The corrosion resistance of fiber reinforced polymer (FRP) composites is one of their outstanding advantages over ordinary steel, FRP bars are considered as an ideal alternative to steel bars which are prone to corrosion in extremely corrosive environments. However, FRP bars, especially the basalt fiber reinforced polymer (BFRP) bars, have a short development and application time. The understanding of their degradation law and degradation mechanism under actual service environment is not enough. As a result, the uncertainty of their long-term performance becomes one of the key problems that plague their wide application. Therefore, it is of great practical significance to carry out researches on the durability of FRP bars reinforced concrete structures. In addition, because FRP bars have excellent resistance to chloride ions attack, the combination of FRP bars and seawater and sea sand concrete (SWSSC) is attracting a great deal of research interest. However, the related durability studies are still very limited. In this paper, accelerated aging test, micro-meso-observation and theoretical calculation and analysis are combined to study the key issues involved in the long-term durability of FRP bars reinforced concrete structures. The main research work and conclusions are listed as follows.
(1) Firstly, accelerated aging tests were conducted to investigate the degradation law and to reveal the inherent degradation mechanism of the BE (Basalt/Epoxy) bar and the BV (Basalt/Vinyl ester) bar by immersing them in 60℃ alkaline solution. The results showed that the durability of BE bars was better than that of BV bars. The fiber/resin debonding was the main reason for the degradation of the mechanical properties of BFRP bars. Then, a comparative study on the degradation of BE bars in two types of alkaline environments (i.e. alkaline solution immersion and moist concrete wrapping) was carried out. The results showed that the degradation rate of BE bars immersed in alkaline solution was 2.67 times higher than that embedded in moist concrete. Finally, the durability of the BE bar under the combined effect of stress and corrosive solution (i.e. acid, alkali, salt and water) was investigated, and the long-term performance of the BE bar was predicted based on Arrhenius theory. The results showed that the durability of BFRP bars exposed to acid, salt and de-ionized water were less affected than that of bars exposed to alkaline solution. The effects of sustained stress on the degradation of BFRP bars were not obvious when the stress level was less than 20% of the ultimate strength. The predicted times required for a tensile strength reduction of 50% for BE bars immersed in alkaline solution at an area with a northern latitude of 30°, 40° and 50°, were 4.2 years, 7.4 years and 16.1 years, respectively.
(2) Firstly, the effects of fiber types, resin matrix types and surface treatment methods on the bond durability of FRP bars embedded in normal concrete in marine environment were studied. The results showed that the bond durability of BFRP bars and glass fiber reinforced polymer (GFRP) bars were basically the same, and the bond durability of carbon fiber reinforced polymer (CFRP) bars was the best. The bond durability of FRP bars with epoxy matrix was better than that of FRP bars with vinyl ester matrix. Sand-coating the FRP bar improved the long-term bond durability. Then, the bond durability of the new steel-FRP composite bar (SFCB) embedded in river sand or sea sand concrete in marine environment was further studied. The results showed that the salt ions contained in the sea sand concrete, which can reduce the alkalinity of the microporous solution in concrete, may had some positive effect on the bond durability of the SFCB by reducing the degradation rate of the surface BFRP layer of the SFCB. Finally, accelerated aging tests were performed on the bond durability of BFRP bars and SFCBs with SWSSC under two types of pull-out tests scheme (i.e. direct pull out and eccentric pull out). The results showed that under the same conditions, the ultimate bond strengths in direct pull out condition were all higher than those in eccentric pull out conditon. Overall, the bond degradation in seawater immersion environment was worse than that in the wet-dry cycling environment.
(3) Several batches of accelerated aging tests were conducted on the long-term flexural performance of FRP bars reinforced concrete (RC) beams in simulated ocean environment. The influences of aging times, types of tensile reinforcements, types of concrete (i.e. normal concrete or seawater and sea sand concrete), types of stirrups (i.e. FRP bar or steel bar) and types of accelerated environment (i.e. wet-dry cycling or immersion) were analyzed and compared. Experimental results showed that the retentions of flexural capacity of GFRP bars, BFRP bars and SFCB RC beams in batch I (normal concrete) were 72%, 58% and 62%, respectively, after conditioned in sustained load and sea water wet-dry cycling coupled environment for a maximum of 1 year, and the retention of the yield load of SFCB reinforced beam was 75%. After conditioned in load and sea water coupled environment for a maximum of 3 months, there were no significant changes in the characteristic loads of ordinary steel bars and SFCB reinforced sea sand concrete beams in batch II (sea sand concrete). Both the flexural stiffness and energy ductility of all the conditioned beams increased, and these improvements were more significant for the SFCB beams. Finally, in batch III (SWSSC), a systematic study was conducted on the changes of flexural behavior of SWSSC beams reinforced with all-composite reinforcements (i.e. longitudinal reinforcement, stirrups and hanger bars are all FRP bars) under accelerated ocean environment. The results showed that the failure modes of BFRP bars RC beams and CFRP bars RC beams gradually changed from concrete crushed to shear-compression coupled failure, and the failure mode of SFCB RC beam gradually changed from concrete crushed to the SFCB ruptured. The retentions of flexural capacity of BFRP bars RC beams, CFRP bars RC beams and SFCB RC beams in batch III were 79%, 70% and 89%, respectively, after a maximum of 9 months’ environmental condition.
(4) By referring to the existing prediction methods, a more refined long-term performance prediction model for FRP bars was proposed with the influence of concrete-wrap and seasonal temperature fluctuations further considered. Based on the proposed prediction model and the existing accelerated aging data of BFRP bars available in the literature, the detailed steps for the application of the model was illustrated, and a recommended environmental reduction factor for the new BFRP bar was given. In addition, the degradation law of the ultimate bond strength of BFRP bars under actual service temperature and humidity was predicted. Based on the short-term bond test data of BFRP bars in this paper and in the literature, a calculation formula for the short-term ultimate bond strength was obtained by fitting method. Then, by introducing the damage factors obtained from the degradation law of the ultimate bond strength to the formula, calculation methods for the basic anchorage length of the BFRP bar located at different environments were given.
(5) The calculation formulas of the flexural stiffness and the maximum crack width of FRP bars reinforced concrete beams under service load were systematically checked and modified. Based on the experimental data in this paper and the experimental data in the existing literature, a modified formula for calculating the short-term flexural stiffness of FRP bars reinforced concrete beams was proposed according to theoretical analysis and calculation. A conceptual analysis of the effect of changes of bond properties of FRP bars on the calculation of the flexural stiffness was carried out. Based on the measured data of crack widths of FRP bars RC flexural members at home and abroad in recent years, the values of the coefficients implied in the calculation formula of the maximum crack width were systematically checked, and a modified formula for the calculating of the maximum crack width of FRP bars RC beams was proposed.