13th International Conference on Fracture June 16–21, 2013, Beijing, China 3 hardness and strength [16, 17]. In this study, the distribution of indentation modulus was used as a basis for the property characteristics of old ITZ and new ITZ, such as the thickness, elastic modulus and strength related to old and new mortar matrix, respectively. Figure 3 (c) and (d) show the indentation modulus distributions with the distance across old ITZ and new ITZ. In this test, greater porosity in the ITZs caused the reduced elastic modulus and strength, relative to those of related mortar matrix. (a) Indent area in old ITZ of RAC (b) Indent area in new ITZ of RAC 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 Modulus (GPa) Distance (m) Natural aggregate Old ITZ Old mortar matrix 50 m 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 Modulus (GPa) Distance (m) Old mortar matrix New ITZ New mortar matrix 60 m (c) Modulus distribution across old ITZ (d) Modulus distribution across new ITZ Figure 3. Nanomechanical properties of ITZs in RAC [9, 35] The old ITZ and new ITZ thickness was estimated by locating the places where there is slight variation in the indentation modulus with the distance from the natural aggregate or old mortar matrix surface, and the indentation modulus distribution of the ITZs seem to be close to those of corresponding old mortar matrix and new mortar matrix. Based on the nanoindentation results, the thicknesses of old ITZ and new ITZ are found to be around 50 μm and 60 μm, respectively. Moreover, the average indentation modulus of old ITZ and new ITZ were found to be approximately 80% and 85% of those of old mortar matrix and new mortar matrix, respectively. We choose a linear relationship between indentation hardness and strength. Considering the hardness has a similar distribution with the modulus in the ITZs regions, the strengths of old ITZ and new ITZ are assumed to be 80% and 85% of those of old mortar matrix and new mortar matrix, respectively. For the mortar matrix, elastic modulus, the strength (peak stress), and deformation capacity (peak strain and ultimate strain) parameters are obtained according the experimental data from Refs. [5, 14]. In case of the ITZs, the relative mechanical properties (elastic modulus and strength) to those of mortar matrix can be provided by the nanoindentation test. Combining the descriptions in this and above sections, the predicted uniaxial tensile and compressive stress-strain relationships for both ITZs and mortar matrix based on the plastic-damage constitutive model are shown in Figure. 4.
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