| The addition of short carbon fibers to concrete was found to increase the flexural strength, flexural toughness and freeze-thaw durability, decrease the drying shrinkage, volume electrical resistivity and steel-concrete contact electrical resistivity and provide strain/stress sensing ability, even at a fiber content as low as 0.2 vol.%. Effective use of the fibers requires dispersion of the fibers in the concrete and is achieved by the use of latex, methylcellulose and/or silica fume. The combined use of methylcellulose and silica fume for fiber dispersion provided the highest degree of fiber dispersion, the highest compressive strength, the lowest chloride ion permeability and the greatest strain/stress sensing ability, compared to the use of methylcellulose alone or the use of latex alone for fiber dispersion. The use of latex gave the lowest degree of fiber dispersion, the lowest strain/stress sensing ability and the highest cost, but it gave the highest tensile strength, the lowest void content, the lowest average void size and the highest debonding strength between old concrete and the carbon fiber reinforced new concrete; the debonding strength was higher than the cases where the new concrete contained latex or epoxy, but no fibers. The increase of the concrete-concrete debonding strength due to the fiber addition is because of the low drying shrinkage of the new concrete due to the fiber addition. The fiber addition increased the void content and the average void size, thus decreasing the compressive strength, increasing the permeability and decreasing the corrosion resistance, though increasing the freeze-thaw durability. However, at a low fiber content (as low as 0.2 vol.%), the negative effects of the fiber addition were negligible and were overshadowed by the counteracting positive effects of the silica fume addition, which greatly increased the compressive strength, decreased the permeability and increased the corrosion resistance. The aggregate size had little effect on the effectiveness of the fibers in increasing the flexural strength, but the strain/stress sensing ability was decreased by the presence of a large aggregate. The sensing ability is due to the volume electrical resistivity increase during crack opening and the reverse during crack closing. Reversible crack opening and closing occurred during cyclic elastic deformation, and was made possible by the fibers' control of the cracking. Without the fibers, there was no sensing ability. Prior to the first crack opening, weakening of the fiber-matrix interface took place and resulted in an irreversible resistivity increase, which provided memory of the first deformation. The stress/strain when the interface weakening started was smaller under tension than compression. |
