Exploring the Durability of GFRP Bars in Natural Seawater

Chloride ions in maritime conditions enter the concrete protective layer and rust rebars, jeopardizing their structural safety and longevity. A new study in the journal Buildings considers how continuous glass fiber and resin are used to make glass fiber reinforced polymer (GFRP) bars. They are corrosion-resistant, have high tensile strength, and are light in weight.

Study: Assessment and Prediction Model of GFRP Bars’ Durability Performance in Seawater Environment. Image Credit: Dmitry Markov152/Shutterstock.com

GFRP induces microstructure damage and a macro-mechanical property decrease when employed in a high alkali concrete solution and marine environment, despite their corrosion resistance. This is due to damage to glass fiber, resin, and their interface phase caused by the service environment. GFRP bars of vinyl-ester, polyester, and epoxy resin in the alkali environment at 60 °C showed signs of deterioration after 5000 hours.

When free OH⁻ and water molecules diffuse in GFRP bars, they produce a chemical reaction with esters groups in the resin matrix, resulting in the degradation of GFRPs.

Researchers have been studying the endurance of GFRP bars in the maritime environment in recent years. They discovered that the resin matrix had a significant influence on the durability of the materials.

Due to the alternating impact of seawater wet-dry environment on offshore engineering, seawater concrete was utilized to wrap GFRPs and accelerated aging at 60 °C, based on an exponential degradation model. The tensile strength of GFRP bars was retained at a rate of 72%.

The goal of this research was to look at the durability of GFRP bars that had been treated in natural seawater, an artificial saline-alkali solution, and concrete. Temperature (25, 40, and 60 °C), as well as age duration (15, 30, 60, 90, and 183 days), were studied.

The study also planned to use interlaminar shear strength (ILSS) to investigate the mechanical property deterioration of GFRP bars under various test settings. Finally, a model was devised for predicting the long-term mechanical characteristics of GFRP bars in a saltwater environment.

Methodology

As illustrated in Figure 1, the samples were put in an incubator for accelerated aging.

Figure 1. Specimens. Image Credit: Li, et al., 2022

The WAW-1000D electro-hydraulic servo universal testing equipment was used to conduct the short-beam shear test, which followed ASTM D4475. Figure 2 shows the span, which was set at 48 mm.

Figure 2. Short-beam shear test. Image Credit: Li, et al., 2022

Results and Discussion

Figure 3 illustrates the surface morphologies of the specimens after 183 days of aging in three environments at 60° C.

Figure 3. Surface morphology of GFRP bars after aging in different environments at 60 °C for 183 days: (a) SW; (b) SA; and (c) SWC. Image Credit: Li, et al., 2022

Unconditioned bars had an ILSS of 46.93 MPa on average. The ILSS retention rate in Table 1 is the ratio of the aged sample’s ILSS to the unconditioned bars’ ILSS. Table 1 lists the test results.

Table 1. ILSS retention (%). Source: Li, et al., 2022

Figure 4 shows that in the three environments of SW, SWC, and SA, the ILSS retention of GFRP bars reduces.

Figure 4. Comparison of ILSS retention of GFRP bars in three environments: (a) 25 °C; (b) 40 °C; (c) 60 °C. Image Credit: Li, et al., 2022

Figure 5 depicts the effect of aging temperature on GFRP bar strength retention.

Figure 5. Comparison of ILSS retention of GFRP bars in three temperatures: (a) SW; (b) SWC; and (c) SA. Image Credit: Li, et al., 2022

Figure 6 illustrates the microstructures of the samples after 183 days of accelerated aging at 60 °C in three environments.

Figure 6. SEM photos of GFRP bars aged at 60 °C for 183 days: (a) Cross section; (b) Longitudinal section. Image Credit: Li, et al., 2022

T_g represents the glass transition temperature. T_g1 and T_g2 were assigned to the two temperature rise mechanisms, as illustrated in Table 2.

Table 2. The T_g of GFRP bars in different environments. Source: Li, et al., 2022

T_g2 of all samples is larger than T_g1, as seen in Figure 7. After aging at 60 °C for 183 days in the three environments, T_g2 of GFRP bars reduced by 1.7%, 7.8%, and 7.0%, respectively, compared to unconditioned samples.

Figure 7. Comparison of T_g of GFRP bars in different aging environments. Image Credit: Li, et al., 2022

The infrared spectra of GFRP bars aged in the environments at 60 °C for 183 days and in a non-aged condition were tested using FTIR, as illustrated in Figure 8.

Figure 8. FTIR of GFRP bars. Image Credit: Li, et al., 2022

The test data were fitted to produce the fitting parameter (i.e., τ), and Figure 9 depicts the fitting curve. Table 3 shows the results of theτand correlation coefficients.

Figure 9. Fitting of long-term mechanical property test data for GFRP bars based on Model 2 in different environments: (a) SW; (b) SA; and (c) SWC. Image Credit: Li, et al., 2022

Table 3. Data fitting of long-term mechanical property test for GFRP bars based on Model 2. Source: Li, et al., 2022

Figure 10 shows the Arrhenius straight-line fit, and Table 4 lists the straight-line slope and correlation coefficient.

Figure 10. Arrhenius line for durability prediction model of GFRP bars in different environments: (a) SW; (b) SA; and (c) SWC. Image Credit: Li, et al., 2022

Table 4. Fitting of Equation (6) for long-term mechanical property prediction of GFRP bars. Source: Li, et al., 2022

Table 5 shows the TSF for three distinct temperatures and environments.

Table 5. TSF of GFRP bars at different temperatures. Source: Li, et al., 2022

The master curve of the mechanical model of GFRP bars in the offshore area of the Yellow Sea of China was developed using the data and test results in Table 5, as shown in Figure 11, and the regression equation parameters are listed in Table 6.

Figure 11. Master curve of long-term mechanical property degradation of GFRP bars in different environments at 12.3 °C. Image Credit: Li, et al., 2022

Table 6. Main curve parameters of regression equation of long-term mechanical model of GFRP bars in the Yellow Sea area of China. Source: Li, et al., 2022

Conclusion

The mechanical deterioration law and mechanism of GFRP bars in different aging environments were investigated in this work.

In the same environment, GFRP bars degraded the fastest in the saline-alkali solution (SA), followed by wrapped with concrete, and then submerged in natural seawater (SWC), and finally natural seawater (SW). The basic reason is that the OH^- radical degrades the glass fiber when it combines with SiO₂. In the SWC environment, the concrete had a protective effect, resulting in reduced damage.

The rate of ILSS deterioration increases as the temperature rises in all three conditions. Water molecules immediately diffused to the GFRP bars during the early stages of aging, decreasing the resin and fiber’s interfacial bonding capacity. The rate of strength deterioration in the early stages was quicker than in the latter stages.

The microstructures of the samples were examined following accelerated aging in three conditions for 183 days. After aging in SA and SWC conditions, the sample fiber and resin were separated to varying degrees. Different degrees of resin matrix expansion after water absorption generated this phenomenon, and seepage pressure will damage the interface phase as well.

The fiber surface of samples in the SA and SWC environments was extensively affected after aging, while samples in the SW environment had very little aging damage.

T_g of GFRP bars aged in the three environments at 60° C for 183 days. In three environments, the hydrolysis degree of GFRP bars dropped by 1.7%, 7.8%, and 7.0%, respectively, when compared to T_g of conventional samples. In the Yellow Sea area of China, a master curve of a long-term mechanical model of GFRP bars was constructed, which can forecast the strength retention of GFRP bars serving in three environments.

Journal Reference

Li, W., Wen, F., Zhou, M., Liu, F., Jiao, Y., Wu, Q., Liu, H. (2022) Assessment and Prediction Model of GFRP Bars’ Durability Performance in Seawater Environment. Buildings, 12(2), p. 127. Available Online: https://www.mdpi.com/2075-5309/12/2/127/htm

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