The Effectiveness of Polymer Reinforced Concrete Masonry - Essay Example

The effectiveness of Polymer reinforced Concrete Masonry (CMU) Walls as Compared to unreinforced CMU Walls Recent events in the world have demonstrated that rigidity of buildings to blast loads is currently under great consideration. A great number of old buildings have unreinforced masonry walls (URM). As a result of their low flexing ability and brittle mode of failure, such kind of walls doesn't possess enough resistance to out-of-plane loads, especially to a blast load. Consequently, an effort has been made to view retrofit methods that can increase their out-of-plane resistance. The application of outwards bonded and mounted near surface Fiber Reinforced Polymer (FRP) (NSM) laminates and rods have shown good results to heighten the out-of-plane load ability. Using of FRP composites have been viewed as a suitable and cost-effective way for strengthening URM. Seismic design in the USA is nearly completely grounded on the consideration that the structural system gives a flexible failure mode. Masonry walls strengthened by FRP actually have fragile failure modes as a result of the nature of the strengthening system itself. The idea explored in our research paper is the introduction of flexibility using some kind of hybrid strengthening system. We based our investigation on the experiments held by J.J. Myers and P. Carney (cited in Tumialan, 2005). The research study investigated the practicability of developing continuity between the FRP and the surrounding reinforced concrete frame system. In the paper, we evaluated strengthened URM wall's functioning using static tests as tools for our investigation. The authors whose works we used for the examination utilized 2 strengthening methods including the application of glass FRP (GFRP) laminates to the wall's surface and the installation of near surface mounted (NSM) GFRP rods. In both methods, the strengthening material was anchored to boundary members above and below the wall on some of the specimens in the research program. A shear retrofit, the effects of bond pattern, and the effects of FRP laminate strip width were also investigated in our paper. The development of continuity between the FRP materials and the surrounding framing system is important to improving the blast resistance of URM infill walls. Keywords: FRP strengthening; blast resistance; masonry wall retrofits; masonry wall connections. 1. INTRODUCTION 1.1. BACKGROUND Recent events in the world have attracted attention to the vulnerability and sustainability of buildings and infrastructure to acts of terrorism. Our infrastructure is vital to this nation's economy and way of life. Any damage to it would and has had drastic effects on our culture. Attacks may cause a variety of results ranging from minor building damage to complete structural failure and considerable loss of life. Some examples within the United States include the bombing of the Murrah Federal Building in Oklahoma City (1995) and the bombing and attacks on the World Trade Center in New York City (1993, 2001). Abroad, numerous attacks have been directed toward embassies, and suicide car bombers have been used to targetpopulated areas. In the cases where complete structural failure is not an issue, the dangers of flying debris have resulted in loss of life or injury to numerous civilians. Of particular concern are unreinforced masonry (URM) infill walls. Structural systems composed of a reinforced concrete (RC) framing system with URM infill walls makes up a significant portion of the building inventory in the United States and around the world. Since there is no reinforcement within these walls, they have little resistance to out-of-plane loads such as a blast load. As a result, an effort has been undertaken to examine retrofit methods that are feasible to enhance their out-of-plane resistance. One method of strengthening URM walls is the application of fiber reinforced polymers (FRP) to the surface of the wall to improve their performance. Today, FRP is considered an emerging technology. Its use began becoming more widespread following World War II when the aerospace industry began to make use of its unique properties. FRP was a material that could have a very high strength, but was still lightweight, making it an ideal material for use in this industry (Pinder & Pinder, 1990, p. 23). Following this, the use of FRP became even more widespread as it was used in the manufacturing of golf clubs and fishing poles, and in the 1960s FRP was considered for use in reinforced concrete as concerns of rebar corrosion began to escalate. Epoxy coated rebar became the acceptable solution at the time, but in the 1990s the long-term effectiveness of epoxy coated bars began to be questioned. As a result, FRP is being considered as a long-term solution (ACI 440.1R-01). Not only can FRP be used within concrete, but it also can be applied to the surface. Externally bonded FRP systems have been investigated and been used since the 1980s. External bonding is a developing technology and has been used to strengthen a wide variety of structural systems and members. Externally bonded FRP may be applied to more than just concrete. Other materials that have been strengthened include wood, steel, and masonry. External bonding of FRP was first used as an alternate to bonding steel to the surface of a material. These FRP systems can be applied to concrete columns to provide additional confinement or applied to beams for flexural or shear strengthening (ACI 440.2R-02). Since the effects of a blast cause a pressure to be exerted on the surface of a wall, the flexural behavior of the wall can be observed. This makes it appropriate to strengthen the walls to improve their flexural capacity. The application of externally bonded FRP materials have been shown to improve the flexural capacity of walls with and without arching action (Detailing of Facades, 2005, p. 279), but the development of continuity between the wall system and surrounding boundary members needs to be investigated. Strengthening of walls is not the only step included in the process of reducing a building's vulnerability to blast loadings. Proper risk assessment must also be performed to determine the level of vulnerability of a structure. One must also determine the level of damage that is acceptable for the structure to sustain. The characteristics of an explosion are key in assessing this vulnerability. The pressures that are developed as a result of an explosion are a function of the weight of the charge and the distance from the explosion, commonly called the standoff distance. The charge weight is expressed in terms of equivalent weight of trinitrotoluene (TNT). As you increase the charge weight the pressures that are developed are also increased. Similarly, as the standoff decreases, the pressures on a surface increase. For a given charge weight, the effects may be drastically different if the standoff distance is changed (Hughes Brothers, Inc., 2001, 17). For a very small standoff distance, strengthening the wall per say may have little effect; rather the addition of significant mass in the form of thick walls is often the approach. However, it may be more appropriate to try to increase the standoff distance to a facility by implementing barriers or restricting vehicular access to a structure. Wall strengthening would then allow for a compromise, that is the standoff distance would only have to be increased to the point which the strengthened wall could withstand the pressure from the design blast (Erman, 2004, 71). With the proper assessment and an understanding of the key parameters, the strengthening of URM infill walls with FRP to improve their blast resistance has great potential. 1.2. SCOPE AND OBJECTIVES Previous research that is described later has shown that externally mounted FRP has improved the out-of-plane performance of URM infill walls. This research investigation further investigates the ability of FRP to increase the flexural capacity and ductility of URM infill walls. There are several retrofit techniques proposed by Tumialan that have been investigated in this research paper. The first technique was the application of FRP laminates and near surface mounted (NSM) FRP rods to the surface of the wall to increase the flexural capacity of the walls. The FRP was anchored to the surrounding boundary members for the purpose of developing continuity between the wall system and the surrounding RC framing system. A shear retrofit technique was also observed in an attempt to improve the shear capacity of the masonry in the regions near the boundary members. The effects of bond pattern, stacked versus running bond, were also observed in this research program to examine any impact on the out-of-plane strength of the walls. These out-of-plane tests carried by Tumialan were performed in the laboratory under static loading conditions using an air bag as the loading mechanism. These tests evaluated the effectiveness of anchoring the FRP material to the boundary members for development of continuity. Field blast testing was performed by experts on two wall systems to evaluate the retrofit scheme that was most effective under out-of-plane loading in the lab (Tumialan, 2005, p. 44). 2. MASONRY WALL TESTING The testing of URM walls in the out-of-plane direction can be accomplished in a variety of ways. Several different methods of loading have been utilized in previous research to effectively apply a static load. Some research programs have applied a point load either at center span or used a device to apply two point, or line, loads on either side of the midpoint of the wall (Hamoush et al., 2001. p. 72). A complex, but effective method of applying a uniform load is the use of a pressurized water chamber. A wall can be constructed between two tank sand one of them pressurized to apply a uniform load to the wall. A simpler method is the use of an airbag which is what was used in Phase I of this study. An airbag can be used to apply a load by placing the bag in contact with the test wall and a reaction structure. The airbag used in this research program had deflated dimensions of 36 in (914.4 mm) wide by 48 in (1219.2 mm) tall. They were six ply paper dunnage bags commercially produced by International Paper's Ride Rite Division. They were capable of withstanding pressures of over 20 psi (138 kPa) based on testing by the manufacturer (Tumialan, 2005, p. 49). 3. REVIEW OF LITERATURE 3.1 Test matrix. The development of this test program extended previous research performed at UMR (El-Domiaty et al., 2002, cited in Stennott, 2004, 129). The previous work illustrated that strengthening masonry walls with FRP materials does in fact improve their out-of-plane performance. This research was conducted by J.J. Myers and P. Carney (cited in Tumialan, 2005) to further investigate the effectiveness of strengthening URM walls with several variables. This study was completed in two phases. Phase I was the evaluation of the retrofit techniques under static loading conditions using an airbag to incrementally load the walls to failure. Phase I was divided into two series. Series I consisted of six test walls. Series II was composed of an additional six walls based on the results obtained from Series I. Phase II was the field evaluation of two walls under actual blast loading. The walls in both phases were constructed of 4 in x 8 in x 12 in (101.6mm x 203.2 mm x 304.8 mm) CMU. The overall dimensions of the walls were 48 in (1219.2 mm) tall by 36 in (914.4 mm) wide. These dimensions result in a slenderness ratio of 12. The slenderness ratio of a 10 ft (3.05 m) tall wall constructed out of 8 in (203.2 mm) thick blocks is approximately 15. Therefore, the slenderness ratio used in this research program is comparable to what could be expected in an existing building. The 36 in (914.4 mm) wide dimension allowed for the wall to be three blocks wide giving two vertical, or head, joints in each wall. The FRP was applied along or within each of the head joints. It may be noted that 8 in (203.2 mm) thick walls were not tested due to limitations in the capacity of the air bags to meet the required failure pressure requirements of thicker wall units (cited in Tumialan, 2005, p. 53). After Series I walls were tested, the test program for Series II was developed. Walls 1 and 2 served as unreinforced control walls in Phase I. Walls 7 and 8 in Series II served as strengthened FRP control walls without anchorage details. These were strengthened, but do not make use of the anchorage techniques. This allowed for a direct measure of the increase in capacity associated with the use of anchorage. The walls in Series I were all constructed using a stacked bond pattern. Since many facilities are constructed using a running, or staggered, bond, it was necessary to study the effects of bond pattern. This was done by constructing two of the walls (Walls 9 and 10) using a running bond. Both FRP retrofit techniques with anchorage were tested using this bond pattern. Series II concluded by studying the effects of the reinforcement ratio, or the width of the laminate strip. The final two walls made use of laminate strips that were 4.5 in (114.3 mm) and 6.5 in (165.1 mm) wide to investigate how the capacity of the walls changes as the amount of reinforcement on the walls increases. This change is only possible when using the laminates. The amount of reinforcement in the case of NSM bars cannot be increased without cutting additional grooves in the blocks due to the size limitations of the mortar joints (cited in Tumialan, 2005, p. 19). The research program concluded with Phase II. This was the field blast testing of two walls. Under static loading, the laminates performed better than the NSM rods, so they were selected for use in this phase to evaluate their performance under short dynamic (blast) loading. One wall made use of 2.5 in (63.5 mm) laminates unanchored, while the other wall had the same reinforcement, but the FRP was anchored to the boundary members and the shear retrofit was included. This experimental program investigated the development of continuity between the FRP and the boundary members. Several other variables were also examined, including the effects of bond pattern, and the effects of the width of the laminate strips. The test program is summarized in Table 1. Two FRP strengthening methods were utilized in this research along with anchorage techniques for both methods. The first strengthening technique is the use of externally bonded glass FRP laminates as shown in Figure 1. The laminates (fabrics) are applied vertically to the surface of the wall centered on the two head joints at the two third points. This system includes a primer, putty, saturant, and a glass fiber sheet to form the composite material. Glass fiber sheets were selected by the researchers in lieu of carbon FRP sheets based previous studies of retrofitting masonry systems by the research team. Glass fibers are more economical and provide a more compatible strength than the carbon fibers. The second method was the application of near surface mounted (NSM) glass FRP rods, illustrated in Figure 2. 3.2 Material and application material properties The properties of the FRP rods and FRP fabric used in this study are detailed in Tables 2 and 3, correspondingly. Table 4 details the properties of the application materials. All material properties were evaluated using standard ASTM test methods. The mortar strength was 2000 psi, 1150 psi, and 1250 psi for Phase I-Series I, Phase I-Series II, and Phase II, respectively at test age of the walls. The compressive strength of the RC boundary elements was 4000 psi at test age. The concrete boundaries were one foot square beams, reinforced with three longitudinal 3 steel rebar top and bottom, allowing the beams to have the same strength in both directions. Shear reinforcement consisting of 3 stirrups spaced at 14 in (355.6 mm) on center were used in the boundary elements. 3.3. Phase I test set-up. Phase I tests were performed in the high-bay structural engineering laboratory at UMR. A strong wall was used as a reaction surface to load the URM walls. Concrete block was used to fill the void between the strong wall and the test location. RC beams, 12 in. (305 mm) square, were used as boundary elements on the top and bottom of the walls. The boundary members were post-tensioned to the strong floor as illustrated in Figure 3. The top member was also laterally restrained to the strong wall to limit translation at the top boundary. An air bag was placed between the test specimen and the concrete block fill to act as the loading mechanism. The bag was inflated incrementally and the pressure measured and recorded. As the bag inflated to a nominal level to bear against the wall, a strip along the edges of the wall was left unloaded. Dial gauges were used to measure the out-of-plane deflection (see Figure 3) and strain gauges were placed on the FRP to monitor the strain at each load increment where cracks were expected to form in the wall under out-of-plane load. 3.4 Phase II test set-up. The blast testing of the walls in Phase II took place at the United States Army Base at Fort Leonard Wood (FLW) near St. Roberts, Missouri. The tests were conducted on a certified military explosives range. The infill walls also had boundary members (concrete beam/footing) on the top and bottom, respectively of the wall. A structural steel frame was designed to withstand the blast loading and support the boundary members and was anchored to the footings. The structural steel frame composed of 6 in 6 in 3/8 in (152 mm 152 mm 9.5 mm) tube sections and miscellaneous steel plates and angles is shown in Figure 4 (cited in Tumialan, 2005, p. 59). As the results indicate, strengthening the walls with FRP materials does in fact increase the wall's resistance to out-of-plane loads. Furthermore, the anchorage details allow for the development of continuity between the FRP and the concrete boundary elements. This can be seen by comparing the results of Walls 7 and 8 to Walls 3 through 6 as illustrated in Figure 5. Walls 3 through 6 investigate the condition in which the reinforcement is anchored to the boundary members. In Walls 7 and 8, the same reinforcement is used without the anchorage. For the case of the GFRP sheets, the unanchored condition provides a capacity between the unreinforced case and the anchored case. Some benefit can be obtained for walls with arching action just by applying the laminates to the walls. When anchorage of the sheets is provided, this research suggests additional capacity is gained. This is not true in the case of the NSM rods. When the NSM rods are installed without anchoring them to the boundary, they behave in much the same way as an unreinforced wall. When anchorage is provided, continuity is developed and additional capacity is obtained. To verify the performance of the strengthening systems tested in the lab under blast loads, field blast tests were conducted on two walls. One of the walls was strengthened with GFRP laminates unanchored to the boundary elements, while the other made use of the same reinforcement but included the anchorage detail. Four damage levels have been established to categorize the damage caused by a blast load to test walls (Hamoush et al. 2001, p. 89). Table 5 summarizes the blast loadings undertaken by the walls in this phase, as well as the level of damage the wall sustained under each loading. Wall 2 - Wall 2 was strengthened with 2.5 in (63.5 mm) GFRP laminates that were anchored to the boundary members. This wall survived the first blast of 3 lb (1.4 kg) with light damage consisting of minimal cracking in some of the mortar joints. The following blast event induced heavy damage but did not result in failure of the wall system. Failure occurred when subjected to a 5 lb (2.3 kg) charge. Lateral rotation occurred with the loss of one of block form the wall, however, the anchorage details remained intact. Propagating cracks near the midspan mortar joint indicate that the wall system was approaching the onset of delamination. The end stack of blocks comprising the wall began to rotate. It may be noted that in a continuous full scale wall, this rotation would not have happened due to the fact that the column of blocks would have either been supported by a vertical boundary element or bonded to the next column of blocks. There would not have been an end free to rotate as did the wall in this test program. Despite the rotation, the anchorage detail remained in tack, suggesting that addition capacity could have been obtained had the premature failure not occurred. Even with the rotation, the anchorage clearly provided an increase in capacity over the unanchored wall. The development of continuity between the FRP strengthening material and the surrounding boundary elements is key to increasing a walls out-of-plane strength and blast resistance for walls of similar slenderness ratios with arching action. Obviously this approach induces additional demands on the reinforced concrete frame and demands special attention to examine the overall system behavior of the framing system. CONCLUSION. This research program has demonstrated that FRP composites offer perfect benefits for the strengthening of masonry walls to resist blast loads. FRP systems have been proven to increase the out-of-plane flexure capacity of URM elements to resist a higher level of blast threat levels. However, the study has highlighted the associated need to address proper shear capacity requirements and wall to frame connections. The strengthened wall's shear capacity was the controlling failure mode. It has also been shown that failure of the FRP strengthened walls without side boundary restrictions failed in a safe manner controlling their debris. Shear damage or failure instable manner gave an indication for not surviving additional blast loads. If additional blast loads were applied, the retrofitted walls' collapsed towards the outside direction in contact debris that would help in reducing the hazard of causing possible harm and injury to building occupants while the un-reinforced walls failed in a sudden flexural manner towards the inside direction with scattered debris. References ACI Committee 440 (2001). Guide for the Design and Construction of Concrete Reinforced with FRP Bars (440.1R-01). American Concrete Institute, Farmington Hills, MI. ACI Committee 440 (2002). Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures (440.2R-02). American Concrete Institute, Farmington Hills, MI. Detailing of Facades. (2005). Architectural Science Review, 48(3), 279. Erman, E. (2004). Earthquake Failure of Reinforced Concrete Buildings: The Case of the 1999 Earthquakes in Turkey. Architectural Science Review, 47(1), 71. Hamoush, S. A., McGinley, M. W., Mlakar, P., Scott, D., and Murray, K. (2001). Out-of-Plane Strengthening of Masonry Walls with Reinforced Composites. Journal of Composites for Construction. American Society of Civil Engineers, Reston, VA. Hughes Brothers, Inc. (2001) Glass Fiber Reinforced Polymer (GFRP) Rebar, Aslan 100. Seward, NE. Pinder, A., & Pinder, A. (1990). Beazley's Design and Detail of the Space between Buildings. London: Spon Press. Stennott, R. S. (Ed.). (2004). Encyclopedia of 20th Century Architecture (Vol. 2). New York: Fitzroy Dearborn. Tumialan, J. G. (2005). Strengthening of Masonry Structures with FRP Composites. Doctoral Dissertation, Department of Civil Engineering, University of Missouri - Rolla, Rolla, MO. Figure 5 - Peak Out-Of-Plane Pressure Results for Phase I Test Walls at Failure (All figures and tables used in our paper were taken form the experiments held by J.J. Myers and P. Carney (cited in Tumialan, 2005)) Table 5 Summery of blast events and levels of failure Read More