| 種別 | Paper |
| 主題 | FRACTURE ENERGY EVALUTION OF CONCRETE WITH COMPACT TENSION TEST |
| 副題 | |
| 筆頭著者 | Hirozo MIHASHI |
| 連名者1 | Folker H. WITTMANN |
| 連名者2 | Philippe SIMONIN |
| 連名者3 | Keitetsu ROKUGO |
| 連名者4 | |
| 連名者5 | |
| キーワード | |
| 巻 | 9 |
| 号 | 2 |
| 先頭ページ | 657 |
| 末尾ページ | 662 |
| 年度 | 1987 |
| 要旨 | 1. INTRODUCTION While structural analysis techniques have been rapidly developed, the constitutive laws of concrete are still remained to be rather traditional. Although the tensile strength is not taken into consideration in general structural design calculations, the importance to study the cracking behaviour of concrete has been recently noticed. For example, sheer failure and/or bond failure of reinforced concrete members are strongly influenced by the cracking behaviour. Demands of finite element analysis of concrete structures have been stimulating a great increase of investigations into rational failure criteria and rational constitutive laws (1). Since concrete is an extremely heterogeneous composite material, cracks are arrested and deviate when they encounter aggregates. Such cracking process creates a rather large microcracking zone and crack tip is blunted by the development of the zone. That is one of the reasons why linear elastic fracture mechanics can not be applied to concrete, while fracture mechanics approaches have been employed to study tensile mechanical behaviour of concrete since around 1960. Ordinary nonlinear fracture mechanics approaches are also inapplicable to concrete because it is hardly possible to measure the real crack length (2). One way to quantify the toughness is to introduce the ‘fracture energy GF', which is the energy absorbed in the unit area of fracture surface. In order to measure the real fracture energy GF, the movement has to be stable and the deformation should be increased without any sudden jumps. RILEM Technical Committee 50 ? Fracture Mechanics of Concrete (50FMC) has published a recommendation on the determination of the fracture energy GF by means of three-point bend test on a notched beam. In that connection, totally about 700 beams were tested in 14 laboratories from 9 countries in order to collect information and experience regarding the proposed method (3). While it was proved that the proposed test method seemed to be suitable for a standard test used in normal testing laboratories, it was also recognized that the value of GF was dependent on the size of the ligament, the composition of concrete, the maximum aggregate size, curing conditions, age and so on. However, it is so far not known in detail how and why those parameters influence the fracture energy. Before the concept of fracture energy can be applied to structural analysis, it must be studied how the fracture energy is influenced by such parameters as the specimen size, the composition of concrete and so on. Horvath and Persson studied the influence of the specimen size on GF of concrete by means of three-point bend tests and direct tensile tests (4). They found that GF increased with specimen size for low water-cement ratio but such effect was not found for high water-cement ratio. Mindess did also three-point bend tests using notched beams with four times different sizes (5). He pointed out that the fracture energy of the largest beams tested was about 40% higher than that for the smaller ones. Shortcomings of the three-point bend test to study the influence of specimen size and the maximum aggregate size might be following: 1). The influence of self-weight causes various kinds of vagueness. Consequently RILEM recommendation used to give rather large values of GF (6). 2). Increasing the size causes too large influence of the weight of the beam. Beams with the span which is proportional to the square root of the depth suggested by Hillerborg (3) might have some different arch action effects. The aim of this article is to present an idea to study the influences of specimen size, the composition of concrete, the maximum grain size and the rate of loading direction is horizontal and consequently the specimen weight have no influence on the fracture energy evaluation. Another important information for numerical analysis is the constitutive law of the fracture process zone, which is the strain-softening diagram. Even though the value of the fracture energy GF is the same, various load-displacement curves are drawn with different constitutive laws. It means that only the determination of GF is not sufficient but also the identification of the constitutive law is essential for the application to computerized structural analysis. This is discussed in other articles (7,8). 4. CONCLUSIONS The fracture energy of concrete is influenced by the ligament length, the rate of loading, the water-cement ratio and the maximum grain size. The specific fracture energy increases with increasing ligament length up to a limit, then GF seems to remain constant. The fracture energy increases at higher rates. The value of GF decreases slightly with increasing w/c ratio. Furthermore GF is considerably influenced by the maximum grain size. Concrete with the maximum grain size of 32mm has a higher value of GF by about 40% than the value of concrete with the maximum grain size of 8mm. |
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