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Mitigating stripping in asphalt mixtures

Moisture damage is one of the major types of asphalt pavement distress along with rutting, fatigue cracking, and low-temperature cracking. Moisture, in liquid or gas form, penetrates into the interface between aggregate and asphalt and strips aggregate particles of asphalt coating, resulting in stripping and raveling (Caro et al. 2008; Mehrara and Khodaii 2013; Kakar et al. 2015, Xu et al. 2017). Moisture damage causes the loss of adhesion between aggregate and asphalt mastic (mixture of aggregate particles smaller than 0.075 mm in size and asphalt binder) and/or the loss of cohesion within asphalt mastic. The reduced adhesive and cohesive bonding strength further contributes to other forms of pavement distress, such as fatigue cracking and permanent deformation (rutting) (Caro et al. 2008). The fundamental cause for moisture damage in asphalt mixtures is that aggregate is usually hydrophilic, not hydrophobic. In other words, aggregate attracts water more than asphalt. There is a stronger affinity between aggregate and water than between aggregate and asphalt. Water has a natural propensity to penetrate in between aggregate and asphalt and thus displace asphalt. There are many factors that affect moisture damage of asphalt mixtures, such as compositions and properties of aggregate and asphalt binder, construction quality, traffic loading and environmental conditions, chemical additives (including anti-stripping agents). It has been found that use of lime or liquid anti-stripping agents can improve the resistance of asphalt mixtures to moisture damage. The Tennessee Department of Transportation (TDOT) has long been striving to mitigate moisture damage in its asphalt pavements through performance testing and use of liquid anti-strip agents. However, moisture damage still has been observed despite these efforts. Moisture damage leads to stripping of the asphalt cement from the aggregate which in turn leads to a loss of structure in the pavement. Therefore, there is an urgent need to better identify methods to reduce moisture susceptibility of asphalt mixtures.