Laboratory assessment of the resistance to crack propagation in high-stiffness asphaltic materials

This paper describes laboratory work undertaken to measure the resistance to crack propagation of bituminous materials comprising a range of binder grades from different sources. The compact tension (CT) test has been adapted for use with bituminous materials, and an image analysis technique, based on digital photographs of the specimen taken during testing, has been used to quantify crack length. Linear elastic fracture mechanics (LEFM) principles have been used to calculate the mode I stress intensity factor (SIF), and the Paris law has been used to quantify crack growth. Results show that the general trend is for the Paris law exponent n to decrease and for the multiplier A to increase with increasing temperature for each of the materials, indicating that, the harder the bitumen, the greater is the sensitivity to overloading. A good correlation was found between the values of A and n, and also between the values of A in the Paris law and the stiffness modulus of the asphaltic mixture. It was found that mixtures containing the nominal 15 penetration grade binders were least sensitive to variations in stiffness of the mixture, and mixtures containing the nominal 35 and 50 penetration grade binders were most sensitive to variations in stiffness of the mixture. The number of standard wheel loads required to cause a crack to propagate through the asphalt layer of a typical minor and major flexible pavement structure have been estimated using data from the CT testing. Results show that there is a good correlation between the stiffness modulus of the asphalt mixture and the predicted number of load applications to failure. The predicted number of load applications to failure increases as the stiffness modulus of the asphalt increases.