Tsutomu OTANI

Study on the Evaluation Method of Crack Propagation Rate of Asphalt by Semi-Circular Bending Test

Osamu TAKAHASI

Cracking is known to be one of the main distresses that cause structural failure of pavements. Failure by cracking is caused by interconnected cracks that develop as a result of load-related, insufficient pavement thickness, overloading, moisture damage, or a combination of these factors. These factor combined cause cracks to grow bigger, moving through the different layers of pavement structure, to the point these cracks become visible. The crack grows bigger and it becomes a visible damage state. As a mean of countering this distress, it is important to understand the mechanics that cause cracking to appear and grow through the pavement structure.
In this study, as a way of evaluating the resistance to cracking, a cracking gage was used to obtain the crack propagation rate based on results from the fatigue bending test. 
However, the fatigue bending test is a very time-consuming and expensive experiment. Therefore, this research aimed to evaluate resistance to cracking by means of a simple Semi-Circular Bending test (SCB) that is commonly used outside of Japan. 
In the first phase of the experimental work, the crack propagation rate was obtained from the fatigue bending test. Results revealed that the crack propagation rate of straight asphalt was the larger than that of modified asphalt. Furthermore, the crack propagation rate at high temperature was larger than propagation at low temperature.
After the first phase was completed, it was decided to start work with the SCB test. The main output of the SCB test is a load-displacement curve. From all results, focus was given to the slope of this curve after the load peak. A high correlation was observed between the slope that followed peak values and the crack propagation rate from the fatigue bending test. In addition, when comparing the number of crack occurrence obtained from the fatigue bending test with the slope before the peak value of the load-displacement curve obtained from the SCB test, there was a high correlation between both.
From these results, resistance to the occurrence of cracks can be evaluated by the slope before the peak value. The same can be said about the slope plotted after the peak value is registered, which provides insight into crack propagation process happening in the pavement structure.