Structural performance of complex core systems for FRP-balsa composite sandwich bridge decks

Based on current fiber-reinforced polymer (FRP) composite construction principles, FRP decks fall into two categories: pultruded decks and sandwich decks. Sandwich decks comprise face sheets and either honeycombs or foams reinforced with internal FRP webs for shear resistance. The honeycomb structure and the webs cause debonding between the upper face sheets and the core due to the uneven support of the former. An alternative material that has high shear capacity and can provide uniform support for the upper face sheet is balsa. Balsa panels have therefore been proposed as the core material for sandwich decks in this research work. Balsa panels are produced by adhesively bonding dissimilar balsa blocks, resulting in a non-homogenous and anisotropic material. These inherent characteristics are not taken into account in the current shear behavior of balsa, thus making it unreliable. Balsa also exhibits high ductility when subjected to compressive loads, however, the shear ductility required by engineers to design safe sandwich structures is lacking. Furthermore, currently existing GFRP-balsa sandwich bridge concepts can only be applied to short-span bridges due to high cost and manufacturing challenges in the case of sandwich slab bridges. In hybrid sandwich deck-steel girder bridges, low bending stiffness in the bridge direction and low composite action in the deck have been the drawbacks. The purpose of this research is to develop novel concepts for lightweight, stiffer and stronger sandwich decks, using balsa cores, which can be fabricated with fewer manufacturing challenges and offer longer spans than existing decks. Balsa panels were experimentally investigated to establish their shear properties and shear ductility at the three orthotropic shear planes. The influence of shear plane, balsa density and adhesive joints on the shear properties was quantified. Two new GFRP-balsa sandwich bridge concepts (complex core systems) have been proposed for long-span bridges. In the first concept, the sandwich core comprises high- and low-density balsa cores and an FRP arch reinforced at the core interface. Sandwich beams based on this concept were experimentally investigated to evaluate their structural performance. The beams demonstrated high bending stiffness and strength and were lightweight. Crack initiation and propagation in the balsa blocks of the complex balsa core could finally be explained. A new analytical model to predict the bending behavior of the new sandwich beams was developed. The second bridge concept involves integrating timber inserts into the balsa core of a sandwich deck. GFRP-balsa sandwich beams, with timber inserts, were numerically investigated to evaluate their structural performance. High stress concentrations occurred in the face sheets and cores at the balsa/timber core joints which were eliminated by changing the core joints from butt to scarf. An optimum angle of termination of scarf joints, based on low stress concentrati