CFRP Strengthening of Reinforced Concrete Arches

Introduction

Reinforced concrete arches can be found in a variety of structures, but the main usage of them has been in bridges to allow for larger spans in construction. The main cause of bridge arch deterioration, in the form of cracks and corrosion, is bending moments, which are imposed on an arch because of non-symmetrical actions such as:

High levels of non-uniform traffic jams
Settlement of the foundation and pier
Earthquake
Construction deficiencies
Fatigue under repeated loading
Environmental effects

To increase the service life of arch bridges and getting them to the required safety level of serviceability, strengthening them is of great importance.

Figure 1: Components of an arch

Strengthening of RC Arches:

One of the main techniques of arch strengthening has been the usage of externally bonded CFRP sheets. However, “CFRP debonding” has been a major problem of this technique which results in a significant decrease in the CFRP’s strengthening effects. Using the wrapping or bonding/anchoring methods can help prevent CFRP debonding. ¹
Different strengthening schemes have various strengthening effects. Here is the classification of strengthening schemes of RC arches:

Bonding method (Intrados or extrados bonding & I-style)
Wrapping method (U-style & O-style)
Bonding/anchoring method

Bonding method:
In the bonding method, CFRP sheets are applied on the intrados or extrados of the arch. The I-style scheme is a bonding method, but it’s effectiveness is limited as its strength improvement is only 7.8%.
If the arch is located underground, CFRP strips/sheets can only be installed on the intrados of the arch.

Figure 2: CFRP strengthening of arch – bonding method (intrados bonding)

Wrapping method:

The wrapping method is an excellent solution to limit the debonding issue of the “bonding method”.
In the wrapping method, CFRP strips wrap around the cross-section of the arch.
In the U-style scheme, the intrados and the lateral surfaces of the arch are strengthened by CFRP (Figure 3). CFRP in the web of the arch will not only increase the shear resistance of the arch but will also prevent debonding of the CFRP strips installed on the intrados.
O-style is another scheme of the wrapping method in which the arch’s cross-section is entirely wrapped by CFRP strips (Figure 4). The O-style scheme is three-dimensionally constrained, its ductility is more than other schemes and it is the most efficient method in arch strengthening.
The O-style strengthening scheme can increase the peak load of an RC arch by about 30.7%. Debonding was not observed before material failure of the CFRP.
The ultimate load using the O-style scheme is five times higher than the un-strengthened arch.
The ultimate load using the O-style scheme is 3 times higher than I-style and U-style.
The strengthening effect of GFRP in arches is much smaller than CFRP’s effects. ²

Figure 3: CFRP strengthening of arch – wrapping method (U-style bonding)

Figure 4: CFRP strengthening of arch – wrapping method (O-style bonding)

Bonding/anchoring method:
In underground arches where the wrapping method is not applicable, to eliminate the debonding problem of CFRP strips installed on the intrados, the bonding/anchoring method is the best solution. In this method, mechanical fasteners are installed on the intrados CFRP strips/sheets, which prevent CFRP debonding.

Figure 5: CFRP strengthening of arch – bonding/anchoring method

Facts about CFRP strengthening of RC arches:

The advantages of CFRP in strengthening arches depend on the arch material, the strengthening scheme, and the arch’s rise-to-span ratio. ³
In the bonding method, the efficiency of CFRP is less than 20%. ³
In the bonding/wrapping method, 40% of the CFRP strength will be utilized. ³
Only increasing the strength and reinforcement ratio of the CFRP alone cannot improve its efficiency since the debonding phenomenon greatly limits the contribution of the CFRP’s strengthening effects.
In concrete arches (without steel reinforcement), CFRP strengthening techniques can increase the load-bearing capacity four times higher than the un-strengthened arch. The improvement is so remarkable that the strengthened concrete arch can be compared with the strengthened RC arch in terms of strength.
CFRP-strengthening schemes are efficient and improve the ductility and cracking load of RC arches.

Authors
Parastoo Azad and Dr. Mehrtash Soltani (July 19, 2021)
References

ngChen, W. X. (2015). Blast-loaded behaviors of severely damaged buried arch repaired by anchored CFRP strips. Composite Structures.
2. ZZ., T. (2011). The study on performance of concrete arch reinforced with GFRP (Dissertation). Chongqing: Chongqing Jiaotong University.
Xu Zhang, P. W. (2015). CFRP strengthening reinforced concrete arches: Strengthening methods and experimental studies. Composite Structures.