VAA Virginia Asphalt Fall/Winter 2019

28 FALL/WINTER 2019 Harikrishnan Nair, Ph.D., P.E., Senior Research Scientist, Virginia Transportation Research Council Stacey D. Diefenderfer, Ph.D., P.E., Senior Research Scientist, Virginia Transportation Research Council Benjamin F. Bowers, Ph.D., P.E., Assistant Professor, Auburn University Introduction The use of reclaimed asphalt pavement (RAP) in asphalt mixtures can bring both economic and environmental benefits, especially when large quantities of RAP are locally available. Despite the potential benefits of RAP, a major concern is that since RAP contains aged asphalt binder, mixtures with significant proportions of RAP may not perform as well in-service as mixtures with higher proportions of a virgin binder. The primary concern has been that RAP will overly stiffen mixtures, making them prone to premature cracking. Understanding and quantifying the effects of RAP content on the performance of mixtures are essential for the design of more economical and longer performing mixtures. In 2007, VDOT’s specifications were revised to allow up to 30% RAP in surface mixtures without requiring a change in the virgin binder grade. Initial research (following the change) found that in the laboratory, no significant differences existed between the mixtures with a higher RAP content (21%–30%) and the control mixtures (having a RAP content of 20% or less) for fatigue, rutting, and susceptibility to moisture. A follow-up study evaluated the in-service per- formance of these mixtures after approx- imately seven years. Laboratory testing of field cores from these mixtures, which included extracted binder grading and analysis, also found no trends in the results with respect to RAP content. VDOT researchers wondered if mixtures with up to 30%RAP content were performance- compromised, VDOT researchers wondered whether Virginia contractors could design, produce, and place mixtures with even higher levels of RAP (up to 50%). To explore this question, researchers worked with several contractors during the 2013 and 2014 construction seasons to evaluate a spectrum of mixtures with RAP content ranging from 0% to 45% in several trials around the state. This study also provided an opportunity to establish a baseline against which promising laboratory tests may be able to predict ulti- mate field performance. Field Trials—“Very High RAP” Test sections that represented “very high RAP” content mixtures were constructed in VDOT’s Fredericksburg, Hampton Roads, and Lynchburg districts. These trial instal- lations, which included various binder types and RAP content, are summarized in Table 1. For most sections, a base binder grade of PG 64-22 was used with RAP contents of 30% or greater. For lower RAP contents of 0% and/or 20%, a PG 70-22 base binder was used. In one instance, for CR 639, a softer binder grade (PG 58-28) was used with a 40% RAP mixture to evaluate the effectiveness of binder “bumping” to address the stiffening of the binder with higher RAP contents. Laboratory Evaluation Researchers collected typical quality control data and documented the produc- tion and laydown characteristics of each mixture. The loose mixture was collected during production, and cores were taken of the freshly compacted mat. The loose mixture made it possible to conduct a series of laboratory tests to explore A VIEW FROM THE LAB Very High RAP Plant Mix the effect of different RAP contents on expected performance in terms of cracking resistance, rutting, and durability. Rutting Resistance The mixtures’ resistance to rutting was assessed using two laboratory procedures. The first method, the repeated load/per- manent deformation test, is conducted in accordance with AASHTO TP 79, Standard Method of Test for Determining the Dynamic Modulus and Flow Number. The Flow Number (FN) is expected to correlate with resistance to rutting. It is generally accepted that the higher the FN, the lower the rutting susceptibility. All mixtures showed excellent rutting resistance based on flow number test- ing, regardless of RAP content. In general and as expected, mixtures with higher percent- ages of RAP showed better rutting resistance. The second test for rutting resistance used the Asphalt Pavement Analyzer (APA) and was conducted following Virginia Test Method 110. VDOT specifications limit deformation in the APA rut test to 7 mm and 5.5 mm, respectively, for surface mixtures des- ignated A and D. APA rut test results showed excellent rutting resistance for all mixtures used in this study. Mixtures with higher RAP percentages were especially stable. Resistance to Cracking Cracking resistance was evaluated through three different tests. The first two, beam Table 1. Description of Field Trials Location Mix Type RAP Content (Base Binder Grade) SR 3, King George County SM-12.5D 20% (PG 70-22); 30%, 40%, 45% (PG 64-22) Sherndon Street, City of Hampton SM-9.5A 30%, 40% (PG 64-22) US 60, Cumberland County SM-12.5D 0% (PG 70-22); 30%, 40%, 45% (PG 64-22) CR 639, Caroline County SM-12.5D 40% (PG 58-28)