CHARACTERISATION OF FIBRE RUPTURE FAILURE MODE OF BENT FIBRE REINFORCED POLYMER (FRP) ANCHORS

LIM. J.1, KIM. J.1, DEL REY CASTILLO. E.1, GRIFFITH. M.2, DIZHUR. D.1 AND INGHAM, J.1

1 Department of Civil and Environmental Engineering, University of Auckland, NZ

2 School of Civil, Environmental and Mining Engineering, University of Adelaide, Australia SUMMARY The main problem when strengthening existing reinforced concrete structures using externally bonded fibre reinforced polymers is premature FRP-to-concrete debonding. Bent FRP anchors were introduced as a solution to improve the FRP-to-concrete bond strength, but incomplete information on anchor behaviour and the absence of reliable design equations triggered an on-going investigation to further analyse the characteristics of bent FRP anchors. An ongoing experimental programme is being conducted to develop an optimal design for bent FRP anchors exhibiting fibre rupture failure mode. A mathematical model is proposed based on the relationship between the parameters under study and the anchor capacity. INTRODUCTION FRP composite materials consist of a polymer matrix reinforced with fibres that were initially developed in the 1930s and have been extensively used in the last thirty years, mainly as a method to strengthen and/or repair existing structures. The high strength to weight ratio and corrosion resistant properties of FRP favoured the use of these products, with the Externally Bonded Fibre Reinforced Polymer (EBR-FRP) system being one of the most commonly used methods to strengthen and/or repair existing Reinforced Concrete (RC) structures (Bank 2006). The main drawbacks of the EBR-FRP system are the premature FRP-to-concrete debonding, see for example (Chen & Teng 2001), and the geometrical limitation preventing the structural element to be fully wrapped by the FRP sheets as per Figure 1 detail a. To overcome these problems, FRP anchors were introduced in the past decade, which have several advantages when compared to more traditional anchorage systems that are heavier and prone to corrosion, as explained by Kalfat (2013). FRP anchors consist of a bundle of fibres soaked in epoxy resin, with one end being splayed out in a circular or fan-like shape and bonded to an FRP sheet, and the other end inserted into a pre-drilled hole filled with epoxy resin. FRP anchors can be used following two different approaches, with the anchor installed into the structural element at an insertion angle β (bent anchor) as shown in Figure 1 detail b or with the anchor introduced straight into the structure (straight anchor) as shown in Figure 1 detail c.

Figure 1: Three EBR-FRP installation options for shear strengthening of an RC beam

The anchor end introduced into the hole is known as the dowel, while the end splayed and bonded onto the FRP sheet is referred to as the fan component, with the key portion being the point of transition between these two components as shown in Figure 2. Knowledge and understanding of the behaviour of bent anchors is limited, and no design guidelines currently exist for practicing engineers, limiting wide implementation of FRP anchors for EBR-FRP systems. Previous experiments have been conducted to investigate the behaviour and strength of bent FRP anchors, but the studies focused on the dowel (Eshwar et al. 2008; Zhang & Smith 2012; Mahrenholtz et al. 2015), with the behaviour of the key portion and fan component being poorly investigated to date. Three failure modes were observed by Zhang (2012) for bent FRP anchors, being dowel pull-out, fibre rupture and fan-to-sheet debonding. Reported herein are the results of 24 tests, which are part of on-going research that is aimed at characterising the strength and behaviour of bent FRP anchors when exhibiting the fibre rupture failure mode, depending on the anchor size and the fanning angle α. The insertion angle β remained constant at 90° throughout the test programme as the influence of the angle β has already been investigated (Zhang & Smith 2012). The size of the anchor is determined by the number of bundles used to manufacture the anchor, while the fanning angle α is the angle between the middle of the fan component and the furthermost fibre, as per Figure 2.

Figure 2: Bent anchor and components

TEST SET-UP When the testing set-up was designed two important factors were considered, being the need to replicate the behaviour of the RC and FRP materials in a real structural application and to minimise induced moment or shear stresses in the FRP materials that could trigger premature

(a)

Fully wrapped

beam

(a)

Bent

anchor

(c)

Straight

anchor

Fan component

Dowel

FRP sheet

Key portion

α

β

failure. Taking these factors into account, the experimental set-up was prepared similarly to the anchor configuration shown in Figure 1 detail b, but with a 180-degree rotation to facilitate the construction and testing of the specimens. An RC H-beam and RC block were used to replicate the RC slab and RC beam respectively, as shown in Figure 3. The RC H-beam and the RC block had to be disconnected to transfer the tensile load from the RC block to the bent FRP anchors, with the FRP anchors being the only connection between the RC H-beam and RC block. A bar protruding out of the RC block was attached to a steel plate connector with a nut and with a layer of plaster positioned between the steel plate connector and the RC block to further enhance rigidity. The tensile load was applied onto a loading bar screwed into the steel plate connector that transferred loads from the bar into the RC block and into the bent FRP anchor through the FRP sheet. A steel frame designed specifically for these experiments was used as a reaction frame, with the loading bar passing through a hollow hydraulic jack and the load being recorded using a load cell, see Figure 3. The Digital Image Correlation (DIC) technique was used to measure displacements and strains during the experiments, but only the load results are reported herein.

Steel Frame

Loading bar

Steel plate connector

RC block

FRP Sheet

FRP Anchor

RC H-Beam

Figure 3: Test set-up

SPECIMEN ASSEMBLY PROCESS The surfaces of the RC blocks were grinded to ensure that no loose aggregates could compromise the FRP-to-concrete bond, and to expose the concrete pores. Holes were drilled deep enough into the RC H-beam to prevent pull-out failure and dust particles were then blown out of the hole using compressed air. Plastic sheets were laid over the H-beam to prevent the RC block from bonding onto the RC H-beam before placing the RC block on top of the RC H-beam, with the protruding bar aligned with the drilled hole to prevent rotation of the RC block. The epoxy resin was mixed with the curing agent before soaking the FRP sheets into the epoxy resin, making sure that each tow was soaked before installing the sheets with the bottom of

the sheet placed above the hole for the bent FRP anchors to be fanned upon. The FRP anchors were also soaked in the epoxy resin and then inserted into the hole using a steel rod. After the anchor was fully inserted the free end was fanned onto the FRP sheet and the steel wire was carefully removed out of the hole. The epoxy resin was allowed to cure for at least three days before the tests were undertaken. Consistency in the experiments, besides the two parameters under investigation, was important for the reliability of the results. Properly filling the hole with epoxy resin, without air voids trapped inside, was crucial in order to prevent dowel debonding. Even partial debonding would have induced moment in the dowel and the key portion, triggering premature failure. After several iterations, this problem was solved by injecting epoxy resin into the hole with a syringe before inserting the saturated FRP anchor and blocking the hole with thickened resin immediately after the insertion to prevent the resin from flowing outside. MATERIALS PROPERTIES As previously observed by del Rey Castillo et al (2015), the concrete strength did not have an influence on the FRP anchor final strength, but stronger concrete properties were preferable as more experiments could be performed with the same RC structures. The concrete properties are reported in Table 1 as determined by NZS 3112-2 (1986). The FRP anchor material used in these experiments was supplied as a bundle of fibres with a standard fibre content, which was then combined to produce anchors of different sizes. The net-fibre material properties of the FRP sheets and anchors as specified by the manufacturer are reported in Table 2.

Table 1: Concrete material properties

Strength Standard Deviation

Coefficient of Variation

Characteristic ultimate compressive strength

39.0 MPa 1.23 MPa 0.03

Characteristic ultimate tensile strength 3.0 MPa 0.35 MPa 0.12

Table 2: Manufacturer-specified FRP net-fibre material properties

Net fibre thickness

Tensile Modulus (GPa)

Tensile strength (MPa) Ultimate strain

Ave Design1 Ave Design1 Ave Design1

CFRP sheets

0.331 mm 75.7 68.1 968 833 0.013 0.011

CFRP anchor

28 mm2 - 230 - 2100 - 0.016

1Design values defined as average values minus two standard deviations

RESULTS Each test specimen is denoted by the number of bundles and the α angle measured on the fan component as shown in the example below.

Bn- αa

Where: B-Bent anchor n-number of bundles α-fanning angle a-specimen number The two parameters that were varied throughout the research are the anchor size and the fanning angle α. Anchors of two sizes were used, being one-bundle anchors with a net fibre section equal to the section of the bundle of 28 mm2 and two-bundle anchors, made by combining together two bundles, with a net fibre section equal to 56 mm2. Three variations of

fanning angle α were investigated, being 15 degrees, 27 degrees (the anchor width is equal to the anchor length) and 60 degrees, but due to installation errors and the difficulty of splaying the fan component, the angles were measured to be slightly different from the targeted angle. The angle α of 60 degrees is a large fan angle and is uncommon in real applications, but was chosen to in order to establish how variations of the fanning angle affected the strength of the anchor. Two sub-failure modes were observed during the test programme, both exhibiting fibre rupture but located in different parts of the anchor: fibre rupture occurring at the anchor key portion (Rk), see Figure 4a; and fibre rupture occurring at the anchor mid-section (Rm), see Figure 4b. The sub-failure modes had no significant effect on the final strength of the FRP anchor, but indicated that fan-to-sheet debonding had been initiated at the time of failure for the Rm sub-failure mode.

(a) Rupture at the key portion (Rk) (b) Rupture at mid-section (Rm)

Figure 4. Sub-failure modes observed during experiments The ultimate tensile load was recorded at failure and the real strength was calculated based on the ultimate tensile load and the net-fibre cross section area of the FRP anchors. The efficiency was then obtained as a percentage of the manufacturer-specified strength as given in Table 2 and the real strength calculated and reported below, with the strength of two bundle anchors being obtained by multiplying the manufacturer-specified strength by two. The statistical population for one and two-bundle anchors (cross section area of 28 mm2 and 56 mm2 respectively) are represented in Figure 5, with the pull-out results represented as red dots. As can be observed, most of the pull-outs results were below the minimum load obtained for the fibre rupture tests, but some results reached values within the fibre-rupture results range.

Table 3: Test results

Name UTL (kN)

σreal (MPa)

Eff (%) FM Name UTL (kN)

σreal (MPa)

Eff (%)

FM

B1- 40.1 1432 68.2 Rk B1-47c 38.3 1369 65.2 Rk

B1-13b 50.5 1805 86.0 Rk B2-13a 69.1 1234 58.8 Rk

B1-15c 40.7 1455 69.3 Rk B2-13b 74.9 1338 63.7 Rk

B1-12d 43.9 1568 74.7 Rk B2-11c 65.7 1174 55.9 Rk

B1-15e 45.6 1628 77.5 Rk B2-9c 72.2 1289 61.4 Rk

B1-27a 37.96 1356 64.6 Rm B2-22a 77.3 1381 65.7 Rk

B1-27b 44.6 1592 75.8 Rk B2-23b 70.7 1263 60.1 Rk

B1-32c 40.4 1441 68.6 Rk B2-23c 77.1 1376 65.5 Rk

B1-28d 39.5 1412 67.2 Rk B2-21d 74.3 1328 63.2 Rm

B1-30e 46.1 1646 78.4 Rk B2-35a 80.7 1440 68.6 Rk

B1-48a 43.7 1561 74.3 Rk B2-40b 62.2 1110 52.9 Rk

B1-48b 39.6 1414 67.3 Rk B2-50c 66.6 1189 56.6 Rm Cross section area of: 1 Bundle = 28 mm2, 2 Bundles = 56 mm

UTL = Ultimate Tensile Load, σreal = Real Strength, Eff = Efficiency, FM = Failure Mode (Rk rupture in the key

portion and Rm rupture at mid-length)

Figure 5. Statistical population

FORCE TRANSFER MECHANISM The load path from the FRP sheets to the concrete structure through the FRP anchor is analysed in this section, aimed to help understanding the anchor behaviour and developing the design model for bent FRP anchors exhibiting fibre rupture failure mode. In a real installation, the tensile force is transferred from the FRP sheets to the anchor fan component, and through the whole depth of the anchor dowel into the RC structure in perpendicular to the concrete surface. As a simplification, it was assumed that the load is not distributed through the full dowel depth, but concentrated on the surface of the RC structure, and the force on the anchor dowel (Fdowel) is thus equal to the force on the sheet (Fsheet) to maintain equilibrium, equation 1a. To determine the ultimate tensile load the fanning component is assumed to consist of an infinitesimal number of fibres with each fibre having its own force (fi). Forces in the perpendicular and parallel direction to the applied force develop, as hypothesised by Kobayashi (2001), with the total force transmitted to the dowel being equal to the summatory of the longitudinal forces in the fibres, which are the cosines of the corresponding angle αi multiplied by the distributed forces (fi), equation 1b. The ultimate tensile load of the anchor (Nfr) is therefore calculated as the sum of the longitudinal forces on the fan, which is the FRP tensile stress (σFRP) multiplied by the cross section area of the dowel (Adowel), the efficiency coefficient (ψeff) found through the experiments and the summatory of the cosine of the angle α of each infinitesimal fibre (∑cos(α)), equation 1c.

0sheet dowel

F F F (1a)

0

0

cosdowel i iF f

(1b)

0

0

2 cos( )fr eff dowel FRP i iN A f

(1c)

Figure 6: Force transfer mechanism model The sum of the cosine of α can be calculated using the integral of the cosine in radians between zero and α0 (maximum angle α) which is the sin of α0, equation 2a. However, this equation implies that the quantity of fibres in the anchor increases as the angle α increases, which is incorrect as the number of fibres remains constant for a fixed cross section area - the density of the evenly distributed fibres depends on the angle. This is accounted for by finding an average α’ value, which is equal to sin(α0) divided by α0 in radians, as can be seen in Figure 7, which results in equation 2b. For simplification this average value α’ can be assumed to be equal to the cosine of 0.57α for angles between 0 and 60 degrees, equation 2c.

00

0

000 0

cos( ) cos( ) sin( ) sin( ) sin(0)i d

(2a)

0 0

0 0

sin( ) sin( )' fr eff dowela N A FRP

(2b)

cos(0.57 )fr eff dowelN A FRP (2c)

Figure 7: Explanation for sin(α)/α

Fsheet fi

f I co

s (

αI)

αi

Fdowel

α0

To find the efficiency coefficient for each test, the ultimate tensile load was divided by the tensile strength of the FRP anchor (σFRP) and by the cross section area of the dowel (Adowel). The efficiency coefficient of each test against the correspondent angle α for each anchor size can be seen in Figure 8a and b, and their respective best fit equations are reported in equations 3a and b. The behaviour of the anchors featuring two cross section areas and their efficiency factors can be seen in Figure 8c and the best fit equation is reported in equation 3c. Net cross section area (mm2) Model Equation

28 28 0

0

sin( )0.76eff

(3a)

56 56 0

0

sin( )0.63eff

(3b)

- 0.251.75

dowel

area

eff A (3c)

The final model is obtained by substituting the efficiency coefficient from equation 3c into equation 2c and is reported in equation 4, while the three dimensional representation of the equation, together with the data points, can be seen in Figure 8d. The model underestimates the strength of the bent FRP anchor, with possible reasons being the dispersion of the results and the simplification of the force transfer model mentioned above. While the dispersion of the results is inherent to FRP materials and their installation process, and similar dispersion can be found in the literature (Mahrenholtz et al. 2015), further analysis is needed to obtain a more accurate model.

Final model Equation

0.751.75 cos(0.57 )fr eff dowelN A FRP (4)

(a) Efficiency coefficient vs angle α for one bundle anchors

0 0.2 0.4 0.6 0.8 1 1.2

0.65

0.7

0.75

0.8

0.85

0.9

0.95

Fanning angle (RAD)

Effi

cien

cy C

oef

fici

eny

28 mm2

28 0

0

sin( )0.76

eff

R2 = 0.13

(b) Efficiency coefficient vs angle α for two bundle anchors

(c) Efficiency VS cross section area

(d) Ultimate force from Eq 2b

Figure 8: Theoretical models for one and two bundle anchors and comparison between model prediction and test results

0 0.2 0.4 0.6 0.8 1 1.20.5

0.55

0.6

0.65

0.7

Fanning angle (RAD)

Effi

cien

cy C

oef

fici

eny

56 mm2

0 20 40 60 80 100

0.65

0.7

0.75

0.8

0.85

Cross Section Area (mm2)

Effi

cien

cy C

oef

fici

eny

Efficiency

28 0

0

sin( )0.63

eff

0.251.75

area

eff dowelA

R2 = 0.04

R2 = 1

CONCLUSION AND FUTURE WORKS A total of 24 tests were performed to help further understand the behaviour of bent FRP anchors subjected to tensile forces. By investigating several angles and two anchor sizes, the capacity of the specimens was analysed with the objective of formulating a design equation that considers both the fanning angle and the anchor size. This research was focused on the fibre rupture failure mode, with the dowel pull-out failure mode being prevented by introducing the anchor dowel to a sufficient depth in the RC base and the fan-to-sheet debonding failure mode being prevented by having a large enough fan component. In general, anchors featuring acute fanning angles performed better than those that featured more obtuse angles, as can be deduced by the larger ultimate tensile loads and efficiency coefficients observed for anchors with a larger fanning angle α. The large anchors failed at higher ultimate tensile loads, but were less efficient than the small anchors. The displacements and resulting strains were recorded using the DIC technique but have not been reported here. A model to calculate the maximum load of bent FRP anchors was proposed, but several problems need to be addressed. Large dispersion in the results was observed, with a probable cause being the low reliability inherent to the installation of FRP materials, which was especially significant for larger anchor size and more obtuse fanning angles. It is recommended that the use of bent FRP anchors with large obtuse fanning angles be avoided in the future and that fanning angles be restricted to the range between 15 and 27 degree. The large dispersion measured in the experimental results is likely to be a contributing factor to explain why the current model underestimates the anchor capacity, with the coefficient of variation measured in this study having a similar value as has been reported in other studies. However, some assumptions made regarding the force transfer mechanism may have also influenced model accuracy. Consequently, in future further attention will be given to the force transfer mechanism in an attempt to improve the predictive nature of the design equation. Additionally, larger cross section areas are to be investigated to further understand the behaviour of bent FRP anchors. ACKNOWLEDGEMENTS The authors acknowledge the support of the technical staff of the Department of Civil and Environmental Engineering at the University of Auckland, as well as Contech Limited and Sika (NZ) for supplying the CFRP and epoxy resin used in the experimental campaign reported here. The funding support provided by the New Zealand Earthquake Commission is highly appreciated. REFERENCES Bank, L.C., 2006. Composites for Construction: Structural Design with FRP Materials, John

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