A Multiple Laboratory Study of Hot Mix Asphalt Performance Testing

Transcription

1 A Multiple Laboratory Study of Hot Mix Asphalt Performance Testing M. Heitzman 1, R. McDaniel 2, A. Shah 2, L. Myers McCarthy 3, C. Paugh 3, J. Gudimettla 3, E. Dukatz 4, G. Reinke 5 and V. Goetz 6 Abstract This paper presents the results of a collaborative study between the Iowa Department of Transportation, FHWA, Mathy Construction and the orth Central Superpave Center. All four labs tested the same plant produced mixture in a variety of ways to examine the differences in complex dynamic modulus ( E* ) testing between multiple laboratories. This effort also allowed comparison of E* test results with other performance test methods, comparison of E* from mix design to plant produced mixture, and comparison of measured E* values with predicted E* values. Three different devices were used in conducting the E* testing. The other tests conducted for comparison purposes included the flow time test, complex shear modulus using the Superpave Shear Tester and a torsional test in the Dynamic Shear Rheometer (DSR), DSR Creep Test and Hamburg rutting test. The results suggest that the differing test protocols yielded similar E* values, at least at intermediate temperatures. At higher temperatures, differences in the protocols may be affecting the results. Comparison of laboratory predicted E* with predictive equations showed good agreement at the low and intermediate temperatures. Results of testing mix design specimens0 compared well with testing plant produced mix. The E* test differentiated a mixture containing a small amount of reclaimed asphalt pavement (RAP) from a similar mix without RAP. Both predictive equations and measured E* data showed that typical mixture variation during production should not cause substantial changes in E*. The flow number results for mix design specimens and plant produced mix compared well at high temperatures. The torsional G* test showed promise as it compares well to the G* from the SST. Key Words: Dynamic modulus, E*, Field E* data, Rutting prediction, Witczak Model, Hirsch Model, master curves, hot mix asphalt, Superpave, SPT. 1 Formerly with Iowa Department of Transportation, now at ational Center for Asphalt Technology, 277 Technology Parkway, Auburn, AL, 36830, mah0016@auburn.edu 2 orth Central Superpave Center, 1205 Montgomery Street, P. O. Box 2382, West Lafayette, I 47906, rsmcdani@purdue.edu, bano@purdue.edu 3 Formerly with Office of Pavement Technology, Asphalt Team, FHWA Headquarters, now at Villanova University, Room 144 Tolentine, 800 E. Lancaster Ave., Villlanova, PA, 19085, leslie.mccarthy@villanova.edu 4 Mathy Construction Company, Materials and Research, 915 Commercial Court, Onalaska, WI, 54650, edukatz@mathy.com 5 MTE Services, Inc. 915 Commercial Ct, Onalaska, WI, 54650, greinke@mathy.com 6 Iowa Department of Transportation, 800 Lincoln Way, Ames, Iowa,

2 Introduction This paper discusses the results and findings of a multifaceted collaborative study. The Federal Highway Administration (FHWA) Mobile Asphalt Testing Laboratory (MATL) was invited by the Iowa Department of Transportation (Iowa DOT) in July 2003 to introduce facets of the FHWA Long Life Pavements program in Iowa. Field evaluation of the new asphalt performance testing technology was done on site, evaluating local materials in Iowa. The Dynamic Modulus test and Flow umber tests were run on specimens prepared by the FHWA for testing in the Simple Performance Tester (SPT). The influence of mixture volumetrics on performance in the SPT was investigated. Both laboratory blended and plant produced materials were evaluated by the FHWA laboratory. Plant produced HMA stiffness was tested by four laboratories: FHWA, Iowa DOT, orth Central Superpave Center (CSC) and Mathy Construction using a variety of test techniques. The overall purpose of the investigation was to examine the differences in complex dynamic modulus ( E* ) testing between multiple laboratories. This effort also allowed comparison of E* test results with other performance test methods, comparison of E* from mix design to plant produced mixture, and comparison of measured E* values with predicted E* values. Background The complex dynamic modulus, E*, is defined as the absolute value of the maximum (peak topeak) stress divided by the maximum recoverable (peak to peak) axial strain for a hot mix asphalt (HMA) cylindrical specimen subjected to a sinusoidal loading. The E* test is a stress controlled procedure in which an axial compressive load is applied to an HMA specimen, and the resulting applied stress and recoverable axial strain responses are measured. Views of the SPT device, equipped to run the E* test, are shown in Figure 1, along with the stress strain predictions as a result of the test (1). σ o sinωt φ/ω σ, ε σ ε 0 0 ε o sin(ωt-φ) Time, t σ 0 E* = ε Figure 1. SPT performance test equipment and resulting E* predictions 2 0

3 One benefit of using the E* test for mix design is that it yields values used in models to determine both the rutting and fatigue cracking performance properties of a mix from one test (2, 3). The test is relatively straightforward and quick to run. The SPT equipment produced from the CHRP 9 29 project is capable, on average, of running the E* test in approximately ten minutes per specimen per temperature (combined with prior specimen temperature conditioning time of two hours) (4). One of the reasons the E* test was selected for describing the mixture stiffness as part of the CHRP 1 37A mechanistic empirical design approach is that it incorporates the time response of the mix through time temperature superposition for characterizing the E* response at various temperatures (5). Field Project Description The HMA mixture selected for this study was an Iowa DOT 10 million ESAL base mix for the US 218 (Avenue of the Saints) ashua bypass in north central Iowa. Following Iowa s standard practices, the mix was designed with an ini of 7, des of 86 and max of 134. The 19 mm (¾ inch) mix was a blend of five aggregate components including a crushed limestone (23%), limestone chip (23%), limestone manufactured sand (28%), screened gravel (20%) and RAP (6%). At design, the effective asphalt binder content was 4.4%, the VMA was 14.3%, and the film thickness was 9.4 microns. The Iowa DOT mix design quality control check of the G mm values rated this mix design as excellent. Iowa uses this analysis to evaluate how well the G mm data for the design represents an acceptable range of values. The predominant limestone source had a moderate level of absorption, and the mixture was measured at 0.95% absorbed binder (aggregate basis). At this level of effective asphalt loss, the laboratory procedures must ensure that the proper amount of oven curing takes place before compacting specimens. Mathy Construction produced and placed the mixture. The plant generated approximately 4000 Mg of HMA per day. Three days of production were used to collect four samples for the round robin testing. Table 1 summarizes the results of quality control testing of the HMA for the three days. Table 1. Summary of HMA plant QC testing Measured Property Target July 14 July 16 July 17 o. 4 (% passing) 64% o. 8 (% passing) 49% o. 30 (% passing) 25% o. 200 (% passing) 5.9% Binder Content (P b ) 5.4% 5.2% 5.4% 5.5% Air Voids (P a ) (Range) 4.0% 3.5% 4.2% 4.1% ( ) ( ) ( ) Film Thickness (microns) VMA 14.3% 14.2% 15.0% 15.1% Dust to Asphalt Ratio The FHWA Mobile Asphalt Testing Lab determined the binder properties according to AASTHO M320 and MP1a. Under the M320 protocols, the binder met a PG58 28 grade (continuous grade ). Using the MP1 a protocols, the critical cracking temperature was determined to be 27, so the binder graded out as a PG58 22 (6). 3

4 Sampling and Testing Plan Sampling of the Hot Mix Asphalt (HMA) was conducted at the asphalt plant site location. HMA was loaded onto a small single axle dump truck from which the 19 mm base mixture was split into four sublots using a quartermaster device, as shown in Figure 2. The sublots were placed in cardboard boxes for storage until testing. They were then divided among the FHWA Mobile Asphalt Lab Trailer, Mathy Construction, the Iowa DOT, and the orth Central Superpave Center at Purdue University. A total of four samples were taken representing three separate days of production. Sample #1 was taken on July 14, 2003; Samples #2 and #3 were taken from the same truckload on July 15, 2003; and Sample #4 was taken on July 16, A reheating protocol was established to ensure that the HMA samples were handled in the same manner by all participating laboratories. The procedure called for the mixture in the sample boxes to be reheated in a forced draft oven for four hours at 135 C (275 F). At the end of the four hours, the boxes were removed from the oven; the HMA mixture was reblended and quartered to test sample size for the test being conducted. The test material was then placed back in the oven until the compaction temperature of 135 C was achieved, usually within two hours for the Superpave gyratory specimens. The performance test specimens were compacted approximately one hour after the specimens for volumetric determinations. Figure 2. HMA sample splitting with quartermaster device Testing Protocols The four laboratories participating in this study performed a variety of tests for comparison purposes. In addition, some tests were accomplished using different types of equipment and varied protocols. The differences in the dynamic modulus test protocols are detailed in the next section. The tests conducted included the following: Binder Characterization FHWA MATL following MP1a Complex Dynamic Modulus ( E* ) FHWA MATL used an IPC at 17, 23 and 40 C Iowa used the ottingham Asphalt Tester (AT) at 11, 17, 23 and 40 C CSC used an IPC at 17, 23 and 40 C 4

5 Mixture Complex Shear Modulus ( G* ) CSC used an Interlaken SST to perform Frequency Sweep test at 23 and 40 C Torsional Test Mathy used a Dynamic Shear Rheometer to determine G* and Flow umber (F n ) of the mix Hamburg Wheel Rutting Test Mathy performed at 50 and 58.5 C. The complex dynamic modulus is the focus of this evaluation. The other tests are supplementary tests used for comparison to the dynamic modulus. Table 2 lists the tests conducted and practices used by the FHWA MATL on the HMA samples. In addition, the mix design was replicated by the MATL. The physical properties of the aggregates were also determined. Table 2. HMA tests conducted by the FHWA Mobile Asphalt Lab Trailer Test ASTM D 4125 AASHTO T308 AASHTO PP2 AASHTO T209 AASHTO T269 AASHTO T312 AASHTO T30 AASHTO TP62 CHRP 513 Protocol Description Asphalt Content by the uclear Method Asphalt Binder Content by the Ignition Method Practice for Mixture Conditioning of HMA Maximum Specific Gravity of Bituminous Paving Mixtures, Rice Method with supplemental dry back procedures Percent Air Voids in Compacted Bituminous Paving Mixtures Preparing and Determining the Density of HMA Specimens by means of the Superpave Gyratory Compactor Mechanical Analysis of Extracted Aggregate Determining the Dynamic Modulus of Hot Mix Asphalt Concrete Mixtures Simple Performance Test System Flow umber Test The CSC followed AASHTO TP7 for the Frequency Sweep testing in the Superpave Shear Tester (7). As described earlier, the dynamic modulus test involves applying a sinusoidal axial compressive loading to a mix specimen while measuring the applied stress and recoverable strain response. The frequency sweep test also uses a sinusoidal loading of a mixture specimen, but the load is a shear load rather than an axial load. By shearing the specimen over a range of frequencies, it is possible to determine the complex shear modulus ( G* ) of the mixture and its phase angle. Mathy used their own test protocols for the torsional test and Hamburg testing. For the torsional tests, rectangular specimens are cut from a gyratory specimen or field core. Dimensions of the specimens are nominally 12 mm wide, 10 mm thick and 50 mm long. (A detailed discussion of specimen preparation is given in Reinke et al. (3)) The specimens are mounted in a dynamic shear rheometer (Figure 3). An air circulating chamber or oven is used to control the specimen temperature. An oscillatory test is performed to obtain complex modulus results. A typical test is a frequency sweep covering the range of 0.1 to 100 radians per second over the desired temperature. Strains employed are typically 0.01%. A typical G* result is shown in Figure 4. For this report complex modulus tests were conducted at 40 C to 58 C in 6 C increments and from 10 C to 25 C in 5 degree increments. Data was reported at 10 Hz for comparison to SST testing. For the mix from each sample set, three specimens 5

6 were tested for each of the temperature ranges. The G* results for the three specimens were averaged to yield the final data value. Figure 3. DSR test in the machine IOWA 218 DA 1 40 C G* ( ) [Pa] PhaseAngle ( ) [ ] Freq [rad/s] Figure 4. Torsional G* results Mathy also performed repeated creep testing on mix slices using a dynamic shear rheometer, a test they have named the DSR Creep Test. Details of this test are described in Reinke et al. (3). The tests were performed at the target temperature (40 C) using a loading time of 1 second and a zero load time of 9 seconds with a 68 kpa applied stress. The strain at the end of each 10 second cycle is determined (Figure 5), and then the point of minimum change in strain with time (strain rate) is determined for each plot. This point is analogous to the SPT Flow umber test, but because of the differences in loading times (1 second for the DSR versus 0.1 second for the SPT) and the differences in applied loads (68 kpa for the DSR versus 600 kpa for the SPT) the actual Flow umber values cannot be the same. Other test 6

7 results obtainable from the DSR Creep Test are time to 2% strain and time to 5% strain. For the DSR Creep Test, five specimens were tested, and the trimmed mean was used to obtain the final test result reported. HW 218, DA 2, 40 C, 68 kpa STRESS % Strain Global Time (Seconds) Figure 5. DSR creep result The Hamburg Wheel Tracking test was performed dry at a target test temperature of 58 C (8, 9). Sample #4 was tested first at 50 C, but no rutting occurred so that test was ignored, and the temperature was increased to 58 C for the remainder of the testing. An air heater was used to control the temperature. A wheel load of 702 (158 lbs) was supplied by each wheel to the test specimens. Test specimens (61 mm in height) were compacted from the field mix to a target air void level of 6 7% air voids. The protocols used by the different labs to run dynamic modulus were patterned after the CHRP protocol, but differed in many respects, as detailed below. Differences Between Lab Procedures for Dynamic Modulus The theory of E* measurement implies that the method of testing should not be a factor; if E* is truly a fundamental property, it should be independent of the testing device used. This conclusion will only be valid when two conditions are met. One, the test protocol measures the true response of the HMA mixture to an axial load; and two, the test is not generating a significant permanent strain in the sample. This study tests that theory. The test protocols used in this study were uniquely different, as summarized in the appendix. The table in the appendix outlines the protocol described in Appendix A of CHRP Report 465 and notes differences in the protocol used by each of the labs in the study. Readers should refer to the CHRP report for the specific details of each step. one of the labs in this study used the CHRP protocol exactly. The following paragraps highlight some of the differences. The FHWA lab used the current protocol model of the IPC that was built as a result of the CHRP 9 29 project (10, 11). This device had an environmental chamber that did not accommodate both a test specimen and a dummy specimen with a temperature probe. Therefore, the dummy specimen was kept with the test specimens as they were conditioned in either the oven or water bath. The LVDT system used magnets to mount the LVDTs to the studs fixed on the specimen. The low friction medium between the specimen and the end platens was a silicone coated paper, and the end platens had polished surfaces. The axial deformation instrumented length was only 70 mm. The loading sequence 7

8 applied ten preconditioning cycles and ten test cycles at each frequency. Each preconditioning phase established a new loading rate to meet the micro strain limits. All ten test cycles were used in the calculation of E* and phase angle. The SCS lab equipment followed the CHRP protocol very closely. Parallelism of the cut faces was verified using a set square. The specimen diameter was assumed to be fixed by the core bit diameter. Specimens were stored for more than 14 days before testing, due to equipment problems that delayed testing. The contact load was approximately 10% of the test load. After the preconditioning cycles, the test cycles for each frequency were generally doubled and the last five cycles were captured for analysis. The limiting permanent strain was 1500 με. The Iowa DOT lab procedure involved several deviations from the prescribed protocol. A betaversion of software was written for the AT to operate the equipment and collect the data following the CHRP protocol. The software was not programmed to collect all the data from the last six cycles and did not properly compute the phase angle. Only two LVDTs were mounted to the specimen. The end platens were polished steel, and no low friction medium was used. The specimen was gyratory molded to be 100 mm in diameter by 150 mm in height. Specimens were not wrapped during storage and were not discarded after 14 days. The contact load was set at 30. The operating software automatically reduced the test load by 50% during the test when the total permanent strain reached 1000 με, and specimens were not discarded for exceeding that strain level. Each of the test protocols deviate from the CHRP protocol. This study provides some insight into how significant the differences are. The FHWA procedure used magnetic LVDT mounts and significantly limited the number of test cycles at higher frequencies. The CSC procedure significantly increased the number of test cycles and permitted a higher level of permanent strain. The Iowa DOT procedure used a molded specimen and only two LVDTs. All of the dynamic modulus testing was done in the unconfined state, which means that a confining pressure of 0.0 kpa was applied. Calculation of Test Temperatures The dynamic modulus tests were conducted at the effective pavement temperatures for rutting and for fatigue determined for the project location. The effective pavement temperature for rutting is calculated using equations developed by Witczak el al. (12) The T eff rut was calculated as 40 C for a surface lift using that approach. The effective pavement temperature for fatigue can be calculated using two different approaches. The T eff fatigue of the surface layer, computed using the SHRP approach (13, 14), was 23 C. However, there are other existing methods for computing the T eff fatigue and, at this time, the suggested approach has not yet been presented from the CHRP 9 19 project. A second method for predicting T eff fatigue uses Witzcak s Asphalt Institute equation (15). Using this approach, the T eff fatigue was calculated as 11 C, which is below the range of temperature testing capabilities (approximately 13 C to 60 C) of the IPC SPT. In order to approximate for a range of T eff fatigue, the E* test was run incrementally by FHWA at temperatures of 17 and 23 C. In this way, a range of temperatures was bracketed and it was possible to extrapolate down to a T eff fatigue of 11 C and approximate what the E* would be at that temperature. Iowa was able to test at 11 C in the AT. The CSC tested at 17, 23 and 40 C. In order to compare the complex shear modulus from the SST to the dynamic modulus, the CSC ran the Frequency Sweep test at 23 and 40 C. These temperatures are reasonable temperatures at which to run this test, as it is typically run at 20 and 40 C. 8

9 Mathy performed the torsional tests for complex shear modulus at a range of temperatures from 40 to 58 C at 6 increments and a 10 to 25 C at 5 increments. This allows comparison of their G* results to the CSC results at 40 C. Mathy also performed the DSR creep test at 40 C. The Hamburg test was conducted for comparison purposes at 58 C. Test Results The data from the various laboratories is summarized individually, then the results are compared between labs and between test methods. The statistical analysis of the individual labs E* data is presented in the section on Dynamic Modulus Comparisons Between Labs and Between Tests. Standard Analysis of Variance (AOVA) techniques were used to compare the test results from different laboratories, different samples and different tests. The SAS software was used for this analysis. In order to compare mean values from two different treatments, the mean and variance (standard deviation) must be calculated for each treatment. Once these are known, hypothesis testing or significance testing can be used to determine if the mean values are the same or different. The so called null hypothesis, H 0, is that there is no difference between the means. The alternative hypothesis, H 1, is that the means are different. The p value is a statistic commonly used to draw conclusions from a comparison of means at any given α level. The p value is the probability that result of a test could have been more extreme than the observed result, if the means are in fact equal (null hypothesis is true). If the p value is greater than the desired α level, the null hypothesis is accepted. If the p value is lower than the desired α level, the null hypothesis is rejected and the means are not equal. In other words, if the p value is higher than the α level (often set at 0.05), it indicates the result is consistent with the null hypothesis. If the p value is low, it implies that the result is very unlikely (low probability) if the means are equal and so the null hypothesis should be rejected. Comparison of means tests can be used to determine which means are different. These tests, like the Bonferroni and Scheffe tests and others, group data into groupings that are not significantly different from each other. For example, if Samples #1 through #4 fall into the same group, there is no significant difference between the means. If Sample #1 is in a different group from Samples #2, #3 and #4, then there is a statistically significant difference between Sample #1 and the other samples. Sometimes means will be divided into groups that overlap, making differentiation between the groups problematic. This generally means that the variability in the test results is such that clear conclusions cannot be drawn. In addition, for some data, linear regression was used to estimate trend lines and correlations (R 2 values). FHWA Test Results The following summarizes the FHWA s experimental testing plan and the results of dynamic modulus and flow number testing. A test matrix was selected that allowed FHWA to compare the mix design material with plantproduced HMA. Four replicate specimens were prepared from each of the four sets of samples. This sampling approach yielded 16 compacted specimens. 9

10 Four mix design specimens were produced at each binder content (4.9, 5.4 and 5.9%), based on the mixture with RAP, yielding 12 compacted specimens. In addition to the specimens prepared above, some additional specimens were prepared for each mix design replication and plant produced sample in order to be tested in the Flow umber test without prior performance of the Dynamic Modulus test. The Dynamic Modulus test was run in accordance with the existing draft protocols from the CHRP 9 19 and CHRP 9 29 projects. The testing specifications indicated that the Dynamic Modulus test be run at five frequencies (0.1, 0.5, 1, 5, 10, and 25 Hz), representing traffic loads traveling at low to high speeds. An axial stress of 600 kpa was applied to simulate mixed traffic loading with an average tire contact stress of approximately 85 psi. Dynamic modulus tests were run in order to obtain the E* for a pavement both at the effective pavement temperature for fatigue damage and at that for rutting. The repeated load test (Flow umber test) was run at an axial stress of 600 kpa and a contact stress of 30 kpa. The pulsating load was applied for 0.1 second, followed by a 0.9 second rest period. This Flow umber test is used to predict the mixture performance in terms of rutting; therefore, in this experiment it was run at the effective pavement temperature for rutting for the surface layer. This test was also run on specimens in the unconfined mode. Though the Dynamic Modulus test is a non destructive test, it imparts some amount of strain on the specimen. To observe if the Dynamic Modulus test had any effect on the Flow umber Test results, some specimens were run in the Flow umber test alone. The test temperature for these specimens was the same as the effective pavement temperature for rutting. The complex dynamic modulus E* was determined at the effective temperatures for fatigue cracking and rutting (23 C and 40 C, respectively). Additionally the dynamic modulus was also measured at 17 C. The average E* values measured for the three point mixture design specimens and the plant produced specimens are plotted in Figure E* (MPa) 1000 Average E* for Mix Designs at 17 C Average E* for Plant Produced at 17 C 100 Average E* for Mix Designs at 23 C Average E* for Plant Produced at 23 C Average E* for Mix Designs at 40 C Frequency (Hz) Average E* for Plant Produced at 40 C Figure 6. Average E* for mix design and plant produced HMA specimens. The mix design replication specimens exhibited higher E* values than the plant produced mix specimens at the temperatures of 23 C and 40 C and lower values at 17 C. The individual and average values of E* for the plant produced specimens are plotted along with the average values for the mix design specimens in Figure 7 for fatigue and in Figure 8 for rutting. It should be noted that the mixture produced on Day 1 (Sample #1) was a virgin mix without RAP; the 10

11 modulus of this material is lower, as expected in the absence of the stiffening effect of the RAP. An overall observation of Figures 6, 7 and 8 is that there is relatively little difference between the mix design replicates and the plant produced mix E* (MPa) 1000 Mix Design Average Plant Produced Average sample1, Plant Produced sample2, plant produced sample2, Plant Produced sample3 Plant produced Frequency (Hz) Figure 7. E* for mix design and plant produced HMA specimen at 23 C E* (MPa) M ix design average Plant produced average Day 1 plant produced Day 2 plant produced Day 2 plant produced Day 3 plant produced Frequency (Hz) Figure 8. E* for mix design and plant produced HMA specimens at 40 C Flow umber test results for the mix design and plant produced mixtures are presented in Figure 9. The average microstrains at the flow point are plotted for specimens tested at 40 C. Little variation between the mix design specimens and the plant produced mix is observed. A lower amount of microstrains at flow point is indicative of more cycles required to induce flow and consequently better rut resistance. These results correlate well with the Dynamic Modulus test results which also indicated that the mix design replicates were performing slightly better than the plant produced mixes at higher temperatures. 11

12 This data indicates that results from the performance specimens are strongly tied to volumetrics. The plot confirms that binder content has an influence on the amount of flow that occurs in the specimen. The mix design specimens show increasing levels of microstrains at flow with increasing amount of binder content. In the plant mixes, results from the flow number test detect the absence of RAP in Day 1 mixture, as the high number of microstrains at flow indicates. The other observation to be made is that again the mix design replicates and plant produced specimens exhibit similar flow characteristics Microstrains at Flow Point MD4.9% MD5.4% MD5.9% Plant Day1 Samp1 Plant Day2 Samp2 Plant Day2 Samp3 Plant Day3 Samp4 Figure 9. Microstrains at Flow umber for mix design and plant produced mixes Figure 10 plots the microstrain at flow number values for specimens that were subjected to prior Dynamic Modulus tests and specimens that were not. The plot illustrates that the overall trend for both appears to be similar. However, the magnitude of the microstrains at flow number was greater in specimens which were not subjected to Dynamic Modulus tests. This may be because specimens subjected to Dynamic Modulus tests have already undergone some amount of strain during the Dynamic Modulus test. 12

13 Microstrains at Flow Point Samples with prior E* Test Samples without E* Test 0 MD4.9% MD5.4% MD5.9% Plant Day1 Samp1 Plant Day2 Samp2 Plant Day2 Samp3 Plant Day3 Samp4 Figure 10. Effect of Dynamic Modulus test on the Flow umber test Iowa Test Results For each sample of plant produced HMA collected, four individual specimens were molded and tested by the Iowa DOT. The Dynamic Modulus ( E* ) was measured for each of the different specimens. The measurements over a range of frequencies were made at temperatures of 11 C, 17 C, 23 C, and 40 C. The resulting average E* values for all 16 specimens of each frequency/temperature combination are plotted in Figure Dynamic Modulus E* (Mpa) E* at 11ºC E* at 17ºC E* at 23ºC E* at 40ºC Frequency (Hz) Figure 11. Average E* for plant produced HMA samples 13

14 Figure 12 illustrates the comparison of the values of E* at 23 C for Samples #1 though #4 (average of 4 specimens each) to the 16 specimen mixture average. Similar results were obtained at 11 and 17 C Dynamic Modulus E* (Mpa) ºC Average 23ºC Sample 1 23ºC Sample 2 23ºC Sample 3 23ºC Sample Load Frequency (Hz) Figure 12. E* for Samples #1 to #4 and Average at 23 C As can be noted in Figure 12, there is a significant difference between the stiffness measured for Sample #1 compared to Samples #2, #3, and #4. The material collected for Sample #1 did not contain any RAP, and the results indicate that the presence of RAP in the material impacts the stiffness of the pavement at the lower temperatures. This difference increases as the test frequency increases. These trends were clearly observed at all test temperatures, though it was not as pronounced at 40 C due to greater variability in the results for Samples #2, #3, and #4. As was stated earlier, four individual specimens were formed for each of the four samples collected from the plant produced HMA mix. The complex dynamic modulus measurements taken from each specimen at 11 C are shown in Figure 13. This figure illustrates the overall variability in the testing results for the individual specimens from each sample of mix. There do not appear to be significant differences in the range of replicate specimen E* values for most of the samples. 14

15 15000 Dynamic Modulus E* (Mpa) Sample 1, 4 Specimens Sample 2, 4 Specimens Sample 3, 4 Specimens Sample 4, 4 Specimens Frequency (Hz) Figure 13. E* variation between specimens at 11 C Overall, the Iowa DOT E* testing generated reasonable results. The testing properly distinguished between significant mixture differences (with and without RAP). The three samples of similar mixture have comparable results. CSC Test Results below. The results of the CSC complex dynamic modulus and complex shear testing are described Dynamic Modulus Results The E* test was conducted at the CSC at 17, 23 and 40 C on four replicate specimens from each set of samples. As illustrated in Figure 14, there was less variability at 40 C than at the cooler temperatures. Typically, the results from Sample #1, the mixture without RAP, showed lower moduli than the other samples at all temperatures and frequencies. The coefficient of variation in E* values ranged between 3% and 22%, which is higher than that observed in the other laboratories. Figure 15 shows the complex dynamic modulus as a function of frequency. In general, the Sample #3 data showed higher E*, while Sample #1 data showed the lowest at all temperatures and frequencies. As the frequency increases, the E* increases, as expected. The moduli at 23 C are somewhat lower than the results at 17 C, while the results at 40 C are much lower, showing the dependence of modulus on temperature. 15

16 E* at 25 Hz, MPa C 23 C 40 C Test Temperature sample1 sample2 sample3 sample4 Figure 14. Complex dynamic modulus versus temperature C 23 C E*, MPa C sample1 sample2 sample3 sample Frequency, Hz Figure 15. Complex dynamic modulus versus frequency CSC Complex Shear Modulus Results At the CSC, frequency sweep testing was conducted at 23 C and 40 C on three replicate samples per each mix, compacted to 6 ± 0.5% air voids. The results of this testing at 10 Hz are shown in Table 4. 16

17 Table 4. Statistical summary of the G* data at 23 C and 40 C Temperature C ( F) 23 (73) 40 (104) Sample M ean G* Std. Deviation Coefficient of Variation, MPa (psi) MPa (psi) % (227408) 122 (17726) (184329) 38 (5480) (202165) 218 (31659) (235586) 400 (58062) (34751) 26 (3752) (44864) 24 (3467) (33409) 12 (1718) (53857) 71 (10263) 19.1 The variability in shear modulus ranged from 3% to 24.6% (in terms of coefficient of variation). At the higher test temperature of 40 C, the variability appears to be slightly lower than that obtained at the lower test temperature. Sample #4 had the highest modulus and highest variability at all temperatures and frequencies. Since Samples #2 and #3 were taken from the same truckload of mix, the material would be expected to be the same. Therefore, a simple two sample t test was run on Samples #2 and #3 data to verify the null hypothesis that these two sample sets were similar, i. e., showed no difference in mean G*. The test indicated that the two samples were statistically indistinguishable at 23 C (p value = ), as would be expected. This did not hold true at 40 C (p value = ), perhaps because of high variability. AOVA on CSC G* Data AOVA tests were run on G* data obtained by CSC lab at 23 C and 40 C to look for differences between the samples. These data were analyzed in three different combinations. First, the mean G* of the non RAP sample set (#1) was compared with the combined mean G* of the other three RAP samples. o differences in their means could be detected at an α level of In the second case, data from the samples obtained on Day 2 (Samples #2 and #3) were combined and treated as one sample set. This mean G* was compared with the mean G* of the non RAP sample (#1) and that of Sample #4. In this case, AOVA indicated no differences in the mean G* at 23 C. While the test at 40 C indicated that the mean G* was significantly different, further tests (Bonferroni and Schéffé) did not indicate clear demarcation between the three different sample sets (#2/3, #1 and #4). Lastly, when each of the four sample sets was treated as a unique set, the means at 40 C were significantly different, but comparison of means tests showed overlapping groups, as in the second case. Comparison between CSC Mix G* and E* The complex shear moduli obtained from the CSC testing described above were compared with the complex dynamic moduli of the same mixtures, at corresponding test temperatures and at the common frequency of 10 Hz. Regression lines were plotted to obtain R2 values at both test temperatures. (See Figures 16 and 17.) Comparable R 2 values of about 83% were obtained at both temperatures. The slope of the regression line at 40 C was 2.5. This is in general agreement with the common assumption that E* 3 G*. At 23 C, the slope value was 7.5, which does not correspond to the general rule of thumb. 17

18 E*, MPa y = x R 2 = G*, MPa Figure 16. E* versus G* at 40ºC and 10 Hz E*, MPa y = 7.47x R 2 = G*, MPa Figure 17. E* versus G* at 23ºC and 10 Hz Mathy Test Results The results of torsional, DSR Creep and Hamburg rutting tests performed by Mathy are described below. Mathy s results from testing the mix in the DSR to determine the complex shear modulus are shown in Figures 18 and 19. The results presented show the generally good agreement of G* results for each day s mix at 40 C. (The two samples taken on Day 2, Sample #2 and #3, were combined for the Mathy testing.) Graph 4 clearly shows that for the mixes tested at the Mathy lab the Day 3 mix (Sample #4) was substantially higher in modulus than the mixes for the other two days. 18

19 Hwy 218, Day 1, 40 C, 3 TESTS and AVERAGE Hwy 218, Day 2, 40 C, 3 TESTS and AVERAGE G* Hwy 218, Day 2, 40 C, 3 TESTS and AVERAGE Temp = 40 C Hwy 218, Day 2, , 5_7% AV #2 Temp = 40 C Hwy 218, Day 2, , 5_7% AV #3 Average Temp = 40 C Hwy 218, Day 2, , 5_7% AV G* ( ) [Pa] G* Hwy 218, Day 1, 40 C, 3 TESTS and AVERAGE Temp = 40 C Hwy 218, Day 1, , RAP, 7% AV, #3 Temp = 40 C Hwy 218, Day 1, , RAP, 7% AV, #2 Average Temp = 40 C Hwy 218, Day 1 G* ( ) [Pa] Freq [rad/s] Freq [rad/s] 10 9 Hwy 218, DA 3, G* 40 C, 3 TESTS & AVERAGE 10 9 Average G* RESULTS Temp = 40 C Hwy 218, Day 1, 2, 3 G* ( ) [Pa] 10 8 G* Hwy 218, Day 3, TEST #1 Temp = 40 C Average Hwy 218, Day 3, , A-1,2, 6_8% 40C Hwy 218, Day 3, TEST #2 Temp = 40 C Hwy 218, Day 3, TEST #3 Temp = 40 C G* ( ) [Pa] 10 8 G* Average Temp = 40 C Hwy 218, Day 1, 2, 3 Average Hwy 218, Day 3, , A-1,2, 6_8% 40C Average Temp = 40 C Hwy 218, Day 2, , 5_7% AV Freq [rad/s] Freq [rad/s] Figure 18. G* data from torsional tests plotted as a function of frequency at 40 C for Days 1, 2 and 3 and averages at 40 C 0

20 Table 6 summarizes the results of the Hamburg testing along with torsional G* test results at 23 and 40 C and DSR Creep results at 40 C. Figure 19 shows regression lines of the torsional and DSR creep results versus the Hamburg rutting results. Since there are only three samples of data to plot, the regression data should be viewed more as trends than predictive. The correlation of Hamburg rutting to complex shear modulus, flow time and time to 5% strain is quite good, as indicated by the high R 2 values. The trends also agree with expectations; that is, as modulus increases, the rut depth decreases. Table 6. Summary of the Hamburg rutting test Sample Rut at HWT G* at 40 C, G* at 23 C, Flow time at Time to 5% strain passes, mm MPa MPa 40 C, s at 40 C, s / Figure 19. Hamburg rut depth versus number of passes R 2 = R 2 = R 2 = R 2 = G*@40 C, MPa G*@23 C, MPa Flow 40 C Sec to 5% Strain Log. (G*@40 C, MPa) Log. (G*@23 C, MPa) Power (Flow 40 C) Power (Sec to 5% Strain) HWT Rut Passes, mm Dynamic Modulus Comparisons Dynamic modulus testing was conducted at 17 C, 23 C and 40 C by three labs: the FHWA, IDOT and CSC. (In addition, IDOT also ran tests at 11 C, but because they were the only lab to actually test at that temperature, those results are not presented here.) In most cases, four replicate samples were tested with 6 ± 0.5% air voids. Table 7 shows the summary statistics for the E* results at 25 Hz from all three labs. AOVA tests were performed using these data from all the labs to look for (a) differences in mean E* between labs at each test temperature and each sample set, and (b) differences in mean E* between sample sets at each lab and at each test temperature. 0

21 Tables 8 and 9 show the p values obtained corresponding to the null hypotheses being tested. Results shown in Table 8 indicate that Sample #1 (non RAP) and tested at the three labs produced similar E* values; whereas the Sample 4 mixtures showed more variability and dissimilar E* values between the labs, at all test temperatures. Samples #2 and #3 were collected on the same day. These samples produced mixed results. The discussions of individual lab s results below refer to the analysis in Table 9. AOVA on IDOT E* Data IDOT test results indicated that the mean E* of mixes obtained on different sample sets were not similar. Bonferroni and Schéffe comparison methods were used to examine which samples were similar or dissimilar. These tests showed that the E* values from Sample #1 were significantly different from the other samples at all test temperatures. AOVA on CSC E* Data Similar AOVA tests were conducted on data from CSC lab. At the lowest temperature (17 C), no significant difference in mean E* was observed between the four sets of samples. At higher temperatures (23 C and 40 C), a difference in the mean E* values between the four sample sets was indicated by the low p value; however, no clear grouping was obtained. This could be due to the high variability in the test results, which masks any true differences that might exist between the mixtures. The E* of the specimens from Sample #1 did rank the lowest, as expected for mixture without RAP. AOVA on FHWA E* Data FHWA results showed no significant difference at high temperature (40 C). At lower temperatures (17 C and 23 C), low p values were obtained; but the groups overlapped. Again Sample #1 mixtures, without RAP, ranked the lowest. Two Sample t test on E* Data Two sample t tests were conducted on the data from each lab to verify the null hypothesis that samples #2 and #3 are similar, in terms of E*. In almost all cases, the results indicate that there is no significant difference between samples #2 and #3 (p values between and 0.675). For the FHWA at 17 C and the CSC at 23 C, low p values (0.032 and 0.016) indicate there may be some difference in the means of samples #2 and #3. This may be due to variations in sampling the mixture from the truck, sample preparation or testing. onetheless, the preponderance of the results strongly suggests that there is no significant difference between the two samples, as expected for samples from the same truckload. This suggests that normal variation in the mixture will not cause significant differences in E* results. 1

22 Temperature C ( F) 17 (63) 23 (73) 40 (104) Table 7. Statistical summary of the E* data (25 Hz) for all labs, at all test temperatures Mean E*, σ Mean E*, σ c. v., Mean E*, σ c. v., % Sample MPa (psi) MPa (psi) MPa (psi) MPa (psi) % MPa (psi) MPa (psi) IDOT FHWA CSC ( ) (57435) ( ) (57870) ( ) (82816) ( ) (93259) ( ) (83687) ( ) (325175) ( ) (148954) ( ) (68313) ( ) (197251) ( ) (63091) ( ) (138221) ( ) (42931) (791181) (66572) (677906) (98771) (759853) (105587) ( ) (66717) (807425) (90068) (784219) (140106) (974509) (92099) (913448) (52794) ( ) (169984) ( ) (42496) (776387) (62076) (920700) (24511) (225824) (18420) (208129) (23786) (223068) (21321) (314297) (20740) (239312) (27412) (255702) (46122) (289060) (25962) (260198) (36114) (311541) (30023) (368541) (30748) 3 (241053) (23061) (250625) (38580) c. v., %

23 Table 8. AOVA results for comparison of mean E* between labs, on each day, at different test temperatures Temperature ull hypothesis 17 C μ ia = μ sc = μ fh 23 C μ ia = μ sc = μ fh 40 C μ ia = μ sc = μ fh Temperature ull hypothesis 17 C μ ia = μ sc = μ fh 23 C μ ia = μ sc = μ fh 40 C μ ia = μ sc = μ fh ull hypothesis statement o differences in mean E* between labs o differences in mean E* between labs o differences in mean E* between labs ull hypothesis statement o differences in mean E* between labs o differences in mean E* between labs o differences in mean E* between labs p value Conclusion Grouping p value Conclusion Grouping SAMPLE #1 SAMPLE # Accept Accept Accept Reject Accept Reject SAMPLE #3 SAMPLE # Accept Reject Reject Overlap [SC, IA] [IA, FH] Reject Accept Reject [IA] [FH, SC] Overlap [IA, SC] [SC, FH] Overlap [IA, SC] [SC, FH] [IA] [SC] [FH] [IA] [SC, FH] ote: IA = Iowa, SC = Superpave Center, FH = FHWA 0

24 Table 9. AOVA results for comparison of mean E* Fixed variables ull hypothesis ull hypothesis statement p value Conclusion Grouping IDOT 17 C, IDOT μ 1 = μ 2 = μ 3 = μ 4 o differences in mean E* between days Reject [4, 2, 3] [1] 23 C, IDOT μ 1 = μ 2 = μ 3 = μ 4 o differences in mean E* between days Reject [4, 2, 3] [1] 40 C, IDOT μ 1 = μ 2 = μ 3 = μ 4 o differences in mean E* between days Reject [4, 2] [2, 3] [1] FHWA 17 C, FHWA μ 1 = μ 2 = μ 3 = μ 4 o differences in mean E* between days Reject 23 C, FHWA μ 1 = μ 2 = μ 3 = μ 4 o differences in mean E* between days Reject 40 C, FHWA μ 1 = μ 2 = μ 3 = μ 4 o differences in mean E* between days Accept CSC 17 C, CSC μ 1 = μ 2 = μ 3 = μ 4 o differences in mean E* between days Accept 23 C, CSC μ 1 = μ 2 = μ 3 = μ 4 o differences in mean E* between days Accept 40 C, CSC μ 1 = μ 2 = μ 3 = μ 4 o differences in mean E* between days Reject Overlap [3, 2] [2, 4, 1] Overlap [3, 2, 4] [2, 4, 1] Overlap [3, 4] [4, 2, 1] Overlap [3, 2, 4] [2, 4, 1] 1

25 Comparison of Measured and Predicted E* Data In addition to comparing measured values of E* among laboratories and test procedures, an investigation was also conducted to determine the correlation between measured and predicted E*, by utilizing both the Witczak (16) and Hirsch models (17, 18). Figure 20 shows the E* values predicted by the Witczak and Hirsch models along with the measured data from the FHWA, Iowa and CSC labs at different temperatures for Sample #1. The measured E* values obtained from the three labs at 25 Hz generally compared well with the predicted values obtained using the two models. At higher temperatures, the two models appear to yield E* values that are close to each other. At lower temperatures, the differences in predicted E* widen. Data from the Iowa DOT tended to be higher than the other labs, except for Sample #3, where the CSC data was highest. In all cases, data from the FHWA MATL agreed closely with the predicted E* values. It should be noted that the MATL is the most experienced lab in running the dynamic modulus test, compared to the other labs. E* (MPa) Witczak Hirsch FHWA IDOT CSC Temperature ( C) Figure 20. Sample #1 predicted versus measured E* Mathy did a comparison of the E* values predicted for plant produced mixtures at average, minimum and maximum gradation and volumetric results from the plant quality control data. The comparison showed that the predicted values, using both the Witczak and Hirsch models, were insensitive to normal construction variability, as illustrated by one example in Figure 21. This is reassuring, as one would not expect reasonable amounts of production variability to cause large differences in mixture performance. This also agrees with the two sample t test results comparing measured E* values for Samples #2 and #3. 0

26 E* (psi) Mix Design-Virgin Mix Design-RAP Production-Average Production-minimum Production-Maximum Temperature, C Figure 21. Predicted E* for mix design with and without RAP and plant produced mix at upper, lower and average gradation and volumetric properties Other Comparisons The complex shear moduli determined from the SST and the torsional test at 10 Hz were compared. At 23 C, the p value was 0.177, indicating the means are not significantly different. At 40 C, the p value was 0.572, again indicating no significant difference in means. This is a limited comparison, but does suggest that the torsional test compares well with the SST results of the Frequency Sweep test. Conclusions and Recommendations Several observations were made as a result of the multi laboratory research investigation. For example, although the E* test protocol for each laboratory was different, all laboratories recorded similar material responses at low to intermediate temperatures which indicates that different E* testing protocols might be used to achieve comparable measured E* values. However, the laboratories measured material responses at higher temperatures which differed, indicating that some criteria of E* testing protocol are critical for ensuring that the test measures the true response of the HMA. Likewise, comparison of laboratory predicted E* with the predictive equations showed good agreement at the low and intermediate temperatures. Both material from laboratory blended mixture design and plant produced samples was evaluated in the FHWA laboratory, but in the multi laboratory study mix design aggregate and binder samples were obtained from the plant during production. Although some statistical differences were found, comparisons between laboratories and between production days were quite good overall. It was important to find that laboratory testing was capable of detecting the difference in mixture stiffness for the mix without the RAP (Sample #1), despite the fact that the percentage of RAP was fairly low on the other production days. Dynamic modulus test results appear to be sensitive and reproducible at the intermediate temperatures used to define fatigue cracking performance. However, differences between test results at the high temperature used to define rutting performance may indicate that dynamic 1