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Effects of Curing Time and Reheating onPerformance of Warm Stone-Matrix AsphaltImad L. Al-Qadi, Dist.M.ASCE1; Hao Wang, A.M.ASCE2; Jongeun Baek3;
Zhen Leng4; Matt Doyen5; and Steve Gillen6
Abstract: The effects of curing time and reheating on the short-term performance of stone-matrix asphalt (SMA) mixtures with variouswarm-mix additives (i.e., Evotherm, Sasobit, and foamed asphalt) was evaluated in laboratory performance tests. The laboratory tests in-cluded complex modulus, loaded wheel track, indirect tensile (IDT) strength, and semicircular beam (SCB) fracture. In the tests, plant-produced mixes that were sampled from a field overlay project were compacted in the laboratory with and without reheating, and performancetests were conducted at various curing times after compaction. The effect of curing time on mixture characteristics was dependent on themixture type and performance test considered. The mixtures containing warm-mix additives showed similar variations in mixture propertiesdue to curing time compared to the control mixture. The reheating process caused asphalt mixtures to have greater modulus, tensile strength,and rutting resistance but smaller fracture resistance. Among the mixtures containing various warm-mix additives, the mixture containingSasobit showed the relatively smallest changes in mixture properties attributable to reheating. Finally, it was discovered that the warm SMAmixtures showed variations in different performance characteristics, depending on the type of warm-mix additives and recycled materials,than the control mixture. DOI: 10.1061/(ASCE)MT.1943-5533.0000513. © 2012 American Society of Civil Engineers.
CE Database subject headings: Asphalts; Mixing; Material tests; Curing.
Author keywords: Stone matrix asphalt; Warm mix asphalt; Performance test; Curing; Reheating.
Introduction
Recently, increased environmental awareness and rising energycosts have inspired the development of alternate paving materialsthat allow lower operating temperatures but possess similar in-service performance to hot-mix asphalt (HMA). Warm-mix asphalt(WMA), which originated in Europe in the mid 1990s, appears tobe a promising paving material that addresses these issues. WMA isan asphalt mixture that can be mixed and compacted at tempera-tures lower than the required temperatures for conventional HMA.It has been proven that WMA techniques can provide a number ofbenefits, including reduction of fuel consumption and emission,extension of construction seasons, improved compaction, and
increased use of reclaimed asphalt pavement (RAP) (D’Angeloet al. 2008; Chowdhury and Button 2008).
Physical and chemical properties of the mixture can be altered,because of the different mechanisms of WMA preparation tech-niques, which can result in different short-term and long-termmechanical behaviors. A series of early studies was conductedat the National Center for Asphalt Technology (NCAT) to evaluatethe laboratory performance of asphalt mixtures with various warm-mix additives (i.e., Evotherm, Sasobit, and Aspha-Min) (Hurleyand Prowell 2005a, b, 2006). Many other researchers have evalu-ated the performance of WMA with regard to various pavementdistresses, especially the susceptibility of WMA to permanentdeformation and moisture-induced damage (Prowell et al. 2007;Wasiuddin et al. 2007; Mallick et al. 2008; Lee et al. 2009;Wielinski et al. 2009; Xiao et al. 2010).
The early-age performance of WMA is another concern becauseof its curing process. After a relatively short-period of time follow-ing construction, a time-dependent hardening, called curing, canoccur in WMA as the asphalt binder regains its original apparentviscosity and/or a certain amount of entrapped moisture is evapo-rated from theWMA. An insufficient curing time can cause deterio-ration (such as rutting) of WMA at an early stage, which canconsequently affect its long-term performance. The curing timeof WMA can be affected by additives, asphalt binder, aggregate,temperature, and other design parameters. The effect of curing timeon the moisture content and the stability of WMA prepared usingfoaming techniques has been reported by earlier researchers(Ruckel et al. 1983; Brennen et al. 1983). Their work showed astrong correlation between moisture content and mixture strength.A considerable gain in strength was observed for specimens sub-jected to short-term (i.e., within 1 day) and intermediate-term(i.e., 1–7 days) curing periods. Hurley and Prowell (2005a, b)found that the rutting potential of WMA with Aspha-Min and a2 h curing period was higher than HMA, and the tensile strength
1Founding Professor of Engineering, Dept. of Civil and EnvironmentalEngineering, Univ. of Illinois, Urbana-Champaign, IL. E-mail: [email protected]
2Assistant Professor, Dept. of Civil and Environmental Engineering,Rutgers, The State Univ. of New Jersey, New Brunswick, NJ (correspond-ing author). E-mail: [email protected]
3Research Professor, Sejong Univ., Seoul, Korea. E-mail:[email protected]
4Postdoctoral Research Associate, Dept. of Civil and EnvironmentalEngineering, Univ. of Illinois, Urbana-Champaign, IL. E-mail: [email protected]
5Graduate Research Assistant, Dept. of Civil and EnvironmentalEngineering, Univ. of Illinois, Urbana-Champaign, IL. E-mail: [email protected]
6Materials Manager, Illinois State Toll Highway Authority, DownersGrove, IL. E-mail: [email protected]
Note. This manuscript was submitted on October 23, 2011; approved onMarch 7, 2012; published online on March 10, 2012. Discussion periodopen until April 1, 2013; separate discussions must be submitted for indi-vidual papers. This paper is part of the Journal of Materials in Civil En-gineering, Vol. 24, No. 11, November 1, 2012. © ASCE, ISSN 0899-1561/2012/11-1422-1428/$25.00.
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of WMA with Sasobit within 5 days of curing was lower than orequivalent to HMA. Hence, to minimize premature deformation,WMA pavements should not be opened to heavy traffic until theWMA gains adequate stiffness.
The performance ofWMA in previous studies was mostly evalu-ated with laboratory-prepared specimens. However, the drawback ofusing laboratory-prepared specimens is that they may not representthe plant-produced mix and may have some practical challenges,especially when a foaming procedure is needed. If the loose-mixsamples are obtained from plants, reheating is needed for laboratorycompaction, which results in inauspicious binder aging and uncon-trolled curing. For example, the rut depth of reheatedWMA sampleswas three times less than that of immediately compacted samples(Wielinski et al. 2009). In another field and laboratory study con-ducted by the Virginia Department of Transportation (Diefenderferand Hearon 2008), volumetric properties and performance param-eters were tested for the WMA samples prepared immediately aftermixing in the plant and in the laboratory after being reheated. Thestudy clearly showed an increase in strength and rutting resistancewith time and reheating. Hence, the WMA specimens should betested without reheating and as soon as possible after being manu-factured in a plant or sampled from the field.
Objective and Scope
The primary objective of this study is to investigate the effectof curing time and reheating on the short-term performance ofstone-matrix asphalt (SMA) mixtures produced with various WMAadditives. To achieve this objective, the following research taskswere conducted:• Evaluated the mechanical properties of warm SMA in terms of
laboratory performance tests Comprehensive laboratory testswere conducted on the control SMA mixture and the SMA mix-tures produced with three warm-mix additives (i.e., Sasobit,Evotherm, and foamed asphalt). These tests included complexmodulus, loaded wheel track, indirect tensile (IDT) strength,and semicircular bending (SCB) fracture.
• Investigated the effect of curing time on mechanical propertiesof WMA On-site sampling and laboratory compaction wereconducted at each producer’s plant without reheating. The com-pacted samples were transported to the laboratory for conduct-ing performance tests at different curing periods (3, 6, and 12 h;and 1, 3, and 7 days) after compaction. Given that fracture prop-erties are not time-critical for traffic opening, the fracture testswere conducted at 12 h; 1, 3, and 7 days; and 3, 6, and 12 weeksafter compaction.
• Investigated the effect of sample reheating on the mechanicalproperties of WMA Loose mixes collected from the field werereheated and compacted in the laboratory. The same set ofperformance tests were conducted for the reheated specimensby following the same curing time as the mixtures that werein-situ compacted right after field sampling.
Test Materials
Warm-Mix Additives
Numerous warm-mix techniques have been developed with thegoal of either reducing the effective viscosity of the binder orallowing better workability to enable full coating and compactabil-ity at lower temperatures than a typical HMA allows. These tech-niques are typically classified into three categories: (1) organic orwax additives; (2) chemical additives; and (3) foaming techniques.
In this study, three warm-mix techniques (i.e., Sasobit, Evotherm,and foaming), one from each category, were used to produce warmSMA mixes at each producer’s plant.
Sasobit is a Fischer-Troph wax in the form of a white powderor granulate, which is a product of Sasol Wax. Sasobit has a con-gealing temperature of approximately 102°C and is completelysoluble in an asphalt binder at temperatures higher than 120°C.It produces a long-chain aliphatic hydrocarbon wax with a meltingpoint between 85 and 115°C. At temperatures below its meltingpoint, Sasobit forms a crystalline network structure in the binderand increases the viscosity and stiffness of the binder. Sasol recom-mends that Sasobit be added at a rate of 0.8–3% by mass of thebinder (Hurley and Prowell 2005b; Damm et al. 2004).
Evotherm, developed by MeadWestvaco, is a chemical packageof emulsification agent and antistripping agent. Evotherm is used toimprove aggregate coating, mixture workability, and compaction.Evotherm is typically delivered with a relatively high asphalt res-idue (approximately 70%) and stored at 80°C (Hurley and Prowell2006). Most of the water in the emulsion flashes off as steam whenthe emulsion is introduced to the heated aggregates. The productiontemperature typically ranges between 85 and 115°C.
The foaming process is produced at plant using the Accu-Sheardevice manufactured by Stansteel Inc. The Accu-Shear is a multi-purpose device that can blend multiple liquids (i.e., 70% water plus30% antistripping agent, and AD-here in this study) with liquidasphalt by mechanical shearing. By mechanically blending insteadof simply injecting, the production avoids the inherent nature oflaminar fluid flow and the foaming action is improved. Therefore,this process provides a more uniform coating of asphalt on theaggregates and increases the workability of the asphalt mixture.
Stone-Matrix Asphalt Mix Design
The SMA control mixture was a 12.5-mm binder mix and con-tained crushed gravel coarse aggregates and fractionated reclaimed-asphalt pavement (FRAP). The SMA control mixture had thenominal maximum aggregate size (NMAS) of 12.5 mm and 6.2%total asphalt content. The virgin asphalt was PG 64-22 asphaltbinder modified with 12% ground tire rubber (GTR) by weightof asphalt binder, except that the PG 70-22 binder (SBS modified)was used for the mix containing Sasobit. GTR is usually usedto improve the high temperature properties of the virgin binder.Previous research has shown that the addition of 10% crumb rubberby weight of the virgin binder can increase the PG grade of thebinder by at least one grade (e.g., from PG 64 to PG 70) (Putmanet al. 2005). No antistripping agent was used in the SMA controlmixture.
Table 1 shows the composition of asphalt mixtures producedwith three different warm-mix additives and the SMA control mix-ture. These mixes were used by Chicago area contractors on severallarge-scale expressway overlay projects. The control mixture andthe mixture containing Evotherm had 8% FRAP. However, the mix-ture containing Sasobit had 5% FRAP and 5% recycled-asphaltshingle (RAS), whereas the mixture containing foamed asphalt had13% FRAP. Aggregate gradations of the four SMA mixtures areshown in Fig. 1. The aggregate gradation met the standard IllinoisDepartment of Transportation (IDOT) requirements for SMA mixdesign, as modified by the Illinois Tollway special provisions.
Specimen Preparation
One day prior to preparing samples in the field, mixes were tested atthe production sites for maximum specific gravity and for mix volu-metrics to ensure production accuracy. Compaction was conducted
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in the asphalt plant immediately after sampling from a truck andthen transported to the quality control (Q=C) laboratory. A totalof six Pine Superpave portable gyratory compactors were used inthe compaction. After compaction and a brief cooling period, thegyratory samples were packed using PVC molds and industrialhose clamps to avoid undesirable deformations at hot temperaturesduring transportation. The specimens that needed to be tested 3 hafter compaction were transported to the laboratory at the Universityof Illinois by chartered airplane. The remaining samples were trans-ported by truck. Small coolers with ice were used for the samplesthat needed to be tested 3 and 6 h after compaction to ensure that thesamples were cool enough to be cut and tested at the specified time.The time and sample temperatures were recorded at each step ofthe process from sampling through final testing of each specimen.
In addition to the field compaction process, loose mixtures werecollected to investigate the effect of sample reheating on mixtureproperties. The loose mixtures were first preheated for 3 h and thenblended and split into smaller samples. As soon as the mixturereached the field compaction temperature, the samples wereweighed and compacted by using the portable gyratory compactors,following the same procedures used at the field plants. The targetweights used for the reheated samples were the same as the field-compacted samples.
The air voids of the compacted gyratory specimens werechecked for the specimens prepared for various laboratory tests.The results showed that the air voids were within the range of6.0� 0.5%. Generally, the control-mixture specimens had rela-tively higher air void contents, whereas the mixture specimens pre-pared with foamed asphalt had relatively lower air void contents.
Laboratory Performance Tests
Mechanical testing is a critical component in the design of asphaltmixtures and the evaluation of pavement performance. In this study,laboratory performance testing included the evaluation of modulus,
strength, rutting resistance, and fracture resistance by usingdynamic modulus, IDT strength, loaded wheel track, and SCB frac-ture tests (Fig. 2). Because of project constraints and strict timeframes, specific testing parameters were agreed upon and main-tained to ensure consistency in the testing program.
Complex (Dynamic) Modulus Test
The dynamic modulus test provides a structural characterization ofasphalt mixtures, and it is used as a major input for the AASHTOapproved mechanistic-empirical pavement design guide (MEPDG).The dynamic modulus test is performed by measuring the recov-erable vertical strain when sinusoidal vertical loads are applied tothe specimen at different frequencies. The AASHTO TP-62 wasfollowed for dynamic modulus test. For each mixture and curingperiod, three replicates were prepared for testing. In this study, dy-namic modulus tests were conducted at room temperature (25°C)with frequencies of 25, 10, 5, 1, 0.5 and 0.1 Hz. The dynamicmodulus tests were conducted by using a controlled stress mode,which produced strains smaller than 100 microstrain. This ensuredthat the response of the asphalt mixture was within the linearviscoelastic range.
Loaded Wheel Track Test
A Hamburg-type loaded wheel tester, manufactured by PMW, Inc.,was used to assess the rutting performance of mixtures. The testwas conducted in accordance with a Texas Department of Trans-portation (TxDOT) procedure (Tex-242-F), with the exception ofbeing conducted in a dry condition at 30°C. The dry condition wasselected to better represent the short-term performance immediatelyafter construction. The test was performed by rolling a 738 N(158 lb) steel wheel on the specimen surface at 50 passes a minutefor 20,000 total passes. This test is considered a torture test to com-pare potential rutting performance between different mixtures. Therut depth at a specified number of wheel passes or the number ofpasses until failure was reported.
Indirect Tensile Strength Test
The IDT strength test was used to determine the tensile strength ofasphalt mixtures at biaxial stress states. This test was performed inaccordance with AASHTOT-322-07 on a universal testing machinemanufactured by Instron, Inc. In the strength test, the specimen wasloaded until failure at a rate of 12.7 mm=min. The IDT strength wascalculated. The applied load could exceed 10 kN because of thelimit of the load cell used in the test. The strength test was con-ducted at room temperature (25°C).
Semicircular Bending Fracture Test
The fracture characterization of the asphalt mixture was conductedby using the SCB fracture test. For this test, cylindrical specimenswere sliced into 50-mm thick cylinders and cut in half along the
Table 1. Composition of Asphalt Mixtures with Various Warm-Mix Additives
Mix Ndes NMAS (mm) Binder RAP (%) RAS (%)Compactiontemp. (°C)
Warm-mix additive(% of binder)
Control SMA 80 12.5 PG 64-22 12% GTR 8 — 150 —Evotherm SMA PG 64-22 12% GTR — 125 0.5Sasobit SMA PG 70-22 (SBS modified) 5 5 125–140 1.5Foamed SMA PG 64-22 12% GTR 13 — 125 1.0
Note: NMAS = nominal maximum aggregate size; RAP = reclaimed asphalt pavement; RAS = recycled asphalt shingle; GTR = ground tire rubber; andSMA = stone-matrix asphalt.
0
20
40
60
80
100
Sieve Size (mm)
Per
cen
t P
assi
ng
Control mix and mix with EvothermMix with foamed asphaltMix with Sasobit
19.0
12.59.5
4.75
2.36
1.180.
6
0.15 0.
3
0.07
5
Fig. 1. Aggregate gradation for SMA mixtures
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diameter. A 15-mm notch was cut into each half of the specimen.The test was performed at a temperature of−12°C, which was 10°Cwarmer than the low-temperature binder grade. The test was con-ducted in a constant crack mouth opening displacement (CMOD)rate mode (0.7 mm=min) and the load, displacement, and CMODwere recorded. The work of fracture was calculated from theSCB test by taking the area under the load-CMOD curve (Li andMarasteanu 2004).
Test Results Analysis and Discussion
Variation of Mixture Properties Attributable to CuringTime
The mechanical properties of asphalt mixtures are affected by nu-merous factors, including aggregate type and gradation, bitumengrade, compaction temperature, curing and aging, antistrippingtreatments, and volumetric parameters. In this study, laboratory testresults were used to compare the performance of SMA mixturescontaining various warm-mix additives and recycled material thatwere compacted at lower temperatures than the control SMA mix-ture. The variation of mixture properties with curing time was an-alyzed. The curing time describes the possible oxidative hardeningand strength gain processes after compaction.
Figure 3(a) compares the measured dynamic modulus at 25°Cand 10 Hz for various SMA mixtures. (The modulus at otherfrequencies follow the similar trend as the modulus at 10 Hz.)The columns indicate the average value from the replicates,whereas the error bars indicate the spread of data within one stan-dard deviation. The results showed that the modulus trends withcuring time were not the same across the four mixtures. The dy-namic modulus showed an increasing trend as the curing timeincreased for the control mixture and the mixture containingEvotherm. However, for the mixtures containing foamed asphalt
and Sasobit, the dynamic modulus increased initially and then de-creased as the curing time increased. For the mixtures containingfoamed asphalt and Sasobit, the modulus after a 7-day curing timewas closer to the modulus after a 3-h curing time.
Figure 3(b) compares the measured rut depths (dry condition) at30°C for various SMA mixtures after 20,000 cycles. Generally, noclear trend was observed between the mixtures’ rutting potentialand curing time, although the rut depths of the mixtures containingfoamed asphalt and Sasobit increased after a 7-day curing time.
Figure 3(c) shows the measured IDT strength at 25°C for variousSMA mixtures. The actual tensile strength of the mixtures contain-ing Sasobit could have been greater than the measured values be-cause the maximum load limit (10 kN) was reached in the test. Itwas found that the tensile strength increased with curing time forthe control mixture and the mixture containing Evotherm. How-ever, for the mixtures containing foamed asphalt and Sasobit,the tensile strengths increased initially and then decreased as thecuring time increased. The trend of tensile strength changing withcuring time is consistent with the trend observed from the modu-lus data.
Figure 3(d) shows the measured work of fracture at −12°C forvarious SMA mixtures. Two groups of test data with Evotherm hadvery high variations and were excluded from the analysis. Giventhat fracture properties are not time-critical for traffic opening, thefracture test was conducted at a relatively longer time period aftercompaction. Generally, the mixtures’ fracture resistance decreasedslightly as the curing time increased.
Table 2 summarizes the variations of mixture properties attrib-uted to curing time for various SMA mixtures. It was usuallyhypothesized that warm mix may continue to increase in strengthover time and thus, need prolonged curing time before opening totraffic. However, the results showed that the mixtures containingwarm-mix additives had similar variations in their mechanicalproperties because of their curing times compared to the controlmixture.
Fig. 2. Laboratory test setup: (a) dynamic modulus test; (b) loaded wheel test; (c) indirect tensile strength test; (d) semi-circular beam fracture test
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The test results were analyzed by using the statistical analysissystem (SAS) program. A two-way analysis of variance (ANOVA)was performed to analyze test results with each mixture property asthe response variable. The factors considered in the analysis werecuring time and mixture type. A Fisher least significant difference(LSD) test was performed with ANOVA for multiple comparisonswithin each factor at a significant level of 0.05. The statistical sig-nificance of the changes in the mixture properties as a function ofcuring time or mixture type was analyzed. The test results wereranked using letters, and the letter was changed when the meanwas statistically different from others. The letter A was assignedto the best performer followed by other letters in alphabetical order.A double letter, such as A=B, indicates that the difference in themeans is not statistically significant and that the results could fallin either group (Ott and Longnecker 2000).
Table 3 shows the effect of curing time on each mixture propertywhen the data are clustered for all mixture types. The dynamicmodulus at 10 Hz and the rut depth measured in the load wheel
test at 20,000 cycles were used in the analysis. The results indicatedthat curing time was responsible for the significant difference inmodulus and tensile strength but only for a small or insignificantdifference in fracture and rutting resistance. As the curing time in-creased, the modulus increased but the fracture resistance slightlydecreased; whereas the tensile strength initially increased and thenslightly decreased over time. The results indicated that for compar-ing the performance among different asphalt mixtures, the mixturesshould be tested after a consistent amount of storage time followingcompaction. Otherwise, the mixture performance may be falselyranked because the mixture properties may change with curingtime.
Effect of Aging on Mixture Properties Attributable toReheating
The loose mixtures collected from the asphalt plant were reheatedin the laboratory to investigate the influence of sample reheating onmixture properties. The reheating process can artificially age the
Fig. 3.Material properties of SMA mixtures for (a) dynamic modulus at 10 Hz; (b) rut depth after 20,000 cycles; (c) tensile strength; and (d) work offracture
Table 2. Variation of Mixture Properties Attributable to Curing Time
Mixture property Data
SMA mixturea
Control Evotherm Foam Sasobit
Modulus at 10 Hz(MPa)
Average 3,838 3,761 4,380 5,838COV 19% 16% 13% 15%
Rut depth at 20,000cycles (mm)
Average 3.4 3.6 2.9 2.2COV 12% 7% 8% 13%
IDT strength (MPa) Average 0.52 0.48 0.61 0.76COV 14% 13% 16% 9%
Work of fracture(kN-mm)
Average 3.7 3.3 3.5 2.6COV 6% 3% 12% 6%
Note: COV = Coefficient of Variation.aMixture design is not the same, except for control and Evotherm.
Table 3. Fisher LSD Test Results for the Effect of Curing Time on MixtureProperties
Mixture property
Curing time
3 h 6 h 12 h 1 day 3 days 7 days
Modulus D C=D B=C A A A=BRut resistance / A A A A ATensile strength D B=C A A A=B C
Mixture property
Curing time
1 day 3 days 7 days 21 days 42 days 84 days
Fracture resistance A A=B A=B A=B B A=B
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mixture because chemical reactions may take place in the reheatingprocess regardless of the duration of heating. The reheated speci-mens were tested at the same curing time (3, 6, and 12 h; and 1, 3,and 7 days after compaction) as the specimens tested directly aftercompaction in the plant. Table 4 summarizes the variation of mix-ture properties attributable to curing time for various SMAmixturesafter reheating. Similar to the findings for the mixtures withoutreheating, the reheated mixtures containing warm-mix additivesshowed similar variations with respect to the reheated control mix-ture because of curing time.
An aging ratio was used to quantify the extent of the binderhardening effect on the mixture properties attributable to reheating.The aging ratio was calculated as the ratio of the mixture propertiestested by using the reheated specimens with respect to the mixtureproperties tested by using the specimens without reheating. Theaging ratios were calculated by using the mixture properties mea-sured at different curing times after compaction. Figure 4 comparesthe average aging ratios attributable to sample reheating for variousSMA mixtures. The results showed that the reheating processcaused asphalt mixtures to have greater modulus, tensile strength,and rutting resistance, but smaller fracture resistance. This wasexpected because the viscosity of the binder could significantlyincrease during the reheating process, making the binder stifferand more brittle. Among various mixture properties, the reheatingcaused relatively greater changes in the modulus, compared to theother mixture properties.
The results showed that the effect of reheating was more signifi-cant for the control mixture than the mixtures containing warm-mixadditives. This was likely because the reheating temperature for thecontrol mixture (150°C) was higher than the reheating temperaturefor the mixtures containing warm-mix additives (125 or 140°C),which corresponded to their compaction temperatures. Amongthe mixtures containing various warm-mix additives, the mixturecontaining Sasobit had the relatively smaller changes in mixtureproperties attributable to reheating. This is probably because the
mixtures containing Evotherm and foamed asphalt incorporatedadditional moisture to promote aggregate coating, workability,and compaction; and they were heated to a lower temperature thanthe SMA with Sasobit during mix production.
Laboratory Performance of Warm SMA
The laboratory performance of SMA mixtures with various warm-mix additives and recycled materials was compared by using theANOVAwith Fisher LSD test based on laboratory performance testresults. Table 5 shows the rank of mixture performance for eachmixture property when the data are clustered for all curing timesfor the specimens without reheating and for the reheated speci-mens, respectively. The results showed that the mixture containingEvotherm had a statistically similar modulus but a smaller ruttingresistance and fracture resistance for the specimens without reheat-ing compared to the control mixture. This could be because of theeffect of the less-aged binder and the residual moisture in the as-phalt mixture containing Evotherm; these two mixtures had thesame mixture components except for the Evotherm additive.
For the specimens without reheating, the mixtures containingSasobit and foamed asphalt showed superior performance in ruttingresistance but worse performance in fracture resistance than thecontrol mixture. This could be attributable to the combined effectsof warm-mix additives and recycled materials. The control mixturehad 8% FRAP. However, the mixture containing foamed asphalthad 13% FRAP, and the mixture containing Sasobit had 5% FRAPand 5% RAS. Previous research with RAP found that higher RAPcontent can increase mixture stiffness but decrease fracture resis-tance (Al-Qadi et al. 2009). Previous research has also shown thatthe use of roof shingles results in the increase of modulus and rut-ting resistance but lower fatigue resistance and low-temperaturecracking resistance. This is probably because of the use of higherviscosity asphalt in the shingles with the reinforcing effect of thefiber (Sengoz and Topal 2005).
However, the performance ranking among various mixtureschanges because of reheating. For example, the control mixtureand the mixture containing Sasobit had a statistically similar modu-lus after reheating. The reheated mixture containing foamed asphaltshowed superior performance in the tensile strength, rutting resis-tance, and fracture resistance. This indicates that the binder agingmechanism may change when warm-mix additives are included inthe mixture.
Conclusions
The following conclusions were obtained from this study:• The effect of curing time on mixture characteristics was depen-
dent on the mixture type and performance test considered.
Table 4. Variation of Mixture Properties Attributable to Curing Time(Reheated)
Mixture property Data
SMA mixturea
Control Evotherm Foam Sasobit
Modulus at 10 Hz(MPa)
Average 6,985 5,649 5,463 7,293COV 13% 6% 11% 11%
Rut depth at 20,000cycles (mm)
Average 2.1 2.4 1.4 1.6COV 20% 13% 11% 9%
IDT strength (MPa) Average 0.65 0.66 0.79 0.77COV 14% 17% 8% 7%
Work of fracture(kN-mm)
Average 2.5 2.7 3.3 2.3COV 9% 9% 7% 12%
aMixture design is not the same, except for control and Evotherm.
Fig. 4. Aging ratios of mixture properties attributable to reheating
Table 5. Fisher LSD Test Results for the Effect of Mix Type on MixtureProperties
Mixture property
SMA mixture
Control Evotherm Foam Sasobit
Modulus C C B AModulus (reheated) A B B ARut resistance C D B ARut resistance (reheated) C D A BTensile strength C D B ATensile strength (reheated) B B A AFracture resistance A B A=B CFracture resistance (reheated) C B A C
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The mixtures containing warm-mix additives showed similarvariations in mixture properties attributable to curing time com-pared to the control mixture. The laboratory test results indi-cated no evidence that a significantly longer curing time wasneeded before allowing traffic on WMA pavements.
• The reheating process caused asphalt mixtures to have greatermodulus, tensile strength, and rutting resistance but less fractureresistance. Among the mixtures containing various warm-mixadditives, the mixture containing Sasobit showed the relativelysmallest changes in mixture properties attributable to reheating.In addition, the performance ranking between various mixtureschanged after reheating.
• Adding Evotherm to the control mixture resulted in a statisti-cally similar modulus but less rutting and fracture resistance.Compared to the control mixture, the mixtures containingSasobit and foamed asphalt showed superior performance inrutting resistance but worse performance in fracture resistancebecause of the combined effects of warm-mix additives andrecycled materials.To further validate these conclusions, the performance of asphalt
mixtures with a broad range of binder types, aggregate sources,compaction temperatures, and various percentages of warm-mixadditives will be evaluated in future studies.
Acknowledgments
The authors would like to acknowledge the sponsorship providedby the Illinois State Toll Highway Authority. The help provided bythe staff and many students at the Illinois Center for Transportation;the S.T.A.T.E. Testing, LLC; the Applied Research Associates,Inc.; and the K-Five Construction, Geneva Construction, and RockRoad asphalt plants in the Chicago area was greatly appreciated.The contents of this study do not necessarily reflect the officialviews or policies of the Illinois Center for Transportation, theIllinois State Toll Highway Authority, or the Federal HighwayAdministration. This paper does not constitute a standard, specifi-cation, or regulation.
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