Concrete--Cracking

This thesis presents an experimental and analytical investigation of concrete structural members strengthened with externally bonded composite laminates with varying configurations. Parameters, such as size, type of laminate, debond, etc., are evaluated from the viewpoint of stress patterns and their influence on interfacial debonds. Stress patterns in the structure and stress intensity factors around crack tips are determined using a finite element model developed for this purpose. The study also includes a precise description of cracking and the failure function of each parameter investigated. Besides the development of an innovative finite element program, which enables the study of interfacial cracks in structures with highly nonlinear behavior and multiple irregular cracking patterns, the significant contributions include the effect of laminate geometry, the inefficiency of laminate prestressing, the negative effect of end debond, and the insignificant effect of midspan debond on the cracking and the strength of a laminated concrete structural member.

The acoustic emission investigations reported herein are divided into two experimental sets. In the first, the concrete specimens were stressed internally, and the resulting acoustic emissions were monitored. In the second, specimens were subjected to fracture mechanics tests and simultaneously studied for AE signals. For both sets, two kinds of concrete were studied--regular concrete and concrete containing fly ash. The specimens were stressed internally by subjecting them to an accelerated state of corrosion in a marine environment. The corroded specimens were then tested for impact and flexural strengths to study the effect of fly ash replacement on rebar corrosion in a marine environment, and to ascertain any correlation between the monitored AE signals and residual strength. Furthermore, an attempt was made to predict the remaining life of the specimens. For the externally stressed specimens, AE was used to determine the load at initiation of unstable crack propagation terms of ultimate load. These tests have immediate and relevant applications to field problems.

This thesis presents the experimental investigation of durability and fracture toughness (K IC) of fly ash concrete in the marine environment. The findings indicate that the deterioration rate of durability parameters, such as compressive strength, weight loss, and dynamic modulus of elasticity, due to 450 wet and dry cycles exposure (the Accelerated Durability Testing), was inversely proportional to the amount of fly ash replacement. On the other hand, tensile strength properties, such as modulus of rupture and fracture toughness, were independent of fly ash replacement, but increased with the period of accelerated testing. The mean K IC values of fly ash concrete mixes showed that they are closely related to their compressive strengths and size effects. According to AE, unstable crack propagation initiated at 93-97% maximum load. With SEM observations, it was found that crystallized particles were precipitated in the void spaces due to chemical reaction between the cement paste and seawater.