Comparative Study On Influences Of Various Steel Fibers On Mechanical Properties Of Ultra-high Performance Concrete (UHPC)

ABSTRACT:In this thesis, an experimental investigation was conducted to study the influence of various steel fibers on mechanical properties of ultra-high performance concretes.After28days of standard curing, the basic mechanical properties (compressive strength, tensile splitting strength, static modulus of elasticity under compression) and fracture energy of concretes at two different water-binder ratios (0.18,0.26) and six types of steel fiber with different tensile strength and different shapes were investigated. The effect of different kinds of steel fiber was studied on mechanical properties of those concretes.The results of basic mechanical property tests reveal that the compressive strength of HPC with0.26W/B ratio and UHPC with0.18W/B ratio is slightly increased. The increase of compressive strength of HPC with hooked-ends steel fiber is quite more than the others, and the crimped steel fiber does the most significant work in UHPC compressive strength. The increase of splitting tensile strength is mainly positive correlate with the tensile strength and the length to diameter ratio of steel fibers. The rubber-attached crimped steel fiber over6kinds of steel fibers has the most outstanding effect on improving the toughness of HPC. The two kinds of steel fiber with590MPa tensile strength and relatively low length to diameter ratios are not working so well. Without apparent ascending of mechanical properties, but with reduction of compressive strength of UHPC, it indicates that this type of steel fiber is apparently not applicable to ultra-high performance concrete.The results of fracture energy tests reveal that the existing of rubber attached on the environmental crimped steel fiber can reduce concrete cracks and more effectively enhance the fracture energy of HPC and UHPC. Because of the low deformation degrees while steel fiber with2000MPa tensile strength is pulling out, the crimped steel fiber is better playing its role by bridging the gap than the hooked-ends steel fiber does. Between the two kinds of steel fiber with1100MPa tensile strength, the disparity in UHPC fracture energy is not that evident, however in HPC, the hooked-ends steel fiber is clearly more stability and more effective in bridging. The two kinds of steel fiber with relatively low length to diameter ratios are not significant in the promotion of UHPC fracture energy. The reason is mainly correlated with the low steel strength and high brittleness, which leads to weak interfaces in UHPC.The above reveal that the effect of steel fiber in HPC and UHPC is related to the steel tensile strength, length to diameter ratio and the shape of fiber. With the different influence of mechanical properties by all sorts of steel fiber in different water-binder ratios, it is necessary to comprehensively consider the matching between various factors of steel fiber and the strength of concrete itself. Combine the basic mechanical property tests and fracture energy tests, it appears that the crimped steel fiber and thread steel fiber with590MPa tensile strength are not fit for UHPC. The crimped steel fiber with high tensile strength and the hooked-ends steel fiber with high length to diameter ratio have the most obvious improvement effect on mechanical property of high performance concretes.