J. of Architecture and Urban Design, Kyushu University, No.22, pp.145~ 153, July. 2012. Strength and Ductility of Masonry Prism and Triplet Retrofitted with.
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J. of Architecture and Urban Design, Kyushu University, No.22, pp.145~ 153, July. 2012
Strength and Ductility of Masonry Prism and Triplet Retrofitted with Engineered Cementitious Composites ~?.::
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Gholamreza ZAMANI AHARI *, Kentaro YAMAGUCHI**, Masakatsu MIY AJIMA *** and Hiromi NISHIYAMA ****
Unreinforced masonry (URM) buildings were not designed to resist earthquake loads resulting in extensive collapses and should be considered for seismic retrofitting. Engineered cementitious composite (ECC) with multiple fine cracks is a cement"-based composite material which exhibits a strain - hardening tensile stress-strain . behavior. Improving low tensile strength, strain-softening and brittle behavior of URM walls with such a ductile strain-hardening material was the main motivation of this research work. Two series of tests such as compression and shear tests were conducted on the masonry specimens retrofitted with ECC overlay. It was found out that the shear strength and stiffness of the specimens increased considerably. Also this technique changed the brittle failure mode ofURM to a ductile one along with a high deformation capacity. Therefore ECC retrofitting can be considered as one ofthe suitable methods which improve ductility and in-plane strength of masonry wall.
Keywords: Unreinforced masonry, Seismic retrofitting, Engineered cementitious composites, Shear strength, Ductility ~*Hi5!1i*Hfff~,
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1. INTRODUCTION
Unreinforced masonry (URM) structures have shown low lateral resistance and ductility in earthquake events and should be seismically retrofitted. Currently, several methods are being applied for the seismic retrofitting of URM structures and some are under study process. The philosophies of these techniques are improvement of strength and deformability or the combination of both. Their approaches are mainly based on the failure mechanisms of URM structures. Unreinforced masonry walls are one of the most vulnerable parts of URM structures. Their weak seismic in-plane and out-of-plane response are known as the most important reason of the major URM damages and failures. Many researchers around world have conducted static and dynamic experiments to grasp the structural behavior of URM wall and examined some retrofitting methods to improve its seismic performance. The results of these studies have been reported frequently for some retrofitting techniques such as
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the use of fiber reinforced polymers (FRP). In spite of the improvements exhibited by· these methods, there are some limitations and disadvantages on the application of them. Engineered Cementitious Composite (ECC) - also refers to as High Performance Fiber Reinforced Cement Composite (HPFRCC), Strain-Hardening Cement-based Composite (SHCC) and bendable concrete - with multiple fme cracks is a cement-based composite material with a strain-hardening tensile behavior and an excellent capability to control the width of crack[!, 21. This composite material has shown a high strain capacity and can absorb and dissipate high amounts of energyC3l. Improving the low tensile strength, strain-softening and brittle behavior of URM walls by surface retrofitting with such a ductile strain-hardening material was the main motivation of this research work. The effect of ECC retrofitting on the structural performance of the existing URM walls should be studied through smallscale and full scale testing of the retrofitted wall specimens. Improvement in the basic behavioral characteristics of the retrofitted masonry can be evaluated through series of tests on small-size specimens retrofitted with ECC. Kyriakides and BillingtonC4J studied ECC retrofitting for concrete frame-infill masonry walls. They conducted a series of experiments in order to examine the impact of a
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thin layer of ECC applied to masonry infill wall as well
Table 1 ECC mortar mixture (for 1 liter mortar)
as when it is applied on a masonry wall bounded by a KJ(g) SP(g) UN(g) ECC(g) Water(g) AE(g) Sample 1.728 CECl-9, BECl-3 1600 331-2 20.02 10.02 6.016 non- ductile reinforced concrete frame. The study results CECPl-3, BECPl-3 20.02 1.728 1600 331.2 10.02 9.024 showed that ECC can help keep unreinforced masonry Table 2 ECC test sample properties walls in tact to large lateral drifts, adding significant ductility to the entire structural system under cyclic loading. Sample Test Dimension (mm) Age (days) CEC I, CEC2, CEC3 7 The effect of ECC mixture components on its retrofit 14 CEC4, CECS, CEC6 functionality for masonry walls was studied by Bruedem et Compression 50 x 100 28 CEC7, CEC8, CEC9 al. [5]. The test results showed that the shear load bearing 28 CECP1,CECP2,CECP3 capacity and the energy absorption capacity of masonry BEC 1, BEC2, BEC3 28 Bending 40 X40 X 160 28 BECP1,BECP2,BECP3 increased by the application of a thin ECC layer. Maalej et al. 161 studied the ECC retrofitting for URM walls Table 3 Mechanical properties ofECC mortar under impact loading. The quasi-static loading test results Sample Age (days) Property Amount showed that the ECC-strengthening system improves the 1.53 CECl-9 & BECl-3 3 Fresh Specific Weight (g/cm ) out-of-plane resistance of masonry walls significantly. 1.64 CECPl-3 & BECPl-3 130 CECl-9 & BECl-3 Moreover with the use of ECC overlay, fragmentations due Mortar Flow(mm) Fresh 134 CECPl-3 & BECPl-3 to impact were also reduced significantly. Average ofCEC7-9 12.07 28 Elastic Modulus (kN/mm2) Also Lin et al. Pl conducted some in-plane and out-of-plane 15.8 Average ofCECPI-2 tests on the ECC retrofitted masonry specimens· and Average ofCEC7-9 0.210 28 Poisson's Ratio 0.203 Average ofCECPl-2 examined a two story URM building shotcreted with ECC in Average ofCEC7-9 22.1 New Zealand. As a result of out-of-plane tests, an increase 28 Compressive Strength (N/mm2) 24.6 Average ofCECPl-2 in maximum load of 1.6 times the strength of the bare wall Average ofBECl-3 7.50 28 Flextiral Strength (N/mm2) 8.33 Average ofBECPl-3 was observed when ECC retrofitting was applied on the compression surface and an increase of 13 .2 times when it 2.1 Characterization of ECC was applied on the tension side. In this study only the properties of ECC mortar were In order to grasp the shear and compressive impact of ECC investigated based on the mixture plan provided by the retrofitting on URM walls, an experimental investigation producing company which is shown in Table 1. About the was conducted on the efficiency of this technique. components, ECC is a fiber cementitious powder and KJ, Experiment process and corresponding results have been UN, SP and AE are liquid additives. Information about the discussed in this paper. effect of ECC components on its overall performance could The structural performance of the masonry wall retrofitted be found on other research worksp,s, 91. with ECC was evaluated through a series of tests on small Some tests were conducted on ECC mortar to find the size URM specimens such as prism and-triplets. specific weight, mortar flow, compressive and flexural 2. MATERIAL AND SPECIMEN SPECIFICATIONS In order to understand the retrofit capability of ECC on URM structures, uniaxial compression and shear triplet tests were conducted on the wall specimens. The results of such an experimental study are necessary to predict the structural behavior of bare and retrofitted masonry wall and consequently the whole URM structure. Two types of specimens were constructed and tested such as unretrofitted (refers to as U specimens) and retrofitted (refers to as R specimens). Comparison between these two test result data was used to evaluate the retrofit efficiency. Basic material properties of masonry components and ECC mortar are necessary to grasp the behavior of the retrofitted masonry. Mechanical characteristics such as strength and stressstrain relationship of masonry unit bricks, bed joint mortar and ECC overlay mortar were obtained through testing as it is discussed in following parts.
strength, ultimate tensile strain, modulus of elasticity and Poisson's ratio. Due to the relatively low workability of ECC mortar, for retrofitting purpose the amount of super plasticizer (SP) in the mixture was increased up to 1.5 times (as shown in Tables 1and2 as CECPl-3 and BECPl-3 samples). Compression tests were conducted on the cylindrical samples ofECC mortar (samples shown in Table 2). Mechanical properties of ECC mortar and information about the tested samples are shown in Table 3. Compressive stress-strain diagram of ECC mortar (samples CEC7-9 & CECP 1-2) is shown in Figure 1. Also extreme fiber stressstrain diagram of it (samples BECl-3 & BECPl-3) is shown in Figure 2. The compressive strain range of ECC was close to one of concrete or mortar as shown in Figure 1. Also as it can be seen from Figure 2, strains more than 1% for the most of the test samples were recorded.
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In case of sample CECP3, because of non-uniform distribution of compressive loading as one of the main possible reasons, strain values obtained from two side strain gauges were so different and consequently its compressive stress-strain diagram was not included in Figure 1. The average flexural strength of samples BECl-3 and BECPI-3 is . indicated in Table 3. The flexural strain was shown until the decrease of strain, while stress increases in diagrams of Figure 2. Also the average compressive strength versus time diagram of the mortar is shown in Figure 3. Due to the existence of fiber in ECC mortar, a very slow failure mode along with a high ultimate tensile strain was observed. However it must be mentioned that the mortar mixing process and its workability and treatment method are different of usual cement mortars. So there is a need to special application system especially in case of shotcrete on the wall surface as it was reported by Lin et al. [7J. 30 ~
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(b) Unit brick series UBB
Figure 4 Masonry unit brick specimens
Table 6 Bed joint mortar test sample properties
Table 4 Properties of masonry unit brick series UBH and RBH Sample no. Age (days)
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Dimensions (mm)
Length
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99.06 98.42 99.69 112:84 120.23 114.7
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The lower bond of this deformation capacity for the retrofitted specimens with 10 and 20 mm thick ECC overlay was obtained as about 20 and 27 times of the unretrofitted ones. The position of the above mentioned 80% strength was shown as point marks in all diagrams of Figure 13. In some retrofitted specimens with ECC thickness of 20mm, detachment of ECC overlay from brick surface was observed. However in some of them, vertical tension cracks were observed in side bricks prior to the detachment and resulted in their splitting.
4.3 Prism tests
Figure 13 Shear stress-strain diagram of masonry triplet specimens aged 378 days
Failure mode in bare prism specimen was represented by vertical tensile crack_s parallel to the loading direction. They appeared mostly on the longer sides of prism such as shown in Figure 16(a). In case of the retrofitted specimens, due to the confining effect of ECC overlay, failure condition was similar to buckling behavior as shown in Figure 16(b). Moreover, it was observed that in case of ECC overlay of 20 mm thickness, detachment of ECC overlay from brick surface was started before the above mentioned buckling behavior.
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