Fracture mechanics analysis of concrete structures

Abstract: This thesis presents results from numerical and analytical analyses of concrete structures as well as results from laboratory tests. Numerical methods based on the finite element method and fracture mechanics are used. The thesis comprises five papers and a general introductory part. The five papers deal with the following topics: Material properties for concrete at low temperatures, Paper A: This paper presents results from an investigation of the fatigue strength and the fracture energy of water saturated unreinforced concrete. The tests were carried out both at room temperature and under cold conditions. The experiments show that the load bearing capacity of water saturated specimens is strongly affected of low temperatures. The water in the pores freezes and generates a load bearing ice skeleton. The compressive and splitting strength of the concrete will as a result increase. Also the fracture energy increases at low temperatures. Both the absolute and the relative fatigue strength increase for water saturated concrete. Bond properties of concrete, Paper B: The paper discusses the influence of splitting failure on the bond properties of deformed reinforcement bars. Laboratory tests with normal and high strength concrete are presented. The tests were designed to obtain a high degree of confinement with the intention to establish the upper limit of the bond strength. The bond strength has been normalized with regard to the concrete splitting strength. The high strength concrete has a higher normalized bond strength than the normal strength concrete for bar diameters of 8 and 16 mm. A theoretical model, based on fracture mechanics, is presented. The model gives relationships between the pressure generated by the mechanical interaction and the length of the splitting cracks. Modelling of concrete shaft in a gravity based offshore structure. Influence of hydraulic pressure on fracture mechanics parameters, Paper C: The paper describes an analysis carried out in order to predict the risk of concrete delamination in the shaft of the Draugen Gravity Based Structure (GBS) platform situated in the northern Atlantic. The analysis formed part of an independent verification of the platform by VERITEC in Norway and was performed using a nonlinear finite element program (DIANA) with a material model based on fracture mechanics. The analysis shows that there was no risk of progressive cracking or delamination in the shaft of the studied gravity based offshore structure. Modelling of mixed mode fracture, Paper E: This paper describes numerical modelling of fracture formation under combined tensile and shear loading. The numerical model used is based on the Inner Softening Band Approach, see Klisinski et al. (1991). The results are compared with test results presented in Hassanzadeh (1992) and Nooru-Mohamed (1992). The model reproduces crack pattern and nominal stress displacement curves. The compressive stresses due to dilation can also be modelled. The stress rotations are generally small. In the final stage of the test,the stress rotation increases in some elements, especially in the smallest elements, so that more cracks should be introduced in each element in order to avoid interlocking effects. Anchor bolts in concrete, Papers D and E: The first paper [D] presents results from anchor bolt tests, comprising both plane and axisymmetric specimens. Different design formulas are discussed. Results from finite element analyses are presented. The second paper [E] presents analyses and tests on a plane anchor bolt specimen. The analyses are based on the Inner Softening Band Approach, see Klisinski et al. (1991). The analysis reproduces deformations at maximum load and crack pattern at an early stage of the test. Peak loads are overestimated. The post peak behaviour is not properly captured due to stress interlocking even before the peak load is reached. The possibility to introduce more then one crack in an element seems to be an essential step for further development of the analysis.