Early Age Fracture Mechanics and Cracking of Concrete: Experiments and Modelling

Modern high performance concretes have low water cement ratios and do often includesilica fume. Also early age high strength cements are often applied and when all thesefactors sum up, it turns out that the cracking sensibility is dramatically increased in com-parison with ordinary concrete. The reasons are the increased autogenous deformation,the high rate of heat evolution and a higher brittleness of these concretes. Due to these adverse mechanisms the interest in the full description of the behavior ofearly age concrete has increased dramatically in the last two or three decades. Almost all the governing material parameters have undergone intensive research and the body of knowledge provides today a basis for calculation of the stress evolution and thus, represents a tool for prediction of whether cracking will occur or not. However, the experimental investigation and the modelling of the early age concrete aftercrack initiation has occurred, is scarce. What are the constitutive properties for the crackin early age and how can they be determined? This is the subject for the present thesis. A detailed analysis of five important test methods and their application for early age concrete have been carried out and experimental results generated. These test methods comprise the uniaxial tension test, the three point bending test, the split cylinder test,the wedge splitting test and a method for determination of the bond properties in early age between reinforcement and concrete. The analysis has been carried out utilizing the finite element method and through analytical modelling. The uniaxial tension test is not suited for early age concrete, but it serves as an essential tool for comparison. The most well suited method for determination of early age fracture properties of concrete is the wedge splitting test. For this method a fast and simple method for interpretation and inverse analysis has been developed and calibrated versus the finite element method and experimental results. The analysis of the split cylinder test has shown that this test method produces erroneous results if applied in early age. The results are only valid after 24 hours for fast and normal hardening cements and after 48 hours for slow hardening cements. This is confirmed in a finite element model. The fracture properties of early age concrete have been determined. The framework of the investigations has been the fictitious crack model and the aim has been experimentally to determine the fracture mechanical properties related to this model. The results provide interesting and important insight into the development of the fracture properties in early age. It is found that the characteristic length has moments of low values in early age, which means that the cracking sensibility is higher at those time points. The possible influence of time-dependent effects in the fracture mechanical properties on the cracking behavior in early age has also been investigated. The reason for this has been the known fact that viscoelasticity of the bulk material is very important for the determination of the stress evolution in early age, and thus, this could also be the case for the crack. However, it has not been possible to determine any rate effects or viscoelastic effects of the crack in early age. The observed time dependent deformations can entirely be ascribed the bulk creep effects. The work on the bond properties between concrete and reinforcement in early age has resulted in the development of a method for interpretation and inverse analysis and also in a experimental setup, which is suited for determination of the early age properties.