Review of retrofitting techniques of deteriorated reinforced concrete columns

To ensure sustainability, all types of building structures should be kept in accepted condition throughout their life of service. Rehabilitation has been the subject of extensive research recently because of rising spending on construction and maintenance of constructed infrastructure. The most recent methods to reinforce current reinforced concrete columns, including both conventional and cutting-edge strengthening procedures, are presented in this study. Frequently, general materials used in repairing on-service reinforced concrete columns are concrete, steel, and different kind of Fiber Reinforced Polymer (FRP) composites. Conventional methods and procedures for different mode of failures strengthening, including all types of FRP procedures Near Surface Mounting (NSM) reinforcement, Externally Bonded (EB) FRP, and textile reinforced concrete will all be included in the discussion, and FRP pre-stressing methods for modifying existing structures.


Introduction
A column is a vertical structural part that typically supports compressive loads and has a length-toleast-cross-sectional-dimension ratio exceeds three [1].In certain situations, columns may have bending moments about one or both cross-sectional axes.The general behavior of columns is influenced by different parameters, including the strength of the concrete that was used, the conditions of the supports, the slenderness ratio, and the quantity of steel reinforcement that is available in both directions [2].Presence of steel fibers in columns enhances confinement and reduce cover spalling.The column is a component that carries axial stress (the weight of the base or sustaining part of the construction).The eventual collapse of the entire structure is one of the disastrous effects that might occur when columns fail.There are numerous column failure types that can be differentiated, involving compression, combined stress, buckling, and shear failure (Figure 1), failure from insufficient confinement reinforcement, torsional failures, failures caused by construction issues, and failures caused by mistakes during construction.The principal load-bearing structural elements of the structure are Reinforced Concrete (RC) columns, making them particularly sensitive to deterioration.As time goes on, RC columns need to be strengthened to withstand earthquakes, increased loads, shifting facility service types, or rising bridge traffic.Strengthening techniques are influenced by structure types and loading plans.For structures primarily subjected to static load, strengthening in axial direction as well as flexural direction should be considered, whereas it is more practical to increase shear and flexural strength for structures primarily subjected to dynamic force [4].
For RC columns, several repair and retrofit techniques, such as Carbon Fiber-Reinforced Polymer (CFRP).Because its advantageous and notable qualities (including its exceptional ratio of its strength to its weight, high level resistance of corrosion, and longevity).Many concrete columns have been repaired and reinforced using CFRP [5].A matrix made of Fiber-Reinforced Cement (FRCM), glass, aramid, or carbon fiber can typically be used to construct FRCM.Several researches have utilized Glass Fiber-Reinforced polymer (GFRP) for strengthening in the past 20 years and identified its advantages [6].Jacketing is the method that is most frequently used to reinforce construction columns.Steel jackets, reinforced concrete jackets, and composite jackets made of FRP are the three widely used forms of jackets.The purpose of this work is to present a critical review of several RC column strengthening and repairing techniques that have been put forth by various researchers over the past 20 years.

CFRP
The previous two decades, CFRP, out of all FRP composites, have been the most frequently employed for RC column strengthening and maintenance.Most studies regarding strengthened RC columns using CFRP was externally bonded.The numerous conclusions of earlier research regarding the typical behavior of CFRP-strengthened and repaired columns are covered in this section.Moreover, a comparison of CFRP wrapping's method performance to other methods is also provided.Owing to the abundance of studies on strengthening CFRP, there are three main categories for the studies.strengthening CFRP, repairing CFRP, and comparing with additional materials including CFRP.Several studies have looked at how strengthening of CFRP affects the overall performance and failure mechanisms of improved RC columns.
Ye et al. [7] observed that by adding CFRP sheets, an enhancement in the shear strength of the RC column with inadequate shear reinforcement was noticed.Also, the CFRP's mechanism of shear resistance was equivalent to that of reinforcing hoops, which began to function when concrete started to develop diagonal shear cracks.
Ma et al. [8] discovered that damaged RC columns with externally CFRP jacketing showed steady flexural reaction, enhanced flexibility, and increased dissipation of energy capacity, avoiding rigid shear failure.
El Maaddawy et al. [9] performed an intriguing experiment to test how the cross section geometry (round, square, and rectangular) affects how effectively reinforced concrete (RC) elements contained in CFRP were strengthened.Elements that had two different ratios were compressed axially and eccentrically (e/h = 0.46 and=0.60).The ductility and axial ultimate capacity rely on the cross-section geometry, as the authors observed.In comparison to rectangular elements, higher increases in axial deformation and load ultimate capacity.The effectiveness of eccentrically compressed parts is also enhanced by the CFRP strengthening, albeit to a much lesser extent.The geometry of the cross-section had little bearing on the ductility of the non-axially loaded elements, according to the authors.
Comparing rectangular portions to round and square ones, they were less ductile .
Lee et al. [6] showed that the use CFRP fabric for RC column repaired with varying degrees of rebar corrosion led to the discovery that confinement and shear strengthening rendered the repaired columns more resistant to the development of shear and bond-splitting cracks and increased their ductility.

GFRP
Because GFRP bars are more affordable than other forms of FRP materials, the building sector is becoming more interested in this material.Recently, GFRP bars have been successfully employed as primary flexural and shear reinforcement RC columns.
Tobbi et.al reported that the utilizing of GFRP reinforcement with sufficient confinement, the GFRP bars contributed by almost one-tenth of the column capacity compared to twelve percent of steel bars' contribution [10].
N Elmessalami et al. showed that at equivalent axial stiffness for FRP bars, RC columns displayed capacities lower than those of steel RC columns by a range of (7 to 8%), indicating that both types of columns had similar compressive behavior [11] .
Youm et al. [3] discovered that GFRP enhanced the seismic performance of lap-spliced columns by stabilizing the hysteresis response and increasing the displacement ductility factor to 8.3 vs. 2.6 for unretrofitted columns that collapsed early as a result of lap splice bond failure.The yielding of spliced bars was demonstrated to be unaffected by the thickness of the GFRP layer, which significantly delayed bond slip failure.
In a related analysis, Afifi et al. [12] calculated the ductility indices of RC columns with circular section reinforced with GFRP bars.They discovered a variance in range of (1.19 and 4.75) with respect to the degree of confinement and GFRP reinforcement ratio that were given.

Concrete jacketing
Lehman et al. [13] used a new poured concrete, headed reinforcement, and mechanical couplers, to restore the strength of RC columns with moderately and severe damage.Although they restored the moderately damaged columns' ductility and strength, their stiffness was not completely restored.The behavior of highly damaged specimens could not be restored using this repair method.
Liu et al. [14] suggested using one asymmetric concrete section to reinforce RC columns.Using anchor rebars or extra-strong bolts, the portion was connected to the existing part.It was found that the modified specimen's ultimate strength and ductility had significantly increased.This technique is said to reduce the difference in stress between the original part and the part that has been changed.
Ou and Troung [15] strengthened the RC columns by added flanges to the weak axis as shown in Figure 2.They showed that the repaired columns had a ductile failure mode and had greater lateral strength than original columns.Meda et al. [16] suggested to repair a corroded-damaged RC columns using fibrous RC with high performance, and it was claimed that the reconstructed column's strength had been improved somewhat.

Steel jacketing
Steel plates, angles, and batten plates or strips with various combinations typically make up steel jackets as shown in Figure 3.This method can be used where there is a restriction regarding the dimensions of the columns cross-section.It was found and proved that this method is useful for improving response of old bridge columns during seismic analysis.It can be done using two shell parts, which are then welded together in the area surrounding the column.Unfortunately, this approach necessitates challenging welding, and in the long run, corrosion may still be a concern.
Julio Garzo n-Rocaet et al. [17] reported the findings of several tests performed on strengthened columns using steel caging.It was discovered that steel caging improves the ductility and failure load of the reinforced columns.
Bsisu [18] investigated 20 steel-jacketed RC columns.All test specimens were put through axial loading that was concentric.According to the author, adding entire steel jackets to square reinforced concrete columns during retrofitting increased their compressive strength by more than twice that of the un-jacketed column.Moreover, confining RC columns with steel jackets improved their ductility.
Pasala Nagaprasad et al. [19] offered a strategy for proportioning the steel cage that took into account the concrete column's effect of confinement.The author concluded that steel caging technology can significantly improve the defective RC columns under axial and cyclic lateral stress.The suggested design method was deemed reliable and efficient.The general behavior of the column under lateral pressures was improved by carefully detailing the steel cage's end battens, which were situated in the possible zone for a plastic hinge.The wider end battens had a little impact on the energy dissipation potential but considerably improved the plastic's ability to rotate and its resistance to lateral pressures.
Choi et al. [20] found that split prefabricated steel wrapping jackets for circular RC columns improved seismic performance nearly as much as whole jackets, and were therefore more cost-and time-effective.A cable and a cross device were used to apply an external restricting pressure to the steel wrapping jacket in this investigation.

FRCM
The FRCM system as shown in Figure 4 is made up of fabric meshes with fibers arranged in two orthogonal orientations that are externally bonded to the surface of the concrete using cement-based mortars.To make it easier for the mortar to penetrate and moisten the composite fibers, as well as to strengthen the link between the two materials, fibers are frequently fashioned into fabric meshes and grids.Such a retrofitting method can be applied to enhance bending [22] and shear capacity of RC elements [23].It also provides a high level of ductility and therefore enhance their seismic performance.
Triantafillou et al. [24] examined the performance of specimens of plain concrete restrained by textile reinforced mortar (TRM) and compared it to specimens restrained by FRP jackets.Findings showed; (a) TRM jacketing significantly improves the axial compressive strength and deformation capacity and (b) TRM jackets may be slightly less efficient than their FRP jacket.
Bournas et al. [25] examined the textile reinforced mortar (TRM) system's efficiency in RC columns.Because to the longitudinal bars' potential for buckling, RC columns with restricted capacity that had been strengthened with TRM and FRP jackets underwent testing.Findings indicated that by preventing longitudinal bar buckling, TRM confining jackets significantly increase compressive strength and deformation capacity.The compressive testing results on short columns demonstrated that this method is marginally less effective regarding strength increase and deformation capacity when compared to FRP jackets of equal stiffness and strength.Testing large-scale columns subjected to cyclic uniaxial flexure, on the other hand, revealed that TRM jacketing was just as effective as FRP jackets of comparable strength and stiffness.
Basalo et al. [26] performed study on concrete cylinders that which had two different GFRP architecture types (uni and bi-directional fabric) confined within them and attached to the concrete with various types of grouts.The findings indicated that FRCM systems considerably improved the strength and deformability of the tested cylinders.
Ortlepp et al. [27] tested columns with various fillet radii and square and circular cross sections.The area close to the section corners is not adequately contained.According to experimental findings, as the jacketing system's effectiveness decreases the cross-section changes from a circle to a square.

Conclusion
This study included a comprehensive overview of several methods for reinforcing and restoring columns built of reinforced concrete.These strategies can aid in ensuring the long-term viability of existing infrastructure made of reinforced concrete by maximizing their current capacity without requiring reconstruction or replacement.Each tactic is detailed in detail, including advantages and disadvantages.An examination of the results of numerous studies shows that, even though the strength, ductility, and drift capacity of damaged columns can be restored and even improved through repair, it is very hard to get the damaged column back to its original stiffness.Moreover, the scientists predict that hybrid jacketing procedures, can leverage numerous methods of material reinforcement and restoration, will be most effective in the future based on their evaluation of various strengthening and repair processes.Overall, it was found that recovering the entire columns' resistance to axial compressions using the suggested retrofitting and repairing technique for the repair of GFRPreinforced columns that have been damaged was successful.Higher load eccentricity values were used in GFRP reinforced columns, which revealed virtually 10% a lesser capacity for strength restoration compared to columns with steel reinforcement.Underneath eccentric and concentric compression loadings, the columns' axial stiffness's could not be fully recovered by the FRP retrofitting technique that was demonstrated.Although columns reinforced with GFRP subjected to concentric loads had a 15% less weight carrying capacity than steel reinforced columns at comparable stiffness indices, columns with GFRP reinforcement are frequently most efficient because it is relatively quick to install, can significantly increase drift, ductility, and strength, and can preserve the structure's primary geometry and configuration.When investigating the axial behavior of RC columns enclosed in FRCM jackets, the geometric configuration, transverse reinforcement ratio, and number of fiber layers were considered as important variables.The cross-section has a significant impact on the results of the fractured pattern produced by the specimens.The highest performance for cylinder columns is shown by the confined compressive strength fcc, which grows in proportion to the number of layers.According to the strain records, the FRCM-layer jacket's count has a favorable impact on ductility.The stirrups and fibers show an interaction between the FRCM-jacket and internal transverse reinforcement.Consequently, based on the measured strain values, the lateral pressure applied by the two confining mechanisms is calculated.To determine how much the two confining methods contribute to increased strength and ductility, however, more research is required.

2 Figure 1 .
Figure 1.Some types of column failures [ 3 ] .The principal load-bearing structural elements of the structure are Reinforced Concrete (RC) columns, making them particularly sensitive to deterioration.As time goes on, RC columns need to be strengthened to withstand earthquakes, increased loads, shifting facility service types, or rising bridge traffic.Strengthening techniques are influenced by structure types and loading plans.For structures primarily subjected to static load, strengthening in axial direction as well as flexural direction should be considered, whereas it is more practical to increase shear and flexural strength for structures primarily subjected to dynamic force[4].For RC columns, several repair and retrofit techniques, such as Carbon Fiber-Reinforced Polymer (CFRP).Because its advantageous and notable qualities (including its exceptional ratio of its strength to its weight, high level resistance of corrosion, and longevity).Many concrete columns have been repaired and reinforced using CFRP[5].A matrix made of Fiber-Reinforced Cement (FRCM), glass, aramid, or carbon fiber can typically be used to construct FRCM.Several researches have utilized Glass Fiber-Reinforced polymer (GFRP) for strengthening in the past 20 years and identified its advantages[6].Jacketing is the method that is most frequently used to reinforce construction columns.Steel jackets, reinforced concrete jackets, and composite jackets made of FRP are the three widely used forms of jackets.The purpose of this work is to present a critical review of several RC column strengthening and repairing techniques that have been put forth by various researchers over the past 20 years.