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1 Background Economic, environmental, and engineering benefits moti- vate the reduction of production and placement temperatures for asphalt mixtures. The latest technology that has been rap- idly adopted for this purpose is warm mix asphalt (WMA), which is traditionally defined as an asphalt concrete paving material produced and placed at temperatures approximately 50°F (28°C) cooler than those used for hot mix asphalt (HMA). However, in the context of this project, WMA is defined as an asphalt mixture produced with warm mix additives (i.e., sur- factant, wax, etc.) or mechanical foaming processes regardless of the production temperature. WMA technologies offer many benefits such as improved workability and compactability, reduced aging, and better resistance to cracking and raveling. However, there has always been a concern regarding the early life performance of WMA mixtures, especially with respect to the potentially higher susceptibility to rutting and mois- ture damage due to the incomplete drying of the aggregates, reduced binder absorption by the aggregates at lower produc- tion temperatures, or the incorporation of additional moisture in the foaming process. NCHRP Project 9-49 âPerformance of WMA Technologies: Stage IâMoisture Susceptibilityâ focused on the evaluation of moisture susceptibility of WMA mixtures. Laboratory-mixed laboratory-compacted (LMLC) specimens, plant-mixed laboratory-compacted (PMLC) specimens, and field cores obtained from four field projects were evaluated to develop guidelines for identifying and limiting moisture susceptibility in WMA mixtures. The main product from NCHRP Proj- ect 9-49 is summarized in Figure 1, which details the proposed laboratory short-term oven aging protocols on asphalt loose mix and long-term oven aging protocols on compacted asphalt mixtures and thresholds for three different standard laboratory tests used to assess moisture susceptibility of WMA mixtures (Epps Martin et al., 2014). These thresholds were established by discriminating nine WMA mixtures from four field projects with good and poor field performance in terms of moisture sus- ceptibility (i.e., raveling). The flow chart presented in Figure 1 was produced as a set of guidelines for mix design and quality assurance (QA) of WMA mixtures. Since the aging protocols and moisture susceptibility thresholds were developed based on a limited number of field projects, further validation of the flow chart or use on a trial basis was recommended prior to adoption. The continuation work from NCHRP Project 9-49B âPerfor- mance of WMA Technologies: Stage IâMoisture Susceptibility Validationâ described in this report focused on further cor- roboration of the moisture susceptibility thresholds included in the flow chart. A follow-up web-survey of state DOTs and contractors to the one conducted in NCHRP Project 9-49 was performed to identify WMA mixtures with available field performance plus mix design and/or QA data including wet indirect tensile (IDT) strength and tensile strength ratio (TSR) by AASHTO T 283, wet resilient modulus (MR) and MR ratio by modified ASTM D 7369, and Hamburg wheel tracking test (HWTT) per AASHTO T 324. Reports from related NCHRP Projects 9-47A âProperties and Performance of WMA Tech- nologiesâ and 9-49A âPerformance of WMA Technologies: Stage IIâLong-Term Field Performanceâ were reviewed to identify additional WMA mixtures having this same type of information. In addition, recent relevant literature on field per- formance of WMA mixtures, laboratory moisture susceptibil- ity tests, and moisture conditioning procedures were reviewed. Finally, a laboratory experiment was performed to assess addi- tional moisture conditioning protocols as alternatives to the modified Lottman protocol per AASHTO T 283, and to explore various specimen-drying methods and their effects on the rec- ommended moisture susceptibility parameters. C H A P T E R 1
Figure 1. NCHRP Project 9-49 Proposed WMA moisture susceptibility evaluation for mix design and QA (Epps Martin et al., 2014).
