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 concentrations at the joints, low costs and manufacturing challenges, was recommended in the design of GFRP-balsa sandwich decks. An existing analytical model for predicting axial stress concentrations in face sheets at butt core joints was extended to scarf joints and a new analytical model was developed to predict shear stress concentrations in the cores at both butt and scarf joints. Finally, the results obtained from the experimental work, the new models and the two proposed sandwich bridge concepts were implemented in the design of new GFRP-balsa sandwich slab bridges and decks. The structural limits of the new bridges were established and the potential of the new GFRP-balsa sandwich deck to replace a reinforced concrete deck was explored. Taking manufacturing limits into account (800-mm slab thickness) and using the proposed complex core system, sandwich bridge slabs of up to approximately 19 m can be constructed. Furthermore, GFRP-balsa sandwich decks bonded on steel girders can reach spans of up to 30 m. The findings of this research work therefore establish that deteriorated reinforced concrete decks with long spans of even up to 30 m can be replaced by GFRP-balsa sandwich decks. Also, based on this work, in the future, existing reinforced concrete decks can be widened using the proposed sandwich decks due to their lightweight, which has been extensively demonstrated. This will lead to cost saving since the bridge substructure will no longer need to be reconstructed.