Compression Behavior Of FRP Confined Ultra High Performance Concrete

Ultra-high performance concrete(UHPC), as a new type of cement-based composite mater ial, is significantly superior to high-strength concrete and high-performance concrete in strengt h, durability, toughness and other aspects, with broad application prospects. The toughness of U HPC is greatly improved by adding steel fiber, but the ductility of compression components ma de of UHPC still needs to be improved. Fiber reinforced polymer(FRP) is widely used in the fie ld of civil engineering because of its light weight, high strength, corrosion resistance and other e xcellent properties. With the development of FRP confined concrete technology and theory, the use of FRP confined UHPC can not only improve the bearing capacity of UHPC members, but a lso greatly improve their ductility. At present, there are few researches on the mechanical prope rties of FRP confined UHPC under compression, so this paper will use experimental research, n umerical simulation and theoretical analysis to systematically study the mechanical properties o f FRP confined UHPC under axial and eccentric pressure. The main contents and conclusions ar e as follows:(1) Through the axial compression tests of 42 GFRP tube confined UHPC specimens and 12 unconfined UHPC specimens, the effects of curing method, steel fiber content and type in UH PC, fiber orientation and loading method on the axial compression mechanical properties of spe cimens were studied. The test results show that the UHPC specimen confined by FRP has highe r ductility and compressive strength than that without FRP. When the restraint is strong, hot wat er curing can improve the overall performance of the specimen. In the transition stage of stressstrain curve, the strength of the specimens without steel fiber decreased sharply, and its compres sive strength and ultimate axial strain were lower than those of the specimens with steel fiber. In creasing the filament winding angle can effectively improve the compressive properties of the s pecimens. Compared with loading only UHPC core, full cross-section loading will reduce the u ltimate strain of thicker FRP tube, but for thinner FRP tube, the effect can be ignored.(2) Taking FRP thickness, eccentricity and fiber type as test variables, 22 GFRP tube confi ned specimens and 35 CFRP tube confined specimens were tested. The test results show that th e mechanical properties of FRP confined UHPC can be improved by increasing the thickness of the tube, while the larger the eccentricity is, the lower the strength and ultimate strain will be. I n the study, when the eccentricity(e/R) is less than 0.27, the strengthened section will app ear in the load displacement curve of the specimen; when the eccentricity(e/R) is greater than 0.27, the specimen will not enter the strengthening section. The small eccentricity fail ure mode is FRP fracture in the middle of compression side, UHPC is crushed, and the large ecc entricity failure mode is FRP interlaminar tear in tension side. The effect of FRP type on the circ umferential fracture strain of FRP is small under the similar constraint ratio.(3) On the basis of experimental study, a FRP confined UHPC micro finite element model including FRP, UHPC matrix and steel fiber was established by LS-DYNA finite element analy sis software to study the whole process of specimen failure. By comparing the simulation result s with the test results, it shows that the model in this paper can simulate the whole process of the compression response of the specimen well, and the failure mode and load displacement curve are basically consistent with the test results. The results of parameter analysis show that the incr ease of FRP thickness or filament winding angle is beneficial to increase the bearing capacity of the specimens. Under sufficient restraint stiffness, the large eccentric specimen appears load str engthening section after the initial peak load. The larger slenderness ratio leads to the decrease of stiffness and bearing capacity, but the increase of ductility.(4) The existing FRP confined concrete strength and strain models are evaluated by collect ing the existing test data, and the results show that the berthet model has the highest prediction a ccuracy for the strength and ultimate strain of FRP confined UHPC specimens. Based on the Al i fallah pour stress-strain model, a simplified stress-strain design model for FRP confined UHP C is proposed, which has a good prediction effect. The equivalent axial stress-strain curve is obt ained by using the section analysis method. The results show that with the increase of ecc entricity,the second stiffness and ultimate stress decrease and the ultimate strain incre ase,but the influence on the stress and strain at the turning point is not significant.