A total of nine full-scale concrete deep beam specimens with shear span to depth ratios a/d of 1, 1.5 and 2 were constructed and tested under three-point loading to failure. The studied variables were the shear span to depth ratio and the quantity of web shear reinforcement. The behaviour of deep beams is indicated by their levels of ultimate shear strength, mid deflection, FRP reinforcement strain, crack propagation, and by their type of failure. The test results are also compared to predictions based on the design procedures of the ACI and the CSA design and construction code for building structures with fibre-reinforced polymers. Based on the review of available experimental studies and comparison of code provisions for conventional deep beams, and the experimental study conducted on FRP-RC deep beams, the following conclusions were drawn.
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7.2.1. Conclusions based on the behaviour of conventional deep beams and relevant code provisions:
The Strut and Tie Models as provided in three prominent codes, namely ACI-318- 08 [2008], CSA-A23.3-04 [2004], and Eurocode EN1992-1-1 [2004], are generally found to be appropriate methods for the design and evaluation of the shear strength capacity of concrete deep beams with a shear–span to depth ratio less than or equal to two.
Although the effect of web reinforcement is not accounted for in some of the code provisions, experimental studies show that such reinforcements improve the capacity of concrete deep beams.
The code provisions may not produce accurate results in the prediction of the mode and location of failure as observed in the experimental studies.
The provisions of the Canadian Code appear to be the most conservative in estimating the capacity of concrete deep beams.
When the Eurocode method is modified by multiplying the ultimate load by a factor β as provided in the code, it provides a reasonable and conservative estimate of capacity similar to that obtained by using the provisions of the Canadian code. The procedure of ACI improves significantly when bottled-shaped struts are used
instead of uniform cross-section struts, and shows conservative results. However, the code does not provide guidance on when to use the bottle-shaped or the uniform strut sections.
The STM design procedures in Appendix A of the ACI 318-085 codes, after modifying the efficiency factors of bottled shape struts and calculated as function
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of shear span-to-depth ratio as proposed in the present work, provides an improvement in the conservative estimate of the capacity of a concrete deep beam.
7.2.2. Conclusions based on the present experimental study of FRP-RC deep beams and relevant design provisions:
The deflection gradually increases for beams with higher shear span-to-depth ratio
a/d, i.e. the behavior of beams with higher shear span-to-depth ratio a/d becomes more flexible.
When the shear-span to depth ratio a/d decreases, the influence of the shear behavior becomes dominant.
The higher load-resisting capacity was observed after the first diagonal crack for beams with the smaller shear span depth ratio.
Although the three specimens A1, B1.5and C2 with 100% of web reinforcement failed in shear-compression mode, the effect of the a/d ratio was reflected in the severity of the pre-failure damage.
Web reinforcement has a significant effect on controlling the crack propagation, and the pre-failure damage appears to be more severe for beams without or with less web reinforcement.
The ultimate shear strength of the tested beams was increased due to the web reinforcement.
Web reinforcement has a significant effect on the beam stiffness, where the deflection gradually increased for beams with web reinforcement in two groups (A and C).
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The beams’ failure modes are mainly affected by the shear span-to-depth ratio a/d
and by the web reinforcement.
The code provisions of ACI 440.1R-06 [2006] produce very conservative results to predict the ultimate shear strength compared to the experimental studies, as expected.
The reduction of the ultimate tensile strength Fu according to clause 8.5.3.1 of the CAN/CSA-S806-12 code [2012] appears sufficient when compared to the experimental results.
The STM design procedures in Appendix A of the ACI 318-08 code [2008], after modifying the tie strength according to FRP properties, constitute a conservative and convenient design method for FRP-reinforced concrete deep beams.
The significant increase in the tensile strain in web reinforcements the region of the assumed direction of the main struts and in the main longitudinal FRP rebars indicates that the Strut-and-Tie Model (STM) is the appropriate method for the design of FRP reinforced concrete deep beams with (a/d) less than or equal to two. It was also observed that the strain in the both layers of the longitudinal rebars were similar in a given specimen, indicating that the longitudinal rebars acted in a group as the tie in the STM model.
The STM design procedure in the CAN/CSA-S806-12 [2012] code provides a practical and reliable design method for FRP-reinforced concrete deep beams. However, there are some aspects of the provisions that are inconsistent. For example, the way strut capacity is calculated, and the stress limit in a straight FRP bar is defined. Also there is concern about the required minimum quantity of web
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reinforcements specified in the standard, which seems quite conservative and may lead to very close spacing.
Based on the work presented in this thesis an equation has been proposed to calculate the contribution of the FRP web reinforcement to the ultimate shear capacity of FRP-reinforced concrete deep beams.