Static Pushover Test On FRP-retrofitted RC Columns With Sustained Loading

Carbon Fiber Reinforced Polymer (CFRP) laminates have been taking significant attentions for use in civil infrastructure due to its unique properties, such as the high strength to weight ratio, and they have been widely applied to strengthen or retrofit reinforced concrete structures.corrosion resistance, fatigue strength and tailor ability. It is well known that FRP wraps increase the load-carrying capability and the ductility of concrete columns. However, most part of those activities is aimed at CFRP-confined concrete columns without preloading. Strengthening existing reinforced concrete (RC) structures with carbon fiber reinforced polymer (CFRP) laminates is always conducted with the existence of preloading and residual deformations since the applied loads on the structures cannot be removed completely prior to strengthening. Researchs have indicate that in some cases, sustained loading is of much significance for confinement effectiveness. To make a comprehensive and reasonable understanding of those static pushover behavior of FRP-retrofitted RC columns with sustained loading, Ten square concrete column specimens, classified into four groups, were tested 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. Furthemore, considering the safety of using electric, the test used hydraulic loading apparatus controlled by manualcleaning separator pump to maintain constant compression stresses for long time. On initial stage, the load is unstable, so the constant compression stresses needs the manualcleaning separator pump press the oil ino the hydraulic loading apparatus in order to maintain the compression stresses fooat in a small scale. The vertical load was measured and controlled by load sensor that fixed on the hydraulic loading apparatus with a mersurement range of 2000KN. The unidirection horizontal load was provided by electro-hydraulic loading system MTS with its measure rang 250KN, and the experimental data was collected and outputed by its collection system, besides in the loading process, it can be monitored all long. The static pushover behavior of preloaded square concrete columns with CFRP Confinement include lateral force, lateral deflection, failure mode, yielding, ultimate capacity, ductility etc. In the test, four nominal axial stress levels(0,0.5,0.75 and 1.0), two volumetric lateral reinforcement ratios and three confining stresses are identified. The main purpose of this test is to study the effect of FRP-retrofitted RC columns under preload loading with these factors. The test results indicate that confinement caused by CFRP laminates can improve the ductility of concrete columns significantly, while the improvement of the the ultimate axial capacity is not significantly; As the preloading stress level increases, the ultimate axial capacities of the tested columns increase while the ductility decreases; when the nominal axial stress level is 1.0, the sustained loading increase a little as the ductility decrease in a small range. Generally, the crack and yielding behavior of CFRP-confined columns seems not to be influenced by nominal axial stress level. Insufficient amount of lateral reinforcement can also be compensated by CFRP laminates both for lateral resistance and ductility capacity. In addition, compared to the rusult obtained from the test, we can get the conclusion that the flexural strength of the reinforced concrete column is much larger than the rusult obtained from code fomula, and this phenomenon in the CFRP-retrofitted square RC columns under hinger nominal axial stress levels is also very prominent. Finally, the in-depth research of whole force process of beam-column component is given base on previous studies, and a reasonable constitutive relationship of reinforced concrete is selected, thus a non-linear model of the force of the the entire process of reinforced concrete structures is established. And based on the Matlab as a platform to reinforced, nonlinear sectional analysis of FRP-retrofitted reinforced concrete columns are carried out and corresponding program in Matlab language is made. The simplified moment vs. curvature formulations corresponding to concrete cracking, steel tension yielding, steel compressive yielding, concrete crushing and FRP longitudinal rupture of FRP retrofitted columns are derived based on some assumptions. Finally, a comparison ismade between the theoretical results calculated and the experimental data. The procedure ignored the effects of shear deformation, and good agreement between theoretical results calculated and the rusult obtained from code fomula is achieved.,which shows that the procedure have a higher accuracy.