Research On The Effects Of Hybrid Fiber On Reactive Powder Concrete

Reactive Powder Concrete(RPC), prepared with high-quality fine quartz sand, low water-cement ratio, high-performance silica fume and other active admixture, exhibits a high compressive and rupture strength, low porosity, ultra- high durability. But meanwhile RPC also becomes more brittle. In order to improve the mechanical properties of RPC, including rupture strength, fracture toughness as well as to improve the brittle characteristics of RPC, copper-plating fine steel fibers are usually added in the RPC. In addition, PVA fibers can improve the bending toughness and the tensile deformation of concrete. this paper aim to research on the e ffects of hybrid fiber on reactive powder concrete.The main work in this paper was preparing RPC specimens with different copper-plating thin steel fiber/PVA fiber ratio and taking their compressive and rupture strength tests, beam bending test, impact to ughness test, salt freezing experiment and hardness, microstructure analysis, results show:With copper-plating fine steel fiber content increasing from 0% to 2.0%, the compressive and rupture strength and flexural properties of RPC specimens were significantly enhanced; the impact toughness and the salt frost resistance of reactive powder concrete firstly increased and then decreased by increasing steel fiber. the additions of steel fiber is 1.5%, the impact toughness and the salt frost resistance of reactive powder concrete shows good performance. Wherein, there is a slight increase in the first crack, then a multiplication in the final crack. Hardness inside steel fiber is about 3.5 times of the cement, the interfacial transition zone is fully dense, there is obviously a reinforcement area within 30 micron, and the microhardness is slightly larger than cement.With a certain amount of copper-plating fine steel fibers, Deformation peak in beam bending test and the flexural strength of all three kinds of RPCs mixed with different PVA fibers increased with increasing content from 0.5%, 1.0% to 1.25%, the load-deflection curve segment slowed by the steep decline, while compressive strength was consistent with steel fiber only RPC. When PVA fiber content increased to 2.0%, the compressive strength and rupture strength both significantly decreased, the peak loading, peak deformation decreased significantly, declining section slowed down as well. All three kinds of PVA fiber can slow down the first crack of the RPC, but the final crack numbers of PVA-HT-12 fiber and PVA-HT-6 fiber significantly decreased. Both the first and final crack of PVA-ECC-12 with only 1.25% content is significantly better than the copper-plating thin steel fiber only ones. With increasing copper-plating fine steel fiber content from 0% to 2.0%, PVA-HT-12 fiber showed better salt-frost resistance performance than steel fiber only one at early stage, but a poor performance at late stage. Short fibers exhibited better salt- frost resistance performance than long- fiber ones. Hardness inside the PVA-ECC-12 fiber is about 1/4 of the cement, the interfacial transition zone is loose structure, there is obviously a weak region within 30 micron, while the microhardness of interfacial transition zone is roughly half of the cement. These agreed with the result of digital microscope.Based on the above rules, the final ratio of the fibers mixture is given : 1.5%wt copper fine steel fiber, 1.25% PVA fiber, and the overall performance of PVA- ECC-12 fiber rank the place.