Tack Coat Bond Strength Evaluation Methods and Mechanistic Design of the Interface for Multilayer Asphalt Pavement

Asphalt pavements are layered structures with layers bonded together using tack coat materials. Several distresses including slippage and delamination are due to the failure of the interface between asphalt layers. To prevent the distresses associated with the debonding of the pavement layers, tack coat should be selected based on the state of stresses at the interface between pavement layers. Currently, tack coats are chosen based on the manufactures recommendation or empirical methods and there are no mechanistic methods for selection of tack coat materials and design of the interface.;In this study, the mechanisms of the development of the crescent shape slippage cracks and delamination distress in the asphalt pavement structure were investigated by viscoelastic finite element models. Besides, a mechanistic design methodology for the design of the interface between layers in the multilayer hot mix asphalt (HMA) pavement structure is presented. The presented mechanistic design methodology incorporates proper materials testing and structural analysis considering complex hot mix asphalt behavior.;During last decades, several test methods were developed by various researchers to evaluate the shear bond strength of the tack-coated interface between asphalt pavement layers. Some devices, evaluate the shear bond strength in direct shear with normal confinement and other devices, test the interface shear bond strength in direct shear without normal confining pressure or use passive confinement to evaluate the interlayer bond strength. It was found that for the design of the interface direct shear tests with normal confining pressure should be used. A methodology is presented for selecting the appropriate level of normal confinement for testing the shear bond strength of the interface between asphalt pavement layers.;The effects of the debonded pavement surface layer on the behavior of the pavement structure were illustrated and the mechanism of the slippage and delamination of the pavement surface layer due to the interface failure explained. In a pavement with a bonded interface, it was shown that the horizontal tensile stress in the surface layer could cause crescent tensile cracks in the surface layer. Therefore, designing of the pavement structure for tensile stress due to the braking force at the critical condition of high friction coefficient, high temperature and low speed of the braking wheel should be incorporated into the pavement design procedure.;Despite the importance of the tack coat application on the bond strength of the asphalt interface and the long-term behavior of the pavement structure, there is no standard quality control method to ensure the proper application of tack coat in situ. In this research, two existing instruments for testing tack coats performance in the field, i.e. ATacker TM and TACKY were evaluated. The tests results of the instruments were scattered and with the current performance, the instruments were not recommended for the standard field tack coat material quality control and further improvement of the instrument are needed. It was proposed to place an aluminum plate on the existing asphalt surface before the application of tack coat and evaluating the bond strength of the sample collected on the aluminum plate by The Pneumatic Adhesion Tensile Testing Instrument (PATTI) that is an ASTM D 4145 adhesion tester. PATTI test procedure and results indicate a promising method that can be used to evaluate the bond strength of different tack coat materials in the field for the quality control of materials or design purposes. The shear bond strength of the asphalt pavement interface was predicted by correlating the tensile bond strength of the samples to the interface shear bond strength.