Confinement of Hollow Brick Walls with CFRP Sheets

Strengthening and retrofitting of existing reinforced concrete (RC) structures against earthquake effects has come into focus during the last decade. Credible earthquakes have shown that non-engineered RC frames are particularly vulnerable to seismic action and are a major cause of loss of lives. Structures with varying quality and function all age and deteriorate over time. However, deterioration alone is not the only factor making retrofitting necessary. Errors during design and/or construction might be the main reasons for retrofitting in order to prevent fatal disasters during earthquakes. The general profile of low-rise structures in our country is mainly infilled RC frames. They lack not only an engineering process and construction quality but also compliance with design code requirements. Some of these structures might need reconstruction since they are in such poor condition. However, instead of reconstruction, carbon fiber reinforced polymers (CFRP) has proven through many academic studies to be an effective material for strengthening of RC structures. A retrofitting proposal by Ozkaynak (2010) with CFRP sheets is presented in Figure 1. In the case of confining infill walls by the application of diamond bracing type CFRP scheme, the earthquake performance and energy dissipation characteristics of existing RC structures could be considerably improved. Figure 1. Carbon fibre reinforced polymers (CFRP) scheme proposed by Ozkaynak (2010) The advantages of CFRP material are low weight, ease of application, non-corrosiveness and high strength. Direct observation of damaged structures following the Kocaeli, Turkey 1999 earthquake showed that main damage usually occurred within the vicinity of beam-column joints, but some did not collapse because they included infill walls. These observations on infilled RC structures prove that infill walls are essential in improving the stiffness, strength and damping properties as well as the energy dissipation capacities of structures provided their out-of-plane movement was restricted. Moreover, using CFRP for retrofitting is of interest since it is very easy to apply to infill walls without disturbing the occupants living inside the buildings. The most practical and efficient way to increase the strength level and decrease structural damage is to increase the internal energy dissipation properties of the global structure. Integration of infill walls into frame structures provides a great source of energy dissipation capacity. The friction occurring through the interface of infill walls and the structure ensures an increase in structural inherent damping properties both in linear and nonlinear regions, [2]. Past earthquakes and recent literature have proven that infill walls are a great source of stiffness, strength and energy dissipation unless they are located such that they are placed uniformly and do not cause any short column effects or torsional discontinuities, [3]. Consequently, it should be noted that if infill walls are very well integrated into existing frame structures and if their possible out-of-plane behavior was restricted, infill walls turn out to be the most essential non-structural components and have great advantages during earthquakes. Here, several academic studies are summarized, and in order to demonstrate the effects of CFRP sheets application on the mechanical characteristics of hollow brick wall mockups, some experimental results are presented. Ghanem et al., (1993) conducted a comprehensive test program including fourteen 1/3 scaled partially reinforced concrete masonry shear walls under in-plane monotonic lateral loads with and without axial effects. The test results showed that axial compression has a significant effect on failure mode, cracking load and load carrying capacity. Lafuente et al. (1998) investigated the seismic behavior of masonry walls experimentally and analytically. They performed four full-scale concrete block structural masonry walls against lateral loads. The analytical study was based on finite element methods. Two of the specimens were internally reinforced and the remaining two were built with confined masonry. Conclusions do not support the procedure of taking advantage of the inelastic behavior of the material for seismic resistant design. They suggested exploration of other alternatives to obtain more ductile mechanisms. Alshebani and Sinha, (1999) carried out a series of tests on half-scale sand plast brickwork panels under uniaxial cyclic loading. Forty-two square panels were tested until failure for two cases of loading: normal to bed joint and parallel to bed joint. It was concluded that the peak stress of the stability point curve can be regarded as the maximum permissible stress, which is almost equal to two-thirds of the failure stress. The permissible stress level depends on the plastic strain level present in the material due to cyclic loading. H.H. Knutsson and J. Nielsen (1995) developed a standard test method for the determination of the modulus of elasticity and ultimate strength for masonry in compression. The curved stress-strain diagram for the material is approximated both by parabola and logarithmic curve. John Jai et al. (2000) developed a mathematical model to take advantage of fiber-reinforced composite materials when used in retrofitting masonry buildings. The analytical model could predict the behavior of masonry walls reinforced with the composite layer subjected to static in-plane normal and shear loads. These models could provide guidance to designers on how different types of reinforcement could affect the load carrying capacity of a masonry wall. In the same context of the research study, one individual investigation was performed on determining the experimental behavior of reinforced masonry walls. Three types of test specimens were constructed: i) walls made of clay bricks and mortar and reinforced with chopped glass mat and epoxy resin, ii) walls made of wood bricks and reinforced with paper, and iii) beams made of wood bricks and reinforced with paper. There were good convergencies between experimental and analytical results. Reinhorn et al. (1985) carried out experimental research based on 23 masonry specimens in two stages. The first involved 10 pairs of thin ferrocement sheets which were used to define bonding, connector size and spacing. The remaining 13 brick specimens were subjected to diagonal split tests. The results demonstrated good performance for the strength and ductility of the ferrocement-brick composite. Santa-Maria et al. (2004) performed experiments on twenty-four unreinforced masonry (URM) panels reinforced with carbon fiber reinforced polymer (CFRP) laminates and sheets under diagonal compression effects. Panels with two configurations of reinforcement were subjected to monotonic and cyclic loading. The study reported results in terms of strength, failure mechanism, stiffness and energy dissipation. External CFRP application minimized crack thickness and improved shear strength and stiffness. Vintzileou and Tassios (1995) investigated the effects of injecting cement-based grouts in three-leaf stone masonry on wallettes subjected to vertical and diagonal compression. A drastic improvement of mechanical characteristics was observed due to the homogenization of the three-leaf masonry after grout injection. Sathiparan et al. (2009) proposed a retrofitting technique based on the use of polypropylene bands (PP bands), which are commonly utilized for packing. In order to evaluate the beneficial effects of the proposed PP band mesh retrofitting method, diagonal shear tests and out-of-plane tests were carried out on masonry wallettes with and without retrofitting. In diagonal shear tests, masonry wallettes were retrofitted with meshes whose borders were connected with either epoxy resin or simply by overlapping to evaluate whether the connection type influences retrofitting performance. From both test results, PP band retrofitted specimens resulted in larger residual strength after the first crack in both in-plane and out-of-plane loading. This retrofitting technique improved the overall stability and ductility of the structure. Gurgain et al. (2007) simulated brick masonry wall behavior. The effect of aspect ratio, which governs the failure pattern, was adopted into the analytical model. The simulation of PP-band retrofitting was done with simplified modeling of PP-band meshes. Comparison of behaviors between retrofitted and non-retrofitted cases numerically proved that PP-band application improved earthquake safety. Krevaikas and Triantafillou (2005) investigated the application of fiber-reinforced polymer (FRP) to increase the axial capacity of masonry through confinement. Four series of uniaxial compression tests, with a total of 42 specimens, were conducted on model masonry columns in terms of layer numbers, radius at the corners, cross-section aspect ratio, and types of fibers. It was concluded that FRP-confined masonry behaved similarly to FRP-confined concrete. Confinement increased the load bearing capacity and deformability of masonry almost in the elastic region. El-Dakhakhni et al. (2006) conducted several experiments in various directions to determine the behavior of reinforced masonry (RM) and unreinforced masonry (URM) wall elements and infilled steel frames. Experiments performed on approximately 24 specimens were tested under axial and shear effects. Composite laminate elements were used for retrofitting. During the experiments, out-of-plane movements were kept limited and significant improvements in terms of stiffness, strength and post-peak behavior were ensured by the application of composite laminate. The experiments resulted in composite laminates being effective in increasing deformation ability and limiting severe damage. Farooq et al. (2006) tested masonry walls reinforced with steel plates in different configurations under axial and shear effects in two different groups. The arrangement intervals of the steel plates were the parameters of the research. It was concluded that significant increases in compressive and shear strength could be achieved with steel plates anchored to the masonry wall surface. Alcaino and Santa Maria (2008) tested 16 full-scale masonry walls under shear effects. CFRP composites were applied to masonry walls in different directions and test results were evaluated in terms of strength, stiffness, ductility and energy consumption. Test results showed that strength could be increased by 13 to 84% and displacement capability could be increased by 51 to 146% by the use of strengthening. In conclusion, CFRP application improved the energy dissipation properties of the specimens. In the experimental study, equivalent damping rate was also determined. It was proposed that the average displacement of the peak displacement of 0.29% could be taken as 8% on average. Taghdi et al. (2000) constructed and tested four concrete block masonry wall elements and two reinforced concrete masonry wall elements to represent non-ductile masonry and wall concretes. One of each wall element is surrounded by steel plates in vertical or transverse direction. Anchor bolts and steel elements were used for connection to the foundation and the upper part of the element. According to test results, strengthening, ductility and energy consumption properties were increased for retrofitted specimens with steel elements. Chang Wei et al. (2007) constructed three masonry wall elements with dimensions of 1.5×1.5 m in order to determine the ductility and horizontal load carrying capacity of masonry walls. One of three specimens was not reinforced with CFRP and the other two were retrofitted and tested under the effects of constant vertical loads and lateral reversible static loads. Reinforcement techniques increased the ductility, horizontal load carrying capacity and energy dissipation characteristics of the wall elements. Elgawady et al. (2007) investigated the behavior of seven ½-scale masonry walls before and after CFRP application, under the effects of constant vertical loads and reversed cyclic horizontal displacement pattern. Axial stiffness and the amount of fiber polymer materials are important parameters in lateral load bearing capacity, stiffness and energy consumption. Using an over-dense amount of FRP causes the wall to exhibit very brittle behavior. It was found that the damping rates of retrofitted specimens were higher. A comprehensive experimental program was conducted in the Structural Earthquake Engineering Laboratory (STEELab) of Istanbul Technical University in Turkey, [21]. The main aim of the study was to determine the mechanical properties of hollow brick walls. In order to achieve this purpose, several wall mockups were produced with dimensions of 350×350×70 mm³. Hollow clay bricks, which had a void ratio of approximately 40%, were used to build the walls. The unit weight of CFRP is 0.0015 g/cm². The tensile strength is 3900 MPa and the modulus of elasticity of CFRP is 230 GPa. Fiber reinforced polymers were applied to the wall mockups. Some photos during the application of CFRP sheets on the wall mockups are presented in Figure 2. Surface Preparation Semi CFRP Application Full CFRP Application Figure 2. CFRP application stages The tests were performed in two groups in order to determine both shear and compressive strength. The amount of CFRP is the parameter in the first group experiments. The testing set-up and the specimens for the case of shear experiments can be seen in Figure 3. Bare Specimen Semi-CFRP Full CFRP Figure 3. First group specimens The force vs. displacement relations of compression tests derived from the first group mock-up tests are given in Figure 4. Displacement data were recorded from displacement transducers located in vertical and horizontal directions. Figure 4. Load vs. displacement relations derived from first group experiments The improvement in shear strength was outstanding and almost similar with both the strip type (Semi-CFRP) and full surface type (Full CFRP) retrofitting. It could be concluded that semi-CFRP confinement type of infill walls resulted in better shear deformation capacity than full CFRP application. The specimens from the second group experiments are given in Figure 5. The main aim of this group of experiments is to determine the effects of CFRP application on the compressive strength of the specimens. The load was applied in parallel direction to the brick holes. Bare Specimen Semi-CFRP Figure 5. Second group specimens The compressive stress vs. vertical strain relations derived from second group experiments are given in Figure 6. The strain values were extracted from displacement transducers which were vertically mounted on the wall mock-ups.   Figure 6. Compressive stress vs. vertical strain relations derived from second group experiments CFRP retrofitting provided great improvement in terms of compressive and shear strength and deformability of infill walls. There were a few local CFRP tears at the head joint of both types of retrofitted wallettes with some head mortar and brick crushing. Globally, similar damages were observed on all specimens. It can be concluded that by using CFRP sheets, great improvement of overall performance can be achieved. These retrofitting techniques can minimize crack widths and result in more ductile behavior. CFRP application could prevent brittle shear failures of infill walls. In practical terms, CFRP retrofitting techniques have advantages not only in the technical scope (strength, stiffness, energy dissipation) but also in labor and material costs. In this context, total cost could be considerably reduced when compared with other conventional techniques such as constructing shear RC walls and/or jacketing structural elements. Acknowledgments This study is based on test results conducted at the Material Laboratory and the Structural and Earthquake Engineering Laboratory of Istanbul Technical University. It was sponsored by research Project 106M050 of the Scientific and Technological Research Council of Turkey (TUBITAK) and by BASF. The author is grateful to the supporters and to Prof. Dr. Ercan Yüksel, Prof. Dr. Alper İlki and Dr. Medine İspir. Associate Professor Dr. Hasan Özkaynak - Beykent University Faculty of Engineering and Architecture Civil Engineering Department

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