Behaviour of reinforced concrete corbels strengthened by FRP composites and steel bars: a critical review

In reinforced concrete (RC) structures, corbels that extend from the faces of reinforced concrete columns are commonly employed to support primary beams and girders. Frequently, the use of corbels is limited to cantilevers with a shear span-depth ratio that is often less than 2. Because of this relatively low ratio, corbel strength is normally dominated by shear, which is comparable to deep beams. However, owing to some reasons (e.g., design and/or construction faults, modification of facility use, deterioration, and aging), strengthening of existing corbels is becoming of prime importance. The aim of the present paper is to conduct a critical review of commonly proposed techniques that have been implemented to strengthen RC corbels. These techniques comprised near surface mounted (NSM) steel bars, NSM fibre reinforced polymer (FRP) bars, and FRP sheets. The latter strategy included carbon (CFRP) and glass (GFRP) sheets. Hybrid strengthening (combining FRP sheets and FRP/steel bars) was also highlighted in this paper. The strengthened (upgraded) RC corbels were evaluated considering several parameters, including shear span to depth ratio (a/d), quantity and configuration of reinforcement, FRP type, and levels of damage in the tested specimens.


Introduction
Corbels can be considered as short cantilevers that typically perform as deep beams or simple trusses [1].Pre-cast structural members, such as pre-cast beams and pre-stressed beams, can be supported by corbels or brackets.Corbels or brackets, which are normally cast with the column or wall members, have become a standard feature in the construction of buildings.Corbels are primarily intended to withstand the beam's ultimate shear force as well as the ultimate horizontal action caused by beam shrinking.Figure 1 shows a sample of a reinforced concrete (RC) corbel.Typically, there are six basic ways that the structural failure of corbels is related, as shown in figure 2. These are: concrete crushed on the strut, bending, shearing at the point where the corbel meets the column, loss of the main tie reinforcement's anchorage, crushing of concrete under the bearing plate, and the horizontal load [2].Many research studies have highlighted the behaviour and reaction of reinforced concrete corbels.Such research examined the most influencing parameters, which includes a/d ratio, shear and longitudinal reinforcement ratios, concrete's compressive strength, and the type of stress.It was demonstrated that for corbels with the same a/d ratio, increasing the quantity of shear reinforcement (horizontal stirrup bars) plays an essential role in enhancing the ultimate cracking and loads [3].In this regard, by adding secondary reinforcement, the stiffness of the corbel is enhanced, the width of the crack is decreased, and the ductility is also enhanced [4].As stated by Mattock et al. [5], the brittle failure of RC corbels was avoided by using a limited number of horizontal stirrups.Such stirrups must be consistently arranged within two thirds of the effective depth of the corbel section, near the primary steel reinforcement.It is worth mentioning that shear friction (SF) can be used to design corbels that have an a/d equal to or less than 1.In addition, it was concluded that a more precise prediction of ultimate load and behaviour was attained by using the STM technique as compared to the shear-friction technique [6].On the other hand, the strut and tie method (STM) is utilise to designing corbels with an a/d less than two [1].On this basis, Abdul-Razzaq and Dawood [7] conducted experimental testing on RC corbels differing in shear span to effective depth ratios (a/d).They analysed the tested specimens using both STM and SF methods and concluded that both methods could successfully be adopted in the case of a/d <1.However, when a/d was between 1 and 2, STM was more precise than SF.Here, the former method was found to be more conservative because of its safety aspects.Furthermore, when a/d is more than 2, the corbel has to be dealt with as a standard cantilever beam.It is worth mentioning that the structural inferiority could take place in reinforced concrete corbels in the same manner as other structural members.This has arisen for several reasons, such as changing the usage of the building and/or its ability to carry greater loads, steel and concrete deterioration, and mistakes that occurred during the planning or construction stages.Therefore, it is necessary to strengthen or repair the deteriorated structure prior to the failure taking place.

Research significance
The damage to RC structures have be repaired prior the failure affects the constructed members.In the civil engineering, repairing is of prime importance to mitigate the inferiority of aged, damaged, deteriorated reinforced concrete.The methods for strengthening and repairing the structures are dependent on their state of damage.Thanks to modern technology, new composite materials have been developed to improve the behaviour of RC structures by improving their flexural and shear strengths [8].Strengthening with advanced composite materials, namely, fibre reinforced polymer (FRP) bars and laminates, in addition to steel bars, has become a widely used strategy in the upgrade and rehabilitation of reinforced concrete structures.In this regard, these strategies have been found to have a positive impact on strengthening reinforced concrete corbels, which will be discussed in detail in the following sections.

Strengthening of RC corbels using near surface mounted
During the past few decades, a number of research works have shed light on strengthening RC corbels by implementing near-surface mounted (NSM) techniques with CFRP and steel bars.Shakir and Kamonna [9] have conducted experimental tests to investigate the behaviour of high strength reinforced self-compacting concrete (SCC) corbels strengthened using the NSM steel bar technique (figure 3).The findings have revealed that the failure load of the RC corbel was significantly improved by about 57 % for a/d ratio of 0.85 and 41% for those of 1.25, by the strengthening with NSM steel bars method.The results have also discovered that for small a/d ratios (a/d less than 1), the V shaped configuration was more efficient, while for large a/d (a/d more than 1), the strengthening with horizontal bars configuration was more effective.Shakir and Abdlsaheb [10] have investigated the effectiveness of strengthening the partially damaged corbels made of high-strength SCC by using CFRP sheets and NSM steel bars (figure 4).The specimens were strengthened by using different configurations and alignments with two levels of damage.Results have shown that within the early damage levels, the NSM steel bars method could be used as a suitable technique, while for the largely damaged corbels, the externally bonded CFRP sheet was more adequate.It has also been discovered that the configuration of the repair has a substantial impact on the capacity of the NSM steel bars, whereas this impact was lower for externally bonded CFRP sheets because of the effectiveness of different material, as shown in figure 5.  Another experimental study [12] was conducted to examine the behaviour of reactive powder concrete (RPC) corbels, strengthened by varying orientations (inclined and horizontal) of NSM carbon fibre reinforcement polymer (CFRP) strips.The findings demonstrated that the ultimate load and stiffness of strengthened reinforced RPC corbels were considerably improved and enhanced, compared with control specimens (without RPC).Results have also shown that the mode of failure of the strengthened concrete corbels can change from brittle to more ductile when RPC is incorporated.Shakir and Abdlsaheb [13] have investigated the competence of repairing the partially damaged corbels made of high-strength SCC by using CFRP sheets and NSM steel bars.Results have shown that within the early damage levels, the NSM steel bars method could be used as a suitable technique, while for the largely damaged corbels, the externally bonded (EB) CFRP sheet was more adequate.It has also been discovered that the configuration of the repair substantially impact on the behaviour of the NSM steel bars, whereas this impact is lower for externally bonded CFRP sheets.

Externally bonded strengthening of RC corbels
Externally bonded strengthening techniques, in the shape of laminates or fabrics, have been employed to strengthen most of the RC structure's elements, such as beams, columns, and slabs.However, studies dedicated to examine the performance of strengthened RC corbels (using FRP laminates or fabrics) are still limited.In this regard, an experimental research was conducted by Attiya and Ali [14] who stated that the behaviour and carrying capacity of RC corbels improved significantly when they were strengthened with externally bonded CFRP.Similar tendency (figure 6) was also observed by Mohammed and Hassan [15].The behaviour of RC corbels strengthened with CFRP fabrics was also investigated by Randhavan et al. [16].The corbels were designed using the strut and tie method suggested by the ACI code.Experimental findings have shown that the behaviour of corbels were improved by strengthening with inclined and horizontal CFRP sheets.It has also been shown that the strengthening achieved by unidirectional full wrapping horizontal CFRP sheets was considered the best configuration compared with that of inclined CFRP fabrics (45°).Souza et al. [17] suggested a strengthening technique that included a localised repair using carbon fibre reinforced polymer (CFRP) 6 sheets.The suggested technique allowed the concrete zone that was at risk of spalling to be contained with an acceptable level of safety.Comparing to the other strengthening methods, such as section enlargement with concrete jacketing, the proposed methodology was quick, dependable, useful, and less expensive.Assih et al. [8] investigated the theoretical and experimental response for repairing damaged RC corbels by gluing carbon fibre fabrics.Five RC corbels were investigated to failure.According to the obtained results, composite fibre fabrics with bonding techniques could be an effective strategy for concrete structures.Al-Fadhli [18] investigated the effect of using hybrid reinforcement (a combination of steel and CFRP bars) on RC corbels, in addition to use of CFRP sheets for external corbel reinforcement.It was discovered that when both CFRP sheets and bars were used, the corbels' ultimate strength could be increased by 50%, whereas an improvement of about 30% could be reached when CFRP bars or CFRP sheets were individually used.Abdulqader et al. [19] investigated the behaviour of SCC corbels externally strengthened by CFRP strips.Test results have shown that for strengthened and unstrengthened corbels, decreasing the a/d ratio increased the ultimate and cracking loads with the same horizontal reinforcement stirrups.While, at the same a/d ratio, increasing horizontal secondary reinforcement stirrups increased both ultimate and cracking loads.It was also found that the ultimate load was increased by 50% by repairing RC corbels with external CFRP strips.In this concept, the techniques of wrapping CFRP increased the energy absorption of RC corbels, as reported in [20].However, a slight improvement (7%-13%) in failure load was observed in CFRP upgraded RC corbels [21].
El-Maaddawy and Sharif [22] reported that the ultimate load of RC corbel was improved by up to 40% by using external CFRP composite reinforcement.However, the contribution of such reinforcement to load carrying capacity decreased when internal steel bars were included in the tested RC corbels.In this study, it was also shown that the ultimate loads were increased by the additional CFRP fabrics parallel to the primary steel bars, whereas there was no additional strength gain when the CFRP sheets were included at the mid height of the corbels.Further, it was found that the propagation of shear cracks was restricted by the diagonal CFRP sheets and that the gain in load capacity was increased.In this regard, additional strength (27%) in RC corbel specimens, externally strengthened with CFRP sheets, was gained by adding secondary steel bar reinforcement at the mid height of the tested specimens [23].
On the other hand, the applicability of using epoxy bonded glass fibre reinforced polymer (GFRP) overlays in strengthening RC corbels was also investigated [24], as shown in figure 7. The a/d ratio, reinforcement ratio, and the quantity and direction of the GFRP fibres were all considered as tested variables.Test results have shown that GFRP overlays can be utilised to increase corbel load bearing capability.It was also found that the two main factors influencing the reached load capacity in the corbels were the main reinforcement ratio and the amount of GFRP sheets.

Conclusion
Based on the above reviewed literature, the following conclusions can be drawn: 1.The efficiency of RC corbels repaired with NSM steel bars is dominated by strengthening configurations and the span to depth ratio (a/d).For small a/d ratios (less than 1), the V-shaped configurations were more effective, while for large a/d (exceeding 1), the strengthening with horizontal bars configuration was more effective.2. The effectiveness of repairing RC corbel specimens by using CFRP sheets and NSM steel bars is highly dependent on the levels of damage in the tested specimens.At the early damage levels, the NSM steel bars method could be used as a suitable technique, whereas for the largely damaged corbels, the externally bonded CFRP sheet was more adequate.3.With regard to the behaviour of corbel specimens strengthened with NSM carbon fibre reinforced polymer (CFRP), it was revealed that the specimens having inclined NSM-CRFP bars (at 45°) demonstrated the greatest ultimate load capacity compared with other strengthening configurations.4. The implication of reactive powder concrete (RPC) in RC corbels, strengthened with NSM-CRFP, considerably improved their load capacities and stiffness, as well as changing their behaviour from brittle failure to more ductile failure. 5. Comparing to the other strengthening methods (e.g., section enlargement with concrete jacketing), strengthening of RC corbels by CFRP sheet techniques could be an effective strategy at different configurations, although the strengthening by horizontal CFRP sheets is considered the best configuration compared with that of inclined CFRP fabrics (45°).6. Combining CFRP bars and CFRP sheets to strengthen RC corbels (i.e., hybrid strengthening) increases the ultimate strength by 50%, whereas an enhancement of 30% can be reached when CFRP sheets or CFRP bars are separately implemented.
7. The contribution of CFRP composite strengthening to the load carrying capacity of RC corbels decreases when additional internal steel bars are included.Contrary, when reinforced steel bars were externally imbedded to the specimens, additional strength in RC corbel specimens was considerably gained.8. Epoxy bonded glass fibre reinforced polymer (GFRP) overlays can be successfully utilised to increase corbel load bearing capability.The two main factors influencing the degree of strength gain in the corbel specimens were the main reinforcement ratio and the amount of GFRP sheets.