Retrofitting Methods for Shear Strengthening of Reinforced Concrete Beams Using CFRP

Abstract

Due to the ageing of infrastructures, there is a need for repair and rehabilitation of the structures at later stages. The usage of carbon fiber-reinforced polymer for strengthening is one such practice that has become common in the Indian scenario. This paper specifically discusses the comparison of different application methods of carbon fiber-reinforced polymers for enhancing the shear capacity of the reinforced concrete beams. Near surface mounted carbon fiber-reinforced polymer strips, externally bonded carbon fiber-reinforced polymer strips and externally bonded carbon fiber-reinforced polymer strips with U–wrap are the methods adopted for strengthening reinforced concrete beams for shear. Three-point loading flexure tests were conducted on the strengthened beams, and it was found that the near surface mounted method exhibited a 28.5% increase in the shear capacity of the beam. In contrast, the externally bonded method exhibited an 18.5% increase, and externally bonded with U–wrap exhibited a 142% increase in the shear capacity of the beam compared to the control beam.

1 Introduction

During the life of a structure, various situations would arise that may not have been considered during the initial design stage. One such example is changing the structure's functionality leading to heavier loads that may not have been considered in the initial stage. For such conditions, demolishing and constructing a new building will incur more time and cost. Thus, retrofitting and strengthening have become the ideal solution to these situations. Though various methods of strengthening techniques are available, strengthening using Carbon Fibre Reinforced Polymer Composites (CFRP) has become a prominent solution because of its strength and application time. Based on the intended applications of the CFRP, they are classified as Bond-critical and Contact-critical applications. The bond-critical application requires an adhesive bond between the FRP and concrete, which can be used for flexural or shear strengthening of beams, columns and slabs. The contact-critical application requires intimate contact between FRP and concrete, such as column confinement.

Here in this study, discussions on the bond critical applications and the methods of strengthening using CFRP laminates which are Near Surface Mounted CFRP (NSM), Externally Bonded CFRP (EB), and Externally bonded CFRP strips with U–Wrap CFRP (EBU), are critically presented. Briefly, the NSM method involves cutting grooves in the concrete for the required depth to install the CFRP laminates with epoxy adhesives. In the EB method, the CFRP laminate is glued with epoxy adhesive on the surface of the Reinforced Concrete (RC) members. Similarly, the U-Wrap involves the installation of CFRP laminates on the entire member for the required length except the top surface using epoxy resins. For the installation of CFRP, ACI 440 2R.17 (2017) suggests various parameters to be adhered to. One such parameter is that the minimum tensile strength and compressive strength of concrete should be 1.4 MPa and 17 MPa, respectively, to carry out the bond critical applications. Whereas the same is not required for contact-critical applications.

Further profiling of the concrete surface for CFRP laminates is stipulated by ACI 546R (2017) and ICRI 03,730 (2008). They need to be compatible with adhesion to concrete surfaces as well as FRPs and should be able to resist environmental impact and be highly workable. Various studies have been done in the field of retrofitting using CFRP. Barros (2006) concluded that the NSM method is an effective method of shear strengthening of beams in comparison with the results of three variations in the specimens—without shear reinforcement, with steel shear reinforcement and with NSM CFRP laminate shear reinforcement. As a result, it was found that steel shear reinforcement exhibited an 85% increase in the shear capacity, whereas NSM CFRP exhibited a 102% increase in the shear capacity than the control specimen. Further, Zhang (2017) found that the bond between the epoxy and CFRP laminate was better in the NSM method than in the EB method. This was because the area of contact is more in the NSM method, and hence the separation failure of the concrete cover was prominent in the NSM method. In a recent study related to the groove spacing of CFRP laminates, Zhang (2017) observed that the bond strength of the CFRP laminate increases with an increase in the groove spacing. Comparing the different factors affecting the bond strength, Sharaky (2013) suggested that in the NSM method of CFRP, the failure load can be increased by 17% with an increase in 25% bond length. From the above discussions, the NSM method is observed to be a viable method for the shear strengthening of RC beams. Various past studies have established the enhancement in the flexural strength of the strengthened RC beam with different methods of CFRP, but further studies are required to interpret the performance of shear strengthening of RC beams using different techniques. Therefore, this study compares the shear strength enhancement of RC beams strengthened using three existing approaches such as NSM, EB and EBU, which can be effectively practiced at the site.

2 Experimental Program

2.1 Design and Casting of Beam Specimens

A total of 8 RC beams (4 pairs) were cast, along with 3 cubes and 3 cylinders. The beams cast were control beam specimens, beam specimens strengthened by the NSM method, beam specimens strengthened by the EB method and beam specimens strengthened by the EBU method. Cubes and cylinders were cast to ascertain the mechanical properties of concrete. All beams were of length 1m, width 0.1 m and depth 0.2 m. The grade of concrete adopted was M40. The reinforcement details for all the beams adopted were Fe 415 grade steel with 2 numbers of 12 mm diameter bars as the main reinforcement with 2 numbers of 12 mm diameter bars as hanger bars and stirrups of 8 mm diameter bars at 125 mm c/c. Figures 1a, b and 2 show the cross-section details, reinforcement arrangement and casting of RC beams.

Fig. 1

a Cross-section details. b Reinforcement details

Fig. 2

Cast RC beams

2.2 Preparation of RC Beam Specimens

The preparation procedure for strengthening the beam specimens using the three methods is explained below. The properties of the CFRP laminates and ply are stipulated in Table 1. A thixotropic epoxy adhesive resin named Sikadur-30 LP which is tested according to EN 1504-4 was utilised for the study.

Table 1 Properties of CFRP ply and laminates

2.2.1 Near Surface Mounting Method (NSM)

Near Surface Mounting method involves cutting a groove through the concrete (Fig. 3) and placing CFRP laminate inside the groove using an epoxy adhesive (Fig. 4). On a trial basis, two grooves were cut for the CFRP installation. Care was taken while cutting to ensure that no damage was caused to the steel reinforcement. As per ACI 440 2R.17 (2017), the groove’s size should be at least 3 times the width and 1.5 times the depth of CFRP laminate. Therefore, in order to place a 1.4 mm thick and 25 mm wide CFRP laminate, a 4 mm wide and 35 mm deep groove cut was made. This was due to the fact that, since it was a bond-critical application, the bond between the FRP and concrete should be 100% and also should ensure perfect load transfer between FRP and concrete. Similarly, the spacing between the two grooves was kept at twice the depth of the groove, which is 80 mm. The grooves were cleaned completely by removing all the dust from the groove. The two-component epoxy adhesive was mixed and filled in the groove completely, and further adhesive was applied on the surface of the CFRP ply and firmly placed inside the groove. The adhesive was allowed to cure overnight.

Fig. 3

Grooves for NSM

Fig. 4

Installation of CFRP

2.2.2 Externally Bonded Method (EB)

The externally bonded method involves preparing the surface and bonding the laminate. Being a bond-critical application, the bond between the FRP and concrete is very vital. The surface was prepared as stipulated by ACI 546R (2017), Concrete Surface Preparation Level 3 (CSP 3). Grinding of the surface was done extensively by using an electrically operated grinding machine to remove any form of lattice and dust. On a trial basis, it was decided to place CFRP laminates for the entire beam width by stacking together two laminates having a width of 50 mm and a thickness of 1.4 mm. Before attaching the laminate, the undulations on the concrete surface were leveled using thixotropic epoxy putty to ensure perfect bonding of the FRP and concrete. The adhesive was mixed thoroughly until a uniform colour was obtained using a mixer machine. The surplus adhesive was applied on the soffit of the beam, and the laminates were installed effectively using rollers to remove any entrapped air bubbles and to ensure that the contact area of FRP was completely coated with the epoxy adhesive, as shown in Figs. 5 and 6.

Fig. 5

Applying Epoxy adhesive for installation of CFRP

Fig. 6

CFRP laminate installed to the beam using EB Method

2.2.3 Externally Bonded with U—Wrap (EBU)

CFRP laminates were installed as described in EB, and the CFRP plies were applied as U-wraps to compare the performance of the combined action of externally bonded laminates and U-wraps. Additionally, the corners of the beams were rounded to an average radius of 15 mm for reducing the stress concentration in the FRP system as per ACI 44-2R.17 (2017) after the installation of the U-wrap. All the undulations on the concrete surface were leveled using thixotropic epoxy putty to ensure a uniform level surface for the U-wrap installation. Post installation of the CFRP laminate by EB method, epoxy primer was applied uniformly to the surface and allowed to cure for 24 h. A wet layup system of application was adopted for the installation of the CFRP U-wrap plies. 450 GSM CFRP fiber layers were saturated with epoxy matrix and laid over the concrete surface using a hand layup process. Rollers were used to ensure that the saturant passed through the fibers to provide a uniform layer of wrapping over the beam. The beam was left undisturbed for a period of 48 h. Figure 7 shows the beam specimen strengthened using the EBU method.

Fig. 7

CFRP laminate installed to the beam using the EBU Method

2.3 Experimental Setup

The three-point loading test was carried out using a Computerized Universal Testing Machine (UTM) of 100 ton capacity. Tests were carried out on the 28th day of casting and curing for all specimens. The three-point loading has been performed on the beam specimens to ascertain the shear capacity of the control beam and the strengthened beams, as shown in Fig. 8. The displacement was recorded at the mid-span of the beam corresponding to every load increment. The displacement readings were noted based on the UTM values as there were constraints in fitting LVDT or dial gauge. The deflection pertaining to the beam was based on the column movement in the UTM. The working length of the beam was taken as 0.8 m, and the specimens were loaded till the failure at a loading rate of 1.8 kN/min.

Fig. 8

Schematic diagram of three-point loading setup

3 Test Results and Discussions

The load–deflection plots of the tested RC specimens are shown in Fig. 9. It was observed that the control specimen exhibited diagonal cracks of 45 degrees from a distance of L/3 from the support region, which implies pure shear failure and the crack width increased with an increase in the applied load. The specimen failed in a brittle manner at a load of 70 kN. The NSM specimen exhibited the same cracking and failure pattern with a relative increase of 28.5% in the load-carrying capacity.

Fig. 9

Load versus Deflection for all specimens

The increase in the shear resistance is due to the dowel action of CFRP laminates. Concrete crushing was noticed in both specimen types at the loading point. Figures 10, 11, 12 and 13 showcase the different failure modes observed in the tested specimens. In the case of the EB strengthened beam, the increase in shear capacity was observed as 18.5%, which is marginally more than the control but lesser than NSM strengthened specimens. The EBU specimen showed a very high increase in the shear capacity. During testing, delamination occurred in the CFRP laminates, and cohesive failure was witnessed in the concrete layer that was bonded to CFRP laminates at the failure stage. The very high increase of 142% in shear was due to the confinement provided by the CFRP U-wraps, which in turn increased the compressive strength of the concrete by accounting for the increase in the shear strength of the specimen and also the presence of the CFRP laminate placed through EB method would account to the increase in the shear capacity of the EBU specimen.

Fig. 10

Concrete Crushing in control beam

Fig. 11

Cracking in NSM specimen

Fig. 12

Shear crack in EB specimen

Fig. 13

CFRP de-lamination and concrete crushing in EBU specimen

4 Summary and Conclusions

In this study, three different types of retrofitting techniques focused on enhancing the shear capacity of an RC beam are discussed. Based on the test results, the following conclusions are drawn.

• The EBU method of retrofitting showed a very prominent increase in the shear capacity of the beam compared to other techniques primarily due to the confinement provided by the CFRP wrap systems.

• The NSM is another efficient method that can be adopted to improve the shear capacity, which enhanced the performance by 25% due to its dowel action and higher stiffness (vertical orientation).

• The EB method did not show much increment in the shear strength compared to other methods because of delamination and lower stiffness (horizontal orientation) of CFRP laminates. Hence, EBU and NSM are better techniques to improve the shear strength of the RC beam.

• Concrete crushing and de-lamination of CFRP is the most prominent failure mode noticed in all cases. Surface preparation of the substrate is critical for the efficient performance of CFRP to prevent bond failure between concrete and CFRP.