Flexural Behavior of RC Beams Strengthened by NSM-CFRP Laminates or Bars

The strengthening and enhancing the structures represents an important aspect in the construction industry due to the growing need to increase the tolerability of origin to a specific level and within the required rehabilitation and maintenance work. This paper assessed the performance and effectiveness of the Near Surface Mounted (NSM) strengthening technique for the reinforced concrete beams. Three (140x260x2700 mm) reinforced concrete beams were strengthened in flexure with NSM strengthening systems using Carbon Fiber Reinforced Polymer (CFRP) strips, bars, and cement-based adhesive as a binding materials. The flexural behaviour of the beams was evaluated by testing the specimens under three-point loading to failure. The structural performance, deflection, ductility, stiffness, and modes of failure of the tested beams are presented and discussed in this paper. The test results indicate that using NSM-CFRP strips and bars is practical and significantly improves the stiffness and increases the flexural capacity of reinforced concrete beams. The strength increments were 48, 42, and 15 percent recorded with CFRP bars, rough strips, and smooth strips respectively. The deflection of the strengthened beams was reduced by about 66, 48, and 58 percent for CFRP smooth strips, rough strips, and CFRP bars respectively, compared with the control beam due to the increased stiffness of the strengthened beams.

section_content: The NSM FRP has been considered as an attractive method for increasing their flexural and shear strength w of RC members when strengthened with the ACI Committee 440 [1] are considering the design guidelines for this technique. The NSM Strengthening technique is about placing CFRP members, bars or strips, inside a pre-cut groove made within the concrete cover in the region of tensile stresses of the reinforced concrete member and then using a high strength epoxy adhesive or cementitious grout to bond them to the three sides of the groove using [2]. A variety of applications involved the NSM FRP technique and it showed several advantages over the external bonded FRP technique in strengthening concrete structures [3, 4]. The application of NSM has many advantages; one of the significant advantages is that it does not require any surface preparation work except grooving. when the NSM reinforcements is inside the grooves the concrete cover will protect them, which leads to strengthening the tensile zone of beams and slabs. As, a result, the harm resulting from fire, mechanical damages and other effects is significantly decreased. However, more initial costs might be required for NSM strengthening techniques compared to conventional externally bonded CFRP sheet application [5]. Beside the cost increase for groove cutting labor in NSM technique, the high cost of material and viscosity of adhesives in overhead situations can result in making the technique more expensive then externally bonded systems. The effect of grove size and the use of cementitious adhesives (or injection epoxies) rather than epoxy putty or paste adhesives could reduce the cost of the technique thus and additional research is needed. Furthermore, taking into account the superior characteristics of NSM strengthening, it becomes particularly interesting in seismic retrofit of RC column-beam joints providing either additional strength or durability when moving the failure zone from the column to the beam [6]. The NSM technique mostly involved the use epoxy resin as bonding material. On the other hand, cement paste has low viscosity which leads to make it hard to use in practice and that is why cement paste has not been used before with NSM technique. Hashimi and Al-Mahaidi [7] have developed a cement-based adhesive that is suitable to be used with NSM technique; significant bond properties were achieved by the addition of polymer to the mortar of the adhesive, which increased its viscosity also an excellent ductile behaviour of all specimens, was observed.
section_name: Introduction

section_name: Experimental Program

section_content: A total of four, simply supported concrete beams, 2700mm in length, 140mm wide, and 260mm high were constructed and tested using a 5MN MTS machine under three-point loading up to failure at the Smart Structures Laboratory, Swinburne University of Technology using a displacement-controlled rate of 2 mm/min. The beam had a 2300mm effective span and a 900mm shear span of. The bottom tension reinforcements were 3Ø12 deformed steel bars, and 2Ø12 at the top of the beam to hold the stirrups. To prevent any shear failure prior to flexural failure an adequate shear reinforcement was used with Ø10stirrups deformed steel bar were distributed along the beam every 125mm center-to-center. Beam dimensions and reinforcement details are shown in Figure 1.
section_name: I. Specimen Dimensions and Details

section_content: The concrete material was supplied by a ready-mixed concrete local supplier (Hanson-Heidelberg Cement Group) in order to minimize the variations of all constituents and to achieve good quality control. Batch proportion of concrete shown in Table 1. To specify the properties of concrete used in the reinforced concrete beam specimens, an average of three (100x200) mm cylinders were tested for each property. Table 2 lists the properties of the concrete and cement-based adhesive cylinders specimens.
section_name: II. Materials 1. Concrete

section_content: Cement-based adhesive was chosen as a binder with CFRP in the strengthening system. For strengthening beams with NSM-CFRP, a cement-based adhesive consisting of cement, micro cement, silica fume, silica filler primer, super-plasticizer and water was used. The mix design is provided in Table 3. Mortar cylinders were cast, 50mm in diameter and 100mm in height.
section_name: Cement-Based Adhesive

section_content: Tensile tests in accordance with AS1391 (2007) [12] were carried out to determine the properties of the steel bars. The properties of the tested steel bars are shown in Table 4.
section_name: Steel reinforcement

section_content: CFRP rough bar and laminate (rough and smooth) were manufactured by BASF Construction Chemicals Australia. The mechanical properties were determined using testing procedures in accordance with ACI 440. 3R-04 [13] and Table 5 is showing the results.
section_name: Carbon Fiber Reinforced Polymer, CFRP

section_content: To assess the flexural strength improvement resulting from the NSM reinforcements, one beam was tested without strengthening (C) to serve as the control specimen for comparison purposes. Three beams (SL, RL, and RB) were strengthened using NSM CFRP smooth strips, rough strips, and rough bars, respectively. Table 6 shows a summary of these beams. The embedment lengths of all the NSM FRP bars and strips were kept constant in all beams at 2300mm. Moreover, the same area, for all FRP reinforcement was kept constant to ensure the same area of fiber in each beam. The deflection at mid span was monitored on both sides of the beam by using two linear differential transducers (LDTs). Readings from LDTs were recorded during testing every second using a data acquisition system connected to a computer. To measure the strain in the FRP reinforcement during testing, four gauges were installed on each CFRP (strip and bars) and each beam had two strips making a total of 8 gauges for each beam. The strain gauge layout is shown in Figure 2.
section_name: Test Specimens and Designation

section_content: The NSM CFRP rebar and strips installation started with making the grooves, two longitudinal grooves on the concrete tension face of each beam except the control beam were carefully cut to 2300 ± 10 mm lengths using a tuck-point grinder in conjunction with a diamond concrete blade. Two different groove sizes were used in the current experimental work, 8mm width x 24mm depth for CFRP laminate and 12mm width x 20mm depth for CFRP bars, as recommended by ACI 440. 2R-08 [1]. First, the beams were placed upside down; the grooves were wetted before inserting the adhesive. The strengthening procedure was carried out as follows: i. A layer of cement-based adhesive was placed on the fibre. 

ii. The groove was filled to half its depth with cement-based adhesive and the CFRP strip/ bar was then placed in the groove and slightly pressed. 

iii. The positioning of CFRP strip/ bar at the center and distributing the adhesive layer equally between the fibre and the surface of the groove were carefully considered. iv. After positioning the CFRP strip, the installation was completed by adding more cement-based adhesive to fully fill the grooves and the surface was levelled. v. The beams were covered with moist burlap and plastic sheets for 7 days.
section_name: III. Installation of the FRP Reinforcements

section_name: Test Results and Discussion

section_content: Table 7 shows the results of the tested beams in terms of ultimate load Pu, cracking load Pcr, yielding load py, load efficiency at Py (ηy) and load efficiency at Pu (ηu)%, following the procedure proposed by Sharaky et al. [14], deflection, and mode of failure.
section_name: I. Load-Deflection Curve

section_content: The load-deflection curve for the control beam, C, is shown in Figure 3. It can be seen that the beam exhibit a linear response until cracking occurs in the concrete in the initial stages at the load of 56 kN approximately. The mid-span deflection then increases at a higher rate until reaching the maximum 68. 8 mm due to decreasing stiffness resulting from cracking in the concrete beam. At this stage, the stresses in steel increase at a faster rate until the steel rebars yield. The ultimate capacity reaches its peak once the steel rebar enters the yielding plateau. The ultimate failure load was 99. 8 kN. The flexural cracks spread upward an enlarged with load increment, as shown in Figure 4. The failure mode was concrete crushing after the yielding of steel, which is a typical failure mode in unstrengthened RC beams. 5 illustrate the load-deflection scheme of the SL beam. The strength showed slight increases over that of the reference beam, with an ultimate load of 114. 5 kN. The increment was approximately 15%. The beam showed linear load-deflection behaviour until steel yielding, followed by nonlinear performance up to failure. 

After the first crack that started at a load of 61 kN and typically formed at mid-span, the loaddeflection behaviour was influenced by the formation of more flexural cracks formed at the load points and the excessive flexural cracks that formed between the midspan and the internal supports. Steel yielding started the failure mode, followed by concrete crushing. Subsequently, debonding occurred within the fiber and adhesive through the interfacial zone accompanied by cracks along the adhesive. Figures 6 and 7 present the two failure modes.
section_name: Control beam, C

section_content: The load versus deflection curve for the RL beam shown in Figure 8. The rough surface laminates strips enhanced the flexural strength of the beam more than the smooth surface laminate strips did. This can be attributed to the nature of the surface of the CFRP strips, as rough surface CFRP strips provided more bond than the smooth surface CFRP strips. The increases in ultimate load was approximately 24 and 42 percent greater than those of SL and C beams respectively. The ultimate load was 140. 9 kN. Initial cracks started on RL beam at load of 68kN, which was higher than those of the SL and reference beams. The NSM-CFRP rough surfaces beam exhibited higher yield load, approximately 36% and 61% greater than those of the SL beam and the reference beam, respectively. Moreover, the stiffness increased significantly over the reference beam within the post-cracking stage, and was more noticeable than that achieved by the SL beam. After debonding of the NSM-CFRP bar, the load dropped, similar to the control beam C. This load represents the residual frictional stresses between the fibre and the adhesive. As shown in Figures 9 and 10, the RL beams failed by concrete crushing at mid-span followed by adhesive-concrete interface debonding and centralize splitting of the cement-based adhesive.
section_name: Strengthened Beam with Rough Laminate, RL

section_content: Beam RB (i. e. the beam strengthened with NSM-CFRP bars) the highest strength increment was shown with ultimate load of 146. 9 kN. Higher yield load, approximately 29% and 52% over the SL beam and the reference beam respectively but slightly lower than that of the RL beam of about 5. 8%. Initial cracks started at load of 70 kN with further increases in loading propagation of the cracks took place. Figure 11 shows approximately linear behavior with regard to concrete, cracking and steel yielding phases. First, before the cracking phase. Similar to the control beam, the strengthened beam exhibited linear-elastic behaviour like. The NSM-CFRP bars had little effect on the cracks formed at mid-span of the beam. In the second phase, between cracking and steel yielding, the NSM-CFRP bars increased the yielding load and the stiffness of the beam when compared with the control beam. In this phase, the load increased about 48 percent of the reference beam, C. The third phase covered the time after steel yielding until failure. This phase showed nonlinear behavior; higher range increase in deflection was shown than in the previous phase. It's obvious from the curve that there are two drops in the load after the ultimate value but again ascending up till failure. However, fiber still provided friction stress and the CFRP bars still have the ability to carry out loads higher than the unstrengthened beam did with more deflection rate, and this lead to make the beam showing better performance even at failure. The failure mode was debonding of the bar at the adhesive-bar interface. Severe cracking occurred in the adhesive layer (refer to Figures 12 and 13 ). Major cracks formed between the mid-span and the load point and failure occurred after the bond between the FRP bars and the surrounding adhesive was lost associated with cracking and peeling of the adhesive at the surface. The failure of the CFRP was localized along the groove, which indicated the existence of an effective failure zone in the vicinity of the installed NSM-CFRP bars. Post failure examination of the samples showed that on the failure bond plane, large portions of the grout adhesive bonded Figure 15 illustrates comparison of the maximum load (Pu) and the corresponding deflection of the strengthened beams. The strengthened beams experienced higher load capacities than the control beam, whereas the deflection was less. 

The deflection of the strengthened beams was reduced by a maximum of about 48%, 66% and 58% for the SL, RL, and RB strengthened beams respectively, compared with the control beam due to the increased stiffness of the strengthened beams.
section_name: Strengthened Beam with Rough CFRP Bar, RB

section_content: The strain distributions along CFRPs are plotted at different load levels in Figures 16 to 18 for the SL, RL, and BR beams respectively. In each specimen, the strain value increasing rate (the slope of strain vs. load curve) was very small at lower loads. The concrete did not crack within this load range and the CFRP did not contribute very much. The section almost worked as an ordinary reinforced concrete section. When the concrete cracked at around (61-70) kN, the CFRP started contributing effectively and the slope of strain vs. curve changed. As shown in Figure 16, the peak recorded strain of the SL beam in the CFRP strips which occurred at the mid-span was 6105 microstrain, since the failure in the fiber occurred under this From Figure 17, the RL beam showed the maximum strain value of 7789 microstrain. The peak occurred at the center of the beam. This strain value is very close to that recorded under the load point, especially at loads of 120 and 140 kN, as the cracks in the high moment region became wider and deeper than when loads are lower. This means that the CFRP strips in this region act as a tie supported by the rest of the bond length located in the shear span. The peak recorded strain value of CFRP in the BR beam was 7098 microstrain under the load point. This value is lower than the corresponding value of that in beam RL because the failure point occurred at mid-distance between the beam center and the load point, and the maximum value was not recorded at the failure point. With reference to Figure 4. 18, SG4: strain gauge at the centre of the beam, SG3: is the strain gauge at 450mm distance from SG4, SG2: is the strain gauge at 125mm distance from SG3, and SG1: is the strain gauge at 125mm distance from SG2, as illustrated previously in Figure 2.
section_name: II. Strain Distribution along the CFRPs:

section_content: Figure 19 shows the relative stiffness of the strengthened beams compared to that of the control beam (C), using Equation 1 [14]. 

(

where δc and δstr are the deflection of the control and strengthened beams, respectively. It can be seen from the figure above that all strengthened beams exhibited considerably stiffer post-cracking behaviour than the control beam. However, the cracks were spaced more evenly and closer compared to those in the control beams. The stiffness of the strengthened beams increased by an amount that depended mainly on the surface preparation of CFRP and the bonded area of CFRP material. Moreover, when the tensile strength and modulus of CFRP increased, so did the stiffness of the strengthened beams. 

Beams strengthened with CFRP rough surface strips (RL) experienced higher stiffness than those of specimens strengthened with smooth surface strips (SL) and with CFRP bar (BR) in post cracking stage, while the SL beam show higher stiffness at yielding stage than the RL and BR beams. Increase in stiffness caused lower curvatures resulting in deflection reduction. From the viewpoint of ductility, although CFRP has high strength, it is very brittle. When loaded in tension, CFRP exhibits a linear stress-strain behaviour up to failure without exhibiting any indication of impending failure. As CFRP behave differently from steel, it consequently suffers from a significant loss in beam ductility, particularly when CFRP is used for the flexural strengthening of RC beams. When applied to RC members, the term ductility implies the ability to sustain significant inelastic deformation prior to collapse. The ductility index in this study was obtained based on deflection computation [16] expressed as follows:

(2)

where Δ and Δ is the displacement ductility indices at ultimate load and load at failure, respectively. Δ, Δ, Δ is the mid span deflections at ultimate load, failure load and yield load respectively. Table 7 shows the ductility ratios of the tested beams. 

The ductility of the RC beam specimens decreased with increased strengthening reinforcement [17]. This was due to the nonductile performance of CFRP with the linear stress-strain relationship and the comparatively brittle failure modes owing to the separation of the strengthening reinforcement from the concrete. It was observed that the ductility indices of all strengthened beams were lower than that of the control specimen at both ultimate and failure loads. However, the specimens strengthened with NSM-CFRP strips had slightly higher ductility indices than the beam strengthened with NSM-CFRP bars. contrast, the stiffness and the yielding load of the strengthened beam were increased by the NSM-CFRP in the pre-cracking stage the CFRP materials provides constraints to the opening of cracks and reduce the deflection.
section_name: III. Stiffness and Ductility

section_name: Acknowledgments

section_content: The authors greatly acknowledge the support of Dr. Riadh Al-Mahaidi who provided the opportunity and helped in the implementation and the financial support. Laboratory assistance on the test from the laboratory technicians of the Smart Structures Laboratory, Swinburne University of Technology is greatly appreciated.
section_name: Acknowledgments

section_name: Notation

section_content: The following symbols are used in this paper: CFRP: Carbon Fiber Reinforced Polymer NSM: Near Surface Mounted RC: Reinforced concrete Pcr: Cracking load, kN Pu: Ultimate load, kN Py: Yielding load, kN δc: Deflection of the control beam, mm δ cr: Deflection at cracking, mm δstr: Deflection of the strengthened beam, mm δ u: Deflection at ultimate, mm Δ : Mid span deflections at failure load, mm Δ : Mid span deflections at ultimate load, mm Δ: Mid span deflections at yield load, mm ηu: load efficiency at Pu ηy: load efficiency at Py Δ : Displacement ductility index at load at failure Δ : Displacement ductility index at ultimate load
section_name: Notation