Cracking in Reinforced Concrete Structures Due to Imposed Deformations

Abstract: This thesis is concerned with modeling of the cracking process in reinforced concrete due to imposed deformations. Cracking is investigated both at early ages, during hydration, and at mature age when the final properties of the concrete are reached. One of the most important material characteristics of the concrete at early ages, the Young’s modulus is determined by means of a dynamic method called the resonance frequency method. The thesis consists of an introduction and four papers with the following contents: Paper I Is a presentation of a simple constitutive model describing the thermal stress development in reinforced concrete at early ages. The most important input parameters are temperature history, stiffness development and tensile strength development. By means of a parametric investigation it is shown that the magnitude of thermal stresses is very sensitive to the timing between temperature rise and stiffness growth. Paper II Presents results from E-modulus measurements at early ages. A simple dynamic method is used, based on measurement of resonance frequency and damping in a concrete prism. Using the dynamic test results an empirical conversion formula is suggested for prediction of the static E-modulus. Application of the conversion formula for two concrete mixtures showed consistency and good agreement with the static test results. Paper III Deals with modeling of softening and cracking of reinforced concrete at mature age due to imposed deformations. Medium-thick structures provided with minimum reinforcement and exposed to sudden cooling corresponding to a drying shrinkage process are studied. It is shown that the force actually arising is about 0.5-0.7 of the nominal tension capacity of the wall and consequently may lead to a considerable reduction of the minimum reinforcement. This is valid for cases with imposed strain large enough to cause a through crack regardless the external restraint. Paper IV Is a further development on the study of cracking due to cooling and shrinkage. The effect of slow cooling, the presence of wind and the influence of different geometries are investigated. It is concluded that in most practical cases cooling processes are slow leading to small internal restraint. For structures exposed to slow cooling processes and small external restraint stiffness relative to the axial stiffness of the wall, the crack reinforcement can be omitted. Crack reinforcement should be provided for structures with large dimensions in the direction of the restraint.