Size effect in asphalt mixture at low temperature: Types I and II

Low-temperature cracking is a serious distress for asphalt pavement built in cold regions, such as northern USA. and northern Europe. Therefore, accurate assessment of the strength and fracture properties of asphalt mixtures is fundamental for ensuring the long-term integrity of the entire pavement structure. It has been shown that asphalt mixtures behave in a quasi-brittle manner at low temperatures and, consequently, its nominal strength strongly depends on the structure size. Most of the research performed in the past has experimentally addressed this phenomenon either on unnotched or deep-notched specimens corresponding to Type I and Type II size effects, respectively. However, the evolution of the pavement conditions during the service life can lead to the formation and propagation of cracks which strongly affect the response of the material and, eventually, determine a complex size effect. In this paper, a comprehensive experimental study is performed to evaluate the effect of different notch depths on the scaling law for quasi-brittle fracture. Three-point bending tests are performed on plain and notched asphalt mixture beams of different sizes at low temperature. The results on unnotched specimens are analysed through the Type I energetic-statistical size effect law (SEL) in combination with the Weakest Link Model, while, in the case of deep-notched specimens, Type II SEL is used. A significant variation in the evolution of the SEL is observed with a dramatic decrease in strength over size and notch length. In addition, an empirical relation between the initial fracture energy, G_f, and the total fracture energy, G_F, is found, and the characteristic length, c_f, associated with the length of the fracture process zone, is determined.