Pullout Test of Concrete Blocks Strengthened with Near Surface Mounted FRP Bars

Transcription

1 NSERC Industrial Research Chair in Innovative FRP Composites for Infrastructures Pullout Test of Concrete Blocks Strengthened with Near Surface Mounted FRP Bars Progress Report No 1 Prepared by: Shehab M. Soliman, Ehab El-Salakawy, and Brahim Benmokrane Department of Civil Engineering Faculty of Engineering Université de Sherbrooke, Sherbrooke, Québec, Canada J1K 2R1 Tel: (819) Fax: (819) Brahim.Benmokrane@USherbrooke.ca October 2006

2 Table of Contents 1. Introduction Experimental program Material properties Concrete FRP bars Adhesive Concrete blocks test specimens Cutting the grove Installation of the CFRP bar Test setup Instrumentation Test results Failure mode Bond stress-slip relation Conclusion References i -

3 List of Figures Figure 1: Two components adhesive package Figure 2: Test specimen Figure 3: Cutting the grove Figure 4: Installation of FRP bar Figure 5: Test setup Figure 6: Pullout load versus bonded length... 8 Figure 7: Failure modes Figure 8: Average bond stress-slip for E Figure 9: Bar of length dx Figure 10: Strain distribution along the bar bonded length Figure 11: Bond stress distribution along the bar bonded length ii -

4 List of Tables Table 1: Mechanical properties of the FRP bars Table 2: Material specifications (Hilti 2005). 3 Table 3: Description of test specimens for the pullout test and Test results iii -

5 1. Introduction As we move into the twenty-first century, the renewal of our lifelines or deterioration of concrete infrastructure becomes a topic of critical importance. It may have to carry larger loads or require change in building use. Or it may also suffer steel corrosion. Or may have errors made during the design or construction phases. The structure may need to be repaired or strengthened before it can be used. The most common way to strengthen structures is to place Fibre- Reinforced Polymer (FRP) sheets or laminates on the surface of the structure. However, a disadvantage of such a technique is that the surface bonded materials are subjected to fire, accidents or vandalism. But if the FRP composite material is placed in slots in the concrete cover some of these disadvantages can be overcome. This method can be designated as Near Surface Mounted Reinforcement (NSMR). The main objectives of this research program are: 1) to utilize new near surface mounted system by using Carbon FRP V-ROD bars manufactured by Pultrall Inc. (Thetford Mines, Quebec, Canada)) and adhesives manufactured by Hilti Inc.(Montreal, Quebec, Canada), and 2) to investigate the effect of different parameters on the bond performance of this system. 2. Experimental Program The experimental program consisted of two phases. The first phase included the pullout testing of 36 C-shape concrete blocks. The second phase included testing of 24 flexural strengthened concrete beams using NSM system. The first phase included the following parameters: 1. Grove size: two different grove sizes were used; namely 1.50 and 2.0 times the diameter of the FRP bar. 2. Bonded length: three different bonded lengths were used; namely 12, 18 and 24 times the diameter of the FRP bar. 3. Type of adhesive: two types of adhesive were used epoxy and cement-based. The main objectives of this phase were: 1) to investigate the effect of the different parameters affecting the bond performance of the NSM strengthening system, 2) to develop a bond slip model for the CFRP V-ROD bars, which can be used in a finite element model. The second phase will include testing of 24 full scale beams which represent the field application of this strengthening system

6 This phase will include following parameters: 1. Type of FRP reinforcement: CFRP and GFRP 2. Diameter of FRP bar: two main diameters will be used for each type of reinforcement, No.10 (9.5mm) and No.13 (12.7 mm). 3. Grove size: two different grove sizes will be used; namely 1.50 and 2.0 times the diameter of the FRP bar. 4. Bonded length: four different bonded lengths will be used 12, 18, 24 and 48 times the diameter of the FRP bar. 5. Type of adhesive: two types of adhesive will be used; namely epoxy and cement-based adhesives. The main objectives of this phase are: 1) to investigate the different parameters affecting the behaviour of reinforced concrete beam strengthened in flexure with NSM FRP bars, and 2) to establish design recommendations for the use of CFRP reinforcement in the NSM method. This progress report describes the results obtained in the first phase of this research project (pullout testing). 3. Material properties 3.1. Concrete All test specimens were constructed using a ready mix concrete in order to minimize the variations of all constituents, the targeted concrete compressive strength after 28 days was 35 MPa. The actual concrete compressive and tensile strength were determined based on the average values of three cylinder specimens (150x300mm). The cylinders were cast from the same patch used for casting the blocks. These cylinders were cured under the same environmental conditions as those of the blocks. The cylinders and the blocks were tested the same day of testing of the blocks. The average concrete compressive and tensile strength were 42 and 4.3 MPa respectively CFRP bars The Carbon FRP V-ROD bars used in this study were manufactured by Pultrall Inc. (2005). The CFRP bars were tested for tensile strength and the modulus of elasticity according to the ACI 440.1R-03. Table 1 provides the tensile properties of the No. 10 (9.5 mm diameter) and No. 13 (12.7 mm diameter) CFRP V-ROD bars

7 Table 1: Mechanical properties of the CFRP V-ROD bars Bar type CFRP Bar diameter mm Bar area mm Modulus of elasticity, GPa Ultimate tensile strength, MPa Ultimate strain % Adhesive Two adhesive types produced by Hilti Inc. were used in this investigation. The first type, HIT RE 500 is a high strength two part epoxy based adhesive, resin and hardener as shown in Figure 1. This type is specially designed for fastening into solid base materials in a wide range of material temperatures ranging from 49 o C down to -5 o C. It may be also used in underwater fastening for oversized holes up to twice the bar diameter but with a maximum hole diameter of 76 mm. The HIT RE 500 can be used on wet or dry surfaces and characterized by having excellent weathering resistance and high temperature resistance. The second type of adhesive, HIT HY 150, is a hybrid adhesive consisting of methaccrylate resin, hardener, cement and water. It is formulated for fast curing and installation in a wide range of material temperatures ranging from 40 o C down to -5 o C (Hilti Inc. 2005). The specifications of these adhesives are listed in Table 1. Figure 1: Two components adhesive package Adhesive Table 2: Material specifications of adhesives (Hilti 2005) Compressive Strength (MPa) Tensile Strength (MPa) Modulus of Elasticity (MPa) Bond Strength (MPa) Absorption (%) Resistance (Ω/m) HIT RE HIT HY Not Specified

8 4. Concrete blocks test specimens The specimen configuration used in this investigation is similar to the ones developed by De Lorenzis et al, 2002 as shown in Figure 2. A typical specimen consisted of a C shaped concrete block with external dimensions of 340 x 340 mm and inner dimensions of 170 x 170 mm and a height of 500 mm. The NSM bar grove was made after curing in the middle. 500 Elevation Plan Figure 2: Test specimen 5. Cutting the grove A special concrete saw with a diamond blade was used to cut the grove in the middle of the C shape with fixed depth 20 mm and variable width. The grove was made by cutting two lines using the concrete saw and removing the concrete in between by a power hammer as shown in Figure

9 Figure 3: Cutting the grove 6. Installation of the CFRP bar A pressurized air was used to ensure that the grove is clean. The epoxy was injected into the grove to cover 2/3 of the grove depth. The bars were placed in the grove over a foam support outside the bonded length to maintain the thickness of the epoxy and gently pressed to displace the bonding agent. Extra epoxy was added to fill the grove. The excess epoxy was then removed as shown in Figure

10 a) The epoxy injected into the grove to b) The bars were placed in the grove over cover 2/3 of the grove depth foam support a) Extra epoxy was added to fill the grove. b) The excess epoxy was then removed Figure 4: Installation of FRP bar 7. Test setup The concrete blocks were tested in MTS BALDWIN machine as shown in Figure 5. The applied load was reacted by means of four steel threaded bars inserted into holes made in the block passing through two steel plates over and under the block and the bar was pulled up as shown in Figure Instrumentation Two LVDTs were used to measure the free end and the loaded end slip of the bar. Electrical resistance strain gauges with gauge length of 6 mm were attached to the bar to capture the strain distribution along the bonded length. Reading from the load cell, LVDTs and the electric strain gauges were collected using an automatic data acquisitioning system connected to a computer

11 Figure 5: Test setup 9. Test results Only 18 specimens out of the 36 specimens were tested. These 18 specimens utilized epoxy adhesive. Test results are listed in Table 3 with respect to pullout load and average bond stress and mode of failure. Every specimen was given a code in the format X The first character represented the type of adhesive. E for epoxy and C for cement, the first number represented the grove depth in a multiple of the bar diameter and the second number represents the bonded length also in a multiple of the bar diameter. 9.1 Failure mode The main failure for most of the specimens was concrete shear tension failure (semi-cone failure) accompanied by or without cracking in epoxy. This concrete tension failure was due to tensile stresses along the inclined planes in the concrete surrounding the grove. Internal cracks were observed in the epoxy in many specimens after failure. These cracks indicated the path of compressive force from the bar and spread into the epoxy. Specimens E failed by bar rapture. As shown from Figure 6, as the bonded length was increased the force required to pullout the bar was increased, and the bond stress was decreased. Increasing the grove size from 1.5d to - 7 -

12 2.0d did not have a great influence on the pullout load. This was due to the fact that failure mostly controlled by the tensile strength of concrete not the splitting of the epoxy cover, even when it happened in some specimens it happened after the formation of inclined cracks in concrete. Figure 7 shows some of the failure that happened in the specimens. Table 3: Description of Test Specimens for the Pullout Test and Test Results Specimen code Grove filling Grove dimension L b Pullout load, kn Average bond stress, MPa Failure E d C E Epoxy 1.50d 18d C+S E d R E d C+S E Epoxy 2.00d 18d C+S E d C C= Concrete tension failure, R= Rupture of bar, S= Splitting of epoxy pullout load, kn Grove=1.5d Grove=2.0d bonded length, Ld Figure 6: Pullout load versus bonded length - 8 -

13 a) Concrete tension failure b) concrete tension failure with epoxy cracking c) Splitting of epoxy cover d) Rupture of FRP bar Figure 7: Failure mode of some specimens 9.2 Bond stress slip relation Figure 8 shows the experimental average bond stress versus slip diagrams for the tested specimens. The data obtained from readings of the strain gauges can be used to obtain the local bond slip relation. The importance of developing the local bond-slip constitutive law is that it completely characterizes the bond behaviour of the system. It can be used, for example, to evaluate the development length of a given type of FRP bar for each different groove size

14 14 Average bond stress, MPa Free end slip Loaded end slip Slip, mm Figure 8: Average bond stress-slip for E Equilibrium of a piece of bar of length dx is shown in Figure 9. It is assumed that behaviour is linearly elastic. This leads to the following: Figure 9: Bar of length dx d d ε f (x) τ =.E f. 4 dx τ xi+ x = j f ( ) 2 d ε fj ε fi.e. 4 x x fj fi Where x = coordinate along the longitudinal axis of the FRP bar within the bonded length, Therefore, the bond stress profile diagram at a given load level can be obtained from the first derivative of the strain profile at that load level multiplied by the elastic modulus, E f and the diameter of the FRP bar, d

15 And from the definition of slip: S l =u f - u e - u c Where u f, u e and u c is the displacement of FRP bar, epoxy and concrete respectively, since du f due duc ε f =, ε e = and ε c = dx dx dx and by assuming that the epoxy strain, ε e and concrete, ε c may be considered negligible when compared to that of the FRP, ε, which can be calculated as follows: f dsl ε f = dx S(x) S(0) ε (x)dx l l f 0 x = + where S l (0) is the free-end slip of the FRP bar. Therefore, the slip profile for each given load level can be obtained by integrating the strain versus location curve and adding the free-end slip at that load level. Figure 10 shows the strain distribution along the bar bonded length for specimen E Each curve corresponded to a specific load represented as a percent of the ultimate load. The X-axis started from the free end to the loaded end along the bonded length of the bar. The strain distribution along the bond length, highly nonlinear at lower load levels, gradually approached an almost linear shape as the load increased. This means that, as the load increased, redistributions of the bond stress along the bond length occurred as a result of the changes in the state of the bond. Micro cracking at the bar epoxy interface and the consequent slip of the FRP tend to produce a more even distribution of the bond stress. Figure 11 shows the bond stress distribution along the bar bonded length for specimen E corresponding to a specific load represented as a percent of the ultimate load. At low load levels the bond stress at the bar free end was close to zero, as the load increases, the peak of the bond stress gradually shifts towards the free end. The presence of more than one peak in the bond stress distributions during the last loading stages is probably related to the presence of transverse concrete cracks which introduce local disturbances to the bond behavior

16 Strain,ue %Pu 30%Pu 50%Pu 70%Pu 90%Pu 20%Pu 40%Pu 60%Pu 80%Pu 100%Pu 0 0 Free end Distance along the bar, mm Loaded 180 end Figure 10: Strain distribution along the bar bonded length for specimen E % Pu 20%Pu 30%Pu 40%Pu 50%Pu 60%Pu 70%Pu 80%Pu 90%Pu Pu Bond stress, MPa Free end Distance along the bar, mm Loaded end Figure 11: Bond stress distribution along the bar bonded length for specimen E Conclusion The following conclusions could be drawn from the experimental investigation on bond behaviour of NSM FRP bars in concrete: The adopted system, CFRP V-ROD bars and Hilti adhesives appeared to perform well in the NSM strengthening technique. The adopted test methods appeared to be efficient and gave consistent results

17 The main failure for most of the specimens was concrete shear tension failure (semi-cone failure) accompanied by or without epoxy cracking (splitting). One specimen with the longer bonded length failed by bar rapture. Increasing the bonded length increased the pullout load and the bond stress was decreased due to larger length distribution. Increasing the grove size did not have a great influence on the pullout load as the factor that controlled failure was the tensile strength of concrete. The next progress report No 2 will include the results of the remaining 18 concrete blocks and the 28 beam tests

18 Acknowledgements The authors would like to express their special thanks and gratitude for the helpful of: Natural Science and Engineering Research Council of Canada (NSERC) ISIS Canada Pultrall Inc. (Thetford Mines, Québec) Hilti Inc. (Montréal, Québec) Technical Staff at the structural lab in Department of Civil engineering of University of Sherbrooke

19 11. References 1] ACI 440.2R, 2002, Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures. ACI Committee 440, American Concrete Institute, Farmington Hills Michigan, 45p. 2 Asplund, S., 1949, Strengthening Bridge Slabs with Grouted Reinforcement. ACI Structural Journal, American Concrete Institute, Vol.20, No.6, pp [3] Carolin, A., 2003, Carbon Fiber Reinforced Polymers for Strengthening of Structural Element. Doctoral Thesis, Luleå University of Technology, Sweden 247p. 4] Cruz, S., Barros, O., and Antonio, J., 2004a, Modeling of Bond Between Near-Surface Mounted CFRP Laminate Strips and Concrete. Computers and Structures, Vol.82, No.17, pp [5] Cruz, S., Manuel, J., Barros, O., and Antonio J., 2004b, Bond Between Near-Surface Mounted Carbon-Fiber-Reinforced Polymer Laminate Strips and Concrete. ASCE, Journal of Composites for Construction, Vol. 8, No. 6, pp [6] El-Hacha, R. and Rizkalla, S., 2004, Near Surface Mounted Fiber Reinforced Polymer Reinforcements for Flexural Strengthening of Concrete Structures. ACI Structural Journal, American Concrete Institute, Vol.101, No.5, pp ] Hassan, T. and Rizkalla, S., 2003, Investigation of Bond in Concrete Structures Strengthened With Near Surface Mounted Carbon Fiber Reinforced Polymer Strips. Journal of Composites for Construction, Vol.7, No. 3, pp ] Hassan, T. and Rizkalla, S., 2004, Bond Mechanism of Near-Surface-Mounted Fiber- Reinforced Polymer Bars for Flexural Strengthening of Concrete Structures. ACI Structural Journal, American Concrete Institute, Vol. 101, No. 6, pp Hilti Inc. (2005). Product Technical Guide, [10] ISIS CANADA, 2001, Strengthening Reinforced Concrete Structures with External Bonded Fibre Reinforced Polymers. Design Manual No.4, Winnipeg, Manitoba, Canada, 96p

20 [11] Lorenzis, L., 2004, Anchorage Length of Near-Surface Mounted Fiber-Reinforced Polymer Rods for Concrete Strengthening, Analytical Modeling, ACI Structural Journal, American Concrete Institute, Vol.101, No.3, pp Lorenzis, L. and Nanni, A., 2001a, Shear Strengthening of Reinforced Concrete Beams with Near-Surface Mounted Fiber-Reinforced Polymer Rods. ACI Structural Journal, American Concrete Institute, Vol. 98, No.1, pp [12] Lorenzis, L. and Nanni, A., 2001b, TECHNICAL PAPERS - Characterization of FRP Rods as Near-Surface Mounted Reinforcement. Journal of Composites for Construction, Vol. 5, No. 2, pp ] Lorenzis, L. and Nanni, A., 2001c, Characterization of FRP Rods as Near-Surface Mounted Reinforcement. Journal of Composites for Construction, Vol.5, No.2, pp [14] Lorenzis, L. and Nanni, A., 2002, Bond between Near-Surface Mounted Fiber- Reinforced Polymer Rods and Concrete in Structural Strengthening. ACI Structural Journal, American Concrete Institute, Vol. 99, No. 2, pp Lorenzis, L., Rizzo, A., La Tegola, A., 2002, A modified pull-out test for bond of nearsurface mounted FRP rods in concrete. Composites Engineering Part B., Vol.33, No. 8, pp [] Lorenzis, L., Lundgren, K., Rizzo, A., 2004b, Anchorage Length of Near-Surface Mounted Fiber-Reinforced Polymer Bars for Concrete Strengthening, Experimental Investigation and Numerical Modeling. ACI Structural Journal, American Concrete Institute, Vol.101, No. 2, 269p. Pultrall Inc. (2005). Product Technical Specifications, 15] Stone, D., Tumialan, G., Nanni, A., Parretti, R., 2002, Reinforcement and Accessories, Near Surface Mounted FRP Reinforcement: Application of an Emerging Technology. Concrete Crowthorne, Vol. 36, No. 5, pp

21 16] Taljsten, B., Carolin, A., Nordin, H., 2003, Concrete Structures Strengthened with Near Surface Mounted Reinforcement of CFRP. Advances in Structural Engineering, Vol.6, No.3, pp Yost, J., R., Gross, S., P., 2004 Near Surface Mounted CFRP Renforcement for the Structural Retrofit of Concrete Flexural Members. Advanced Composite Materials in Bridges and Structures, Calgary, Alberta, July 20-23, 2004, 8 p