Strengthening of RC columns under eccentric loads using NSM steel and GFRP bars

The behavior of eccentrically loaded RC columns strengthened by NSM steel and glass fiber reinforced polymer (GFRP) rebars is investigated in this study. A total of six RC column specimens have been cast in this study with dimensions of (150×150×1200) mm (width × height × length). Two column specimens are unstrengthened and considered as controls, while the other specimens are strengthened by the NSM method using steel bars and hybrid steel and GFRP rebars. All column specimens have been tested under concentric and eccentric loads with an eccentricity of 75mm up to failure. Experimental results in terms of crack patterns, modes of failure, and load-displacement curves are all monitored, recorded, and assessed. The experimental results have shown an improvement in the ultimate load capacity for all strengthened column specimens. Further, results have demonstrated that, under concentric load, the almost similar enhancement in the strength of NSM-strengthened RC column specimens using steel bars only and hybrid steel and GFRP bars. However, at eccentric load, the ultimate load capacity of the strengthened RC column was increased by 62% and 79.45% compared to the control column using NSM steel and hybrid steel, and GFRP, respectively. Finally, this study found that at e/h = 0.5 using GFRP in the tension zone increased the ultimate load by about 11% when compared to steel.


Introduction
Reinforced concrete columns are crucial components of buildings.Therefore, the building fails as a result of its deterioration.So, it is crucial to strengthen these components.In recent years, the utilization of fiber-reinforced polymers (FRP) to strengthen existing RC columns has become more common, and the results are very promising.Using the technique of near-surface mounted (NSM) FRP bars, RC columns' flexural capacity can be enhanced.In this strengthening approach, FRP bars are placed into concrete surface grooves and bonded there with adhesive resin.In comparison to the externally bonded (EB) technique, NSM is a quick installation process and does not require surface preparation work [1][2][3][4][5].GFRP bars have high tensile strength, but their compressive strength is negligible [6].Also, GFRP has the ability to withstand high temperatures [7].
Bournas and Triantafillou [8] have presented an experimental study to evaluate the flexural capacity of eleven RC columns strengthened by the near-surface mounted (NSM) technique using GFRP bars, CFRP strips, and stainless steel rebars.A constant axial load and lateral cyclic loading were applied to all column specimens.The experimental tests have shown that GFRP bars, CFRP strips, and stainless steel strengthening enhanced the flexural resistance of RC columns, while the stainless rebars showed more increase in stiffness and dissipated energy.Also, the NSM GFRP rebars and CFRP strips were equal in strength, but GFRP showed a slightly superior deformation capacity.Moreover, results have revealed that the jacketing at the ends of columns provided lateral resistance against the buckling in the strips and stainless steel bars, which also increased the deformation capacity.Furthermore, test results have demonstrated that the epoxy resin had a higher strength than mortar.Finally, the best specimen was that which had the combination of stainless steel bars, jacketing, and epoxy-bonded.Etman [9] has conducted experimental tests to study the performance of RC columns strengthened by the near-surface mounted (NSM) technique using either longitudinal steel bars or carbon fiber reinforced polymer (CFRP).Twenty-seven RC columns have been cast with cross-section dimensions of (100 × 200) mm and a total high of 1600 mm.The RC columns were reinforced with three ratios for longitudinal steel bars which are 1%, 1.57%, and 2.26% respectively.All columns were subjected to eccentrically loaded with a constant eccentricity ratio of 0.25.Test results have revealed that the strengthening with NSM significantly improved the ultimate load capacity of slender columns, especially group three, which was strengthened by NSM steel bars confined partially with CFRP sheets and demonstrated a higher increase in ultimate load than group two.Moreover, the ductility was improved for slender columns strengthened with NSM rather than control specimens and it showed significantly improved in group two.El-Maaddawy and El-Dieb [10] have presented an experimental work to examine the efficiency of NSM-GFRP rebars confined with CFRP sheets to enhance the behavior of repaired and strengthened nine RC columns under axial load and biaxial bending.Two groups were tested under an eccentric load with an eccentricity ratio of 0.75, while the third group was tested with an eccentricity ratio of 1.0 at the direction of the principal axis.Test results have revealed that the level of CFRP confinement and the eccentricity of the load had a significant effect on the efficiency of the NSM-GFRP strengthening.Further, the increase in load capacity was greater at a lower eccentricity for columns with a high level of CFRP confinement.In addition, the increase in load capacity was greater for columns with a low level of CFRP confinement at a higher eccentricity.Nabil et al. [11] have conducted experimental tests to investigate the performance of concrete columns strengthened by the NSM method, externally bonded GFRP confinement and a combination of both.Sixteen circular concrete columns were subjected to concentric axial load up to failure.Experimental test results showed a significant increase in the loadcarrying capacity of strengthened concrete columns, and the best results were obtained when using only four layers of GFRP jackets than using NSM and four layers of GFRP jackets.This may be due to the NSM steel bars yielding and buckling.Sarafraz [12] presented an experimental study to evaluate the flexural capacity of RC columns strengthened with GFRP bars by using the near-surface mounted (NSM) technique with a low longitudinal reinforcement ratio.A total of four column specimens with square cross-section (200 × 200) mm and a total length of 1000 mm.All column specimens were subjected to a combined axial compressive load and lateral cyclic displacement.The experimental results have demonstrated that the lateral load capacity of the strengthened concrete columns increases with increasing the diameter of GFRP.It is also found that the NSM strengthening method enhances the flexural strength of RC columns.Moreover, the maximum measured strain at the GFRP bars was found to be below the ultimate strain because a bond failure occurred between GFRP bars and epoxy paste due to the cracking of the epoxy paste.Therefore, it was concluded that the interface behavior between the GFRP bar-epoxy paste and the epoxy-coated concrete surface affects how the retrofitted specimen performs.Al-Naqeeb and Al-Thairy [13] have investigated experimental work to examine the behavior of normal and lightweight RC columns with different values of temperature under eccentric load.Six RC columns with a square cross-section (150mm×150mm) with a high of 1250 mm were tested.It has been determined from the results that all column specimens collapsed in flexural buckling with the crushing of concrete on the compression side.Saadoon [14] presented an experimental work to study the behavior of eight RC deep beams strengthened with CFRP strips.all beams were created and put through testing using a four-point loading scenario.The RC deep beams were strengthened using CFRP strips in three different orientations: inclined, horizontal, and vertical.It has been proven that FRP strips strengthening with vertical fabric direction are more effective than strengthening with inclined or horizontal direction because the vertical direction offers the greatest load-bearing capacity, thinnest crack width, and greatest deflections at ultimate load.In addition, applying the FRP strips horizontally proved insufficient for the strengthening objectives.
It can be noted from the above-mentioned review that most of the previous research studies have only considered FRP composites in strengthening the RC column due to their high tensile strength.However, using hybrid NSM GFRP and steel in the strengthening of the columns has not been investigated yes.In fact, using steel bars in a compression zone surpasses the FRP bars due to the high compressive properties of the steel compared to FRP.

Experimental program
An experimental program was conducted to investigate the behavior of RC columns strengthened by the near surface-mounted (NSM) GFRP and steel rebars under eccentric and concentric axial loads.The geometrical, reinforcing, and properties of the material that utilizing in RC column specimens, as well as the application of the NSM technique, are presented in the following sections .

Column specimens
The experimental program involved the design, casting, curing, and testing of six reinforced concrete columns with 150 mm × 150 mm square cross sections, a total length of 1200 mm, and a 720 mm distance between corbels.The columns were constructed with corbels at their top and bottom ends.by ACI Code 318 [15] requirements to prevent the local collapse of the concrete's bearings or crushing due to stress concentration under the applied compressive load.The dimensions of the corbel zones were (275×150) mm (width × depth).All column specimens can be categorized as long or slender columns depending on the column's slenderness ratio which is kl/r =27.713 is more than 22, as recommended by ACI 318 [15], at which the P-δ has a significant impact on the column's behavior and failure when subjected to an axial compression load.Where l is the unsupported length of the column, k is the effective length factored, and r is the radius of gyration of a cross-section.The columns were reinforced with 4Ø10 mm longitudinal steel bars located at each corner with a cover of 25 mm and Ø8 mm stirrups at a spacing of 125 mm c/c.The specimens consist of six columns, with the designations, geometrical reinforcement, and strengthening details shown in Table 1 and Figure 1.The columns 2S2Se0 and 2S2Se0.5 were strengthened by 2Ø10 mm steel bars inserted on the specimen's two sides, while the specimens 2G2Se0 and 2G2Se0.5 were strengthened by 2Ø10 mm steel bars in the compression side and 2Ø10 mm of GFRP rebars in the tension side.All columns were tested under an axial compressive load with two eccentricity ratios of 0 and 0.5.
Table 1.Details of column specimens considered in the study.

Concrete mix
Three trail mix proportions, each with three cubes, were prepared, cast, cured, and tested as shown in Table 2.Each cube had a dimension of (150×150×150) mm.The cubes were de-molded and submerged in water after 24 hours.Table 2 illustrates the specifics of the trail mix and the average compressive strength results for each mix.The second mix has been chosen that gives compressive strength of approximately 45 MPa.

Portland cement
The cement used was ordinary Portland cement named AL-GISSER, which is readily available in the local market.In the Al-Sebtayn construction engineering laboratory in the Diwaniyah governorate, physical and chemical testing was performed.Physical and chemical properties tests show that the used cement conforms to the Iraqi specification requirements (ASTM-C150M-2019).

Fine and coarse aggregate
In this study, naturally coarse and fine aggregates were used.In Al-Sebtayn construction engineering laboratory in the Diwaniyah governorate, both types of aggregate were tested for physical, chemical, and sieve analytical tests.These test results satisfied the Iraqi Specification's standards (IQS NO. 45, 1984).

Superplasticizer
For increased workability and improved compressive strength, a superplasticizer known as a waterreducing admixture by the trade name Hyperplast PC175 was added to the concrete mixtures.The superplasticizer complies with ASTM C-494/C494M types F and G.

GFRP rebars
For NSM strengthening RC column specimens, glass fiber reinforced polymer (GFRP) rebar with a diameter of 10 mm was employed in tension zones.Table 3 below lists the properties of GFRP according to the datasheet provided by the manufacturer.

Steel bars
Three different diameters of steel bars 8mm, 10mm, and 16mm were utilized to reinforce the column specimens in both the main and transverse directions, in addition to its use in the NSM process.The uniaxial tensile tests have been carried out to find the yield and ultimate stress using a tensile testing machine at the laboratory of the College of Engineering at Al-Qadisiyah University.Table 4 summarizes the findings according to ASTM C370a.

Casting and curing column specimens
Casting was conducted on six cylinders, six cubes, and six column specimens, and then cured in water for 28 days, (see Figure 2).The cylinder and cubes were cast for each mixture to find the splitting tensile strength and compressive strength, respectively.The corresponding average tensile and compressive strengths were 5.7 MPa and 45 MPa, respectively.All column specimens were strengthened except two columns that were unstrengthened and considered as reference columns, which are designed CRe0 and CRe0.5.To apply the NSM technique, an electrical grinder was utilized to cut two grooves into the column's surface tension and compression sides.The dimensions of all grooves were (20 × 20) mm (width× depth), respectively according to American Concrete Institute Technical Committee 440-2R [13].Then, the grooves were cleaned with a brush, washed with water, and left to dry.After that, the NSM steel bars with diameters of 10 mm were embedded into the grooves at each side of the 2S2Se0 and 2S2Se0.5 specimens.However, in the remaining column specimens, the NSM steel bars were embedded in compression sides only and NSM GFRP bars were placed at the tension zone of the column section, as explained in Table 1.Epoxy Sikadur-31/41 CF Slow was used as a bonding material between the concrete grooves and NSM bars after mixing it with an electric mixer.After filling up the grooves with the first epoxy primer layer and placing NSM bars, a final epoxy layer is added and carefully sanded with a trowel.The steps of application of the NSM technique are demonstrated in Figure 3.

Test Setup
All column specimens were tested under axial compressive concentric and eccentric loading until failure.In the current investigation, two values of the eccentricity ratio (e/h=0 and e/h=0.5)were employed to examine the behavior of the RC columns under eccentric and concentric loads.Two bearing steel plates with dimensions of (150×150×20) mm were placed at the column's top and bottom ends to prevent stress condensation and distribution appropriately on the surface of the column, which acts as simply supports.To obtain the needed eccentricity value, the bearing steel plate's size and placement were chosen so that the distance between its center lines and the column section equals the desired eccentricity.see Figure 4. (b) e/h=0.5.A hydraulic pressure gauge with a maximum capacity of 1500 kN was utilized to measure the static load, which was applied to the surface of the bottom end of the columns as seen in Figure 5.Many data were recorded and monitored during the whole test, including the load at the initial crack, crack width, axial displacement, lateral displacement, and the ultimate load.

Results and discussion:
The major objective of this part is to present and discuss the experimental test results regarding the performance of RC columns strengthened with NSM hybrid steel and GFRP bars compared with those strengthened with steel only and un-strengthened columns.Also, this section discusses and examined the impact of the eccentricity ratio on the modes of failure and crack pattern of the NSM GFRP bars strengthened RC columns.Table 6 summarizes the results of all the RC columns, including failure mode.Table 6.Results of the experimental tests of the RC column specimens. Column

Crack patterns and failure modes
The crack patterns and modes of failure of the axially compressed RC columns strengthened by NSM steel or hybrid steel and GFRP bars along with the control specimen are demonstrated in Figure 6.For the control specimen (CRe0), the initial crack appeared at a load level of 272.5 kN.Afterward, more cracks began to appear at the corbel under the bearing plate as the axial load increased, and extended longitudinally in the orientation of the applied load as seen in Figure 6.However, the initial cracks appeared in column specimens 2S2Se0 and 2G2Se0 at load values of 329.18kN and 283.75kN, respectively.These load values are considerably higher than those of columns under eccentric load.In general, the strengthening with NSM rebars delays the appearance of initial cracks, but the 2S2Se0 specimen gives greater increases in the first visible crack than 2G2Se0 because steel bars have high compressive strength.The failure of the control specimen (CRe0.5)occurred gradually due to the buckling of the steel reinforcement, as demonstrated in Figure 6(a).Moreover, this figure also illustrates the failure mode of the 2S2Se0 column is the same as that in the 2G2Se0 column, which occurred by crushing of concrete at un-strengthened sides combined with de-bonding between concrete and adhesive resin in the tension zone.

Load -displacement relationships
The load-axial displacement curves of the RC column specimens and the load-lateral displacement curves strengthened by NSM steel or hybrid steel and GFRP bars with a comparison with those of the control specimen are shown in Figures 7a and 7b.These figures have indicated that the increase in the ultimate load capacity of the strengthened RC specimens compared with the control specimen is very close, at 12.5% and 13.5% for 2S2Se0 and 2G2Se0, respectively.However, it can be noted from Figure 7a-b that the increase in axial and lateral displacements are about (22% and 12.6%) and (5.8% and 54%) respectively, for 2S2Se0 and 2G2Se0, respectively compared with the control specimen.Further, Figure (7) illustrates the three stages of the performance of the relationship between the load and axial or lateral displacement.The initial stage of the elastic phase is represented by the first straight portion of the load-displacement curve.While the second stage (elastic-plastic stage) is a nonlinear portion with a clear change in slope with growing lateral displacements.The third stage is a nonlinear portion with a little increase in load that causes greater lateral displacements (representing the plastic stage).3.2.Eccentrically loaded RC column at (e/h=0.5)

Crack patterns and failure mode
The crack patterns and modes of failure of three RC column specimens tested under eccentric load at e/h=0.5 are illustrated in Figure 8. From this figure, it is shown that the number of cracks in the specimens increased while the crack width decreased compared with the columns subjected to concentric load.For eccentrically loaded columns, the initial cracks started to show up at loads of 90.8kN, 136.2kN, and 138.2kN for CRe0.5, 2S2Se0.5, and 2G2Se0.5,respectively.When the axial load increased, cracks began to appear on the tension side of the middle section of the column (the region between the corbels) and eventually spread to the compression side.The failure mode of all these RC column specimens happened as a result of concrete crushing in the compression zone.

Load -displacement relationships
Figures 9a and 9b demonstrated the load-axial displacement curves of the RC column specimens and the load-lateral displacement curves specimens strengthened by NSM steel or hybrid steel and GFRP bars in a comparison with the control specimen which tested under eccentric load.These figures have demonstrated that the ultimate load capacity increased by 62% and 79.45% for 2S2Se0.5 and 2G2Se0.5,respectively compared with the control specimen.Further, specimen 2G2Se0.5 gives an increase of 11% higher ultimate load capacity than 2S2Se0.5.This difference can be related to the high tensile strength of GFRP compared with that of steel, which is clearer when the load is eccentric.Also, the axial and lateral displacements increased by 7% and 38.8% for 2S2Se0.5 and 6.67% and 32% for 2G2Se0.5,respectively.

Conclusion
This paper investigated the behavior and modes of failure of strengthened RC columns using NSM steel and hybrid GFRP and steel rebars.Six RC column specimens tested under concentric and eccentric loads with eccentricity ratio e/h= 0.5 were considered in the study.The experimental testing considers two parameters, including the eccentricity ratio and the kind of NSM bars (steel or hybrid steel and GFRP).From the current investigation, the following conclusion can be listed: 1. NSM is a very effective technique for improving the load-bearing capacity of RC columns, demonstrating that this technique can be used to strengthen and rehabilitate RC columns to prevent their deterioration.Results have shown increases in the ultimate load up to 79.45% of the NSM bars strengthened RC column compared with the control specimen.Also, strengthening with NSM rebars was found to delay the appearance of initial cracks.2. As compared to the strengthened specimen, the increase in ultimate load for the concentrically loaded (e/h=0) RC column specimens enhanced with steel alone and with hybrid steel and GFRP is very close.3.For the eccentrically loaded RC column, the ultimate load of the NSM strengthened columns using NSM steel and hybrid steel and GFRP increased by 62% and 79.45%, respectively, compared with the control specimen.That means that using GFRP in the tension zone enhances the ultimate load by about 11% higher than steel, which can be related to the high tensile strength of GFRP, which is clearer when the load is eccentric.In addition, there is an increase in the number of cracks with a decreasing crack width for the eccentrically loaded RC column.

Figure 1 .
(a) A side view of the columns (b) Details of reinforcement (c) Cross-section of specimens.

a b Figure 2 .
RC column specimens (a) during curing time (b) after casting.

Figure 3 .
Figure 3.The procedure of the NSM technique.

Figure 4 .
Figure 4. Location of applying loads with different eccentricities (all dimensions in mm).(a) e/h=0(b) e/h=0.5.A hydraulic pressure gauge with a maximum capacity of 1500 kN was utilized to measure the static load, which was applied to the surface of the bottom end of the columns as seen in Figure5.Many data were recorded and monitored during the whole test, including the load at the initial crack, crack width, axial displacement, lateral displacement, and the ultimate load.

Figure 5 .
Figure 5. Experimental device and test setup used in the current investigation.

Figure 6 .Figure 7 .
Figure 6.The failure mode of the RC column tested under a concentric load.

Figure 8 .Figure 9 .
Figure 8.The failure mode of the RC column tested under eccentric load.

Table 2 .
Trial mixture proportions and related compressive strengths.

Table 3 .
Properties of GFRP bars utilized in this study based on manufactured data.

Table 4 .
Steel reinforcing bar results of uniaxial tensile tests.Epoxy type Sikadur-31/41 CF Slow is used as an adhesive material between the concrete surface and steel bars or GFRP bars in the NSM technique.Table 5 displays the main characteristics as specified by the manufacturer.

Table 5 .
Characteristics of adhesive materials.