Issue 48

S. Bhowmik et alii, Frattura ed Integrità Strutturale, 48 (2019) 419-428; DOI: 10.3221/IGF-ESIS.48.40 420 analysis of load-deflection curves and acoustic emission analysis. Extension of crack has been calculated using the stiffness and AE record equivalence. It has been found that the size and evolution of FPZ can be determined through the difference of equivalence. Further it has been observed that the length of fracture process zone increases when the crack extension is small. Tejchman et al. [4] have investigated the properties of fracture process zone in notched concrete beams under quasi-static three-point bending test at meso-scale level. A numerical model has been used to study the strain localization phenomenon in concrete. The numerical model is further strengthened by incorporating the effect of characteristic length of micro-structure. The results obtained through the model has been compared with the results of digital image correlation method. Shah et al. [6] have investigated different fracture properties of concrete-concrete interfaces using digital image correlation technique. In their study, mode I and mode II fracture toughness and critical energy release rate have been determined using DIC technique. Different parameters such as surface displacements, strain components, crack opening and sliding displacements, load-point displacement, crack length and crack tip location have been determined through the analysis of images captured using DIC. The results obtained from the DIC analysis have been compared with the experimental results. Fracture process zone properties of concrete have been investigated by Wu et al. [15] using digital image correlation technique. In their study, it was observed that the length of FPZ increases until it is fully developed and decreases thereafter. Another study has been carried out by Skarzynski and Tejchman [16] to investigate the fracture processes in plain and reinforced concrete beam using DIC technique. Beam specimens were tested under quasi-static three-point bending test with eight concrete mixes. In addition, the effects of aggregate shape, volume, and size in concrete have been considered. Digital image correlation technique has been used to measure the surface displacement of concrete. Width of a localized zone has been found to increase with the increase in the maximum aggregate size. Ohno et al. [17] have applied acoustic emission technique on notched concrete beam specimen tested under three point bending. AE signals were analysed through Green′s functions for moment tensor analysis. From the experimental results it has been confirmed that, fracture energy increases with the increase of maximum size of the aggregate. Additionally, the width of fracture process zone has been determined through results of AE source location. An increase in energy has been observed with the wider size of fracture process zone . Alam et al. [18, 19] have estimated crack opening and size of the fracture process zone using both digital image correlation and acoustic emission technique. DIC has been used to predict the crack opening at various stages of cracking, wherein AE technique has been used to mark the locations of crack growth due to micro/macro crack. Both the techniques have been found to be effective for the understanding of fracture processes in concrete structures. The same research group [20] in another work have addressed the effect of structural size on propagation of cracks in reinforced concrete beams using DIC technique. Trivedi et al. [21], have performed experiments on pre-notched concrete beam specimens under three-point bending. The Authors have used DIC technique and proposed a new scheme called Optical Crack Profile (OCP) to evaluate the full field of displacement and strains for visualizing the fracture growth, fracture propagation in plain concrete. The length and width of the fracture process zone are observed to be in agreement with the guideline of ACI-446 and literature study. Xie et al. [22] have conducted three-point bending tests on notched beam specimens to study the fracture behavior of fly ash based geopolymer concrete. Crack opening displacement, crack extensions and mid span deflections have been measured through DIC analysis. S PECIMEN PREPARATION AND EXPERIMENTAL DETAILS eometrically similar beams of small, medium and large size as shown in Fig. 1, have been casted using ordinary Portland cement. In the mix design, the maximum size of coarse aggregates used is 12.5 mm . The specific gravity of fine and coarse aggregates are determined to be 2.71 and 2.62 respectively. Cube compressive strength values after 28 days of curing have been found to be 34.5 MPa. The values of modulus of elasticity and the tensile strength of concrete are 25000 MPa and 3.5 MPa , respectively. Pre-notching has been done in the beam at the time of casting itself. The specimen dimensions have been decided according to RILEM recommendations and are provided in Table 1. The span to depth ratio S/D in each specimen has been kept equal to 4 and the width B is kept 50 mm . The notch to depth ratio (a 0 /D) is kept 0.2. The schematic diagram of a typical beam has been shown in Fig. 2. Monotonic and fatigue testing have been carried out on concrete beams using 250 kN servo-hydraulic MTS testing facility. The DIC set-up consists of a pair of digital camera and a VIC-2D software to analyses the images. During testing process, the whole specimen is illuminated by providing white light. The entire arrangement is shown in Fig. 3. Pre-notched beam specimens of small, medium and large sizes have been subjected to three point bending tests under the action of monotonic increasing load. The testing has been performed under crack mouth opening displacement (CMOD) controlled manner at an approximate rate of 0.0005 mm/sec . The CMOD measurements have been obtained using a cantilever type clip gauge placed between G