Cracking Concrete Virtually

I have been spending alot of my time recently, trying to ensure that the model will simulate cracking correctly. I began by using a simple fixed-fixed model, as shown below. The model uses ANSYS SOLID65 nonlinear concrete solid elements to model the concrete and LINK8 3D spar elements to model a single piece of reinforcement. The reinforcement is modeled deiscretely rather than using the SOLID65 element's smeared reinforcement option.

Since the finite element model cannot explicitly impose shrinkage strains, the thermal coefficient of expansion is used and by applying body temperature loads to the model, shrinkage of the concrete can be simulated. The model has a coeffiecient of thermal expansion of 6 x 10^-6 for concrete and zero for the reinforcing steel. The steel's coeffiecient is given a value of zero to simulate the fact that steel does not shrink due to moisture loss. As the model becomes more sophisticated and thermal effects are considered in combination with drying shrinkage, an adjusted coefficient of thermal expansion will have to be used. A temperature change of 50 F is applied to the model. This corresponds to a shrinkage strain of 300 microstrain. The figure below shows the stress in the model in the long dimension of the specimen. Stresses are uniform along the length of the specimen, expecept at the ends where the boundary conditions induce stress concentrations.

After observing the stresses in the model, the locations of the cracks are plotted. The following two figures show the cracks plotted by ANSYS. All cracks form at the ends of the specimen, in the regions affected by the stress concentrations from the applied boundary conditions.