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Experimental and analytical investigation into the stress performance of composite anchors for CFRP tendons
Accepted Manuscript Experimental and analytical investigation into the stress performance of composite anchors for CFRP tendons Dong-sheng Cai, Jie Yin, Rong-gui Liu PII:
S1359-8368(15)00309-1
DOI:
10.1016/j.compositesb.2015.05.014
Reference:
JCOMB 3602
To appear in:
Composites Part B
Received Date: 16 March 2015 Revised Date:
6 May 2015
Accepted Date: 8 May 2015
Please cite this article as: Cai D-s, Yin J, Liu R-g, Experimental and analytical investigation into the stress performance of composite anchors for CFRP tendons, Composites Part B (2015), doi: 10.1016/ j.compositesb.2015.05.014. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Experimental and analytical investigation into the stress performance of composite anchors for CFRP tendons Dong-sheng Cai1, Jie Yin1,2,*, Rong-gui Liu1
1 Department of Civil Engineering, Faculty of Civil Engineering and Mechanics, Jiangsu University,
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Zhenjiang 212013 China 2 Department of Civil and Environmental Engineering, University of Wisconsin–Madison, Madison, WI 53706 USA *
Corresponding Author, e-mail:[email protected]; Tel: +1 608 422 3373
1. Introduction
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Abstract: This paper presents a study of the internal stress distribution and its stress transfer mechanism of composite anchors for carbon fiber-reinforced polymer (CFRP) tendons. One set of static tensile test for composite anchor was introduced and carried out. Three zones designated as tension zone, holding zone and compression zone respectively were divided to analyze the stress distribution separately based on some assumptions. Test and analysis results show that the tensile stress and its variation on the steel tube surface will reflect the internal stress distribution of composite anchor. Peak tensile stress exists in the tension zone test points indicates the bonding failure occurs between adhesive and CFRP tendons. The radial clamping action of wedge could effectively enhance the anchoring effect. The obvious decrease of stress curve in the compression zone indicates the bonding damage of entire composite anchor occurs. Keywords: CFRP tendons; Adhension; Stress transfer; Static tensile test; Composite anchors.
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In civil engineering, the employment of fiber reinforced polymer (FRP) materials is increasing due to their notably advantages over conventional materials inclusive of light weight, high mechanical properties and corrosion resistance (Hollaway, 2008; Schmidt et al, 2012; Dolan & Swanson 2002; Li et al. 2007). Extensive studies have been made to examine the overall behavior of FRP-reinforced and strengthened concrete members (Mufti, 2003; Elrefai et al, 2007; Seo et al, 2013; Nigro et al,2014; Neto et al, 2014; D’Antino & Pellegrino, 2014; ; Cai and Aref 2015;
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Faleh et al. 2012; Kim et al. 2014; Koo et al. 2014; Mahroug et al. 2014; Wahab et al. 2015; Yang et al. 2012). In the field of external prestressing, FRP composites are promising to be widely used as
external tendons for the rehabilitation and construction of various engineering structures(Lou, et al, 2014). Among the FRP composites, carbon FRP (CFRP) is considered as replacement for the conventional prestressed steel (Arockiasamy et al. 2000; Crouch et al. 2013; Puigvert et al, 2014; Lou, et al, 2014). Currently, three anchor systems can be used to attach the tendon of composite material to the concrete structure: mechanical (wedge-type) anchors, adhesively bonded anchors and composite anchors. The first is based on the current anchors for steel tendons and is not considered entirely successful because the wedges tend to dig into the composite material causing premature failure(Schmidt , et al., 2009; Schmidt, et al., 2012). For this reason, adhesively bonded anchorages are being investigated to attach composite material tendons to the anchor structure. An adhesive bond-type anchorage consists of a steel tube inside which single or multiple tendons are 1
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bonded with an adhesive. These joints are increasingly being utilized because of their recognized advantages over the mechanical anchorages Puigvert et al, 2014 . The overall properties of a bond-type anchorage depend mainly on the geometry of the materials involved in the joint and the properties of the adhesive (Nanni, et al., 1996; Bahei-El-Din & Dvorak 2001 ; Pincheira & Woyak, 2001). The third anchor system is composite anchor, which combine the advantages of adhesively bonded anchorages and mechanical clamping anchorages. Therefore, it has better anchoring performance (Elrefai et al., 2007; Lü et al., 2007; Liu et al. 2009). This paper presents a test investigation conducted to evaluate the internal stress distribution and stress transfer mechanism in the composite anchor system.
2. Experimental work 2.1. Composite anchors
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The sketch map of the composite anchor, as shown in Figure 1, was tested under static tensile load in this study. The CFRP rod, which has a surface roughness, is 8 mm in diameter. The content of fibre is around 40% by volume. The tensile strength of the CFRP was 2720 MPa and the average limit breaking force is 138.6 kN. The adhesive used was a low viscosity polyamine cured epoxy. The modulus of elasticity of the adhesive was 260 MPa. The thickness of the adhesive is 2 mm. The tensile and compressive yield stresses are 17.3 and 27.2 MPa respectively. The CFRP rods were set in No. 20 # seamless steel tubes through the adhesive epoxy. The length of the tube is 400 mm, and the tensile strength is 410MPa and the modulus of elasticity 21.1 GPa. An anchor ring, which was clamped by mechanical wedge, was set outside the tube. The diameter of the anchor ring is 18 mm and the length of wedge clamping is 65 mm.
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1-Steel tube 2- Anchor ring
3-Wedge clamping
4- Adhesive 5-CFRP rod
Figure 1 Sketch map of the composite anchor
Compared to the mechanical anchors and adhesively bonded anchors, the composite anchors used in this study would be preferable since the wedge clamping can increase the friction force and improve the anchoring efficiency. Meanwhile, the steel tube and adhesive bonded to the CFRP rod can protect the tendon from damaging of wedge clamping.
2.2 Test setup Figure 2 shows the test up using the composite anchor, where 6 strain gauges were installed along the outer wall of steel tube. According to the strength performance of steel tube, three different zones can be observed, which were denoted as tension zone, holding zone and compression zone
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ACCEPTED MANUSCRIPT respectively. Gauges A-1, A-2 and A-3 were pasted in the tension zone and Gauges A-4, A-5 and A-6 were in the compression zone.
Compression zone
Holding zone
Tension zone
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Loading part
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Strain gauges
Figure 2 Sketch map of test set up (in mm)
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Static tensile load was applied in increments of 20 kN until the failure occurs to the CFRP tendon using center hole hydraulic jack with loading rate varying form 300 MPa/min to 400 MPa/min. When each incremental load was applied, allow 5 minutes between load increments and record the strain gauge reading.
2.3 Test results
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During the test, when the tensile load was about 110 kN, a slight sound of fiber fracture can be heard. Finally a sliding failure between CFRP tendon and adhesive can be observed at the load of about 138.6.7 kN and the corresponding ultimate strength was 2720 MPa. The efficiency coefficient of the CFRP tendon-anchorage assembly can be calculated according to the technical specification (JGJ 85-2002). The calculated = 98.6% > 95% using the equation of = /( ), where Fapu is the measured ultimate tensile force of tendon-anchorage
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assembly; Fpu is the ultimate tensile force of the tendons; ηp refers to the efficiency coefficient of the tendons, when the number of tendons or bars vary from 1 to 5, and here ηp=1.
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Based on the data of strain recorded by six strain gauges under different applied tensile load, tensile stress can be calculated. Figure 3 shows the relationship between the tensile stress calculated and the applied tensile load. Under different load increments, designated as 20kN, 40kN, 60kN, 80kN, 100kN, 120kN, 130kN, strains were measured and tensile stresses were calculated at different strain gauges. Figure 4 shows the tensile stress distributions along the steel tube under different load increments.
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Tensile stress /MPa
300 200 100 0 40
20
60
100
80
120
A-1 A-2 A-3
-100
A-4 A-5 A-6
-300 -400
Applied tensile load /kN
Figure 3 Axial stress variation curves 400
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300
100
0 0
-200
-300
-400
40
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Tensile stress /MPa
200
-100
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-200
80
120
160
200
240
280
320
360
400
20kN 40kN 60kN
80kN 100kN 120kN 130kN
Distance along the steel tube /mm
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Figure 4 Tensile stress distributions along the steel tube
3 Stress distribution analysis of composite anchor
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The stress distribution of composite anchor is more complicated than mechanical anchors or adhesively bonded anchors due to its complicated structure, as shown in figure 5. It can be seen in figure 5 that three different zones inclusive of compression zone, holding zone and tension zone respectively were differentiated due to the load-bearing mechanism. In figure 5, Fr is the radial holding force of anchor ring and wedge clamping, Ft is the shear force between wedge clamping and steel tube, l1, l2 and l3 are the lengths of tension zone, holding zone and compression zone, respectively. The stress performance at different zones was analyzed in the following sections. Compression
Adhesive
Steel tube
CFRP rod
Figure 5 Sketch map of stress analysis for composite anchor
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ACCEPTED MANUSCRIPT 3.1 Tension zone The stress analysis diagram for composite anchor and separated parts in tension zone under external tensile load (T) is shown in figure 6. σ 1′ is the tensile stress under the applied load, which can be calculated by tensile force T over the cross section area(A1) of CFRP rod, σ 1 is
tube. The length of tension zone is l1 and using the length of x1 stress analysis. CFRP rod
0 < x1 ≤ l1
when conduct
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Steel tube Adhesive
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normal stress of the rod, σ 2 is the normal stress of adhesive and is the normal stress of steel
Figure 6 Stress analysis diagram in tension zone
Because the thicknesses for steel tube and adhesive are less than the axial length, two assumptions
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are supposed as follows: (1) Both σ 2 and σ 3 follow the uniform distribution; (2) Bonding between steel tube and adhesive is good enough to be regarded as a whole. Hence, for steel tube
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and adhesive, the force equilibrium equations can be written as
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σ 3 A3 + σ 2 A2 = π d ∫ τ ( x1 )dx1
1
Then
σ3 =
where d is diameter of CFRP rod;
π d ∫ τ ( x1 )dx1 σ 2 A2 A3
−
A3
2
τ ( x1 ) is function of the shear stress between CFRP rod and
adhesive in tension zone; A2 is the cross section area of adhesive; A3 is the cross section area for the steel tube.
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ACCEPTED MANUSCRIPT According to the deformation compatibility relationship between steel tube and adhesive due to their coupling condition, Eq. (3) can be obtained.
= where
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ε 2 is the mean normal strain of adhesive section; ε 3 is the mean normal strain of the
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steel tube section. Constitutive relation equations for adhesive and steel tube can be given as
σ 3 = E3ε 3
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σ 2 = E2ε 2
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5
where E2 is the elastic modulus of adhesive; E3 is the elastic modulus of steel tube.
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Then
= = × = 1.2% ×
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Obviously, the normal adhesive stress σ 2 is much less than the steel normal tube stress σ 3 , which means the adhesive can bear the shear stress more than normal stress. Meanwhile, A2 is
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smaller than A3. Hence Eq. (2) can be simplified as follow
σ3 =
πd A3
∫τ ( x )dx 1
1
= C ∫ τ ( x1 )dx1
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Normal stress of steel tube at given distance m can be written as
πd A3
∫
m
0
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σ 3,m = where C =
πd A3
τ ( x1 )dx =
is constant,
πd A3
∫
m
0
τ ( x1 )dx1 = ( ) =
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= ( ) is the integral area of the bond shear stress
along the CFRP rod at a given distance m. Bond shear stress distribution curve along the steel tube had been measured in the previous study for adhesively bonded anchor (Zhang & Benmkorane, 2000), which revealed that with the increase of the load, the peak shear stress of FRP tendon will gradually move to the free end of the anchor and keep the peak shear stress with no change. In this study, when the tensile stress measured at a given point of the steel tube surface increase to the peak value, it reflects the bond shear stress between CFRP rod and adhesive also reach its peak value at the same point. And then the broken between adhesive and CFRP rod will occur. So it can be concluded that the 6
ACCEPTED MANUSCRIPT measured stress variation at the surface of steel tube will indirectly reflect the shear stress variation between CFRP rod and adhesive, which can provide a reference for monitoring the internal stress of the anchor.
3.2 Holding zone In the holding zone, anchor ring and wedge clamping (see figure 1) will exert a radial holding
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force Fr on the steel tube as well as a shear force Ft between wedge clamping and steel tube. Therefore, regarding anchor ring and wedge clamping as a whole integrity, its stress analysis diagram can be seen in figure 8.
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Wedge clamping
Figure 7 Stress analysis diagram for anchor ring and wedge clamping
Based on the force equilibrium condition,
l2
T ′ = σ A = π D ⋅ ∫ τ t ( x2 )dx2
0 < x2 ≤ l2
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0
where, τ t ( x2 ) is the bonding shear or tangential stress distribution function for wedge clamping; σ n ( x2 ) is the normal stress distribution function for wedge clamping T ′ is the tensile reaction of
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anchor ring σ is the end normal stress of anchor ring, A is the end cross section area of anchor ring D is the outer diameter of steel tube.
Figure 8 Stress analysis diagram in holding zone
Figure 8 shows the stress performance of whole composite anchor and each part of anchor in the holding zone. Taking the steel tube and adhesive as the research object, also assuming the adhesive mainly bear the shear force, axial force equilibrium equation can be given as
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ACCEPTED MANUSCRIPT ( + " )
= #$ % ( ) − # ( )
10
where τ ( x2 ) is the bonding shear stress distribution function between CFRP tendon and adhesive in holding zone.
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According to the measured axial tensile stress variation curves of steel tube (see figure 4), when the applied load ranges from 80kN to 110kN, a small change of tensile stress can be found for six test points. Based on the analysis steel tube in holding zone, we can conclude that when the peak value of bonding shear stress moves from tension zone the holding zone, the bonding shear stress in tension zone only shows the bonding residual stress between adhesive and steel tube, thus will have little change in amplitude of variation. The relative deformation between CFRP tendon and adhesive in compression zone is relatively small so that the bonding shear stress is also small. The above analyses explain the tress variation trend observed in the test. Hence, we can assume that
(σ 3 + σ 3′ ) A3 = C ′ is a constant and the Eq. (10) can be rewritten as -
' ( )* ( +,
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( ) = -
∆ ( ) =
∆' ( +,
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It can be known in Eq. (12) that the increment of tensile reaction of anchor ring is controlled by the shear force between CFRP tendon and adhesive. It can be concluded that if we increase the
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radial holding force Fr, we can increase τ ( x2 ) and improve the anchoring effect. The main factors affecting the radial holding force include steel tube thickness, wedge angle and wedge length, etc. Reasonable thickness for steel tube should guarantee the adhesive do not crush, while
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making the radial holding stress as high as it can be. Meanwhile, reasonable wedge angle can make the radial stress distribution be more uniform in holding zone. Larger wedge length can
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result in larger loading area and enhances the overall anchoring behavior. The impacts of above factors on anchoring effect need further studying through the laboratory tests and theoretical analysis in order to achieve an optimal parameter combination.
3.3 Compression zone
The stress performance for composite anchor in compression zone is similar to that of the tensile zone. Figure 9 shows the stress analysis diagram of whole and each part in the compressive zone, taking the length is x3
0 < x3 ≤ l3
.
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Figure 9 Stress analysis diagram in compression zone
σ 3 A3 = π d ∫ τ ( x3 )dx3
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τ ( x3 ) is the bonding shear stress distribution function between CFRP rod and adhesive in
the compression zone, then
σ3 =
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where
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Taking the steel tube and adhesive as the research object, also assuming the adhesive mainly bear the shear force, the axial force equilibrium equation can be written as
πd A3
∫τ ( x )dx 3
3
= C ∫ τ ( x3 )dx3
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According to the measured axial tensile stress variation curves in figure 3, when the applied load reaches 110 kN, test points in compression zone show a larger decrease of tensile stress, which suggests that a increase of bonding shear stress and peak bonding shear stress value have already moved to this zone, followed by an overall breakage. Besides, based on the theoretical equations (8) and (14) for tension zone and compression zone respectively, the stresses can be calculated at different positions. By comparison the stresses calculated and measured for Gauges A-1, A-2 and A-3 in the tension zone and Gauges A-4, A-5 and A-6 in the compression zone, generally good agreement was obtained.
4 Conclusions
Combined with static tensile load test on composite anchor and stress distribution analysis on three different zones of anchor, some conclusions are summarized as follows: 1 The variation of the tensile stress measured on steel tube surface can reflect the internal stress distribution. When the measured stress increased to the peak value in tension zone, the bonding shear stress between adhesive and CFRP tendon at the test point also approach the peak value and followed by the bonding failure.
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ACCEPTED MANUSCRIPT 2 The radial wedge clamping force can increase the whole anchoring effect in the holding zone. Steel tube thickness, wedge angle and wedge length will affect the anchoring effect, which need further studying.
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(3) When peak shear stress moved to the compression zone, all of the test points show a larger increase of compression stress and the composite anchor will suffer overall destruction. Acknowledgements
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This study was financially sponsored by the National Natural Science Foundation of China (51478209, 41402251) and Jiangsu Government Scholarship for Overseas Studies (JS-2013-092). All of these supports are gratefully acknowledged.
References
Arockiasamy, M., Chidambaram, S., Amer, A., and Shahawy, M. (2000). "Time-dependent
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deformations of concrete beams reinforced with CFRP bars." Composites Part B-Engineering, 31(6-7), 577-592.
Bahei-El-Din, Y. A., Dvorak, G. J. New designs of adhesive joints for thick composite laminates. Compos Sci Technol 2001; 61:19-40. Cai, H. W., and Aref, A. J. (2015). "On the design and optimization of hybrid carbon fiber reinforced polymer-steel cable system for cable-stayed bridges." Composites Part B-Engineering, 68, 146-152.
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Crouch, R. D., Clay, S. B., and Oskay, C. (2013). "Experimental and computational investigation of progressive damage accumulation in CFRP composites." Composites Part B-Engineering, 48, 59-67. D’Antino, T., Pellegrino, C. Bond between FRP composites and concrete: assessment of design procedures and analytical models. Compos B Eng 2014, 60: 440-456. Dolan, C. W., Swanson, D. Development of flexural capacity of a FRP prestressed beam with vertically
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distributed tendons, Composites Part B: Engineering, 2002, 33(1): 1-6. Elrefai, A., West, J. S., Soudki K.. Performance of CFRP tendon-anchor assembly under fatigue loading.
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Composite Structures. 2007,80:352-360. Faleh, H., Al-Mahaidi, R., and Shen, L. M. (2012). "Fabrication and characterization of nano-particles-enhanced epoxy." Composites Part B-Engineering, 43(8), 3076-3080. Hollaway, L.C. A review of the present and future utilization of FRP composites in the civil infrastructure with reference to their important in-service properties. Construction and Building Materials, 2010,24: 2419-2455.
JGJ 85-2002, Technical specification for application of anchorage, grip and coupler for prestressing tendons. Beijing, Building Industry Press, 2002. (in Chinese) Kim, D. J., Lee, J., and Lee, Y. H. (2014). "Effectiveness factor of strut-and-tie model for concrete deep beams reinforced with FRP rebars." Composites Part B-Engineering, 56, 117-125.
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ACCEPTED MANUSCRIPT Koo, J. M., Choi, J. H., and Seok, C. S. (2014). "Prediction of post-impact residual strength and fatigue characteristics after impact of CFRP composite structures." Composites Part B-Engineering, 61, 300-306. Li, R., Cho, Y. S., and Zhang, S. M. (2007). "Punching shear behavior of concrete flat plate slab reinforced with carbon fiber reinforced polymer rods." Composites Part B-Engineering, 38(5-6), 712-719.
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Liu, R.G., Xu, F.; Cai, D.S.;. Analysis of Static Load Test for Cable-stayed Bridge with CFRP Cables. China Journal of Highway and Transport,2009, 22(2):48-52. (in Chinese) Lü, Z.T., Mei, K.H. First application of CFRP cables for a cable-stayed bridge in China. China Civil Engineering Journal, 2007, (1):54-58.(in Chinese) Mufti, A.A. FRPs and FOSs lead to innovation in Canadian civil engineering structures. Consttuction
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and Building Materials, 2003, 17:379-387.
Neto P, Alfaiate J, Vinagre J. A three-dimensional analysis of CFRP–concrete bond behavior. Compos B Eng 2014;59:153-165.
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Nanni, A., Bakis, C. E., O’Neil, E.F. Dixon TO. Performance of FRP tendon anchor systems for prestessing concrete structures. PCI J, 1996: 34-44. Nigro, E., Cefarelli, G., Bilotta, A., Manfredi, G., Cosenza, E. Guidelines for flexural resistance of FRP reinforced concrete slabs and beams in fire. Compos B Eng 2014, 58:103-112.
Puigvert, F., Gil, L., Escrig, C., Bernat, E. Stress relaxation analysis of adhesively bonded anchorages for CFRP tendons, Construction and Building Materials, Volume 66, 2014, 313-322
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Puigvert, F., Crocombe, A.D., Gil, L. Fatigue and creep analyses of adhesively bonded anchorages for CFRP tendons, International Journal of Adhesion and Adhesives, Volume 54, 2014, 143-154. Seo, S. Y., Feo, L., Hui, D. Bond strength of near surface-mounted FRP plate for retrofit of concrete structures. Compos Struct 2013, 95: 719-727.
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Schmidt, J. W., Täljsten, B., Bennitz, A. Pedersen H. FRP tendon anchorage in posttensioned concrete structures. Concr Repair, Rehab Retrofitting, 2009: 1181-1196. Schmidt, J. W., Bennitz, A., Taljsten, B., Goltermann, P. Pedersen H. Mechanical anchorage of FRP
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tendons – a literature review. Constr Build Mater, 2012, 32: 110-121.
Pincheira, J. A., Woyak, J. Anchorage of carbon fiber reinforced polymer (CFRP) tendons using cold-swaged sleeves. PCI J 2001:100-111. Lou, T. J., Lopes, S. M. R., Lopes, A. V. Factors affecting moment redistribution at ultimate in continuous, Composites Part B: Engineering, 2014, 66: 136-146. Lou, T. J., Lopes, S. M. R., Lopes, A. V. External CFRP tendon members: Secondary reactions and moment redistribution, Composites Part B: Engineering, 2014, 57: 250-261. Mahroug, M. E. M., Ashour, A. F., and Lam, D. (2014). "Tests of continuous concrete slabs reinforced with carbon fibre reinforced polymer bars." Composites Part B-Engineering, 66, 348-357. Wahab, N., Topper, T., and Soudki, K. A. (2015). "Modelling experimental bond fatigue failures of concrete beams strengthened with NSM CFRP rods." Composites Part B-Engineering, 70, 113-121.
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ACCEPTED MANUSCRIPT Yang, J. M., Min, K. H., Shin, H. O., and Yoon, Y. S. (2012). "Effect of steel and synthetic fibers on flexural behavior of high-strength concrete beams reinforced with FRP bars." Composites Part B-Engineering, 43(3), 1077-1086.
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Zhang, B., Benmkorane, B., Prediction of tensile capacity of bond anchorages for FRP tensions. Journal of Composites for Construction(ASCE), 2000, 4(2): 39-47
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S1359-8368(15)00309-1
DOI:
10.1016/j.compositesb.2015.05.014
Reference:
JCOMB 3602
To appear in:
Composites Part B
Received Date: 16 March 2015 Revised Date:
6 May 2015
Accepted Date: 8 May 2015
Please cite this article as: Cai D-s, Yin J, Liu R-g, Experimental and analytical investigation into the stress performance of composite anchors for CFRP tendons, Composites Part B (2015), doi: 10.1016/ j.compositesb.2015.05.014. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Experimental and analytical investigation into the stress performance of composite anchors for CFRP tendons Dong-sheng Cai1, Jie Yin1,2,*, Rong-gui Liu1
1 Department of Civil Engineering, Faculty of Civil Engineering and Mechanics, Jiangsu University,
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Zhenjiang 212013 China 2 Department of Civil and Environmental Engineering, University of Wisconsin–Madison, Madison, WI 53706 USA *
Corresponding Author, e-mail:[email protected]; Tel: +1 608 422 3373
1. Introduction
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Abstract: This paper presents a study of the internal stress distribution and its stress transfer mechanism of composite anchors for carbon fiber-reinforced polymer (CFRP) tendons. One set of static tensile test for composite anchor was introduced and carried out. Three zones designated as tension zone, holding zone and compression zone respectively were divided to analyze the stress distribution separately based on some assumptions. Test and analysis results show that the tensile stress and its variation on the steel tube surface will reflect the internal stress distribution of composite anchor. Peak tensile stress exists in the tension zone test points indicates the bonding failure occurs between adhesive and CFRP tendons. The radial clamping action of wedge could effectively enhance the anchoring effect. The obvious decrease of stress curve in the compression zone indicates the bonding damage of entire composite anchor occurs. Keywords: CFRP tendons; Adhension; Stress transfer; Static tensile test; Composite anchors.
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In civil engineering, the employment of fiber reinforced polymer (FRP) materials is increasing due to their notably advantages over conventional materials inclusive of light weight, high mechanical properties and corrosion resistance (Hollaway, 2008; Schmidt et al, 2012; Dolan & Swanson 2002; Li et al. 2007). Extensive studies have been made to examine the overall behavior of FRP-reinforced and strengthened concrete members (Mufti, 2003; Elrefai et al, 2007; Seo et al, 2013; Nigro et al,2014; Neto et al, 2014; D’Antino & Pellegrino, 2014; ; Cai and Aref 2015;
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Faleh et al. 2012; Kim et al. 2014; Koo et al. 2014; Mahroug et al. 2014; Wahab et al. 2015; Yang et al. 2012). In the field of external prestressing, FRP composites are promising to be widely used as
external tendons for the rehabilitation and construction of various engineering structures(Lou, et al, 2014). Among the FRP composites, carbon FRP (CFRP) is considered as replacement for the conventional prestressed steel (Arockiasamy et al. 2000; Crouch et al. 2013; Puigvert et al, 2014; Lou, et al, 2014). Currently, three anchor systems can be used to attach the tendon of composite material to the concrete structure: mechanical (wedge-type) anchors, adhesively bonded anchors and composite anchors. The first is based on the current anchors for steel tendons and is not considered entirely successful because the wedges tend to dig into the composite material causing premature failure(Schmidt , et al., 2009; Schmidt, et al., 2012). For this reason, adhesively bonded anchorages are being investigated to attach composite material tendons to the anchor structure. An adhesive bond-type anchorage consists of a steel tube inside which single or multiple tendons are 1
ACCEPTED MANUSCRIPT
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bonded with an adhesive. These joints are increasingly being utilized because of their recognized advantages over the mechanical anchorages Puigvert et al, 2014 . The overall properties of a bond-type anchorage depend mainly on the geometry of the materials involved in the joint and the properties of the adhesive (Nanni, et al., 1996; Bahei-El-Din & Dvorak 2001 ; Pincheira & Woyak, 2001). The third anchor system is composite anchor, which combine the advantages of adhesively bonded anchorages and mechanical clamping anchorages. Therefore, it has better anchoring performance (Elrefai et al., 2007; Lü et al., 2007; Liu et al. 2009). This paper presents a test investigation conducted to evaluate the internal stress distribution and stress transfer mechanism in the composite anchor system.
2. Experimental work 2.1. Composite anchors
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The sketch map of the composite anchor, as shown in Figure 1, was tested under static tensile load in this study. The CFRP rod, which has a surface roughness, is 8 mm in diameter. The content of fibre is around 40% by volume. The tensile strength of the CFRP was 2720 MPa and the average limit breaking force is 138.6 kN. The adhesive used was a low viscosity polyamine cured epoxy. The modulus of elasticity of the adhesive was 260 MPa. The thickness of the adhesive is 2 mm. The tensile and compressive yield stresses are 17.3 and 27.2 MPa respectively. The CFRP rods were set in No. 20 # seamless steel tubes through the adhesive epoxy. The length of the tube is 400 mm, and the tensile strength is 410MPa and the modulus of elasticity 21.1 GPa. An anchor ring, which was clamped by mechanical wedge, was set outside the tube. The diameter of the anchor ring is 18 mm and the length of wedge clamping is 65 mm.
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1-Steel tube 2- Anchor ring
3-Wedge clamping
4- Adhesive 5-CFRP rod
Figure 1 Sketch map of the composite anchor
Compared to the mechanical anchors and adhesively bonded anchors, the composite anchors used in this study would be preferable since the wedge clamping can increase the friction force and improve the anchoring efficiency. Meanwhile, the steel tube and adhesive bonded to the CFRP rod can protect the tendon from damaging of wedge clamping.
2.2 Test setup Figure 2 shows the test up using the composite anchor, where 6 strain gauges were installed along the outer wall of steel tube. According to the strength performance of steel tube, three different zones can be observed, which were denoted as tension zone, holding zone and compression zone
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ACCEPTED MANUSCRIPT respectively. Gauges A-1, A-2 and A-3 were pasted in the tension zone and Gauges A-4, A-5 and A-6 were in the compression zone.
Compression zone
Holding zone
Tension zone
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Loading part
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Strain gauges
Figure 2 Sketch map of test set up (in mm)
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Static tensile load was applied in increments of 20 kN until the failure occurs to the CFRP tendon using center hole hydraulic jack with loading rate varying form 300 MPa/min to 400 MPa/min. When each incremental load was applied, allow 5 minutes between load increments and record the strain gauge reading.
2.3 Test results
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During the test, when the tensile load was about 110 kN, a slight sound of fiber fracture can be heard. Finally a sliding failure between CFRP tendon and adhesive can be observed at the load of about 138.6.7 kN and the corresponding ultimate strength was 2720 MPa. The efficiency coefficient of the CFRP tendon-anchorage assembly can be calculated according to the technical specification (JGJ 85-2002). The calculated = 98.6% > 95% using the equation of = /( ), where Fapu is the measured ultimate tensile force of tendon-anchorage
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assembly; Fpu is the ultimate tensile force of the tendons; ηp refers to the efficiency coefficient of the tendons, when the number of tendons or bars vary from 1 to 5, and here ηp=1.
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Based on the data of strain recorded by six strain gauges under different applied tensile load, tensile stress can be calculated. Figure 3 shows the relationship between the tensile stress calculated and the applied tensile load. Under different load increments, designated as 20kN, 40kN, 60kN, 80kN, 100kN, 120kN, 130kN, strains were measured and tensile stresses were calculated at different strain gauges. Figure 4 shows the tensile stress distributions along the steel tube under different load increments.
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Tensile stress /MPa
300 200 100 0 40
20
60
100
80
120
A-1 A-2 A-3
-100
A-4 A-5 A-6
-300 -400
Applied tensile load /kN
Figure 3 Axial stress variation curves 400
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300
100
0 0
-200
-300
-400
40
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Tensile stress /MPa
200
-100
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-200
80
120
160
200
240
280
320
360
400
20kN 40kN 60kN
80kN 100kN 120kN 130kN
Distance along the steel tube /mm
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Figure 4 Tensile stress distributions along the steel tube
3 Stress distribution analysis of composite anchor
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The stress distribution of composite anchor is more complicated than mechanical anchors or adhesively bonded anchors due to its complicated structure, as shown in figure 5. It can be seen in figure 5 that three different zones inclusive of compression zone, holding zone and tension zone respectively were differentiated due to the load-bearing mechanism. In figure 5, Fr is the radial holding force of anchor ring and wedge clamping, Ft is the shear force between wedge clamping and steel tube, l1, l2 and l3 are the lengths of tension zone, holding zone and compression zone, respectively. The stress performance at different zones was analyzed in the following sections. Compression
Adhesive
Steel tube
CFRP rod
Figure 5 Sketch map of stress analysis for composite anchor
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ACCEPTED MANUSCRIPT 3.1 Tension zone The stress analysis diagram for composite anchor and separated parts in tension zone under external tensile load (T) is shown in figure 6. σ 1′ is the tensile stress under the applied load, which can be calculated by tensile force T over the cross section area(A1) of CFRP rod, σ 1 is
tube. The length of tension zone is l1 and using the length of x1 stress analysis. CFRP rod
0 < x1 ≤ l1
when conduct
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Steel tube Adhesive
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normal stress of the rod, σ 2 is the normal stress of adhesive and is the normal stress of steel
Figure 6 Stress analysis diagram in tension zone
Because the thicknesses for steel tube and adhesive are less than the axial length, two assumptions
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are supposed as follows: (1) Both σ 2 and σ 3 follow the uniform distribution; (2) Bonding between steel tube and adhesive is good enough to be regarded as a whole. Hence, for steel tube
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and adhesive, the force equilibrium equations can be written as
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σ 3 A3 + σ 2 A2 = π d ∫ τ ( x1 )dx1
1
Then
σ3 =
where d is diameter of CFRP rod;
π d ∫ τ ( x1 )dx1 σ 2 A2 A3
−
A3
2
τ ( x1 ) is function of the shear stress between CFRP rod and
adhesive in tension zone; A2 is the cross section area of adhesive; A3 is the cross section area for the steel tube.
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ACCEPTED MANUSCRIPT According to the deformation compatibility relationship between steel tube and adhesive due to their coupling condition, Eq. (3) can be obtained.
= where
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ε 2 is the mean normal strain of adhesive section; ε 3 is the mean normal strain of the
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steel tube section. Constitutive relation equations for adhesive and steel tube can be given as
σ 3 = E3ε 3
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σ 2 = E2ε 2
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5
where E2 is the elastic modulus of adhesive; E3 is the elastic modulus of steel tube.
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Then
= = × = 1.2% ×
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Obviously, the normal adhesive stress σ 2 is much less than the steel normal tube stress σ 3 , which means the adhesive can bear the shear stress more than normal stress. Meanwhile, A2 is
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smaller than A3. Hence Eq. (2) can be simplified as follow
σ3 =
πd A3
∫τ ( x )dx 1
1
= C ∫ τ ( x1 )dx1
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Normal stress of steel tube at given distance m can be written as
πd A3
∫
m
0
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σ 3,m = where C =
πd A3
τ ( x1 )dx =
is constant,
πd A3
∫
m
0
τ ( x1 )dx1 = ( ) =
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= ( ) is the integral area of the bond shear stress
along the CFRP rod at a given distance m. Bond shear stress distribution curve along the steel tube had been measured in the previous study for adhesively bonded anchor (Zhang & Benmkorane, 2000), which revealed that with the increase of the load, the peak shear stress of FRP tendon will gradually move to the free end of the anchor and keep the peak shear stress with no change. In this study, when the tensile stress measured at a given point of the steel tube surface increase to the peak value, it reflects the bond shear stress between CFRP rod and adhesive also reach its peak value at the same point. And then the broken between adhesive and CFRP rod will occur. So it can be concluded that the 6
ACCEPTED MANUSCRIPT measured stress variation at the surface of steel tube will indirectly reflect the shear stress variation between CFRP rod and adhesive, which can provide a reference for monitoring the internal stress of the anchor.
3.2 Holding zone In the holding zone, anchor ring and wedge clamping (see figure 1) will exert a radial holding
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force Fr on the steel tube as well as a shear force Ft between wedge clamping and steel tube. Therefore, regarding anchor ring and wedge clamping as a whole integrity, its stress analysis diagram can be seen in figure 8.
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Wedge clamping
Figure 7 Stress analysis diagram for anchor ring and wedge clamping
Based on the force equilibrium condition,
l2
T ′ = σ A = π D ⋅ ∫ τ t ( x2 )dx2
0 < x2 ≤ l2
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0
where, τ t ( x2 ) is the bonding shear or tangential stress distribution function for wedge clamping; σ n ( x2 ) is the normal stress distribution function for wedge clamping T ′ is the tensile reaction of
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anchor ring σ is the end normal stress of anchor ring, A is the end cross section area of anchor ring D is the outer diameter of steel tube.
Figure 8 Stress analysis diagram in holding zone
Figure 8 shows the stress performance of whole composite anchor and each part of anchor in the holding zone. Taking the steel tube and adhesive as the research object, also assuming the adhesive mainly bear the shear force, axial force equilibrium equation can be given as
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ACCEPTED MANUSCRIPT ( + " )
= #$ % ( ) − # ( )
10
where τ ( x2 ) is the bonding shear stress distribution function between CFRP tendon and adhesive in holding zone.
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According to the measured axial tensile stress variation curves of steel tube (see figure 4), when the applied load ranges from 80kN to 110kN, a small change of tensile stress can be found for six test points. Based on the analysis steel tube in holding zone, we can conclude that when the peak value of bonding shear stress moves from tension zone the holding zone, the bonding shear stress in tension zone only shows the bonding residual stress between adhesive and steel tube, thus will have little change in amplitude of variation. The relative deformation between CFRP tendon and adhesive in compression zone is relatively small so that the bonding shear stress is also small. The above analyses explain the tress variation trend observed in the test. Hence, we can assume that
(σ 3 + σ 3′ ) A3 = C ′ is a constant and the Eq. (10) can be rewritten as -
' ( )* ( +,
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( ) = -
∆ ( ) =
∆' ( +,
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It can be known in Eq. (12) that the increment of tensile reaction of anchor ring is controlled by the shear force between CFRP tendon and adhesive. It can be concluded that if we increase the
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radial holding force Fr, we can increase τ ( x2 ) and improve the anchoring effect. The main factors affecting the radial holding force include steel tube thickness, wedge angle and wedge length, etc. Reasonable thickness for steel tube should guarantee the adhesive do not crush, while
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making the radial holding stress as high as it can be. Meanwhile, reasonable wedge angle can make the radial stress distribution be more uniform in holding zone. Larger wedge length can
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result in larger loading area and enhances the overall anchoring behavior. The impacts of above factors on anchoring effect need further studying through the laboratory tests and theoretical analysis in order to achieve an optimal parameter combination.
3.3 Compression zone
The stress performance for composite anchor in compression zone is similar to that of the tensile zone. Figure 9 shows the stress analysis diagram of whole and each part in the compressive zone, taking the length is x3
0 < x3 ≤ l3
.
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Figure 9 Stress analysis diagram in compression zone
σ 3 A3 = π d ∫ τ ( x3 )dx3
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τ ( x3 ) is the bonding shear stress distribution function between CFRP rod and adhesive in
the compression zone, then
σ3 =
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where
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Taking the steel tube and adhesive as the research object, also assuming the adhesive mainly bear the shear force, the axial force equilibrium equation can be written as
πd A3
∫τ ( x )dx 3
3
= C ∫ τ ( x3 )dx3
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According to the measured axial tensile stress variation curves in figure 3, when the applied load reaches 110 kN, test points in compression zone show a larger decrease of tensile stress, which suggests that a increase of bonding shear stress and peak bonding shear stress value have already moved to this zone, followed by an overall breakage. Besides, based on the theoretical equations (8) and (14) for tension zone and compression zone respectively, the stresses can be calculated at different positions. By comparison the stresses calculated and measured for Gauges A-1, A-2 and A-3 in the tension zone and Gauges A-4, A-5 and A-6 in the compression zone, generally good agreement was obtained.
4 Conclusions
Combined with static tensile load test on composite anchor and stress distribution analysis on three different zones of anchor, some conclusions are summarized as follows: 1 The variation of the tensile stress measured on steel tube surface can reflect the internal stress distribution. When the measured stress increased to the peak value in tension zone, the bonding shear stress between adhesive and CFRP tendon at the test point also approach the peak value and followed by the bonding failure.
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ACCEPTED MANUSCRIPT 2 The radial wedge clamping force can increase the whole anchoring effect in the holding zone. Steel tube thickness, wedge angle and wedge length will affect the anchoring effect, which need further studying.
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(3) When peak shear stress moved to the compression zone, all of the test points show a larger increase of compression stress and the composite anchor will suffer overall destruction. Acknowledgements
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This study was financially sponsored by the National Natural Science Foundation of China (51478209, 41402251) and Jiangsu Government Scholarship for Overseas Studies (JS-2013-092). All of these supports are gratefully acknowledged.
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