📖 ABSTRACT/OVERVIEW
Natural fibre-reinforced composite structural elements offer significant sustainability and cost advantages for building construction in Nigeria, but their susceptibility to moisture-induced dimensional change and stiffness reduction remains a barrier to engineering adoption without rigorous predictive modelling capabilities. This research develops a theoretical framework for multiphysics modelling of moisture transport and hygroscopic swelling in natural fibre composite structural elements, validated for building applications across the diverse humidity environments of Nigeria's six geopolitical zones. The framework couples non-Fickian anomalous diffusion models with anisotropic hygroscopic swelling constitutive equations and moisture-dependent mechanical property reduction functions. Diffusion parameters including saturation concentration, diffusion coefficient, and relaxation time were experimentally determined for kenaf, sisal, and bamboo fibre-reinforced polypropylene and epoxy composites across relative humidity ranges of 40 to 95 percent and temperatures of 25 to 55 degrees Celsius representing Nigerian climatic extremes. Free and stressed swelling measurements established swelling stress development in restrained configurations representative of structural joint connections. Multiphysics simulations were implemented in COMSOL Multiphysics environment. Structural performance reduction factors were developed from coupled structural analysis under moisture-induced eigenstrains. Results demonstrate that non-Fickian two-stage diffusion model with power-law relaxation term provided significantly improved fit to experimental data compared to classical Fickian model, reducing root mean square error by 58 percent. Swelling anisotropy ratios ranged from 1.4 to 3.1 depending on fibre orientation relative to moisture exposure direction. Structural simulation confirmed that moisture-induced stiffness reduction of 18 to 34 percent must be incorporated in design calculations for humid coastal applications. The theoretical framework provides the first comprehensive multiphysics tool specifically developed for predicting moisture effects in natural fibre composite building structures in Nigeria. Keywords: moisture transport, hygroscopic swelling, natural fibre composite, multiphysics modelling, building application
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