RC slabs are critical components in structures, and their cracking or failure can cause long-term degradation of the strength and performance of the whole structure. Common failure modes of two-way slabs include flexural and punching shear which are related to the slab-to-steel reinforcement ratio (figure 1).

Two-way slabs with low or medium reinforcement generally fail in flexural, while slabs with reinforcement ratios of more than 1.0% fail in punching shear. ¹

In this article we have reviewed the effectiveness of strengthening RC slabs using CFRP.
Figure 1: Typical slab failures – a) Flexural failure, b) Punching shear failure

Why use CFRP?

  1. It is possible to increase the average load-bearing capacity of RC slabs by about 40%. ²
  2. The flexural capacity of CFRP-strengthened two-way slabs increases by about 35.5%. ²
  3. Strengthening RC slabs with CFRP reduces deflection and crack width. ³
  4. By adding more layers of CFRP to a certain limit, there is a significant increase in RC slab strength without losing eligible ductility. ⁴
  5. By increasing the number of CFRP layers, there is a slight reduction in ductility. Ductility reduction is directly related to the increase in stiffness. ⁴ By increasing the number of CFRP layers, the stiffness of the linear part (elastic stage) of the CFRP-strengthened slab increases by about 8 times.
  6. Strengthening with CFRP strips has a lower reduction in ductility, but using CFRP sheets increases the ultimate carrying capacity more than CFRP strips. Therefore, using CFRP sheets can be more helpful in slab strengthening with rational ductility reduction. ⁴

Effect of CFRP on strengthening one-way slabs with an opening

Openings in one-way slabs can be due to elements such as stairs, elevators or any other services such as air heating or conditioning ducts. The presence of openings can decrease the load-bearing capacity of the RC slab by about 50%. By using CFRP sheets for strengthening these kinds of slabs, it is possible to enhance their strength. ⁵ The effects of using CFRP on one-way slabs are summarized below:
  1. The ultimate load-bearing capacity increases by about 24~92%.
  2. By increasing the stiffness, the deflection reduces 40~49% at service load, and 47~62% at ultimate load.
  3. Crack loads increase by about 74~88% and crack widths are reduced by about 44~76% at service load, and 86~95% at ultimate load.
  4. The concrete compressive strain at service and ultimate load stages decreases by about 8~23%, and 30~56% respectively.
  5. Covering four sides of the opening with CFRP has better results in crack load, ultimate load, crack width and deflection compared with using CFRP on only 2 sides of the opening.

Figures 2: RC Slab

Punching shear behavior of slab-column connections strengthened with CFRP strips

Five different types of orientations and configurations of CFRP strips are shown in figure 3.
Figure 3- Various CFRP Orientations in RC slab-column connection
The following results have been observed: ⁶
  1. Strengthening slabs with CFRP increases their punching shear capacity by up to 29% (figure 4).
  2. Stiffness of CFRP-strengthened slabs increases significantly; hence there will be lower amounts of deflection in the strengthened slab.
  3. Placing CFRP closer to the column results in higher stiffness, while placing CFRP at a distance from the column edge increases the punching shear capacity.
  4. The most effective orientation of CFRP strips is the diagonal strips, placed at a distance from the column edge.
  5.  It is worth mentioning that adding more CFRP strips does not necessarily lead to a higher capacity of the slab.
Figure 4: CFRP Orientations in RC slab-column connection to reduce punching shear
Parastoo Azad and Dr. Mehrtash Soltani (April 23, 2021)
  1. Marzouk, H. a. (1991). Experimental Investigation on the Behavior of High-Strength Concrete Slabs. ACI Structural Journal, 701-713.
  2. Ebead U, M. H. (2004). Fiber-reinforced polymer strengthening of two-way slabs. ACI Structures, 650-659.
  3. Tan KY, T. G. (2003). Evaluation of externally bonded CFRP systems for the strengthening of RC slabs. Department of Civil Engineering. University of Missouri, Rolla.
  4. Al-Rousan, M. I. (2012). Performance of reinforced concrete slabs strengthened with different types and configurations of CFRP. Composites Part B: Engineering.
  5. Wissam D. Salman, A. A. (2018). BEHAVIOR OF REINFORCED CONCRETE ONE-WAY SLABS STRENGTHENED BY CFRP SHEETS IN FLEXURAL ZONE. International Journal of Civil Engineering and Technology (IJCIET), 1872–1881.
  6. Khaled Soudki, A. K.-S. (2011). Strengthening of concrete slab-column connections using CFRP strips. Journal of King Saud University – Engineering Sciences.
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