Comparison of asphalt mixture specimen fabrication methods and binder tests for cracking evaluation of field mixtures

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1 Road Materials and Pavement Design ISSN: (Print) (Online) Journal homepage: Comparison of asphalt mixture specimen fabrication methods and binder tests for cracking evaluation of field mixtures Jo Sias Daniel, Matthew Corrigan, Christopher Jacques, Rasool Nemati, Eshan V. Dave & Ashton Congalton To cite this article: Jo Sias Daniel, Matthew Corrigan, Christopher Jacques, Rasool Nemati, Eshan V. Dave & Ashton Congalton (2018): Comparison of asphalt mixture specimen fabrication methods and binder tests for cracking evaluation of field mixtures, Road Materials and Pavement Design, DOI: / To link to this article: Published online: 02 Feb Submit your article to this journal View related articles View Crossmark data Full Terms & Conditions of access and use can be found at

2 Road Materials and Pavement Design, Comparison of asphalt mixture specimen fabrication methods and binder tests for cracking evaluation of field mixtures Jo Sias Daniel a, Matthew Corrigan b, Christopher Jacques c, Rasool Nemati a, Eshan V. Dave and Ashton Congalton a a a Department of Civil and Environmental Engineering, University of New Hampshire, Durham, NH, USA b Federal Highway Administration, Washington, DC, USA c AECOM, Chelmsford, MA, USA (Received 30 July 2017; accepted 3 January 2018) One of the most influential variables in determining the performance-based properties of the asphalt mixtures is the specimen fabrication method. This study investigates the impact of specimen fabrication methods through a comprehensive study of six test sections constructed in 2011that include a range of RAP contents and two different virgin binders. Complex modulus and fatigue characterisation was conducted on asphalt binders and mixtures. Three specimen fabrication methods were evaluated: specimens compacted from plant sampled loose mix with and without reheating, and specimens fabricated from raw materials in the laboratory. Predicted performance from lab tests were compared to field performance. The mixtures with PG binder showed expected trends with increasing RAP content (higher modulus, lower phase angle); however, mixes with PG did not. Similar trends were observed for specimens fabricated with plant mix (not reheated), and the specimens fabricated with raw materials in the lab. Binder results and performance prediction using the plant mix agree with the observations in the field. Keywords: cracking; RAP; specimen type; binder testing; mixture testing 1. Introduction Researchers and practitioners have always recognised that results obtained from testing of binders and mixtures can be vastly different and that differences in the methods used to produce asphalt concrete specimens for laboratory testing can influence the measured material properties and predicted performance. As agencies move towards performance-based design and using performance tests for acceptance, it is increasingly important to understand the impact of specimen preparation on the measured laboratory parameters and to determine what binder properties may be appropriate for use. Material handling, mixing temperatures and equipment, and compaction methods vary with different specimen preparation methods and can all impact the measured material properties. Three of the most common methods used to prepare asphalt mixture test specimens were evaluated in this study: Laboratory mix (LM): The specimens are mixed and compacted in the laboratory using conditioning methods that are intended to simulate what happens in the plant and are generally used for mix design purposes. *Corresponding author. jo.daniel@unh.edu 2018 Informa UK Limited, trading as Taylor & Francis Group

3 2 J.S. Daniel et al. Plant mix (PM): Loose mix is sampled at the plant and the specimens are compacted using gyratory compactor in a laboratory immediately following production without reheating of the loose mixture. Reheated plant mix (RPM): Loose mix is sampled at the plant and the specimens are fabricated in the laboratory by reheating and compacting the loose mix produced at the plant. Most studies conducted on plant- and lab-produced mixtures show the lab-produced specimens are stiffer than plant-produced specimens. Johnson et al. (2010) evaluated asphalt mixtures containing reclaimed asphalt pavement (RAP) and recycled asphalt shingle (RAS) and showed that the dynamic modulus ( E* ) of plant-produced specimens are lower than those of lab-produced mixtures. Mogawer et al. (2012) showed that reheating mixtures in the laboratory (RPM) caused a significant increase in stiffness among recycled asphalt pavement (RAP) mixtures compared to those that were not reheated (PM). Results also showed that while lab-compacted mixtures were stiffer than plant-compacted, the plant-compacted mixtures saw a larger increase in modulus with an increase in RAP content. Xiao, Putnam, and Amirkhanian (2014) evaluated plant-foamed asphalt mixtures containing RAP and found that the measured rut depth of RPM specimens were lower than PM specimens and that warmer failure temperatures were measured on the binders recovered from the plant-produced materials. Various lab ageing methods have been evaluated by Islam and Tarefder (2015) through beam fatigue tests. In this study, loose mixture appeared to have more ageing compared to the compacted samples. The main objective of laboratory performance testing is to replicate characteristics of field mixture so that lab measured properties can be used to either make decisions on whether to approve a particular design or to predict field performance. Field core specimens would be ideal for performance testing because they represent actual in situ mixture and its characteristics (such as compaction and ageing). However, this is not possible for purposes of design and project acceptance. The use of RAP in hot mix asphalt mixtures has become common due to the cost savings and environmental benefits. RAP contains asphalt binder that has undergone ageing in the field, which typically results in stiffening the binder and increased brittleness. The major contributing factor to this ageing is the loss of lighter hydrocarbon fraction through volatilisation and an oxidation process at the molecular level. Also, it has been shown that the stiffening effect from long-term oven ageing on RAP mixtures is less than virgin mixtures, most likely due to the already-aged binder that stiffens at a slower rate (Daniel, Gibson, Tarbox, Copeland, & Andriescu, 2013; Tarbox & Daniel, 2012). Due to the stiffer and more brittle properties of RAP, there are concerns about low temperature and fatigue performance and the need to use softer binder grades at the higher RAP contents to mitigate the stiffness increases. Several recent studies have explored the performance of plant- and lab-produced high RAP mixtures. Sabouri, Bennert, Daniel, and Kim (2015) showed that using a soft base binder and maintaining the optimum asphalt binder content and/or increasing the asphalt layer thickness are effective strategies in producing a high RAP mixture that performs well and is economical. Diefenderfer and Nair (2014) found that mixtures containing up to 45% RAP can be successfully constructed if proper procedures are followed. It has been shown that the low-temperature performance does change with increasing RAP content. For example, in research utilising up to 40% RAP, critical cracking temperatures were shown to get warmer with more RAP (McDaniel, Shah, Huber, & Copeland, 2012; Mensching et al., 2014). The primary objective of this paper is to evaluate differences in material performance properties due to specimen fabrication type and compare those to actual performance in the field. A secondary objective is to evaluate the impact of RAP content and virgin binder grade on

4 Road Materials and Pavement Design 3 differences observed in the binder and mixture properties, predicted performance, and field performance of the different mixtures and specimen types. This research presents the results of a study evaluating six test sections that were constructed in 2011 on the southbound lanes on I-93 between Exits 30 and 32 in Woodstock and Lincoln, New Hampshire. The test sections include a range of RAP contents and two different virgin PG binders: Virgin PG 58-28, 15% RAP with PG binder, 25% RAP with PG binder, 25% RAP with PG binder, 30% RAP with PG binder, 40% RAP with PG binder. Constituent materials (binder, aggregate, and RAP) were collected for fabrication of LM specimens and the loose mix was sampled during production to fabricate PM and RPM specimens. Testing was conducted on both asphalt binders (tank sampled, and extracted and recovered from mixtures) and mixtures. 2. Materials and methods 2.1. Mixture information The mixtures were produced at a batch plant with tons per hour capacity. The mixtures produced had a nominal maximum aggregate size of 12.5 mm with an optimum asphalt content of 5.8%. Six different mixtures were produced using two different virgin binder grades and different RAP contents. The RAP used in the mixtures had an average asphalt content of 5.36% and a continuous PG grade of Table 1 shows the mixture design as well as production volumetric information. During production, the asphalt content for all mixtures was higher than Table 1. Mixture volumetric data. Mix Mixing/ discharge temperature ( C) Total binder content P b (%) Recycled binder ratio Air void level at design gyrations V a (%) Voids in mineral aggregate (%) Voids filled with asphalt (%) Mixture Virgin Design 15% RAP % RAP % RAP % RAP % RAP Production Virgin % RAP % RAP % RAP % RAP % RAP

5 4 J.S. Daniel et al. the optimum, with the largest difference of 0.4% for the 30% and 40% RAP PG mixtures. The mixture design gradations are similar for all six mixtures, with the differences of no more than 5% in the 4.75, 2.38, and 1.19 mm sieves; the differences were up to 10% during production. The 30% and 40% RAP PG mixtures had the finest gradations during production, and the 25% RAP mixtures were the coarsest Specimen fabrication All laboratory compacted test specimens (LM, PM, and RPM) were fabricated using a Superpave gyratory compactor. Specimens 150 mm in diameter and approximately 180 mm tall were compacted and then cut and cored to achieve final test specimens with a target air void content of 6 ± 0.5%. Specimens for four mixtures (virgin, 25% RAP PG 58-28, 25% RAP PG 52-34, 40% RAP PG 52-34) were fabricated using constituent materials (aggregate, RAP, and binder) to produce the LM specimens. The materials were batched using the mixture design proportions, mixed at the recommended temperatures, and short-term oven aged at 135 C for 4 h before being compacted. The loose mix was sampled at the plant and then compacted immediately without reheating to produce the PM specimens. The RPM specimens were produced by reheating the loose mix to 10 C below the discharge temperature, dividing into the appropriate weights and then heating to compaction temperature. Mixtures were not reheated for more than four hours and were not cooled and reheated Binder tests Asphalt binders were subject to dynamic shear rheometer (DSR) and bending beam rheometer (BBR) testing; analysis included performance grading and evaluation of the T cr and Glover- Rowe parameters. The performance grades of the binders were determined in accordance with AASHTO M320. All tank sampled asphalt binders were subject to both Rolling Thin Film Oven (RTFO), and Pressure Aging Vessel (PAV) ageing. The recovered asphalt binders were only PAV aged under the assumption that short-term ageing occurred during the plant production. The asphalt binder master curves were constructed at a reference temperature of 21.1 C using the RHEA software package. Anderson, King, Hanson, and Blankenship (2011) identified the difference between the bending beam rheometer (BBR) stiffness (S) and m-value critical low temperature as a means of indexing the non-load associated cracking potential of asphalt binders. Asphalt binders that exhibit a greater difference between the S and m-slope low temperature have been recognised as being prone to non-load associated cracking. The parameter, defined as T cr, is shown in Equation (1): DTcr = Tcr (S) Tcr (m - value), (1) where T cr is the difference in critical low-temperature PG grade; T cr (S) is the critical lowtemperature grade predicted using the BBR Stiffness (S); and T cr (m-value) is the critical lowtemperature grade predicted using the BBR m-value. In Equation (1), as the T cr decreases (becomes more negative), the asphalt binder is considered to be more prone to non-load associated cracking. Anderson et al. (2011) set a limit of T cr 2.5 C for when there is an identifiable risk of cracking and preventative action should be considered. Rowe, King, and Anderson (2014) recommended that at a T cr 5 C immediate remediation should be considered. Glover et al. (2005) proposed the rheological parameter, G /(η /G ), as an indicator of ductility based on a derivation of a mechanical analogy to represent the ductility test consisting of springs and dashpots. Rowe (2011) re-defined the Glover parameter in terms of

6 Road Materials and Pavement Design 5 G* and δ based on analysis of a Black Space diagram and suggested use of the parameter G (cos δ) 2 /sin δ, termed the Glover-Rowe (G-R) parameter, in place of the original Glover parameter. Rowe (2011) proposed measuring the G-R parameter based on the construction of a master curve from frequency sweep testing at 5 C, 15 C, and 25 C in the DSR and interpolating to find the value of G-R at 15 C and rad/sec to assess binder brittleness. A higher G-R value indicates increased brittleness. It has been proposed that a G-R parameter value of 180 kpa corresponds to damage onset whereas a G-R value exceeding 600 kpa corresponds to significant cracking based on a study relating binder ductility to field block cracking and surface ravelling by Anderson et al. (2011). The test results generated during the master stiffness curve analysis were utilised to determine the G R parameter Mixture tests Complex modulus Complex modulus testing was performed using an Asphalt Mixture Performance Tester (AMPT) in load-controlled mode in axial compression following the protocol given in AASHTO T 342. Tests were completed for all mixtures at a minimum of three temperatures (typically 4.4 C, 21.1 C, and 37.8 C) and a range of frequencies (typically 25, 10, 5, 1, 0.5, and 0.1 Hz). The LM, PM, and RPM specimens were 100 mm in diameter and 150 mm tall with a 70 mm gauge length and master curves were constructed using RHEA software Fatigue using direct tension cyclic test: simplified viscoelastic continuum damage (S-VECD) approach Simplified VECD (S-VECD) model is a mode-of-loading independent, mechanistic model that allows the prediction of fatigue cracking performance under various stress/strain amplitudes at different temperatures. The S-VECD model is composed of two material properties: the damage characteristic curve (C vs S) that defines how fatigue damage evolves in a mixture and the energybased failure criterion (G R ). The S-VECD testing was conducted using the AMPT in controlledcrosshead direct tension mode on 100 mm diameter, 130 mm tall cylindrical specimens. Details of the test method can be found in AASHTO TP 107. All cyclic tests were performed at a minimum of three different amplitudes to cover a range of numbers of cycles to failure (N f ). Once the fatigue tests were conducted, the damage characteristic curves were developed by calculating the secant pseudo stiffness (C) and cross-plotting with the damage parameter (S) at each cycle of loading. The S-VECD fatigue failure criterion, called the G R method, uses the released pseudo strain energy concept. The G R characterises the overall rate of damage accumulation during fatigue testing. A characteristic relationship can be derived between the rate of change of the averaged released pseudo strain energy during fatigue testing (G R ) and the final fatigue life (N f ). The equation to calculate G R is shown below, G R = (1/2) N f 0 (εr 0,ta )2 i (1 F i), (2) N 2 f where (ε R 0,ta ) i is the pseudo strain amplitude at cycle i, F i i, and N f is the total number of cycles to failure.

7 6 J.S. Daniel et al. The analysis of S-VECD fatigue is conducted using the Alpha-Fatigue software. Using the G R relationship and the S-VECD model, the fatigue life of asphalt concrete under different modes of loading and at different temperatures and strain amplitudes can be predicted Pavement evaluation Layered Viscoelastic Pavement Design for Critical Distresses (LVECD) is a programme developed by North Carolina State University to calculate responses and predict the fatigue and rutting behaviour of asphalt pavements (Eslaminia, Thirunavukkarasu, Guddati, & Kim, 2012). To assess the fatigue behaviour, this 3D finite element based software uses the inputs from the S- VECD approach. One of the most useful outputs of this programme is damage factor calculated based on a cumulative damage model and Miner s rule. The damage factor (Equation (3)) varies in magnitude from 0 (no damage) to 1 (completely damaged element), Damage factor = T i=1 N i N fi, (3) where T is the total number of periods, N i is the traffic for period i, and N fi is the allowable failure repetitions under the conditions that prevail in period i (Sabouri et al., 2015). The pavement cross-section used for LVECD analysis in this study is shown in Figure 1. The measured complex modulus and S-VECD fatigue parameters for each of the six mixtures are input for the surface layer. The remaining layer material properties were kept constant. The Whitefield, NH weather station climate input used for the analysis as it is the closest to the field site. Figure 1. I-93 Pavement cross-section for LVECD analysis.

8 Road Materials and Pavement Design 7 3. Results and discussion 3.1. Binder results The results of the testing conducted on the virgin binders and the binders that were extracted and recovered from the plant-produced mixture are presented in this section. The asphalt binder from the loose mix was extracted and recovered in accordance with AASHTO T 164 method A using Toluene and after the third wash, an 85/15 blend of Toluene/Ethanol. Testing was not conducted on binders recovered from lab-produced mixtures or from reheated plant-produced mixture Superpave performance grade and T cr The continuous high and low PG temperatures and the T cr values are shown in Figure 2. The addition of RAP stiffens the continuous high-temperature grades, but there is not a consistent trend with RAP content for these mixtures. The low-temperature grades for the PG mixtures show little impact from the addition of RAP; the PG mixtures show slightly warmer low-temperature grades with RAP, but no trend with increasing RAP content. The two virgin binders and the 15% RAP mixture have positive T cr values, indicating they are S-controlled. The higher RAP contents and the PG materials become more m-controlled (negative values) indicating these materials would be more susceptible to cracking Asphalt binder rheological behaviour The shear modulus master curves are shown in Figure 3. The extracted and recovered RAP binder has the highest stiffness, as expected. The PG virgin binder is stiffer than the PG binder over most of the frequency range, although the two binders have similar stiffness at low frequencies. The extracted and recovered binders from the PG base binder mixture show increasing stiffness with RAP content, except at low frequencies where the 25% RAP mixture shows a stiffer response. The extracted and recovered binders from the PG base binder mixtures are stiffer than the PG base binder materials at high frequencies, but have similar response at low frequencies. The Glover-Rowe parameters for all the materials are plotted in Figure 4. Based on the current recommended threshold values, the PG materials either exceed or are extremely close to the onset of cracking threshold with the 15% RAP material exceeding the significant cracking threshold. The PG materials are similar and are further from the cracking threshold than the other materials Mixture results Complex modulus LM specimens The dynamic modulus of LM specimens was measured on four of the six mixtures. Four replicate specimens were fabricated and tested for each mixture; the average dynamic modulus master curves for the four mixtures are shown in Figure 5(a). The virgin and the 25% RAP PG mixture have similar curves, with the 25% RAP mixture showing slightly stiffer response over the mid to high-frequency range. The two mixtures with the PG base binder show softer response than the PG base binder mixtures, with a slight increase in stiffness at the higher RAP content. Figure 6(a) shows the average Black Space curves for the LM specimens. The phase angles for the virgin and the 25% RAP PG curves follow the expected trend that the addition of RAP decreases the maximum phase angle. The 25% RAP PG mixture has a smaller phase angle than the 25% RAP PG mixture, which typically is not expected with the softer binder, but

9 8 J.S. Daniel et al. Figure 2. Continuous high (a) and low (b) temperature grade and T cr values (c) for virgin and extracted and recovered binders. follows the trends seen with the virgin PG binder. In summary, the PG grade of the base binder shows a larger impact on the dynamic modulus and phase angle than the RAP percentage PM specimens Four replicate specimens were produced and tested for each mixture during each day of production. The average dynamic modulus curves for the six mixtures over all three production days are shown in Figure 5(b). Each curve represents the average of 12 specimens. The PG base binder mixtures have similar responses with minimal impact of RAP on the average stiffness of the mixtures. The PG base binder mixtures all show softer response than the virgin PG mixture and show slight increases in stiffness with increasing RAP content. The PG base binder mixtures are statistically similar (using t-test evaluated

10 Road Materials and Pavement Design 9 Figure 3. Shear modulus master curves at 21 C for virgin and extracted and recovered binders. Figure 4. Glover-Rowe parameter for virgin and extracted and recovered binders. at 95% confidence level) to each other over most of the master curve range, as are the PG base binder mixtures. All the dynamic modulus values for PG base binder mixtures are statistically different from all of the PG base binder mixtures. The statistical analysis of the phase angle values is similar, with the exception that most mixtures showed statistically similar phase angle values at the 21 C testing temperature and 5-25 Hz frequency range. In summary, the base binder grade shows a larger, statistically significant impact on the dynamic modulus than the RAP content. The average Black Space curves for the six plant mixed mixtures are shown in Figure 6(b). The three mixtures with the PG binder are very similar in Black Space, with a slight decrease in the phase angle with increasing RAP content. The mixtures with PG binder

11 10 J.S. Daniel et al. Figure 5. Average dynamic modulus master curves at 21 C for (a) LM specimens, (b) PM specimens, and (c) RPM. Figure 6. Average black space curves for (a) LM specimens, (b) PM specimens, and (c) RPM specimens.

12 Road Materials and Pavement Design 11 have lower phase angles than the PG mixtures and also show an increase in phase angle with increasing RAP content. This is similar to the trends observed with the LM specimens RPM specimens The loose mixture sampled at the plant during production was brought back to the lab reheated to produce three replicate specimens for each mixture. Production and compaction occurred several days apart, so the loose mix was at room temperature prior to reheating. The average dynamic modulus curves for the six mixtures are shown in Figure 5(c). The stiffness of both the PG and PG base binder mixtures show a decrease in average stiffness as the RAP content increases. The 25% RAP mixture has a higher stiffness than the 25% RAP mixture. The dynamic modulus for the 25% RAP and 40% RAP mixtures are statistically similar over the intermediate and high-frequency range, as are the 25% RAP and 30% RAP mixtures. The others are statistically different over most of the intermediate to high-frequency range. The phase angle measurements are statistically different at the intermediate temperature for most mixtures, but are similar at the low and high test temperatures. The average Black Space curves for the six reheated plant mixed mixtures are shown in Figure 6(c). There are no discernible trends with respect to RAP content or base binder grade with these results. These results do not follow expected trends with RAP content and binder grade and are different than what was observed with the LM and PM materials. A possible reason for the differences is the impact of the reheating process that was required to fabricate specimens from the loose mix Comparison of all complex modulus results In this section, the dynamic modulus for all of the different specimen types is compared in Figure 7. The LM, PM, and RPM specimens all have air void contents that were controlled in the laboratory and are in the 6 ± 0.5% range. Impact of reheating loose mix (PM vs RPM): The impact of reheating the loose mixture for compaction in the laboratory is shown by comparing the PM and RPM data. The reheated specimens have higher stiffness and the difference between the PM and RPM stiffness decreases with higher RAP contents with both virgin binders. This is expected, as the presence of a higher proportion of aged RAP binder in the mixtures means there is less additional ageing that can occur during the reheating process. The results show that mixtures with the PG virgin binder have larger differences, indicating that the PG virgin binder ages more during the reheating process than the PG virgin binder. The PM and RPM dynamic modulus curves are statistically different over the whole frequency range for all mixtures except the 25% RAP mixture. The phase angles for the virgin 58-28, 25% RAP and 30% RAP mixtures are significantly different at the low and intermediate temperatures; all other phase angles are statistically similar. This indicates that the ageing occurring during the reheating process primarily impacts measured stiffness instead of phase angle for these materials. Mix design vs production: The difference between measurements that would be made during the mix design process and those made on the material produced at a plant can be evaluated by comparing the LM and PM specimens. This comparison was only done for the virgin 58-28, 25% RAP 58-28, 25% RAP 52-34, and 40% RAP mixtures. All of the LM master curves are stiffer than the PM master curves, and are statistically different. The PG mixtures show larger differences than the PG mixtures between the LM and PM master curves. The mixtures with lower RAP contents also show larger differences between the LM and PM master curves. One likely reason for the differences in LM and PM master curves is the differences in ageing; the LM mixtures were subject to short-term oven ageing while the PM mixtures were subject to ageing through plant production. The higher asphalt content and finer gradations during production likely also contribute the differences observed.

13 12 J.S. Daniel et al. Figure 7. Average dynamic modulus master curves at 21 C for LM, RPM, and PM SVECD fatigue Fatigue testing was conducted in uniaxial tension mode using the AMPT. The analysis was performed using the S-VECD approach developed by Underwood and Kim (2010). The damage characteristic curves of the six mixtures for the PM and RPM specimens are compared using an exponential fit in Figure 8. There is no specific trend with respect to RAP content or virgin binder grade for any of the specimen types. The damage characteristic curves for the PM and RPM specimens are similar for the mixtures with up to 25% RAP. The curves for the two specimen types are different for the 30% and 40% RAP mixtures. The relationship between the S-VECD failure criterion, G R, and the number of cycles to failure for the different specimen types are shown in Figure 9. In general, mixtures that have shallower slopes and are further towards the upper right would be expected to have better fatigue performance. However, the actual field performance will depend upon the structure in which the mixture is placed and the traffic and environmental loadings. The RPM specimens do not show any trends with respect to RAP content or virgin binder grade; however, the virgin mixture has a shallower slope than the RAP mixtures. The virgin and 15% RAP PM specimens are grouped together with a shallower slope than the remaining RAP specimens that show similar expected performance.

14 Road Materials and Pavement Design 13 Figure 8. Comparison of PM and RPM and C versus S curves for each mixture LVECD pavement evaluation The LVECD analysis for the different mixtures and specimen types only showed failed elements (number of load repetitions [N] to reach the number of allowable repetition [N f ], i.e. N/N f = 1.0) for three different mixture-specimen type combinations with the given cross-section, traffic, and climate. To allow for comparison of relative performance of all of the mixture-specimen type combinations, the maximum N/N f value in the surface layer at the end of the 20-year analysis is used to provide mixture rankings. This indicates the extent of damage at a point in the asphalt concrete layer that has undergone greatest amount of damage. The rankings of the mixtures for each specimen type and the magnitude of the maximum N/N f value along with the field performance ranking of the mixtures are shown in Table 2 below. The comprehensive field performance analysis of mixtures is described in the next section. The rankings for the PM and RPM materials are relatively similar, with the exception of the two 25% RAP mixtures. These two specimen types generally show that the PG mixtures are expected to perform better than the PG mixtures; this is similar to the results of the

15 14 J.S. Daniel et al. Figure 9. Comparison of PM and RPM G R versus number of cycles to failure curves for each mixture. binder testing. However, the magnitudes in the maximum N/N f values is different for the PM and RPM specimen types, so the amount of actual field cracking that would be predicted for each section would be different 3.4. Field performance Field performance of the sections has been monitored yearly since construction using an automated pavement distress data collection van by New Hampshire DOT. The measurements from the field core air voids are presented in Table 3. Both fatigue cracking and transverse cracking at three severity levels are tracked. The weighted crack length for each section is calculated using Equation (4): Weighted crack length (m/km) = (Severity 1 crack length) + 2(Severity 2 crack length) + 3(Severity 3 crack length) + (Sealed crack length). (4)

16 Road Materials and Pavement Design 15 Figure 10. Field cracking performance since construction. Table 2. Mixture Rankings from LVECD analysis and field performance on different specimen types and mixtures. PM LVECD ranking Fieldperformance ranking RPM Fatigue cracking Rank (best Rank (best Rank (best to worst) Max N/N f to worst) Max N/N f to worst) Transverse cracking Rank (best to worst) Virgin E E % RAP E % RAP E % RAP E % RAP % RAP E Table 3. Measured field air void. Mixture Virgin % RAP % RAP % RAP % RAP % RAP Field air void (%) The amount of fatigue and transverse cracking in each section are shown in Figure 10. Some amount of fatigue and transverse cracking has been observed in each section. As it was also indicated in Table 2, the ranking from field performance complies well with that of LVECD. The lower RAP content mixtures have less cracking and the PG binder appears to be performing better than the PG binder (as seen from the two 25% RAP mixes). The 30% RAP section appears to have the worst performance overall, which may also be due to the relatively higher air void content as measured from the field cores provided in Table 3. For this set of mixtures, the binder T cr values (Figure 2) clearly identify the mixtures that are more prone to cracking and the relative ranking of the different mixtures. The Glover-Rowe parameter (Figure 4) identifies the difference in performance between the PG and PG materials, but does not show exactly the same rankings. The LVECD analysis of the PM and RMP mixture specimens also provides relatively similar rankings as observed in the field.

17 16 J.S. Daniel et al. 4. Summary and conclusions This study includes the evaluation of six different mixtures with a range of RAP contents and two different virgin binders. Three different specimen fabrication methods were used to evaluate differences in measured material properties: using laboratory mixed material (LM), plant-produced material (PM), and reheated plant mix (RPM). Recovered binder testing was conducted on the plant-produced material and mixture testing on all materials included complex modulus and uniaxial fatigue. Using the measured material properties, viscoelastic pavement analysis was conducted to evaluate expected relative differences in fatigue damage in the actual pavement structure. The performance of the mixtures in the field has been monitored for five years. Based on the results of measurements and pavement performance analysis along with the field condition the following conclusions are made: Differences in the viscoelastic and fatigue properties measured from the LM, PM, and RPM materials were observed, and the relative differences depend on both the RAP content and virgin binder grade. Generally, larger differences were observed with the lower RAP content and softer virgin binder grade. The impact of the change in binder grade on stiffness was greater than the impact of the change in RAP content for the LM and PM specimens. The trends observed with the RPM specimens were different, possibly due to the impact of reheating the material in the laboratory; the lower RAP content mixtures and PG base binder mixtures were affected by the reheating to a greater extent. This is because the larger proportion of virgin binder and softer binders will undergo a greater change in stiffness due to the reheating than already-aged RAP material. This has also been observed in other research projects. The S-VECD fatigue show differences in the characteristic curves and failure criterion plots measured from the PM and RPM materials. However, there are not any discernible trends with respect to RAP content or virgin binder grade for the PM and RPM. These differences translate to different magnitudes of expected damage as predicted through the LVECD pavement analysis, but result in similar relative rankings of the mixtures. The materials with the PG materials were shown to be more prone to cracking from the binder and mixture analysis and did not always follow expected trends with respect to RAP content. Performance of these mixtures to date in the field shows that the PG mixtures are performing better than the PG mixtures with respect to both transverse and fatigue cracking and that the amount of RAP does show performance differences. The use of the softer PG binder at the higher RAP contents did not help improve the performance of these materials, and in fact, appeared to make it worse. The extracted binder testing and the Black space plots for all of the mixture-specimen types identified a possible issue with the PG materials. The different binder indices and mixture-specimen types produced different magnitudes of response in some cases and different trends with respect to RAP content and virgin binder grade (sometimes matching with observations and other times not). This emphasises the need for a better understanding of the impact of binder versus mixture tests and mixture-specimen fabrication technique on the resulting properties measured in the laboratory and performance predictions from those measured properties. The results also indicate that some enhancement of the binder grading procedure is needed to identify possible performance issues with materials like PG binders like the one used in this study.

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