Masato TAKAHASHI

Study on Mechanism of Top-down Cracking in Asphalt Pavement

Osamu TAKAHASHI, Takeshi NAKAMURA

 The main destruction forms in asphalt pavement are cracks and rutting. Cracks are classified into several categories depending on factors of occurrence and way of progress. One of the destructive forms of cracks is generated from the surface of the surface course called top-down cracking. Generally, when a load is applied from above the structure and tensile stress acts of the bottom due to the bending moment. Therefore, cracks are thought to progress from the bottom to the top. However, top-down cracking generate from the surface of the surface course.
 The factors of the occurrence of the top-down cracking are known as stress relaxation of the asphalt, pavement temperature, pavement depth, and load bearing capacity of base course, and subgrade. These are based on past experience and many of parts are unclear. Therefore, this study consider exist factors of occurrence of top-down cracking and clarify the mechanism of occurrence of this phenomenon. As a consideration on stress relaxation of asphalt, only compression strain flexural fatigue test was conducted. And then cracks generated from the surface of the specimen and stress behavior on the specimen were observed. For other factors, tensile stress generated in the surface course by multilayer elastic analysis was estimated, and the terms which top-down cracking occurred were considered.
 It was confirmed that cracks occurred from the surface of the specimen by only compression strain flexural fatigue test. Moreover, when the stress acting on the specimen was confirmed, compressive stress and tensile stress occurred although only compressive strain was applied. This phenomenon is caused by stress relaxation of HMA, and it is considered that cracks have occurred from the surface due to periodic tension fatigue.
 As a result of analysis, it show that a high possibility that top-down cracking will occur because the tensile stress on the surface of the surface course becomes large when the pavement structure with the base course as the asphalt base course and load bearing capacity of base course. From the above, it was found that stress relaxation of HMA is largely related to the occurrence mechanism of top-down cracking, and tensile stress acts on the surface of the pavement due to running of the vehicle and cracks are generated by accumulation thereof.