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Permeability of fiber reinforced concrete under stress Bhargava, Ankit

Abstract

Durability of concrete structures is one of the most significant problems currently facing the civil engineering community. It depends on the interactions with the service environment, in which the penetration of deleterious substances is highly significant. The latter can be considered solely controlled by permeation properties of the near surface concrete. Material transport properties, especially permeability, affect the durability and integrity of the structure. High permeability, due to porosity or cracking, provides ingress for water, chlorides, and other corrosive agents. If such agents reach reinforcing bars within the structure, the bars corrode, thus compromising the ability of the structure to withstand loads, which eventually leads to structural failure. Fiber reinforced concrete is becoming an increasingly popular construction material due to its improved mechanical properties over un-reinforced concrete and its ability to enhance the mechanical performance of conventionally reinforced concrete. Though much research has been performed to identify, investigate, and understand the mechanical traits of fiber-reinforced concrete, however, relatively little research has been done on the transport properties of fiber reinforced concrete under an applied compressive stress. The goal of this research work was to quantify the influence of compressive stress on transport properties of concrete, to investigate if the presence of cellulose fiber reinforcement in concrete helps maintain or reduce water permeability of concrete when under an applied stress, and finally to assess the durability of fiber reinforced concrete by using mathematical models. The above objectives were achieved by using a novel test procedure for measuring concrete permeability under stress. In this technique, two hollow-core concrete cylinders are simultaneously tested—one with stress and the other without—using identical flow conditions. A special design of the permeability cell eliminates leakage and allows the specimen to achieve conditions of flow equilibrium early in the test. The role of fibers in preserving water permeability of concrete in the presence and the in absence of compressive stress was investigated in two tests phases using the above described test procedure. In First phase of the experiments, water permeability of plain concrete was compared with fiber reinforced concrete for different fiber volume fractions of cellulose fibers using two identical hollow cylindrical samples-one with plain concrete and other with fiber reinforced concrete. In second phase, permeability of a pair of identical hollow cylindrical samples -one under stress and other under zero stress condition was compared. Fibers were found to be effective in controlling permeability of concrete under both stressed and unstressed conditions. A deterioration mechanism comprising chloride induced corrosion was proposed to investigate the durability of concrete. Further, a relationship between the water permeability and the chloride ion diffusion coefficient was developed to calculate chloride ion diffusion coefficient. Values of diffusion coefficient obtained by using above established relationship were used in a mathematical model to predict service life of concrete, which in turn was used to evaluate durability factor. Fibers seemed to play a significant role in preserving the permeable nature of concrete. Fiber reinforced concrete was found to be less permeable under the influence of an applied stress as compared to plain concrete. It was found that fiber reinforcement enhances the capability of concrete to withstand higher level of stress with a significant improvement in its durability.

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