Compact Tension Testing of Asphalt Binders at Low Temperatures

Abstract

This paper documents and discusses the development of a compact tension (CT) test for the grading of asphalt binders. The geometry was chosen because it provides an easy way to obtain the plane-strain fracture toughness ( KIc), fracture energy ( Gf), and crack tip opening displacement (δ t) in brittle failure on a small sample. It is believed that the method will allow for a better ranking of binders in regard to their low-temperature fracture resistance. CT specimens were prepared in different sizes with varying notch depths. KIc was found to be constant, regardless of the notch depth or specimen width for both straight and modified binders. Gf was found to decrease with notch depth; this is thought to be the result of energy-absorbing mechanisms away from the crack tip. Deeper notches or an energy correction is able to account for that issue. A slightly different fracture energy, Jf, was obtained in a more direct fashion from the slope of a plot of the normalized failure energy ( Uf/B) versus notch depth ( a). Jf provided results close to those obtained with an energy correction from a single notch depth. Reproducibility of the fracture test was found to be good with pooled standard deviations of 5% to 10% for KIc and 15% to 20% for Gf, which is typical for such tests. Given the fact that brittle fracture properties can vary by orders of magnitude for binders of the same Superpave® grade, it is concluded that the test method has a high ability to reveal statistically significant differences in toughness.