Experimental Study On The Axial Mechanical Behavior Of Preloaded Circular Concrete Columns With CFRP Confinement

In the past decades, carbon fiber reinforced polymer (CFRP) laminates have been widely applied to strengthen or retrofit reinforced concrete structures since they have some advantages over other conventional buildings materials, e.g. high tensile strength, low mass density, corrosion resistance, fatigue endurance, nonmagnetic, easy treatment during construction, etc. When subjected to axial loading, CFRP-confined concrete columns are under triaxial compressive state so that their compressive strengths and ductility capacities can be significantly increased. So far, many research activities have centered on the investigation of the mechanical behavior of CFRP-confined concrete columns and some substantial achievements have been already obtained. However, most part of those activities is aimed at CFRP-confined concrete columns without preloading. Study on the behavior of preloaded concrete columns with confinement is still at starting point. So, the effect of preloading stress on the behavior of CFRP-confinement concrete columns is not well known. Note that the loads applied on concrete columns can not be completely removed prior to field application of confinement and that this preloading stress level varies greatly from case to case for columns, such effect should be clearly identified. By taking this into consideration, a total of twenty-nine circular concrete columns, classified into four groups, are tested in this paper.In view of relative long curing period of CFRP laminates during preloading process, the commonly-used hydraulic loading apparatus, e.g. hydraulic jack, is not practical any more. Two special loading devices are designed and manufactured to be used for applying constant compression stresses for long time. Addditionally, two 1000kN mechanical jacks are introduced to speed up the test process of concrete columns with a diameter of 100mm. Two-stage axial compression test, i.e. preloading process for unconfined concrete and subsequent loading process for CFRP-confined concrete, is carried out on four groups of specimens with different preloading stress levels and different sectional diameters of CFRP-confined plain and reinforced circular concrete columns. From the test results, the effects of preloading stress level, sectional dimension and reinforcement ratio on the ultimate axial capacity, failure mode and ductility of CFRP-confined concrete columns are analyzed in details. The test results have revealed again that confinement caused by CFRP laminate can improve the ultimate axial capacity and ductility of concrete columns significantly and that all tested columns fail in a distinct brittle mode of CFRP rupture. As preloading stress level increases from 0 to 0.85, generally, the ultimate axial capacities of the tested columns firstly increase and then decrease. The preloading stress level corresponding to transition point is about 0.70. As preloading stress level increases, the ultimate axial strains of the tested columns change in the similar way as the ultimate axial capacities of those columns. As for the ultimate expansion ratios of the tested columns, they firstly decrease and then increase as preloading stress level increases. The transition point occurs where preloading stress level is approximately equal to confining ratio.Based on the test data, some available predictive models for ultimate axial compressive strength, ultimate axial compressive strain, expansion ratio and axial stress-strain relation of FRP-confined concrete columns are compared. Thereafter, a strength predictive model with the consideration of preloading stress level is proposed in which the concept of so-called "residual strength" is incorporated into the predictive model as specified in design guideline CSA S806-02. In addition, the influence factor function regressed from test data for the ultimate expansion ratio of concrete columns with preloading is used to develop the corresponding predictive models for the ultimate axial strain, complete expansion ratio and axial stress-strain relation, on the basis of their originals proposed by Teng et al, Lam and Teng for CFRP-confined concrete columns without preloading. The predictive models fit well with the test results. The corresponding design suggestions are put forward in the end of this paper.