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Technical Papers
Nov 7, 2022

Novel Embedded FRP Anchor for RC Beams Strengthened in Flexure with NSM FRP Bars: Concept and Behavior

Authors: Y. Ke https://orcid.org/0000-0003-1831-5409, S. S. Zhang, Aff.M.ASCE https://orcid.org/0000-0003-2218-1917 [email protected], S. T. Smith, F.ASCE https://orcid.org/0000-0002-8837-1214, and T. Yu, M.ASCEAuthor Affiliations
Publication: Journal of Composites for Construction
Volume 27, Issue 1
https://doi.org/10.1061/(ASCE)CC.1943-5614.0001279
PDF

Abstract

The near-surface-mounted (NSM) fiber-reinforced polymer (FRP) strengthening technique for reinforced-concrete (RC) beams has attracted worldwide research attention and application in the past two decades. In spite of improved bond strength between the NSM FRP and concrete, compared with the externally bonded (EB) FRP method, premature debonding failure still occurs, and, hence, the efficiency of the NSM FRP intervention is limited. An intuitive means to enhance efficiency is to apply anchorage devices to the RC beam; however, the availability of viable anchorage is limited for NSM applications. A novel anchorage device is therefore presented in this paper, which is referred to as an embedded FRP anchor (EFA). The concept, manufacture, and installation of the EFAs were initially presented. The effectiveness of EFAs in NSM FRP-strengthened RC beams was then ascertained through full-scale beam tests and comparison with an NSM FRP-strengthened beam anchored with a U-jacket. Moreover, the effects of EFA parameters on the behavior of anchored beam were also investigated. The test results not only prove the high anchoring efficiency of EFA but also reveal the significant effects of EFA parameters on the failure mode, load–deflection response, and NSM FRP strain of the EFA anchored beam.

Introduction

The strengthening of reinforced-concrete (RC) beams by using externally bonded (EB) fiber-reinforced polymer (FRP) plates has attracted extensive research studies and an increasing number of field applications to date (Teng et al. 2002; Hollaway and Teng 2008; Smith and Kim 2009; Hawileh et al. 2014; Wang et al. 2017; Mohammad 2022; Nie et al. 2022). A number of design guidelines have also been published to date (CS 2012; ACI 2017). A promising alternative to the EB FRP method is the near-surface mounting (NSM) of FRP materials (herein NSM FRP, unless indicated otherwise) such as bars and strips into the concrete cover layer of RC members. In the NSM FRP method, grooves are first cut into the surface of RC members, and then FRP bars are embedded into the grooves with high-viscosity adhesive. The NSM technique offers distinct advantages over EB FRP laminates, such as higher bond efficiency due to greater bond contact between the NSM FRP and concrete, and the concrete cover may better protect the FRP reinforcement from adverse physical and/or environmental effects, such as accidental impact, fire, and ultraviolet radiation (Barros and Fortes 2005; De Lorenzis and Teng 2007; Zhang et al. 2017).
Flexural strengthening is one of the main applications of the NSM FRP method that has resulted in numerous experimental studies (Hassan and Rizkalla 2003; Barros and Fortes 2005; Teng et al. 2006; Seracino et al. 2007; Bilotta et al. 2011; Peng et al. 2014; Sharaky et al. 2014; Dias et al. 2018; Zhao et al. 2020), numerical studies (Zhang and Teng 2014, 2016; Shabana et al. 2018; Sharaky et al. 2018), and theoretical studies (Zhang and Teng 2013; Teng et al. 2016; Zhang and Yu 2016). Existing studies on RC beams strengthened in flexure with NSM FRP bars have consistently demonstrated that debonding failure limits the effectiveness of FRP. Debonding has been commonly observed to initiate at the end of the NSM FRP bars in two main modes, namely (1) FRP end interfacial debonding (Hassan and Rizkalla 2003; Peng et al. 2014; Sharaky et al. 2014), and (2) FRP end concrete cover separation (Barros and Fortes 2005; Teng et al. 2006; Seracino et al. 2007; Peng et al. 2014; Sharaky et al. 2014; Dias et al. 2018).
The failure mechanisms of the two types of end debonding failures are summarized as follows. (1) FRP end interfacial debonding [Fig. 1(a)] is induced by high interfacial shear and normal stresses that develop near the NSM FRP end as a result of the abrupt FRP termination. Then, cracks form in the concrete near the FRP–concrete interface that finally leads to FRP–concrete interfacial debonding (Zhang and Teng 2013; Zhang and Yu 2016). (2) FRP end concrete cover separation [Fig. 1(b)] is initiated by the formation of a critical flexural crack near the NSM FRP end, and then a horizontal crack forms and propagates along the level of the longitudinal tension steel bars toward the beam midspan that finally causes detachment of the NSM FRP with the concrete cover attached (Zhang and Teng 2014; Teng et al. 2016). The different failure planes of interfacial debonding and concrete cover separation are schematically shown in Fig. 1(c). Concrete cover separation was found to be more common than interfacial debonding in NSM FRP flexurally strengthened RC beams (Zhang et al. 2017).
Fig. 1. Schematic representation of FRP end debonding failures: (a) FRP-concrete interfacial debonding; (b) concrete cover separation; and (c) sectional view.
A number of end anchorage measures in NSM FRP flexurally strengthened RC beams have been investigated to prevent or delay debonding failures (El-Hacha and Rizkalla 2004; Wu et al. 2014; Hosen et al. 2015; Sharaky et al. 2018; Shabana et al. 2018; Zhang et al. 2021). Wu et al. (2014) studied the effect of a steel anchorage device consisting of steel sleeves connected to an NSM bar with steel hoops bolted on the beam on the behavior of an NSM FRP flexurally strengthened RC beam. It was found that the use of such steel devices can prevent end debonding failure and enhance the load capacity of the beam by approximately 10% over the strengthened control beam without anchorage. Some researchers (El-Hacha and Rizkalla 2004; Shabana et al. 2018) utilized thermoplastic FRP bars that contained end hooks formed by bending the end parts of the bars to create 90° hooks for anchorage. They reported that end debonding, including concrete cover separation and interfacial debonding, was successfully delayed for NSM FRP bars containing such end-hook anchorage.
Hosen et al. (2015) and Sharaky et al. (2018) reported that the use of carbon fiber–reinforced polymer (CFRP) U-jackets can delay FRP end concrete cover separation and therefore lead to an enhancement in the load capacity by 14.5% to 33.1% and an increase in midspan deflection at the failure of the beam by 88.2% to 150% over the strengthened control beam. More recently, Zhang et al. (2021) reported the details of a systematic experimental investigation on full-scale RC beams strengthened with NSM CFRP strips that were anchored with FRP U-jackets of different material and geometrical configurations. The tests revealed that FRP U-jackets could delay or prevent concrete cover separation failure, and the strengthened beams experienced increases of 3.6% to 33.9% in load capacity and 17.7% to 79.9% in deformation capacity above the strengthened control. The performance of the strengthened beams was found to be dependent on the layout (number and amount) and inclination angle of the FRP U-jackets.
These studies have confirmed the effectiveness of several end anchorage measures for delaying debonding failure and improving the behavior of NSM FRP flexurally strengthened RC beams. However, a range of deficiencies of such existing end anchorage measures warrant further investigation. The key issues are as follows. (1) Steel anchorages usually suffer from corrosion and thus are unsuitable for aggressive environments unless a protective coating, that is usually costly, can be provided. (2) FRP U-jackets are effective in preventing concrete cover separation failure but less effective in preventing interfacial debonding between NSM FRP bars and concrete (Zhang et al. 2021). They are also not suitable for slabs that require flexural strengthening. Furthermore, a coating may be necessary to prevent FRP U-jackets from incisions/cuts and ultraviolet radiation if used in outdoor environments. (3) The mechanical bending of thermoplastic FRP bars usually involves a relatively complicated process that leads to a significant loss of bar strength at the bend.
Against this background, a novel embedded FRP anchor (EFA) for NSM FRP flexurally strengthened RC beams is proposed. This anchor, made mainly from fiber sheets and embedded in the beam, is proposed against the deficiencies of existing anchorage measures. The concept, manufacture, and installation process of the EFA are introduced in detail herewith. The details of an experimental study consisting of nine full-scale RC beams are also presented, including one unstrengthened control beam, one strengthened control beam, one beam containing U-jacket anchorage for comparison, and six beams containing EFA anchors with different parameters. The test results not only showed excellent performance of the EFA in delaying end debonding failures and thereby enhancing load and deformation capacities of the strengthened beams but also revealed the significant effects of EFA parameters on the behavior of EFA anchored beams.

Concept, Manufacture, and Installation of EFA

Concept

Fig. 2 provides a schematic representation of an installed EFA anchor as well as a close-up view. Two forms of EFAs, vertical EFA and inclined EFA, are shown in Fig. 2(a). The EFA is composed of sleeve and spike components as shown in Fig. 2(b), which is conveniently made by a rectangular fiber sheet through a wet lay-up process. The sleeve is formed by wrapping one end of an adhesive saturated rectangular fiber sheet onto the NSM FRP bar, and the spike is formed by twisting the remaining portion of the saturated fiber sheet into a bundle, then spirally binding it with a steel wire to maintain the shape of the spike. The EFA is adhesively embedded into the beam vertically or at a certain angle with respect to the beam axis. Fig. 2 shows that the EFA can be applied at the ends of NSM FRP bar where they are anchored into the core of the beam.
Fig. 2. Schematic representation of EFA: (a) installed anchor; and (b) close-up view of anchor.
Compared with existing end anchorage measures as reviewed in the “Introduction” (e.g., steel hoops, FRP U-jackets, FRP bar end hooks), the advantages of the proposed EFA include the following: (1) the EFA is not physically visible because it is embedded within the beam; thus, the appearance of the beam is not influenced. More importantly, the anchoring strength of EFAs can be higher than externally bonded FRP U-jackets if the same type and amount of FRP material is used for the anchor, as the former is less likely to debond from the RC beam; (2) the EFA is directly connected with the NSM FRP bar; thus, it can be effective for preventing/mitigating FRP end concrete cover separation and interfacial debonding failures, while FRP U-jackets are much less effective for the latter due to the limited bond contact between externally bonded U-jacket and NSM FRP bar; (3) the embedded EFAs are better protected by the concrete beam from possible physical and/or environmental attacks during service; (4) EFAs can be applied in NSM FRP flexurally strengthened RC slabs that do not have side surfaces to apply hoops or U-jackets; and (5) the manufacture and installation processes of EFAs are not overly complicated, considering the time-consuming treatment on the concrete surface for externally bonded anchorages (Wu et al. 2014; Fu et al. 2017a, b) is unnecessary for EFA; therefore, time and labor can be saved. The extent of hole-drilling required can be completed on-site with reasonably high quality and acceptable time.

Manufacture and Installation Process of EFAs

This section discusses manufacturing and installation of EFAs onto NSM strips. The approach is the same for application to NSM bars and other cross-sectional shapes.
The manufacturing process of an EFA is summarized in Fig. 3 as follows:
1.
Impregnate a rectangular fiber sheet with adhesive by using a roller. Make sure to squeeze out air bubbles during the process.
2.
Clean the surface of the NSM strip that will bond with the sleeve of the EFA using Aceton. Uniformly apply the adhesive to the surface area of the strip and limit the thickness to approximately 1 mm, as shown in Fig. 3(a). The adhesive thickness is maintained flush with 1-mm-thick steel wires that entwine the strip.
3.
Wrap one end of the NSM strip with the saturated fiber sheet to form the sleeve of designated length L1 and thickness, as shown in Fig. 3(b).
4.
Twist and bind the remaining portion of the fiber sheet into a bundle with a permanent spirally applied steel wire to form the spike of designated length L2, as shown in Fig. 3(c).
Fig. 3. Manufacture process of an EFA: (a) application of adhesive; (b) formation of sleeve component; and (c) formation of spike component.
The installation process of an NSM strip with EFAs is provided in relation to Fig. 4 as follows:
1.
Cut the groove using a circular saw near the beam surface where the NSM strip is to be mounted. Drill the anchor hole with a diameter of approximately 1.5 times the spike diameter and a depth of approximately 1.2 times the spike length at the position corresponding to the NSM strip end, as shown in Fig. 4(b). The anchor holes will be at a designated angle to the beam surface (45° or 90° in the present study). The exact direction of the anchor hole can be slightly adjusted in practical applications to avoid clashing with internal reinforcement.
2.
Clean the groove and anchor holes using a compressed air gun. Then, inject adhesive into the anchor holes by using a syringe and then fill the groove with adhesive halfway, as shown in Fig. 4(c).
3.
Insert the EFA spikes into the anchor holes at both ends of the NSM strip, and at the same time press the NSM strip into the groove, as shown in Fig. 4(d). Then, apply additional adhesive to fully fill the groove. It should be noted that the installation of the NSM strip with inclined EFAs should be conducted prior to setting of the adhesive on the EFA, when the EFA spike are still flexible to be bent into position.
Fig. 4. Installation steps for NSM strip with an EFA: (a) schematic of installation; (b) saw-cut groove and drill anchor hole; (c) fill groove and anchor hole with adhesive; and (d) insert NSM strip with EFA.

Experimental Program

Test Specimens

The experimental program consisted of testing nine RC beams under four-point bending to failure. Fig. 5 shows the nominal dimensions and steel reinforcement details of the beams. All the beams were designed to contain a low longitudinal steel reinforcement ratio, in order to simulate RC beams with insufficient flexural capacity that could be caused by incorrect design, construction errors, and corrosion of steel bars. A compound NSM CFRP strip was used for the strengthened beams, which was formed by adhering two single strips of 2 mm in thickness back to back with an approximately 1-mm-thick adhesive layer in between, leading to a total thickness of approximately 5 mm. The length and height of the NSM strips were 2,200 and 16 mm, respectively. The NSM strip was inserted into a groove located at the center of the beam soffit and also placed symmetrically in the longitudinal direction with respect to the beam midspan. The NSM-strengthening was designed with reference to a previous study (Teng et al. 2006) to ensure FRP end concrete cover separation was the critical failure mode for the NSM FRP-strengthened beam without anchorage.
Fig. 5. Dimensions and reinforcement details of test beams (unit in mm): (a) elevation; (b) unstrengthened section; and (c) strengthened section.
One beam was not strengthened with an NSM strip, thus serving as the control beam. The remaining eight beams were all strengthened in flexure with the same amount of NSM strip material. One of these strengthened beams did not contain end anchorage and hence also served as a control. The end anchorage schemes adopted for the remaining seven beams are shown in Fig. 6. In Fig. 6, Specimen UB contained vertical carbon FRP (CFRP) U-jackets at the ends of the NSM strip, where the strip was anchored with a 100-mm-wide U-jacket of the test end, while the other end was anchored with a 200-mm-wide U-jacket to ensure that failure did not occur at this end. This design strategy, which enables test observation and U-jacket strain measurement to be focused on only one end of the NSM strengthening strips, has been adopted by other research groups to date (e.g., Fu et al. 2017a, b). The remaining six beams contained EFAs of different design parameters at the ends of the NSM strip. The material type and plies of EFA, length of sleeve, and inclination angle of spike were chosen as the studied parameters, while the same spike length of 150 mm was adopted for all the EFAs to ensure that the spikes reach the beam core. The width of the rectangular carbon-fiber sheet used for making the EFAs was determined by the number of plies of EFA multiplying the sectional perimeter of NSM strip, and the length of that was determined by the sum of spike length and sleeve length. The carbon-fiber sheet used for 1-ply U-jacket located at the test end of Specimen UB had a width of 100 mm and a total length of 750 mm (i.e., 2 × 300 mm for two side components plus 150 mm for the bottom component).
Fig. 6. NSM FRP and end anchorage details (unit in mm): (a) UB; (b) E-C-2-120-90 and E-G-2-120-90; (c) E-C-4-120-90; (d) E-C-4-120-45; (e) E-C-4-200-45; and (f) E-C-8-200-45.
The details of all specimens are provided in Table 1, in which CB = unstrengthened control beam, SB = control beam strengthened in flexure with NSM strip but without anchorage (referred to as strengthened control beam hereafter), and UB = NSM-strengthened beam anchored with FRP U-jackets. The naming of specimens anchored with EFAs start with a letter “E,” followed by a letter “C” or “G” to represent the EFA made of CFRP sheet and GFRP sheet, respectively, followed by the numeral “2” or “4” to represent the number of plies of EFA, followed by the numeral “120” or “200” to represent the length of EFA sleeve, followed by the numeral “45” or “90” to represent the inclination angle of EFA spike. For example, E-C-2-120-90 refers to the beam anchored with 2-ply CFRP EFAs composed of a 120-mm sleeve and a spike vertical to the beam axis.
Table 1. Specimen and anchorage details
SpecimenFlexural NSM strengtheningAnchorageSurface area of fiber sheet used to make anchorage (mm2)
Anchorage typeMaterial typeNumber of pliesSleeve length (mm)Spike length (mm)Inclined angle of spikea (°)
CB————————
SBCFRP strip———————
UBFRP U-jacketCFRP1———75,000
E-C-2-120-90EFA21201509021,600
E-G-2-120-90GFRP21209021,600
E-C-4-120-90CFRP41209043,200
E-C-4-120-45 41204543,200
E-C-4-200-45 42004556,000
E-C-8-200-45 820045112,000
aRelative to longitudinal beam axis.

Material Properties

Table 2 provides details of the cylinder compressive strength and elastic modulus properties of the concrete contained in each beam. These results were averaged from three plain cylinders with a diameter of 150 mm and a height of 300 mm tested on the same day of each beam test following the appropriate British standard (BSI 2009, 2013). The tensile strength and elastic modulus of the steel bars were tested in accordance with BS EN ISO 6892-1 (BSI 2019). The yield stress, ultimate stress, and elastic modulus are, respectively, 502, 607 MPa, and 192 GPa for the 14-mm deformed steel bars, and 497, 598 MPa, and 195 GPa for the 10-mm deformed steel bars.
Table 2. Summary of test results
Specimenfc (MPa)Ec (GPa)Concrete crackingYield of tension steel barsBeam failure (ultimate)ηload-CB (%)ηload-SB (%)ηdeflection-SB (%)αstrip (%)Failure mode
Pc (kN)δc (mm)Py (kN)δy (mm)Pu (kN)δu (mm)εstrip (µε)εconcrete (µε)
CB48.230.126.21.693.116.297.0———0———TC
SB47.630.224.31.7109.518.2117.324.44,642−1,692210035CCS
UB47.630.221.61.5107.218.5128.233.06,823−2,0313293552STD + CCS
E-C-2-120-9048.930.425.11.4109.317.7135.235.07,529−2,21239154357RE + CCS
E-G-2-120-9048.930.425.81.8110.218.1132.133.47,239−2,17036133755RE + CCS
E-C-4-120-9048.730.623.21.9109.218.7142.037.68,703−2,24746215466SLD + CCS
E-C-4-120-4548.730.625.31.8113.919.1143.537.98,994−2,31148225568DS + CCS
E-C-4-200-4549.230.624.81.7110.018.3151.041.49,412−2,54656297071RE + CCS
E-C-8-200-4549.230.625.11.6113.016.0158.237.610,181−2,66363355477SSD + CCS
Notes: fc = concrete cylinder compressive strength; Ec = concrete elastic modulus; Pc, Py, Pu = load at first cracking, yield and ultimate; δc, δy, δu = midspan deflection at first cracking, yield and ultimate; εstrip = maximum tensile strain in NSM strip; εconcrete = maximum compressive strain of concrete; ηload-CB = % increase in load Pu compared with Specimen CB; ηload-SB = % increase in load Pu compared with Specimen SB; ηdeflection-SB = % increase in midspan deflection δu compared with Specimen SB; αstrip = % utilization of ultimate tensile strain of NSM strip (tested ultimate tensile strain of CFRP strip = 13,172 µε); TC = yield of the longitudinal tension steel bars followed by concrete compressive crushing on the top surface of beam; CCS = concrete cover separation at end of NSM strip; STD + CCS = strip-concrete interfacial debonding at end of NSM strip followed by concrete cover separation; RE + CCS = Rupture of EFA followed by concrete cover separation; SLD + CCS = sleeve-concrete interfacial debonding and concrete cover separation; DS + CCS = delamination of sleeve followed by concrete cover separation; SSD + CCS = strip-sleeve interfacial debonding followed by concrete cover separation nearby.
The tensile properties of the CFRP strips as well as the CFRP sheets (formed by a wet lay-up procedure from one layer of a 0.167-mm nominally thick carbon-fiber sheet) used for the FRP U-jackets and EFAs, were tested according to ASTM D3039/D3039M-17 (ASTM 2017) and ASTM D7565/D7565M-10 (ASTM 2010), respectively. The tensile strength, elastic modulus, and ultimate tensile strain averaged from five tensile coupons are 1,989 MPa, 151 GPa, and 1.32%, respectively, for the compound CFRP strips, and 3,868 MPa, 247 GPa, and 1.59%, respectively, for the CFRP sheets; it should be noted that the tensile properties of CFRP sheets were calculated based on the nominal thickness of the carbon-fiber sheet (0.167 mm). The tensile properties of the adhesives were tested according to ASTM D638-14 (ASTM 2014). The corresponding tensile strength, elastic modulus, and ultimate tensile strain averaged from five coupon tests are 31.2 MPa, 3.4 GPa, and 1.10%, respectively, for the epoxy adhesive (Sika-330) used for the installation of the NSM strips and EFAs, and 49.7 MPa, 3.2 GPa, and 1.85%, respectively, for the adhesive (Lica-100A/B) used for the installation of the U-jackets.

Preparation of Specimens

All the RC beams were cast together from one batch of concrete. For all NSM FRP-strengthened beams, a groove of 22 mm in depth and 10 mm in width was cut with a concrete saw along the centerline of the beam soffit to install the FRP strip and adhesive.

Installation of U-Jackets

Seven days after the NSM strip was inserted into the groove with adhesive, the FRP U-jackets were installed. First, the bond region of the concrete was roughened by a compressed air-driven needle gun to remove the weak cement laitance on the surface. Second, the corners of the beam soffit were rounded to a radius of 15 mm to reduce stress concentration in the U-jacket. Finally, the FRP U-jackets were installed onto the beam using adhesive through the wet lay-up process.

Installation of EFAs

The manufacturing and installation processes of EFA has been detailed. In addition, (1) the groove near the NSM strip end was locally widened to approximately 15 mm to accommodate the EFA, as shown in Fig. 4(b); and (2) the connection regions between anchor holes and the grooves were rounded to a radius of approximately 10 mm using a portable concrete grinder to alleviate the stress concentration at the EFA bend region.

Test Set-Up and Instrumentation

The tests were conducted within a closed-loop test frame with the load applied via a 1,000-kN capacity servo-controlled actuator, as shown in Fig. 7(a). The load was applied in a load control manner of 5-kN load increments until yield of the longitudinal tension steel bars, after which the load was applied using displacement control with 1 mm increments until beam failure. The load application of each step was completed in about 1 min, then the load was paused for approximately 1 min to mark the formation and development of cracking of the beams. Such a loading and pausing scheme has been widely adopted in previous experimental studies (Teng et al. 2006; Hosen et al. 2015; Fu et al. 2017a) on the behavior of FRP flexurally strengthened beams, based on which the load–vdeflection response and crack development of beam can be well recorded.
Fig. 7. Test setup and strain gauge layout (unit in mm): (a) test set-up; (b) strain gauges applied to concrete and steel bars; (c) strain gauges applied to NSM strip; and (d) strain gauges applied to FRP U-jacket.
Linear variable displacement transducers (LVDTs) were placed at supports, load points, and midspan of the beam to measure beam deflections at these locations, as shown in Fig. 7(b).
Strain gauges were bonded onto the concrete, steel bars, NSM CFRP strips, and FRP U-jackets. As shown in Fig. 7(b), two parallel strain gauges of a 100-mm gauge length (SG-1 and SG-2) were attached to the top surface of the beam to measure the concrete compressive strains. Four 6-mm-length strain gauges were attached to the longitudinal tension steel bars to record their strain development.
As shown in Fig. 7(c), strain gauges of a 6-mm gauge length were bonded to each NSM strip. The gauges were sandwiched between the two strips which were used to form the compound NSM strip. The gauges were spaced closer near the end of strip to capture the deep strain gradient therein. As shown in Fig. 7(d), 20-mm-length strain gauges were adhered onto both U-jacket legs at the test end of Specimen UB to capture the strain distributions along the centerline and width of U-jackets.

Test Results and Discussion

Failure Modes and Failure Processes

The control beam without strengthening (Specimen CB) failed in accordance with the design expectation, namely by yield of the longitudinal tension steel bars followed by concrete compressive crushing on the top surface of the beam (TC), as shown in Fig. 8. The ultimate load (i.e., the load at concrete compressive crushing) of Specimen CB is 97 kN, which is close to the theoretical one obtained by a sectional analysis (95 kN). Photographs of the failure modes of the remaining eight NSM-strengthened beams are shown in Figs. 9–15. A detailed account of the failure process of each strengthened beam, including commentary on the influence of each anchorage, is provided in the following sections.
Fig. 8. Failure mode of Specimen CB (TC).
Fig. 9. Failure mode of Specimen SB (CCS): (a) overall view; and (b) close-up view.
Fig. 10. Failure mode of Specimen UB (STD + CCS): (a) soffit view; and (b) side view.
Fig. 11. Failure mode of Specimens E-C-2-120-90 and E-G-2-120-90 (RE + CCS): (a) E-C-2-120-90; and (b) E-G-2-120-90.
Fig. 12. Failure mode of Specimen E-C-4-120-90 (SLD + CCS): (a) soffit view; and (b) close-up view of EFA (post-test).
Fig. 13. Failure mode of Specimen E-C-4-120-45 (DS + CCS): (a) close-up view (post-test); and (b) cutaway view (post-test).
Fig. 14. Failure mode of Specimen E-C-4-200-45 (RE + CCS): (a) side view; and (b) cutaway view (post-test).
Fig. 15. Failure mode of Specimen E-C-8-200-45 (SSD + CCS): (a) side view; and (b) close-up view.

Specimen SB

The strengthened beam without end anchorage failed by concrete cover separation (CCS) at the NSM strip end, as shown in Fig. 9. The failure process is summarized as follows: (1) a flexural–shear crack formed at the NSM strip end at a load of approximately 55 kN; (2) as the load increased to 100 kN, a horizontal crack formed at the intersection of this flexural–shear crack and the longitudinal tension steel bars, which then started to propagate along the plane of the bottom steel bars toward the beam midspan; and (3) complete concrete cover separation [Fig. 9(b)] occurred at a load of 117.3 kN (i.e., the ultimate load recorded).

Specimen UB

The beam anchored with FRP U-jackets failed by strip-concrete interfacial debonding at the NSM strip end (referred to as STD) followed by concrete cover separation nearby (collectively referred to as STD + CCS) in the test end of beam as shown in Fig. 10, while no visible failure occurred in the non-test end during the test. The failure process was as follows: (1) at a load of 55 kN, a critical flexural–shear crack [Crack No.13 in Fig. 10(b)] formed near the inner edge (i.e., the edge closer to the beam midspan) of the U-jacket at the test end; (2) as the load increased, high normal and shear stresses developed along the strip–concrete interface near the NSM strip end; (3) at a load of 128.2 kN (i.e., the ultimate load recorded), longitudinal concrete cracks formed near the strip–concrete interface at the NSM strip end; thus, the end portion of the NSM strip between the critical flexural–shear crack and the NSM strip end was partially “pulled out” from the concrete cover confined by the U-jacket [Fig. 10(a)]; and (4) the concrete cover separation then rapidly occurred near the inner edge of the U-jacket and propagated to the beam midspan [Fig. 10(b)].

Specimens E-C-2-120-90 and E-G-2-120-90

The strengthened beams anchored with 2-ply (CFRP and GFRP) EFAs composed of a 120-mm sleeve and vertical spike both failed on one side of the beam by rupture of EFA (RE) followed by concrete cover separation (collectively referred to as RE + CCS), as shown in Fig. 11. The failure process is as follows: (1) a flexural–shear crack formed at the end of NSM strip and crossed the joint of sleeve and spike; and (2) large tensile and shear stresses generated at the joint of sleeve and spike with the widening of this crack and finally led to rupture of EFA therein.

Specimen E-C-4-120-90

The strengthened beam anchored with 4-ply CFRP EFAs composed of a 120-mm sleeve and vertical spike failed on one side of the beam by combined sleeve–concrete interfacial debonding (SLD) and concrete cover separation (collectively referred to as SLD + CCS), as shown in Fig. 12. The failure process is as follows: (1) a flexural–shear crack formed near the inner edge (i.e., the edge closer to the beam midspan) of the sleeve at a load of 55 kN that widened to 0.74 mm at a load of 130 kN; (2) as the applied load further increased, longitudinal cracks developed in the concrete near the sleeve-concrete interface and finally induced sleeve-concrete interfacial debonding at a load of 142 kN (i.e., the ultimate load recorded). Almost at the same load, concrete cover separated from the beam located near the sleeve [Fig. 12(a)]. A post-test inspection on the EFA is shown in Fig. 12(b), from which it can be seen that the sleeve–concrete interfacial debonding occurred in the concrete side near the sleeve–concrete interface.

Specimen E-C-4-120-45

The strengthened beam anchored with 4-ply CFRP EFAs composed of a 120-mm sleeve and inclined spike failed on one side of the beam by delamination (i.e., interlayer debonding) of the sleeve, followed by concrete cover separation nearby (referred to as DS + CCS), as shown in Fig. 13. The failure process occurred as follows: (1) a flexural–shear crack formed near the inner edge of the sleeve at a load of approximately 55 kN; (2) as the load increased to 143.5 kN (i.e., the ultimate load recorded), interlayer debonding in the fiber-sheet layers of sleeve occurred, then visible sliding of the NSM strip can be observed, as the end portion of NSM strip with part of sleeve was “pulled out” from the concrete cover. Finally, concrete cover separation occurred near the inner sleeve edge and propagated to the beam midspan, as shown in Fig. 13(a). As a result of a post-test inspection, a portion of the concrete cover near the NSM strip end was removed, as shown in Fig. 13(b). Here, EFA sleeve delamination can be seen where part of the sleeve remained bonded with the end of the NSM strip, and the other part of the sleeve remained attached to the RC beam.

Specimen E-C-4-200-45

The strengthened beam anchored with 4-ply CFRP EFAs composed of a 200-mm sleeve and inclined spike failed on one side of the beam by rupture of EFA followed by concrete cover separation (RE + CCS), as shown in Fig. 14. Similar with Specimens E-C-2-120-90 and E-G-2-120-90, the rupture of EFA also occurred at the joint of sleeve and spike in Specimen E-C-4-200-45. However, the ultimate load of the latter is higher than those of the formers, due to the fact that the EFAs used for the latter is thicker.

Specimen E-C-8-200-45

The strengthened beam anchored with 8-ply CFRP EFAs composed of a 200-mm sleeve and inclined spike failed on one side of the beam by strip–sleeve interfacial debonding followed by CCS nearby (referred to as SSD + CCS), as shown in Fig. 15. The failure process of Specimen E-C-8-200-45 is similar with that of Specimen E-C-4-120-45, but the debonding occurred at the strip–sleeve interface in Specimen E-C-8-200-45.

Load–Deflection Responses

The load versus midspan deflection responses of all specimens are plotted in Fig. 16. In addition, a summary of key results including the load versus midspan deflection as well as strain data of each specimen at critical stages including concrete cracking, yield of longitudinal tension steel bars, and failure of beam is provided in Table 2. Note that the failure of beam refers to the ultimate load for the NSM-strengthened beams, while for the control beam (Specimen CB), the failure of beam refers to compressive crushing of concrete on the account that the load almost remained constant after yield of the tension steel bars.
Fig. 16. Load–deflection responses.
The load–deflection curves of all the beams exhibit a three-segment response, including two turning points corresponding to concrete cracking and yield of tension steel bars, respectively. Compared with the control beam CB, a slightly larger gradient and a higher load at yield of the tension steel bars can be observed in the strengthened beams. In addition, the load continues to increase with an increase in displacement after yield of the tension steel bars in the NSM-strengthened beams due to the NSM strip contribution, which is different from that of control beam CB.
As presented in Table 2, addition of the NSM strip can enhance the load capacity of the beam where the ultimate load of Specimen SB is 21% higher than Specimen CB. With the application of U-jackets at the NSM strip ends, the behavior of the strengthened beam was improved where the ultimate load and midspan deflection at failure of Specimen UB was increased by 9% and 35%, respectively, compared with those of Specimen SB. Upon application of the EFA devices, the behavior of the strengthened beam was further improved, with the increases in ultimate load being 13% to 35%, and the increases in midspan deflection at failure being 37% to 70%, compared with Specimen SB. Although the amount of fiber sheet used for making EFAs in all the EFA anchored specimens except Specimen E-C-8-200-45 is less than that used for the U-jacket in Specimen UB. Therefore, the proposed novel EFA can be a more cost-effective end anchor, on a material basis than U-jackets, for NSM FRP flexurally strengthened beams.

Strain and Shear Bond Stress Distributions

Strain Distributions in NSM Strips

The distributions of strains along the NSM strips for all strengthened beams at the ultimate load are shown in Fig. 17. The distributions exhibit an ascending branch from the NSM strip end to the loading point and then exhibit a near uniform trend in the pure bending region.
Fig. 17. Strain distributions along NSM strips at failure of NSM-strengthened beams.
Table 2 presents that the maximum percentage utilization of the ultimate tensile strain of the NSM strip is from 55% to 77% for the specimens anchored with EFAs, while the efficiency is approximately 52% for the specimen containing U-jackets (Specimen UB) and only 35% for the specimen without end anchorage (Specimen SB). These results indicate the excellent performance of EFAs in enhancing the debonding resistance of NSM strips when compared with standard U-jacket anchorage. It should be noted that the test debonding strain (4,642 µε) of NSM FRP strip in the specimen without end anchorage was found to be very close to the prediction (4,314 µε) by De Lorenzis and Nanni’s (2003) model. For NSM FRP system with end anchorage, however, there has been no calculation method for predicting the FRP debonding strain so far, indicating the necessity of future studies.

Strain Distributions in FRP U-Jacket

Fig. 18 plots the strain distributions in the U-jacket at concrete cracking, yield of longitudinal tension steel bars, and beam failure. In this figure, the strain levels along the U-jacket centerline [Fig. 18(a)] are very low at and before yield of the longitudinal tension steel bars. At beam failure, the maximum strain along the U-jacket centerline peaks at 50 mm above the beam bottom (i.e., the level of longitudinal tension steel bars), indicating that a large horizontal crack existed therein. The three strain readings in the U-jacket at 50 mm above the beam bottom are plotted in Fig. 18(b), which shows that the maximum strain in the U-jacket occurred near the inner edge of the U-jacket (i.e., the edge closer to beam midspan). This finding is due to concrete cover separation that occurred near the inner edge of the U-jacket, as shown in Fig. 10(b).
Fig. 18. Strain distributions on U-jacket on test side of Specimen UB: (a) centerline of U-jacket; and (b) 50 mm above beam bottom.
The maximum strain in the U-jacket at beam failure is approximately 2,100 με, which is only 13% of its ultimate tensile strain (i.e., 15,943 µε). The relatively low utilization can be attributed to the following reason. The U-jacket can effectively restrain concrete cover separation in its vicinity but not NSM strip-concrete interfacial debonding, on the account that the U-jackets and NSM strip were not bonded together. Once NSM strip-concrete debonding occurs in the U-jacket region, the subsequent concrete cover separation occurs nearby.

Shear Bond Stress Distributions along NSM Strips

The shear bond stresses τ (referred to as bond stress or bond stresses hereafter) on the strip-concrete interface can be calculated based on the strain distribution along the NSM strip using the following equations (Teng et al. 2013):
τ0=(ε1−ε0)EstripAstripCgrooveD0,1
(1)
τi=EstripAstrip2Cgroove×(εi−εi−1Di−1,i+εi+1−εiDi,i+1),(i=1,2,…,12)
(2)
τ13=(ε13−ε12)EstripAstripCgrooveD12,13
(3)
where Estrip, Astrip, and Cstrip = elastic modulus, cross-sectional area, and cross-sectional perimeter of groove (i.e., the total length of the three sides of the groove), respectively; i = number of strain-measuring points commencing from the NSM strip end to the beam midspan (i = 0 refers to the strip end where the strain ɛ0 is assumed to be zero); ɛi = measured strain at the ith point; and Dm,n = distance between the mth and nth measuring points. Note that the bond stresses obtained using Eqs. (1) and (3) are average stresses at midlength between two adjacent measuring points. Therefore, τ0 = bond stress located 10 mm away from the strip end, while τ13 = bond stress located 150 mm away from the midspan of beam, as shown in Fig. 7(c).
The bond stress distribution along the NSM strip in each strengthened beam at the ultimate load is shown in Fig. 19. In the pure bending region, the bond stresses are nearly zero, and in the shear span region, the bond stresses fluctuate on account of the presence of cracks. For the specimens with end anchorages, the bond stresses are above 10 MPa at 10 mm away from the strip end and rapidly drop to about 4 MPa at approximately 50 mm away from the strip end. Then, they gradually decrease to nearly zero near the section corresponding to the loading point. For the specimen without end anchorage, the bond stress is zero near the strip end and gradually increases to approximately 5 MPa at around 150 mm away from the strip end. Then, it drops to nearly zero near the loading point.
Fig. 19. Shear bond stress distributions along NSM strips at beam failure.
To explore the reason for this phenomenon, the bond stress τ0 is plotted against the applied load in Fig. 20 for all strengthened beams. This figure shows that the stress τ0 in the specimen without end anchorage (i.e., Specimen SB) increases with the applied load until a load of 85 kN is reached. Then, it gradually drops to zero due to the initiation and propagation of the horizontal crack along the longitudinal tension steel bars near the strip end which finally causes concrete cover separation failure. The stress τ0 in the U-jacket specimen (i.e., Specimen UB) increases with the applied load until a load of 55 kN is reached. It then starts to decrease slightly as a result of the initiation of NSM strip–concrete interfacial debonding. Friction existed between the NSM strip and concrete due to the confinement effect of the U-jacket onto the concrete cover, thereby resulting in the stress τ0 reaching 10.9 MPa at beam failure. The stress τ0 for the specimens with EFAs continues to increase during the entire loading process and finally reaches 12.4 to 18.2 MPa, which is higher than that in Specimen UB. This comparison clearly reveals that the proposed EFA is more effective than a traditional U-jacket in delaying end debonding failure, as the former is directly connected with the NSM strip and deeply embedded in the concrete.
Fig. 20. Shear bond stresses at 10 mm from the end of NSM strip versus applied load.

Effects of EFA Parameters

The design parameters of EFA including material type and the thickness of EFA, length of sleeve, and inclination angle of spike were found to have effects on the behavior of NSM-strengthened beams anchored with EFAs. Therefore, these effects are clarified through comparisons and analyses in the following sections.

Material Type of EFA

The EFAs of Specimens E-C-2-120-90 and E-G-2-120-90 are made of CFRP and GFRP sheet respectively. The strength and elastic modulus of the former are both higher than those of the latter. Both specimens failed by rupture of EFA, but Specimen E-C-2-120-90 shows a slightly higher ultimate load (135.2 kN) than Specimen E-G-2-120-90 (132.1 kN), the maximum strain in NSM strip of the former (7,529 με) is also slightly higher than the latter (7,239 με). This indicates a better performance of CFRP EFA than GFRP EFA. However, the number of plies of EFAs for these two specimens is small (only 2 plies); the difference between their performances is, therefore, less obvious.

Thickness of EFA

Specimens E-C-2-120-90, E-C-4-120-90, E-C-4-200-45, and E-C-8-200-45 are compared in order to investigate the effect of thickness of EFA on the performance of the anchored beam.
The EFAs of Specimens E-C-2-120-90 and E-C-4-120-90 are 2 and 4 ply, respectively, with the other parameters kept the same. Specimen E-C-2-120-90 failed by a rupture at the joint of EFA sleeve and spike, while Specimen E-C-4-120-90 failed by sleeve–concrete interfacial debonding. The ultimate load of Specimen E-C-4-120-90 (142.0 kN) is higher than that of Specimen E-C-2-120-90 (135.2 kN) by 5%; the maximum strain in strip of the former (8,703 με) is also higher than that of the latter (7,529 με) by 16%.
The EFAs of Specimens E-C-4-200-45 and E-C-8-200-45 are 4 and 8 ply, respectively, with the other parameters kept the same. Specimen E-C-4-200-45 failed by rupture at the joint of EFA sleeve and spike, while Specimen E-C-8-200-45 failed by strip-sleeve interfacial debonding. The ultimate load (158.2 kN) and maximum strain in the NSM strip (10,181 με) of Specimen E-C-8-200-45 are higher than those of Specimen E-C-4-200-45 (151.0 kN and 9,412 με) by 5% and 8%, respectively. And it can be seen from Fig. 16 that the stiffness of Specimen E-C-8-200-45 is obviously higher than the other specimens.
This indicates that increasing the thickness of EFAs could avoid the rupture of EFA at the joint of sleeve and spike; however, the failure was shifted to the sleeve side. In general, the increase of EFA thickness improved the load capacity of the anchored beam, and the flexural stiffness of beam was significantly improved when relatively thick EFAs were used for anchoring.

Length of Sleeve

The sleeve lengths of EFAs for Specimens E-C-4-120-45 and E-C-4-200-45 are 120 and 200 mm, respectively. The test results show that Specimen E-C-4-120-45 with a shorter sleeve length (120 mm) failed by delamination of sleeve, while Specimen E-C-4-200-45 with a longer sleeve length (200 mm) failed by EFA rupture at the joint of sleeve and spike. The ultimate load (151.0 kN) and corresponding midspan deflection (41.4 mm) of Specimen E-C-4-200-45 are higher than those of Specimen E-C-4-120-45 (143.5 kN and 37.9 mm) by 5% and 9%, respectively, and the maximum strain in NSM FRP strip of the former (9,412 με) is also higher than that of the latter (8,994 με) by 5%. This indicates that the increase of sleeve length enhanced the strength on the sleeve side, leading to a better performance of the anchored beam, but the failure was shifted to the joint of sleeve and spike again.

Inclination Angle of Spike

Different spike inclination angles (i.e., vertical or 45° to the beam axis) resulted in similar ultimate loads and corresponding midspan deflections (i.e., 142.0 kN and 37.6 mm for Specimen E-C-4-120-90; 143.5 kN and 37.9 mm for Specimen E-C-4-120-45). However, different failure modes were observed, as Specimen E-C-4-120-90 failed by interfacial debonding between sleeve and concrete, while Specimen E-C-4-120-45 failed by delamination of sleeve. This finding indicates that the spike inclination angle has an influence on the behavior of beams with EFAs. However, the sleeve length in the studied cases is relatively short; thus, the difference between the failure strengths of sleeve–concrete interfacial debonding and sleeve delamination is less evident, thereby leading to similar load–deflection curves of Specimens E-C-4-120-90 and E-C-4-120-45. It is, therefore, not unreasonable to expect that if a longer sleeve, which means a larger bond length between sleeve and strip, was used in EFAs, the effect of spike inclination on beam performance could be more significant. Moreover, it can be observed from Fig. 20 that the stress τ0 in Specimen E-C-4-120-45 is much higher than that of E-C-4-120-90 from the load of 60 kN to the ultimate load of beam, indicating that the inclined EFAs were activated at a lower load level compared with the vertical EFAs. This can be explained as the inclined spike is better in transferring the stresses at the strip end into the concrete core as a result of its horizontal projection.

Conclusions

This paper presents the details of a proof-of-concept experimental study on a novel embedded FRP anchor (EFA) for preventing or mitigating FRP end debonding failures of NSM FRP flexurally strengthened RC beams. The concept, manufacturing, and installation of the novel EFA are introduced in detail. The superiority of the EFA over traditional FRP U-jackets in terms of improving the flexural behavior of the beams is verified through an experimental study containing nine full-scale RC beam tests, in which the effects of EFA parameters on the beam behaviors are also investigated. Based on the test results, the following conclusions can be drawn:
1.
The EFA can be applied at the ends of the NSM FRP bar, and its various parameters can be designed to satisfy different strength requirements. Such parameters include the material type, length, and thickness of the spike and/or sleeve, as well as the inclination angle of the spike. In addition, this research program confirmed that the manufacturing and installation process of the proposed EFA was not onerous and is, therefore, a viable method for field applications.
2.
In the present study, the use of EFAs led to an increase in the load-carrying capacity of the NSM-strengthened beam by 13% to 35%, an increase in midspan deflection at beam failure by 37% to 70%, and an increase in the utilization percentage of the tensile strength of NSM strip by 56% to 119%, compared with the NSM-strengthened beam without end anchorage. The corresponding increases arising from the use of FRP U-jackets were only 9%, 35%, and 47%, respectively, although the amount of fiber sheet used for making the EFAs in all the EFA-anchored specimens except Specimen E-C-8-200-45 is less than that for making the U-jacket in Specimen UB.
3.
The effect of material type of EFA on the behavior of anchored beam is less obvious in the present study. This is because that the thickness of EFAs used in the studied cases is too small to reflect the difference in beam behavior caused by different EFA material type.
4.
The increase of EFA thickness and sleeve length both improved the load capacity and maximum NSM FRP strain of EFA anchored beam. The failure mode was also significantly affected by the EFA thickness and sleeve length: the increase of EFA thickness and sleeve length led to debonding on the sleeve side and rupture at the joint of sleeve and spike, respectively.
5.
The spike inclination angle of EFA also has significant effect on the beam behavior. In the present study, this effect was mainly reflected by the failure modes of beams, as the beam with vertical EFAs failed by the sleeve–concrete interfacial debonding, while the beam with inclined EFAs failed by delamination of the sleeve.
6.
The present study presents the manufacturing and installation process of the proposed EFA in considerable detail so as to allow other researchers to carry out more experimental studies on the EFA. The results of the full-scale strengthened RC test beams are also presented with sufficient detail in order for numerical/analytical models to be reliably developed in the future. Such numerical/analytical modeling will facilitate a deeper understanding of the relationship between the EFA device with the NSM and surrounding RC beam. Moreover, although the current study is confined to NSM strips, the EFA concept is also applicable to NSM bars of other shapes (e.g., round bars). Future studies on the performance of the proposed EFA for NSM bars of other shapes are, therefore, needed.

Data Availability Statement

All data, models, and code generated or used during the study appear in the published article.

Acknowledgments

The authors are grateful for the financial support received from the National Natural Science Foundation of China (Project No. 51878310). The authors are also grateful to Mr. Mateusz Jan Jedrzejko at Huazhong University of Science and Technology for his assistance with the testing.

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Information & Authors

Information

Published In

Go to Journal of Composites for Construction
Journal of Composites for Construction
Volume 27 • Issue 1 • February 2023

Copyright

This work is made available under the terms of the Creative Commons Attribution 4.0 International license, https://creativecommons.org/licenses/by/4.0/.

History

Received: Apr 16, 2021
Accepted: Aug 19, 2022
Published online: Nov 7, 2022
Published in print: Feb 1, 2023
Discussion open until: Apr 7, 2023

ASCE Technical Topics:

  1. Anchorages
  2. Anchors
  3. Beams
  4. Bonding
  5. Concrete
  6. Concrete beams
  7. Engineering fundamentals
  8. Engineering materials (by type)
  9. Equipment and machinery
  10. Fiber reinforced polymer
  11. Hydraulic engineering
  12. Hydraulic structures
  13. Materials engineering
  14. Materials processing
  15. Polymer
  16. Ports and harbors
  17. Reinforced concrete
  18. Structural behavior
  19. Structural engineering
  20. Structural members
  21. Structural strength
  22. Structural systems
  23. Synthetic materials
  24. Water and water resources

Authors

Affiliations

Y. Ke https://orcid.org/0000-0003-1831-5409
Ph.D. Candidate, School of Civil and Hydraulic Engineering, Huazhong Univ. of Science and Technology, Wuhan 430074, China. ORCID: https://orcid.org/0000-0003-1831-5409
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S. S. Zhang, Aff.M.ASCE https://orcid.org/0000-0003-2218-1917 [email protected]
Professor, School of Civil and Hydraulic Engineering, Huazhong Univ. of Science and Technology, Wuhan 430074, China; Professor, National Center of Technology Innovation for Digital Construction, Huazhong Univ. of Science and Technology, Wuhan 430074, China (corresponding author). ORCID: https://orcid.org/0000-0003-2218-1917. Email: [email protected]
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S. T. Smith, F.ASCE https://orcid.org/0000-0002-8837-1214
Professor, School of Civil, Environmental and Mining Engineering, The Univ. of Adelaide, South Australia 5005, Australia. ORCID: https://orcid.org/0000-0002-8837-1214
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T. Yu, M.ASCE
Professor, Dept. of Civil and Environmental Engineering, The Hong Kong Polytechnic Univ., Hung Hom, Kowloon, Hong Kong 999077, China.
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Cited by

  • S.S. Zhang, M.J. Jedrzejko, Y. Ke, T. Yu, X.F. Nie, Shear strengthening of RC beams with NSM FRP strips: Concept and behaviour of novel FRP anchors, Composite Structures, 10.1016/j.compstruct.2023.116790, 312, (116790), (2023).
    Crossref

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