Experimental Investigation On The Mechanical Properties Of Mortar-aggregate Interface In Concrete

On the meso-scale, concrete is generally considered as consisting of mortar, aggregate and the interfacial transition zone between mortar and aggregate. Compared with mortar, interfacial transition zone has the characteristics of high porosity, low density and low strength. As a result, the interfacial transition zone always behaves like the "weakest region" in concrete. It is generally recognized that the mechanical properties of mortar-aggregate interface in concrete have great effects on the concrete failure mode and its macro-mechanical behaviors when subjected to loading or environmental actions. In addition, the constitutive relationship of mortar-aggregate interface is the basis for meso-scale numerical simulations of concrete. Therefore, in order to predict the macro fracture process of concrete with the meso-scale strategy, it is desirable to establish the mechanical properties of these three phases through experiment. In the current study, the mechanical property of interface of mortar-aggregate is studied on the meso and macro level, and the main contributions are summarized as follows:(1) The composite mortar-aggregate specimens withdifferent aggregate surface roughness and different mortar strength were used to investigate the bond strengths of interface by means of the splitting tensile test and direct shear test respectively. The influence of the aggregate surface roughness and the strength grade of mortar on the bond strengths of interface were examined. Test results indicated that the interface between mortar and aggregate was the "weakest region" in concrete. The bond strength of the interface was lower remarkably than mortar strength. The bond strength of interface relied on the aggregates surface roughness and the type of mortar. With the increase of mortar strength, the bond strength of the interface also increased. The bond strength of the interface showed an increasing tendency with the value of interface roughness. In addition, based on the failure mode of the specimen, the bonding mechanism of the interface was analyzed. The interface between mortar and aggregate should be improved in order to get high bond strength.(2) Based on the RILEM recommends, the three point bending test for a notched beam was used to evaluate the Mode I fracture parameters of mortar-aggregate interface. Sequentially, the softening curve model for mortar-aggregate interface under tensile stress is developed based on the test results. It provides an analysis model for the numerical simulations of concrete on the meso-scale. (3) Aiming to investigate the bond slip characteristic of concrete under loading, a new experiment method was proposed. The optical fiber sensors were used to measure the strains of mortar and aggregate. Through the comparison, it was shown that the results obtained by the optical fiber sensor were more accurate than that obtained by the traditional sensor, and the relative slip in the compression test was larger than the tensile test. Based on the experimental data, the bond slip model between mortar and aggregate was established. It can provide a convenient and effective analysis model for the numerical simulations.(4) Compared with the constitutive relationships of mortar-aggregate interface and mechanical parameters used in the meso-scale numerical simulation, the proposed constitutive relationships and mechanical parameters can provide a convenient and effective analysis model for the meso-scale numerical simulations.