Hardening Of Ultra-high Strength Concrete

As a new cement-based material, UHPC has a character of high strength and superior durability. Ultra high-performance concrete (UHPC) can show compressive strength more than 150 MPa, approximately 3 times as that of conventional concrete. Due to incorporation of steel fiber, the ductility and energy absorption of UHPC is typical 300 times greater than that of high performance concrete (HPC). UHPC is nearly impermeable to carbon dioxide, chlorides and sulphates. Its superior durability leads to long service life with reduced maintenance. Though UHPC possesses many outstanding properties, it also has some weaknesses. High binder content of about 800-1000 kg/m3 affects not only the production costs, but also high heat of hydration, causing shrinkage problems. Traditionlly, UHPC was usually composed of cement, silica fume, quartz sand, quartz powder, steel fiber and superplastizer, et al, and its cost was several times than that of ordinary concrete. In order to improve the acitivity of supplementary cementitious materials in UHPC, preparing UHPC was often cured by steam or autoclave curing. It not only improved the cost of UHPC, but also limited the application of UHPC in practical engineering. In order to reduce the cost of UHPC and simplify its craft, preparing UHPC by romm temperature is becoming a hot topic. Due to difference of raw materials and craft, there may be a large difference between hardening of UHPC prepared by room temperature and that of UHPC prepared by heat curing. Therefore, in this paper, the hardening of UHPC prepared by room temperature was investigated. Since steel fiber has little effect on hardening of UHPC, hardening of ultra-high strength concrete (UHSC) without steel fiber was investigated in this paper.Compressive strength of UHSC prepared by natural quartz sand reached to 131.5MPa when sand-binder ratio was 1.0. Compressive strength of UHSC prepared by crushed quartz sand reached to 139.8MPa when sand-binder ratio was 1.1. Although the flowabilities and compressive strengths of UHSC prepared by crushed quartz sand were larger than that of UHSC prepared by natural quartz sand, excellent properties of UHSC could be achieved by using natural quartz sand. When the ratio of sand to binder was 1, the compressive strength of UHSC prepared by natural quartz sand was 131.5MPa. Considering the cost and properties of UHSC synthetically, UHSC were prepared by natural quartz sand in this paper, and the ratio of sand to binder was 1.The composition of the binder was designed using seven-batch factorial design method. The relationships between the binder composition and the properties were expressed in contours. Proper silica fume content could improve the flowability and compressive strength of UHSC with cement-silica fume-slag binder, reduce the porosity and calcium hydroxide content of UHSC. Slag reduced the flowability, compressive strength, porosity, and calcium hydroxide content of UHSC to certain extent. The silica fume and slag demonstrated positive synergistic effects on the flowability and 3d compressive strength, but have negative synergistic effects on the total heat of hydration, hydration heat when the time is infinitely long(Po),56d compressive strength, porosity and calcium hydroxide content of UHSC.The composition of the binder was designed using seven-batch factorial design method. The relationships between the binder composition and the properties were expressed in contours. Proper silica fume content could improve the flowability and compressive strength of UHSC with cement-silica fume-fly ash binder, reduce the porosity, calcium hydroxide content and the time of acceleration period of hydration of UHSC. Fly ash increased the flowability and early porosity, but decreased early compressive strength and calcium hydroxide content of UHSC to certain extent. The silica fume and fly ash demonstrated positive synergistic effects on the flowability and compressive strength, but have negative synergistic effects on the total heat of hydration, hydration heat when the time is infinitely long(Po), porosity and calcium hydroxide content of UHSC.In order to study the hardening process of ultra-high strength concrete(UHSC) with cement-silica fume-slag-fly ash four binder, water-binder ratio, silica fume content, slag content and fly ash content were selected as four factors and four levels in this paper. Factorial design was applied to the mix proportion of UHSC, and the hardening process of UHSC was studied. With the increase of water-binder ratio, the flowabilities of UHSC increased significantly, compressive strengths at different ages decreased, porosities at different ages increased, and calcium hydroxide content at different ages increased. With the increase of silica fume content, the flowabilities of UHSC decreased significantly, compressive strengths increased first and then decreased, porosities at different ages decreased, and calcium hydroxide content at different ages decreased. With the increase of slag content, the flowabilities of UHSC had no obvious change, compressive strengths increased first and then decreased, porosities decreased at early age but increased at later age, and calcium hydroxide content at different ages decreased. With the increase of fly ash content, the flowabilities of UHSC increased first and then decreased, compressive strengths at early age decreased evidently, compressive strengths at later age increased first and then decreased, porosities increased at early age but had no obvious change at later age, and calcium hydroxide content at different ages decreased.The flowabilities of UHSC were reduced by incorporating nano-SiO2 and nano-CaCO3. After incorporating nano-SiO2, the hydration heat flow of UHSC increased, and the hydration heat of UHSC increased first and decreased later. After incorporating nano-CaCO3, the hydration heat flow of UHSC increased, and the hydration heat of UHSC decreased. With increase of nano-SiO2 content, the compressive strengths of UHSC increased first and decreased later. With increase of nano-CaCO3 content, the compressive strengths of UHSC increased first and decreased later. With increase of nano-SiO2 content, the porosities of UHSC decreased first and increased later. With increase of nano-CaCO3 content, the porosities of UHSC decreased first and increased later. With increase of nano-SiO2 content, the calcium hydroxide content of UHSC decreased. With increase of nano-CaCO3 content, the calcium hydroxide content of UHSC increased.