Moisture damage characteristics of warm mix asphalt mixtures
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Warm Mix Asphalt (WMA) mixtures are produced and placed at significantly lower temperatures than conventional Hot Mix Asphalt (HMA) mixtures. WMA mixtures can be produced by means of various technologies such as foaming (moisture-based) agents, wax-based additives, emulsion-based products, and surfactants. The lower temperatures provide several benefits such as reduced emissions and odors, decreased energy consumption for production, and improved environmental working conditions at plants and paving sites. A concern that arises from the use of WMA technologies is the effect of lower production and placement temperatures on the moisture susceptibility of asphalt mixtures. This concern is primarily based on the fact that lower temperatures could lead to the possibility of inadequate drying of aggregates. Furthermore, some WMA technologies introduce water into the mixture to increase its workability. Aggregates that are not sufficiently dried and/or the introduction of water into the mixture can adversely impact the adhesion between the aggregates and the asphalt binder and the cohesion of the mixture. Adhesion and cohesion failures can lead to moisture damage. This study consisted of two parts. The objective of the first part was to conduct an internet survey of materials and construction engineers in each of the fifty state Department of Transportation (DOT) agencies to determine if moisture damage related distresses have occurred in WMA field projects placed in the United States. Based on the responses received there was no evidence of moisture damage related distresses in the field. The second part of the study involved conducting laboratory measurements of the resistance of various WMA mixtures to moisture damage. Moisture susceptibility was assessed using the Hamburg Wheel Tracking Device (HWTD); E*stiffness ratio (ESR); adhesive bond energy based on surface energy measurements of asphalt binders and aggregates; and fracture characteristics of the mixtures tested under dry and wet conditions. These methods were selected because they are believed to assess different mechanisms of moisture damage. The tests were conducted on specimens that were aged at different times and different temperatures. In general, the results showed that most WMA technologies have good resistance to moisture damage. The difference in moisture susceptibility between the WMA and control (hot mix asphalt) mixtures was dependent on the aging time. An increase in aging time decreased the difference in performance between WMA and HMA mixtures. The HWTD results indicated that the resistance to moisture damage improved with the incorporation of anti-strip agents (chemical and hydrated lime).