Chunjiang et alii, Frattura ed Integrità Strutturale, 35 (2016) 500-508; DOI: 10.3221/IGF-ESIS.35.56 501 instance, decision of stability of various thorough cracks in concrete pouring blocks and calculation of cracking load of reinforced concrete (RC) in diagonal direction, etc. People are expected to improve design methods of gravity dam and arch dam using fracture mechanics [2]. Two-parameter model considers actual crack in combination with micro-crack zone as an effective crack, then obtains results using theory of linear elastic fracture mechanics and combines it with numerical computation method. Its essence lies in using valid crack tip opening displacement (CTOD) to reach critical valid CTOD, i.e., fracture criterion. Xu Shilang [3], a scholar from China, put forward a simple and applicable fracture criterion in 1999, namely, double-K fracture criterion. This criterion, belonging to the first type, not only corrects linear elastic fracture mechanics model based on a complete theory, but also decides its fracture parameters with the help of simple test methods, which is expected to be popularized and applied in practical engineering. Two fracture parameters are introduced into this criterion, initial fracture toughness ini IC K and unstable fracture toughness un IC K . Test indicates that those two parameters without size effect under a certain size, as fracture parameters, can be well applied in the analysis of concrete structure crack extension, and have drawn much attention [4-7]. A relevant trial [8] reveals that maximum seam strain is 10 times of average value of ordinary bending ultimate strain, which is exactly a ratio of theoretical strength and breaking stress of a point of concrete material. This thesis aims to explore characteristics of double-K fracture parameters (initial fracture toughness ini IC K and unstable fracture toughness un IC K ) of concrete using data obtained from test. D ESIGN OF CONCRETE FRACTURE STIFFNESS TEST WITH WEDGE SPLITTING METHOD Material Selection and Mix Proportion aterials include tap water for life, 32.5 ordinary Portland cement, fly ash (level II), natural medium sand with particle size of over 5mm, macadam with maximum particle size of 20 mm (level I), high efficiency slushing agent suitable for mass concrete, and mix proportion is shown in Tab. 1. Wedge splitting specimen [14, 15] is adopted in this test (Fig. 1) and Tab. 2 displays specimen parameters. All specimens are divided into two categories containing 12 kinds of working conditions, the quantity of specimen in each working condition is presented in Tab. 2 and testing devices are in Fig. 2. Materials Tap water Cement Fly ash Sand Stones Additives RCC kg/m 3 130 134 90 805 1280 0.75% Common concrete kg/m 3 203.7 407 0 535 1245 0 Table 1 : Mix proportion of roller compacted concrete (RCC) and common concrete. RCC Prefabricated crack Construction structural adhesives Marble Figure 1 : Shape and size of specimen. M

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