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

Comparison of test results with shear design equations

The shear resistance due to the CFRP obtained by tests (refer to Table 5) is compared to the nominal shear resistance predicted by the guidelines in Table 6. Such a comparison is useful as it allows to verification of the reliability of the requirements contained in the codes and standards.

For slender specimens with no transverse steel (Series SB-S0), it is observed that the shear resistance obtained by tests is of the same order as the one predicted by both ACI 440.2R-02 and CSA-S806-02, for 0.5L and 1L. For these cases, the shear resistance predicted by fib TG9.3 is higher than the experimental value. In contrast, a significant deviation between experimental and guidelines values is observed for 2L schemes: while the gain obtained by test is negligible as the number of layers is increased from 1L to 2L, the guidelines overestimate the shear resistance. This is particularly true for the CSA-S806-02, where the prediction equations do not include the effect of the FRP stiffness. Generally, it can be concluded that the guidelines fail to adequately predict the shear resistance of strengthened beams when the thickness of FRP (and hence the stiffness) is high. In such cases, premature debonding can prevail, inhibiting thereby the gain due to FRP to reach its optimum value. Crushing of moderate strength concrete struts can also limit the gain due to FRP, as was the case for the specimens tested in this study. But this failure mode is not appropriately covered in the guidelines.

In contrast, for the slender specimens with transverse steel (SB-S1), the shear resistance due to FRP obtained by tests is negligible. This is not reflected in the guidelines because the shear resistance due to FRP is the same, regardless of the presence of transverse steel.

Likewise, the influence of the ratio a/d on the shear resistance due to FRP is clearly indicated by the test results but not included in the guidelines.

These observations are in agreement with the conclusions drawn by the authors (Bousselham at al. 2003), who compared, in an extensive state of the art review, more than a hundred test results reported in the literature with the values predicted by the guidelines. In these conclusions, it was noted that the research studies devoted to the shear strengthening yielded interesting results that were included in the guidelines. Nevertheless, it remains that comparison of the resistance predicted by the guidelines with test results clearly shows that major aspects, such as the transverse steel and the ratio a/d, are not captured by the guidelines.

CONCLUSIONS

This study presents results of an experimental investigation on the behavior of reinforced concrete T-beams retrofitted in shear with externally bonded CFRP composite. The influence of the following parameters was studied: 1) the CFRP ratio (that is, the number of CFRP layers); 2) the transverse steel ratio (that is, spacing); and 3) the shear length to the beam depth ratio, a/d (that is, deep beam effect). The following conclusions were reached: