Over the last three decades there has been a growing awareness amongst civil and structural engineers of the importance of advanced polymer composites in the civil infrastructure. The application of fibre-reinforced polymer (FRP) plates and jackets for the strengthening and repair of RC beams and columns has now become commercially viable. Further developments of FRP composites have resulted in new constructions being conceived and developed, with considerable emphasis on bridges. The aim of the current study was to develop, manufacture and determine the structural properties of an advanced composite/concrete (duplex) beam. This beam comprised of a compressive concrete section and a tensile advanced composite box section. The webs consisted of a sandwich construction of +/-45° glass fibre reinforced polymer (GFRP) composite and rigid foam to enhance the buckling capacity. The tensile flange of the box section consisted of a hybrid uni-directional carbon fibre reinforced polymer (CFRP) and +/-45° GFRP composite. The webs were continued into the compressive section to provide permanent formwork for the concrete section. The structural properties of the beam system were determined by flexural tests on 1.5m span beams under static, creep and fatigue load regimes and long-term environmental exposure tests on 0.75m span beams. The short-term strength of the beam was designed and shown to be similar to that of an RC beam of similar dimensions with 2.5% of tensile steel reinforcement. The creep and fatigue properties (strength and stiffness) were shown to be superior to those of the equivalent RC beam due to the elimination of the tensile concrete and steel reinforcement. Exposure to freeze/thaw cycling and a warm, humid environment had no significant effect on the strength or stiffness of the duplex beam. The experimental tests were supplemented with linear and non-linear FE analyses that successfully modelled the short and long-term behaviour of the duplex beam. It was concluded that the duplex beam was particularly suitable in applications where a severe environment is present and provides a superior alternative to RC beams. This is of vital importance considering the large stock of deteriorated RC structures present world-wide. Therefore, the duplex beam forms an economic and durable structural solution for the global civil infrastructure.