Torsion Strengthening of RC Beams using CFRP
Although Reinforced Concrete (RC) is one of the most favorable materials in the construction industry, it has potential problems such as the concrete cracking due to shrinkage and expansion, seismic damage and failures. Concrete cracks permit moisture to penetrate into the concrete resulting in the corrosion and rusting of steel reinforcement which eventually results in loss of structural strength. In many cases, the damaged parts of structures can be repaired. Also, it is possible to strengthen concrete structures to accommodate changes in load variation or code revisions.
Strengthening of RC structures using CFRP
Strengthening concrete structures by externally bonding CFRP sheets on the surfaces of RC members is an excellent reinforcement technique. One of the most common applications of CFRP is known as the “external bonding reinforcement” (EBR) technique, in which the CFRP composites are adhesively (in general, with epoxy) bonded to the face of the structure.
CFRP has many advantages such as ease of transportation, formability and flexibility, resistance against corrosive environmental conditions, high strength to weight ratio, high modulus of elasticity, light weight, advanced mechanical properties, high resistance to tension and shear, resistance to moisture, and high durability and stability. The easy-to-apply character of CFRP allows for its use for upgrading or repairing inadequate or damaged RC members and structures.
The importance of torsional resistance of RC beams
When increasing the strength of the beam-column connection, the twisting angle must be considered. The angle of twist and torsional resistance of RC members have had a great effect on the stiffness and lateral strength of buildings and therefore their effects on seismic behavior of structures must be considered during analysis and design.
Applying CFRP for torsion strengthening reinforces RC beams against torsional loads imposed by lateral earthquake loads and existing service loads. Strengthening with CFRP raises the energy absorption of a building during an earthquake so the building will have a higher resistance. ¹
Torsional failure of beams typically occurs in seismically-affected areas and similar to shear failure, failure is sudden & brittle. Recent studies have shown that externally bonding CFRP limits the width of cracks and widens the spacing between cracks which will increase the concrete’s contribution to torsion strength.
Figure 1: Spalling of the concrete cover due to torsion ⁴
Advantages of CFRP as a torsional strengthening system
- The torsional capacity of RC beams strengthened with CFRP sheets increased 92.62 percent. ¹
- The increase in torsional capacity of RC beams strengthened with one layer of CFRP (54 percent surface coverage) is up to 91 percent and in RC beams strengthened with two layers of CFRP (60 percent surface coverage) the increase in tortional capacity is up to 111 percent. ¹
- The torsional capacity of CFRP-strengthened RC beams can be increased by up to 116.7 percent while the twisting angle of the beams increases about 66.4 percent. ¹
- Strengthening RC beams with CFRP reduces cracking by up to 40 percent and the ultimate strength increases by up to 78 percent. ²
- The ultimate strength of RC beams externally reinforced by a full-wrap CFRP scheme is much greater than that of beams strengthened by a U-wrap CFRP scheme. In the case that existing slabs are positioned in a way making it not possible to use a full-wrap CFRP scheme, using a continuous U-wrap CFRP scheme (Figure 2) is better than using U-wrap CFRP strips (Figure 3). ³
Figure 2: Continuous U-wrap CFRP configuration for torsional strengthening of RC beams (EBR technique)
- When a beam is strengthened using CFRP strips, the torsional strength will increase but the failure will inevitably occur in the unwrapped spaces between strips.
- In the case that continuous wrapping of the beam with CFRP is not economical, a 45° spiral U-wrap is much more efficient than vertical strips of CFRP. The inclined CFRP wraps are in tension up to failure, while the vertical CFRP wraps are subjected to forces that are not along the direction of the fibers (Figure 3).
- The 45° CFRP wrap should be applied continuously in a spiral form all around the beam.
6. Applying 45° strips of CFRP on only one side of RC beams, due to the discontinuity of the fiber wrap, is not an efficient strengthening scheme. The 45° strips should be applied continuously in a spiral form all around the beam to be torsional resistant. In other words, CFRP bonded on each side of the beam separately without continuity does not have significant torsional resistance. ³ Torsional cracks follow a spiral pattern, propagating in opposite directions on both sides of the beam.
7. The best type of CFRP wrap to increase RC beam ductility is full wrapping along the entire length with two layers of CFRP. In this case, the torsion strength of the retrofitted beam will be upgraded by up to 92.62 percent. ²
Figure 3: Torsion strengthening of an RC beam with 45° EBR CFRP Strips
Parastoo Azad and Dr. Mehrtash Soltani (June 9, 2021)
- Shokri, M., & Edalati, M. (2017). Comparison of Twisting Angle-Torsional Moment in Unstrengthened Reinforced Concrete Beams with Reinforced Concrete Beams Strengthened with CFRP Sheets. Journal of History Culture and Art Research, 6(1), 43-58. doi:http://dx.doi.org/10.7596/taksad.v6i1.702
- Elwan, S. K. (2016). Torsion Strengthening of RC Beams using CFRP(Parametric Study). Journal of Civil Engineering.
- Ghobarah, A., Ghorbel, M. N., & Chidiac, a. S. (2002). Upgrading Torsional Resistance of Reinforced Concrete Beams Using Fiber-Reinforced Polymer. Journal of Composites for Construction.
- Michael P. Collins, D. M. (n.d.). Shear and Torsion Design of Prestressed and Non-Prestressed Concrete Beams. Toronto, Ontario & Montreal, Quebec: pci.org.