Analyses of Resilient Behavior of Unbound Materials for the Purpose of Predicting Permanent Deformation Behavior

Doctoral thesis, 2011

Post compaction is the unintended compaction of the unbound material that commences immediately after a road is brought into service and caused by densification of the material. Analogies to this post compaction can also be seen in laboratory and field tests. The objective of the thesis is to identify the conditions for post compaction and separate it from permanent deformation caused by shearing/dilatation to facilitate process understanding. The relationship between permanent and resilient deformation is analyzed. The resilient performance is then described through the stress hardening behavior of the unbound materials using the so called k- model. Input for the analysis of stress hardening behavior and permanent deformation comes from small scale laboratory tests and full scale field tests performed by VTI. The laboratory tests used are repeated load triaxial tests and the field tests are accelerated pavement tests using Heavy Vehicle Simulator and Falling Weight Deflectometer readings. A conceptual model of unbound material was set up to facilitate the analysis of the continuous stress hardening behavior from peak value observations in both the laboratory and the field tests. The analysis shows that it is possible to describe the permanent deformation behavior of unbound road material by examining resilient behavior. However, it is not sufficient to use only resilient deformation or resilient modulus. For an adequate description it is necessary to evaluate the stress hardening behavior. This can be evaluated by using the k- model and a graphic presentation of its stress hardening parameters k1 and k2. In such an analysis the stress hardening caused by post compaction can be separated from stress hardening caused by resting. The analysis also shows that there is a specific bulk stress level that separates compaction behavior and shearing behavior as a result of repeated loading.