Effect of Tack and Prime Coats, and Baghouse Fines on Composite Asphalt Pavements

Abstract

This investigation was undertaken to develop a mechanistic design procedure to minimize the interfacial delamination distress, and to evaluate the contribution of baghouse fines to delamination in composite pavements. The need for this research was based on extensive occurrence of delamination problems in Division 13 of NCDOT. Pavements in Buncombe County, where emulsions were used as tack coat, had higher occurrence of such distresses compared to Rutherford County where PG64-22 binder was used as tack coat. In addition to use of tack coats, the asphalt mixes in these counties were prepared by an intermittent purging of baghouse fines.

Results from particle analyzer indicated similar gradations for baghouse fines and regular mineral fillers. DSR testing of mastics indicated a similar performance of mastics prepared with regular fillers and baghouses. SST (FSCH and RSCH) and APA tests results on the mixtures with and without baghouse fines did not indicate a significant difference between the two mixes. However, AASHTO T283 test indicated that mixes with baghouse fines were moisture sensitive than mixes without baghouse fines. It could be possible that dosage of anti-strip additive might not have been adequate to counteract moisture damage.

Composite AC-AC, AC-PCC and AC-CTB samples were fabricated in the laboratory and the interfacial bond strength was measured using the SST. For tack coats, it was observed that PG64-22 performed better on AC-AC interfaces, whereas CMS-2 performed better on AC-PCC interface. CSS-1h performed better than EA-P and EPR-1 as a prime coat. Non-bonded surfaces could not resist any interfacial shear.

Using a 3-D computer program, the interfacial shear stresses were computed at various thickness, loading and temperature conditions. The results indicated that delamination will not be a problem at lower temperature (20 celsius), but at elevated temperatures a minimum AC layer thickness is necessary to reduce the interfacial shear stress to laboratory measured values. For thin pavement sections, the use of prime coat was recommended to increase the bond between the AC and underlying layers.