Experimental and Numerical Investigation of Patch Anchors used to Enhance the Performance of FRP Laminates in Concrete Structures

Transcription

1 Experimental and Numerical Investigation of Patch Anchors used to Enhance the Performance of FRP Laminates in Concrete Structures R. Kalfat 1 and R. Al-Mahaidi 1 1 Swinburne University of Technology, Hawthorn, Vic, Australia Abstract: Research has demonstrated that bi-directional fiber patch anchors are one of the most efficient and practical forms of anchors which can be used to increase the efficiency of Advanced Fiber Reinforced Composites (FRP s) systems when bonded to concrete. However, such anchors have found applications to a limited number of projects due to the absence of sufficient experimental data to promote the development of theoretical models. The study will present the first extensive experimental program to investigate the parameters of patch anchor size and spacing on overall anchorage performance. Numerical simulations such as non-linear finite element method (FEM) have been presented to simulate the full response of reinforced concrete (RC) members strengthened with fibre reinforced polymers (FRP s). It has been shown that calibrated numerical models have the potential to reduce the number of experimental tests and parametric studies can provide extensive data to evaluate the influence of key parameters 1. Introduction Over the last two decades, extensive research has demonstrated the effectiveness of externally bonded (EB) fibre-reinforced polymer (FRP) composites for strengthening and repairing (in addition to seismic retrofitting) reinforced concrete (RC) structures. Numerous field applications have been completed to date, in particular the strengthening of buildings and bridges with EB FRP (Hollaway & Teng 28; Williams, Al-Mahaidi & Kalfat 211). The main advantages of EB FRP for strengthening, when compared to strengthening using traditional engineering materials such as steel, are their high strength-to-weight ratio (up to ten times stronger than steel and about 2% of the weight) and high corrosion resistance. A commonly documented failure mode of FRP strengthened RC is by premature debonding, which generally occurs at fibre strains well below the tensile strain capacity of the FRP. Failure by debonding is usually rapid and represents an underutilisation of the materials high tensile strength. Design guidelines around the world are strongly influenced by such behaviour and place quite strict limitations on the usable strain of the bonded FRP that may be utilised in design. A logical means to enhance the strain capacity of externally bonded FRP is by anchorage (Kalfat et al. 212). However, research on anchorage systems has been extremely limited to date. Such limitations have inspired the present research and further experimental studies into bi-directional fibre patch anchorages with the aim of building a greater experimental database and to investigate effects of patch anchor size on the overall anchorage strength. Recent tests by (Al-Mahaidi and Kalfat 211; Kalfat et al. 211) have shown that ±45º oriented bi-directional fabric anchorages (herein Patch Anchors) resulted in gradual debonding of FRP laminates as a result of FRP-adhesive stresses being distributed over a greater area of the concrete. The ±45º oriented bi-directional fabric configuration was successfully applied to strengthen the West Gate Bridge in Melbourne (Williams et al 211) which represents the world s largest application of FRP strengthening to date. It is well established research practice to use numerical simulations in order to reduce the number of experimental tests, which are costly in terms of time and money. Finite element simulations by (Pham and Al-Mahaidi 25, 27; Al-Mahaidi and Hii 27) has shown that such a tool is capable of adequately modelling the full response of FRPstrengthened RC. 2. Experimental Program 2.1 Specimen Design The current specimen design was largely based from that used in an earlier experimental study (stage 1), the results of which were published in (Al-Mahaidi & Kalfat 211). Here (stage 2), emphasis was placed on investigating a more commonly used laminate thickness (1.4mm). The effect of laminate spacing was investigated by the use of three alternative concrete block sizes of width: 42, 32 and 22mm respectively. Appropriate boundary conditions of symmetry at the concrete block edges were

2 applied by replicating restraint normal to the concrete sides (x direction) whilst allowing movement in the vertical plane (y direction). This was accomplished by the construction of steel angle slotted movement joints, the details of which are presented in figure 1. Each angle (1x1x1mm) contained 2 no. 5 x 11mm slots which were placed between two greased steel plates with 1mm holding dowels to create the movement joint. The result of this symmetric boundary was that the effects of the patch anchors used to anchor multiple laminates spaced at 42, 32 and 22mm apart could be investigated by simulating symmetry. In addition, 2 different patch anchor lengths: 3 mm in types (1, 3 and 4) and 25mm (type 2) were investigated. Figure 1. Specimen summary 2.3 Test Preparation and Material properties Concrete blocks were reinforced nominally with 4 no.12mm diameter bars at 1mm centres each face. The reinforcement cover used was 3mm. All specimens consisted of a single laminate strip bonded to the surface of the concrete block with a bond length of 37mm. Table 1 summarises the material properties used as per manufacturers specifications. Properties Table 1. Adhesives, Saturant and FRP Properties data Laminate Adhesive Saturant Bidirectional FRP (±45 ) FRP Laminate Unidirectional FRP Units Compressive >6 > Strength MPa Tensile Strength 32 > MPa Tensile Modulus 1 > GPa Ult. Elongation % Thickness mm Width mm

3 2.4 Instrumentation and loading procedure The specimens were loaded under displacement control at a load rate of 1mm/minute. Strain and load data was obtained from surface mounted strain gauges and a 3D non-contact measuring technique based on image correlation photogrammetry (Correlated-Solutions 21). A series of 7 strain gauges (G1-G7) were applied to the length of the FRP laminate at 5 mm intervals. An additional 4 gauges were placed either side of the laminate (2 each side) to measure strains in the bi-directional fibers (G8- G11). Gauges G1 and G12 were installed at the front and back of the laminate to monitor any bending in the FRP plate during testing indicating the presence of tilting. The 3D photogrammetry measurements were taken using a pair of high resolution, digital CCD (charged couple device) cameras. A measuring step of 1 second was used between recording intervals. 3. Experimental Results 3.1 Quality Control Tests A total of 12 concrete cylinders were tested to assess the concrete compressive strength. After 53 days curing at room temperature, the average compressive strength of the concrete was 69.2MPa. Pull off tests conducted prior to testing of the specimens indicated that laminate bond failure also occurred within the concrete at a bond pressure of 4.5 MPa. 3.2 Failure Modes Both control specimens failed by debond within the concrete cover zone within the initial 5mm of bond length. Further along the laminate the failure plane shifted to between the concrete and adhesive interface, refer figure 3(a). Two alternative failure modes were observed in the specimens anchored with bi-directional fibres. All anchored specimens exhibited partial debonding between the concrete and adhesive, over the unanchored 5mm bond length at a load level of 9-1 kn. Load was sustained as stresses were dispersed further along the laminate and through the bidirectional fibres. The final failure modes observed were: (1) complete debonding of the sandwiched laminate and bidirectional fabric structure from the concrete block, refer figure 3(b); or (2) slippage of the laminate from between the two layers of bidirectional fibres, refer figure 3(c). (a) (b) (c) Figure 3. Specimen failure summary; (a) control specimen; (b) patch anchor pull-off failure; (c) laminate slippage It was observed that specimens with a higher concentration of aggregate at the bond interface fail by laminate slippage, whereas specimens with a lower concentration of aggregate failed by complete patch anchor debonding. 3.4 Results Overview Table 2 and figure 4 summarises the failure loads and maximum FRP elongations reached in all specimens tested. In tables and figures which follow reference is made to V3D (Photogrammetry) and

4 SG (strain gauge). These refer to the two data acquisition techniques used in the experimental program (a) (b) (c) (d) (e) (f) (g) (h) (i) Figure 4. Strain vs distance along Laminate; (a) Spec.1, (b) Spec 1.1, (c) Spec 1.2, (d) Spec 2.1, (e) Spec 2.2, (f) Spec 3.1, (g) Spec 3.2, (h) Spec 4.1, (i) Spec 4.2 Specimen Width of Patch Anchor (mm) 4kN(V3D) 6kN(V3D) 84kN(V3D) 6kN (SG) 84kN (SG) 4kN(V3D) 8kN(V3D) 17.2kN(V3D) 8kN (SG) 17.2kN (SG) 4kN(V3D) 8kN(V3D) 138kN(V3D) 8kN (SG) 138kN (SG) Table 2. Results summary Length of Patch Anchor(mm) Distance along Laminate (mm Failure Load (kn) 4kN(V3D) 8kN(V3D) 13kN(V3D) 8kN (SG) 13kN (SG) 4kN(V3D) 8kN(V3D) 127.9kN(V3D) 8kN (SG) 127.9kN (SG) 4kN(V3D) 1kN(V3D) 14kN(V3D) 1kN (SG) 1 Max Limate strain (µå) Increase in Load (%) Failure mode.1 NA (Control) NA (Control) Pull Pull Pull Slip Slip Slip Slip Slip Pull kN(V3D) 8kN(V3D) 14kN(V3D) 8kN (SG) 1 4kN(V3D) 8kN(V3D) 151kN(V3D) 8kN (SG) 151kN (SG) Distance From Edge (mm) 8kN (SG) 12kN (SG)

5 Microstrain (με) Microstrain (με) Microstrain (με) Microstrain (με) From the above table it is clear that the patch anchors exhibited a marked improvement in the failure load and strain reached prior to debond. Specimens 2.1 and 2.2 were designed with a lower patch anchorage length (25mm) and exhibited slippage at a lower load. Specimens with anchorage lengths of 3mm failed by either slippage at a higher load or patch anchor debond. As a result, 3mm was considered to be the optimal anchor length for use in further study. Specimens with 3mm anchor lengths demonstrated that slippage occurred at load levels ranging between kn (9.5% variation). Patch anchor debond was observed at similarly high levels, kn for anchor widths of 4mm and at a lower load of kn for 2mm wide anchors. Design guidelines such as the (ACI 44.2R-8 28) section recommends that for shear strengthened members, the maximum effective FRP design strain should not exceed 2365 µε for the laminate properties shown in table 1. This level of strain is only 16.9% of the laminate rupture strain, further highlighting the low level of permissible utilization in such applications. All patch anchors, which used an anchorage length of 3mm exceeded µε prior to failure. As a result, material utilization can be increased to 31-38% when patch anchors are provided resulting in significant reductions in material required to reach a given level of strengthening. Strain in Bi-directional fibres Measurements were conducted using surface mounted strain gauges and photogrammetry to determine the strains in the bi-directional fibres either side of the laminate at progressive stages of loading. It should be noted that the strains are orientated at ±45 degree angles from the longitudinal axis parallel to the direction of bi-directional fibres. The results are summarized in figure 5 for each of the 4 types of specimens tested. Specimen type 1, which failed by debonding of the bi-directional fibre sheet, showed a greater engagement of fibre strains at a distance of 5mm away from the laminate edge when compared to the specimens that failed by slippage. It is also clear that in all cases, the bidirectional fibre strains are concentrated within the first 5mm distance away from the laminate edge and reduce to zero at a distance of 1mm. These results provide insight into the minimum spacing that patch anchored FRP laminates may be placed beside one another without a reduction of anchorage pull-off strength. The minimum spacing recommended is 25mm. 6kN (SG) 6kN (SG) 8kN (SG) 1kN (SG) 11kN (SG) 8kN (SG) 9kN (SG) 1kN (SG) Distance ±45º from centre of laminate (mm) (a) 6kN (SG) Distance ±45º from centre of laminate (mm) (b) 6kN (SG) 8kN (SG) 1kN (SG) 13kN (SG) 8kN (SG) 1kN (SG) 12kN (SG) Distance ±45º from centre of laminate (mm) Distance ±45º from centre of laminate (mm) (c) (d) Figure 5 Strain of 45º Bidirectional FRP either side of laminate; (a) Spec 1.1, (b) Spec 2.1, (c) Spec 3.1, (d) Spec 4.1

6 4. The Proposed Finite Element Model The FE model was implemented in ATENA 3D (Cervenka 27) and utilised axis-symmetric boundary conditions through the centre line of the specimen (refer figure 4(c)) to reduce model size and solution time. The components of the model included the definition of material models for concrete, FRP laminates, FRP unidirectional and bi-directional fabric, steel reinforcement and an interface bond law between the fibres and the concrete. 4.1 Modeling of concrete The non-linear compressive behavior of concrete can be captured using numerical non-linear plasticity models. Inclusion of cracking response can be simulated using fracture-plastic material models currently available in many FE packages. Extensive research on the numerical modeling of concrete cracking has resulted in two main crack models being investigated: (a) discrete crack model and (b) smeared crack model. In general, the smeared crack approach has grown more popular and demonstrated greater advantages than the discrete crack method. However, the smeared crack strategy tends to spread crack formation over a band of elements and fails to predict localised fracture. As a result, the proposed material model utilised in this study was based on the smeared crack model and refined crack band theory. The rotating crack model was based on a non-linear plasticity fracture material model utilising fracture energy. Concrete cracking was considered as part of a three stage fracture process: Uncracked, potential crack in progress and crack opening after complete release of stress. The compressive failure was simulated using a biaxial stress failure criterion based on Kupfer et al. (1969). A reduction of compressive strength and shear stiffness after cracking was also considered. The input parameters for the required by the concrete material model were: Young's modulus (E s ), compressive strength (f c, ), tensile strength (f ct ), Poisson s ratio (v) and specific fracture energy (G F ). The chosen parameters are summarised in table 3. With the exception of fracture energy, all input parameters could be determined from the experimental measurements. Table 3. Concrete material model parameters used in numerical model Concrete properties Model Young's modulus, E s (MPa) 421 Mean compressive strength, f cm 69.2 Characteristic (MPa) tensile strength, f ct 4.76 Poisson (MPa) ratio, v.2 Specific fracture energy, G F (N/m) Shear factor coefficient 2 A fracture energy of (G F = N/m) was adopted in this study was based on (Trunk & Wittmann 1998) and using a maximum aggregate size of 1mm. In order to define the relationship between normal and shear crack stiffness, a shear factor coefficient of 2 was specified based on experimental work by (Walraven 1981). 4.2 Modeling FRP Patch Anchors FRP bidirectional fibre sheets consisted of loosely woven fibres orientated in the ±45 directions and embedded within a saturant matrix. The material was simplified in the FE model by defining a saturant base material, comprising three dimensional brick elements of equivalent.86mm sheet thickness for a single layer. An orthotropic linear elastic material model with Von Mises plasticity hardening was assigned to the saturant material. The fibres were defined using smeared reinforcement in perpendicular orientations (within the saturant elements) representing the embedded fibres. The orientation of the smeared reinforcement could be defined such that it was ±45º to the direction of the laminate. A fibre fraction of 19.7% in each direction was used to replicate the orthogonal fibres. This figure was obtained by dividing the total area of loose fibres by the area of saturant to arrive at the correct force per unit width. The FRP laminate was modelled as an orthotropic linear elastic material with properties described in the experimental aspect of the study. Uni-directional patch anchors were modelled in a similar fashion to their bi-directional counterparts. The uni-directional fibres were also

7 Bond Stress (MPa) defined using a.86mm thick homogeneous saturant base material which included embedded fibres as smeared reinforcement with a fibre fraction of 27.36%. Material properties for the fibre and saturant materials were based on manufacturer s specifications. 4.3 Modeling steel reinforcement The steel reinforcement was assumed to be elastic, perfectly plastic and was defined using a bi-linear stress-strain law. Steel bars were modelled individually as embedded reinforcements in the concrete elements, which implied a perfect bond exists between the steel bars and concrete due to full strain compatibility. This was a reasonable assumption, since the majority of the tensile and shear stresses during loading were concentrated at the FRP-to-concrete interface and the steel reinforcement was not expected to develop any significant stress levels. 4.4 Modeling FRP-to-Concrete Interface A commonly used approach is to simulate FRP debonding by the definition of interface elements between the FRP and the concrete. A constitutive bond-slip model or shear traction-separation law is typically assigned to the interface elements which can be calibrated using experimental data or by utilising available theoretical models based on linear or non-linear fracture. This bond-slip relationship consists of two stages: an initially elastic stage in which the interfacial stress increases with the slip until it reaches the strength of the interface, and a softening stage in which interfacial stress decreases with the slip resulting in debonding (Wang & Zhang 28). This method was utilized in this study as a result of the ease of calibration with experimental data. The present study uses interface elements as contact between two surfaces (concrete and FRP). The interface material was based on Mohr- Coulomb criterion with tension cut off. The constitutive relation for a general three-dimensional case is given in terms of tractions on interface planes, relative sliding and opening displacements. The initial failure surface corresponds to a Mohr-Coulomb condition, where after stresses violate the tension cutoff limit, the surface collapses to a residual surface which corresponds to dry friction. Tensile and shear softening of the interface was also considered using a multi-linear softening law calibrated with the experimental data, refer Fig. 6(a) and (b). Note that the properties for cohesion (c) used in the interface model were the average values of those obtained from all experimental specimens. The parameters utilised in the interface model are presented in table V (a) (b) Figure 6. Typical interface model behaviour in shear with cohesion softening law; (a) Numerical definition (Cervenka 27); (b) shear-slip curve for interface derived from experimental data (Kalfat, 21). Where: τ = interfacial shear stress, σ = normal stress, Φ = friction angel, K tt = tangential stiffness, G I F = mode 1 fracture energy Table 4. Interface material model parameters used in numerical model mm (GAUGE) 175mm (FEM) Value adopted Parameter Interface Tangential stiffness, K tt (MN/m 3 ) 6x1 5 Characteristic tensile strength, f t (MPa) (MPa) (MPa) 4.76 Cohesion, c (FRP-to-Concrete Interface) (MPa) 5.49 Cohesion, c (Patch Anchor-to-Laminate Interface) (MPa) 7.29

8 Load (kn) Load (kn) Load (kn) The experimental bond slip curve was used to determine the numerical parameters of, tangential stiffness (K tt ), cohesion (c) and friction coefficient (ϕ) for the shear-displacement function in figure 6(a). The multi-linear softening component was also derived from the same experimental data and used to define the mode I fracture energy (G F I ) negating the need of a friction coefficient as there is no longer a sudden collapse of cohesive strength to the dry friction value. Failure was replicated in the FE model by the definition of an interface bond law between the adhesive and concrete materials and by assigning a perfect bond between all other subsequent layers. 4.5 Bi-directional fabric specimen Results A review of the strains along the length of the laminates presented in figure 8 shows good correlations with FE predictions. (a) 75 Kn (b) 1 kn (c) 14 kn Figure 7. Patch Anchor (FEM Model) exaggerated deformations G2 (FEM) G2 (SG) G2 (V3D) G4 (FEM) G3 (FEM) G4 (SG) G3 (SG) 2 G4 (V3D) G3 (V3D) (a) (b) (c) Figure 8. Load vs strain distribution, Patch Anchor, Type 1; (a) Gauge G2; (b) Gauge G3; (c) Gauge G4; 4.6 Parametric studies It is well know that parametric studies based on a well calibrated numerical model can be used to expand the existing experimental data while maintaining a minimal number of tests. Parametric studies were conducted on anchorage type 5 from stage 1 of the experimental study to investigate the effects of concrete strength on the maximum strain reached prior to debond. Three alternative concrete strengths were chosen (32, 45, 62 MPa) and corresponding parameters within the concrete and interface material models were adjusted accordingly and presented in table 5. Concrete fracture energy was adjusted accordingly for each concrete strength, while maintaining the same model used by (Trunk & Wittmann 1998) with a reduction in aggregate size.

9 Concrete strength (f'c) Table 5. Summary of parametric study on concrete strength for anchorage type 5 f' c (MPa) E (MPa) f t (MPa) G F (N/m) C (MPa) Laminate dimensions (mm) 12x2 12x2 12x2 ɛ,max An examination of figure 9 reveals an approximately linear relationship between the concrete strength and the maximum laminate strain reached prior to debond. It is therefore reasonable to assume that the numerical data can be used to extrapolate anchorage strain efficiencies for the concrete strengths within range microstrain (μɛ) 5 Figure 9. Anchorage Type 5 parametric study Concrete strength vs max laminate strain prior to de-bond. 5. Conclusion Anchorage systems are a relatively new area of study with potential to improve the performance of FRP materials bonded to concrete. This research has focused on the use of patch anchors made of bidirectional fibers to distribute FRP-to-concrete bond stresses within the FRP anchorage zone over a greater area of concrete. As a result, anchorage strength is increased, while facilitating a higher level of fiber strain prior to debond. The results and discussions presented throughout the paper allow the following conclusions to be made: Patch anchorage lengths of 25mm exhibited slippage at a lower load. As a result it was recommended that 3mm be the ideal patch anchorage length. Specimens anchored with 3mm long anchorages (which failed due to slippage) exhibited increases in load of 53-81%. 3mm long anchorage joints which failed by anchor debond, exhibited similar increases in load of 56-67% for specimen type 1. However the effect of reduction in concrete block width (2mm) used in specimen type 4, resulted in pull-off failure at a lower load. By examining the strain distributions within the bi-directional fibres it is expected that laminates could be spaced as close a 25mm without any reduction of anchorage strength. The present paper has demonstrated the effectiveness of FE models in predicting the pre-peak and post-peak responses of FRP-to-concrete joints anchored using bi-directional fibres. 6. Acknowledgements The authors would like to acknowledge the staff and services offered at the Smart Structures Laboratory at Swinburne University.

10 7. References 1. ACI 44.2R-8 28, 'Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures', American Concrete Institute, Farmington Hills, Michigan. 2. Al-Mahaidi, R. and Hii, A. (27). An investigation on bond behaviour of CFRP reinforcement for torsional strengthening of solid and box-section RC beams, Composites Part B: Engineering, 38(5-6); Al-Mahaidi, R & Kalfat, R 211, 'Investigation into CFRP laminate anchorage systems utilising bi-directional fabric wrap', Composite Structures, vol. 93, no. 4, pp Cervenka, V, Cervenka, J 27, 'ATENA program documentation: Part 2-1, User s Manual for Atena 2D, Cervenka Consulting, Prague'. 5. Correlated-Solutions 21, 'Vic-3D Referance Manual', 6. Hollaway, LC & Teng, JG 28, 'Strengthening and rehabilitation of civil infrastructures using fibre-reinforced polymer (FRP) composites', Woodhead Publishing, Cambridge, UK. 7. Kalfat, R, Al-Mahaidi, R and Williams, G. (211). "Investigation of efficient Anchorage systems for shear and torsional retrofitting of box girder bridges." FRPRCS-1, International Symposium on Fiber Reinforced Polymer, Tampa, Florida, Kalfat, R, Al-Mahaidi, R and Smith, S.T. (212) Anchorage devices used to improve the performance of concrete structures retrofitted with CFRP composites: A-state-of-the-art review, Journal of Composites for Construction, ASCE, accepted for publication. 9. Kupfer, H, Hilsdorf, H.K and Rusch, H. (1969) - Behavior of Concrete under Biaxial Stress, Journal ACI, Proc. V.66,No.8, Aug., pp Pham, H and Al-Mahaidi, R. (25) Experimental and finite element analysis of RC beams retrofitted with CFRP fabrics ACI Special Publication SP-23, USA; Pham, H.B and Al-Mahaidi, R. (27) Modeling of CFRP-concrete shear-lap tests, Construction and Building Materials, 21; Trunk, B & Wittmann, FH 1998, 'Experimental investigation into the size dependence of fracture mechanics parameters. Third international conference of fracture mechanics of concrete structures, D-Freiburg: Aedificatio Publ.', pp Wang, J & Zhang, C 28, 'Nonlinear fracture mechanics of flexural-shear crack induced debonding of FRP strengthened concrete beams', International Journal of Solids and Structures, vol. 45, no. 1, pp Williams, G, Al-Mahaidi, R & Kalfat, R 211, 'The Westgate Bridge: Strengthening of a 2th century bridge for 21st century loading', 1th Int. Symp. on FRP Reinforcement for concrete structures, FRPRCS-1, Tampa, USA, 2-4 April