THESIS
2015

iii leaves, iv-xvii, 167 pages : illustrations ; 30 cm

**Abstract**
Steel corrosion in reinforced concrete structures can lead to concrete cracking when corrosion products expand in the presence of water. Finite element model and boundary element model are established in this research to study crack propagation in concrete as corrosion progresses. The models consider non-uniform corrosion distribution around the steel cross section which better reflects the real situation when chlorides are penetrating from the member surface. In such a case, corrosion does not initiate simultaneously at all points around the steel but rather starts at different times because the time taken for each surface point on steel to reach the critical chloride concentration is different. As the non-uniform rust distribution varies with time, geometry models need to be changed a...[

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Steel corrosion in reinforced concrete structures can lead to concrete cracking when corrosion products expand in the presence of water. Finite element model and boundary element model are established in this research to study crack propagation in concrete as corrosion progresses. The models consider non-uniform corrosion distribution around the steel cross section which better reflects the real situation when chlorides are penetrating from the member surface. In such a case, corrosion does not initiate simultaneously at all points around the steel but rather starts at different times because the time taken for each surface point on steel to reach the critical chloride concentration is different. As the non-uniform rust distribution varies with time, geometry models need to be changed at each time step or corrosion level for analyzing the mechanical effects of steel corrosion.

The corrosion initiation time is obtained from a diffusion analysis while the thickness of rust at each point around the steel is assumed to increase at a constant rate after corrosion starts. Mechanical analysis is first performed with the finite element software ATENA. To account for the change in rust volume and geometry, the model has to be set up repeatedly and re-meshed at each corrosion level, which takes a lot of computational time. Discrete crack model is used to simulate the fracture process zone in concrete. The cracking mechanisms of concrete cover under different boundary conditions are examined. To highlight the necessity to consider non-uniform corrosion, the surface crack width evolution for concrete under non-uniform corrosion under different boundary conditions is compared with that under uniform corrosion. It is shown that non-uniform corrosion results in wider crack opening than uniform corrosion for the same amount of corrosion products formed.

To establish a more efficient and convenient model that can take into account the time-varying non-uniform corrosion distribution, a program based on boundary element method is also developed. Fictitious stress method and displacement discontinuity method are used to model smooth boundary and crack boundary of concrete respectively. The bilinear cohesive law is imposed on the crack elements to consider the aggregate bridging effect. Corrosion products are simulated as combined normal springs and shear springs connecting concrete and uncorroded steel. Cracking in concrete and rust generation with corrosion are simulated as boundary changes, which results in the change of the influence coefficients matrices and known boundary quantities matrix. Crack width evolutions obtained in the boundary element program are compared with those in finite element analysis for uniform corrosion. The boundary element model gives results close to finite element modeling, but with much more convenience in model set-up and much faster computation speed. For non-uniform corrosion, crack width is less than that obtained in the finite element analysis at the same corrosion level. The interface materials put on the contact between concrete/rust and concrete/steel to allow the discrete cracks opening in the finite element modeling lead to the overestimate of crack width compared with results in the boundary element modeling. The amount of rust penetrating into cracks is also considered to study its effect on the relation between crack width and corroded percentage of steel. In the end, discussions on service life time prediction based on the coupled diffusion-mechanical model developed in this study are given.

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