On Engineering the Microstructure of High-Performance Concretes to Improve Strength, Rheology, Toughness, and FrangibilityPublic Deposited
The research reported in this dissertation covers the development of two high-performance concrete materials, Very-High-Strength Concrete (VHSC) and Frangible Concrete, and the philosophy of engineering the microstructure of these materials to produce their specific beneficial traits. The underlying problem addressed was how to optimize their design to maximize their desirable properties and minimize their detrimental ones. VHSC is a concrete material that has been engineered to produce very-high strength and good ductility or toughness while maintaining rheological properties suitable for normal construction practices. Research was conducted to enhance and optimize its compressive strength by studying and varying its component materials of cement, sand, and silica fume; by researching high-range water-reducing admixtures to maintain useable plasticity in the fresh state; and by employing toughening mechanisms to combat the undesirable traits of brittleness. Specific toughening mechanisms included: types of aggregate, distributed steel fibers, and microinclusions of wollastonite microfibers and high-strength ceramic microspheres. Results showed that by choosing component materials that enhance strength and toughness, it was possible to reliably increase the compressive strength of VHSC to 266 MPa. Frangible Concrete lies at the other end of the high-performance material spectrum and is believed to be a new category of material that possesses adequate structural strength yet is designed to break into small, non-lethal fragments under dynamic blast pressures. Research was conducted to engineer the properties of slag-based cement pastes to develop a system of pre-induced microcracks that would weaken but not cripple the static load bearing capacity of the material. However, under dynamic load the fracture process would continue, producing tiny fragments much like automotive safety-glass. Laboratory and field blast tests results are presented and show that when small-diameter, mono-sized aggregate particles are coated with the pre-engineered slag paste in a no-fines-concrete approach, the Frangible Concrete material achieves its goal.