concrete composites

Accepted Manuscript Effects of surface roughness and bond enhancing techniques on flexural performance of cfrp/concrete composites M.R.E.F. Ariyachandra, J.C.P.H. Gamage, Riadh Al-Mahaidi, Robin Kalfat PII: DOI: Reference:

S0263-8223(16)31231-4 http://dx.doi.org/10.1016/j.compstruct.2017.07.028 COST 8683

To appear in:

Composite Structures

Received Date: Revised Date: Accepted Date:

18 July 2016 4 May 2017 12 July 2017

Please cite this article as: Ariyachandra, M.R.E.F., Gamage, J.C.P.H., Al-Mahaidi, R., Kalfat, R., Effects of surface roughness and bond enhancing techniques on flexural performance of cfrp/concrete composites, Composite Structures (2017), doi: http://dx.doi.org/10.1016/j.compstruct.2017.07.028

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EFFECTS OF SURFACE ROUGHNESS AND BOND ENHANCING TECHNIQUES ON FLEXURAL PERFORMANCE OF CFRP/CONCRETE COMPOSITES M.R.E.F. Ariyachandra1, J.C.P.H. Gamage2, Riadh Al-Mahaidi3, Robin Kalfat4 1

Postgraduate, Department of Civil Engineering, University of Moratuwa, Sri Lanka Telephone: +94719085399, +94716038433 E-mail: [email protected] 2 Senior Lecturer, Department of Civil Engineering, University of Moratuwa, Sri Lanka Telephone: +94112650567-8 (Ext 2201); Fax: +94112651216 E-mail: [email protected] 3 Professor, Faculty of Science, Engineering and Technology, Swinburne University of Technology, Australia Telephone: +61 3 9214 8429 E-mail: [email protected] 4 Research Fellow, Faculty of Science, Engineering and Technology, Swinburne University of Technology, Australia Telephone: +61 3 9214 4945 E-mail: [email protected]

Abstract This paper reports a detailed test programme conducted on enhancing the flexural performance of Carbon Fiber Reinforced Polymer (CFRP) strengthened reinforced concrete beams to ensure prolonged composite action between CFRP laminate and concrete. The effects of surface preparation and alternative bonding techniques on the composite performance were investigated. In this regards, a total of 28 small-scale CFRP strengthened and non-strengthened reinforced concrete beams were tested using the three-point bending test method. This study clearly shows the importance of surface preparation and level of roughness on bond performance of CFRP/Concrete composites. The results also indicate the better performance in CFRP/epoxy/Polyester mesh hybrid arrangement when compare with CFRP strengthened concrete beams. Keywords: CFRP, End debonding, Polyester mesh, Surface roughness, Transverse U wraps

1. Introduction Retrofitting of degraded concrete structures using CFRP is an excellent solution due to their superior characteristics such as: light weight, high tensile strength, durability, low maintenance and economic use. The full capacity of CFRP strengthened concrete members cannot be achieved due to pre-mature failure of bond. Seven modes of failure have been observed from CFRP strengthened flexural members [1] which have been summarized by: concrete crushing, CFRP rupture [2,3,4], shear failure, concrete cover separation failure, plate end interfacial debonding, intermediate flexural or flexural shear crack-induced interfacial debonding [5] and shear- induced debonding. All these failure modes are due to premature nature of bond except the first two failure modes. In addition to those modes, pure bond failure may occur due to poor bond resulted from improper curing, poor surface preparation or low quality workmanship. Different techniques such as mechanical fasteners and Fiber reinforced polymer (FRP) anchorage systems have been explored to delay the premature failure [6, 7]. The use of FRP anchors offers an excellent method to prevent or delay debonding failure while transferring higher loads with greater design strengths [8]. The effectiveness of FRP anchoring systems in terms of bond strength and ductility has been shown in numerous research investigations. For example, the performance of fan and dowel anchors [9 , 10], metallic anchors [11], Pi-shaped anchors [12], L-shaped anchors [13], patch anchors [14, 15] and L- and U-shaped wraps [16] is well comprehended in terms of delaying premature debonding. These research studies were focused on anchors which bent beyond the neutral axis of the beam (i.e. long legs). In some practical applications, it is very hard to install the long U wraps as anchors due to site condition even though it provides excellent performance. The current study focuses on the bond performance of CFRP strengthened concrete beams with use of small U wraps as anchors (i.e. short legs). Other factors governing the performance of FRP-to-concrete bond line are the properties of the epoxy matrix and the concrete substrate [17].The characteristics of the concrete substrate, including its strength, roughness,

and cleanliness can also affect on the bond strength. Hence, excellent preparation of the concrete substrate often improves the interface bond properties. The desired level of surface preparation should be based on the anticipated application of the FRP system, i.e. whether it is bond-critical or contact-critical application. For both of these applications, ACI 546R [18] recommends several methods to improve the concrete substrates prior to the bonding of CFRP sheets by means of either mechanical preparation or abrasive preparation. The use of tools such as breakers, scabblers, grinders and scarifiers can be categorized as mechanical preparation methods, where as techniques such as sand-blasting, shot-blasting or high-pressure water blasting are abrasive preparation methods. In addition, the ACI Committee 440 report [19] defines a minimum intensity of surface roughness similar to Concrete Surface Profiles no. 3 (CSP-3) in accordance with the International Concrete Repair Institute (ICRI) surface-profile chips [20] for bond-critical applications. Furthermore, localized out-of-plane variations, including form lines, should be limited to1 mm or the tolerance recommended by the FRP system manufacturer [21]. However there are limitations in implementing these surface improvements in practise. Slight variations in surface profile may considerably influences on bond performance of the composite. Therefore it is important to identify a range of surface roughness level which provides optimum performance which is one of the objectives of this study. Provision of bond enhancing medium may useful to minimize stress concentrations at bond line [22, 23] which delay debonding of laminate. This study also focuses on achieving a cost effective solution that would ensure a prolonged composite action via a bond enhancement technique or an alternative FRP arrangement. Three such alternative arrangements were proposed with the utilization of an inexpensive polyester mesh for bond enhancement under flexure in the current research. 2. Experimental program 2.1. Overview This investigation consisted of two test series comprising of 28 small-scale reinforced concrete (RC) beams strengthened with CFRP using different bonding techniques and surface preparation techniques. All these test beams were prepared using three concrete batches due to resource restrictions. The summary of test specimens is listed in Table 1. Table 1: Details of test specimens Test series

1. Improvement of interfacial bond properties

2. Provision of transverse FRP end U wraps

Strengthening scheme (a) Use of polyester mesh

(b) Improving concrete substrates

Use of vertical “U” wraps

Description

Non-strengthened control CFRP-strengthened control Polyester mesh at two ends Polyester mesh at full length Non-strengthened control With sand papers Sand-blasted Chipped Non-strengthened control CFRP-strengthened control 100 mm wide CFRP U wraps 100 mm wide GFRP U wraps 50 mm wide CFRP U wraps

Beam designation B/1a/CC B/1a/CF B/1a/CFENDS B/1a/CFFULL B/1b/CC B/1b/CFSP B/1b/CFSB B/1b/CFCH B/2/CC B/2/CF B/2/CF100 B/2/GF100 B/2/CF50

No. of Samples 2 2 3 3 2 2 2 2 2 2 2 2 2

Batch No.

1

2

3

2.2. Materials and specimen preparation Small scale concrete beams, size 100 mm × 150 mm × 750 mm (width × depth × length) were cast using grade 30 concrete (1 cement: 1 sand: 2 aggregate). Selected water-cement ratio was 0.55. Steel moulds were used as formwork and concrete was compacted using a 25 mm diameter poker vibrator. The steel moulds were removed 24 hours after pouring concrete and the beams were immersed in a water tank for 28 days. Four concrete cubes, 150 mm × 150 mm × 150 mm from each batch were also prepared to determine the compressive strength of concrete. The average 28 days compressive strength noted from batch 1, 2 and 3 are 33.2 N/mm2, 33.6 N/mm2 and 32.0 N/mm2, respectively. Mild steel bars 6 mm in diameter were used as the longitudinal reinforcement and galvanized iron bars 4 mm in diameter were used to prepare the shear links (Fig. 1) to avoid shear failure and awaiting flexural failure from strengthened beams with such small

dimensions according to theoretical calculations. The properties of these reinforcement bars were measured using standard laboratory tests (Table 2).

Fig.1 Schematic diagram – Reinforcement details

Table 2 Measured properties of steel

Type of reinforcement Tension /compression Shear links

Bar diameter (mm) 6 mm 4 mm

Measured tensile strength (N/mm2) 250 363

Both CFRP and Glass Fibre Reinforced Polymer (GFRP) sheets were used for strengthening. A commercially available two-part epoxy adhesive was used to bond the FRP sheets. The manufacturer provided material properties of FRP and epoxy adhesive are listed in Table 3 [21]. Table 3 Material FRP sheet

properties [21] Two-part epoxy adhesive

Parameters

CFRP

GFRP

Parameters

Primer

Saturant

Fibre Density (g/cm3)

2.1

2.6

Yield Strength(MPa)

24.1

138

FibreModulus (GPa) Thickness (mm)

240 0.19

73 0.31

Strain at Yield (%) Elastic Modulus(MPa)

4% 595

3.8% 3724

Ultimate Tensile Strength (MPa) Ultimate Tensile Elongation (%)

2600 0.4

2400 4.5

Ultimate Strength(MPa) Glass Transition Temperature (°C)

24.1 77

138 71

The bottom surfaces of the concrete beams were sand-blasted prior to bonding of the FRP sheets in order to remove the outer-most weak layer of concrete except in the test specimens B/1b/CFSP and B/1b/CFCH. The wet lay-up method [19] was used to bond CFRP sheets onto the concrete surfaces. Primer part A (hardener) and part B (base) were mixed at a1:2 ratio by weight. A thin layer of primer was applied immediately after cleaning (Fig. 2(a)), which resulted in a dry and slightly sticky film. The prepared concrete beams were set aside for about 45 minutes. Saturant part A (hardener) and part B (base) were mixed using a 1:2 ratio by weight as per the manufacturer’s specifications [21]. The prepared concrete substrates and FRP sheets were saturated with epoxy saturant. FRP sheets impregnated with saturant were pressed onto the concrete substrates and a ribbed roller was used to remove the air entrapped in the bond line (Fig. 2(b)). Ribbed rolling was carried out in the direction parallel to the fibres. The specimens were cured for 7 days at room temperature. (a)

(b)

(c)

Fig. 2(a) Applying primer on improved concrete surfaces, (b) installation of CFRP

3. Strengthening schemes 3.1. Test series 1(a) - Effects of bond enhancing medium The key objective was to introduce an economical bond enhancing medium which adheres well to both the concrete substrate and the CFRP sheet by overcoming surface irregularities. Hence, a commercially available 1 mm thick light weight (lighter than CFRP sheet) polyester mesh, mesh size 4 mm x 4 mm was used. The measured elastic modulus of the polyester mesh was 568.7 MPa. Firstly, two test specimens (B/1a/CFENDS) were prepared comprising two segments of 150 mm long and 90 mm wide polyester mesh positioned at the ends, as shown in Fig. 3 (a) and Fig. 4 (a). The next two specimens B/1a/CFFULL consisted of a 60 mm × 600 mm polyester mesh placed along the full bond length between the CFRP sheet and the concrete, as shown in Fig. 3(b) and Fig. 4(b).

(a)

(b)

Fig. 3 Schematic diagram (Plan view) of CFRP-concrete beams with polyester meshes at, (a) ends and (b) full bond length

a

b Fig. 4 Positions of polyester mesh prior to the bonding of CFRP sheet at, (a) two ends, (b) full bonded length

3.2. Test series 1(b) – Effects of surface profile The main purpose of this test series 1(b) was to examine the effects of dissimilar surface roughness levels of the bond line on the flexural performance of CFRP-concrete composites. In some practical situations, it is very hard to maintain the recommended roughness level. Another barrier is to carry out sand blasting while operating the building. In such situations, use of sand papers or chipping may be an ideal solution. A total of six CFRP-strengthened concrete beams were prepared. The bottom surfaces of the concrete beams were improved prior to bonding of the CFRP sheets to achieve three different surface roughness levels. Firstly, the concrete substrates of two test beams (B/1b/CFSP) were improved slightly using 60 grit sandpaper as shown in Fig.5 (a). Next, two concrete beams (B/1b/CFSB) were sand-blasted to remove the top most weak layer to expose the aggregates (Fig. 5 (b)). The remaining two concrete beams (B/1b/CFCH) were manually chipped to remove the weak concrete layer (Fig.5 (c)).

(a)

(b)

(c)

Fig.5 Prepared concrete substrates (a) With sandpaper (b) Sand-blasted (c) Chipped

3.3. Test series 2 - Effects of polymer anchorages Provision of end anchorages effectively delays the end debonding failure of composites. In some practical situations, provision of end anchorages at least to the depth of neutral axis of the member is also difficult. The main objective of this test series was to study the effectiveness of FRP U wraps with small vertical leg

dimensions beyond ACI440 recommendations [19]. A total of eight CFRP-strengthened concrete test specimens were strengthened using three different FRP bond arrangements containing vertical CFRP and GFRP U-shaped wraps in addition to the longitudinal CFRP sheet of 100 mm × 450 mm in the transverse direction. Firstly, two vertical CFRP U wraps 50 mm × 100 mm in dimensions were bonded on top of CFRPstrengthened concrete beams (B/2a/CF50), as illustrated in Fig. 6(a). Then, another two test specimens were anchored using two vertical GFRP U wraps, resulting in a CFRP-GFRP hybrid system (B/2a/GF100) as indicated in Fig. 6(b). The remaining two CFRP-concrete test specimens were strengthened by means of two 100 mm wide and 50 mm long vertical CFRP U-shaped wraps (B/2a/CF100), as shown in Fig. 6(c).

(a)

25 mm

25 mm

150 mm

100 mm

(b)

(c) Fig. 6: Schematic diagrams of specimens

4. Test Results Specimens were tested using the three point bending test. Mid-span deflection with transient loading was recorded and initiation of cracks was observed. The average failure loads of non-strengthened concrete control beams were 12.36 kN, 13.70 kN and 14.71 kN for batch No. 1, batch No. 2 and batch No. 3, respectively. 4.1. Failure mechanisms 4.1.1.CFRP-Polyester-Concrete hybrid system Debonding is the most dominant failure mode in flexural strengthening. Debonding failure can be classified into three types based on the initial starting point of delamination. End debond failure originates near the plate end and propagates in the concrete either along the tension steel reinforcement (end cover peeling) or near the bond line (end interfacial debond) [15]. Intermediate span debond starts either due to a wide flexural crack (flexure crack debond), a wide flexure-shear crack (flexure-shear crack debond) or a wide diagonal shear crack (shear crack debond). Mid span debond is mainly due to initiation of a wide flexural crack at the mid span. In both cases, failure propagates from the crack tip to the laminate end parallel to the adhesive/concrete interface. It is noticeable that the formation of the mid-span flexure crack in the CFRP

strengthened control beams initiated the mid-span debonding which soon propagated towards the supports, causing a complete debond from the concrete substrate as depicted in Fig. 7(a). As the polyester mesh has a fine lattice structure and is highly flexible [7], it seems to have distributed the interfacial stresses well and enhanced the percentage of contact area by effectively countering minor surface irregularities on the substrate resulted in higher load capacity (Figs.7(b) and 7(c)). B/1a/CFFULL had the highest gains due to the bonding medium being reinforced to its full applied length by the polyester mesh. However, B/1a/CFENDS succumbed to failure earlier than B/1a/CFFULL, due to not being reinforced at the critical mid-span location where debonding initiates. Both of these test beams failed due to initiation of major flexural cracks at midspan, followed by rupture in the CFRP sheet.

a

b

c

d Fig.7: Failure mechanisms at 0.3 mm wide crack initiation of; (a) Control, (b) CFRP strengthened, (c) CFRP strengthened with polyester meshes at ends, (d) CFRP strengthened with polyester mesh in full bond

In CFRP-concrete composite beams where end delamination has been effectively mitigated, debonding may initiate at flexural cracks, flexural/shear cracks, or both, near the region of maximum moment [10]. This demonstrates the capability of polyester mesh to dissipate stresses uniformly by delaying premature debonding in CFRP-concrete composite beams. Considering the results obtained, it is apparent that Alternative 01 [B/1a/CFEnds] and Alternative 02 [B/1a/CFFull] have had significant strength gains in comparison to the Standard Practice Beam [B/1a/CF], over the Control Beam [B/1a/CC] and have effectively delayed debonding in flexure. The reason for the delay in debonding and its subsequent strength gain can be directly attributed towards the presence of the Polyester mesh, as it seems to have worked as a reinforcing medium for the bonding agent, being an epoxy adhesive, as does reinforcing steel to concrete. It is noticeable that the formation of the mid span flexure crack, in CFRP strengthened elements, had initiated a mid span debonding which soon propagated towards the supports causing a complete debond from the concrete substrate. The formation of these relatively high interfacial stresses could not be resisted by the bonding agent alone. The Polyester mesh having a fine lattice structure and being highly flexible seems to have distributed the interfacial stresses well and seems to have enhanced the percentage of contact by effectively

countering minor surface irregularities on the substrate. When specifically considering the proposed alternatives, Alternative 02 [B/1a/CFFull] had the highest gains due to the bonding medium being reinforced to its full applied length by the Polyester Mesh. However, Alternative 01 [B/1a/CFEnds] had succumbed to failure earlier than Alternative 02 [B/1a/CFFull], due to not being reinforced at the critical locations where debonding initiates. Qeshta et al [22] also conducted an experimental investigation to examine the structural performance of a beam strengthened using a hybrid of wire mesh-epoxy and CFRP sheet. In contrast to the present study, the welded wire mesh was bonded on top of the CFRP sheet, without placing in the CFRPconcrete interface as in the current study. The CFRP sheet was applied first to the concrete surface, followed by bonding the wire mesh-epoxy laminate using epoxy resin. Their main idea was to provide a clamping effect against delamination of the CFRP sheet from the concrete substrate. Therefore, the CFRP sheet could be fully utilized for improving the performance of the strengthened specimen. The results revealed that the use of the hybrid wire mesh-epoxy-carbon fibre composite can enhance post-yield behavior while successfully preventing the debonding of the CFRP sheet. The polyester mesh placed over the full bond length of specimens in the current study showed 82.4% strength gain with respect to non strengthened specimens. The wire mesh bonded on top of the CFRP sheet in Qeshta et al study had shown 41.98% strength increment due to additional clamping effect [22]. 4.1.2.Degree of Surface roughness When compare the failure mechanism of the specimens with different roughness levels, both sand-blasted and chipped concrete beams failed due to the cover separation (Fig.8). In test specimens improved with sandpaper (B/1b/CFSP), CFRP sheet was delaminated with a thin layer of concrete at ends, i.e. end debonding failure (Fig.8). Non strengthened concrete beams exhibited the flexural failure. a

b

c

d

Fig.8: Failure mechanism; (a) Non strengthened (control), and strengthened beams with (b) Sand blasted, (c) Sand grit, (d) Chipped surfaces 4.1.3.Effects of polymer anchorages Surfaces to be bonded in this test series were prepared by sand blasting. With transient loading, a small crack initiated at 60 mm from the end of the bond length of CFRP strengthened specimens without end anchors (Fig.9 (b)). With increased loading, those specimens indicated end debonding failure. Initiation of flexural shear cracks very close to the bond ends were noted from the specimens anchored with CFRP sheets (Fig.9 (c) & (d)). Both specimens showed cover separation at ends with increased loading. However the anchors with long legs showed a higher load at failure. The specimens which are end anchored with Glass Fibre Reinforced Polymer (GFRP) showed similar performance as observed in strengthened beams without anchors. Delamination of GFRP anchors was noted before delaminating the ends of CFRP sheet (Fig.9(e)). Separation of CFRP sheet from concrete at ends was observable.

a

b

d

c

e

Fig.9: Failure mechanism; (a) Non strengthened (control), and Strengthened beam with (b) no anchors, (c) CFRP end anchors with 100 mm long legs, (d) CFRP anchors with 25 mm long legs, (e) GFRP anchors with 25 mm long legs 4.2. Failure loads and load-displacement relationships 4.2.1.CFRP-Polyester-Concrete hybrid system Failure loads observed from the test series 1(a), which used to examine the effectiveness of a polyester mesh as a medium for bond enhancement is shown in Table 4. Appearance of 0.3 mm wide crack during testing was considered as the failure criteria of all these specimens. The strengthened beams without a polyester mesh showed a strength gain of 33% with respect to non strengthened beam. The strength gain was relatively high with the provision of a polyester mesh in the bond line. These results indicate the provision of a polyester mesh over the full bond length is more effective than limiting those to the ends of the bond lengths. Table 4: Summary of failure loads and respective deflections at the mid span of beam Sample

Failure load (kN) (0.3 mm wide crack initiation)

B/1a/CC-1

11.77

Average failure load (kN) (at 0.3 mm wide crack initiation)

Mid span deflection (mm) (at 0.3 mm wide crack initiation) 85

12.36 B/1a/CC-2

12.95

B/1a/CF-1

15.70

65 86

16.46 B/1a/CF-2

17.22

B/1a/CF-Ends1

19.62

33% 89 125

19.89 B/1a/CF-Ends2

20.11

B/1a/CF-full1

22.56

60.9% 102 140

22.54 B/1a/CF-full2

22.52

% Strength increment with respect to non strengthened beam

82.4% 104

Load-deflection relationships of these specimens are shown in Fig.10. Initially all these specimens indicated the similar trend at low load levels which is less than 5 kN. Then the slopes of these curves vary slightly with the bonding technique. These graphs further exaggerate the effectiveness of a polyester mesh as a bond enhancing medium.

Figure 10: Load Vs. Mid span deflection 4.2.2. Degree of Surface roughness Test results of series 1(b) indicated the dependency of bond performance with the level of surface preparation. Average failure load of control specimens (on strengthened) in this series was 13.7 kN. The highest average failure load of 40.2 kN was reported from the sand-blasted test specimens. The second highest average failure load of 39.2 kN corresponded to the test specimens with chipped surface before bonding CFRP sheet, while the least average failure load of 33.4 kN was reported for the test specimens, which had been improved with 60 grit sandpaper. The specimens with chipped and sand blasted surfaces indicated similar improvements. Hence, chipping can also be recommended in the situations that the sand blasting cannot be applied. This can be further verified with the guidelines given in the ACI 440 Committee report [19], which states that the localized out-of-plane variations, including form lines, should be limited to 1 mm or the tolerances recommended by the FRP system manufacturer. 4.2.3.Effects of polymer anchorages The test results of test series clearly indicate the importance of having U wrap anchors with longer leg length. Average failure load noted from CFRP strengthened specimens without anchorages was 40.2 kN. Non anchored specimens and the specimens strengthened with GFRP U wrap anchors with 25 mm long legs at ends indicated similar results(41.2 kN) as in strengthened specimens without anchorages which shows non effectiveness of GFRP as U wrap anchors. This may due to non penetration of adhesive because of high thickness of GFRP sheet leading to delamination from the substrate. Provision of CFRP anchors with small legs which indicated the same results (41.2 kN) as observed in specimens with GFRP anchorages is also not effective. This was insufficient to provide confinement against the cover separation at ends. The highest average failure load was noted from the specimens with CFRP U wrap anchors which contains long legs. Therefore, it is important to extend U wrap anchors beyond the tensile reinforcement level. 4.3. Comparison of test results The summary of average strength gains observed in this investigation is shown in Table 5. The comparison was made with respect to CFRP strengthened control beam. These control specimens in all three series were prepared in the same way (a CFRP sheet was attached to the sand blasted soffit of the beam). These

specimens showed failure loads in the range between 33 kN and 40 kN. In test series 1(b), maximum performance was noted from the specimens those substrate prepared with sand blasting. Maximum performance was noted from the specimens with polyester mesh placed in the full bond length as a bond facilitator. The provision of transverse U wraps enhances the flexural performance of concrete beams by a maximum of 14.6 % compared to CFRP strengthened concrete beams without anchorages. Specimens which contain end anchors with shorter legs did not indicate significant improvements. The use of polyester mesh increased the failure loads by 42% when it was placed over the full bond length rather than at the two ends. The level of surface roughness of concrete substrates has a significant effect on the bond strength of CFRPconcrete beams leading to higher strength gains. Sand- blasting and chipping improved the failure loads. Table 5: Comparison of test results

Strengthening scheme

1.(a) Use of polyester mesh 1.(b) Improving concrete substrates

2.Use of vertical CFRP and GFRP U wraps

% strength gain with respect to CFRP strengthened concrete beam

Description

Beam designation

Average Failure load (kN)

CFRP strengthened control Polyester mesh at two ends Polyester mesh at full length Sand blasted (CFRP strengthened control) With sand papers Chipped

B/1a/CF B/1a/CFENDS B/1a/CFFULL B/1b/CFSB B/1b/CFSP B/1b/CFCH

33.2 41.3 47.2 40.2 33.3 39.2

12.4% 42.0% -17.1% -2.5%

CFRP strengthened control 25 mm x 100 mm CFRP U wraps 100 mm x 100 mm CFRP U wraps 25mm x 100 mm GFRP U wraps

B/2/CF B/2/CF25 B/2/CF100 B/2/GF25

40.2 41.2 46.1 41.2

2.4% 14.6% 2.4%

5. Conclusions CFRP-concrete composite beams exhibit several types of failure modes, of which premature end-debonding is the most critical type of failure, which takes place at the interface of the bond line or within the concrete cover zone. This paper presents an experimental study on enhancing flexural performance of CFRP/Concrete composites. In this regards, Authors attempted to improve the interfacial bond properties in three approaches. Based on the experimental investigations, the following conclusions can be drawn:
The provision of polyester mesh underneath the externally-bonded CFRP sheet can distribute the interfacial stresses well by enhancing the percentage of contact, effectively countering minor surface irregularities on the substrate. Higher strength gains can be achieved with a polyester mesh placed at the full length rather than placing it only at the two ends of the CFRP sheet. In addition, the insertion of a polyester mesh at the interface as a bond enhancement medium is more effective than placing it outside the externally-bonded CFRP laminate, in order to provide a clamping effect to delay premature end-debonding failure. With provision of a polyester mesh in the full bond length, on average 42% strength gain can be achieved with respect to CFRP strengthened concrete beam.
The level of surface roughness of the concrete substrates has a significant effect on the bond strength of CFRP-concrete composites. Sand-blasting and chipping are recommended as the most suitable surface improvement techniques for CFRP-concrete composites. Use of san papers for surface preparation is not recommended.
The provision of transverse FRP end U wraps produces higher strength gains. Nevertheless, the lack of design guidelines restricts the widespread use of transverse end U wraps in the flexural strengthening of CFRP-concrete beams. On average 14% of strength gain was noted from the specimens with end anchors which extended beyond the neutral axis of the beam. Similar behavior can be expected from the beams without end anchors and with end anchors with short legs.


Maximum performance was noted from the specimens with a polyester mesh placed over the full bond length as a bond enhancing medium. 6. Acknowledgements This study was financially supported by a University Research Grant (Grant No. SRC/LT/2013/05) from the Senate Research Committee, University of Moratuwa, Sri Lanka. The authors would like to convey their appreciation to former undergraduate students of the department, Miss. H.R.D Premarathne and Mr. S. Srisangeerthanan.

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