Study On Seismic Strengthening Of Corroded Reinforced Concrete Columns

It has been confirmed by many investigations about rebar corrosions that chloride-induced corrosion of steel bars in reinforced concrete exposed to marine environments and de-icing salts has become one of the major causes of deterioration in many existing reinforced concrete (RC) structures. It can lead to further structural distress due to the loss of the reinforcing steel cross-sectional area as well as loss of bond along the steel-concrete interface. As a result, the RC structure damaged by rebars corrosion suffered extensive structural damage and even collapse when subjected to a strong ground motion. Therefore, it is necessary to investigate the technique to strengthen the corroded column in order to upgrade the seismic performance. Based on that, the following aspects are carried out in this thesis:(1) The seismic behavior of circular RC columns damaged by rebar corrosion was studied by a cyclic horizontal loading test. The variables studied in this program included effects of corrosion degree of the rebars and level of axial load. Experimental results showed that the ductility and energy dissipation capacity, as well as areas of hysteresis loops of circle corroded RC columns can be significantly reduced along with the increase of rebar corrosion extent and axial load. If the circular RC columns contained extremely corroded rebars, the column would be prone to brittle shear failure under seismic loads.(2) An experimental study was conducted on corroded reinforced concrete columns confined with five different strengthened techniques, including CFRP, syeel jacket, combined CFRP and steel jacket, concrete jacket and combined CFRP and concrete jacket, to evaluate their seismic resistance behavior. The variables studied in this program included effects of corrosion degree of the rebars, level of axial load and the amount of retrofitted materials. The effectivities of the five retrofit techniques have been proved. The retrofit technique using CFRP can improve the ductility and dissipated energy capacity of RC columns with corroded rebars, and the other four retrofit techniques not only improve the ductility and dissipated energy capacity, but also enhance the load-carrying capacity considerably. Besides, the combined CFRP and concrete jacket or steel jacket retrofitting technique was found to be effective in enhancing the seismic resistance of the columns and resulted in more stable hysteresis curves with lower stiffness and strength degradations as compared with the corroded columns strengthened only with single material.(3) Based on the experimental results above, considering the bond slip between corroded rebars and concrete and between retrofitted materials and concrete by multiplying factor, the calculation formula of skeleton envelope is suggested, and the model of restoring force was made.(4) Based on the experimental results aout strengthened corroded columns, considering coefficient of effective actions of the corroded rebars retrofitted materials, the simplified equations of calculating loading capacity was provided by amending the specification formula.(5) According to the experimental results aout strengthened corroded columns, considering coefficient of effective actions of the corroded stirrups and retrofitted materials, the extreme strain of the confined concrete was calculated by amending the formula provided by Priestley, and the simplified equations of calculating ductility factor was provided.(6) Based on the model of restoring force obtained above, the elastic-plasticity response was analysed to evaluate the effectiveness of the retrofitted thecniques. The research proved that the assessment of the effectiveness of the retrofitted thecniques was inaccurate only using acceleration ground motion or single seismic wave. The effectiveness of the retrofitted thecniques relying on the the amount of retrofitted materials and the frequency spectrum features ground motion.