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purely Hot Mix Asphalt (HMA) paving mixes reduces the requirement for quality binder and ...... Recycled Asphalt Shingles, and Warm-Mix Asphalt Technology.
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EFFECTS OF POLYMER MODIFIED BINDER ON THE DEFORMATION AND CRACKING PERFORMANCE OF RECYCLED ASPHALT PAVING MIXES

Sachi Kodippily, PhD. Postdoctoral Research Fellow Department of Civil and Environmental Engineering The University of Auckland Private Bag 92019, Auckland 1142, New Zealand E-mail: [email protected] Glynn Holleran Technical Divisional Manager Fulton Hogan Ltd Private Bag 11900, Auckland 1542 New Zealand E-mail: [email protected] Douglas Wilson, PhD Senior Lecturer Department of Civil and Environmental Engineering The University of Auckland Private Bag 92019, Auckland 1142, New Zealand E-Mail: [email protected] Theunis FP Henning, PhD Senior Lecturer Department of Civil and Environmental Engineering The University of Auckland Private Bag 92019, Auckland 1142, New Zealand E-mail: [email protected]

Word Count: Manuscript 5,130 (abstract = 249) + 3 Tables/6 Figures @ 250 each = 7,380 Prepared for presentation at the Transportation Research Board 94th Annual Meeting, Washington D.C. and possible publication in the Journal of the Transportation Research Board

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ABSTRACT Developing technologies that provide sustainable solutions for future pavement construction is vital given the ever-increasing demand on the supply of bitumen and good-quality pavement construction materials. Recycled Asphalt Pavements (RAP) is a technology that presents many benefits in terms of both cost and environmental savings. The presented study investigated the effect of polymers on the deformation (rutting) resistance and cracking resistance of RAP mixes. In this study, laboratory testing was conducted on six paving mixes manufactured using various proportions of RAP and polymers. The mixes contained 15%RAP, 30%RAP, a control mix, 15%RAP+polymer, 30%RAP+polymer and polymer only mix. Samples were subjected to dynamic modulus testing, flow number testing and overlay testing using an Asphalt Mixture Performance Testing machine. The inclusion of RAP resulted in notable increases in the stiffness of the mixes, particularly with a 30% RAP proportion. The addition of polymers reduced the stiffness of the mixes, most notably in the 30% RAP mix while no change in stiffness was observed for the 15% RAP mix. The presence of polymers improved the rutting resistance of all the mixes, although the presence of polymers in conjunction with RAP decreased the reflective cracking resistance of the mixes, where when the RAP content was high, the negative effects of polymers on the cracking performance of the mixes were more significant. The research results provided a valuable understanding of the behaviour of RAP mixes, and in particular, this study provided valuable performance specifications for RAP mixes for the pavement construction industry in New Zealand.

Key words: RAP, recycling, asphalt, pavements, polymers, dynamic modulus

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INTRODUCTION Recycled Asphalt Pavements (RAP) are a sustainable paving method that has many advantages to the pavement industry. The production process for recycled pavement material consists of milling and recovery of existing asphalt pavement layers during pavement rehabilitations, and this recovered material is then combined with new bituminous binder and aggregates to produce recycled material that is suitable for use in asphalt pavement construction. The use of RAP as an alternative to purely Hot Mix Asphalt (HMA) paving mixes reduces the requirement for quality binder and aggregates for pavement construction and the recycling of existing material preserves valuable resources. In addition to relieving pressure on bitumen supplies, the RAP technology has many benefits by providing both economic savings and environmental benefits. The attractiveness of RAP as a sustainable paving material means an increasing number of pavement practitioners and contractors are utilizing this technology as a method to relieve the pressures on the demand for quality bitumen and aggregates. It has been estimated that in the United States of America (USA) approximately 80% of reclaimed asphalt is recycled as pavement material (1) and in Australia it is estimated that approximately 50% of reclaimed asphalt is being used to produce RAP mixes (2). In New Zealand, RAP has been utilized in pavement construction for several years. Presently, the allowable proportion of reclaimed material in a RAP mix is between 15% - 30%, however, the practical use of RAP is still limited to around 15% (3). The use of higher proportions of reclaimed material requires the approval of NZ Transport Agency, where it is necessary to demonstrate adequate performance of the mixes in trial settings in order to achieve approval for use in practice (4). The need for approval to use higher quantities of reclaimed material can limit the widespread use of higher quantities of reclaimed material. Many economic benefits have been observed by the use of 15% of reclaimed material, and it is expected that these benefits can be further increased if higher quantities of RAP are used. It is imperative that testing protocols and performance limits are established for the context of New Zealand’s paving practices that can demonstrate the performance of RAP mixes in order to encourage the use of higher quantities of RAP in practice. The use of polymer-modified binders (PMB) in HMA mixes is typically carried out to improve the HMA properties and allow the mixes to perform satisfactorily under a range of loading and environmental conditions. The use of polymer modified binder in pavement mixes has become a standard technique in pavement construction. The use of PMB in RAP mixes is an area that needs to be investigated for the context of New Zealand, as this new mix of PMB and RAP has the potential to further increase the performance benefits that are often gained by using RAP. OBJECTIVES AND SCOPE The primary objective of the presented study were to characterise the rutting resistance and cracking performance of HMA mixes containing different proportions of RAP for the New Zealand context, and to investigate the effects of polymer inclusion on the rutting and cracking performance of RAP mixes. The performance assessments that were conducted as part of this study were based on laboratory prepared samples and it was intended that the results from the laboratory testing would form the basis for establishing performance limits for RAP mixes. Presented in this paper are the initial results from a larger study that is investigating the performance of RAP mixes for application on New Zealand pavements. The results that were obtained from the presented study were benchmarked against RAP performance observed in international studies in order to validate the study findings. CHARACTERISING THE MECHANICAL PROPERTIES OF RAP MATERIAL The inclusion of recycled paving material in the production of HMA paving mixes alters its characteristics, specifically, the deformation performance of HMA. By its nature, RAP is old material and the binder that is present in RAP is aged and stiffer, hence the inclusion of this aged binder in to virgin HMA material results in a modified mix that has the potential to be stiffer. The increased

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stiffness provided by RAP has its benefits, particularly when considering the rutting resistance of paving mixes. It has been documented that the inclusion of higher quantities of RAP notably increases the dynamic modulus of HMA mixes when compared to virgin HMA mixes (5; 6). Some studies have reported that the inclusion of small quantities of RAP, for example 5% to 10% has similar deformation performance properties as completely virgin HMA mixes, although as the RAP proportions increased, the improved rutting resistance became more noticeable (7). The increased stiffness provided by RAP is extremely beneficial as it delivers the necessary stiffness requirements for pavement layers while at the same time minimising the need for good-quality pavement aggregates. The use of PMB particularly improves the deformation performance of HMA mixes (8), however when polymers are used in conjunction with RAP, the behaviour of the resulting mix can vary depending on the type of polymer binder that is used, and published literature have indicated mixed results when considering the use of RAP with polymers (9-11). In the study by Bernier et al. (9), it was indicated that at lower levels of polymer modification the addition of RAP results in increased rutting resistance, while at higher levels of polymer modification the quantity of RAP does not have a significant effect on the rutting resistance of the HMA mix. Other studies that have investigated the effects of polymer modification on the performance of RAP mixes have indicated that the presence of polymers can have a negative effect on the deformation performance of the RAP mixes and it has been recommended that either changes are made to the grade of the virgin binder prior to the addition of RAP or that PMB is not used when RAP is used in HMA mixes (12; 13). Polymer modification is also routinely used to improve the cracking performance of HMA mixes, where the presence of polymers enhances the elastic properties of the bitumen thus improving the bond strength between the materials and prolonging the service life of a pavement. Studies that have investigated the performance of polymer modification to HMA mixes have indicated that the presence of polymers reduced the low temperature cracking susceptibility of HMA mixes, while at the same time enhanced the fatigue performance of the mixes (14; 15). While polymer modification can improve the performance of virgin binder and HMA mixes, when aged, recycled material is present, the performance can be different. As noted earlier, polymer modification can improve stiffness of HMA mixes, and while this may be beneficial when considering deformation performance, the increased stiffness may pose a problem when considering the cracking performance of the same mix. Hence, there is a need to investigate the influence of polymers on the cracking performance of HMA and the documented research was conducted to answer some of these outstanding questions. MIX DESIGN METHOD Performance testing was conducted on six HMA mixes containing various proportions of RAP and PMB. TABLE 1 shows the volumetric mix design properties of the mixes. TABLE 1 Constituents of the test mixes Mix ID

Mix classification

RAP content

Binder type

Binder content

Mix 1 Mix 2 Mix 3 Mix 4 Mix 5

Virgin HMA HMA + 15%RAP HMA + 30%RAP HMA(PMB) HMA(PMB) + 15%RAP HMA(PMB) + 30%RAP

0% 15% 30% 0% 15%

Virgin binder - PGT64 Virgin binder - PGT64 Virgin binder - PGT64 PMB - Paveflex AXA-H binder PMB - Paveflex AXA-H binder

30%

PMB - Paveflex AXA-H binder

Mix 6

5.3% 5.4% 5.0% 5.3% 5.4%

Voids in Mineral Aggregate (VMA) 17.9 17.4 16.7 18.1 16.6

Voids Filled with Asphalt (VFA) 60.4 60.8 61.1 59.7 58.9

5.0%

16.2

58.6

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The mix design process of the test mixes were conducted according to the specifications set out in NZTA M/10:2010 (3). The mix designs were optimised for the RAP contents of the mixes and incorporated differing job-mix formulae, which were within the mix envelopes set out in NZTA M/10:2010. All six test mixes were prepared according to the test methods set out in standards AS 2891.2.1 – 1995 and AS 2891.2.2 – 1995 (16; 17). TABLE 2 and TABLE 3 show the aggregate grading of the virgin aggregates and RAP aggregates that were used for the test mixes. TABLE 2 Aggregate grading of the virgin aggregates Percentage passing Sieve size Virgin HMA 30% RAP 15% RAP 37.5 26.5 19.0 16.0 13.2 9.5 6.7 4.75 2.36 1.18 0.600 0.300 0.150 0.075

100.0 100.0 100 98 96 75 61 52 35 25 16 11 8.0 6.1

100.0 100.0 100 98 96 80 63 53 38 26 18 13 8.9 6.1

TABLE 3 Grading of the RAP aggregate Sieve size

Percentage passing

37.5 26.5 19.0 16.0 13.2 9.5 6.7 4.75 2.36 1.18 0.600 0.300 0.150 0.075

100.0 100.0 100.0 100.0 100.0 100.0 91.0 80.0 59.0 42.0 30.0 22.0 15.0 10.0

100.0 100.0 100 98 96 80 63 53 38 25 16 11 8.1 5.9

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PERFORMANCE TESTING METHOD Modulus / Stiffness Testing Performance testing of the mixes was conducted using an Asphalt Mixture Performance Testing (AMPT) machine (18), and the tests that were conducted included dynamic modulus testing, flow number testing and overlay testing. The dynamic modulus, E*, is a material property that is used to describe the viscoelastic behaviour of asphalt concrete paving mixes, and it is a measure of the stiffness of an asphalt mix. Studies that have investigated the accuracy of dynamic modulus of paving mixes have indicated that there is a strong correlation between laboratory-observed dynamic modulus values and field-obtained dynamic modulus values (8). The dynamic modulus tests in the reported study were conducted according to the test method set out in AASHTO TP 79-09 (19). Each test that was conducted in the AMPT machine required samples with specific dimensions and target air void volumes. For the dynamic modulus test, three samples having a target air void volume of 7.0 ± 1.0% and dimensions of 100 mm diameter and 150 mm height were prepared from each RAP mix. During the dynamic modulus test each sample was subjected to continuous sinusoidal, stress-controlled loading at frequencies of 10 Hz, 1 Hz and 0.1 Hz at temperatures of 4°C and 20°C, and 10Hz, 1Hz, 0.1Hz and 0.01Hz at a temperature of 35°C, and the applied stresses and the resulting strains were recorded by the AMPT machine during the testing. The measured dynamic modulus values from the three samples of each mix were used to determine the average dynamic modulus value for each mix, and the average dynamic modulus values of the mixes were compared using a t-test. The 95% Confidence Intervals (95% CI) were calculated for dynamic modulus values of each test mix, and the reported differences were significant at p-value < 0.005. Additionally, the recorded stress and strain measurements from the dynamic modulus tests were used to develop dynamic modulus master curve for the test mixes. Deformation Resistance Testing using Flow Number Test The flow number test is commonly conducted to assess the deformation (rutting) resistance of asphalt mixes. The results that are obtained by conducting flow number tests in the laboratory have been found to have strong correlations to field rutting performance of asphalt paving mixes for various traffic levels, and flow number has been utilised to assess and compare the rutting susceptibility of different asphalt mixes (20). In the presented study, the flow number test was conducted to assess the rutting resistance of the RAP mixes in comparison to virgin HMA and polymer mixes. The flow number tests were conducted according to the method set out in AASHTO TP 79-09 (19). The samples for the flow number tests had dimensions of 100 mm diameter and 150 mm height. The flow number test was conducted at a temperature of 60°C, and during the test, a sample was subjected to a repeated axial stress of 600 kPa, where the sample was loaded for a period of 0.1 s followed by a rest period of 0.9 s. The resulting permanent axial strain for each load pulse was recorded in the AMPT machine and these measurements were used to calculate the flow number of each mix by determining the maximum number of cycles that were needed to reach the minimum axial strain rate (flow point). The flow number test was conducted on three replicate samples for each test mix, and the average flow number test cycles of each mix was compared to the other mixes.

Cracking Resistance Testing The overlay test is designed to measure the susceptibility of asphalt mixes to strain-controlled reflective cracking and this test was conducted as part of the presented study to compare the cracking susceptibility of RAP mixes containing PMB. The cracking estimations of the overlay test have been found to have close correlations to field performance of HMA mixes (9; 21). The overlay tests were conducted according to the method set out in the standard Tex-248-F (22). Samples having a thickness of 38 mm and length of 150 mm were used for the overlay tests. The AMPT overlay test

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apparatus consisted of two steel plates with a joint between the plates, and a sample was attached to the two plates with epoxy. During the overlay test, one plate was held stationary while the other plate was pulled to open the joint to a maximum distance of 0.63 mm, and the plate was then pushed back to the original location. Each opening and closing motion was considered one cycle and the samples were subjected to cyclic loading in 10-second cycles. During each cycle the load required to move the plates to the specified displacement was recorded, and when the load was reduced by 93% percent of the first recorded load or when 1200 loading cycles were reached the test was automatically terminated. The overlay test was conducted at a temperature of 25°C and four samples were tested for each mix. For each sample, the number of cycles to failure and the failure curve were recorded in the AMPT software and these results were compared between the mixes. RESULTS Stiffness Comparison between RAP and Polymer Mixes The results of the dynamic modulus testing are shown in FIGURE 1a and FIGURE 1b, which show the average dynamic modulus values for each test mix at test temperatures of 4°C and 20°C. When observing the dynamic modulus results of the mixes at 4°C, it can be seen that for all three frequencies, the highest dynamic modulus is reached by the HMA+30%RAP mix (Mix 3), while the lowest dynamic modulus was reached by the mix that contained HMA(PMB) mix (Mix 4). The high modulus reached by the HMA+30%RAP mix is not surprising as it has been identified that a higher quantity of RAP results in a stiffer mix due to the high proportion of aged material.

a) Modulus testing at 4°C

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b) Modulus testing at 20°C FIGURE 1 Dynamic modulus comparison of RAP mixes

When observing the effects of RAP proportion on the dynamic modulus values at 4°C (FIGURE 1a), it can be seen that for all three frequencies, when 15% RAP was used (ie, Mix 2 vs Mix 5) the resulting modulus value was similar between the two mixes regardless of the presence of PMB in the mix. However, when 30% RAP was used (ie, Mix 3 vs Mix 6) the modulus value reached by the HMA+30%RAP mix (Mix 3) was significantly higher (p value< 0.001) than the HMA(PMB)+30%RAP (Mix 6). For example, at 10Hz at 4°C, the modulus of HMA+30%RAP mix (Mix 3) was 2210 ksi while the HMA(PMB)+30%RAP mix (Mix 6) reached a modulus of 1880 ksi. This observation indicated that at lower RAP proportions, ie, 15%, the addition of PMB does not have a notable effect on the stiffness of a mix, particularly when the temperature is low, ie 4°C, and that the characteristics of RAP are more dominant than polymers in the mix behaviour at lower temperatures. At the higher temperature of 20°C (FIGURE 1b), it was observed that when 15% RAP was used, the modulus of the HMA(PMB)+15%RAP (Mix 5) (800 ksi at 10Hz) was higher than HMA+15%RAP mix (Mix 2) (690 ksi at 10Hz). The difference in modulus values of the 30% RAP mixes at 20°C were the same as for the lower temperature where the HMA+30%RAP mix (Mix 3) (1060 ksi at 10Hz) achieved a higher modulus value than the HMA(PMB)+30%RAP mix (Mix 6) (930 ksi at Hz). This observation indicated that at higher temperatures, the contribution of RAP and polymers towards the behaviour of the mix is variable depending on the proportion of RAP that is being used. Additionally, when comparing the effect of RAP proportion on the stiffness of HMA mixes, it was observed that at 4°C (10Hz), the HMA+30%RAP mix had a modulus value that was 25% higher than the HMA+15%RAP mix, while at 20°C (10Hz), the HMA+30%RAP mix had a modulus that was 53% higher than the HMA+15%RAP mix. However, when polymers were present in the mixes, the modulus improvements that were achieved by using 30% RAP compared to 15% RAP was much lower. For example, at 4°C (10Hz) the HMA(PMB)+30%RAP mix had a modulus value that was 11% higher than the HMA(PMB)+15%RAP mix. At the higher temperature of 20°C (10Hz) the HMA(PMB)+30%RAP mix achieved a modulus value that was only 16% higher than the HMA(PMB)+15%RAP mix. These observations confirmed that when polymers are present in an asphalt mix, the addition of further RAP does not necessarily result in a large increase in the stiffness of the mix which is often desired for satisfactory cracking performance.

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The dynamic modulus master curves for the test mixes are shown in FIGURE 2. Analysis of the master curves shows that at the lower frequencies there is a notable difference in modulus values between the six mixes. Both the HMA(PMB) mix and the HMA(PMB)+15%RAP mix has similar moduli at lower frequencies, although the modulus of the HMA(PMB)+15%RAP mix increases more than the HMA(PMB) mix with increased frequency. It is interesting to note that the modulus of the Virgin HMA mix was higher than the HMA(PMB) mix at lower frequencies, however as the frequency increases the modulus of the Virgin HMA mix decreases below that of the HMA(PMB) mix. This observation is also present in the mixes that contain 30% RAP. However, for the mixes that contain 15% RAP, the opposite pattern is present where the modulus of the HMA(PMB)+15%RAP mix stays higher than the HMA+15%RAP mix for all frequencies below 50Hz. It is also worth noting that the Virgin HMA mix and the HMA+15%RAP mix have similar moduli for all the frequencies. The results in FIGURE 2 clearly show how the addition of PMB changes the behaviour of the mixes, where the presence of PMB lowers the modulus of a mix containing a larger proportion of RAP, ie 30%, thus reducing the over-stiffening that often occurs due to the high proportion of aged RAP material.

FIGURE 2 Dynamic modulus master curves for test mixes; reference temperature: 20°C

A comparison of the phase angle master curves (FIGURE 3) of the mixes was used to further investigate how the stiffening effects of RAP were affected by the addition of polymers. The phase angle can be used to assess the stiffness of the mixes with varying temperature, where a high phase angle value correlates to lower stiffness of the mix. The phase angle comparison of the tested mixes showed that the incorporation of polymers into the RAP mix reduced the stiffness of the RAP mix, where at the higher frequency zone, which correlates to low temperature loading, the HMA(PMB)+30%RAP mix had a higher phase angle than the HMA+30%RAP mix. At the lower frequency zone, which correlates to high temperature loading, the phase angle of the RAP and polymer mix was marginally lower than the RAP only mix, indicating higher stiffness of the RAP and polymer mix at higher temperatures. This effect of polymer addition to RAP is ideal as preserves the stiffening effects of RAP on virgin asphalt at high temperatures while possibly reducing the likelihood of over-stiffening at lower temperatures which would be undesirable for cracking performance.

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FIGURE 3 Phase angle master curves comparison between the Virgin HMA mix, HMA+30%RAP mix and HMA(PMB)+30%RAP mix

Deformation Performance of the RAP Mixes in Comparison to Polymer Modified Mixes Flow number tests were conducted on the RAP mixes to investigate the rutting resistance of the mixes, and the results are shown in FIGURE 4. During a flow number test, the microstrains that were occurring within each test sample was recorded by the AMPT machine, and these measurements were used to determine the flow point, which is commonly referred to as the point at which tertiary flow occurs in a mix, specifically the point at which the mix deforms or ruts (23). The flow point refers to the number of loading cycles corresponding to the minimum rate of change of axial strain within each sample, and the subsequent microstain of the sample at flow point. When comparing the flow number test results of the RAP mixes, it can be seen that there are notable differences in the flow point measurements between the various RAP mixes containing polymers and the mixes without polymers. The HMA(PMB)+30%RAP mix had a flow point at 601 cycles while the HMA+30%RAP mix had its flow point at 131 cycles. The microstrains at flow point were similar for both 30% RAP mixes, of approximately 35000. The high flow number achieved by the HMA(PMB)+30%RAP mix shows that this mix has the most resistance to deformation out of all the tested mixes, and this observation aligns with findings from previous studies which had indicated that the presence of polymers improve the rutting resistance of HMA mixes. As was noted earlier in the dynamic modulus results analysis, the HMA(PMB)+30%RAP mix was notably lower in stiffness than the HMA+30%RAP mix, however, the higher flow number results indicate that even though the stiffness may be less, the presence of polymers still achieves better rutting resistance than purely RAP mixes. Additionally, when comparing the HMA+30%RAP mix and the HMA(PMB)+15%RAP mix, it was observed that both these mixes had similar flow points (131 and 172). This result indicated that the presence of polymers provides the same level of rutting resistance with a lower amount of RAP as it does when a higher amount of RAP is used without polymers.

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FIGURE 4 Flow number test results showing the rutting resistance of test mixes

Reflective Cracking Resistance of RAP Mixes Overlay tests were conducted on the six mixes, and the trend in the reduction in tensile loads of mixes during the overlay tests are shown in FIGURE 5. The pattern of the reduction in tensile load is similar between the Virgin HMA mix, HMA+15%RAP mix, and HMA(PMB) mix. This observation indicated that the pattern of strain-controlled crack propagation is similar in an HMA mix with or without polymers when there is no RAP present. In the case of a smaller quantity of RAP being present, ie 15% RAP, then as long as there are no polymers, then the crack initiation and propagation is similar to a virgin HMA mix. When polymers are present in conjunction with RAP, even when the RAP quantity is low, ie 15%, then the crack propagation pattern changes, and the mix leads to strain-controlled reflective cracking failure faster than when there are no polymers present in the binder. The initial tensile load of the HMA(PMB)+15%RAP mix, 1.86 kN, was higher than that of the HMA+15%RAP mix, 1.58 kN, which indicated that polymers improve the initial resistance to cracking of RAP mixes, however, this improvement is only temporary. The tensile load reduction of the two mixes that contained 30% RAP, ie HMA+30%RAP mix and HMA(PMB)+30%RAP mix, were notably different to the other mixes. Both 30% RAP mixes had higher initial tensile loads, 2.69 kN and 2.11 kN, than the other mixes, however, the overall cracking resistance of these two mixes were much lower than the other mixes. The presence of 30% RAP in conjunction with polymers appears to cause a rapid reduction in the tensile load tolerated by the mix, whereas the 30% RAP mix without polymers achieved a comparatively slower reduction in tensile load initially, although this trend increased with increasing loading. These overlay test results clearly indicated that in general the addition of higher quantities of RAP, particularly 30%, markedly increased the cracking susceptibility of an HMA mix, while the presence of polymers further increased the cracking susceptibility of a mix. This observation can be directly attributed to the proportion of aged material in the 30% RAP mix in the RAP mixes, which are often stiffer thus, making them more prone to cracking. The observed result was validated by findings from earlier published studies, where it has been found that lower quantities of RAP, such as 10% - 15%, had comparable cracking susceptibilities to virgin asphalt mixes, while the addition of higher quantities of RAP increased the likelihood of reflective cracking (24-27). It was expected that the presence of polymers would aid with the cracking resistance of the 30% RAP mix as it was observed in the earlier presented dynamic modulus and phase angle results that the presence of polymers reduced the stiffness of a mix, making the mix less stiff. However, it appears that the polymers in fact reduce the cracking resistance of the mix. This result is contrary to expected performance of polymers which are

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often used in virgin asphalt mixes to improve cracking resistance. It was concluded that the use of polymers in conjunction with RAP does not achieve the cracking performance improvements that are typically achieved by the use of polymers alone and it is recommended that polymers are not used along with RAP due to the poor strain-controlled cracking resistance.

FIGURE 5 Reduction in tensile load during overlay tests FIGURE 6 shows the number of loading cycles that were reached in the overlay test, where the results were used to assess the strain-controlled reflective cracking susceptibility of each mix. Generally, a higher number of loading cycles indicated increased resistance to cracking (7). As can be seen in FIGURE 6, there is a clear distinction in the cracking behaviour of the six mixes. As noted earlier, the Virgin HMA mix, HMA(PMB) mix and the HMA+15%RAP mix exhibited similar properties and a high resistance to reflective cracking, and the average overlay loading cycles that were reached by these three mixes were 1197 for Virgin HMA mix, 1199 for HMA+15%RAP mix, and 1161 for HMA(PMB) mix. The 15% RAP mix with polymers only reached 722 loading cycles which was notably lower than the cycles reached by the 15% RAP mix without polymers. The 30% RAP mix with polymers showed the lowest resistance to reflective cracking susceptibility, with the mix reaching only 138 overlay cycles on average. This result confirmed that polymers actually increase the reflective cracking susceptibility of mixes containing higher proportions of RAP, particularly when 30% RAP is present. This observation was contrary to expectations and findings noted in previously published literature (28), where it was noted that PMB improved the cracking resistance of RAP mixes. This observation further confirmed that the presence of PMB is actually retards the cracking resistance of RAP mixes. It can be seen that the presence of polymers caused the cracking resistance of the mixes to vary much more than when polymers were not present. The difference between the overlay cycles of the HMA(PMB) mix and the HMA(PMB)+15%RAP mix was significant (1161 vs 722; p value< 0.001). This means than even when there is a small quantity of RAP present in a mix containing polymers, the cracking performance of the mix is negatively affected to a large extent. This observation is opposite to the performance of RAP mixes without polymers, where the addition of 15% RAP did not have a significant variability in the cracking performance when compared to a virgin HMA mix.

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The variance of the number of loading cycles experienced by the test samples were all below the limit of 30% specified for the overlay test (19) which indicated that the test results were statistically satisfactory.

FIGURE 6 Number of loading cycles during overlay test The strain-controlled cracking performance results that were observed in the reported study may be explained by investigating the binder blending process that occurs between the RAP binder and the added binder. Previously published studies that have investigated binder blending have indicated that it can be assumed that nearly complete blending occurs when virgin binder is added to RAP material, and that the extent of binder blending varies depending on the quantity of RAP that is being used (29). This means that when there is a small quantity of RAP being used, ie 10%, then the contribution of binder from the RAP material to the performance of the newly blended binder is minimal. In contrast, when a higher quantity of RAP is being used, ie 40%, then the binder from RAP material has a more notable effect on the performance of the final mix. If it is assumed that a large amount of binder blending occurs, then that means the new binder has to be softer to compensate for the stiffened binder from the RAP. This binder blending phenomenon can be used to explain the unexpected cracking performance of the RAP + PMB mixes that was observed in the presented study. It is likely that the expected softening effects of the polymers were counteracted to a large extent by the stiffer binder in the RAP such that the resulting mix in fact had much stiffer binder than wholly virgin binder. This supposition was confirmed by the overlay test results which showed the Virgin HMA mix and HMA(PMB) mix having similar reflective cracking performances, while the mixes incorporating RAP and PMB had poor cracking performances when compared to the mixes that had only RAP. Additionally, the observed performance of the RAP mixes can be put down to overstiffening of the polymer + RAP mixes at higher test temperatures such as at 20°C, which is close to the test temperature of the overlay test (25°C), such that the polymer + RAP mix performed poorly when it came to reflective cracking. Investigation of the effects of binder blending was outside the scope of the presented study. It is recommended that future studies on the subject of RAP further explore the effects of binder blending on the performance of RAP + polymer mixes. CONCLUSIONS The presented study was conducted to investigate the performance of RAP mixes containing polymer modified binders, particularly investigating the effect on the rutting resistance and cracking resistance of HMA mixes. The test methodology of the study included conducting dynamic modulus tests, flow number tests and overlay tests using an AMPT machine. The results were compared

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between RAP mixes with and without polymers to determine the effect of polymers on the mechanical performance of RAP mixes. The practice of using polymer modified binders in pavement construction is commonly done to gain added performance improvements from the materials, where polymer modification has been shown to provide improved deformation and cracking resistance to HMA mixes. Overall, it was found that the presence of polymers in RAP mixes produced mixed results. One area where the polymers were seen to improve the mix performance was in the rutting resistance of the mixes, where it was found that polymers delayed the flow point of the mixes which was a measure of the rutting resistance of a mix. The polymers produced notable improvements in rutting resistance of RAP mixes at high temperatures. On the other hand, the decreases in stiffness that were observed in the RAP + polymer mixes at low temperature tests were promising, as it showed that polymers counteracted the stiffening effects of aged RAP material. When considering the reflective cracking susceptibility of RAP, it was observed that the addition of RAP, particularly 30% had a notable reduction in the cracking resistance of an HMA mix, while the use of 15% of RAP to HMA had similar cracking resistance properties of a virgin HMA mix. It was expected that the RAP + polymer mixes would have better cracking resistance than RAPonly mixes as the presence of polymers were seen to counteract the stiffening effects of RAP. However, it was observed that polymers did not improve the reflective cracking performance of RAP mixes, in fact, the test results indicated that the presence of polymers reduced the cracking resistance of the RAP mixes. The observed cracking test results were used to conclude that polymers should not be used in conjunction with RAP due to the poor cracking performance it produces. One of the reasons for the observed poor cracking performance of RAP + polymer mixes was identified as effects of blending between RAP binder and polymer binder, where is was hypothesised that the softening effects of polymers were reduced by the higher quantity of RAP and the test temperature of 25°C in the overlay test may have caused some stiffening of the RAP + polymer mix. Overall, the findings of the presented study provided valuable understandings in to the performance of RAP material, particularly the effect of polymers on the deformation and cracking performance of RAP mixes. The study also highlighted other important factors that can affect the performance characteristics which need further investigations, such as the degree of blending between RAP binder and polymer binder and how this blending process can be used to understand the performance of RAP and polymer mixes. The study findings can be used as a starting point to benchmarking field performance of RAP mixes.

ACKNOWLEDGEMENTS The authors would like to acknowledge the contribution of Fulton Hogan Ltd’s Research and Development team to the presented study. REFERENCES

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