Experimental Study On Mechanical Properties Of RPC-FST Under Direct Tension Load

With the increasing application of concrete-filled steel tubular (CFST) truss girder in long-span structures, more and more attentions have been paid to research on mechanical properties of CFST member in tension. Concrete filled in steel tube helps to increase chord’s load-carrying capacity and rigidity, however, the mechanical property enhancement is limited because concrete is weak in tensile strength and deformation. Furthermore, the enhancement is usually reduced or even offset completely by the defects like increased structure weight, construction difficulty and material consumption due to filling concrete in chord tube in tension. In order to improve structural performances of CFST member in tension, steel fiber had been added into concrete in some construction projects. However, the application effect is not so good as planed.In order to improve the cracking strength, ultimate load-carrying capacity and rigidity of CFST member in tension, thus promote engineering application and theoretical research of CFST structures, the mix proportion of a kind of reactive powder concrete (RPC) with high tensile strength and ductility, and the research results about mechanical properties of reactive powder concrete-filled steel tubular (RPC-FST) member in tension, were summarized in this paper. Brief introduction to the research work and results is given as below:①RPC-FST test specimens were designed in light of the superstructure of a CFST continuous truss girder bridge. Improvement of loading method of CFST direct tensile test, force transferring configuration installed in both ends of test specimen, thinning of steel tube, and deformation measurement devices, were made for the test. RPC with high tensile strength and ductility was successfully prepared by use of fine sand from local Minjiang River as the aggregate. A total of 23 RPC-FST test specimens and 21 RPC test specimens with different steel fiber volume fractions (0%, 1%,2% and 3%) and different curing temperatures (20℃,60℃,90℃,180℃), as well as three steel tube specimens were fabricated for the direct tensile test.②Based on the full range stress vs. strain curves of RPC that were successfully obtained from RPC direct tensile test, tensile modulus of elasticity, stress as well as strain at cracking load, stress as well as strain at peak load, and high strength strain zone, were specified.③By comparing the mechanical properties of steel tubular specimen, RPC specimen and RPC-FST specimen under axial direct tension load, the individual working condition of and the interaction between steel tube and RPC were studied. By comparing the characteristic parameters, the influence of steel fiber and curing temperature on RPC-FST specimen in tension were investigated, respectively. It was revealed from the test that RPC composed from local materials can have a good workability and mechanical properties, and that RPC-FST has higher bearing capacities and ductilities than CFST, which satisfies the expectation of engineering design.④ Based on the experimental study results, and with reference to tensile resistance formulae of steel fiber reinforced concrete-filled steel tube from literatures, practical algorithms for calculating load-carrying capacity and initial rigidity of RPC-FST in tension were worked out and verified by test results. The proposed algorithms are simple and have high computational accuracy.