The usage of CFRP in strengthening beams for shear
1. Shear in concrete beams
Some reinforced concrete beams have shear weaknesses due to various reasons and need strengthening. Reasons such as insufficient shear reinforcement, reduction in cross-section of reinforcement due to corrosion, increase in service loads, design and or construction defects. Normally, shear reinforcement of the beam is provided by stirrups, and in some cases, inclined bent-up bars are also used along with stirrups to strengthen the shear resistance of the beam in support locations (Figure.1).
Figure 1- Shear reinforcement of a concrete beam
Since shear failure is sudden, brittle and without any advanced warning, beams must be designed in a way that the shear strength of the member exceeds the flexural strength. Shear failure occurs when the beam has shear resistance lower than flexural strength and the shear force exceeds the shear capacity of different materials of the beam. Once a diagonal shear crack appears, the worst thing is that the crack propagates throughout the compression zone and makes a sudden, brittle and entire failure.
Therefore, in this article, the beam’s shear strengthening with CFRP to stop diagonal cracks from developing or at least to limit their widths as a result of increasing shear strength of the beam has been reviewed.
2. Shear strengthening of concrete Beam
For shear strengthening of concrete structures, we should be familiar with shear behavior and shear failure of the structure. Figure No.2 is a concrete beam and its possible shear crack. Concrete beams develop a diagonal crack from the loading point to the bottom of the fixed end, leading to the shear failure of the whole structure. The location of the crack is an indication of the failure mode of the structural element. Shear failure of the beam is brittle and without any forewarning, so beams should be designed in a way that bending failure with large deformations occurs before shear failure. However, according to various investigations on this issue, it has been proven that over-strengthening in shear resulted in bending failure in some cases, but after strengthening the beams for bending, shear failure was achieved. The experimental results which evaluated the contribution of CFRP towards the shear strength of continuous reinforced concrete (RC) beams have shown that the shear strength of the beams was significantly increased by using CFRP and placing the CFRP strips at 45° to the axis of the beam is beneficial (I. A. Bukhari, 2009).
Figure 2- The shear crack in a concrete beam
3. How to calculate the Shear strength of the CFRP strengthened RC beam?
According to ACI318 for RC beams with stirrups, the shear capacity is as the sum of the concrete contribution Vc and the stirrup contribution Vs.
According to ACI440.2R the nominal shear strength of CFRP strengthened RC beam is as the following:
Vd = Vc + Vs + Vf
Vd : Nominal shear strength
Vc : Shear strength of concrete
Vs : Shear strength of stirrups
Vf : Shear strength of CFRP composite
The design strength is obtained by multiplying the nominal shear strength to the reduction factor 0.85 for concrete and steel and 0.95 or 0.85 for CFRP reinforcement (ACI318 & ACI440.2R).
4. Types of CFRP wrapping
According to ACI440.2R, there are 3 types of wrapping of CFRP to increase the shear strength of beam elements (Figures 3 and 4).
The effective parameters in shear strengthening with CFRP include the effect of direction, thickness and angle of fibers relative to the beam axis which will be chosen according to the structural situation. On the reports of various studies around the world, it is difficult to conclude that thicker CFRP has higher strengthening effects and it depends on the situation of the element on-hand as well (Täljsten, 2003).
Figure 3- Two-sided wrapped beam with CFRP perpendicular to shear cracks
Figure 4- Four or Three-sided wrapped beam with CFRP in a vertical direction
- It is worthwhile to mention that the contribution of spaced CFRP strips (Figure-3) in shear shall be determined. Spacing between CFRP strips (S) should satisfy the limitations of steel stirrups according to ACI318.
- It should be emphasized that in all types of CFRP wrapping, especially 4-sided wrapping, the total shear strength provided by stirrups and CFRP should not exceed the limits of steel alone criteria in ACI318. Please be aware that the large amount of CFRP does not necessarily mean a better strengthening effect.
- In the shear design of externally bonded CFRP, the beam is considered to lose the shear capacity at the instant when the debonding occurs in the vicinity of the crack, even though the debonding has not yet developed through the whole length. Excellent bonding between CFRP and concrete surface is of great importance.
Figure 5- Flexural & Shear strengthening of RC beams by Rhino Carbon Fiber™ CFRP (U-wrapped)
5. The importance of CFRP strips direction
- According to (Abdel-Jaber, 2003) covering the entire shear span of the beam with CFRP arranged horizontally, vertically or diagonally has the greatest shear strengthening of the beam. It increases shear strength between 109~122%.
- It has been proven that placing CFRP strips perpendicular to the shear crack changes the type of failure of the RC beam from brittle to ductile but with a smaller increment in shear strength (Figure 6-a). Arrangement of CFRP strips has the highest ductility index which is about 4 times greater than other strip arrangements. The CFRP is perpendicular to the diagonal shear cracks so it can increase aggregate interlock, reduce cracks widths and hence improves the ductility of RC beams and postpones crack propagation.
- Please be aware that when CFRP strips are fixed diagonally, horizontally or vertically the shear strength of the beam will be calculated according to the bonding area and tensile strength of the concrete.
- In the case of beam shear design, placing CFRP makes the beam over-strengthened in shear; the vertical placing of CFRP strips is a better strengthening choice (Figure 6-b).
- According to (Täljsten, 2003) placing CFRP strips in a horizontal direction has the lowest shear strengthening effect because this way strips are along the shear cracks and do not have that much strengthening effect (Figure 6-c). On the other hand, based upon (Abdel-Jaber, 2003) using two horizontal CFRP strips on the ¾ beam depth increases shear strength of the beam by about 55%; due to the high cost of using CFRP for the entire shear span of the beam, this can be an economic solution.
Figure 6- CFRP directions
Studies show that shear cracked beams strengthened with CFRP can carry as much load as an uncracked beam (Täljsten, 2003).
Rhino Carbon Fiber™ has 3 types of CFRP with residential and commercial applications as bellow image:
Figure 7- Types of Rhino Carbon Fiber™ CFRP
For more information about application instructions and product benefits please refer to this link:CFRP Products
Figure 8- Rhino Carbon Fiber™ CFRP applications
Last but not least, the most important advantage of the CFRP strengthened beam is the increase of shear strength and ductility simultaneously. Studies have shown that sudden shear failure of concrete beams without CFRP reinforcement is a normal phenomenon, but after strengthening with CFRP and increasing ductility, first flexural cracks at the tension zone will appear and warn you about the failure so that you have time to strengthen and repair the beam.
Parastoo Azad and Dr. Mehrtash Soltani (January 15, 2020)
- ACI440. (2017). Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures. Michigan, USA: American Concrete Institute.
- Abdel-Jaber, M. W. (2003). Shear strengthening of reinforced concrete beams using different configurations of externally bonded carbon fibre reinforced plates. Materials and Structures, 291–301.
- Al-Nasra, M. &. (2013). Shear Reinforcements in the Reinforced Concrete Beams. American Journal of Engineering Research (AJER), 191-199.
- I. A. Bukhari, R. L. (2009). SHEAR STRENGTHENING OF REINFORCED CONCRETE BEAMS WITH CFRP. Magazine of Concrete Research.
- Täljsten, B. (2003). Strengthening concrete beams for shear with CFRP sheets. Construction and Building Materials, 15-26.