Experimental Investigations Into Fracture and Fatigue Behavior Of Plain and Reinforced Concrete Beams Using Acoustic Emission Technique

Abstract

Concrete is the most widely used material in the world for construction of infrastructures and there are quite a few gaps in understanding its behaviour under different loads. Fracture in concrete occurs at pre-existing crack tip upon the formation of a fracture process zone (FPZ) with several toughening mechanisms such as micro-cracking, aggregate bridging, crack branching etc., taking place resulting in energy dissipation, which resists further crack propagation. This FPZ is responsible for the post-peak softening response under tension and size effect.

The behaviour of reinforced concrete depends on the combined action of concrete and its embedded longitudinal reinforcement. The study of fracture in reinforced concrete is much more complex due to micro-structural changes in concrete, interaction between the concrete and steel and bond between them. There may be other failure mechanisms involved, such as yielding and slippage of steel, and de-lamination between steel and concrete.

Under inadequate provision of stirrups or in the case of deep beams, a beam subjected to transverse loads tends to fail by shear. There is a need to develop analytical models which can address failure under shear using the fracture mechanics theory in order to reflect the size effect and the failure mechanisms. In addition, combined flexural and shear mode of failure in reinforced concrete structures also needs to be studied by considering the internal microcracking mechanisms and the effect of size.

Bridge decks, highway pavements, airport pavements and offshore structures made of reinforced concrete are subjected to variable amplitude fatigue loading. The mechanism of fatigue in concrete is not yet clearly understood when compared to metallic materials. Hence, it is important to characterize the behaviour of concrete structures subjected to fatigue loading and understand the fracture mechanisms.

In this research work, the problems listed above are addressed through experimental investigations into fracture and fatigue behavior of plain and reinforced concrete using the acoustic emission technique. Important elastic and fracture properties of plain concrete including the size independent fracture energy, fracture toughness, critical crack tip opening displacements, critical crack length and size of process zone are determined. These serve as input parameters in the finite element based fracture mechanics models for analysis of concrete structures. Furthermore, the mechanisms of micro-crack formation, their coalescence, macrocrack formation and eventual failure under monotonically increasing and fatigue loading are determined. The evolution of damage under different loading are studied. The effect of varying beam size (depth) and reinforcement ratios are studied in order to understand the fracture mechanisms for failure under flexure, shear, combined flexure-shear and fatigue. It is seen that the acoustic emission technique could provide vital information on the micro-cracking characteristics in concrete.