13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- The cracking of concrete is schematically presented in Fig. 2. The shaded area in Fig. 2, relates to the micro-cracked material. The part of the micro-cracked material, lying in front of the macroscopic crack (including initial crack length a0 and extended crack Δa), constitutes the active fracture process zone. In this region the material cohesion is weakened due to the micro-cracking. The effect of the fracture process zone is to make the specimen sense the crack as being longer than a0+Δa. The fracture process zone causes thus an effective increase in the crack driving force. The effect of the fracture process zone is similar in nature to the Irwin plasticity correction [3] for metals. Figure 2. Schematic presentation of the fracture process of concrete. The initial crack length is a0 and the macroscopic extended crack is Δa. PZ is the size of the active fracture process zone. The part of the micro-cracked material that becomes engulfed in the crack wake, is no longer actively affecting the crack driving force, but it has absorbed energy related to the micro-crack surfaces. This has the effect of increasing the materials effective fracture resistance (Fig. 3). Figure 3. Schematic definition of the difference between an ideally brittle and a quasi-brittle material. Often, fracture toughness tests of concrete are actually assessed by estimating an effective stress intensity factor and effective crack length [4], to derive an effective K-R curve in line with the ASTM E561 – 10 test standard [5]. This procedure is, however, not widely used, partly due to the question regarding the significance of the effective crack length and effective crack driving force. Another method called the double-K or double-G criterion is based on the estimate of the stress intensity at the onset of first crack extension (with a non-developed fracture process zone) and the
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