This thesis presents a new concept in steel-concrete-steel sandwich construction in which a bi-directional corrugated-strip core is proposed as an alternative inclined shear connector. The focus is on the feasibility study of fabrication techniques and the theoretical study of the structural responses of both unfilled and concrete-filled steel-concrete-steel sandwich beams under static flexural loading using numerical and analytical methods. Two possible fabrication techniques to create the proposed bi-directional corrugatedstrip core are presented. The unfilled sandwich beam is studied using a finite element method and three analytical methods referred to as the modified stiffness matrix, the braced frame analogy, and the discrete beam methods The finite element method is used to investigate the stiffness and strength behaviour of the unfilled sandwich beam. The modified stiffness matrix method provides good correlation with the finite element method. The other two analytical methods are less accurate. The assessment of the effect of geometrical parameters defining the bi-directional corrugated-strip core is carried out. The responses of the strength and stiffness, especially the transverse shear stiffness, are examined and discussed. The optimum configuration of the core is found at the angle of the inclined part of the corrugation is about 45o. The concrete-filled sandwich beam is studied using the fnite element method. The finite element method is used to investigate the transverse shear strength and the crack development of a four-point loaded concrete-filled sandwich beam. The assessment of the effect of geometrical parameters defining the inclined shear connectors is carried out. The responses of the concrete-filled sandwich beam are examined and discussed. The optimum advantage of the transverse shear strength of the concrete-filled sandwich beam is also found when the inclined shear connectors align at an angle 45o. It is found that creating the proposed core with a 45o pattern provides a great advantage in transverse shear stiffness and strength in both the unfilled and concrete-filled sandwich beams.

In the light of extreme events of natural disasters (earthquakes or hurricanes) and accidents (fire or explosion), repairing and strengthening of existing concrete structures has become more common during the last decade due to the increasing knowledge and confidence in the use of composite advanced repairing materials. The past experience from real fires shows that it is exceptional for a concrete building to collapse as a result of fire and most fire-damaged concrete structures can be repaired economically rather than completely replacing or demolishing them. In this connection an experimental study was conducted to investigate the effectiveness of fibre reinforced polymer jackets for axial compressive and seismic shear performance of post-heated columns. This study also investigates the effectiveness of ferrocement laminate for the repairing of post-heated reinforced concrete columns.A total of thirty-five reinforced concrete columns were constructed and then tested after categorising them into three main groups: un-heated, post-heated and post-heated repaired. The post-heated columns were initially damaged by heating (to a uniform temperature of 500°C). The concrete cubes were also heated to various temperatures to develop the relation between compressive strength and ultrasonic pulse velocity. The residual compressive strength of the concrete cubes and reinforced concrete columns were determined by ultrasonic testing. The post-heated columns were subsequently repaired with unidirectional glass or carbon fibre reinforced polymer and ferrocement jackets. The experimental programme was divided into two parts. The columns of experimental part-1 were tested under axial compressive loading. The columns of experimental part-2 with a shear span to depth ratio of 2.5 were tested under constant axial and reversed lateral cyclic loading. The results indicated that the trend of reduction in ultrasonic pulse velocity values and in residual compressive strength of concrete was similar with increasing temperature. The reduction in residual stiffness of both post-heated square and circular columns was greater than the reduction in ultimate load. The circular sections benefited more compared to the square cross-sections with fibre reinforced polymers for improving the performance of post-heated columns in terms of compressive strength and ductility tested under axial compression. GFRP and CFRP jackets performed in an excellent way for increasing the shear capacity, lateral strength, ductility, energy dissipation and slowed the rate of strength and stiffness degradation of fire damaged reinforced concrete square and circular columns tested under combined constant axial and reversed lateral cycle loading. However, the effect of a single layer of glass or carbon fibre reinforced polymer on the axial stiffness of post-heated square and circular columns was negligible. The use of a ferrocement jacket for the repairing of post-heated square and circular columns enhanced the axial stiffness and ultimate load carrying capacity of columns significantly.