Fracture process zone in concrete tension specimen
Introduction
A general consensus exists related to the dependence of fracture behavior of concrete on a fracture process zone that develops around and ahead of the crack tip. However, a direct quantitative relationship between the process-zone characteristics and the measured toughness has not been established yet. It has recently been recognized that an important task in the future development of the fracture mechanics of concrete is to standardize the testing procedure. The length and the width of the fracture process zone are crucial in selecting the geometry and dimensions of the standard test specimen.
Then, what are the mechanisms to develop the fracture process zone and what happens in it? For the answer, some concepts about the fracture process zone are presented. One of them, published by Mihashi [1], considers that around the branches of the main crack there are closed microcracks (see Fig. 1) and in front of them opening microcracks exist. Another concept, published by Wittmann [2], is shown in Fig. 2. He separated an inner microcracking zone from a surrounding isolated microcracking zone. These concepts are valuable but do not define the fracture process zone completely. Therefore, it is necessary to obtain a reliable experimental evidence. There are significant discrepancies in existing results, probably resulting from the differences in the techniques employed, such as optical microscopy, electron microscopy, acoustic emission techniques, laser speckle techniques and so forth [3]. It is, therefore, very important to observe the internal fracture in concrete. Otsuka [4], [5], developed an inspection technique based on X-rays using contrast media, which can directly inspect internal cracks. Using this technique, the authors carried out some experiments that are reported elsewhere [6], [7], [8], [9]. Fig. 3 shows one of the results obtained where several microcracks are observed ahead of the notch tip. The authors insist that this microcrack zone corresponds to a fracture process zone or at least must be a main part of the fracture process zone. To confirm this point, the present research is conducted
This paper reports a further investigation of the fracture process zone using two methods: direct observation with the X-ray technique and indirect observation through an AE technique. The results obtained are compared and correlated to provide a clear view of the fracture process zone of concrete. To start this study, extensive and worthy works carried out by Shah and coworkers [10], Mihashi and coworkers [11], [12] and Ohtsu and coworkers [13] gave a lot of suggestion.
Section snippets
Specimens
Concrete containing high-early-strength Portland cement was used. The fine aggregate was a river sand and the coarse aggregate was crushed stone. The maximum aggregate size was 5, 10, 15 and 20 mm. All specimens were made from concrete with a specified cylinder strength of 20 MPa. Relatively low-strength concrete was used to accentuate effects of the fracture behavior of concrete.
Fig. 4 shows the configuration of the specimens used in the test. Four different-sized specimens, S-type, M-type,
Microcracks obtained by X-ray inspection
Fig. 8 shows relationships between load and crack opening displacement (COD) measured in S-type, M-type, and L-type specimens.
Fig. 9 shows results obtained by X-ray inspection in M-type specimen with a maximum aggregate size of 10 mm. X-ray inspections were made at points P1 (70% of peak load), P2 (peak load), P3 (70% of peak load), and P4 (30% of peak load) on the curve in Fig. 8. On the X-ray films, microcracks formed at the notch tip started to be observed at about 80% of peak load. Fig. 9(a)
Conclusions
Experiments were carried out by X-ray and AE techniques to investigate the behavior of the fracture process zone in concrete. From the results of these experiments, the following conclusions can be drawn.
- 1.
Microcracks in a complex configuration near the tip of the notch were observed by the X-ray technique with contrast medium. Locations of AE event sources were determined by the 3D AE technique.
- 2.
The energy of each individual AE event was calculated. It led to the fact that many of the events had
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