# RC Columns Strengthened with NSM-CFRP Strips and CFRP Wraps under Axial and Uniaxial Bending: Experimental Investigation and Capacity Models | Journal of Composites for Construction

Abstract

The efficiency of the use of near-surface mounted (NSM) carbon fiber-reinforced polymer (CFRP) strips confined with CFRP wraps to strengthen reinforced concrete (RC) columns will be experimentally and analytically investigated in this paper. A total of 15 specimens of RC columns strengthened with different numbers of NSM-CFRP strips (0, 4, and 8) and confined with different numbers of CFRP wraps (0, 2, and 4), will be prepared and tested under concentric and eccentric loading with different eccentricity ratios of 0, 0.25, 0.50, and 0.75, respectively. The axial load capacity, lateral deflection (Δ), axial deflection, and longitudinal and transverse strains will be measured. The effects of NSM-CFRP strips, the number of CFRP layers, and the eccentricity ratio on the capacity of the specimen will be investigated. The strengthened specimens showed a significant increase in load-carrying capacity and ductility over the control specimens. The increase in the load-carrying capacity of the confined specimens strengthened with strips in axial and uniaxial bending reached 49% and 95%, respectively, over the control specimen. It was concluded that strengthening RC columns with NSM-CFRP strips wrapped with CFRP composite sheets enhanced both the capacity and ductility consistently, for all applied eccentricity ratios. In addition, an analytical model will be developed to predict the strength of NSM-CFRP-strengthened RC columns and will be validated using the obtained experimental results.

Introduction

Several studies have investigated the effect of CFRP confinement on the flexural strength of RC columns. The hoop confinement provided in concentrically and eccentrically loaded RC columns enhances the load-carrying capacity and ductility to a point that depends on the applied eccentricity ratio. This motivated researchers to focus on the provision of flexural improvement to RC columns using longitudinal CFRP laminates, composite sheets, or both. Longitudinal fibers that were externally bonded along the tension side of columns enhanced the ductility and flexural capacity of RC columns (Quiertant and Clement 2011; Hadi and Widiarsa 2012; Hadi and Le 2014; Mostofinejad and Ilia 2014; Mostofinejad and Torabian 2016). In addition, providing longitudinal CFRP laminates, composite sheets, or both with adequate hoop direction confinement was found to enhance the RC columns’ performance in both strength and ductility (Tan 2002; Mandal et al. 2005; Sadeghian et al. 2008; Issa et al. 2009; Sadeghian et al. 2010; Siddiqui et al. 2014). Moreover, using this type of strengthening scheme provides better seismic resistivity and energy absorption than when each system is applied separately (Olivova and Bilcik 2009; Parvin and Brighton 2014; Faustino et al. 2016).

Apart from externally bonded reinforcement, near-surface mounted (NSM) CFRP bar and strip reinforcement have been used to increase the flexural capacity of columns under combined axial compression and uniaxial bending. For instance, Barros et al. (2008) conducted tests on columns strengthened with NSM-CFRP strips and subjected them to combined cyclic bending and compression loading. Their results showed a significant increase in the capacity of columns due to the use of the NSM technique, especially for lower ratios of conventional steel reinforcement. In contrast, energy absorption was inefficient because the columns were not confined.

Based on the data available in the literature, further investigations and tests are required to study the emerging technique of NSM-FRP reinforcement to strengthen RC columns. This paper will investigate and assess the efficiency of the use of a hybrid system of NSM-CFRP strips with CFRP confinement to strengthen RC columns that are subjected to concentric and eccentric loadings.

This paper aims to investigate the use of NSM-CFRP strips with CFRP fabric confinement as a strengthening technique for RC columns under combined axial compression and uniaxial bending with different eccentricity ratios. CFRP strips are used in this paper instead of CFRP bars, due to their unique advantage of maximizing the bonded surface area compared with the sectional area, which minimizes the possibility of debonding (De Lorenzis and Teng 2007). The CFRP strips were then confined with CFRP sheets to provide higher bonding, ductility, and energy absorption to the strengthened structural element.

Experimental Program

Test Specimen

The test specimens used in this paper were based on a short RC column with a square cross section of 200 × 200 mm and a height of 1,000 mm. The reinforcement used had a yielding strength of 500 MPa and a modulus of elasticity of 200 GPa. It consisted of 4Ø12 mm longitudinal bars over the full length of the specimen, which provided a reinforcement ratio of ρ = 1.13% with a concrete cover of 25 mm. This was to satisfy the minimum reinforcement ratio of RC columns according to the ACI 318-19 guidelines (ACI 2019). In addition, closed ties of Ø6 mm @ 200 mm c/c were provided along the specimen height. To achieve the objectives of this paper, a special arrangement was required to simulate short columns subjected to eccentric loading. This was to ensure that the failure and maximum loads occurred at the test zone, which was in the midheight vicinity of the specimens. To achieve the uniaxial eccentric effect on the test specimens, two corbels were provided at the end of each test specimen and cast monolithically with the specimens as shown in Fig. 1. The corbels were designed to sustain the failure load of the strengthened RC columns and to ensure controlled failure along the test zone. The corbels had a square cross section of 350 × 350 mm and a depth of 300 mm each. In addition, two ties were provided at each end of the specimen to avoid premature failure between the specimen and the corbel and to provide adequate joint strength with the corbel, which resulted in a total of eight ties per specimen. The specimens were positioned at the center of one corbel edge to provide sufficient space for eccentricity in the lateral direction. A schematic diagram of the specimen dimensions and reinforcement detailing is shown in Fig. 1.

Test Matrix

Table 1 gives the test matrix used in this paper. It is composed of 15 identical specimens in terms of dimensions, concrete grade, and reinforcement detailing. The specimens were divided into two main groups: (1) concentrically loaded (axial); and (2) eccentrically loaded (uniaxial). The concentrically (axially) loaded specimens have zero eccentricity and they are designated by the prefix Zx (i.e., ex/bx = 0.0) where ex is the applied eccentricity in the x-direction (mm) and bx is the width of the specimen cross section in the x-direction (mm). This group contained three specimens; one control specimen with no NSM-CFRP strips and no wraps designated as Zx-0S0W, where S is the NSM-CFRP Strip and W is the CFRP Wrap layer of confinement. This specimen was used as a benchmark to measure the performance of the strengthened specimens. The remaining concentrically loaded specimens had four NSM-CFRP strips wrapped with two and four CFRP composite sheets and designated as Zx-4S2W and Zx-4S4W, respectively. The group of eccentrically (uniaxial) loaded specimens consisted of three subgroups, each contained four specimens. Each subgroup represented a specific eccentricity ratio for small, medium, and large eccentricity ratios and was designated with the prefix Sx for a small eccentricity ratio of ex/bx = 0.25, Mx for a medium eccentricity ratio of ex/bx = 0.50, and Lx for a large eccentricity ratio of ex/bx = 0.75. The specimens within each subgroup were further classified based on the number of NSM-CFRP strips (S) and the number of CFRP wraps (W). For example, Sx-4S2W was a specimen with a small eccentricity ratio Sx (ex/bx = 0.25), four CFRP strips, and two layers of CFRP wraps. Table 1 provides the details and summarizes the test matrix. For each eccentricity ratio, the specimen with 0S0W was the control specimen for its subgroup. The strengthening mechanism with eight CFRP strips and two layers of CFRP wraps considered in the eccentrically loaded specimens was not accounted for in the concentrically loaded specimens, which assumed that the longitudinal NSM-CFRP reinforcement would not significantly contribute to the axial load carrying capacity for concentrically loaded specimens, as previously reported by other researchers (Tan 2002; Sadeghian et al. 2008; Issa et al. 2009; ACI 2019).

Table 1. Test matrix

Group Eccentricity ratio Specimen designation No. of CFRP longitudinal strips No. of layers of CFRP wraps
Axial Zx (ex/bx = 0.0) Zx-0S0W 0 0
Zx-4S2W 4 2
Zx-4S4W 4 4
Uniaxial Sx (ex/bx = 0.25) Sx-0S0W 0 0
Sx-4S2W 4 2
Sx-4S4W 4 4
Sx-8S2W 8 2
Mx (ex/bx = 0.50) Mx-0S0W 0 0
Mx-4S2W 4 2
Mx-4S4W 4 4
Mx-8S2W 8 2
Lx (ex/bx = 0.75) Lx-0S0W 0 0
Lx-4S2W 4 2
Lx-4S4W 4 4
Lx-8S2W 8 2
Materials

Ready-mixed concrete with a target strength of 20 MPa was used to cast the specimens in this paper. Three cylinders 100 × 200 mm were cast and submerged in a water tank for 28 days. The average 28-day concrete compressive strength of the tested cylinders was 22 MPa, as given in Table 2. Steel reinforcement bars with a tensile strength of 500 MPa and modulus of elasticity of 200 GPa were used for the specimen preparation. The main longitudinal reinforcement consisted of 12 mm diameter deformed bars and the lateral ties consisted of 6 mm diameter plane bars. The measured yield and tensile strength of the longitudinal deformed bars were 500 and 540 MPa, respectively, as summarized in Table 3. CFRP strips (Sika 2009) were used to strengthen the RC columns by providing secondary enhancement to the bending capacity for the entire length of the specimen with an adhesive material (Sika 2006). The CFRP strips were manufactured as a plate material with a width and thickness of 15 and 2.5 mm, respectively.

Concrete compressive strength at 28 days

Table 2. Concrete compressive strength at 28 days

Cylinder specimen no.

$fc′$

(MPa)

Specimen 1 22.4
Specimen 2 22.2
Specimen 3 22.4
Average 22.3

Steel rebar tensile test results

Table 3. Steel rebar tensile test results

Cylinder specimen no. Steel ƒy (MPa) Steel Es (GPa)
Specimen 1 588.5 200.0
Specimen 2 587.4 200.0
Specimen 3 595.1 200.4
Average 590.3 200.1

CFRP wraps (Sika 2003) were used to strengthen the RC columns by providing confinement to the entire length of the specimen with an adhesive material (Sika 2014), which was used in combination with the CFRP wraps to provide the required confinement to the specimens. The mechanical properties of CFRP strips and wraps, along with their corresponding adhesives, are provided by the manufacturer’s technical data sheet and are summarized in Table 4 (Sika 2003, 2006, 2009, 2014). It is noted that the properties of CFRP laminates (wraps and their corresponding adhesives) were measured using five coupons from the same materials, and they had an ultimate tensile strength of approximately 740 MPa and strain at rupture of 1.12%. The results of the CFRP laminate coupon test are summarized in Table 5.

Summary of mechanical material properties of CFRP strips, wraps, and epoxy

Table 4. Summary of mechanical material properties of CFRP strips, wraps, and epoxy

Type Thickness tƒ (mm) Eƒ (GPa) ƒƒu (MPa) ɛƒr (%)
Plate (Sika 2009) 1.5 165 3,100 >1.7
Epoxy adhesive (Sika 2006) 10 >25
Composite sheet (Sika 2003) 0.17 230 3,900 1.5
Epoxy adhesive (Sika 2014) 4.5 30 0.9

Results of CFRP wrap coupon tensile test

Table 5. Results of CFRP wrap coupon tensile test

Cylinder specimen no. CFRP wrap ƒfu (MPa) CFRP wrap ɛƒr (%)
Specimen 1 762.2 1.1
Specimen 2 744.8 1.1
Specimen 3 833.7 1.2
Specimen 4 672.6 1.1
Specimen 5 686.1 1.1
Average 739.9 1.1
Fabrication and Specimen Preparation

Because the NSM technique implies a groove on each side of the specimen, grooves were engraved and made as required on the sides of the specimen using a HILTI DC-SE20 grooving machine (HILTI 2013) width 10 mm and depth 25 mm. Both width and depth satisfied the minimum values of 3ab = 7.5 mm and 1.5bb = 22.5 mm, respectively, as recommended by the ACI 440.2R-17 (ACI 2017) guidelines, where ab and bb are the thickness and depth of CFRP strips, respectively. Grooves were made throughout the entire length of the specimen on the long and tension sides of the specimens, with 100 mm penetration through the corbels on each end of the short and compression sides of the specimens, as shown in Fig. 2. This figure illustrates the CFRP strips strengthening mechanism layout. In addition, ACI 440.2R-17 provides guidelines for the minimum development length required for CFRP bars on the tension side of 317.8 mm (ACI 2019). Although the provided development length was less than that required, it was acceptable due to the corbel depth limitation of 300 mm as well as the presence of CFRP wrap confinement that surrounded the specimen. In addition, ACI 440.2R-17 (ACI 2017) recommends a minimum clear groove spacing greater than twice the provided groove depth. Hence, the 60 mm clear groove spacing provided was greater than the minimum requirement of 50 mm (2 × 25 mm). Then, CFRP strips (Sika 2009) were placed in the grooves and bonded with adhesive (Sika 2006) that filled the grooves for correct bonding between the concrete and the CFRP strips. Then, CFRP wraps (Sika 2003) combined with adhesive (Sika 2014) were applied onto the specimen’s outer surface to provide the required confinement to the specimens. It is noted that the CRP wrap (Sika 2003) was 500 mm wide, which implied the specific recommended overlap length to provide adequate confinement to the full height of 1,000 mm. The overlap length provided was 100 mm, which was greater than the minimum required development length for CFRP wraps, as defined by the ACI 440.2R-17 guidelines (ACI 2017). Each layer of CFRP wrap followed the pattern shown in Fig. 3 to provide the required confinement ratio and to satisfy the overlap length recommended by the ACI 440.2R-17 guidelines. As shown in Fig. 3, the CFRP confinement configuration for each layer was achieved using a 500 mm wide layer [Layer (a)] at midheight with a length of 900 mm. This was followed by two 300 mm wide layers [Layer (b)]; one at each end of the specimen that allowed for 50 mm overlap with Layer (a) until the required number of layers was satisfied. Finally, four layers 100 mm wide [Layer (c)], two at each end of the specimen to prevent premature failure, were applied. The specimens were cured for 2 weeks at room temperature. It is noted that the specimen’s corners were smoothed and rounded throughout the height of the specimen. This was carried out to avoid stress concentration at the sharp corners and to avoid premature failure of the CFRP wraps. This resulted in increased utilization of the CFRP tensile strength, which led to further enhancement of the strength of the columns.

Test Setup and Instrumentation

Fig. 4 shows the loading setup and instrumentation for each tested specimen. The locations of the mounted linear variable differential transformers (LVDTs) to measure lateral displacement, and the strain gauges to measure longitudinal and lateral strains, are shown in Fig. 4. As shown Fig. 4, strain gauges SG #1 and SG #2 were installed to measure the longitudinal strain in concrete and lateral (hoop) strain in the CFRP wrap and SG #3 and SG #4 were installed to measure the longitudinal strains of the steel rebars. Strain gauges SG #5, SG#6, and SG #7 were installed to measure the longitudinal strains (tension or compression) in the CFRP strips. An LVDT was installed at the midheight of the tension side of each specimen to measure the lateral displacement, as shown in Fig. 4 and the universal testing machine (UTM) automatically recorded the load and axial deformation (δ) for each specimen.

All specimens were tested using an Instron UTM, at a loading rate of 2 mm/min. The load was applied to the specimens in the form of a point load that used fabricated pin support of 50 mm diameter at both ends of the corbels, as shown in Fig. 5. Each corbel was capped with a 40 mm thick steel plate to avoid any stress concentration and premature failure at the loading point.

Summary of Experimental Results and Discussion

Axial Load Capacity, δ, and Ductility

Table 6 presents a summary of the experimental results of concentrically (axially) and eccentrically (uniaxially) loaded test specimens. It shows the ultimate axial load (Pu), the corresponding ultimate axial deformation (δu), the axial deformation at failure (δr), and the axial ductility (μaxial = δr/δu) together with the percentage increase or decrease in Pu and μaxial. The unstrengthened and unwrapped specimens in each subgroup (Zx-0S0W, Sx-0S0W, Mx-0S0W, and Lx-0S0W) were the control specimens for their subgroup and each was used as a benchmark to measure the performance of the other specimens within or outside the subgroup. It was observed that confinement with CFRP wraps increased the axial load capacity of the specimen when the load was concentrically applied. With two and four layers of wrapping, the enhancement in the axial load carrying capacity of specimens Zx-4S2W and Zx-4S4W were 36% and 49%, respectively, compared with the control specimen (Zx-0S0W). Similarly, the enhancement in axial load capacity for specimens with a small eccentricity ratio (ex/bx = 0.25) was 65.2% and 63.3% for specimens Sx-4S2W and Sx-4S4W, respectively, compared with their control specimen (Sx-0S0W). The enhancement in axial load capacity for specimens with medium and large eccentricity (ex/bx = 0.50 and ex/bx = 0.75) were 74.2%, 55.3%, 59.5% and 73.4% for specimens Mx-4S2W, Mx-4S4W, Lx-4S2W, and Lx-4S4W, respectively, compared with their corresponding control specimens (Mx-0S0W and Lx-0S0W). For specimens with eight CFRP strips and two layers of CFRP wraps (8S2W), the increase in the axial load capacity for specimens with small, medium, and large eccentricity ratios (Sx-8S2W, Mx-8S2W, Lx-8S2W) was 48.0%, 95.3%, and 91.2%, respectively, compared with their respective control specimens (Sx-0S0W, Mx-0S0W, and Lx-0S0W). The test results of these specimens further proved that hybrid confinement with CFRP wraps and the use of CFRP strips increased the axial load capacity of all specimens.

Axial load capacity, deformation, and ductility.

Table 6. Axial load capacity, deformation, and ductility.

Specimen name Pu (kN) δu (mm) δr (mm) Axial ductility μaxial = δr/δu Increase in axial load capacity (%) Increase in axial ductility (%)
Zx-0S0W 1,117 26.8 27.0 1.0
Zx-4S2W 1,520 25.1 34.5 1.4 36.1 36.6
Zx-4S4W 1,667 20.6 31.6 1.5 49.3 52.5
Sx-0S0W 546 10.0 18.1 1.8
Sx-4S2W 902 12.2 28.9 2.4 65.2 31.3
Sx-4S4W 891 13.6 43.3 3.2 63.3 77.2
Sx-8S2W 808 12.3 29.7 2.4 48.0 34.7
Mx-0S0W 272 12.9 19.9 1.5
Mx-4S2W 474 15.7 35.4 2.3 74.2 46.6
Mx-4S4W 423 12.2 29.5 2.4 55.3 56.7
Mx-8S2W 531 15.3 36.4 2.4 95.3 54.1
Lx-0S0W 182 9.5 32.0 3.4
Lx-4S2W 290 20.4 52.5 2.6 59.5 −23.2
Lx-4S4W 316 14.2 53.2 3.8 73.4 11.6
Lx-8S2W 348 24.7 45.4 1.8 91.2 −45.4

The use of hybrid CFRP strips and CFRP wraps significantly increased μaxial of all specimens compared with specimens without CFRP strips and CFRP wraps, except for the large eccentricity ratio where ductility showed a small increase in one specimen and a slight decrease in the other two specimens. The increase in μaxial for concentrically loaded specimens was between 36.6% and 52.5% and it was between 31.4% and 77.2% for specimens with a small eccentricity ratio, and between 46.6% and 56.7% for specimens with medium eccentricity ratio. For specimens with a large eccentricity ratio, the decrease in ductility was between 23.2% and 45.4%.

Flexural Capacity, Lateral Deflection, and Ductility

Table 7 presents a summary of the experimental results for the eccentrically (uniaxially) loaded specimens. It shows the Pu, corresponding ultimate lateral deflection (Δu), flexural strength at ultimate load (Mu = Pu × Δu), lateral deflection at failure (Δr), and lateral ductility (μlateral = Δru) together with the percentage increase or decrease in Mu and μlateral. The specimens in each subgroup (Sx-0S0W, Mx-0S0W, Lx-0S0W), for instance, unstrengthened and unwrapped were the control specimens for their subgroup and each was used as a benchmark to measure the performance of other specimens within or outside the subgroup. It was observed that confinement with CFRP wraps and the use of CFRP strips increased the flexural capacity of the specimens; however, the increase in eccentricity ratio decreased the flexural capacity. The enhancement in flexural capacity for specimens with a small eccentricity ratio (ex/bx = 0.25) was 177.8% and 182.7% for specimens Sx-4S2W and Sx-4S4W, respectively, compared with their control specimen (Sx-0S0W). The enhancement in flexural capacity for specimens with medium and large eccentricity (ex/bx = 0.50 and ex/bx = 0.75) were 289.5%, 167.5%, 327.3%, and 206.5% for specimens Mx-4S2W, Sx-4S4W, Lx-4S2W, and Lx-4S4W, respectively, compared with their corresponding control specimens (Mx-0S0W and Lx-0S0W). For specimens with eight CFRP strips and two layers of CFRP wraps (8S2W), the increase in the flexural capacity for specimens with small, medium, and large eccentricity ratios (Sx-8S2W, Mx-8S2W, and Lx-8S2W) were 423.8%, 356.2%, and 589.0%, respectively, compared with their respective control specimens (Sx-0S0W, Mx-0S0W, and Lx-0S0W). Increasing the number of CFRP strips had more influence on flexural capacity than axial capacity. This might have resulted from the balanced failure of this specimen compared with the others. Specimens with four CFRP strips and four CFRP wraps (Sx-4S4W, Mx-4S4W, Lx-4S4W) under medium and large eccentricity showed less flexural capacity compared with those with four CFRP strips and two CFRP wraps (Sx-4S2W, Mx-4S2W, Lx-4S2W). This could be attributed to the presence of higher eccentricity, which offsets the effect of the CFRP confinement, as well as the increase in the confinement compressive stress and ultimate strains of the confined concrete, which reduced the bending capacity. In addition, the experimental results demonstrated that as the eccentricity ratio increased, the effect of NSM-CFRP strips to enhance flexural capacity decreased.

Test results of specimens in uniaxial group

Table 7. Test results of specimens in uniaxial group

Specimen name Pu (kN) Δux (mm) Δrx (mm) Mu (kN · m) Mr (kN · m) Lateral ductility μlateral = Δrxux Increase in Mu (%) Increase in Mr (%) Increase in lateral ductility (%)
Sx-0S0W 546 6.3 25.3 3.4 13.8 4.1
Sx-4S2W 902 10.5 44.5 9.5 40.1 4.2 178 190 4.40
Sx-4S4W 891 10.8 47.7 9.7 42.5 4.4 183 208 8.78
Sx-8S2W 808 22.2 46.8 17.9 37.8 2.1 424 174 −47.8
Mx-0S0W 272 9.4 34.2 2.6 9.3 3.6
Mx-4S2W 474 21.0 56.0 10.0 26.5 2.7 290 185 −26.7
Mx-4S4W 423 16.2 52.7 6.8 22.3 3.3 168 139 −10.5
Mx-8S2W 531 21.9 54.8 11.7 29.1 2.5 356 213 −31.4
Lx-0S0W 182 10.3 47.1 1.9 8.6 4.6
Lx-4S2W 290 27.6 60.1 8.0 17.5 2.2 327 104 −52.4
Lx-4S4W 316 18.2 44.0 5.8 13.9 2.4 207 62 −47.1
Lx-8S2W 348 37.1 38.3 12.9 13.3 1.0 589 56 −77.4

The use of CFRP strips and CFRP wraps slightly increased the μlateral of specimens with a small eccentricity ratio (Sx-4S2W, Sx-4S4W) by 4.4% and 8.8%, respectively, compared with the control specimen (Sx-0S0W). However, for all other strengthened specimens, the lateral ductility μlateral decreased by percentages between 10.5% and 77.4%.

Table 8 presents a summary of the experimental results, which show the Pu, corresponding axial CFRP wrap strain (ɛ,axial), transverse CFRP wrap strain (ɛ,trans), compressive strain in the CFRP strip at ultimate load (ɛ,strip,c), tensile or compressive strain in the CFRP side strip at ultimate load (ɛ,strip,s), tensile strain in tensile CFRP strip at ultimate load (ɛ,strip,t), and tensile strain in the steel reinforcement at ultimate load (ɛus) together with the mode of failure of the tested specimens. It was observed that the compressive strain in the CFRP strip at ɛ,strip,c and tensile strain in tensile CFRP strip at ɛ,strip,t show the largest increase, especially at large eccentricities. The maximum increase in compressive and tensile strains reached 10.3% and 12%, respectively.

Test results and mode of failure of specimens in uniaxial group

Table 8. Test results and mode of failure of specimens in uniaxial group

Specimen name Pu (kN) ɛur,conc (%) ɛur,axial (%) ɛur,trans (%) ɛur,strip,c (%) ɛur,strip,s (%) ɛur,strip,t (%) ɛus (%) Mode of failure
Zx-0S0W 1,117 −0.21 Concrete crushing
Zx-4S2W 1,520 −1.18 0.023 −1.5 Rupture of CFRP wrap
Zx-4S4W 1,667 −0.89 0.077 Rupture of CFRP wrap
Sx-0S0W 546 0.02 0.32 Compression
Sx-4S2W 902 6.00 −0.08 −3.75 −0.72 1.47 Tension
Sx-4S4W 891 0.76 −0.05 −5.00 −1.16 1.1 Tension
Sx-8S2W 808 1.99 0.03 −4.40 1.20 4.90 Balanced
Mx-0S0W 272 0.01 2.03 Compression
Mx-4S2W 474 0.10 −0.02 −6.27 −0.02 6.74 1.14 Tension
Mx-4S4W 423 1.59 −0.19 −5.57 −0.08 0.85 Tension
Mx-8S2W 531 1.93 −0.08 −7.02 2.22 Tension
Lx-0S0W 182 −0.23 0.34 Compression
Lx-4S2W 290 0.22 −0.04 −10.29 3.81 12.00 1.91 Tension
Lx-4S4W 313 0.45 0.01 −2.90 1.59 7.09 1.24 Tension
Lx-8S2W 348 1.19 −0.08 −9.06 5.80 10.57 Tension

The gauges along the NSM-CFRP strips captured the enhancement of the specimen’s bending capacity. In addition, the CFRP wrapping composite sheet increased the axial capacity of the specimens, which was reflected by the strain gauges’ findings for the CFRP wrapping axial and hoop strains. Some strain gauges were damaged during specimen preparation, which resulted in the results omitted in Table 8.

Effect of NSM Strengthening Strips and Confinement Wraps

Fig. 6 shows the variation of axial load (P) with δ for concentrically and eccentrically loaded specimens with different eccentricity ratios of 0.0, 0.25, 0.50, and 0.75, respectively. It can be seen in Fig. 6(a) that, where the eccentricity ratio is zero (ex/bx = 0.0), the axial load capacity increased with the increase in the number of layers of CFRP wraps; however, δ for specimens with a different number of layers of CFRP wraps showed some insignificant variations. Fig. 6(b) shows the P versus δ for specimens with a small eccentricity ratio (Sx with ex/bx = 0.25). It can be seen in Fig. 6(b) that the confinement increased the axial load carrying capacity of specimens; however, the number of confinement layers (either two or four wraps) had little effect on the axial load capacity for this eccentricity ratio. This could be due to the presence of eccentricity that eliminated the effect of CFRP wraps, as discussed by other researchers (El Maaddawy 2009; Yazici and Hadi 2009; Bisby and Ranger 2010). However, δ increased considerably with the increase in confinement layers. Figs. 6(c and d) show P versus δ for specimens with medium (Mx with ex/bx = 0.50) and large (Lx with ex/bx = 0.75) eccentricity ratios, respectively. Observations similar to those made for the specimens with small eccentricity ratio could be applied to specimens with medium and large eccentricity ratios. However, the specimens with eight CFRP strips showed an increase in axial load capacity compared with those with four CFRP strips. In addition, the test results for the specimens with CFRP strips showed that P demonstrated a consistent pattern of sudden, sharp decreases at different stages of loading, as shown in Figs. 6(b–d). The number of sharp decreases increased along with the increase in eccentricity ratio. This type of phenomena could be attributed to the stepwise progressive debonding of the CFRP strips before final failure.

Axial Load Capacity and Lateral Deflection

Fig. 7 shows the variation of P with lateral deflection (Δ) for concentrically and eccentrically loaded specimens with different eccentricity ratios of 0.0, 0.25, 0.50, and 0.75, respectively. It can be seen in Fig. 7 that, in general, the axial load capacity and Δ both increased with the increase in the number of CFRP layers. However, the axial load capacity decreased with the increase in eccentricity ratio and Δ increased with the increase in the eccentricity ratio. In addition, it can be seen in Figs. 7(b and c) that specimens with eight CFRP strips (8S2W) had a greater axial load capacity than those with four CFRP strips (4S2W). This indicated that the influence of the number of CFRP strips on increasing the axial load capacity became more evident for specimens with medium and large eccentricity ratios. In addition, Fig. 7(a) shows that the axial load capacity for small eccentricity for specimens with four CFRP strips, and two and four CFRP layers of wraps, were comparable. It was concluded that confinement increased the axial load carrying capacity of the tested specimens; however, the number of confinement layers (two wraps or four wraps) had little effect on the magnitude of axial load capacity, especially for small eccentricity ratio, as shown in Fig. 6(a). For medium and large eccentricity ratios (Mx and Lx), the effect of the number of confinement layers (specimens with 4S2W and 4S4W) on the axial load capacity was more apparent, as shown in Figs. 7(b and c). In addition, the test results show that specimens with CFRP strips demonstrate a pattern of sudden, sharp decreases at various stages of loading, irrespective of the eccentricity ratio. This could be attributed to the successive debonding of CFRP sheets before the failure of column specimens, as shown in Fig. 7.

Effect of Eccentricity Ratio

Fig. 8 shows P versus δ for specimens with different numbers of CFRP strips and different numbers of layers of CFRP wraps. It can be seen in Fig. 8 that for all groups (0S0W, 2S2W, 4S4W, and 8S2W) the axial load capacity increased with the decrease in the eccentricity ratio and δ increased with the increases in the eccentricity ratio. In addition, as the eccentricity ratio increased, the axial load capacity decreased in a stepwise fashion, for instance, a pattern of sudden, sharp decreases. This is a clear depiction of the effect of the progressive debonding of CFRP strips, as shown in Figs. 8(b–d). Specimens without CFRP strips had one progressive increase and one progressive decrease in their load–axial deformation relationship, irrespective of the eccentricity ratio, as shown in Fig. 8(a).

Fig. 9 shows P versus Δ for specimens with different numbers of CFRP strips and layers of CFRP wraps. It can be seen in Fig. 9 that for all specimens (0S0W, 2S2W, 4S4W, and 8S2W) the axial load capacity increased with the decrease in the eccentricity ratio and Δ increased with the increase in the eccentricity ratio. However, Δ remained almost constant for specimens with small and medium eccentricity ratios and decreased for those with large eccentricity as the number of confinement layers increased. This could be attributed to the specimens with large eccentricity failing at smaller P before the development of significant lateral deflection.

Plots for axial load–strain relationships are shown in Figs. 10 and 11. It can be seen Figs. 10 and 11 that the strain gauges on the steel captured and confirmed their yielding plateau. In addition, the specimens overall behavior and failure modes were captured by the distribution of strain gauges along the specimens critical points.

Figs. 10 and 11 show the axial load–strain relationships for 0S0W and 4S2W specimens, respectively. As illustrated in Table 8 and Fig. 10, as the eccentricity ratios increased, the effect of axial strains measured for the concrete diminished. This was in contrast with the steel strains, which controlled the tensile axial forces transformed by the increase of eccentricity. This was the main phenomenon for the benchmark specimens (0S0W), where compression was the dominating mode of failure. Moreover, single strain gauges did not reflect the exact measure of the axial and hoop strains in the concrete, because cracks led to the debonding of the gauges from the concrete surface.

However, the strengthened group with the 4S2W pattern load strain results indicated a tension-controlled failure, as given in Table 8 and Fig. 11. The hoop and axial strains of the CFRP wraps followed the same pattern as the concrete strains in the control specimens, but with higher values. It is noted that the strain gauges on the CFRP wraps in the axial direction were perpendicular to the fibers; no strength in the sheet was provided in that direction. Similarly, the hoop strains were transformed into axial strains due to the applied eccentricity. This led to a space of further deflection with less cracking. In addition, the strains experienced in the NSM-CFRP strips were much higher than that exhibited by the steel reinforcement, which provided room for additional loading before the failure of each specimen.

Failure Modes

The five observed modes of failure for all specimens are summarized in Table 9. The control specimen (Zx-0S0W) of the concentrically loaded specimens failed in a bursting manner by concrete crushing (Mode 1), as shown in Fig. 12(a). In contrast, the other two specimens in this group, Zx-4S2W, and Zx-4S4W failed in a brittle manner by rupture of the CFRP wrap and CFRP strips (Mode 2). The rupture of the CFRP wrap was initiated at the corners of the specimen, as shown in Figs. 12(c and d). This could be due to stress concentration at the corners of a square RC confined column. However, the mode of failure of the uniaxial group’s unconfined specimen was characterized by a brittle compression failure of the concrete cover (Mode 1), as shown in Fig. 12(b). This mode of failure was evident from the behavior of the specimens; as the load increased, the ultimate compression strain of concrete was achieved, which led to the sudden crushing of the concrete in the compression zone; thereafter, the steel experienced tension and reached its tensile yielding strain. However, specimen Sx-8S2W failed in a balanced manner where the crushing of concrete and rupture of the CFRP wraps was accompanied by yielding of the tensile steel reinforcement, Mode 1, 4, and 3, respectively, as shown in Fig. 12(e). This could be due to the presence of extra NSM-CFRP strips on all sides of the specimen. The behavior of the specimens in this subgroup could be summarized as follows: as the load increased, the concrete crushed around the front NSM-CFRP strip, then the load was transferred to the side strip, where the concrete was crushed around it. Then, the load was carried by the back strip and bonding between the CFRP strip and epoxy reached its ultimate stress; the strip slipped, which led to rupture of the CFRP wraps around the specimen, before the yielding of the steel reinforcement. However, as mentioned previously, specimen Sx-4S2W behaved in the same manner; however, the back NSM-CFRP strip slipped (Mode 5), before crushing the concrete around the side strip, which led to yielding of the tensile steel reinforcement. Fig. 12(f) shows the slip failure mode of the NSM-CFRP strip. As indicated, Zx-4S4W failed in a brittle manner by rupture of the CFRP confinement wrap and the CFRP strips (Mode 2), as shown in Fig. 12(i). The rupture of the CFRP wrap was initiated at the corners of the specimen. This could be due to stress concentration at the corners of a square RC confined column. It is noted, none of the NSM-CFRP strips ruptured, because they always slipped when the strain reached approximately 1.4%, slightly below their specified rupture strain of 1.7%. However, with higher eccentricities (0.50 × and 0.75 × ), the main failure phenomenon was tension failure, where the steel yielded before the whole system collapsed (e.g., Modes 2 and 3. The behavior of all specimens followed the same pattern as the Sx-8S2W specimen; however, the steel yielded before rupture of the CFRP wraps, as shown in Figs. 12(g and h), for the third subgroup of specimens. It was concluded from this behavior that the NSM-CFRP composite strengthening technique changed the failure mode from a sudden brittle compression failure to a ductile tension failure, especially with higher eccentricities ratios. This could be attributed to the enhancement of the compressive stress and strain of the confined concrete, which confirmed the yielding of the steel reinforcement before the rupture and collapse of the strengthening system.

Modes of failure for the uniaxial group

Table 9. Modes of failure for the uniaxial group

Mode no. Failure pattern
Mode 1 Crushing of concrete cover at the compression side.
Mode 2 Cracking of concrete at the tension side.
Mode 3 Yielding of steel rebars.
Mode 4 Rupture of CFRP wraps.
Mode 5 Slip of CFRP strips in tension side.
An Analytical Prediction Model

An analytical model was developed to predict the axial load capacity and flexural capacity of the tested specimens under axial and uniaxial loading conditions. The analytical model was developed based on a previous model developed by El Sayed and El Maaddawy (2011). The model was modified and extended to account for the effect of the addition of hybrid NSM-CFRP strips as strengthening materials. It is noted that the model was adopted by the ACI 440.2R-17 (ACI 2017) committee and follows the same equations, procedures, and assumptions; however, with minor variations to eliminate any reduction factors proposed by the ACI 318-19 design guidelines (ACI 2019). These variations were considered to determine the nominal capacities of the member and understand its behavior. However, the reduction factors could be used in real-life applications as a safety measure. The model was based on the material properties and constitutive laws of unconfined concrete, confined concrete, steel, and CFRP composite strips and sheets, respectively. In addition, it satisfied all the compatibility and equilibrium requirements for the mechanical behavior of the section presented in this paper. A brief description of the proposed analytical model together with its proposed extensions is presented. Details of the original model are given in the literature (El Sayed and El Maaddawy 2011). The accuracy and validity of the developed model were then verified by comparing its predictions with the obtained experimental results. Of note, the proposed analytical model had its limitations and requires further development to cover real-life applications. For instance, the slenderness effect was not considered because the dimensions of the tested specimen used in this paper fell under the short column’s category. However, according to a study carried out by Jiang and Teng (2012a) on slenderness limits for short FRP-confined circular columns, it was observed that short RC columns might become slender after receiving FRP confinement. This was attributed to the increase in the flexural rigidity of an RC section being limited in comparison with the increase in its strength under FRP confinement. The authors concluded that existing slenderness limit expressions for short RC columns did not apply to FRP-confined RC columns. More details of the underlying rationale and theoretical models for slender FRP-confined RC columns can be found in the literature (Jiang and Teng 2012b) together with the design method for slender FRP-confined circular RC columns (Jiang and Teng 2013). By ignoring the effect of the increase in slenderness due to CFRP confinement in the theoretical model developed in this paper, the accuracy of the predictions was slightly sacrificed. However, the results were within a tolerable range of accuracy. Nevertheless, future investigation into this subject should consider the slenderness effect.

Material Constitutive Laws

Concrete

The stress–strain response of unconfined concrete under compression loading is shown in Fig. 13(a). It follows a parabolic behavior that is expressed by (Hognestad et al. 1955)

$fc=fc′[2εcεco−(εcεco)2]$

where

$εco=(2fc′/Ec)$

; and

$Ec=4,700fc′$

.

However, the confined concrete behaved differently, as shown in Fig. 13(b). This stress–strain response was developed by Lam and Teng (2003) and was used in the developed model presented in this paper. The stress–strain relationship is expressed by the following equation (Lam and Teng 2003):

$fc={Ecεc−(Ec−E2)24fcεc20≤εc≤εt′fc′+E2εcεt′≤εc≤εccu}$

where

$E2=((fcc′−fc′)/εccu)$

;

$εt′=(2fc′/(Ec−E2))$

; E2 = the slope of linear portion of the stress–strain response of CFRP confined concrete (MPa); and

$εt′$

= the transfer strain in CFRP confined concrete that corresponds to

$fc′$

(mm/mm).

As seen in Fig. 13(b) and Eq. (2), the stress–strain response of confined concrete follows two patterns. The first proportion shows a parabolic response that is similar to the unconfined concrete until the strain in concrete reaches the value of the transfer strain in the CFRP confined concrete (

$εt′$

). Then, the behavior moves in a linear trend beyond the strain value of

$εt′$

until it reaches the maximum CFRP strain of confined concrete compressive strain (ɛccu), which corresponds to CFRP confined concrete compressive stress (

$fcc′$

). The

$fcc′$

and ɛccu are given by the following two equations (ACI 2017):

$εccu=εco(1.5+12kbflfc′(εfeεco)0.45)≤0.01$

where fl = (2Efntfɛfe/D); and ɛfe = kɛɛfr. The term D is the diameter of the circular cross section of the column. For the noncircular cross section, the equivalent diameter of the column is given by

$D=bc2+hc2$

(ACI 2017), and bc and hc are the shortest and longest dimensions of the cross section (mm), respectively. It is noted that the maximum CFRP ɛccu, should be limited to 0.01, as illustrated by Eq. (4), to prevent the loss of concrete integrity due to excessive cracking according to the ACI 440.2R-17 guidelines (ACI 2017). The efficiency factors ka, kb, and kɛ are determined by the following two equations (ACI 2017):

$ka=(bchc)2[1−(bc/hc)(hc−2rc)2+(bc/hc)(bc−2rc)23Ag−ρg]×(11−ρg)$

$kb=(bchc)0.5[1−(bc/hc)(hc−2rc)2+(bc/hc)(bc−2rc)23Ag−ρg]×(11−ρg)$

where ka and kb = efficiency factors that account for the effect of the cross section shape of confined concrete on the determination of

$fcc′$

and ɛccu, respectively. The efficiency factor (kɛ), which accounts for the premature failures of the CFRP confinement, is taken as 0.55 as recommended in the ACI guidelines (ACI 2017).

Reinforcing Steel

The stress–strain response of steel reinforcing bars under compression and tension loading conditions is shown in Fig. 13(c). It was idealized with a linear elastic region and a postyield region with strain hardening of 1% for the steel preyield modulus. This is expressed by (ACI 2017)

$fs={Esεspre−yieldstagefy+Esp(εs−εy)post−yieldstage}$

where Esp = 0.01Es.

CFRP Strips and Wraps

The stress–strain response of CFRP strengthening composites under tension loading conditions is shown in Fig. 13(d). It was idealized with a linear elastic behavior until rupture, as expressed by (El Sayed et al. 2011):

where ƒƒ = CFRP strips or wraps stress (MPa); and ɛƒ = CFRP strips or wrap strain that corresponds to ƒƒ (mm/mm).

Compatibility Requirements

The stress and strain distribution over the cross section of an eccentrically loaded column is shown in Figs. 14(a and b) for nonstrengthened and strengthened specimens, respectively.

To satisfy the strain compatibility conditions, a maximum value for the unconfined concrete column of ɛc,max = ɛcu = 0.003 was used in the model, whereas for the confined column ɛc,max = ɛccu computed from Eq. (4) was used (ACI 2017). The strain compatibility model was based on an initial assumed value for the depth of the neutral axis of the cross section (c), where at any distance from the neutral axis (zi) the strain in concrete, steel, CFRP, or both composites (ɛz) is determined by the following equation (El Sayed and El Maaddawy 2011):

where ɛc,max = the strain in concrete at extreme compression fiber.

Equilibrium Condition

To satisfy the equilibrium requirements, the internal compression forces and bending moments were calculated using the following two equations (El Sayed et al. 2011):

$∑i=1i=nfciAi+∑i=1i=nfsiAsi+∑i=1i=nffiAfi=Pn$

$∑i=1i=nfciAidi+∑i=1i=nfsiAsidsi+∑i=1i=nffiAfidfi=Mn$

where ƒci = stress of concrete at the center of the discretized layer i (MPa); Ai = concrete area of layer i (mm2); di = distance between the center of concrete layer i and the plastic centroid (mm); ƒsi = stress of steel rebar i (MPa); Asi = rebar i cross-sectional area (mm2); dsi = distance between the center of steel rebar i and the plastic centroid (mm); ƒƒi = stress in the ith CFRP strips, wraps, or both (MPa); Aƒi = cross-sectional area of the ith CFRP strips, wraps, or both (mm2); dƒi = distance between the center of the ith CFRP strips, wraps, or both and the plastic centroid (mm); Mn = bending moment of the cross section (Nmm); and Pn = axial force of the cross section (N).

The term Pn is calculated by the numerical integration of the forces in the discretized finite layers of concrete under compression, forces in the steel longitudinal rebars, and CFRP strips. In addition, the term Mn is determined by numerical integration of the moments of the forces calculated for the plastic centroid of the cross section from the concrete, steel, and CFRP strips, wraps, or both respectively. For the present model, it was assumed that plane sections remained plane and that concrete tensile strength was neglected, as was the confinement provided by the steel ties.

Comparison Between Experimental Results and Analytical Predictions

Axial Group (Zx)

Computer software was coded and calibrated using the presented model to predict the load-carrying capacity of the specimens tested in this paper. The concentric loaded group predicted capacity can be calculated using the following (El Sayed et al. 2011):

$Pno={fc′(Ag−Ast)+AstfsoUnconfinedfcc′(Ag−Ast)+AstfscCFRPConfined}$

where Pno = concentric loaded columns axial load carrying capacity (N); Ast = total cross-sectional area of steel rebar (mm2); ƒso = stress in steel rebars that corresponds to steel strain (ɛco) (MPa) but ƒsu; and ƒsc = stress in steel rebars that correspond to steel strain (ɛccu) (MPa) but ƒsu.

The results are summarized in Table 10 with a comparison of the values obtained from the experimental tests presented previously. The predicted results showed a good agreement with the experimental ones within a range of error of 7% as given in Table 10.

Comparison between experimental and analytical predictions

Table 10. Comparison between experimental and analytical predictions

Eccentricity ratio Specimen Pn,Exp (kN) PnAnal (kN) MAPEAnal (%)
Zx (ex/bx = 0) Zx-0S0W 1,117 1,051 5.9
Zx-4S2W 1,520 1,412 7.1
Zx-4S4W 1,667 1,641 1.6
Sx (ex/bx = 0.25) Sx-0S0W 546 539 1.3
Sx-4S2W 902 795 11.9
Sx-4S4W 891 924 3.7
Sx-8S2W 808 884 9.4
Mx (ex/bx = 0.50) Mx-0S0W 272 302 11.1
Mx-4S2W 474 487 2.8
Mx-4S4W 423 558 32.1
Mx-8S2W 531 557 4.8
Lx (ex/bx = 0.75) Lx-0S0W 182 204 12.2
Lx-4S2W 290 323 11.3
Lx-4S4W 316 375 18.6
Lx-8S2W 348 385 10.5
Average MAPE 9.6
Uniaxial Group (Sx, Mx, and Lx)

The uniaxial group predicted load-carrying capacity for each specimen was computed using the model presented previously. The eccentric loaded group predicted results are summarized in Table 10 along with a comparison of the experimental results obtained previously. The predicted results showed a good agreement with the experimentally measured data, with a maximum deviation of 32.1%. As given in Table 10, the total results have an average mean absolute percent error of 9.6%, which demonstrated the ability of the proposed model to accurately predict the uniaxial load carrying capacity of RC columns strengthened using NSM-CFRP strips and wraps. This was true for all specimens other than Mx-4S4W, which might have deviated from the error band due to premature debonding because of the presence of four layers.

Axial Load–Bending Moment (P–M) Interaction Diagram

Fig. 15 shows a typical interaction diagram developed analytically for the column used in this paper. The pure bending points for each curve have been interpreted using the same analytical model developed in this paper by the application of infinite eccentricity values to the model to simulate a pure bending condition. As shown in Fig. 15, the use of CFRP wrapping increased the axial capacity of the RC columns. In addition, increasing the number of CFRP strips in the tension face of the column increased the flexural capacity for the same. This was in close agreement with the literature as well as the conclusions derived from this paper.

Limitation of the Hybrid Strengthening Techniques

The hybrid strengthening technique proposed in this paper has some limitations. This type of technique is more suitable for pin-ended (simply supported) columns where the maximum moment occurs near the center and away from the ends due to the needed bond (anchorage) length that is developed by the NSM strips. In real-world columns that are part of frame structures, the maximum moment usually occurs near one of the end joints of the column; therefore, the NSM strips need to be anchored at the beam–column joints to be effective. Anchoring of the NSM strips at the beam–column joints was not implemented in this paper due to its impracticality and the time required. Therefore, this type of limitation needs to be taken into consideration when the results of this paper are interpreted.

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