Behavior of Reinforced Concrete T-Beams Strengthened in Shear with Carbon Fiber-Reinforced Polymer-An Experimental Study

ACI Structural Journal, May/Jun 2006 by Bousselham, Abdelhak, Chaallal, Omar

This paper presents results of a wide and extensive experimental investigation on reinforced concrete (RC) T-beams retrofitted in shear with externally bonded carbon fiber-reinforced polymer (CFRP). In total, 22 tests were performed on 4520 mm-long T-beams. The parameters investigated were as follows: 1) the CFRP ratio (that is, the number of CFRP layers); 2) the internal shear steel reinforcement ratio (that is, spacing); and 3) the shear length to the beam's depth ratio, a/d (that is, deep beam effect). The main objective of the study was to analyze the behavior of RC T-beams strengthened in shear with externally applied CFRP by varying the aforementioned parameters. The results showed that the contribution of the CFRP to the shear resistance is not in proportion to the CFRP thickness (that is, the stiffness) provided, and depends on whether the strengthened beam is reinforced in shear with internal transverse steel reinforcement. Results also confirmed the influence of the ratio a/d on the behavior of RC beams retrofitted in shear with external fiber-reinforced polymer (FRP). Finally, comparison of the shear resistance values predicted by ACI 440.2R-02, CSA S806-02, and fib TG9.3 guidelines, with the test results clearly indicated that the guidelines fail to capture important aspects, such as the presence of the transverse steel and the ratio a/d on the one hand, and overestimates the shear resistance for high FRP thickness (and hence high FRP stiffness), on the other.

Keywords: polymer; reinforced concrete; shear; strain; strengthening.

INTRODUCTION

One of the techniques used to strengthen existing reinforced concrete (RC) members involves externally bonding fiber reinforced polymer (FRP) composite materials by means of epoxy adhesives. This technique improves the structural performance of a member (Neale 2000; Meier 1995). The wide use of this strengthening method for various structures, including buildings and bridges, has demonstrated its efficiency and its convenience (Bakis et al. 2002; Clarke 2000).

Strengthening of beams and slabs in flexure and confinement of circular columns have been well documented. A review of research studies on shear strengthening, however, revealed that experimental investigations are still needed (Bousselham and Chaallal 2004; Matthys and Triantafillou 2001). Research studies carried out in recent years have provided valuable findings, particularly with regard to the effect of the stiffness of the composite on the shear strength enhancement (Triantafillou and Antonopoulos 2000; Khalifa and Nanni 2000). Other parameters that also influence the shear resistance mechanism, however, were not sufficiently studied (Bousselham and Chaallal 2004). Shear steel reinforcement and shear span to depth ratio (a/d) are examples of such parameters.

To address these areas, the authors conducted a large experimental investigation on the shear performance of RC beams strengthened with externally bonded carbon fiberreinforced polymer (CFRP) fabric. The parameters of the study were set as follows: 1) the CFRP ratio (that is, the number of CFRP layers); 2) the internal shear steel reinforcement ratio (that is, spacing); and 3) the shear length to the beam depth ratio, a/d (that is, deep beam effect).

The objectives of this paper are as follows:

* To investigate the shear performance, including the mode of failure, of RC beams strengthened with CFRP in terms of the CFRP; the internal transverse steel reinforcement, hereafter called the transverse steel; and the shear span to depth ratios;

* To analyze the behaviour of the CFRP, the internal transverse and longitudinal steel reinforcement, and the concrete struts, while the above parameters are varied; and

* To verify the reliability of ACI 440.2R-02 (ACI Committee 440 2002), CSA S806-02 (Canadian Standards Association 2002), and fib TG9.3 (2001), hereafter called the guidelines.

RESEARCH SIGNIFICANCE

Most research studies on shear strengthening with FRP composites are mainly focused on the properties and the performance of the FRP and often involve rectangular beam test specimens of reduced sizes. Also, the lack of data on the strains experienced by the different components (FRP, concrete, and steel) makes it difficult, if not impossible, to fully grasp the prevailing shear resistance mechanisms. The proposed research was targeted to address these and other important aspects. It is believed that the findings of this study contribute to the understanding of the resistance mechanisms involved for RC beams strengthened in shear with externally bonded FRP. This understanding is of paramount importance because it leads to a more rigorous approach toward safer and rational design guidelines.

EXPERIMENTAL PROGRAM

The experimental program (Table 1) involves 22 tests performed on 11 full-scale T-beams. The control specimens, not strengthened with CFRP, are labelled 0L, whereas the specimens retrofitted with CFRP are labelled 0.5L, 1L, or 2L, corresponding to 0.5, 1, and 2 bonded layers of CFRP, respectively. The letters DB (deep beam) and SB (slender beam) are used to designate specimens with small and high a/d, respectively. Series S0 is made of specimens with no internal transverse steel reinforcement (that is, no stirrups). Series S1 and S2 correspond to specimens with internal transverse steel stirrups, hereafter called transverse steel, spaced at s = d/2 for S1 and s= d/4 for S2, where d = 350 mm and represents the effective depth of the cross section of the beam. Thus, for instance, Specimen DB-S0-1L features a small a/d, has no transverse steel, and is retrofitted with one layer of CFRP.