Balancing asphalt mixes containing recycled materials for optimum field performance

Asphalt concrete (AC) is the most common material used for the construction of pavements in the U.S. and many other countries in the world. With the increasing use of recycled materials like reclaimed asphalt pavement (RAP) and recycled asphalt shingle (RAS), the cracking resistance of asphalt pavements might be jeopardized. Therefore, such AC mixtures should be carefully investigated to ensure that performance is not compromised in the pursuit of an economical and sustainable solution. This study investigates 17 mixes including five leveling binders (LB) and 12 surface mixes, which have been used by the Illinois Department of Transportation (IDOT) in different rehabilitation projects across the State of Illinois, between 2013 and 2015. These AC mixes contain asphalt binder replacement (ABR) from 15 to 60%, acquired from RAP and RAS, whereas these mixes also contain steel slag and crushed concrete. In order to evaluate these AC mixes, a thorough laboratory testing, including indirect tensile creep compliance and strength, Hamburg wheel tracking test, and the Illinois flexibility index test (I-FIT) was conducted to evaluate thermal cracking susceptibility, rutting, and cracking resistance, on plant-mixed lab-compacted (PMLC) mixes and three sets of field cores. The extensive exploratory data analysis on creep compliance showed that softer binder and higher asphalt content result in more compliant mixes, while recycled materials tend to decrease the compliance of AC mixes. Likewise, softer binder and recycled materials tend to counterbalance each other’s impact. Similarly, the LB has relatively higher creep compliance as expected due to its design which is intended to retard reflective cracking. The IDT strength at -10°C is very similar across all AC mixes irrespective of the amount of recycled materials. Results also show that IDT creep compliance and strength might not distinguish the effect of field aging. The Pavement Mechanistic-Empirical Design Guide (MEPDG) uses the empirically developed model for the prediction of creep compliance as an input for Level 3 analysis, which uses mix and binder properties. However, the impact of recycled materials was not captured in the MEPDG nor in the modified models. This research proposes a new model to predict creep compliance that captures the effect of recycled materials. The I-FIT results show that PMLC mixes have relatively lower flexibility index (FI) as compared to first field cores due to the difference in aging and compaction conditions. FI decreases with the increase in aging and recycled materials, while the strength and secant modulus increase. All AC mixes offered excellent rutting resistance, because they contain high ABR, and the resistance increased with aging as expected. The balanced mix design approach was utilized, based on rut depth and FI, which showed the convergence of AC mixes to stiffer zone with field aging. Similarly, secant modulus was added to the existing balance mix design as a stiffness indicator to control soft mixes. Furthermore, the field results confirm an excellent rutting performance in all sections. In addition, an exponential increase in transverse cracking was observed, which correlates with FI values obtained on the corresponding mixes. Thus, it is believed that I-FIT is a very simple, promising, and distinguishable test for evaluating the cracking resistance of AC mixes, which could be used as a performance evaluation tool along with rutting results to obtain balanced mix designs