FRP Anchorage Systems for Infill Masonry Structures

Transcription

1 FRP Anchorage Systems for Infill Masonry Structures Dillon S. Lunn Graduate Research Assistant, Department of Civil, Construction, and Environmental Engineering, North Carolina State University, Raleigh, North Carolina, USA Sami H. Rizkalla Distinguished Professor, Department of Civil, Construction, and Environmental Engineering, North Carolina State University, Raleigh, North Carolina, USA Shohei Maeda Formerly Graduate Research Assistant, Laboratory of Engineering for Maintenance System, Hokkaido University, Sapporo, Japan Tamon Ueda Professor, Laboratory of Engineering for Maintenance System, Hokkaido University, Sapporo, Japan ABSTRACT This paper summarizes the research program undertaken to examine the behavior of several innovative anchorage systems for Fiber-Reinforced Polymer (FRP) strengthening systems for infill masonry wall structures. The research includes an experimental program comprising twelve infill wall specimens. All specimens consisted of two reinforced concrete (RC) beams, simulating the supporting RC elements of a typical building, which were in-filled with solid brick masonry. The specimens were loaded out-of-plane using an airbag to apply uniformly distributed pressure to the masonry. Glass (), Carbon (CFRP), and a new type of FRP with high fracturing strain, (Polyethylene Terephthalate), were used to evaluate the performance of the various anchorage systems. The types of anchorage investigated in the study include overlap, steel plate, wrapping around an embedded CFRP bar, CFRP fiber anchors, CFRP shear keys, and a continuous near surface mounted (NSM) CFRP system. Experimental results indicate that the type of FRP anchorage had a significant effect on the failure mode, ductility, and load carrying capacity. Strengthening of the infill walls lead to a substantial increase in the load carrying capacity ranging from 1.6 to 7.2 times the capacity of the unstrengthened infill wall. KEYWORD Infill Masonry, FRP Anchorage, Out-of-Plane,,, CFRP

2 1. INTRODUCTION Many existing unreinforced masonry (URM) structures, including infill masonry walls, are vulnerable to extreme out-of-plane loading. Out-of-plane collapse of these structures is often catastrophic and can lead to severe property damage and loss of life. Many existing masonry structures need retrofitting to reduce the risk of collapse under extreme loading such as the differential pressure caused by a tornado. Conventional strengthening techniques are often time-consuming, costly, and add significant weight to the structure [1]. These limitations have driven the development of alternatives such as fiber-reinforced polymer (FRP) strengthening systems, which are lightweight, can be rapidly applied, and do not require prolonged evacuation of the structure. Previous research has demonstrated the effectiveness of FRP in strengthening masonry structures [2]. For infill walls, it has also been shown that the manner in which the FRP strengthening system is anchored to the supporting structure is of vital importance in determining the mode of failure, the load carrying capacity and the ductility of the strengthened wall [3]. Various forms of FRP anchorage have been used successfully in the past for masonry structures, including fiber anchors and embedded bars [4], but to date, their effect on the behavior of infill masonry walls subjected to out-of-plane loading has not been fully developed. This paper explores the behavior of several innovative anchorage systems and FRP types for FRP strengthening of infill masonry wall structures. 2. EXPERIMENTAL PROGRAM 2.1Test Specimens and Material Properties All test specimens consisted of two reinforced concrete (RC) caps, simulating the supporting RC elements of a typical building, which were in-filled with solid brick masonry. Dimensions of the test specimens are given in Fig. 1. One specimen was not strengthened and was used as a control specimen. The other eleven specimens were strengthened with one of the three different types of FRP shown in Fig. 2. Five specimens were strengthened with externally bonded Glass Fiber Reinforced Polymer () sheets, four specimens were strengthened with externally bonded Polyethylene Terephthalate () sheets, and two specimens were strengthened with near surface mounted (NSM) carbon strand sheets (CFRP) placed in grooves 13mm wide by 19mm deep or 19mm wide by 19mm deep, depending on the adhesive. The materials used in the construction and strengthening of the wall specimens were tested to determine the engineering material properties as given in Table 1. It should be noted that material properties for the and are based on the FRP laminate tested by the authors while those for the CFRP are based on manufacturer values for the fibers only. As a result, the elastic moduli for and reported in this paper are different from those reported by their respective manufacturers. This is likely the result of differences in the amount of adhesive used wherein the larger the volume fraction of adhesive, the smaller the expected elastic modulus. For this study, the average measured thickness of the laminate was 2.46 mm and 1.69 mm compared to the mm and 1.3 mm reported by the manufacturers of the and respectively. It should also be noted that while the showed linear behavior, the laminate exhibited a bilinear response. The six different FRP anchorage systems used are shown in Fig. 3 to Fig. 8. Two specimens (one strengthened with and one strengthened with ) were anchored to the RC caps at the top and bottom of the infill with 305 mm of overlap of the FRP onto the RC caps as shown in Fig. 3. A shear restraint anchorage system consisting of 25mm thick by 203mm wide steel plates placed at the top and bottom of the wall and tied to the steel testing frame using steel bolts was used for two specimens strengthened with and respectively as shown in Fig. 4. Two specimens strengthened with and respectively were anchored using 13mm diameter CFRP fiber anchors embedded 51mm into the RC caps as shown in Fig. 5. The anchorage for one strengthened specimen consisted of wrapping the FRP sheet around a 10mm CFRP bar embedded near the surface of the RC cap as shown in Fig. 6. Two specimens strengthened with and respectively were anchored using pultruded CFRP strips (2mm thick by 13mm deep) embedded near the surface of the masonry and RC caps with 152mm of overlap above and below the masonry/rc interface as shear keys to resist shear sliding as shown in Fig. 7. The two specimens strengthened with NSM CFRP were anchored to the RC cap by extending the NSM CFRP 305mm into the RC cap as shown in Fig. 8.

3 (a) Elevation Fig.1 Test Specimens (b) Profile (a) (b) (c) CFRP Fig.2 FRP Types Table 1: Summary of Tested Material Properties Material Prop.* ASTM Standard [5] Average Value** Concrete f c C MPa Concrete f c C39 (34.4 MPa) Masonry f m C MPa Masonry f m C1314 (22.5 MPa) Brick f b C MPa Brick f b C140 (36.1 MPa) Mortar f j C MPa Mortar f j C109 (10.6 MPa) E f D ,775 MPa ε fu D % f f D MPa E f D3039 6,996 MPa ε ft D % E f D3039 2,687 MPa ε fu D % f f D MPa CFRP E f D ,000 MPa*** CFRP f f D3039 3,400 MPa*** *Properties: f c - concrete uniaxial compressive strength, f m - masonry net compressive strength, f b - brick unit uniaxial compressive strength, f j - mortar uniaxial compressive strength, E f - FRP tensile chord modulus of elasticity, ε fu - FRP elongation at break (rupture strain), f f - FRP ultimate tensile strength, ε ft - FRP transition strain, E f - FRP second tensile modulus of elasticity ** For concrete, Cast #2 properties are in parenthesis. For masonry, brick, and mortar Round #2 properties are in parenthesis. See Table 2 for correlation of the specimens with the construction phases. ***Values for fiber only as reported by CFRP manufacturer Fig. 3: Anchorage (b) Profile View (b) Profile View Fig. 4: Shear Restraint Anchorage (b) Profile View Fig. 5: Fiber Anchor Anchorage (b) Profile View Fig. 6: Embedded Bar Anchorage (b) Cross-section View Fig. 7: Shear Key Anchorage (b) Cross-section View Fig. 8: NSM CFRP Anchorage

4 2.2 Test Setup and Procedure The test specimens were loaded out-of-plane with a uniformly distributed pressure to simulate the differential pressure induced by a tornado. An airbag was used to apply static pressure in increasing cycles up to failure. The airbag was placed within a steel frame between the brick wall and the laboratory reaction wall. The test setup is shown in Fig. 9. The laboratory reaction wall is a strong wall fixed to the laboratory strong floor, both of which are extremely rigid compared to the test specimens. The concrete caps were secured to the reaction wall using high strength steel bars (reaction rods) spaced 914mm on center. In addition to the four reaction rods, the RC cap was stiffened using four steel bolts centered on the specimen and located at a 457mm spacing as shown in Fig. 9. These bolts were secured only to the steel frame however, and thus were not part of the external out-of-plane reaction as shown in Fig. 10. This system was used to simulate the out-of-plane rigidity of typical RC buildings. In addition to the out-of-plane rigidity, vertical tie rods were used to simulate the vertical rigidity of RC structures. due to the size and shape of the airbag, at the very large deflections observed for the shear restrained specimen, (more than 120mm) the pressure applied to the masonry did not cover the entire surface. The test was continued until the safe operating pressure of the airbag was reached. Thus, the dashed line at displacements greater than 120mm estimates the load deflection response up to the maximum measured displacement based on the assumption of a constant slope. (a) Test Frame (b) Front View (c) Profile View Fig. 9: Test Setup 3. EXPERIMENTAL RESULTS 3.1 Load Deflection Response The load deflection envelopes for various specimens are shown in Fig. 11. The influence of strengthening is illustrated by comparing the strengthened specimens to the unstrengthened control wall. Similarly, the effect of enhanced FRP anchorage is shown by comparing the response of the specimens with various forms of anchorage to those of the corresponding specimens in which anchorage is provided by means of an overlap of the FRP onto the RC caps, which serves a baseline for strengthened specimens. Fig. 11(a) compares the response of specimens strengthened with either or using either shear restraint or overlap anchorage to the response of the unstrengthened (control) wall. It can be seen that the initial stiffness of the strengthened walls was nearly independent of the type of anchorage. However, after initiation of the shear sliding, the stiffness of the overlapped specimens decreased considerably with respect to the corresponding shear restrained specimens. The second slope of the load deflection curve for the shear restrained specimens depended primarily on the type of FRP. It should also be noted that Fig. 10: Out-of-plane Reaction Rods and Steel Bolts Fig. 11(b) compares the response of the fiber anchor specimens with those of the overlapped and the control. The fiber anchor specimen had a smaller second stiffness than the corresponding specimen with overlap. This is consistent with the observation of the initiation of shear sliding and debonding of the sheet at a lower load level than the corresponding overlapped specimen. This is likely due to variations in construction and material properties which affect the quality of the bed joint and/or the presence of a shrinkage crack at the top interface between the masonry infill and the RC cap. Increase of the applied pressure caused debonding up to the level of the fiber anchors, at which point the response stiffened slightly as the anchors were activated. The specimen with fiber anchors had a second stiffness quite close to that of the corresponding overlapped specimen. At an applied pressure of 37 kpa, the

5 Applied Pressure (kpa) Applied Pressure (kpa) Applied Pressure (kpa) Applied Pressure (kpa) fiber anchor specimen experienced a large slip in the range of 20mm. Before failure, the debonded up to the level of the anchors, after which the stiffness increased and the specimen was able to carry additional load. Fig. 11(c) compares the specimens using shear key anchorage to those with overlap and the control. The initial stiffness is similar for the strengthened specimens, but shear sliding of the overlapped specimens caused a decrease in stiffness relative to the specimens with the shear key anchorage. A reduction in stiffness occurred as the shear keys debonded from the RC caps. Fig. 11(d) compares the two near surface mounted specimens each with one of two types of adhesive, cementitious or epoxy. Both strengthened specimens showed enhanced stiffness and strength relative to the control. For both, shear sliding began at approximately 20 kpa resulting in the initiation of debonding of the NSM CFRP from the RC caps. This resulted in an initial slip, after which both specimens were able to further resist load until debonding was complete and the specimens collapsed. 3.2 Failure Modes The observed mode of failure depended heavily on the type of FRP anchorage that was used as illustrated in Fig. 12. The control specimen collapsed due to the formation of a horizontal crack at a bed joint two courses below midspan as shown in Fig. 12(a). As this crack grew, it split the wall into two panels which then rotated about the top and bottom supports respectively. For the specimen strengthened with and restrained using the steel shear restraint anchorage, the began to rupture one course above mid-span. This rupture quickly propagated along the width of the strip, as shown in Fig. 12(b). The shear restrained specimen was not loaded to ultimate failure due a limitation of the airbag. The remaining specimens failed due to either debonding or pullout of the FRP in the anchorage zone. In each case, the debonding or pullout was a result of shear sliding out-of-plane of the infill wall with respect to the RC caps Shear Restraint Shear Restraint Control Deflection at Mid Point (mm) (a) Shear Restraint Anchorage Fiber Anchor Note: Test terminated at maximum airbag operating pressure. Control Deflection at Mid Point (mm) (b) Fiber Anchor Anchorage Shear Key Shear Key Fiber Anchor 20 Control Deflection at Mid Point (mm) (c) Shear Key Anchorage NSM CFRP (Cementitious Adhesive) NSM CFRP (Epoxy Adhesive) Control Deflection at Mid Point (mm) (d) Near Surface Mounted Fig.11 Load Deflection Envelopes

6 (a) Flexural (b) FRP Rupture at Midspan (c) Debonding of NSM CFRP from RC Cap (d) Debonding of from RC Cap (e) Debonding of Shear Keys from RC Cap (f) Pullout of Fiber Anchors from RC Cap (g) Pullout of Fiber Anchors from RC Cap (h) Debonding of Embedded Bar from RC Cap Fig. 12: Failure Modes (i) Debonding of Embedded Bar from RC Cap

7 Both specimens strengthened with the NSM CFRP failed due to debonding of the CFRP from the RC cap. The debonding initiated at the interface between the top RC cap and the masonry infill and propagated up the cap, as shown in Fig. 12(c), until the NSM CFRP was fully debonded after which the wall collapsed. Similarly for the specimens with overlap anchorage, debonding began at the level of the interface between the RC caps and the masonry and further propagated along the cap as shown in Fig. 12(d). The shear key specimens failed by debonding of the shear keys from the RC cap. Fig. 12(e) shows the underside of the sheet after testing. This photograph reveals that the bond between the sheet and the CFRP shear keys to the masonry and RC caps was sufficient since the debonding failure plane was through the masonry and the concrete rather than through the epoxy or at the interface between the epoxy and the substrate. The specimens strengthened with fiber anchors failed when the anchors pulled out of the RC caps. Fig. 12(f) and (g) show the specimen with fiber anchors just before failure and the underside of the sheet after failure respectively. Fig. 12(f) clearly shows the shear sliding of the masonry infill which results in the peeling of the sheet up to the level of the fiber anchors. Fig. 12(g) illustrates the pullout of the fiber anchors and shows that the bond between the anchors and the remained intact throughout the testing. It should be noted that the anchorage relied entirely on this bond, as the sheet was placed on top of the anchors, rather than cutting a hole and passing the fiber anchors through the. The specimen utilizing the embedded bar anchorage failed when the bar debonded from the RC cap as shown in Fig. 12(h) and (i). In a manner similar to the fiber anchor specimens, the debonded up to the level of the embedded bar as shown in Fig. 12(h). Collapse then occurred when the embedded bar debonded from the RC cap. Fig.12(i) shows the underside of the sheet after failure and the good bond achieved between the and the masonry. 3.3 Strength and Ductility A summary of the experimental results is provided in Table 2. In all cases tested, strengthening lead to an increase in the load carrying capacity as shown in Fig. 13(a). The amount of this increase ranged between 1.6 and 7.2 times the capacity of the control specimen and was highly dependent on the FRP type and the anchorage system. For a given anchorage system and FRP width, typically provided a greater increase in strength. For a given FRP type, the steel shear restraint anchorage provided the largest increase in strength. This should not however be interpreted to mean that or steel shear restraint will always lead to the highest strength enhancement, as the load carrying capacity depends on the design of the strengthening and anchorage systems. The results show that resisting shear sliding is an important factor to consider when strengthening infill walls and that several existing and proposed FRP anchorage systems are successful in this endeavor. Displacement capacity is also of great interest for extreme loading events. Displacement of the strengthened walls at the ultimate pressure ranged from 0.6 to 4.5 times the corresponding displacement of the control specimen as shown in Fig. 13(b). The greatest increase in displacement capacity was provided by two strengthened specimens, one with fiber anchors and the other with the steel shear restraint. The displacement capacity for both specimens was well over three times that of the control specimen. This amounts to displacements in excess of 120 mm as shown in Fig. 14, which is not possible for 92 mm wide URM walls without strengthening. Table 2: Summary of Experimental Results Anchor Type* FRP Type** Constr. Phase: Cast # (Round)*** Ultimate Applied Pressure (kpa) Displ. at Ultimate (mm) N/A N/A 2 (2) O 1 (1) O 1 (1) SR 2 (2) SR 2 (2) FA 2 (2) FA 2 (2) EB 2 (2) SK 1 (2) SK 1 (2) NSM CFRP-E 2 (2) NSM CFRP-C 2 (2) *Anchor types: O-, SR-Shear Restraint, FA-Fiber Anchor, EB-Embedded Bar, SK-Shear Key, NSM-Near Surface Mounted. **FRP types: - Glass fiber, - Polyethylene Terephthalate fiber, CFRP - E - Carbon fiber with Epoxy adhesive, CFRP - C - Carbon fiber with Cementitious adhesive. ***Construction phases: For RC caps, there were two casts and for masonry, there were two rounds of construction. See Table 1 for material properties corresponding to the various phases.

8 Diplacement at Ultimate Normalized w.r.t. Control Ultimate Strength Normalized w.r.t. Control (a) Ultimate Strength (b) Displacement at Ultimate Strength Fig.13 Ultimate Strength and Displacement with respect to the Control Specimen 4. CONCLUSIONS (1) Delaying and/or preventing the debonding of the FRP strengthening systems resulting from shear sliding along the interface between the masonry infill and the RC supporting elements is a key factor in enhancing the load carrying capacity of strengthened infill masonry walls. (2) The proposed anchorage systems have shown excellent potential for this purpose and can provide a significant improvement in the out-of-place load carrying capacity of strengthened infill masonry walls. (3) The design of strengthening solutions should include consideration of the effect of anchorage. Further research is necessary to develop rational design procedures for the various anchorage systems. ACKNOWLEDGEMENTS The authors gratefully acknowledge the NSF I/UCRC Center for Integration of Composites into Infrastructure. The authors would especially like to thank Fyfe Company, LLC, Nippon Steel Materials Co., Ltd., Grancrete, Inc. and Maeda Kosen Co. Ltd. for their support. REFERENCES [1] T.C. Triantafillou: Strengthening of masonry structures using epoxy-bonded FRP laminates, J. Compos. Constr., Vol. 2 (2), pp , [2] N. Galati, G. Tumialan, and A. Nanni: Strengthening with FRP Bars of URM Walls Subject to Out-of-Plane Loads, Constr. Build. Mater., Vol. 20, pp , [3] D.S. Lunn and S.H. Rizkalla: Strengthening of Infill Masonry Walls with FRP Materials, J. Compos. Constr., Vol. 15(2), pp , 2011 (a) Profile View (b) Perspective View Fig.14: Displaced Shape of Specimen with Shear Restraint Anchorage [4] K.H. Tan and M.K.H. Patoary: Strengthening of masonry walls against out-of-plane loads using fiber-reinforced polymer reinforcement, J. Compos. Constr., Vol. 8 (1), pp.79-87, [5] ASTM: Standard test methods, C39-05, C , C140-08, C109-08, D West Conshohocken, PA,