Mechanical characterization of asphalt mixes inter-layer bonding based on reptation theory

Abstract

A bituminous pavement is composed of several layers. The bonding of the interfaces between these layers is essential to ensure the durability of the pavement structure. Un-bonded interfaces can cause early deterioration of the surface courses due to the poor distribution of mechanical loads induce by the traffic onto the base-layers. The bonding between asphalt layers depends on several parameters such as the laying temperature, the use or not of bitumen emulsions to bond the interfaces. Bonding is usually characterized by shear, tensile or torsion tests. This paper focuses on the effects of loading rate, test temperature, specimens’ size and emulsion content on the shear strength evaluated through the shear and tensile tests. In part 1, the shear bond test (SBT) and tensile adhesive test (TAT) were firstly performed on two-layered specimens bonded or not with emulsion. Their diameters were 100 or 150 mm. The tests were carried out at 50 mm.min-1 and 20 °C to evaluate the laying conditions (laying temperature, use or not of emulsion). These results were then compared to those of one-layer specimens without interface. The results of part 1 investigations showed that: i) the shear strengths of small samples are 1.3 times higher than that of big specimens, ii) the shear strength resulting from different laying conditions can be classified, from the highest to the lowest, as follows: hot on hot (H/H) bonding, hot on cold (H/C) bonding with emulsion C69B2 and hot on cold (H/C) bonding without emulsion, iii) the failure strengths of TAT are 2.2 times lower than those of SBT. In part 2, the SBT was performed on two-layered specimens of 100 mm in diameter at three loading rates (0.5, 5, 50 mm.min-1) and test temperatures (−10 °C, 5 °C, 20 °C). In this second test series, 250 and 500 g.m-2 of bitumen emulsion C69B2 and styrene-butadiene rubber (SBR) polymer emulsion were used to bond the interfaces. Based on these results the master curves of shear strength were built based on the frequency-temperature superposition principle (FTSP). Furthermore, an explanation of the interfaces bonding mechanism based on the bitumens’ macromolecules chains mobility across the interface is proposed. This interpretation required the introduction of some interface bonding parameters: the degree of intimate Dic contact, the degree of healing Dh and the degree of bonding Db. Finally, the master curves of the degree of healing and degree of bonding were also built based on the frequency-temperature superposition principle (FTSP).