Compact Tension Testing of Asphalt Binders at Low Temperatures

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 ( K Ic ), fracture energy ( G f ), 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. K Ic was found to be constant, regardless of the notch depth or specimen width for both straight and modified binders. G f 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, J f , was obtained in a more direct fashion from the slope of a plot of the normalized failure energy ( U f /B) versus notch depth ( a). J f 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 K Ic and 15% to 20% for G f , 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.