13th International Conference on Fracture June 16–21, 2013, Beijing, China 2 2. Modeled recycled aggregate concrete The MRAC consists of nine recycled coarse aggregate surrounded by a mortar matrix [5, 14]. Recycled coarse aggregate is idealized to have a round shape. The MRAC used in numerical simulation is shown in Figure 1. The MRAC means the recycled coarse aggregates are embedded within new mortar matrix as a square array. This is an assumption made to simplify the real RAC and provide useful information regarding crack initiation and mechanical response of RAC under loadings. The idealization of regular distribution of recycled coarse aggregates in RAC not only simplifies the problem but also assists in studying the effect of different phases on overall failure behavior of RAC. (a) MRAC specimen (b) MRAC schematic Figure 1. Modeled recycled aggregate concrete (MRAC) 3. Constitutive relationships 3.1. Mortar matrix Mortar matrix shows softening behavior after reaching its peak tensile or compressive stress, which is due to toughening mechanics within the fracture process zone. Both micro-cracking development and irreversible deformations contribute to the nonlinear response of mortar matrix. A plastic-damage constitutive model for mortar matrix is developed by former investigators [11, 15]. Anisotropic damage with a plasticity yield criterion and a damage criterion are introduced to adequately describe the plastic and damaged behavior of mortar matrix. In order to account for different effects under tensile and compressive loadings, two damage criteria are adopted: one for compression and a second for tension such that the total stress is decomposed into tensile and compressive components. The strain equivalence hypothesis is used in deriving the constitutive equations, so that the strains in the effective (undamaged) and damaged configurations are set equal. 3.2. ITZs With the emergence of nanoindentation techniques, it is now possible to directly measure the micromechanical properties of the ITZs. A total of 341 indents were performed in an array at each studied area (Figure 3 (a) and (b)). Microhardness testing on cement paste found that there is a linear relationship between microhardness and compressive strength. The hardness obtained from nanoindentation is assumed to provide a linear relationship between 150mm 9mm 9mm 38mm 9mm 38mm 9mm 38mm r=19.0mm d1=9.0mm d2=9.0mm
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