A recent study published in Engineering has introduced a novel dual-phase model to predict the moisture uptake in glass fiber-reinforced polymer (GFRP) bars, offering a more accurate assessment of their long-term durability. The research, conducted by Zhi-Hao Hao, Jian-Guo Dai, and Jian-Fei Chen, addresses the critical challenge of moisture absorption in GFRP bars, which can significantly impact their mechanical properties, especially in marine environments.

GFRP bars have emerged as a promising alternative to traditional steel reinforcement in marine concrete structures due to their excellent corrosion resistance and relatively low cost. However, their long-term durability remains a concern, as moisture absorption can lead to hydrolysis and plasticization of the polymer matrix, reducing the stiffness and strength of the bars. Additionally, the penetration of harmful ions, such as hydroxide ions (OH–), accelerates the degradation process. Understanding and predicting the moisture absorption behavior of GFRP bars is therefore essential for evaluating their durability.

Previous studies have shown that the initial moisture absorption in GFRP bars follows the Fickian diffusion model, which describes mass transport driven by the concentration gradient. However, prolonged exposure reveals anomalous diffusion behavior, classified as non-Fickian diffusion, characterized by additional moisture uptake beyond the Fickian prediction. Existing models for non-Fickian diffusion have limitations in capturing the underlying mechanisms and providing accurate predictions.

To address these limitations, the researchers proposed a new model named the Weibull relaxation (WR) model. This model incorporates three key parameters related to relaxation: the relaxation rate, maximum relaxation amount, and transition time. The WR model was validated using the particle swarm optimization (PSO) algorithm, which demonstrated better agreement with experimental data compared to existing models.
The experimental part of the study involved gravimetric tests on GFRP bars with diameters of 6, 10, and 14 mm, immersed in portable water at temperatures of approximately 23, 40, and 60 ℃. The results showed that the mass increase of the bars was influenced by both diffusion and relaxation processes. At 23 ℃, the contribution of relaxation was marginal, while at higher temperatures, the relaxation process became more significant, leading to additional mass increases.

The WR model effectively captured the moisture absorption behavior during both the diffusion-dominated and relaxation-dominated stages. The model's parameters, including the relaxation rate and transition time, exhibited temperature-dependent variations, consistent with the experimental observations. The study also confirmed that elevated temperatures accelerated the diffusion process, following the Arrhenius relationship, with activation energies ranging from 30.7 to 36.5 kJ/mol.

The proposed WR model not only provides a better mathematical fit to the experimental data but also offers a clearer physical interpretation of the moisture absorption mechanisms in GFRP bars. This advancement could significantly enhance the durability assessment of GFRP bars in various environmental conditions, contributing to the development of more reliable and long-lasting marine concrete structures.

The paper “A Dual-Phase Model for Predicting the Moisture Uptake in Glass Fiber-Reinforced Polymer Bars,” is authored by Zhi-Hao Hao, Jian-Guo Dai, Jian-Fei Chen. Full text of the open access paper: https://doi.org/10.1016/j.eng.2025.04.008. For more information about Engineering, visit the website at https://www.sciencedirect.com/journal/engineering.