Experimental investigations into tensile properties of large specimens and sensitivity of notch size on the fracture and fatigue behavior of concrete

Abstract

Concrete, which is the second largest material used after water, is a highly heterogeneous quasi-brittle material containing ingredients at different length scales. The fracture and fatigue behavior of this material is highly complex with a strong dependence on the size of the specimen. Concrete being weak in tension, its study on the cracking and fracture behavior under tensile stresses become important. Under fatigue loading, concrete structures suffer from damage in the form of microcracks even at low stress amplitudes. With increasing fatigue cycling, these microcracks grow very slowly and coalesce to form macrocracks leading to a sudden failure. Hence, the assessment of the residual strength and life of fatigue-damaged members becomes necessary. This research begins with the study on large size specimens under direct tension which is very sparsely reported in the literature. Experiments are conducted in a specially designed servo hydraulic testing rig on large concrete specimens under displacement control. Acoustic emission sensors and digital imaging are used to study the evolution of microcracks, their coalescence into macrocracks leading to final failure. Important elastic and fracture properties including the tensile strength, fracture toughness, fracture energy and size effect parameters are determined which show a significant difference in their behavior when compared to the traditional small-sized laboratory specimens. Design of experiments for fatigue loading is a challenging task, especially the notch size, which has a bearing on the failure cycles. Hence, the second part of this research lays focus on the study of notch size on the fracture and fatigue behavior of concrete. Compact tension specimens with three different notch to depth ratios of 0.20, 0.35 and 0.50 are prepared and tested under mode I splitting load using wedge grips. The tests are first conducted under increasing crack mouth opening displacement and the mechanisms of microcracking and failure are studied using acoustic emission. After gaining important insights on the fracture behavior under monotonic loading, tests are conducted with constant amplitude fatigue loading. A counter-intuitive behavior of increase in fatigue life is seen, with increasing notch size. A notch size between 0.20 and 0.35 is recommended for carrying out fatigue tests on plain concrete. Concrete shows a very gradual increase in the growth of microcracks under fatigue loading. With increasing cycles, the microcracks coalesce with each other to form macro- cracks. At this stage, a sudden increase in the hysteretic and acoustic energy is seen indicating the rapid growth of microcracks leading to failure. To avoid the sudden failure under fatigue loading, it is important to know the residual strength as a function of the fatigue induced damage. Experiments are conducted on three different sizes of concrete beams that are damaged under a known number of cyclic loading. These beams are further tested under monotonically increasing displacements to obtain their elastic and post-peak fracture parameters. Important conclusions regarding the strength, stiffness degradation and fracture toughness are obtained. The results obtained from this experimental investigation would be useful in design and failure analysis of large concrete structures subjected to static and fatigue loading.