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This study aims to investigate the efficiency of active and passive protection of the pre-corroded steel reinforcement in reinforced concrete (RC) columns using an externally bonded carbon fibre reinforced polymer (CFRP) wrap. Active protection is a novel technique used to impede corrosion achieved by employing the CFRP wrap as the anode of the impressed current cathodic protection (ICCP). Passive protection denotes the anti-corrosion behaviour of the CFRP fabric. Nine reinforced concrete columns with different degrees of corrosion (theoretical mass losses of 1%, 3%, and 6%) were prepared and then treated with different protection methods (non-protection, passive protection, and active protection) by exposing them to wet–dry cycles. By measuring the steel potential, linear polarization, and electrochemical impedance spectroscopy (EIS) of all investigated specimens, the efficiency of active protection was found to be closely related to the degree of the pre-corrosion of the protected specimens. Furthermore, the efficiency of active protection decreased with time, which may be attributed to the deterioration of the CFRP anode and CFRP/concrete interface. Passive protection was proven to be effective in every case, however, it can only slow down the corrosion process but cannot stop the elicited corrosion response.

Previous studies indicate that the use of hybrid fiber has the potential to improve the properties of concrete. This paper investigates the effect of steel-polypropylene hybrid fiber on the axial compression behavior of concrete-filled steel tube (CFST) columns. Sixteen hybrid fiber reinforced concrete filled steel tube (HFRCFST) stub columns are tested under axial load. The variables considered in the tests are the steel fiber volume percentage and polypropylene fiber quality percentage. The failure mode, ultimate capacity, ductility and energy dissipation capacity of HFRCFST column are investigated. Experimental results show that hybrid fiber has negligible effect on the failure mode and ultimate capacity, delays the failure process and improves the energy dissipation capacity. The fiber hybridization has a favourable reinforcement effect on the ductility over the sum of the individual single fibers. Therefore, using hybrid fiber reinforced concrete is a more effective method to improve the ductility of the CFST columns than single fiber.

As the excellent mechanical performance and easy construction of concrete filled steel tubes (CFST) composite structure, it has the potential to be used to strengthen RC pier columns. Therefore, tests were conducted on 2 reinforcement concrete (RC) stub columns and 9 RC columns strengthened with circular CFST under axial loading. The test results show that the circular CFST strengthening method is effective since the mean bearing capacity of the RC columns is increased at least 3.69 times and the ductility index is significantly improved more than 30%. One of the reasons for enhancement is obvious confinement provided by steel tube besides the additional bearing capacity supplied by the strengthening materials. From the analysis of the enhancement ratio, the strengthening structure has at least an extra 20% amplification except for taking full advantage of the strength of the strengthening material. Through the analysis of confining stress provided by steel tube and the stress–strain relationship of confined concrete, it is found that the strength of the core concrete can be increased by 21–33% and the ultimate strain can be enhanced to beyond 15,000 μe.

This paper presents an experimental study on the effect of anode format (carbon fibre sheet (Cs), mesh (Cm) and plate (Cp)), current density (0, 1, 2, and 3 A/m2 referred to the surface area of reinforcement), and fly ash content (plain concrete, 10%, 20% and 30% replacement by weight of cement) on the efficiency of electrochemical chloride extraction (ECE). The results showed the total extracted chloride contents of samples treated with different anodes had an order of Cs > Cm > Cp. The distribution of anodic reaction products indicated that only a small part of Cs played as anode because of the insulation in its transverse direction while Cm performed better conductivity uniformity and accordingly higher usage. With the increase of current density, the extracted chloride content improved, but the improvement was very limited when the current density kept rising. The incorporation of fly ash led to 12%–153.1% improvement in extracted chlorides. After ECE, the measured residual free chloride content in concrete did not change with current density or fly ash content, except for the cases when current density was increased from 0 to 1 A/m2. The additional extracted chlorides with increasing current density or fly ash content can be attributed to the dissolution of bound chlorides. The contribution of increasing compactness by adding fly ash was offset by the destruction of Friedel’s salt and the resulting release of chloride ions in the alkali environment and under the impressed currents.

The aim of this study was to investigate the combined effects of an aggressive environment and sustained load on the mechanical properties of wet lay-up fiber reinforced polymers (FRP). A total of 390 specimens, including 234 carbon fiber reinforced polymer (CFRP) specimens and 156 glass fiber reinforced polymer (GFRP) specimens, were exposed to freeze–thaw cycles, hygrothermal aging, and wet–dry cycles either in an unstressed state or loaded to about 30% or 60% of the initial ultimate load. Uniaxial tension tests were conducted on the samples after specific exposure time as well as on the control samples; and tensile properties were measured for each specimen. The results showed that the three environmental exposures, particularly hygrothermal aging, led to a significant decrease in tensile strength and elongation of the CFRP and GFRP specimens even for relatively short conditioning periods, and this decrease was markedly exacerbated by higher external loading levels. It was interesting to observe that the tensile modulus of the CFRP and GFRP specimens exhibited an excellent resistance and even appeared to increase slightly after exposure. Finally, predictive values of tensile strength based on the Arrhenius method were compared with the design values of ACI 440.2R-08 and GB 50608-2010. The results showed that both ACI 440.2R-08 and GB 50608-2010 were too conservative and significantly underestimated the tensile strength of FRP materials after an anticipated exposure period.