BOND PERFORMANCE OF STEEL REINFORCED POLYMER AND STEEL REINFORCED GROUT

Transcription

1 Proceedings of International Symposium on Bond Behaviour of FRP in Structures (BBFS 25) 25 International Institute for FRP in Construction BOND PERFORMANCE OF STEEL REINFORCED POLYMER AND STEEL REINFORCED GROUT M. Matana 1, A. Nanni 1, L. Dharani 2, P. Silva 1 and G. Tunis 3 1 Department of Civil, Architectural and Environmental Engineering, University of Missouri-Rolla, Rolla, MO 6549, USA, cies@umr.edu 2 Department of Mechanical and Aerospace Engineering, University of Missouri-Rolla, Rolla, MO 6549, USA 3 Hardwire LLC, 1 Quinn Avenue, Pocomoke City, MD 21851, USA ABSTRACT Steel reinforced polymer (SRP) and steel reinforced grout (SRG) are innovative technologies for structural retrofitting developed as complementary techniques of FRP. This paper aims to present the results of an experimental study to evaluate the bond between SRP/SRG and concrete substrate using direct shear test. The variables included type of reinforcement, concrete surface roughness and bonded length. SRP specimens experienced concrete shearing failure with considerable damage of the concrete, while SRG specimens experienced failure in the grout layer. The existence of the effective bond length after which the load can no longer increase was proven and calculated for SRP specimens. Due to the cracking of the cementitious matrix at low load levels, it was not possible to calculate an effective bond length for SRG specimens. KEYWORDS Bond strength, direct shear test, effective bond length, steel reinforced grout (SRG), steel reinforced polymer (SRP), surface roughness. INTRODUCTION Different aspects of FRP applications for strengthening as well as construction of new concrete members have been widely used and studied over the last couple of decades (Nanni, 24). Although FRP systems have been extensively used, their constituents high cost and flammability are limitations that would be desirable to overcome. With this intention, a new family of composite materials made of high strength twisted steel wires impregnated with polymeric resin or cementitious grout has been recently introduced: steel reinforced polymer (SRP) and steel reinforced grout (SRG) systems. The bond of externally applied reinforcement is of critical importance for the overall performance of the composite system, since it is the means of stress transfer between concrete and the composite in order to develop composite action (De Lorenzis et al. 21). Improper bond characteristics may cause failure of the system resulting in the reinforcement peeling from the concrete substrate. The direct shear test is widely used and an effective means for characterization of the bond between externally bonded reinforcement and the concrete (Miller et al. 1999, Chajes et al. 1996, De Lorenzis et al. 21). Previous research work including direct shear testing concluded that the bond strength is not influenced by the bonded length or by the width of the composite sheet (De Lorenzis et al. 21). The majority of researchers has reported that there is an effective bond length beyond which no further increase in ultimate load can be achieved. In addition, the number of reinforcement plies used to make the composite laminate affects the bond strength, but not proportionally to the number of plies. Chajes and co-workers (1996) and Jeffries (24) concluded that surface preparation does influence the bond strength. Surface preparation technique chosen in this research project was waterjetting. Although the water jet penetrating the concrete surface might create micro cracking of concrete surface due to water pressure, it was nevertheless proven that waterjetting enhances the quality of the bond between concrete and externally applied reinforcement (Yoshizawa et al. 1996, Silfwerbrand, 199). Characterization of the roughened surface was conducted using a laser profilometer that assures objective recording of the data (Maerz et al. 21).

2 This paper aims to present the bond characteristics between the externally applied steel reinforced polymer (SRP) and steel reinforced grout (SRG) and the concrete surface utilizing direct shear test. MATERIAL PROPERTIES OF SRP AND SRG Steel reinforced composites are made of steel cords (Hardwire 25) embedded in either a polymeric resin or a cementitious grout. The two types of steel cords recommended by the manufacturer for the application in SRP and SRG are 3X2 Hardwire and 3SX Hardwire, respectively (Wobbe et al. 24). For SRP, initial research results have shown that while tested under uniaxial tension, failure of the cord at a relatively short development length preempts its pull-out from the embedding polymer matrix (Huang et al. 24), proving high quality of the interfacial mechanical interlock between the cords and the matrix. Both cord types are high carbon steel cords with a Brass-AO (Adhesion Optimized) coating which enhances the corrosion resistance. The unidirectional cords are assembled into sheets and held in place by threads assuring easy handling. Cord sheets come in different densities: low, medium and high, corresponding to 4, 12 and 23 cords per inch (1.6, 4.7 and 9.1 cords per cm), respectively. For SRP, the chosen polymeric resin was Sikadur 33, a two-component, moisture-tolerant, high strength, high modulus structural epoxy adhesive (Sika, 25a). The heat deflection temperature of SRP as reported by the manufacturer was 118ºF. Among many advantages, long pot life as well as long open time, easiness to mix and apply and excellent adhesion to concrete, masonry, metals, wood and most structural materials are reported by the manufacturer. For SRG, the embedding grout was SikaTop 121 PLUS, a two-component polymer modified leveling and pore sealing cementitious grout. This grout is additivated with a penetrating corrosion inhibitor that reduces corrosion even in the adjacent concrete. The major advantage of grout is its non flammability. In addition, it is compatible with coefficient of thermal expansion of the concrete. It has increased freeze thaw durability and resistance to deicing salts (Sika, 25b). Although SRP and SRG are fairly new technologies, some laboratory as well as field characterization has recently been conducted and reported. Barton and co-workers (24) conducted a series of tensile strength tests whose test results have been used for the calculations of basic material properties of SRP and SRG composites presented in Table 1. Table 1. Material properties of SRP and SRG Material property / Lamina type SRP SRG Cord type 3X2 3SX Cord area (in 2 ) Sheet density (cords/in) Average sheet thickness for 1 ply (in) Tensile strength, f fu (ksi) Modulus of elasticity, e f (ksi) Strain at failure, ε fu (%) Note: 1 in = 25.4 mm, 1 ksi = 6.89 MPa Laboratory testing of beams strengthened with SRP and SRG resulted with significant increase in flexural capacity (Wobbe et al. 24, Huang et al. 24, Prota et al. 24). SRP was proven to be more efficient than SRG with respect to adding to the flexural capacity. Field applications included case studies (Casadei et al. 24, Lopez et al. 25) where SRP was used for retrofitting of a parking garage and a bridge superstructure. Field testing of these structures showed increased flexural and shear capacities as expected. ACI 44 provisions (ACI 44.2R 22) were used for predicting the ultimate capacity of SRP and they were proven to be effective and equally applicable. EXPERIMENTAL PROGRAM AND RESULTS Specimen Preparation and Test Set up Direct shear testing was conducted using 24 sets of two unreinforced blocks with dimensions 7.5-in 7.5-in 15.5-in (191-mm 191-mm 394-mm). The average value of the concrete compressive strength was 2115 psi (14.6 MPa) while the average tensile strength was 411 psi (2.8 MPa). Embedment material (resin or grout), bonded length and surface roughness were the parameters investigated. The concrete surfaces were prepared

3 using waterjetting at controlled water pressure, stand-off distance, nozzle diameter and jet pattern and the resulting surface roughness measurement was conducted using laser profilometer. Two blocks at a time were coupled, with a separating gap of 23-in (584-mm). On two opposite, lateral sides of each block, a 2-in (51-mm) wide strip of reinforcement was bonded with three different bonded lengths: 4-in (12-mm), 8-in (23-mm) and 12-in (35-mm), respectively. The reinforcement was left unbonded over the first inch (25-mm) from the edge of the block face to avoid edge effects. The reinforcement was limited to a single unidirectional steel cord ply embedded in either resin or grout according to the respective manufacturer s specifications. After the reinforcement application, the embedding material (resin or grout) was allowed to cure for at least 7 days before testing. A specimen ready to be tested can be seen on Figure.1. Prior to the reinforcement application, the two concrete blocks were placed over the web of a steel channel and anchored with bolts to its flanges to prevent any misalignment. In order to minimize friction over the channel web surface during testing, a layer of lubricant was spread between the two plastic sheets that were placed underneath the concrete blocks. After the initial curing, three of the four bonded reinforcement strips (unmonitored sides of the specimen) were mechanically anchored to the concrete to induce the failure on the fourth strip only (monitored side). After the complete test setup was prepared, a 2,-lb (89-kN) hydraulic jack was placed between the blocks and steel plates were used to spread the load to the block ends. A sheet of plywood was placed underneath each steel plate in order to distribute the load. A day prior to the testing, strain gages were applied to the reinforcement on the monitored side along the centerline every 2-in (51-mm), including one strain gage on the unbonded portion of the strip. Prior to the testing, the bolts used for alignment and anchoring of the blocks to the steel channel were loosened to allow the free movement of the blocks during the test. Testing included loading the blocks monotonically up to failure. The applied load was recorded through a single 2,-lb (89-kN) load cell. Had the bonded length be sufficient to fully anchor the reinforcement strip to cause its rupture, the maximum load required to break the reinforcement would have been 14.5-kip (64.5-kN). Hydraulic jack Load cell Anchored side Monitored side Alignment bolts Figure.1. Direct shear test setup Failure Mode and Bond Strength Two failure mechanisms were consistently observed, depending on the type of embedment material used: shearing within the concrete occurred with SRP reinforcement (Figure 2a), while SRG reinforcement failed by debonding in the layer of grout (Figure 2b). These failure modes occurred consistently for all the specimens irrespective of the bonded length and surface roughness. (a) SRP specimen - concrete shearing (b) SRG specimen grout sharing Figure 2. Direct shear tested specimens - failure mode

4 A summary of the test results is reported in Table 2 and Table 3, for SRP and SRG specimens, respectively. In each table, Column (2) represents the ultimate load and Column (3) the ultimate strength calculated using the following expression: where: σ ult = ultimate reinforcement strength P u = ultimate load A = cross sectional area of reinforcing cords P u σ ult =, (1) 2 A Column (4) reports the ultimate strain calculated from the experimentally measured load according to the Eq. (2) given the knowledge of the elastic modulus: ult Pu 1 ε ult = σ E = 2, (2) AE where: ε ult = ultimate strain in reinforcement strip (%) E = longitudinal modulus of elasticity of reinforcement Column (5) shows the corresponding strain as recorded by the strain gage placed in the unbonded portion of the reinforcement strip. Finally, Column (6) shows the ratio between the experimental strain and the load-derived strain. Table 2. SRP test results (1 23 cords/in density) Specimen Ultimate load (kip) (2) Ultimate strength (ksi) (3) Ultimate strain - calculated (in/in) (4) Ultimate strain - measured (in/in) (5) Ratio (5)/(4) (%) (6) (1) P N/A N/A P P N/A N/A P P N/A N/A P N/A N/A P P P N/A N/A P N/A N/A P N/A N/A P Note: N/A = Not available; 1 in = 25.4 mm, 1 kip = 4.45 kn, 1 ksi = 6.89 MPa Specimen Ultimate load (lb) (2) Table 3. SRG test results (1 12 cords/in density) Ultimate strength (ksi) (3) Ultimate strain - calculated (in/in) (4) Ultimate strain - measured (in/in) (5) Ratio (5)/(4) (%) (6) (1) G G G G N/A N/A G N/A N/A G G N/A N/A G G G G N/A N/A Note: N/A = Not available; 1 in = 25.4 mm, 1 kip = 4.45 kn, 1 ksi = 6.89 MPa

5 SG SG SG SG SG SG SG SG SG SG SG SG SG SG SG SG SG SG SG SG The data collected from the strain gages positioned along the reinforcement were used to develop strain-location curves at given load levels. Five load levels corresponding to 2, 4, 6, 8 and 1% of the ultimate load were selected as representative of the strain profile pattern as shown in the diagrams of Figures 3 and 4. In case of SRP, Table 2 shows that for the specimens in which the strain gage over the unbonded length of the reinforcement was mounted, the measured and calculated strains are reasonably close in terms of magnitude. Conversely, in case of SRG, Table 3 shows that consistently the measured strain is lower than that computed from the applied load. This behavior was the result of matrix cracking in the unbonded part of the SRG strip first and, subsequently, over the portion that debonded under load. When such cracking occurred, the strain measured by the gage was highly dependent on its location between the two surrounding cracks. As shown for example in Figure 3 for specimen G-12-2 (SRG, 12-in bonded length, medium surface roughness), the cracking occurred between.4 P u and.6 P u at strain level of approximately 5 µε. Once this strain level is reached, cracks form and the strain in the bonded part of the SRG strip can grow beyond the value recorded in the unbonded part. If the grout cracks and the strain gage is placed immediately after the crack, it will record strain in the grout that is not fully loaded. Strain in SRG (microstrain) Strain Profiles for SRP Specimens G EDGE OF THE BLOCK.2*Pu.4*Pu.6*Pu.8*Pu 1.*Pu Location from block end (in) Figure 3. Location vs. Strain - SRG Note: 1 in = 25.4 mm Figure 4 presents the typical strain vs. gage location curves for the 4-in (12-mm) and 12-in (35-mm) bonded lengths. The graphs show that at early stages of loading the gages far from the block end do not read any strain. For the case of specimen P-12-4 (Figure 4-b), at approximately a load level of.6 P u, the strain profiles attain a bi-linear shape. The first part of this profile shows a plateau indicating that the reinforcement has debonded over that length. The strain in the remaining bonded portion of the reinforcement decreases linearly from the plateau level to zero. As the load slightly increases, the plateau extends (that is more reinforcement debonds), until a non zero strain is attained at the end of the reinforcement. At that instance, the strip totally detaches. This progressive failure allows defining the effective bond length which corresponds to the portion of the reinforcement providing the resistance capacity irrespective of the physical length of material bonded to the concrete. Strain in SRP (microstrain) P EDGE OF THE BLOCK.2*Pu.4*Pu.6*Pu.8*Pu 1.*Pu Strain in SRP (microstrain) P EDGE OF THE BLOCK.2*Pu.4*Pu.6*Pu.8*Pu 1.*Pu Location from block end (in) Location from block end (in) (a) 4-in (12-mm) bonded length (b) 12-in (35-mm) bonded length Figure 4. Location vs. Strain SRP Note: 1 in = 25.4 mm

6 For the case of specimen P-4-2 (Figure 4-a), once the debonding began, the 4-in (12-mm) bonded length specimen failed suddenly, because it was shorter than the effective bond length (see the calculation in next section). For the longer bonded lengths, i.e. 8-in (23-mm) and 12-in (35-mm), the longer bonded length causes the failure to occur progressively. This can be seen in Figure 4-b from the strain distribution at the line on the graph representing failure load level, 1. P u. The debonding is the result of bond stresses that exceed the shear strength of the concrete surface. Calculation of Effective Bond Length for SRP Specimens The strain distribution shows a bi-linear behavior at the ultimate. The gradient of the descending part of the profile is approximately constant for different bond lengths and is calculated to have the average value of 111 µε/in (44 µε/mm) as seen in Table 4. The effective bond length is calculated using Eq. 3: where: L eff = effective bond length (in) ε max = maximum strain in SRP (µε) (Eq.4 ) L eff ε max = dε dx max 6 max = 1 ( A E), (3) P ε, (4) P max = ultimate load obtained (lb) A = cross sectional area of reinforcing cords (in 2 ) E = longitudinal modulus of elasticity (ksi) dε = strain gradient for linear part of the curve at ultimate stage (µε/in). dx Specimen Table 4. Calculation of effective bond length ε max (µε) dε dx (µε/in) L eff (in) P P P P Average: Note: 1 in = 25.4 mm Figure 5 shows the graphical representation of the effect of bonded length on the ultimate load for SRP and SRG specimens. In the case of SRP (Figure 5-a), the difference between the average values for 8-in (23-mm) and 12-in (35-mm) is practically insignificant. But, the difference between the average value for 4-in (12-mm) bonded length and the remaining two is nevertheless higher. This was explained by the fact that the bonded length of 4-in (12-mm) is less than the effective bond length previously calculated to be 5.1 in (13-mm). Therefore the corresponding ultimate load can not be obtained before the sudden peeling of the SRP strip. It is relevant to note that effective bond length for SRP is somewhat larger than in FRP specimens, where it is reported to be between 3 in (76-mm) and 4 in (12-mm) as reported by De Lorenzis and co-workers (21). In the case of SRG (Figure 5-b), the trend of increasing ultimate load with increasing bonded length is clear: the ultimate capacity of the 4-in (12 mm) specimen grows 26 and 56% as the bonded length goes to 8 in (23 mm) and 12 in (35 mm), respectively. For the case of SRG, it is concluded that the effective bond length is in excess of 12 in (35 mm).

7 16 7 Ultimate load (lb) Note: 1 in = 25.4 mm, 1 lb = 4.45 N Bonded length (in) Ultimate load (lb) Note: 1 in = 25.4 mm, 1 lb = 4.45 N Bonded length (in) (a) SRP specimens (b) SRG specimens Figure 5. Effect of bonded length on ultimate load Effect of Surface Roughness Figure 6 presents the effect of the surface roughness in terms of microaverage inclination angle i A on the ultimate load achieved. It is clear that the surface roughness does not have an impact on the ultimate load for both types of reinforcements. This was expected due to the type of failure mode that was concrete shearing and grout shearing for SRP and SRG specimens, respectively Ultimate load (lb) in bonded length 8 in bonded length 12 in bonded length 2 Note: 1 in = 25.4 mm, 1 lb = 4.45 N i A (deg) Ultimate load (lb) in bonded length 2 8 in bonded length 12 in bonded length 1 Note: 1 in = 25.4 mm, 1 lb = 4.45 N i A (deg) (a) SRP specimens (b) SRG specimens Figure 6. Effect of surface roughness on ultimate load CONCLUSIONS This paper presents the study to investigate the bond behavior of SRP and SRG composites utilizing direct shear test having the surface roughness and bonded length as research variables. Irrespective of bonded length and surface roughness, SRP specimens experienced concrete shearing failure, while SRG specimens failed in the layer of grout, leading to the conclusion that for these material systems a high level of concrete surface preparation other than cleaning may not be required. Effective bond length was calculated for SRP specimens and was found to be about 5 in (127 mm), which is somewhat larger than in FRP specimens, where it is reported to be about 4 in (12 mm). For SRG, the effective bond length is larger than 12 in (35 mm). ACKNOWLEDGMENTS The authors gratefully acknowledge National Science Foundation (NSF) for funding this research project, as well as Hardwire LLC, Pocomoke City, MD, for the donation of the steel material. This project was also partially supported by the NSF I/UCRC Repair of Buildings and Bridges with Composites (RB 2 C) based at the University of Missouri-Rolla.

8 REFERENCES ACI Committee 44, Guide for the design and construction of externally bonded FRP systems for strengthening concrete structures (ACI 44.2R), American Concrete Institute, Farmington Hills, Michigan, 22. Barton, B., Wobbe, E., Dharani, L. R., Silva, P., Birman, V., Nanni, A., Alkhrdaji, T., Thomas, J., and Tunis, G. Characterization of RC beams strengthened by steel reinforced polymer and grout (SRP & SRG) composites, To appear in Materials Science and Engineering A, 25. Casadei, P., Nanni, A., Alkhrdaji, T., and Thomas, J., (25). Behavior of double-t prestressed beams strengthened with steel reinforced polymer, Submitted to Advances in Structural Engineering an International Journal (ASE), In press, April 25. Chajes, M. J., Finch, W. W. Jr., Januszka, T. F., and Thomson, T. A. Jr. Bond and force transfer of composite material plates bonded to concrete, ACI Structural Journal, Vol. 93, No. 2, March-April 1996, pp De Lorenzis, L., Miller, B., and Nanni, A. Bond of FRP laminates to concrete, ACI Materials Journal, Vol. 98, No. 3, May-June 21, pp Hardwire, 25, What is Hardwire, Pocomoke City, Maryland. Huang, X., Birman, V., Nanni, A., and Tunis, G. Properties and potential for application of steel reinforced polymer and steel reinforced grout composites. Composites Part B: Engineering. Vol. 36, No. 1, 24 ; pp Jeffries, J. M., 24, Bond behavior of fiber reinforced polymer laminates to concrete subjected to varied surface preparation, Masters of Science Thesis, University of Missouri-Rolla, Rolla, Missouri. Lopez, A., Galati, N., Nanni, A., and Alkhrdaji, T., 24, Strengthening of a reinforced concrete bridge with externally bonded steel reinforced polymer (SRP), Submitted to Composites Part B: Engineering. Miller, B., 1999, Bond between carbon fiber reinforced polymer sheets and concrete, Masters of Science Thesis, University of Missouri-Rolla, Rolla, Missouri. Nanni, A., 24, Infrastructure strengthening with composites (CE 374). Course held in Spring semester 24 at the University of Missouri-Rolla. Class notes. Prota, A., Tan, K. Y., Nanni, A., Pecce, M., and Manfredi, G. Performance of RC shallow beams externally bonded with steel reinforced polymer. Submitted to ACI Structural Journal, 24. Sika, 25a, Lyndhurst, NJ., Sikadur 33, Product Data Sheet, Edition 7.23, Identification no Sika, 25b, Lyndhurst, NJ., Sika Top 121 Plus, Product Data Sheet, Edition 8.23, Identification no Silfwerbrand, J., 199, Improving concrete bond in repaired bridge decks. Concrete International, Vol. 12, No. 9, pp Wobbe, E., Silva, P., Barton, B. L., Dharani, L. R., Birman, V., Nanni, A., Alkhrdaji, T., Thomas, J., and Tunis, G. Flexural capacity of RC beams externally bonded with SRP and SRG, Proceedings of Society for the Advancement of Materials and Process Engineering, Symposium, Long beach, CA., 21. Yoshizawa, H., Myojo, T., Okoshi, M., Mizukoshi, M., and Kliger, H. S. (1996). Effect of sheet bonding condition on concrete members having externally bonded carbon fiber sheet, Fourth Materials Engineering Conference, ASCE Annual Convention, Washington D.C.