Factors Affecting the Interlayer Shear Strength of Laboratory and Field Samples

Abstract

Asphalt emulsion is the most widely used tack coat material in the U.S. The objective of this study is to investigate factors that may affect the interlayer bond shear strength of asphalt emulsion tack coats of both laboratory and field compacted samples. The laboratory study included six types of tack coat materials applied on two surfaces with two residual application rates. The field study phase involved validation of the interlayer shear performance findings using field cores extracted from paving projects. The field study included taking cores of the existing layer, emulsions used for interlayer bonding, and loose mixes of new asphalt layers. Materials were collected to produce the laboratory prepared specimens for comparison with the field cores. Results of the laboratory study demonstrate that there is a direct relationship between the roughness (texture) of the existing surface and the interlayer shear strength (ISS) between two surfaces. Statistical analysis provided a strong correlation and indicated that 79% of the data variance can be explained with surface texture, emulsion type, application rate, and replicate effects. Comparing field cores with laboratory produced samples showed no clear relationship between the shear strength of laboratory and field specimens. It is speculated that the difference in compaction of the upper layers in the laboratory and field, and effect of shearing during coring of the samples from the field, resulted in higher laboratory shear values relative to field core values. The study highlights major challenges in using laboratory prepared samples to predict field behavior of tack coats.