Healing characteristics of a FAM (asphalt mortar) specimen as a function of rest period and state of damage measured using a cylindrical specimen with the DSR.

 

Researchers Investigating Fatigue Cracking in Mixtures Have Identified Material Properties Related to Healing

Healing is commonly quantified in terms of the percentage gain in the number of load cycles to fatigue failure or gain in the modulus as a function of the duration of the rest period. This is a direct approach that allows comparisons of healing in different asphalt mixes. A limitation to this approach is that the percentage of healing measured depends on the experimental conditions, such as the type of loading (e.g. monotonic, cyclic), rate of loading, and mode of loading (e.g. controlled strain or controlled stress). 

       Consequently, the healing characterized using this direct approach is useful only in the context of the specific test conditions.  For example, the test may reveal that material x heals 10% more than material y; however, this may be true only under the conditions of cyclic loading and rest periods used during the test. 

       A way to understand and resolve these limitations is to consider the fatigue cracking life of mixtures. The number of load cycles to failure for an asphalt mix can be directly measured in the laboratory and used to compare the fatigue cracking resistance of different materials.  However, the fatigue cracking life of a particular mix is limited to the experimental conditions under which it was measured. 

       Over the last few years, researchers at North Carolina State University have developed the viscoelastic continuum damage (VECD) theory to experimentally determine an asphalt mixture’s characteristic damage evolution independent of the mode of loading. The VECD theory allows the user to mathematically distill data from a laboratory test to obtain a damage evolution characteristic for the material that is independent of the mode of loading. 

Researchers at the University of Texas at Austin (UT-Austin) have used the VECD theory to develop an experimental and analytical framework to quantify micro-damage healing characteristics of an asphalt composite independent of the mode of loading. They determined healing characteristics using the dynamic shear rheometer (DSR) with fine aggregate matrix (FAM) specimens.  Specimens were subjected to cyclic loading, and rest periods of different durations were introduced at different states of damage within the specimen. 

They analyzed results using a method based on the VECD theory to identify healing characteristics of the specimen as a function of the duration of the rest period and the state of damage before the rest period.  Verification tests confirmed that the healing was unique to the material and independent of the mode of loading.  The same experimental protocol and analytical method can be used to determine the healing characteristics of full asphalt mixtures.  ARC