During mix design, reclaimed asphalt pavement (RAP) materials are commonly preheated to bring the material to the mixing temperature. However, preheating also ages the RAP binder. Different preheating procedures can lead to variations in aging and mechanical properties of asphalt mixtures.
Currently, there is no standard RAP preheating method for laboratory mix design. Various state departments of transportation (DOTs) and contractors use a range of procedures. As Balanced Mix Design (BMD) gains popularity, differences in RAP preheating procedures may lead to disputes between agency and contractor results. Furthermore, as RAP contents increase, the impact of RAP preheating procedures are likely to be amplified. Therefore, this study aimed to:
This study first conducted a survey to summarize the current RAP preheating practices among DOTs and contractors. Responses received from 39 state DOTs and 46 contractors are categorized and summarized in Table 1. Approximately 90% of state DOTs and contractors reported using one of four RAP preheating methods, detailed in the table below as Methods 1 through 4. Methods 1 and 2 preheat the RAP by blending with preheated or cold virgin aggregate, while Methods 3 and 4 heat the RAP separately from the virgin aggregate.
Additionally, 11% of contractors directly blend cold RAP with hot virgin aggregate without preheating, and 8% of DOTs do not have established RAP preheating procedures due to the absence of a mix design program or prohibition of RAP usage. This study selected the four most widely used RAP preheating procedures (Methods 1 - 4) for the laboratory testing.
Table 1. Summary of RAP Preheating Survey
To evaluate the impact of different RAP preheating procedures on the volumetric and BMD properties of asphalt mixtures, two RAP mix designs were selected with the consideration of binder type, RAP sources, and percentage. One mix design was selected from the 2021 cycle of the NCAT Test Track, and the other was chosen from the 2022 MnROAD construction to represent mix designs in southern and northern climates, which were denoted as NCAT mix design and MnROAD mix design, respectively. Both designs were 12.5 mm NMAS Superpave mixtures with a recycled binder ratio (RBR) of approximately 0.20, and the virgin binder used in NCAT and MnROAD mix designs were PG 76-22 and PG 58-28 binders, respectively.
First, only the RAP materials used in two mix designs were preheated using four selected procedures, and the RAP binder was extracted and characterized using a rotational viscometer, a Dynamic Shear Rheometer (DSR), a Bending Beam Rheometer (BBR), and Fourier Transform Infrared (FT-IR) spectroscopy. Figure 1 presents the selective test results of extracted RAP binders, including viscosity, high-temperature performance grade (PG), Glover-Rowe (G-R) parameter, and FT-IR carbonyl area parameter. Note that the control RAP is the RAP without preheating. As shown, the extracted binders from preheated RAP had higher viscosities, high-temperature PGs, and G-R parameters compared to the control RAP binder, indicating further stiffening and aging during preheating. Method 3 caused the most severe aging based on rheological test results. The FT-IR test showed similar or slightly higher carbonyl area for preheated RAP binders compared to control binders, indicating that oxidation aging was not severe enough to be captured by FT-IR.
Figure 1. Effects of Preheating on RAP Binder Properties
Subsequently, asphalt mixtures were fabricated using the four selected preheating protocols and characterized using volumetric measurements, Indirect Tensile Asphalt Cracking Test (IDEAL-CT), and Hamburg Wheel Tracking Test (HWTT). Figure 2 presents the results of air voids and VMA for mixtures prepared using the four preheating methods. For the NCAT mix design, Figure 2 indicates that the air voids and VMA results were impacted by the four methods, with a maximum difference of around 1%. However, for the MnROAD mix design, there were no significant differences in air voids and VMA among the four methods. These varying trends suggest that the impact of RAP preheating on volumetric properties depends on the mix design.
Figure 2. Effects of RAP Preheating on Volumetric Properties of Asphalt Mixtures
Figure 3 presents the IDEAL-CT and HWTT results for mixtures prepared using the four preheating methods. The statistical group analysis indicated that the NCAT mixture fabricated using Method 3 had a statistically higher CTIndex result compared to the other three methods. For the MnROAD mix, Method 4 had the highest CTindex, but the results were not statistically different from the other three methods. As shown in Figure 3 (b), the mixtures prepared with four methods generally exhibited comparable rut depth results for both mix designs, except that the mixture prepared using Method 1 had approximately a 2 mm lower rut depth than the others for the MnROAD mix design.
Figure 3. Effects of RAP Preheating on BMD Properties of Asphalt Mixtures
In this study, binder testing confirmed that the RAP binder aged further during all preheating processes, and mixture test results indicated that the RAP preheating method affected mixture volumetric properties and CTindex to some degree, but the impact depends on other aspects of the mix design. Mix designers should conduct comparison experiments during production to determine whether their current RAP preheating procedures during mix design align with corresponding plant production regarding the BMD properties of asphalt mixtures. The research team proposed experimental design steps to assist contractors and producers in identifying the appropriate RAP preheating method as documented in the NAPA IS-146 report.
Contact Chen Chen for more information about this research.