Factors affecting the interface shear strength of pavement layers

Flexible pavement structures are built in layers with a certain degree of bonding at the interface, which is affected by several factors which could be either material related, or construction related. The tack coat material type, residual application rate, tack coat coverage, and pavement surface type are attributed as material related factors. On the other hand, cleanliness, moisture, and vertical confinement pressure could be classified as construction related factors. The objective of this study was to quantify the effects of these factors on the pavement interface bonding in terms of interface shear strength. Five types of tack coats, four of which were asphalt emulsions; CRS-1, SS-1h, SS-1, and Trackless, and one paving grade asphalt binder, PG 64-22, were considered at three residual application rates; 0.14 (0.031) l/m2 (gal/yd2), 0.28 (0.062) l/m2 (gal/yd2), and 0.70 (0.155) l/m2 (gal/yd2). The selected tack coats were applied on four different pavement surface types; existing HMA, new HMA, milled HMA, and Portland Cement Concrete (PCC). In addition, for each pavement surface type, a “no tack coat” condition was included in the analysis. The influence of moisture and cleanliness were evaluated by careful application of water and dirt respectively. To simulate these test conditions, test specimens were obtained from the test sections designed and constructed at the Louisiana DOTD Pavement Research Facility using conventional tack coat application method and paving equipment. Specimens were extracted from the research facility and were tested in the laboratory at 25 (77) oC (oF) under two vertical confinement pressures, 0 (0) kPa (psi) and 138 (20) kPa (psi), using a direct shear test device. The study yielded several important conclusions. The results strongly supported the need of applying tack coats for improving pavement interface bond strength. Except for the milled surface, the rest of the surfaces failed to adhere with the HMA overlay, in absence of a tack coat. Trackless emerged as the most effective tack coat, while SS-1 and CRS-1 were among the least effective tack coat materials. On a general basis, an increase in the residual application rate showed an increase in the interface bond strength, which is why an optimum application rate could not be established within the application rate range considered in the study. Among the four pavement surface types, milled HMA surface provided the highest bond strength with the HMA overlay. The presence of dirt at the interface remarkably improved the pavement interface bond strength. Presence of moisture did not have any consequential influence on the interface bond strength. Irrespective of tack coat material type, residual application rate, and pavement surface type, interfaces subjected to 138 (20) kPa (psi) vertical confinement pressure a provided higher shear strength than those to 0 (0) kPa (psi). However, the effect of confinement diminished with an increase in the residual application rate