Natural fiber-reinforced composites embedded with optical sensors for next-generation sustainable pedestrian footbridges

Fiber-reinforced polymer (FRP) composites are gaining popularity over steel and concrete materials in bridge construction due to their lightweight nature, ease of assembly, corrosion resistance, and reduced foundational requirements. Although FRPs are less suitable for vehicle-based bridges due to the high vibrations from traffic, they are widely used in pedestrian footbridges. To meet the 2050 net-zero carbon target, the construction sector is adopting plant-based materials that store carbon. This has led to natural fiber-reinforced polymer (NFRP) composites, which combine natural fibers (for strength) with plant-based or synthetic resin. The structural potential of NFRPs has been displayed in notable projects, such as the Smart Circular Bridge built in Almere, The Netherlands, in 2022. This bridge, constructed with flax fibers embedded with FBG sensors and 24% bio-polyester resin, challenges misconceptions about the durability and moisture sensitivity of NFRPs. Future NFRPs aim to use 60% plant-based resin to boost strength and moisture resistance. However, uncertainties remain about NFRP's long-term performance under stress and weather. Key challenges include lower resistance to repeated loads compared to synthetic composites, natural fibers' tendency to slowly deform over time (creep), and moisture absorption, which weakens fiber-resin bonds. Addressing these issues requires more research to improve NFRP reliability.

Embedding optical sensors in NFRPs enables real-time monitoring of strain and deformation, offering precise, continuous data compared to traditional visual checks. For pedestrian footbridges, these sensors detect hidden damage or strain caused by weather or pedestrian loads. Early detection allows timely repairs, improving safety, extending lifespan, and lowering costs. Vibration sensors can also work alongside optical systems to enhance monitoring.

This study aims to develop sustainable and low-cost NFRP composites with embedded optical sensors for footbridge decks. It will test their manufacturing quality, and durability under varying loads/weather, and compare their environmental, economic, and social impacts to steel and concrete materials. Specific objectives are to: (1) design NFRP composites using vacuum resin infusion (VARI) with embedded sensors; (2) evaluate manufacturing quality (via imaging) and long-term durability; (3) assess sustainability impacts using life cycle assessments (LCSA).

The research aligns with UN Sustainable Development Goals such as SDG 9 (industry, innovation, and infrastructure), SDG 11 (sustainable cities and communities), and SDG 13 (climate action) by reducing infrastructure's carbon footprint, promoting urban resilience, and enabling cost-effective monitoring. By advancing NFRP technology, this work supports greener infrastructure and sets new standards for sustainable engineering.

Keywords: Natural fiber-reinforced polymer (NFRP), pedestrian footbridges, durability, structural health monitoring (SHM), life cycle sustainability assessments (LCSA).