz, 1.59 Hz, 5 Hz,and 10 Hz instead of frequency sweep tests. Table 4 showsthe average air voids and standard deviation of the air voidsof three samples for a specific matrix type. Due to aggregateand probably gap in the passing #4 materials and rectangularmold size, it was difficult to prepare samples consistently atsmaller air voids. Therefore, target air voids for compactingmatrix sample were selected to be 11%. Though this is differ-ent from the target air voids of mastic samples (4.0%), it suf-
Fig. 6. Wet versus dry mastic (PG 70-22).
Table 3. Analysis of variance of air voids for dry mastic samples made of PG 70-22 and tested at 15 °C temperature and1 Hz frequency.
Mastic type% Airvoids
Shear modulus,G* (GPa)
% Air voids (averageand Stdev)
G* (GPa) (averageand Stdev) Fair voids Fcritical
Crushed fines (CF) 4.1 33.54.5 34.5 4.3 34.5 2396.7 7.74.2 35.6 0.21 1.05
Natural fines (NF) 4.1 15.84.2 14 4.2 15.5 190.4 7.74.3 16.8 0.10 1.42
CF+NF mastics 4.3 33.04.1 35 4.1 34.7 1186.8 7.74.0 36 0.15 1.53
Note: If Fx > Fcritical, then x significant.
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fices the purpose of this study, which is to compare wet anddry modulus of matrix but not to compare mastic with matrixmaterials. Of the many matrix samples fabricated, only sam-ples within 0.5% of the target void ratio were used in DSRtesting. One way analysis of variance is performed and re-sults are presented in Table 4. It can be seen that F-statisticsvalue for air voids is greater that the F-critical value. There-fore, the difference in air voids in three samples for a specificmatrix is not significant.Figure 8a is a bar plot of complex shear modulus (G*) of
wet and dry matrix of mix SP-B. The other mixes are notpopulated into this plot to keep it simple. Bars are separatedinto four groups based on oscillation frequencies. Clearly, the
complex modulus is lowest at 1.0 Hz and highest at 10 Hz.Overall, dry matrix samples show higher G* values than thewet matrix samples. Figure 8b shows the ratio of shear moduliof wet and dry matrix samples passing #4 sieve. The ratio ishigh in SP-C, then in SP-B, and lowest in SP-III. This meansthat the highest moisture damage occurred in the SP-III mix,then in SP-B, and lowest moisture damage occurred in SP-Cmatrix. The fact that the ratio of wet to dry shear modulus inSP-II mix is below 0.80 could be a reason why this mix failedin the field. Moisture damage in these matrix samples can alsobe explained by the parent gradation materials. The amount ofmaterial that passes through #4 sieves is about 65% for SP-Cmatrix, 57% for SP-B matrix, and 44% for SP-III matrix. In
Fig. 7. Wet to dry mastic shear modulus ratio.
Table 4. Analysis of variance of air voids for dry matrix samples tested at 23 °C temperature and 1 Hz frequency.
Matrix type% Airvoids
Shear modulus,G* (GPa)
% Air voids (averageand Stdev)
G* (GPa) (averageand Stdev) Fair voids Fcritical
Three SP-Bmatrix
11.2 49.111.5 45.3 11.2 48.8 372.2 7.710.8 52.0 0.35 3.36
Three SP-Cmatrix
11.2 57.611.8 58.9 11.4 57.4 1241.9 7.711.3 55.6 0.32 1.66
Three SP-IIImatrix
11.5 29.711.0 35.2 11.3 32.8 174.3 7.711.3 33.5 0.25 2.82
Note: If Fx> Fcritical, then x significant.
Fig. 8. DSR test results on matrix samples.
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that respect, the SP-C matrix is finer than the SP-B matrix andSP-B matrix is finer than the SP-III matrix. Probably, coarsermix has more accessibility to water and hence susceptible tomore water damage. At any rate, DSR matrix test results con-cur with the field moisture damage behavior.
5.4. Mechanical test results on matrixOnly SP-B matrix materials are considered for tension,
pull-off, and direct shear tests.
5.4.1. Matrix tension testThe cylindrical samples used for the matrix tension test are
molded inside a Harvard miniature mold shown in Fig. 9a.Two samples were prepared, one was kept dry and the otherwas wet conditioned. The testing configuration for the matrixtension test is shown in Fig. 9b. The ends of the cylindricalsample are fixed with a high strength epoxy. An interfaceplate is used to ensure that constant stress is applied over thesample end area. Epoxy is applied to the bottom interfaceplate and the sample is placed on top so that the sample setsevenly on the surface. After the bottom interface hasstrengthened, the sample is fixed to the test frame at the bot-
tom with a pin and epoxy is placed on the top sample sur-face. The crosshead is then raised so that the top post comesinto contact with the epoxy in a load free configuration. Thisprocedure was established after many attempts at preventingfailure between the epoxy and the interface plate.
5.4.2. Matrix pull-off testSample is compacted in a tin and tested in pull-off as
shown in Fig. 10a. As a first step, a coated aggregate issliced parallel to one of the fractured faces of the aggregateand then placed in the oven with the mix to heat to the com-paction temperature. The matrix material is compacted inthree lifts by tamping using a metal rod. Prior to the finaltamping of the top most lift, the fractured face of asphaltcoated aggregate is pressed against the matrix so as to ensurecontact between the matrix material and the aggregate face.Figure 10b shows the physical test setup for the pull-off test.The bottom of the sample is secured with a high strength ep-oxy to a steel plate and the plate is set on top of the tinwhich acts as a spacer that places the sample into the operat-ing range of the crosshead. The steel plate is fixed in the ver-tical direction on both sides of the sample to the crosshead
Fig. 9. Test setup for direct tension or compression of cylindrical matrix sample.
Fig. 10. Test setup for matrix–aggregate in tension.
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using a system of steel bars above and below the crossheadthat are linked with all-thread, nuts, and washers. At the topof the sample, a metal post with a flat surface facing down-ward is pinned to a joint that is designed to eliminate unin-tentional moments on the sample that are introduced duringloading. The sample is then loaded in tension at a rate of0.05 in./min. Load and displacement data are collected.
5.4.3. Matrix direct shear testMatrix material is compacted in a shear box at different
lifts. A coated fractured face of a hot aggregate is pressedonto the surface of the top lift. The top of the shear box is
pushed by a direct shear apparatus. The set screws in theshear box are removed and the height of the top of the shearbox is raised so that no matrix material impedes the shearingof the aggregate. Figure 11a shows the configuration of theapparatus before the test. The bottom of the shear box is ad-vanced to bring the top of the shear box into contact with theaggregate. Figure 11b shows a matrix sample after testing.Figure 12a is an average stress versus strain curve for two
replicate dry and wet samples. Samples are tested in tensionat a strain rate of 0.05 in./min (1.27 mm/min). The samplesfailed at the lift interfaces. The dry sample withstood aslightly higher load (i.e., 89 psi) than the wet sample
Fig. 12. Wet versus dry strength of matrix materials.
Fig. 11. Test configuration for direct shear test of matrix–aggregate interface.
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(84 psi). Tension strength ratio of wet to dry sample is about0.94, which is very close to the SP-B mix tensile strength ratio.Figure 12b shows stress–strain behavior of dry and wet
aggregate–mastic interface in pull-off. Both wet and dry sam-ples exhibit a couple thousandths of an inch of displacementand then drop quickly after the ultimate load. The slopes ofthe curves appear similar, although the ultimate load for thedry sample is (approximately 116 psi) higher than the loadfor the wet sample (65 psi). The strength of the aggregate–matrix interface for the wet sample is approximately half ofthe strength for the dry sample. This observation indicatesthat moisture significantly reduces the tensile strength of theinterface between an aggregate and a matrix.Figure 12c is stress–strain behavior of dry and wet aggre-
gate–matrix interface due to shear. The aggregate is loadedwith a shear force at a rate of 0.01 in./min. The rate was re-duced from 0.05 in./min, which was used in the above threetests, to increase the test duration for data collection pur-poses. It can be noted that during loading the amount of dis-placement of the wet sample is higher than that of the drysample. Probably, moisture has made the sample more ductileand therefore it can accommodate increased strain in shearloading.
5.4.4. RemarksIt is evident that when the samples are wetted, the matrix
tension and pull-off tests encounter a decrease in the maxi-mum stress. In both cases the mode of failure is tension andwetting of the sample led to a decrease in the maximumstress. On the other hand, shear tests show an increase inmaximum stress due to wet conditioning.
6. Conclusions
• Laboratory test results show that wet to dry tensile strengthratios of the mixes are 0.94, 0.93, and 0.93 for SP-B, SP-C, and SP-III mixes, respectively. If the tensile strength ra-tio of a mix is below 0.80, that mix fails. Based on that,none of the SP-B, SP-C, and SP-III mixes included in thisstudy are expected to fail in the field. However, it is al-ready known in this study that mix SP-III shows somemoisture damage failure in the field. Therefore, indirecttension test on mix produces false positive and negativefield moisture damage performances. To resolve this issue,this study included testing of mastic and matrix materialsin different test configurations such as DSR, direct tension,pull-off, and direct shear. It is shown that DSR testing onmastic and matrix better identifies the field moisture da-mage of asphalt mixes.
• Based on DSR complex shear modulus (G*) value, moist-ure damage in NF+CF mastic (representative of fine mix)is shown to be smaller than that in NF or CF mastic (re-presentative of coarse mix) at a temperature below 0 °C.Specifically, shear modulus of wet to dry mastic is above90% in mastics of combined crushed and natural fine mas-tic, above 80% in crushed fine mastics, and below 60% innatural fine mastic samples. Very little moisture damageoccurs in NF+CF mastic samples.
• Based on G* value, coarse SP-III matrix are more suscep-tible to moisture damage than the fine SP-B matrix.
• At high temperature, shear modulus values of wet and dry
mastic samples are almost equal. The shear modulus valueof mastic sample prepared using PG 70-22 did not differsignificantly from that of mastic sample prepared usingPG 76-28.
• Strain amplitude affects the complex shear modulus valueof mastic sample. Currently no standard value is availablefor testing solid samples using DSR in torsion mode. It isshown in this study that a strain rate of 0.007% does notaffect DSR test results and therefore, suitable for solidtorsion testing of asphalt mastic and matrix materials.
AcknowledgementsThis study is supported by the National Science Founda-
tion (NSF) through the prestigious CAREER award. NSFGrant # 0644047 belongs to the Infrastructure Materials andStructural Mechanics program. The authors would like to ex-press sincere gratitude towards Angie of Holy Asphalt Com-pany, and Parveez Anwar, State Asphalt Engineer of the NewMexico Department of Transportation. Thanks to Mr. EvanKias, former graduate student who helped with the mix me-chanical testing.
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