Axial and flexural behaviour of reinforced concrete-filled FRP tubes experimental and theoretical studies

Corrosion of steel reinforcement causes continual degradation to the worldwide infrastructures and it has prompted the need for challenges to those involved with reinforced concrete structures. Recently, the use of fibre-reinforced polymers (FRP) tubes as structurally integrated stay-in-place forms for concrete members, such as beams, columns, bridge piers, piles and fender piles has emerged as an innovative solution to the corrosion problem. In such integrated systems, the FRP tubes may act as a permanent form, often as a protective jacket for concrete, and especially as external reinforcement in the primary and secondary directions such as for confinement. Furthermore, the use of concrete-filled FRP tubes (CFFT) technique is predicated on performance attributes linked to their high strength-to-weight ratios, expand the service life of structures, enhance corrosion resistance, and potentially high durability. This dissertation evaluates the axial and flexural performances of reinforced CFFT through experimental and analytical investigations. The details description and the findings of the investigations are presented through seven articles. To fulfill the objectives of this research, an experimental program has been designed including pure compression tests (33 specimens), axial-eccentric load tests (4 specimens) and pure flexure tests (10 specimens). Experimental investigations of the behaviour of CFFT have generally been carried out without using internal longitudinal reinforcement. The CFFT system of this study consists basically of filament-wound glass FRP tubes filled with concrete and reinforced internally with steel or FRP bars. Five types of new FRP tubes have been used with different thicknesses and two different diameters, 152 and 213 mm. Pure compression tests have been conducted on 40 specimens with a total height ranging from 305 mm to 1520 mm. One of the main objectives of testing these specimens is to evaluate the design equations of the North American codes and design guidelines to predict the ultimate load capacities of reinforced and unreinforced short CFFT columns. In addition, the effect of three parameters and their interactions on the buckling behaviour were investigated for these specimens; namely, the FRP tube thickness, concrete compressive strength, and slenderness ratio. The effect of eccentric load on the behaviour of four CFFT specimens of diameters 152mm and long 912mm, has been evaluated using four different eccentricity values (15, 30, 45 and 60 mm). Based on the finding of experimental and theoretical investigation for the CFFT columns, a new confinement model is proposed for the confined concrete compressive strength of the CFFT cylinders. Also, the design equations are modified to accurately predict the ultimate and yield loads capacities of internally reinforced and unreinforced short CFFT columns. In addition, the theoretical analysis was utilized to correlate the slenderness ratio of the CFFT columns to various material characteristics and geometric properties of the FRP tubes and concrete. It was found that a slenderness ratio of 12 gave a safe value for the design purposes. However a more precise formula for the slenderness ratio was proposed to control the buckling mode of failure. Pure flexural tests have been conducted on 10 RCFFT and RC beams of a total length 2000 mm with constant diameter 213 mm. The test variables were the type of internal reinforcements (steel or GFRP bars), the FRP tube thickness, concrete compressive strength and the type of transverse reinforcements (spiral steel or FRP tubes). The influence of the considered variables on the flexural behaviour of the tested RCFFT beams is presented. A simplified analytical method is developed to predict the yield and resisting moments corresponding to the failure modes of the tested RCFFT beams. The analysis was conducted according to the equations derived from linear elastic analysis. This analysis was found to be acceptable for predicting the ultimate and yield moments capacities of the FRP or steel-RCFFT beams. In addition, an analytical investigation to examine the validity of the available design provisions for predicting the load-deflection response of CFFT is conducted. The effective moments of inertia of the tested beams are analyzed using the different available code, manuals and design guidelines equations. The results of the analysis are compared with the experimental values. It has been found that the predicted tension stiffening for steel or FRP-RCFFT beams using the conventional equations (steel or FRP-RC member) is underestimated and hence the predicted deflections are overestimated. Based on the experimental data obtained in this study, new proposed equations and a modified expression for the effective moment of inertia of a simply supported CFFT beams reinforced with steel or GFRP bars are introduced