Three Dimensional Simulation Of Drying Shrinkage Micro-cracking In Numerical Concrete Reconstructed From CT Images

Drying shrinkage micro-cracks have important effects on the mechanical properties, transport properties and durability of concrete materials and structures. Three dimensional structures of the micro-cracks have to be acquired to study their effect on the properties of concrete. However, only the two dimensional crack patterns in concrete slices, instead of the three dimensional crack patterns in concrete, can be investigated because of the limit of the current techniques of observation. Meanwhile all the present numerical models for micro-cracking in concrete is limited in the two dimensional simulation and the results don't agree with the experimental results because of the discrepancies between numerical concrete and real concrete, the poor convergence of the implicit method, and the limitation of cracking models. Based on the studies on methods of producing numerical concrete, improvement of smeared cracking model and the explicit finite element method, the explicit method of simulating drying shrinkage micro-cracking is established and three dimensional micro-crack patterns are simulated. Main achievements in the dissertation are as follows,Three dimensional realistic aggregate and mortar structures are reconstructed and modeled from CT images of concrete.Aggregate-expansion method to mesh Interfacial Transition Zone (ITZ for short) is developed and three dimensional three-phase numerical concrete is produced based on this method. Thickness of ITZ elements generated by this method is close to the real dimension of ITZ in concrete.Modified smeared cracking model with equivalent fracture energy cracking criterion is proposed, and the brittle cracking model available in ABAQUS is employed for the explicit finite element analysis of two dimensional drying shrinkage micro-cracking of concrete. The simulated crack patterns agree well with experimental results, showing the capability of the method in analysis of drying shrinkage micro-cracking. Modified smeared cracking model is suitable for quasi-brittle materials such as mortar and ITZ in simulation of drying shrinkage micro-cracking in concrete. Results from parametric analysis show that values of fracture energy of mortar and ITZ have important effects on the final drying shrinkage crack patterns.Three dimensional drying shrinkage crack patterns in the three-phase realistic numerical concrete generated from CT images, are obtained employing the above explicit finite element method. The fact that drying damage of concrete calculated from the simulation increases with the increasing of drying age, which is in good agreement with the experimental results, shows the reliability of the numerical three dimensional crack patterns in concrete.