| Fiber-reinforced cementitious composites can be tailored for specific applications with the use of hybrid-fiber reinforcement. Different types of fibers are combined to take advantage of the benefits of each. Glass, polyvinyl alcohol (PVA), and polypropylene fibers are combined to reinforce extruded cementitious composites, cast concrete, and mortar.; The flexural performance of hybrid-fiber reinforced extruded cementitious composites, containing strong, but relatively brittle, glass fibers and weaker, but more ductile, polypropylene fibers is studied. As the glass fiber content increases, the composite strength increases. As the polypropylene fiber content increases the ductility, and thus the toughness, increases. Strain hardening is observed when PVA fibers are added to the composite. Extruded composites are also tested in impact. The optimal hybrid combination for static loading of extruded composites is glass and PVA fibers, while a combination of glass and polypropylene fibers performed best in impact.; Hybrid fiber combinations of glass and polypropylene are tested for shrinkage reinforcement in cast concrete and mortar. First, the matrix is modified to permit the addition of a sufficient amount of fibers without compromising workability. The shrinkage performance of fiber-reinforced concrete is measured with restrained ring tests. Fibers enhance shrinkage crack resistance by causing multiple cracks and limiting crack widths. To amplify the effects of fibers and accelerate testing, hybrid fiber combinations of bundled glass and polypropylene fibers are tested in mortar. The effect of the hybrid-fiber reinforcement is additive with the crack widths falling between those in mortar reinforced with one type of fiber.; A model to predict shrinkage crack widths in restrained ring tests is developed to facilitate the design of hybrid-fiber shrinkage reinforcement. Crack widths are calculated from the free shrinkage profile and post-peak tensile stress-crack width relationship. When creep is incorporated in the model, the predicted crack widths are quite accurate, considering the simplifying assumptions. An analytical fracture mechanics based technique is developed to derive the tensile stress-crack width relationship from flexural test data. This eliminates the need for direct uniaxial tension tests. |
