Wrapping Perforated Brick Walls with Fiber-Reinforced Polymers

Repair and strengthening of existing reinforced concrete (RC) structures against earthquake effects have become important research topics in recent years. Major earthquakes have demonstrated that non-engineered RC structures are vulnerable to seismic effects and constitute a significant cause of loss of life. Over time, structural quality and functionality deteriorate. In addition to such deterioration, errors made during construction and design phases become the cause of potentially fatal disasters during earthquakes and make strengthening work necessary. Common characteristics of low-rise infill wall RC frame structures in Turkey are lack of engineering services, poor construction quality, and non-compliance with design codes. Some structures in such poor condition may require demolition and reconstruction. Many academic studies have proven that using carbon fiber reinforced polymers (CFRP) is an effective method instead of demolition and reconstruction. An application method for carbon fiber strengthening proposed by Özkaynak (2010) is shown in Figure 1. When infill walls of existing RC structures are wrapped using the proposed method, improvements in the structures' seismic performance and energy dissipation properties are achieved.

Figure 1. Use of fiber reinforced polymers in strengthening RC structures against earthquake effects [1]

The most important advantages of fiber reinforced polymers are: light weight, easy application, non-corrosive properties, and high strength. Observations made on existing structures following the 1999 Kocaeli earthquake showed that severe damage concentrated in column-beam joint regions, but most were saved from complete collapse due to the presence of infill walls. These observations demonstrate that infill walls are important elements that increase strength, stiffness, damping properties, and energy dissipation capacity in existing structures. At this point, the use of fiber reinforced polymers in strengthening existing structures is highly attractive because it can be applied without disturbing the people living in the building. Within the scope of earthquake engineering, reducing structural damage and increasing strength while imparting energy dissipation properties to structures are among the most fundamental elements. When infill walls are properly integrated into existing structures, significant improvements in energy dissipation properties can be achieved. During earthquakes, friction mechanisms created between structural elements and infill walls during both elastic and post-elastic behavior significantly increase the internal damping ratios of structures [2]. Past earthquakes have shown that when infill walls are arranged so as not to cause short column and torsion effects, they constitute an important source for structures' energy dissipation properties, strength, and stiffness [3]. In summary, when infill walls are well wrapped and integrated into RC frames and their out-of-plane movement is prevented, they emerge as non-structural elements providing significant advantages during earthquakes. A literature review presenting the effects of fiber reinforced polymers on infill wall behavior and mechanical properties and a portion of an experimental study conducted at Istanbul Technical University Building and Earthquake Engineering Laboratory have been presented here. Ghanem et al. (1993) tested 1/3 scale partially strengthened reinforced concrete shear walls under monotonic loads, taking into account the axial load parameter. The test results showed that the applied axial load was significantly effective on specimen failure mode, cracking load, and load-deformation relationship. Lafuente et al. (1998) conducted experimental and finite element-based analytical studies to investigate the seismic behavior of masonry walls. Within the scope of the experimental study, un-reinforced concrete blocks and structural masonry walls were tested under horizontal load. The experimental results highlighted that nonlinear material behavior cannot be exploited in earthquake-resistant structure design. Different alternatives need to be investigated to ensure ductile behavior. Alshebani and Sinha (1999) performed a series of experimental studies on 1/2 scale brick walls under uniaxial cyclic loading. Tests were conducted on numerous specimens in directions parallel and perpendicular to their axes. The test results showed that the failure load was two-thirds of the maximum allowable wall strength. The study concluded that the allowable stress level of the wall was significantly dependent on the nonlinear strain level. H.H. Knutsson and J. Nielsen (1995) developed a standard test method to determine the elastic modulus of walls and maximum strength under compression. The stress-strain relationship curve of the wall was determined using parabolic and logarithmic curve methods. John Jai et al. (2000) developed a mathematical model that could evaluate the effects of fiber polymer materials in the modeling of masonry structures. The developed model can predict in-plane shear behavior and maximum load-carrying capacity of infill walls strengthened with composite strips under static loads. Reinhorn et al. (1985) conducted experimental studies on a total of 23 masonry walls in two groups. According to the study results, test specimens strengthened with ferrocement panels exhibited excellent behavior in terms of strength and ductility. Santa-Maria et al. (2004) tested fiber polymer strengthened unreinforced masonry panels under monotonically increasing and cyclic displacement effects in the diagonal direction. Test results were evaluated in terms of parameters such as strength, stiffness, failure mechanism, and energy dissipation properties. The use of fiber polymers reduced cracks in walls and significantly increased shear strength. Vintzileou and Tassios (1995) tested strengthened test specimens obtained by injecting cement mortar into three-leaf stone masonry walls under shear and compression effects. It was concluded that cement mortar had a significant effect on homogenizing the stone wall structure and that mechanical properties improved accordingly. Sathiparan et al. (2009) demonstrated that polypropylene bands frequently used for packaging could be used for strengthening masonry walls. To observe the effect of polypropylene bands, they tested produced specimens under shear effects and out-of-plane loads. Test results showed that PP application increased the general stability and ductility properties of the specimen. From in-plane and out-of-plane tests, it was observed that specimens with PP mesh application achieved greater residual strengths after initial crack formation. Gurgain et al. (2007) developed an analytical model for simulating the behavior of wall elements to which PP application was applied. The analytical model includes aspect ratio effects and can enable damage distribution determination. Results obtained from numerical studies showed that PP band application improved wall behavior in a manner consistent with test results. Krevaikas and Triantafillou (2005) investigated increasing axial load-carrying capacity by wrapping fiber polymers onto masonry walls. Single-axis compression tests were performed on a total of 42 test specimens. In the experimental study, the effects of parameters such as number of layers in fiber polymer application, turning angle at corners, aspect ratios in sections, and fiber polymer type were examined. The study results showed that strengthened walls made using fiber polymers behaved very similarly to concrete behavior with fiber polymers. The wrapping application increased the load-carrying capacity and deformation properties of test specimens linearly. El-Dakhakhni et al. (2006) conducted numerous experimental studies on reinforced and unreinforced walls and steel frames with infill walls. Composite plates in different directions were used for strengthening purposes. During the tests, it was observed that out-of-plane deformations of strengthened specimens decreased significantly, and the applied strengthening method improved specimens' strength, stiffness, and post-peak behavior. Experimental results showed that composite plate applications on walls increased the deformation capability of test specimens and significantly limited damage. Farooq et al. (2006) tested masonry walls strengthened with steel plates in different configurations under axial load effects and shear effects. The spacing of steel plate placement was an important parameter of the test. It was observed that steel plates anchored to the specimen surfaces provided significant increases in compression and shear strength. Alcaino and Santa Maria (2008) conducted tests on 16 un-scaled masonry wall elements strengthened with fiber polymers. According to test results, fiber polymer application increased specimen strengths between 13% and 84%, and deformation capacity between 51% and 146%. As a general conclusion, it was found that fiber polymer applications increase structural energy dissipation properties. Experimental results also indicate that equivalent damping ratios can provide approximately 8% increase in equivalent damping ratio at a displacement level of approximately 0.3. Taghdi et al. (2000) tested reinforced and unreinforced concrete block elements to represent ductile and non-ductile masonry walls. Some specimens were strengthened with steel plates in vertical and horizontal directions. Plates were anchored with bolts and reinforcement plates at the base level. Test results showed that steel plate applications improved specimens' ductility and energy dissipation properties. Chang Wei et al. (2007) tested 3 masonry wall elements measuring 1.5×1.5m to determine ductility and horizontal load-carrying capacity. A total of three test specimens, two of which were strengthened with carbon fiber polymers, were tested statically under constant axial load and reversing cyclic horizontal displacement effects. Strengthening using fiber polymers provided significant improvements in specimens' ductility, horizontal load-carrying capacity, and energy dissipation properties. Elgawady et al. (2007) tested 1/2-scale test specimens before and after strengthening with fiber polymer application under constant vertical loads and cyclic horizontal displacement protocol. Specimens' axial stiffness and fiber polymer amount were among the experimental study parameters for determining horizontal load-carrying capacity, stiffness, and energy dissipation capacity. Excessive use of fiber polymers caused test specimens to exhibit brittle behavior. Study results showed that strengthened specimens possessed greater damping properties. A series of experimental studies were conducted at Istanbul Technical University Building and Earthquake Engineering Laboratory to determine the behavior of hollow brick walls strengthened using fiber polymers [2]. The study aimed to determine the mechanical properties of strengthened specimens. To achieve this goal, test specimens consisting of multiple brick elements with dimensions of 350×350×70 mm³ were produced. Hollow bricks have 40% voids and are used in creating partition walls in reinforced concrete frame openings. One of the fiber polymers applied to strengthen brick walls has a weight of 0.0015 g/cm², tensile strength of 3900 MPa, and elastic modulus of 230 GPa. Photographs taken during the application of fiber polymers to brick wall elements are given in Figure 2.

Figure 2. Fiber polymer application stages

The first group of test specimens from the experimental study consisted of the group in which shear tests were performed. In shear tests, the amount of fiber polymer formed the experimental study parameter. The test setup for shear tests and test specimens are given in Figure 3. Unstrengthened Specimen Discontinuous Strengthening Continuous Strengthening

Figure 3. First group of tests

Load-displacement relationships obtained from the first group of test specimens are given in Figure 4. Displacement values were obtained from displacement transducers placed horizontally and vertically on the specimen surfaces.

Figure 4. Load-displacement relationships from shear tests

The first group test results showed that fiber polymer application provided significant increases in shear strength, but fiber polymer density was not very effective in the rate of increase in shear strength. Test results showed that discontinuous wrapping of fiber polymers in particular provided significant increases in deformation capacity of infill wall elements, while continuous use of fiber polymers resulted in more brittle behavior. The second group of test specimens is shown in Figure 5. The purpose of tests in this series was to determine the effect of fiber polymer wrapping on specimen compressive strength. Loading was performed parallel to the holes in the brick elements.

Figure 5. Second group of tests

Relationships between compressive strength and vertical strain obtained from the second group of tests are given in Figure 6. Strains were derived from displacement transducers placed vertically and horizontally on wall elements. Fiber polymers are effective in increasing compression and shear strengths and strain capacity of walls under specific compression levels. It was experimentally determined that the increase in compressive strength was more effective when loading was parallel to brick holes.

Figure 6. Compressive strength and strain relationships from second group of tests

During the tests, some crushing was observed at the points where load was applied in both discontinuous and continuous fiber polymer wrapping applications. In general, significant performance improvements can be achieved when hollow brick walls are wrapped with fiber polymers. Experimental studies demonstrated that these types of strengthening methods make it possible to reduce crack formation occurring within infill walls and to achieve more ductile behavior, eliminating brittleness characteristics. In addition to improving overall structural performance, fiber polymers stand out as an economical and quickly applicable strengthening method due to the ease of application. When compared with conventional strengthening methods such as jacketing and shear wall addition, applications using fiber polymers significantly reduce costs.

Acknowledgments

This work was realized through the facilities provided by the Building Materials Laboratory and Building and Earthquake Engineering Laboratories at Istanbul Technical University, TUBITAK project no. 106M050, and BASF company. In this context, I would like to thank the related institutions, Prof. Dr. Ercan Yüksel, Prof. Dr. Alper İlki, and Dr. Medine İspir.   Associate Professor Hasan Özkaynak Beykent University Faculty of Engineering and Architecture Department of Civil Engineering    

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