OUT OF PLANE BEHAVIOR OF MASONRY INFILL WALLS RETROFITTED WITH A REINFORCED POLYMER GRID AND POLYUREA SYSTEM

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1 OUT OF PLANE BEHAVIOR OF MASONRY INFILL WALLS RETROFITTED WITH A REINFORCED POLYMER GRID AND POLYUREA SYSTEM Trevor Hrynyk Graduate Research Assistant CIES / Department of Civil, Arch., & Env. Engineering University of Missouri-Rolla, Rolla, MO 65409, USA Prof. John J. Myers Associate Professor CIES / Department of Civil, Arch., & Env. Engineering University of Missouri-Rolla, Rolla, MO 65409, USA jmyers@umr.edu KEYWORDS: Masonry retrofit, composite strengthening, masonry out of plane strengthening, polyurea grid system. ABSTRACT Recent world events have illustrated that sustainability of buildings to blast loads is an ever increasing issue. Many older buildings contain unreinforced masonry (URM) infill walls. Due to their low flexural capacity and their brittle mode of failure, these walls have a low resistance to out-of-plane loads, including a blast load. As a result, an effort was undertaken to examine retrofit methods that are feasible to enhance their out-of-plane resistance. In previous masonry infill wall studies conducted by Carney and Myers (2005) and Myers et al. (2002), fiber reinforced polymers (FRP) fabrics and near surface mounted (NSM) techniques demonstrated increases in deformation ductility and energy ductility levels by three fold. However, these retrofit systems did not completely control the scatter of debris which could be a life safety issue. In this on-going research study the use of externally bonded reinforced polymer grid systems bonded with a polyurea is investigated to examine the feasibility of increasing the ductility in these retrofit systems and better controlling the scatter of debris. These systems have shown to increase the out-of-plane load capacity of the wall systems with improved ductility based on both an energy-based and deformation-based definition. INTRODUCTION In light of recent events, it is evident that much of the current infrastructure throughout the world is vulnerable to acts of terrorism. Attacks are commonly directed toward highly populated or diplomatic structures and often involve some form of explosive device. The majority of the damage and injuries from these types of attacks are not attributed directly to the blast from the explosive device, but rather fragmentation and debris scatter of the building envelope tends to result in the most injury and loss of human life. The use of unreinforced masonry (URM) infill walls has been and continues to be common practice in building construction. Typically, infill walls are used externally making up the building envelope, or are used internally within the structural framing of a building. URM walls have low flexural capacities and possess brittle failure modes making them highly susceptible to failure when exposed to out-of-plane loadings such as a blast load. The use of externally bonded FRP systems has shown significant increases in both the flexural capacity as well as the shear capacity of URM walls under different out-ofplane loading conditions and can greatly reduce the debris scatter [Carney and Myers (2005), Galati et al. (2005), Morbin (2002)]. A previous study implemented the use of two different FRP retrofit techniques and has shown a significant increase in the flexural capacity of framed URM infill walls tested under simulated static blast loading as well as field blast testing [Carney and Myers (2005)]. The walls in this study were constructed between two rigid beams simulating the boundary conditions of an infill wall within a concrete frame with the presence of arching action. In this study, the use of both near-surface-mounted (NSM) GFRP rods and surface laminate GFRP strips were investigated and both were found to

2 significantly increase the out-of-plane load capacity of URM walls. It was also found that the use of the FRP surface laminates significantly reduced the amount of debris scatter at failure. Other studies have shown that NSM FRP rods as well as FRP surface laminates used to strengthen URM walls tested under four-point bending where no arching action is present effectively increases both the shear and the flexural capacity of the URM walls [Galati et al. (2005), Morbin (2002)]. Lastly, the use of an FRP grid and polyurea system has been shown to effectively increase the shear resistance in URM walls under in-plane loading as well as the capacity of reinforced concrete beams in bending [Yu et al. (2004)]. RESEARCH OBJECTIVES The main objectives of this research program is to investigate the behavior of URM walls strengthened with a reinforced polymer grid/fabric and polyurea system for the following criteria: (1) effective increase in out-of-plane resistance, (2) increased ductility under out-of plane loading, and (3) reduction in the amount of fragmentation and debris scatter at failure. EXPERIMENTAL PROGRAM The experimental program for this study consists of two phases of URM wall testing. Phase I consists of (9) URM walls loaded under four-point bending. Phase II involves the testing of (6) URM walls under a simulated (static) blast loading. The test matrix for the experimental program is presented in Table 1. The experimental program testing is currently ongoing. This paper presents the preliminary results from Phase I only. Table 1: Test Matrix Specimen Dimensions width x height x thickness in mm (in.) Test Method PHASE I PHASE II CL-GGRP-SL 610x914x70mm (24x 36x 2.75in.) Four-point bending CL-GGRP-DL 610x914x70mm (24x 36x 2.75in.) Four-point bending CMU-GGRP-SL 610x1219x92mm (24in x 48in x 3.625in.) Four-point bending CMU-GGRP-DL 610x1219x92mm (24in x 48in x 3.625in.) Four-point bending CMU-SRP 610x1219x92mm (24in x 48in x 3.625in.) Four-point bending CMU-PU 610x1219x92mm (24in x 48in x 3.625in.) Four-point bending BEB-GGRP-SL 610x1372x105mm (24in x 54in x 4.125in.) Four-point bending BEB-MA 610x1372x105mm (24in x 54in x 4.125in.) Four-point bending CL-control 914x914x70mm (36in x 36in x 2.75in.) Uniform load via airbag CL-GGRP-SL 914x914x70mm (36in x 36in x 2.75in.) Uniform load via airbag CMU-control 914x1219x92mm (36in x 48in x 3.625in.) Uniform load via airbag CMU-GGRP-SL 914x1219x92mm (36in x 48in x 3.625in.) Uniform load via airbag BEB-control 914x914x70mm (36in x 36in x 2.75in.) Uniform load via airbag BEB-GGRP-SL 914x914x70mm (36in x 36in x 2.75in.) Uniform load via airbag The identification system used for the test specimens in Table 1 uniquely describes each of the walls in the experimental program. The first set of characters of the specimen name identifies the masonry material that was used for wall construction. The second set of characters is used to identify the

3 strengthening material that was used to retrofit the wall. The last set of characters, where appropriate, was used to describe the number of grid layers that were used to strengthen the wall. As shown in Table 1, three different materials were used to construct URM walls. Four of the walls were constructed using clay (CL) bricks. The height of the clay brick specimens were 614mm (36in.), and their overall thicknesses were 70mm (2-3/4in.). Six specimens were built using concrete masonry units (CMU s) with a height of 1219mm (48in.) and overall thicknesses of 92mm (3-5/8in.). Lastly, four specimens were built using a ballistic enhanced block (BEB) material. These specimens were 54in (1372mm) high with thicknesses varying from 70mm (2-3/4in.) to 105mm (4-1/8in.). All of the specimens within Phase I of the program had a width of 610mm (24in.), and all of the specimens within Phase II of the program were 914mm (36in.) in width. All of the retrofitted URM walls in this study were strengthened using either a glass FRP grid (GGRP) and polyurea system, or a steel reinforced polymer (SRP) fabric and polyurea system, with the exception of wall specimen BEB-MA and wall specimen CMU-PU. Wall BEB-MA was constructed using a mortar alternative (MA) surface coating. Wall specimen CMU-PU was retrofitted using only polyurea. Walls strengthened with using the GGRP system had either one layer (SL) of grid applied or two layers (DL) of grid applied equal to the width of the wall specimen. The surface coating used to construct wall BEB-MA was applied evenly to both faces of the wall as per directions of the manufacturer. All other wall specimens were constructed using a Type S mortar as defined by ASTM C 270, and the mortar joints were finished flush with the wall surface. A running bond was used for all of the specimens with the exception of the specimens in Phase I built from BEB. These walls were constructed using a stack bond. Material Properties Tests were performed to determine the properties of the materials used for wall construction. The compressive strength of the mortar was verified by testing 50mm (2in.) mortar cubes in accordance with ASTM C109. The mortar strength of the Phase I wall specimens was found to be a value of 12.1MPa (1750psi). Cubes were also tested for the mortar alternative surface coating material. The compressive strength of this material was found to be a value of 22.75MPa (3300psi). To determine the compressive strength of the masonry, prisms were constructed for each of the wall specimens. The prisms were match-cured with the wall specimens and were tested uncapped. The resulting compressive strength of the masonry used in Phase I of the experimental program is shown in Table 2. Table 2: Compressive Strength of Masonry Masonry Material Prism Area cm 2 (in 2 ) (net/gross) Compressive Strength, f m MPa (psi) (net/gross) Clay Brick 141 / 171 (21.8 / 26.5) / (1850 / 1500) Concrete Block 291 / 365 (45.1 / 56.6) / 8.96 (1650 / 1300) Ballistic Enhanced Block (mortar) NA / 316 (NA / 49) NA / 3.79 (NA / 550) Note: Ballistic Enhanced Block are solid units. Recently a design guide has been developed for reinforcing URM walls using GFRP grids [Garbin et al. (2005)]. The design guidelines established the guaranteed mechanical properties of the GFRP grids that were used in the strengthening system for this research. The guaranteed properties from the design guidelines are presented in Table 3.

4 Table 3: Guaranteed Mechanical Properties for GFRP Grid Grid Type Grid Spacing mm (in) Tensile Strength kn/m (lbs/ft) (warp x fill) Cross- Sectional Area mm 2 /m (in 2 /ft) Tensile Strength MPa (ksi) (warp x fill) Elastic Modulus MPa (ksi) Ultimate Strain mm/mm (in/in) (warp x fill) G15000-BX , , (0.437) 1 14, (64.5) 2 (5306) (1) Data Provided by Manufacturer (2) Based on Experimental Results Obtained by Yu et al., (2004). Test Setup Phase 1 Out of plane behavior of wall systems without arching action The test setup for Phase I of the experimental program is shown in Fig. 1. All of the URM walls tested in this phase of the program were loaded vertically under four-point bending. The load was applied to 152mm (6in.) steel plates using a 266.9kN (30-ton) hydraulic jack anchored to a steel frame. The loading points were 102mm (4in.) offof-center for the CMU and clay brick walls, and 114mm (4.5in.) off-of center for the BEB walls. All of the walls tested in this phase had a slenderness ratio of 12. The load applied was recorded using an 889.6kN (200kip) load cell and the deflection was determined using LVDT s placed at midspan on each side of the wall. Fig. 1 Test Setup for Phase I Fig. 2 Test Setup for Phase II Phase 2 Out of plane behavior of wall systems with arching action The test setup for Phase II of the experimental program is presented in Fig. 2. URM walls in this phase were loaded with a horizontal distributed load by means of an air bag acting on one face of the wall specimens. The air bag was placed between the URM test specimens and a stiffened steel plate which reacted against the strong-wall. The URM walls in this phase were fixed between concrete boundary elements which were anchored to the strong-wall and strong-floor to simulate the behavior of an infill wall within a structural frame. The pressure in the air bag was increased incrementally and initially deflections were measured using several dial gauges along the height of the wall. However, once the presence of arching action was noticeable, the dial gauges were then removed and the midspan deflection was recorded through use of a video camera and scale ruler. The wall specimens within this phase of the testing had a slenderness ratio of 12. PRELIMINARY EXPERIMENTAL RESULTS This section presents the preliminary results from Phase I of the experimental program to date. The moment-deflection behavior of the retrofit walls and their associated failure mode of specimens tested to

5 date are discussed. Possible failure modes in this phase could be failure due to masonry/mortar crushing in compression, failure due to shear, failure due to FRP rupture, or failure due to FRP peeling/debonding from the substrate masonry. It should be noted that the FRP grid for the test results presented in this section was effectively anchored at either end of the support condition to delay/prevent a peeling or debonding failure of FRP strengthening system. In an actual infill wall system this would typically be done by anchoring the FRP grid, laminate, or fabric into the concrete boundary system using the NSM rod technique and epoxy. This technique has been found to be sucessful in the past to avoid a premature peeling failure and improve the overall system capacity [Carney and Myers (2005)]. Moment-Deflection Behavior and Failure Mode CMU-GGRP-SL Fig. 3 illustrates the midspan moment-deflection behavior of wall CMU-GGRP-SL. The maximum moment obtained was 11.49kN-m (8.474ft-k). The maximum deflection obtained at failure was 29.46mm (1.16in.). The polyurea-frp grid system increased the load carrying capacity of an unstrengthened CMU wall (based on theoretical calculations) under four point loading by 28.7 times. Two different methods may be used to compare the ductility of the reinforcing techniques. The first method is the deflection ductility. This is calculated by dividing the wall s ultimate deflection (u f ) by its deflection at the apparent yield point (u y ). The deformation ductility of wall CMU-GGRP-SL was The second method is the energy ductility. This is determined by dividing the total area under the loaddeflection plot by the area under the linear portion of the plot. Often, energy ductility is used to characterize and discuss the ductility of composite systems. A ratio of 35.3 is obtained based on energy ductility. As illustrated in Fig. 3, by either definition, the ductility of the CMU has been enhanced dramatically compared to URM CMU wall. The failure mode of the wall system may be characterized as a shear failure as shown in Fig Midspan Moment (k-ft) CMU-GGRP-1L Theoretical Midspan Deflection (in) Conversion Factor: 1in = 25.4 mm; 1 ft-k = kn-m Fig. 3 Moment deflection relationship for wall CMU-GGRP-SL

6 Fig. 4 Failure mode of specimen CMU-GGRP-SL (Left: detail of shear crack; Right: debris scatter) CLAY-GGRP-SL Fig. 5 illustrates the midspan moment-deflection behavior of wall CLAY-GGRP-SL. The maximum moment obtained was 8.24kN-m (6.08 ft-k). The maximum deflection obtained at failure was 41.4mm (1.63in.). The polyurea-frp grid system increased the load carrying capacity of an unstrengthened CMU wall (based on theoretical calculations) under four point loading by 36.4 times. The ductility of the clay masonry wall has been enhanced dramatically compared to URM Clay brick wall as shown in Fig 5. The deformation ductility of wall CLAY-GGRP-SL was The failure mode of the wall system may be characterized as a shear failure as shown in Fig Moment (k-ft) Midspan Deflection (in) CLAY-GGRP-SL Theoretical URM Conversion Factor: 1in = 25.4 mm; 1 ft-k = kn-m Fig. 5 Moment deflection relationship for wall CLAY-GGRP-SL Shear crack formation Fig. 6 Failure mode of specimen CLAY-GGRP-SL (Left: debris scatter; Right: detail of shear crack)

7 CLAY-GGRP-DL Fig. 7 illustrates the midspan moment-deflection behavior of wall CLAY-GGRP- DL. The maximum moment obtained was 11.20kN-m (8.26ft-k). The maximum deflection obtained at failure was 19.6mm (0.77 in.). The polyurea-frp grid system increased the load carrying capacity of an unstrengthened Clay brick wall (based on theoretical calculations) under four point loading by 49.5 times. The ductility of the clay masonry wall has been enhanced dramatically compared to the URM Clay brick wall as shown in Fig 7. The deformation ductility of wall CLAY-GGRP-DL was The failure mode of the wall system may be characterized as a shear failure as shown in Fig Midspan Moment (k-ft) CLAY-GGRP-DL Theoretical URM Midspan Deflection (in) Conversion Factor: 1in = 25.4 mm; 1 ft-k = kn-m Fig. 7 Moment deflection relationship for wall CLAY-GGRP-DL Shear crack formation Fig. 8 Failure mode of specimen CLAY-GGRP-DL (Left: debris scatter; Right: detail of shear crack) Summary Discussion on Results The results presented in the moment-deflection plots and the failure modes for each wall have been summarized and are shown below in Table 4. Table 4: Summary of Current Results Wall Ultimate Moment kn-m (ft-k) Ultimate Shear kn (kips) Failure Mode CL-GGRP-SL 8.24 (6.08) (5.61) Shear Failure CL-GGRP-DL (8.29) (7.65) Shear Failure CMU-GGRP-SL (8.47) (5.65) Shear Failure CONCLUSIONS This research has shown that reinforcing walls with FRP improves the out-of-plane capacity of the walls with the polyurea-frp grid system. The grid system also provided the wall system with an increased

8 ductility and controlled the scatter of debris better than previous retrofit wall tests conducted at the University of Missouri-Rolla (UMR), USA using bonded laminate strips and/or NSM rods. These results, in combination with previous testing at UMR, show that FRP can in fact be utilized in the retrofitting of existing buildings with URM infill walls to improve their resistance to extreme out-ofplane loads such as a blast load. This advancement may be key in the limiting damage to buildings and the loss of life due to terrorist activity in the future. FUTURE WORK This research program is ongoing. Remaining Phase I specimens and Phase II specimens will be tested to investigate the effects of boundary conditions (arching vs. non-arching), masonry substrate material, strengthening material and reinforcement ratio on the capacity of the URM walls. Field blast testing of the most promising walls from Phase I and II will also be conducted in the near future to examine the behavior of the wall systems under short dynamic (blast) behavior. ACKNOWLEDGEMENTS The authors wish to express their gratitude and sincere appreciation to the National Science Foundation (NSF) sponsored Repair of Buildings and Bridges with Composites Cooperative Research Center (RB2C) and the Center for Infrastructure Engineering Studies (CIES) at the University of Missouri- Rolla for supporting this research study. The authors would also like to thank Encore Building Solutions of St. Louis, Missouri, USA for their support of this research project. REFERENCES [1] Carney, P., Myers, J.J. (2005). Static and Blast Resistance of Unreinforced Masonry Wall Connections Strengthened with Fiber Reinforced Polymers, American Concrete Institute Special Publication-230, FRPRC7-Editors C. Shield, J. Busel, S. Walkup, D. Gremel, November 2005, pp [2] Carney, P., Myers, J.J. (2003). Out-of-Plane Static and Blast Resistance of Unreinforced Masonry Wall Connections Strengthened with Fiber Reinforced Polymers, Center for Infrastructure Engineering Studies Report 03-46, University of Missouri-Rolla, Rolla, MO. [3] Galati N, Tumialan G, and A. Nanni. (2005). Strengthening with FRP bars of URM walls subject to out-of-plane loads, Construction and Materials. [4] Garbin, E., Galati, N., and A. Nanni. (2005). Design Guidelines for the Strengthening of Unreinforced Masonry Structures Using Glass Grid Reinforced Polymer (GGRP) Systems. University of Missouri-Rolla, Rolla, MO. [5] Morbin, A. (2002). Strengthening of Masonry Elements with FRP composites. Center for Infrastructure Engineering Studies Report 02-23, University of Missouri-Rolla, Rolla, MO. [6] Yu, P., Silva, P., and A Nanni. (2004). Application of Bondo Polyurea in Structural Strengthening of RC Beams and URM Walls, Center for Infrastructure Engineering Studies Report 04-49, University of Missouri-Rolla, Rolla, MO. [7] Masonry Standards Joint Committee. Building Code Requirements for Masonry Structures. ACI /ASCE 5-02/TMS American Concrete Institute, American Society of Civil Engineers, and The Masonry Society, Detroit, New York, and Boulder; [8] Myers, J.J., Belarbi, A. El-Domiaty, K.A. (2004). Blast Resistance of Un-reinforced Masonry Walls Retrofitted with Fiber Reinforced Polymers, The Masonry Society Journal, Boulder, Colorado, Vol.22, No.1, September 2004, pp 9-26.