A newly developed powerful version of microplane model, labeled model M4, is exploited to study two basic phenomena in fracturing concrete:(a) The vertex effect, I.e., the tangential stiffness for loading increments to the side of a previous radial loading path in the stress space, and (b) the effect of confinement by a steel tube or a spiral on the suppression of softening rsponse of columns. In the former problem, the microplane model is used to simulate the torsional response of concrete cylinders after uniaxial compression preloading to the peak compression load or to a post peak softening state. Comparisons with new tests carried out at Northwestern University show the microplane model to predict the initial torsional stiffness very closely, while the classical tensorial models with invariants overpredict this stiffness several times (in plasticity of metals, this phenomenon is called the vertex effect because its tensorial modeling requires the yield surface to have a vertex or corner at the current state point of the stress space). In the latter problem, microplane model simulations of the so called tube squash tests are presented and analyzed. In these tests, recently performed at Northwestern University, steel tubes of different thicknesses filled by concretre are squashed to about half of their initial length and very large strains with shear angles up to about 70 degrees are achieved. The tests and their simulations show that in order to prevent softening and thus brittle failure and size effect, the cross section of th etube must be at least 16% of the total cross section area, and the volume of the spiral must be at least 14% of the volume of the column. When these conditions are not met, which comprises the typical contemporary designs, one must expect localization of damage and size effect to take place.

Concrete is the most widely used construction material in the world with over 8 billioin tons of it being produced yearly. Much of this concrete is steel reinforced since the concrete/steel composite has improved ductility over concrete alone, and the concrete provides a protective environment for the steel. However, reinforced concrete must be used in severe corrosive environments such as found in marine and deicing salt applications. The ingress of chloride leads to corrosion of the steel resulting in early repairs of the structure. The subsequent costs are over $50 billion/year in the United States, and represent a major drain on infrastructure resources throughout the world. In this paper the use of improved concrete designs to control corrosion of steel in concrete are addressed. These designs incorporate the use of low permeability concrete, corrosion inhibiting admixtures, reduced shrinkage and increased toughness with fiber reinforcement. It is demonstrated that this holistic approach to the concrete design provides a lower life-cycle cost.

Restrained tests are used to evaluate the risk of early age cracking and the craking sensitivity of concrete mixtures. One test that has become common in recent years is the active uniaxial restrained test in which the length change due to shrinkage is recovered by applying external load to maintain the concrete sample at constant length. The length change is measured by linear variable differential transformer (LVDT), which is used as the control signal in this test. In such tests, the dog-bone geometry is used to grip the ends. To ensure a fully restrained test, the LVDT response to the loads and to shrinkage specimen interaction should not interfere with the measurement of deformation, and this depends on the instrumentation and how the LVDT is attached to the concrete specimen. Some experiments in the literature have the LVDT attached to the steel grips, a practice vulnerable to possible error due to the interaction between the grip and the concrete. This study considered two methods of attaching the LVDT. First, the LVDT is attached to the steel grips, second, the LVDT is attached to the concrete within the zone of reduced cross-section. The results indicate that attaching the LVDT to the grips results in errant measurement of the shrinkage stress, creep, and elastic strains due to the grip-specimen interaction. The consequences will be false interpretation of fully restrained shrinkage and creep characteristics because the grip-specimen interaction leads to a partially restrained test. The study suggests mounting the LVDT to the concrete sample away from the grips to achieve a fully restrained test. Results for two concrete mixtures with w/c ratio of 0.51 and 0.56 are discussed for both methods of attaching LVDTs.

A current research project on hydration kinetics, mechanical properties and early age stress behavior of blended cement conducted at the University of Michigan is reviewd in this paper. A number of experiments including calorimetry and differential thermal analysis were performed to investigate hydration kinetics. The mechanical properties investigated included the compressive strength, splitting tensile strength, Young's modulus, creep compliance, relaxation modulus, and coefficient of thermal dilation. The early age stress behavior was studied by measuring the stress developed in a uniaxially restrained concrete member. In addition, the deformation due to autogeneous shrinkage was also measured experimentally. The experimental data could be used to quantify degree of hydration,, and temperature effects on hydration, and could be used as imputs for predicting the early age stress development in concrete.

Researchers and educators who specialize in the material science of concrete will find a lot to interest and challenge them in this Symposium Publication. It contains 35 papers presented at a symposium honoring Surendra P. Shah, Director, Center for Advanced Cement-Based Materials, Northwestern University. Shah has made many contributions to the knowledge of concrete material science, fracture mechanics, high-performance concrete, and fiber-reinforced concrete—all subjects that were covered at the symposium. Topics for the presented papers include: • Advances in fatigue and fracture; • Creep, shrinkage, and early-age cracking; and • Laminated and fiber reinforced cement composites. The last group of papers covers the future of research and education in concrete, with topics ranging from fracture mechanics applications for concrete to sustainable development for concrete. Thus, SP-206 examines the past, present, and future of concrete material science. Note: The individual papers are also available as .pdf downloads.. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP206