How do Porous Interfacial Transition Zones (ITZ) Trigger Elastic Limits of Concrete? - Micromechanics of ITZ Failure and ITZ-Aggregate Separation

Abstract

Concrete aggregates are covered by 15-microns-thick interfacial transition zones (ITZs) exhibiting larger porosity, but smaller stiffness and strength than the adjacent bulk cement paste. This renders the immediate vicinities of the aggregates' surfaces as the weakest links within the microstructure of concrete. Onset of cracking, representing the elastic limit of the material, typically starts in these regions. Post-failure concrete fragments give insight into the debonding processes close to the aggregate surfaces: partly the aggregates have separated cleanly from the ITZ, and partly a thin layer of cement paste remains attached to the aggregates. This is the motivation to investigate the possibility of two different types of debonding: ITZ-aggregate separation and ITZ failure. Therefore, concrete is idealized as a perfectly bonded three-phase composite (consisting of spherical aggregates, surrounded by ITZs, and embedded in a cement paste matrix). At the concrete-related scale, ITZs are represented as 2D interface phases because the ITZ thickness of 15 microns is negligible as compared to the typical dimensions of the aggregates and of the cement paste matrix. At an even smaller scale, however, the ITZs are resolved as three-dimensional spherical shells. Based on this multiscale model, we derive, by means of a continuum micromechanics approach, traction vectors acting on aggregate surfaces and of full three-dimensional stress states within representative ITZ volumes. These microscopic fields of traction vectors and ITZ stresses intervene in microscopic tensile strength criteria for ITZ-aggregate separation and for ITZ failure, respectively. Comparison of model predictions with available experimental data implies that onset of concrete cracking is more likely to happen by means of ITZ failure than by ITZ-aggregate separation, if representative concrete volumes are subjected to compression-dominated types of loading; while under tension-dominated types of loading, also ITZ-aggregate separation might well govern onset of concrete cracking.