Experimental Study On Bond Behavior Of Prestressed Reinforcement Covered With Retard Mortar

The concrete structure with retarded-bonded tendons is a new technical specification. The construction process of producing holes and grouting are cancelled in concrete structures with retarded-bonded tendons which must be in concrete structures with bonded tendons. The fatigue problems of anchorage and prestressing tendons are overcome that are contrasted with unbounded prestressed tendons by bond behavior retard mortar, concrete and steel strand. No researcher is focus on this problem now. In this paper, the fluidity, mechanical properties of retard mortar, the bond behavior of retard-bonded prestressing steel strand under static loading and repeated loading are studied by experimental research and theoretical analysis. The major work performed is detailed as follows:(1) The mechanical properties and fluidity of retard mortar are studied by experiment with different water-cement ratio, sand-cement ratio and the content of silica. The research show that the fluidity will increase with water-cement ratio and reduce with sand-cement ratio. The fluidity will reduce along with the content of silica are reduced. The workability of retard mortar is the most best when the consistency of retard mortar is 25~55mm. The results of grey incidence analysis show that the water-cement ratio influences mechanical properties most, the content of silica influences least and the sand-cement ratio between them.(2) The pullout specimens of 1 hot ribbed steel and 14 steel strand covered with retard mortar are tested under monotonic loading. The whole processes of bond failure are researched. The differences of bond behavior of retard-bonded steel are studied. The p-s curve of hot ribbed steel covered with retard mortar is unimodal curve. The p-s curve of steel strand covered with retard mortar has long linear phase, and is not nearly fall in the whole processes. The bond strength of hot ribbed steel is higher than the bond strength of steel strand obviously. The bond strength of steel strand covered with retard mortar will be increasing because of retard mortar strength, wrap thickness, the high and width of bellow, concrete strength increasing, and will be decreasing with anchorage length. The main influences on bond strength are investigated such as surface configurations, mechanical properties of retard mortar, bond medium, bond length, wrap thickness of retard mortar, the types of bellow and the strength of concrete. The formulas of bond stress and this parameter are derived from the present test data. The bond strength of steel strand covered with retard mortar is higher than cement paste and retard glue, however, lower than concrete and epoxy.(3) The fatigue damage experiments under constant amplitude cyclic loading are tested. The mode of fatigue damage is consistent with the principles of three stages. The cycle numbers when fatigue damage is closely related to high level of stress. The pullout experiments after repeated loading are tested. The trend of bond stress-slip of retard-bonded prestressing steel strand with different stress level is discussed. The degradation of bond strength and bond stiffness of retard-bonded prestressing steel strand is analyzed because of stress level and the number of load cycles.(4) Based on the experimental results, the bond-slip constitutive relation of retard-bonded prestressing steel strand under static load and fatigue load is created. The equation to bond strength and the fatigue life equation for retard-bonded prestressed reinforcement are derived. The exponent is decided by the mothod of mathematical fitting. The basic constitutive model is deduced theoretically by elastic theory as well. At last, the parameters of slip angle, poisson’s ratio and coefficeient of friction are determined. The calculated bond strength are basically consistent with the bond strength by experiment.