International Concrete Abstracts Portal

In this paper, a new approach predicting the punching shear capacity of FRP reinforced concrete flat slabs without shear reinforcement is introduced in the light of the results of an experimental program conducted at the Centre for Cement and Concrete of the University of Sheffield. The new approach is found to accurately predict the punching shear capacity of the tested slabs. Verification of this new approach is undertaken through comparisons with other test results ensures its validity. Comparisons of the new approach with the ACI 318-95 equation calculating the punching shear capacity of reinforced concrete slabs without shear reinforcement show that the current ACI equation is unconservative in case of FRP reinforcement with low reinforcement ratios. A modification is proposed to the current ACI 318-95 code equation for punching shear in order to accommodate low stiffness FRP reinforcement. Comparisons with test results show that the proposed modification of the ACI code equation leads to good predictions.

Results of tests of ten concrete beams reinforced with fiber-reinforced plastic grids fabricated from small pultruded profiles are presented. The beams were designed to investigate the behavior and performance of the grids when used to reinforce beams that develop significant flexural-shear cracking (a/d = 3). Different grids were designed to study the influence of the main bars, vertical bars and transverse bars (cross-rods) of the grid on the failure loads and failure modes. From the results of the testing it was concluded that pultruded grids can provide effective shear reinforcement, however, their design must account for failure of the main bars in the shear span. Overreinforced beams failed in a somewhat more ductile manner than underreinforced beams.

This paper summarizes an experimental program conducted at the University of Manitoba, Canada, to examine the structural performance of fiber reinforced polymer (FRP) stirrups as shear reinforcement for concrete structures. A total of ten large-scale reinforced concrete beams were tested to investigate the modes of failure and the contribution of the FRP stirrups in the beam mechanism. The ten beams included four beams reinforced with carbon FRP stirrups, four beams reinforced with glass FRP, one beam reinforced with steel stirrups and one control beam without shear reinforcement. The variables were the material type of the stirrups, the material type of the flexural reinforcement, and the stirrup spacing. Due to the unidirectional characteristics of FRP, significant reduction in the strength of the stirrup relative to the tensile strength parallel to the fibers is introduced by bending FRP bars into stirrup configuration and by the kinking action due to inclination of the diagonal shear crack with respect to the direction of the stirrups. A total of 40 specially designed panel specimens were tested to investigate the bend effect on the stirrup capacity, along with two control specimens reinforced with steel stirrups. The variables considered in the bend specimens are the material type of the stirrups, the bar diameter, the bend radius, the configuration of the stirrup anchorage, and the tail length beyond the bend portion. A total of 12 specially designed panel specimens were also tested to investigate the effect of the angle of cracks on the stirrup capacity. The two variables considered in this case are material type of the stirrups and the crack angle. Description of the experimental program, test results and design recommendations are presented.

Four 6.5 m long two span continuous beams were tested to investigate the feasibility of using FRP grids as shear reinforcement. The beams were longitudinally reinforced with equal amounts of CFRP reinforcement while transversely two beams were reinforced with steel stirrups and two with CFRP grids. The two spans of each beam had identical amount and disposition of reinforcement, and were symmetrically loaded with a point load at the center of each span. For the beams with steel stirrups, the so-called Av/s = 2.0 mm while for the ones with CFRP grids the equivalent quantity was 1.75 mm. The elastic modulus of the grid was half the elastic modulus of the steel. The beams were monotonically loaded to destruction and they all failed in flexure. Despite their lower shear reinforcement ratio, the beams with the CFRP grid performed as well as those with the steel stirrups. They actually failed at 10% higher load than the beams with steel stirrups. Strain values of up to 0.4% were measured in the grids. Recognizing the practical convenience of grid shear reinforcement over custom manufacturing of stirrups, and considering their satisfactory performance, as observed in the present testing program, the use of FRP grids as shear reinforcement is indeed feasible.

A closed-form high-order analytical approach for the analysis of concrete beams strengthened with externally bonded fiber-reinforced polymer (FRP) stripes, on the tensile faces of the RC beam in positive and negative bending moment regions, is presented. The model is based on equilibrium and compatibility requirements in and between all constituents of the strengthened beam, i.e. the concrete beam, the FRP stripe, the adhesive layers and at the interfaces between the various parts. The governing equations of the mathematical model of the strengthened beam are derived and an analytically closed-form solution is determined. A numerical example of a typical continuous reinforced concrete beam strengthened with externally bonded FRP stripes is discussed with emphasis on the shear and transverse normal (peeling) stress concentrations at the adhesive-concrete and adhesive-FRP interfaces at the edges of the FRP stripes. Results concerning the edge stresses at three characteristic locations that have been determined using the closed-form high-order model are discussed. In the sequel a summary, conclusions, and recommendations for the design of the strengthened beam are presented.