Key Engineering Materials Vols. 629-630

Abstract: The dynamic material properties of high performance hybrid fiber reinforced cementitious composites (HFRCC) with various volumetric fractions of steel and polyvinyl alcohol (PVA) fibers were studied by the Split Hopkinson Press Bar (SHPB) test. The results show that HFRCC with higher volumetric fraction of steel fibers are more sensitive to stain rate and the dynamic compressive strength increase more prominently with the strain rate increasing, but peak strain shows the opposite trend. The PVA fibers increase the ductility of HFRCC more effectively than steel fibers. Compared to PVA fiber reinforced cementitious composites (FRCC), HFRCC present better dynamic material properties under impact loading.

Abstract: The purpose of this study was to reduce production costs for UHPFRC members by using a bundle of longitudinal reinforcement bars as a substitute for steel fiber. Experiments on the ductile behavior of Ultra High Performance Concrete (UHPC) rectangular beams with a combination of steel fiber and longitudinal reinforcement bars were performed.The volume fractions of steel fiber were 0%, 0.7%, 1%, 1.5%, 2%, and the reinforcement ratios of longitudinal reinforcement bars that promoted ductile behavior were 0.0036, 0.016, 0.029 and 0.036.Fifteen UHPC beams were made with the combination of these test factors. Both the steel fiber and the longitudinal reinforcement bars had the effect of induciing ductile behavior for UHPC structural members. The load-deflection relationship, the concrete stress variation and the crack pattern indicated the usefulness of the bundle of longitudinal bars that have a small diameter with close arrangement.

Abstract: Nanotechnology is being used in UHPC for achieving more dense structural packing and better physical and mechanical properties. In this paper, nanosilica particles (0-1wt %) by cement were incorporated in cement pastes and mortars with water-binder ratio of 0.2, and their effect on fresh properties was addressed. The fresh properties of mixtures were investigated by spread, rheology and cohesiveness. In addition, effect of nanosilica on cement hydration was also investigated. The results show that significant increase of yield stress, viscosity and cohesiveness is observed with nanosilica above 0.005 wt% by cement, but nanosilica with content below 0.005% presents an improvement of workability especially with content of 0.002%. Moreover, the addition of nanosilica in cement pastes obviously accelerates the early cement hydration rate, but does not increase the cumulative heat evolution.

Abstract: Ultra-high performance concrete (UHPC) incorporating coarse aggregate was prepared with common raw materials. Fresh concrete had excellent good workability with slump of 265 mm and slump spread of 673 mm. Compressive strength of UHPC at 56 d reached 150 MPa. However, UHPC exhibited high brittleness in terms of spalling failure which occurred during compression loading.The ratio of splitting tensile strength to compressive strength of about 1/18 and the ratio of flexural strength to compressive strength of about 1/14 at 56 d were also associated with the brittleness of UHPC in this research. Mineral admixtures and fluidity of fresh concrete influenced compressive strength of UHPC significantly. Moreover, UHPC had excellent permeation-related durability but considerable shrinkage. Autogenous shrinkage of UHPC was less than half of free shrinkage, for which the reason is unknown and needs further research.

Abstract: This paper presents an experimental investigation on mechanical properties (including compressive strength, tensile splitting strength and fracture energy) of ultra-high performance concrete (UHPC) with recycled steel fiber, compared with none fiber and industrial steel fiber reinforced UHPC. Moreover, the microscopic observation of fracture energy was carried out. All specimens were prepared at 0.18 water /binder (W/B) ratio and the dosage of steel fiber was controlled at 60 kg/m³. The results indicate that recycled steel fiber has a significant effect on enhancing strength and toughness of UHPC. And owing to the crimped shape, higher tensile strength (1800-2000 MPa) and appropriate diameter (1 mm) of recycled steel fiber, the steel fibers of UHPRSFRC will not immediately be pulled off and necking phenomenon is distinct.

Abstract: An experimental investigation on the variation of compressive strength, splitting tensile strength and fracture energy, with the ratios of water to binder (W/B) of ultra-high strength concretes, including the reactive power concrete (RPC) and ultra-high strength concrete with coarse aggregate (UHSC), has been carried out. The W/B varied between 0.14 and 0.22 at a constant increment of 0.02. It was observed that, compressive strength of RPC almost remained the unchanged, when the W/B was between 0.14 and 0.18. However, it decreased dramatically when the ratios were 0.20 and 0.22. For UHSC, the compressive strength was the highest value at the ratio of 0.18. The results of the two concretes could not comply with the Abrams' generalized W/B ratio law. Moreover, splitting tensile strength of RPC and UHSC decreased continually as the ratio increased from 0.14 to 0.22. Fracture energy of RPC was more or less the same when the ratios were between 0.16 and 0.20, and the maximum value was at 0.14. Fracture energy was observed to be almost no variation for UHSC at all ratios

Abstract: Scanning electron microscope, differential thermal analysis, thermo-gravimetric analysis, fluorescent analysis and X-ray CT et.al. have become usual means used in the research of concrete material science. This paper proposed a new method to evaluate the causes of cracking of in-situ concrete based on the investigation by using these comprehensive technologies for analysis of micro-structure. The proposed new method is more reliable and objective than the traditional method which is mainly based on experience and chemical analysis. A case investigation using the new method to explore the causes of cracking in a real project was introduced.

Abstract: The creep of the concrete might cause comprehensive effect on the structure, such as stress redistribution and pre-stress relaxation. To better understanding the significant influence of the creep on the structure, the restriction of the reinforcement should be taken into account. In this paper, the creep of reinforced concrete was studied based on designed experiment with different concrete mixtures and reinforcement configurations. And Dischinger model was used to analysis the experiment results. It was found that, with the increasing of reinforcement ratio, the creep of all concretes could be reduced with a trend that the difference of the creep property between concrete mixtures would be delaminated by the reinforcement. And suggestion was proposed to modify the Dischinger model to better consider the influence of firm structure formed by rebar.

Abstract: In this paper, the performance of two kinds of migrating corrosion inhibitors on the corrosion behavior of steel in concrete under wet-dry cycle was investigated. The wet-dry cycle duration on the open circuit, current density and impedance of carbon steel imbedded in concrete were analyzed by linear polarization resistance (LPR) and electrochemical impedance spectroscopy (EIS). The results show that two kinds of migrating corrosion inhibitors can restrain corrosion of reinforcing steel, the corrosion efficiency of ZX was less more than MCI. Migrating corrosion inhibitor not only can improve carbon steel impedance, also can improve concrete resistant. Key words: migrating corrosion inhibitor;Cl^-;reinforcing steel;linear polarization resistance;electrochemical impedance spectroscopy

Abstract: In this paper, restrained ring test and shrinkage test are carried on three kinds of concrete—high-strength portland cement concrete, high-strength calcium sulfoaluminate cement concrete and high-strength calcium sulfoaluminate cement concrete with internal curing in order to evaluate the shrinkage induced cracking performance of the concretes. The experimental results show that calcium sulfoaluminate cement concrete exhibits lower shrinkage caused by surface drying comparing to portland cement concrete. Internal curing can eliminate most of the autogenous shrinkage of concrete. In the ring test, the latter two concrete did not crack during the whole test history—42 days, while high-strength portland cement concrete cracked at the 13th day after casting. High strength calcium sulfoaluminate cement concrete exhibits better anti-cracking ability than the high strength portland cement concrete with the same strength grade.