Postearthquake reconnaissance studies show that the beam-column joints of substandard reinforced concrete buildings (particularly the corner joints) may fail to resist the demanded forces and deformations under severe seismic actions. In this study, the effectiveness of carbon fiber–reinforced polymer (FRP) composites for retrofitting beam-column joints is demonstrated through testing of four full-scale three-dimensional reinforced concrete frames that are representative of substandard reinforced concrete buildings. Additionally, analytical seismic performance predictions made using available seismic assessment and retrofit guidelines are compared with the test results.

In this study, 18 concrete cylinder specimens were tested either under uniaxial compression at different loading rates or exposed to sustained axial stresses after being jacketed externally with carbon-fiber-reinforced polymer (CFRP) sheets. The specimens were cast using medium strength concrete. All the specimens had identical dimensions and level of confinement. Loading rate and applied sustained stress level were the main test parameters. Applied loading rate varied between 0.0002 and 0.04 strain/min. Four stress levels between 0.52 and $0.85f_{\text{cc}}^{\prime }$ (0.90 and $1.46f_{\text{co}}^{\prime }$ ) were used in short-term creep tests. Test results showed that the stress-strain behavior of CFRP confined concrete was influenced by the change in loading rate, and CFRP confinement provided considerable increase in the creep performance of concrete. The strength enhancement was more pronounced for specimens loaded at higher strain rates, while specimens loaded at slower strain rates exhibited better deformability. Results obtained from short-term monotonic loading tests were also compared with the results of two analytical approaches originally developed for plain concrete. None of the specimens failed during the short-term creep tests. However, the lifetime of the specimen, which was subjected to $0.85f_{\text{cc}}^{\prime }$ $\left(1.46f_{\text{co}}^{\prime }\right)$ sustained axial stress, was predicted as 20 days. Results of residual strength tests showed that specimens did not have any strength loss due to sustained loading.

Although many theoretical and experimental studies are available on external confinement of columns using fiber-reinforced polymer (FRP) jackets, as well as numerous models proposed for the axial stress-axial strain relation of concrete confined with FRP jackets, they have not been validated with a sufficient amount and variety of experimental data obtained through full-scale tests of reinforced concrete (RC) columns with different geometrical and mechanical characteristics. Particularly, no systematical experimental data have been presented on full-scale rectangular substandard RC columns subjected to reversed cyclic lateral loads along either their strong or weak axes. In this study, firstly, test results of five full-scale rectangular substandard RC columns with a cross-sectional aspect ratio of two (300 mm × 600 mm) are briefly summarized. The columns were tested under constant axial load and reversed cyclic lateral loads along their strong or weak axes before and after retrofitting with external FRP jackets. In the second stage, inelastic lateral force-displacement relationships of the columns are obtained analytically, making use of the plastic hinge assumption and different FRP confinement models available in the literature. Finally, the analytical findings are compared with the test results for both strong and weak directions of the columns. Comparisons showed that use of different models for the stress-strain relationship of FRP-confined concrete can yield significantly non-conservative or too conservative retrofit designs, particularly in terms of deformation capacity.

In this study, 36 concrete specimens with square cross-sections and different concrete qualities were tested either under uniaxial compression at different loading rates or subjected to sustained uniaxial stresses after externally jacketing with carbon fiber-reinforced polymer (CFRP) sheets. The main test parameters were the loading rate and the applied sustained stress level. Among these parameters, the loading rate varied in the range of 0.0002 and 0.04 strain/min. In the case of short-term creep tests under sustained loads, three stress levels (between 0.73 f'_cc and 0.90 f'_cc or 2.76 f'_cc and 3.37 f'_cc) for low-strength and four stress levels (between 0.69 f'_cc and 0.92 f'_cc or 0.89 f'_co and 1.20 f'_co) for medium-strength prisms were applied. The test results showed that the stress-strain behavior of CFRP-confined concrete was affected by the change in loading rate, and external CFRP confinement enhanced the creep performance of concrete significantly. For low-strength concrete specimens, higher strain rates did not bring higher strength values; however, an increase in strength was obvious for medium-strength prisms. On the other hand, for both concrete qualities, the specimens loaded at slower strain rates exhibited better deformability. None of the specimens of the medium-strength concrete failed during the short-term creep tests; however, three of the low-strength concrete prisms failed during the tests. The results of residual strength tests showed that sustained loading did not cause a strength or ultimate deformation capacity loss, but affected the residual strain capacities.