One of the most common causes of seal coat failures is the presence of dust on the cover aggregate, which prevents good adhesion between the applied bituminous binder and the aggregate. Precoating the aggregate with a thin film of bituminous binder usually solves the dust problem and provides good adhesion. This research was undertaken (a) to evaluate the adhesion of aggregates precoated to varying degrees so that the optimum precoating requirement can be established, and (b) to develop an end-result type test in lieu of the subjective visual test for accepting precoated aggregates. Five AASHTO No. 8 Aggregates of different mineralogical compositions and absorptive characteristics were used. These aggregates were precoated with MC-30 cutback asphalt to varying degrees (from a salt-and-pepper effect to 90% or more coating). The Pennsylvania aggregate retention test developed in this study was used to evaluate the effect of precoating on aggregate retention loss. Immediate adhesion of the cover aggregate with the bituminous binder was best obtained at 90% or greater precoating. The agreement (reproducibility) between different evaluators who made subjective visual evaluations of the percent precoating was also by far the best at a level of 90% or more. Of the three end-result type tests attempted, dry gradation test of the precoated aggregate was determined to be most appropriate with an acceptance criteria of 0.5% maximum minus 200 (dust). It has been recommended to use AC-20 asphalt cement as a precoating material in lieu of MC-30 cutback asphalt, because it can be mixed at higher temperatures in a hot-mix asphalt (HMA) plant, does not need any curing, and will cause better aggregate retention.
Abstract