| Systematic experiments at high and low strain rates, and micromechanical modeling are used to study the response and failure of concrete in triaxial compression. Appropriate experiments on concrete at high strain-rates are difficult to perform due to its coarse microstructure, which requires the use of large samples. The development of a physically based model poses many challenges due to the complex microstructure of concrete. Made primarily from sand, rock aggregates, cement and water, concrete contains microcracks at several length scales in many directions, as well as voids. The present work uses experimental, analytical and numerical techniques to study and describe the response of concrete and its constituent materials.; Concrete and its constituents, mortar and limestone aggregate, are characterized using ultrasonic techniques, optical microscopy and scanning electron microscopy. High and low strain-rate uniaxial compression tests are performed on each material. In addition, new experimental techniques to study concrete and mortar in triaxial compression at high strain rates are developed. These techniques allow for accurate measurement of the behavior of concrete with these loading conditions. The results clearly show that at high strain rates, concrete can sustain a higher load with increasing confining pressure.; A new model predicts the elastic modulus of concrete based on its microstructure. The elastic modulus is found using a self-consistent implementation of the double-inclusion model, with a newly developed provision for additional strain due to the presence of microcracks, either in the constituents or at their interface. A simplified version of the elastic model has been implemented using the software Mathematica. Important physical parameters are identified through testing, material characterization and the available literature. As concrete is loaded, crack density increases; the increase in crack density may be found using fracture mechanics, based upon the local stress. Crack growth results in both inelastic strain and, since the geometry has changed, a decrease in the elastic modulus. |
