The use of the rheology to describe the asphalt fracture behavior occurring in the Poker Chip test

Abstract: Since the 1960s the importance of relating asphalt binder stiffness with fracture performance has been known. However, fracture tests have often been performed with consideration of the time and temperature effects that define the underlying asphalt binder stiffness.

Recently a Poker chip test has been introduced in the asphalt binder industry as an improved method to define cracking performance. The test measures the force and ductility of an asphalt binder under a constant load rate application test, with a specimen configuration similar to that of a poker chip – but specifically with a specimen dimension that is 57.15mm diameter and with a 1.5875mm thickness made with 4.5+/- 0.05 grams of asphalt binder. The top plate which adheres to the sample has a diameter of 50.8mm. At the required test temperature, the top plate is moved vertically, to maintain a monotonically increasing load at a rate of 2N/s. Most of the testing conducted in the USA to date has been carried out at a room temperature of 25C. However, in this testing we have conducted tests ranging from 0 to 30°C to investigate if this method can be used to define the brittle to ductile/flow fracture behavior of the asphalt binders used in the shingles investigated. We anticipated that as this work was started that we should be able to rank products and see a transition from ductile/flow to brittle behavior over that temperature range which would be strongly correlated to the binder stiffness, expressed as a relaxation modulus. In our review of published data, we observed that binder rheological properties can be strongly correlated to data from the Poker Chip test, thus making this a viable candidate test for assessment of the performance of ultimate properties.

In the initial experimentation it was noticed that the data produced did not have a clear definition of the start time and in addition, for a constant load application test, the shape of the stress strain curve should have a reducing tangent modulus, whereas the tangent modulus slowly increases at the start of the test. This occurs due to slack in the equipment and a scheme was developed to reduce the effect of this and define the start time. An analysis method was then developed and this shows that when a binder is tested and combined with a rheological analysis the poker chip method clearly defines the brittle to ductile transition that is commonly seen in other types of tests, such as the direct tension test and fatigue tests on asphalt binders. The brittle to ductile transition that occurs in this test is consistent with that defined in many other methods. This paper presents that analysis scheme and discusses the importance of temperature and loading rate effects on the definition of fracture properties in this test.