Aggregate morphology affecting strength and permanent deformation behavior of unbound aggregate materials

This paper presents findings of a research study focused on investigating influences of coarse aggregate morphologies on the strength and permanent deformation behavior of unbound aggregate materials. Angularity and surface texture properties of six different types of aggregates were evaluated using two shape indices, angularity index (AI) and surface texture index (ST), based on image analysis. Twenty-one unbound aggregate blends having the same gradation and voids content but different crushed percentages were prepared by mixing the imaging evaluated aggregates and tested using triaxial tests for strength and permanent deformation. The laboratory permanent strain-load application (epsilon(p)-N) curves evaluated using a phenomenological power model showed the A and b parameters to be heavily dependent on applied stress levels. Aggregate blends with higher AI and ST indices were consistently associated with lower magnitudes of A and b. The angularity property was found to contribute mainly to the strength and stability of aggregate structure through confinement by bulk stress, whereas the surface texture property tended to mitigate the dilation effects through increasing friction between individual aggregate particles.