Structural Behaviour of Deteriorated Concrete Structures

Doktorsavhandling, 2010

A growing concern for better assessment of existing concrete structures has revealed a need for improved understanding of the structural effects of deterioration. The two most common causes of deterioration in concrete structures are freezing of the concrete and corrosion of the reinforcement. The aim of this study is to deepen the understanding of the structural effects of deterioration with special attention to the bond between deformed bars and concrete. The effects of freezing on the material properties of concrete and the bond behaviour of bars were investigated through experiments. A significant influence of frost damage was observed on the stress-strain response of concrete in compression, tensile stress-crack opening relation, and bond-slip behaviour. Based on this, a set of methods was introduced to predict the mechanical behaviour of reinforced concrete ‎structures with a measured amount of frost damage. The ‎methodology was applied to frost-damaged beams using non-linear finite element analysis at the structural level. The results‎ indicated that the changes in failure mode and the effect on failure load ‎caused by internal frost damage can be predicted by modelling at the structural level. Corrosion of reinforcement leads to volume expansion of the steel, which can cause cover cracking and spalling; this weakens the bond of the reinforcement. The bond-slip model given in Model Code 1990 was extended to include corroded reinforcement. Analysis of corroded beams using the methodology gave results which are on the safe side. However, for large corrosion penetrations that lead to extensive cover cracking, more detailed modelling of the surrounding concrete and stirrups is required. Under such conditions, when wide cracks develop, the favourable effect of rust flowing through the cracks becomes significant; this decreases the splitting stress around the bar. A previously developed corrosion model was extended to include this phenomenon. The volume flow of rust through a crack was assumed to depend on the splitting stress and the crack width. The splitting stress was evaluated from the strain in the rust, and the crack width was computed from the nodal displacements across the crack. The extended model resulted in more corrosion cracks with smaller crack openings, which better corresponds to the measurements on specimens tested. Eccentric pull-out tests were carried out to study the influence of cover cracking and stirrups on the bond of corroded bars. The extended corrosion model was used in detailed three-dimensional analyses of the tests. The tests and analyses showed an important effect of the cover cracking in terms of loss of confinement and the flow of rust through the cracks. They also indicated that the bond behaviour and the failure were strongly governed by the position of the anchored bar, i.e. corner or middle positions, and the level of the corrosion attack. Stirrups played an important role after cover cracking, as they then became the primary source of confinement. Furthermore, corrosion of stirrups led to a more extensive cover cracking for a relatively low level of corrosion attack. The knowledge gained in this study contributes to better understanding of the effects of deterioration on structures, and can be used primarily for assessment of the load-carrying capacity of existing structures.