Thermal cracking of young concrete : partial coefficients, restraint effects and influence of casting joints

Abstract: Cracks can occur due to temperature induced movements during the hardening phase of concrete. Thermal cracks are not decisive for the load-bearing capacity but cause costs for repair and may reduce the life of the structure. The risk of cracking can be judged through safety values, (partial coefficients) given in building codes. In the thesis, partial coefficients for thermal cracking problems are determined with a probabilistic model. One crucial parameter influencing the risk of cracking is the restraint. In the report, methods are derived and presented for the determination of the restraint. Rotational boundary restraint from elastic foundations is treated as well as the restraint in internal points in the structures. For the determination of the rotational boundary restraint coefficient, applicable and simple graphical tools are presented. The restraint in internal points is based on a so-called plane-section restraint coefficient, a resilience factor for structures with length to height ratios smaller than seven, and a factor for slip failures in joints between the young and old parts. Four experiments of walls cast on slabs are presented in the thesis. On a precast slab, concrete walls are cast and loaded by restraint stresses induced by the temperature during the hydration. Different boundary restraint situations are used for the structure as well as different amount of reinforcement in the joint between the walls and the slab. The restraint and the amount of reinforcement are investigated regarding the risk of through cracking of the wall. The phenomenon of slip failure in joints has been shown in the tests.