Effect of Tack Coat Type and Application Rate on Bond Strength of Flexible Pavement Layers

This research investigates the influence of tack coat type and application rate on the interlayer bond strength between flexible bituminous pavement layers. A strong bond at the interface is essential to ensure structural integrity and long-term performance under repeated traffic loads. Two layer combinations are considered in this study: bituminous concrete (BC) over dense bituminous macadam (DBM) and semi-dense bituminous concrete (SDBC) over bituminous macadam (BM) representing commonly used pavement configurations in practice. To assess the bonding behavior, three types of tack coat materials were utilized: VG-30 paving-grade bitumen, Cationic Rapid Setting (CRS-1) emulsion, and Cationic Medium Setting (CMS-2) emulsion. The specimens were subjected to testing at service temperatures of 25°C, 30°C, 35°C, and 40°C, with varying tack coat application rates. A custom-fabricated bond strength testing device, integrated into a Modified Marshall Stability Apparatus, was employed to evaluate the interlayer shear strength of laboratory-prepared cylindrical specimens. Experimental findings reveal that interlayer bond strength declines with increasing temperature due to reduced material stiffness. The performance of tack coats varied with both emulsion type and dosage, with optimal bond strengths achieved at specific application rates depending on the material properties and mix combinations. Notably, CRS-1 emulsions demonstrated superior performance at lower temperatures, while VG-30 bitumen showed improved bonding at intermediate application rates.

This study provides critical insights into the selection and application of tack coat materials, contributing to enhanced pavement durability, minimized surface distress, and overall improved structural response under traffic loading.