Studies on Flexural Fatigue Behaviour of Plain Concrete – Mechanisms of Crack Growth, Effect of Loading Amplitude, Frequency and Notch Size

Abstract

Concrete structures such as bridges, pavements and off shore structures are subjected to fluctuating loading throughout their service life. These fluctuating loads cause progressive internal damage to structures leading to sudden failure and this phenomenon is termed as fatigue. Despite being designed for strength and serviceability criterion, fatigue loading has resulted in catastrophic failure of our infrastructure in the past. While behaviour of materials such as metals has been well researched for fatigue loading, there are several gaps in understanding the fatigue and fracture mechanisms of a heterogeneous material like concrete. Concrete is a complex hybrid material composed of constituents varying from nano to millimetre length scale. Even in its pristine state concrete consist of several flaws due to shrinkage and expansion during the fabrication process. Under the influence of external loads these existing flaws in concrete trigger the formation of a fracture process zone (FPZ). Several toughening mechanisms such as micro cracking, aggregate bridging, crack tip blunting and crack deflection occur in FPZ making concrete a quasi-brittle material. The damage quantification of concrete structures under fatigue loading poses a difficult challenge owing to the heterogeneity of materials and variability in fatigue loading. Existing experimental research and fatigue models based on mechanistic approaches have predominantly used constant amplitude loading but in reality concrete structures are encountered with variable amplitude loading. Load sequence and load interaction effects due to variable amplitude loading significantly influence the fatigue life of concrete structures. In this research, effect of variable amplitude loading is experimentally and analytically studied. Rigorous understanding of the underlying physical phenomenon is critical to effectively model fracture characteristics of a material. Fatigue failure in concrete is a manifestation of progressive damage from micro scale to macro scale. To understand micro mechanisms and kinetics of macro crack propagation we require effective measurement tools such as digital image correlation (DIC) and acoustic emission (AE). Micro mechanisms of crack nucleation can be better understood with the help of AE technique using which the micro-cracks can be visualized in three-dimensions. DIC technique helps to obtain the surface crack length of the specimen by processing consecutive digital images of the deforming specimen. Fatigue behaviour of concrete is influenced by several parameters such as stress ratio, loading amplitude, loading frequency, sequence of loading, structural size, aggregate size, notch size and so on. Over the years, fatigue models for concrete have evolved incorporating the role of these parameters. Despite decades of pioneering research, there are still several links missing in understanding the above parameter’s influence on the fatigue behaviour. In light of the above discussion, this thesis focuses on investigating the crack growth mechanisms of concrete at micro and macro scale and the effect of variable amplitude loading, frequency and notch size through an extensive experimental program.