Elsevier

Engineering Structures

Volume 31, Issue 7, July 2009, Pages 1535-1542
Engineering Structures

Punching shear strength of interior slab–column connections strengthened with carbon fiber reinforced polymer sheets

https://doi.org/10.1016/j.engstruct.2009.02.021Get rights and content

Abstract

In this paper, punching shear strengthening of flat slabs using Carbon Fiber Reinforced Polymer (CFRP) sheets is studied. Fifteen specimens of reinforced concrete slabs were tested. Thirteen of them were strengthened by CFRP sheets and two specimens were kept as control specimens. Four of these strengthened specimens were tested under cyclic vertical loading. The width of CFRP sheets varied in different specimens. The CFRP sheets were located at the tension side of the slabs in two perpendicular directions. Vertical load was applied downward through a column stub using a hydraulic Jack. In all specimens, no rupture of CFRP sheets was observed. The test results showed that the use of CFRP sheet, in addition to steel reinforcing bars, as flexural reinforcement improves the punching shear strength of slabs. This improvement can be significant for the slabs made of high strength concrete and low steel reinforcement ratio. However, the improvement of punching shear strength due to FRP strengthening reduced under cyclic vertical loading. The test results were compared with the equations proposed by ACI 318 and BS 8110 Codes. The ACI Code underestimates the punching shear strength of slabs and this underestimation becomes more pronounced with the increase in the flexural reinforcement. The BS 8110 Code appropriately accounts for the effect of flexural reinforcement on punching shear strength of slabs. However, for the strengthened slabs, an equivalent reinforcement ratio should be used to include the effect of both steel and CFRP flexural reinforcement.

Introduction

The slab–column connection of a flat plate is susceptible to punching shear failure. Previously, it has been shown that punching shear failure can be divided into two types, i.e. pure shear punching and flexural punching [1], [2]. In the former type which is the case for slabs with large reinforcement ratios, failure occurs suddenly with small displacement. For the case of small reinforcement ratios, flexural punching takes place after relatively large displacement. According to Moe [1], pure shear punching and flexural punching can be calculated by Eqs. (1), (2) as follows: Vu=1.25(10.075cd)fc1+0.44u0dfcVflexu0dVflex=8(bsc0.172)Mnwhere fc is the compressive strength of concrete; d is the effective depth of the slab; c and u0 are the side length and the perimeter of a square column, respectively; b and s are the side dimension and that between supports of a square slab, respectively; Vu is the pure punching shear strength and Vflex is the shear strength corresponding to the flexural punching failure of the slab, which is derived from the yield line analysis. Fig. 1 compares Eq. (1) with Eq. (2), and shows that as the reinforcement ratio increases, the failure type changes from flexural to pure shear punching [1].

Currently, there are various punching shear strength equations for slab–column connections such as those proposed by ACI 318 [3] and BS 8110 [4] Codes. These equations were developed for normal slab–column connections, thus they might not be applicable to strengthened slab–column connections. The classical strengthening techniques for concrete slab–column connections, in order to prevent sudden punching shear failure, include use of steel plates and bolts, transverse pre-stressed reinforcement, the use of an epoxy bonding steel plate, and thickening of the slab around column or use of a large column cross section. Increasing attention has been paid to the application of advanced composite materials for strengthening especially carbon fiber reinforced polymer (CFRP) and glass fiber reinforced polymer (GFRP) in the structural engineering field. Limited research work has been conducted on the strengthening of concrete two-way slabs using FRP materials.

Recently, some experimental and analytical studies have been conducted to propose methods for strengthening of flat slabs with FRP sheets against punching failure [5], [6], [7]. It has been shown that a simple and effective method for strengthening of slabs against punching failure is to use FRP sheets as flexural reinforcement [6], [7]. According to the previous studies [8] and BS 8110 Code [4], flexural reinforcing bars increase the punching shear strength. By applying FRP sheets on the tension side of slabs, the flexural strength of slabs and thus the punching shear strength is expected to increase. Chen and Li [7] used Glass Fiber Reinforced Polymer (GFRP) laminates for shear strengthening of slabs. They showed that flexural strengthening of slabs by GFRP laminates can increase the punching strength, significantly. However, GFRP laminates were more effective for the slabs with low steel reinforcement ratios. In this case, a noticeable increase was observed in the load carrying capacity of these specimens as compared to those having large reinforcement ratios. Based on an analytical method, Chen and Li [7] proposed equations to calculate the punching strength of slabs strengthened with GFRP laminates. In the equations, they introduced two parameters of equivalent reinforcement ratio ρeqv and equivalent depth deqv for slabs. Chen and Li [7] showed that strengthening of a slab having low steel reinforcement ratio could change the mode of failure from flexural punching to pure shear punching.

Section snippets

Existing equations

According to BS 8110 [4] Code, the flexural reinforcement of slabs increases the punching shear strength and proposes Eq. (3) for punching shear strength as follows: Vu=0.79100ρs3400d4u0d where u0 is the rectangular critical perimeter at distance 1.5d from the face of a column; u0=4(c+3d); fcu is the cube compressive strength of concrete; ρs=As/bvd is the reinforcement ratio limited to 0.03; c is the side length of column in mm; bv is the breadth of section in mm and d is the effective depth of

Materials and test specimens

For the slab specimens, two concrete mixtures based on the design compressive strengths of 25 MPa and 50 MPa were used. For each series of casting, the compressive strength was measured by testing of five standard concrete cylinders. Table 1 presents the average concrete compressive strengths of different specimens. Two sizes of reinforcing bars with 12 mm and 16 mm diameters were used in specimens. For each bar size, three samples were tested under tension. The yield strengths of the steel

Results and analysis of tests under monotonic loading

Fig. 4 shows the load versus displacement relationships for different specimens under monotonic loading. All specimens failed in punching shear except that the control Specimen R0.8-C25-F0 with small reinforcement ratio failed in flexural punching after relatively large displacement (Fig. 4). However, the type of failure for the Specimen R1.6-C25-F0 and all strengthened specimens was due to shear punching. These specimens had relatively large flexural reinforcement ratios. Specimens in series

Results and analysis of tests under cyclic loading

Fig. 8 shows the results of tests under vertical cyclic loading. A minimum compressive loading of approximately 60 kN was applied to include the dead load effect. The cyclic loading represents the variation of live load on the slab. The envelops of peak loads for different load–displacement relationships of tests under cyclic loading are shown in Fig. 9, Fig. 10. These figures also include the results of tests under monotonic loading.

As shown in Fig. 9(a) and (b), for slabs with low

Comparison of test results with code equations

In this section, the results of experiments reported in this study are compared with the values calculated by ACI 318 [3] and BS 8110 [4] Codes. In order to account for the effect of the strengthening material values on punching strength predictions by different codes, it is necessary to define the two parameters of the equivalent depth deqv and the equivalent flexural reinforcement ratio ρeqv. Eqs. (4), (5) proposed by Chen and Li [7] are used in this study. The calculated values of ρeqv and d

Conclusions

In this study, the punching shear strength of flat slabs strengthened with CFRP sheets was investigated. Fifteen slab specimens were manufactured and tested under monotonic and vertical cyclic loading. In all specimens, no rupture of CFRP sheets was observed. Steel reinforcement ratio, concrete strength and the value of the strengthening materials varied in different specimens. Based on the test results, the following conclusions are drawn:

  • 1.

    Using CFRP sheet, in addition to steel reinforcing

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