Evaluation of Asphalt Rubber and Reclaimed Tire Rubber in Chip Seal Applications
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2022-04-01
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Edition:Final Report, 05/2018 – 12/2021
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Abstract:Chip sealing is a commonly used pavement maintenance technique that aims to delay pavement deterioration by reducing water infiltration and restoring skid resistance. The aim of this study was to characterize the chemical, molecular, and rheological properties of different asphalt emulsions, evaluate their laboratory performance, short-term field performance, and cost-effectiveness in chip seals prepared with different application rates, and aggregate blends. A newly introduced tire rubber modified asphalt emulsion was evaluated that allows chip seal installation at the same temperature of a standard emulsion. Types of emulsion included a newly introduced tire rubber modified asphalt emulsion (CRS-2TR), a polymer-modified emulsion (CRS-2P), high float polymer modified emulsion (CHFRS-2P), a conventional unmodified emulsion (CRS-2), and an asphalt rubber binder (AC20-5TR). Application rates were selected based on the Louisiana Department of Transportation and Development (DOTD) specifications, the Texas Department of Transportation (TxDOT) specifications as well as from the chip seal design method recommended in NCHRP Report 680. Partial replacement of lightweight aggregate (LWA) and granite aggregate (GA) with rubber aggregate (RA) was also investigated. The loss of aggregate was measured using two laboratory performance tests: the sweep test (ASTM D 7000) and the Pennsylvania Aggregate Retention Test (PART). Superpave Performance Grade (PG), Surface Performance Grade (SPG), and the Multiple Stress Creep Recovery (MSCR) tests were conducted to evaluate the rheological properties of the binder residues. Evaluation of the differences in functional groups, molecular weight distribution, and chemical composition of the asphalt binder residues was conducted using Fourier Transform Infrared Spectroscopy (FTIR), High-Pressure Gel Permeation Chromatography (HP-GPC), and Saturates, Aromatics, Resins and Asphaltenes (SARA) fractionation, respectively. Based on the results of the experimental program, it was found that aggregate retention properties of polymer-modified and tire rubber emulsions were superior to the unmodified emulsion. While CHFRS-2P and AC20-5TR were the best performer in terms of aggregate loss, CRS-2P and CRS-2TR performed similarly followed by the unmodified emulsion. Results of the Bitumen Bond Strength (BBS) test showed a similar rank for the bond strength of the emulsions. It was also observed that the loss of aggregate in chip seal decreased with the increase in application rate. However, incorporation of rubber as aggregate in chip sealing increased the loss of aggregate in the specimens indicating poor adhesion between the emulsions and the rubber aggregate. Chemical and molecular characterization test results indicated that the tire rubber modified emulsion had lower carbonyl indices and colloidal instability indices as compared to the other conventional emulsions, indicating higher resistance to aging. On the other hand, rheological test results showed that the performance of CRS-2TR was comparable to CRS-2P and was expected to perform better than CRS-2. In the field study, seven chip seal sections were successfully constructed and were regularly monitored over an 18-month period as part of the short-term field performance evaluation. In the northbound lane, the chip seal section constructed with CRS-2TR (0.37 gsy) was the best performer statistically. In the southbound lane, the chip seal sections constructed with CRS-2TR and CRS-2P (0.31 gsy) performed similarly. Furthermore, the maximum Service Life Extension (SLE) was predicted for the CRS-2TR (0.31 gsy) chip seal sections; whereas, the chip seal sections constructed with CRS-2 was expected to exhibit the minimum SLE. In addition, the most cost-effective chip seal section was achieved by the application of CRS-2TR emulsion at the DOTD recommended emulsion application rate. Overall, the tire rubber modified asphalt emulsion provided promising results in the laboratory and in the field experiments and is expected to provide equivalent or superior performance in chip seal applications. Based on the results of this study, incorporation of the tire rubber modified asphalt emulsion is recommended in the Louisiana specifications.
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