Fracture Mechanics of High Strength Concrete and Size Effect

从混凝土断裂力学的角度研究了高强混凝土的抗裂性能和尺寸效应。结果表明：1)当采用100×100×8400(Mm)的Rilem标准缺口梁时，虽然混凝土的断裂能增加到抗压强度的80 Mpa左右，但断裂能下降到抗压强度的100 Mpa以上。另一方面，即使对于100 MPP左右的超高强度混凝土，六倍尺寸的楔形劈拉试件的断裂能也高于切口梁的断裂能。为了探讨造成这种差异的机理，分析了切向刚度荷载-CMOD曲线的变化率。结果表明，在大试件断裂前，裂纹尖端前方的断裂过程区已经充分发育，而在小试件中，断裂过程区没有得到充分的发育。2)高强混凝土在细化微观结构的同时，其拉伸软化性能也发生了很大的变化，特别是最大粘结应力的增加。3)由标准试验确定的三个参数定义的脆性指数可以很好地表征不同强度和尺寸试件的抗裂性能。4)钢纤维增强混凝土提供了更高强度和更高抗裂性的混凝土。

Crack resistance and size effect in high strength concrete were studied from the view point of fracture mechanics of concrete. The following results were obtained.1) Although fracture energy of concrete increases up to about 80MPa of the compressive strength, it turns to decrease beyound 100MPa of the compressive strength when RILEM standard notched beams of 100x100x8400 (mm) were used. On the other hand, wedge splitting compact tension specimens with a six times larger size give a higher values of the fracture energy than that of notched beams even for the very high strength concrete of about 100MPa.To investigate the mechanism to couse the difference, the changing rates of tangential stiffness load-CMOD curves were analyzed. It was shown that the fracture process zone in front of the crack tip was fully developed before the fracture in large specimens but not in smaller ones.2) While high strength concrete is realized by making the microstructure finer, it changes strongly the tension softening properties and especially increases the maximum cohesive stress. It was clarified that the maximum cohesive stress is empirically described as a function of the porosity.3) Brittle index defined with three parameters determined by standard tests, can well charcterize the crack resistance properties of concrete with various strength and sizes of specimens.4) Steel fiber reinforcement provides high strength concrete with higher crack resistance.