State-of-the-art test equipment for determining the crack initiation and crack propagation properties of asphalt mixtures
In recent years many mixture design methods have produced materials that are stiffer, leaner and more resistant to rutting, however, such materials are often more susceptible to fatigue and reflection cracking. The Texas Overlay Tester (CRT-TOT) was designed to simulate the expansion and contraction movements that occur in the vicinity of joints or cracks and which result in reflection cracking in overlays. With the CRT-TOT it is possible to characterize both the crack initiation and crack propagation properties of asphalt mixtures. Cooper has developed the CRT-TOT, which is a dedicated, state-of-the-art piece of equipment for carrying out this test.
Increasing bitumen content will significantly improve the reflective cracking resistance of asphalt mixtures and materials with low resistance can fail in minutes. TOT results have correlated well with field performance and have shown good correlation with both beam fatigue test results and the low temperature performance of asphalt mixtures in the field. The test is rapid and has been shown to be repeatable The TOT is performed on standard size samples, typically 6 inch (150 mm) long by 3 inch (75 mm) wide by 0.5 to 1.5 inch (12mm to 38 mm) high. These specimens can be cut from field cores or from laboratory compacted specimens, which can be prepared with the Cooper Gyratory Compactor (CRT-GYR). The specimen is precisely positioned and glued to two base plates using the gluing frame (CRT-TOT-GLUE). This glued assembly is put into the machine using the “zero stress” carrying frame. Sample dimension and test conditions are entered through the surface mounted Cooper TOUCH+ touch-screen panel. A test can be instantly started by the user, or will commence after a target test temperature has been reached.
A triangular waveform in constant deformation mode is applied, alternately extending and compressing the specimen so as to simulate the movement of the cracked, rigid material under the overlay. The test will automatically stop when the test end conditions have been met. It is suggested that the TOT could complement mixture design methods where the emphasis is mainly on resistance to rutting thus ensuring that mixtures are not too lean and prone to cracking. It will also provide the means of assessing different binders and binder modifiers.
A proposed new ASTM International standard for “Determining the Susceptibility of Bituminous Mixtures to Reflective Cracking using the Overlay Tester” is currently under development. It is hoped that this International standard will improve the test method by providing a unified sample preparation procedure as well as overall improved precision. The CRT-TOT will, of course, meet any changes prescribed in this new standard.