Control of Cracking due to Imposed Strains in Concrete Structures

Sammanfattning: This thesis is concerned with crack control due to imposed deformations in concrete structures. The objective of crack control may either be to prevent cracking or to limit the crack widths. The aim here is to increase the knowledge in both fields for some common structures. Five papers are included and the three last concern crack development. Paper I: Friction tests are performed for slabs cast on ground to study the horizontal external restraint. It is shown that the load-slip relations are non-linear and depends on the ground materials and the ground pressure. Paper II: Recommendations for design are proposed to prevent cracking due to imposed deformations in slabs and foundations resting on a frictional surface. The recommendations are based on comparisons of Finite Element results and results from simplified theoretical methods. Restraint stresses are calculated with a two-dimensional FE-model with non-linear contact elements, taking into account both horizontal and vertical movements. Imposed strains with a gradient over the thickness give restraint stresses significantly higher than uniformly imposed strains. The simplified methods give rather good estimations for thin and slender structures. Paper III: A technique for modelling of crack development in reinforced concrete structures exposed to imposed deformations is proposed. A two-dimensional FE-method is used with closing forces in cracks concentrated to spring elements. The spring stiffness is estimated from bond stress - slip relations for reinforcement. Tensile softening of concrete can be neglected. Simplified temperature changes are used as load and the calculations are performed stepwise with opening of nodes and implementation of spring elements. The FE-method is established for a wall fully restrained at the base, but it can also be adapted to other structures and restraints. Paper IV: The influence on crack development of reinforcement and concrete properties as well as geometry is studied for the wall fully restrained at the base. It is shown that the two-dimensional behaviour of the wall only gives about half the crack widths compared to a one dimensional bar with the same percentage of reinforcement. The reason is that the restraint along the base will effectively facilitate distribution of cracking along the wall. The crack widths are limited also at low reinforcement ratios. The crack widths will increase with tensile strength of concrete. The geometry of the wall and the bond stiffness has very little influence on the cracking widths and distribution of cracks in the wall. Paper V: Crack development is studied for different types of structures and varying boundary conditions. It is shown that the type of restraint has significant influence on crack widths. For structures with restraint along the length direction the restraint effectively facilitates distribution of cracking, similar to a wall fully restrained at the bottom. Crack widths always become smaller with increasing reinforcement ratio, but also low reinforcement ratios reduces crack widths. Walls on slabs resting on a frictional surface give significantly lower crack widths compared to a fully restrained structure.