Hybrid FRP-lightweight concrete sandwich system for engineering structures

Hybrid slab systems combining fiber-reinforced polymer (FRP) composites with concrete are promising load-bearing structures, and an increasing number of applications has demonstrated their high potential in terms of structural performance and durability. Hybrid slabs are currently manufactured mainly on site however, which limits their economic advantages. The aims of this research are to develop a novel concept for a lightweight hybrid FRP-concrete sandwich slab system, which can be prefabricated and easily installed on site, and provide a corresponding engineering-adapted design method. The proposed system uses three layers of different materials: an FRP sheet with T-upstands for the bottom skin, which also serves as formwork, lightweight concrete (LC) for the core material and ultra-high performance fiber-reinforced concrete (UHPFRC) for the top skin. No additional shear reinforcements are used, resulting in a simple and cost-effective slab manufacturing process. Analytical and experimental investigations on the proposed system indicate that one of the governing failure mechanisms is shear failure of the LC core. A fracture mechanics-based model to predict the shear resistance of the hybrid sandwich slab is presented. To verify the modeling, experiments were performed on twelve hybrid beams comprising two different types of LC materials for the core: sand lightweight aggregate concrete (SLWAC) and all lightweight aggregate concrete (ALWAC). The proposed model demonstrates good agreement with experimental results and highlights the importance of considering not only the LC static strength, but also fracture mechanics properties such as characteristic length. Furthermore, a continuous direct load transmission model is developed to model the behavior of the sandwich slab with loads next to the support. The model consists of a diagonal bottle-shaped strut with an infinite number of transverse ties and is based on the principles of strut-and-tie models for direct load transmission. The statically indeterminate system allows the stress redistribution resulting from post-peak material softening after concrete cracking to be taken into account. This leads to an accurate modeling of the varying experimental responses of eight hybrid short-span beams. Again, the considerable influence of LC brittleness on load-bearing behavior is demonstrated, something which is not taken into consideration in classic strut-and-tie models. In a final step, design examples demonstrate the feasibility of the hybrid FRP-concrete sandwich slab and illustrate an appropriate selection of material properties in accordance with the proposed design method.