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Asphalt Fatigue Testing

Preventing Cracks on Our Roads The Ministry of Transportation, in partnership with Queen's University, is working on a project to develop a new method for testing and selecting asphalts to improve the fatigue life and low temperature performance of asphalt pavement. Given the high cost of total reconstruction when a road fails through fatigue and low temperature-related distresses, a reliable testing method is required to predict fatigue life and prevent the need for early rehabilitation and reconstruction. The main issue is to discern what factor or factors are responsible for the differences in performance between asphalts that apparently meet the same specification grades. The intent of this project is to identify what factors need to be considered in order to provide MTO with a better tool to design the asphalt for our construction work.

Based on the performance results from test roads in northern Ontario, the current SuperpaveTM asphalt grading method, which is widely used in North America, has proven inadequate at predicting low temperature and fatigue fracture performance. Road failure can be reduced and cost savings realized by the proper selection of asphalt and an optimized mix design to account for low and high temperature and fatigue performance. Therefore, there is a need to develop a test that accurately predicts the fatigue life and low temperature cracking of asphalt. If such a test could replicate the distress behaviors as they occur under various road conditions, MTO could avoid inferior binders or specify superior binders when needed.

The new method, based on a fracture energy analysis, measures the energy released during the fracture process in the asphalt binder. This method has been successfully used to design fracture-resistant polymers, metals, and other composite materials, but so far has not been used in asphalt binder selection. MTO is introducing this more fundamental method to rank a variety of modified and unmodified asphalt binders and mixtures. Because MTO uses a large number of modified asphalt binders, it is difficult to compare these divergent groups of materials without resorting to measuring the fundamental material properties as determined by the fracture energy test.

Asphalt binders and mixtures from several Ontario highways were tested by the fracture energy method. The test was performed by pouring asphalt binders into silicone molds with inserts placed on both ends. The inserts were pulled apart at different speeds and temperatures until fracture occurred. Even though the reproducibility of the mixture tests was poor compared to what it was for the binder tests, clear performance differences were found for the materials tested. The binder test method is highly reproducible and it can show significant fracture performance differences between binders of the same SuperpaveTM grade. These differences highlight the fact that different modification methods lead to different performance.

Highway 17 testing showed that the fracture energy method could predict the likely contributing factors to premature pavement distress. Similarly, for the Highway 118 test sections constructed in 1994, the new test method showed that one section with significant wheel path distress had low resistance to fracture at various temperatures.

Asphalts used in overlay contracts on Highways 401, 416, and 417 showed signs of premature cracking months after construction. Results of testing for fracture resistance explained why these binders failed so soon into their service life.

Highway 631, a carefully constructed test site of 12 years old, was the oldest road monitored. It is interesting to see that while the structural and design factors were the same, the large variations in performance between samples of nearly the same grade were due to differences in the asphalt fracture and aging behavior.

For Highway 655, a newly constructed test road being carefully monitored, MTO is confident that the fracture energy test will account for the performance differences already apparent in its test sections.

The case studies discussed demonstrate that the current low temperature specification system does not adequately reflect the performance potential of asphalts. Premature cracking is often caused by a combination of physical aging and low fracture resistance. The new method is an improvement over current specification systems as it is able to detect chemical and physical aging as well as fatigue cracking as potential aggravating factors in low temperature failure of asphalt mixes. It is important to obtain high fracture energy values if superior low temperature and fatigue resistance are to be obtained.

The development of the new fracture test method has many implications for MTO in regards to improving the performance of asphalt mixes. Testing has shown that the premature transverse and wheel path cracking that occurred on Highways 401, 416, 417, and 655 could have been prevented if the improved binder grading method had been used.

The preliminary results support the proposition that a new binder specification test is not only needed but also feasible. With the ability to test these desirable fracture properties, it is now possible for MTO to incorporate them into its asphalt mix designs to produce longer-lasting roads and to eventually reduce long-term maintenance costs.