RC Columns Strengthened with NSM-CFRP Strips and CFRP Wraps under Axial and Uniaxial Bending: Experimental Investigation and Capacity Models

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.

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., e_x/b_x = 0.0) where e_x is the applied eccentricity in the x-direction (mm) and b_x 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 e_x/b_x = 0.25, Mx for a medium eccentricity ratio of e_x/b_x = 0.50, and Lx for a large eccentricity ratio of e_x/b_x = 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 (e_x/b_x = 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).

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.

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.

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 3a_b = 7.5 mm and 1.5b_b = 22.5 mm, respectively, as recommended by the ACI 440.2R-17 (ACI 2017) guidelines, where a_b and b_b 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.

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.

Table 6 presents a summary of the experimental results of concentrically (axially) and eccentrically (uniaxially) loaded test specimens. It shows the ultimate axial load (P_u), the corresponding ultimate axial deformation (δ_u), the axial deformation at failure (δ_r), and the axial ductility (μ_axial = δ_r/δ_u) together with the percentage increase or decrease in P_u and μ_axial. 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 (e_x/b_x = 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 (e_x/b_x = 0.50 and e_x/b_x = 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.

The use of hybrid CFRP strips and CFRP wraps significantly increased μ_axial 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 μ_axial 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%.

Table 7 presents a summary of the experimental results for the eccentrically (uniaxially) loaded specimens. It shows the P_u, corresponding ultimate lateral deflection (Δ_u), flexural strength at ultimate load (M_u = P_u × Δ_u), lateral deflection at failure (Δ_r), and lateral ductility (μ_lateral = Δ_r/Δ_u) together with the percentage increase or decrease in M_u and μ_lateral. 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 (e_x/b_x = 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 (e_x/b_x = 0.50 and e_x/b_x = 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.

The use of CFRP strips and CFRP wraps slightly increased the μ_lateral 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 μ_lateral decreased by percentages between 10.5% and 77.4%.

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

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.

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.

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.

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.

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.

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.