Study on the Effect of Retrofitting of a Seismically Damaged Building with X-Plate Damper and Geopolymer Concrete

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1 Study on the Effect of Retrofitting of a Seismically Damaged Building with X-Plate Damper and Geopolymer Concrete Pinky Merin Philip¹, Dr. C. K Madheswaran², Prof. Eapen Skaria³ ¹M.Tech student, Saintgits College of Engineering, Kottayam, India ²Senior Principal Scientist, CSIR-Structural Engineering Research Centre, Chennai, India ³HOD, Department of Civil Engineering, Saintgits College of Engineering, Kottayam, India Abstract - Repair, rehabilitation and strengthening of structures have become a major part of construction activity in recent past. Retrofitting is a technique in which damaged structure is strengthened to original design requirements. The recent earthquake occurred in India during 2001 in Gujarat, damaged many buildings that were seismically deficient. So there is an urgent need for retrofitting of damaged and deficient structures. The seismic deficiency of a building can be local as well as global. Global deficiencies include plan irregularities and vertical irregularities. Since open ground storey buildings have stiffness and mass irregularity vertically, an open ground storey building has global deficiency. Local deficiencies are element deficiencies that lead to the failure of individual elements of the building such as crushing of columns, flexure and shear failure of beams, columns and shear walls. Geopolymer concrete (GPC) which is a green concrete that utilize industrial wastes such as Fly Ash and GGBS is a good alternative for concrete. In this paper the feasibility study geopolymer concrete for retrofitting local damages of a seismically damaged building is presented. Also many multi-storey buildings are soft storey buildings by providing open ground storey. The X-plate energy dissipater (XPD) connection at ground storey overcomes the global deficiency of the open storey. And the study of XPD for global retrofitting after local retrofitting is also presented. Natural frequency evaluation is a very good technique to detect the change in rigidity of a structure. Thus the natural frequencies of original soft storey building (building before seismic damage), locally retrofitted building (building after retrofitted with GPC and new infill masonry) and globally retrofitted building (building after connected with XPD in ground storey) is compared in this paper. Keywords- X-plate damper, global retrofitting, soft storey building, geopolymer concrete, local retrofitting. I. INTRODUCTION Portland cement production is under critical review due to high amount of carbon dioxide gas is released to the atmosphere and it is also one among the most energy intensive construction material. So retrofitting of existing damaged and deficient building using eco-friendly material which could promise higher structural performance than the original building is essential. 240 Many retrofitting methods such as epoxy injection, CFRP and GFRP wrappings, concrete and steel jacketing etc. are used currently for local retrofitting, but from several experiments there is a mismatch of their tensile strength, stiffness and thermal response with that of concrete structures being retrofitted. Geopolymer concrete (GPC) that utilize industrial wastes such as Fly Ash and GGBS is an eco-friendly upcoming construction material. Many research works are conducting so as to replace cement concrete with geopolymer concrete. But its structural performance, durability and thermal stability should be studied. Also many multi-storey buildings are soft storey buildings by providing open ground storey. Such open ground storey buildings are easily susceptible to earthquake because of its global deficiency i.e., its stiffness irregularity. The X-plate energy dissipater (XPD) connection at ground storey overcomes the global deficiency of the open storey. The building used for this experimental study is a half scale model building with 4.8m height with three stories each of 1.6m high. It was subjected to an earthquake of 7.99m/s² ground acceleration and the building got damaged due local failures and global deficiency. After retrofitting the building locallyi.e, removing damaged concrete replacing with geopolymer concrete. And again after global retrofitting the reaction mass shaker test was conducted. Natural frequency is directly proportional to stiffness of a structure. Thus the natural frequencies of the cases of building are compared i e., original soft storey building (building before seismic damage), locally retrofitted building (building after retrofitted with GPC and new infill masonry) and globally retrofitted building (building after connected with XPD in ground storey). II. RESEARCH SIGNIFICANCE One of the most applications of geopolymer concrete is in construction industry. However, the suitability of GPC to various structural components is to be established by large number of experimental studies.

2 Various experimental works has done to find out the suitability of geopolymer concrete to replace the Portland cement concrete and based on these results the geopolymer concrete application can create an environmental friendly construction industry. The application of geopolymer concrete as retrofitting material is studied in this paper. Also the suitability of X-shaped elasto-plastic passive energy dissipater as global retrofitting material is studied. III. BUILDING DETAILS The three dimensional reinforced concrete frame structure is having two-bays in X-direction, single bay in Y-direction with a total height of 4.8 m. A photographic view of the reinforced concrete frame before seismic damage is shown in Fig: 1. Each storey is 1.6 m high. The section size of beams and columns are 150mm 150mm, with a 100mm thick slab. The RC frame model is an open ground storey, with second and third floor levels filled fully with the brick infill of 70mm thick. After retrofitting the infill masonry is replaced with 115mm thick with 1450 x 450mm door opening for upper stories. The base of the three dimensional reinforced concrete RC frame model is having a raft foundation. The longitudinal beam and transverse beam reinforcement consist of two numbers of 16 mm diameter and two numbers of 10 mm diameter bars respectively. Columns are reinforced with four numbers of 12mm diameter bars. Lateral ties in the columns and beams are 6 mm diameter two legged stirrups at a spacing of 150 mm c/c. Materials used are M 25 grade OPC concrete and Fe 415 steel. Fig: 1 Original Building Before Seismic Damage IV. RETROFITTING Retrofitting means structural strengthening of a building to a pre-defined performance level. Retrofitting techniques can be classified as global retrofitting and local retrofitting. Local Retrofitting means maintenance of local deficiencies of building like crushing of columns, flexure and shear failure of beams, columns and shear walls, also rebuilding infill masonry. Global Retrofitting means maintenance of global deficiencies of building like plan irregularities and vertical irregularities. A. Local Retrofitting Using Geopolymer Concrete Geopolymer concrete is a non-portland cement binder based on alkaline activation of industrial fly ash and ground granulated blast furnace slag (GGBS). Setting mechanism depends on polymerization. The alkaline liquids consists sodium hydroxide (NaOH), sodium silicate (Na 2 SiO 3 ) and distilled water. The geopolymer concrete used for retrofitting was produced from grade 1(IS: 3812) fly ash with specific gravity 2.2 obtained from Ennore power plant and GGBS with specific gravity 2.9 obtained from Quality polytech, Mangalore. River sand with specific gravity 2.49 was used as fine aggregate and crushed granite stones of size 6mm was used as coarse aggregate. M40 grade mix is used such that binder: sand: coarse aggregate ratio is 1:1.31:2.16. The binder consists 25% fly ash and 75% GGBS. 5Molar ratio of sodium hydroxide alkaline activator is used.the geopolymer concrete is used for local repairing of beam-column and column p portions. The various steps involved in the local retrofitting of the building are: Removal of spelled concrete and damaged brick masonry. Retrofitting the damaged portion of columns with geopolymer concrete (GPC). Air curing at room temperature. Construction of brick masonry walls with door opening. The failure crack pattern of column of ground story the seismically damaged building was mainly damaged severely. The damaged concrete at cracked portion was removed completely by chipping of concrete without causing any disturbance to the reinforcement using a chisel as shown in Fig: 2. After shuttering, the freshly prepared GPC using 5M alkaline solution was used for casting. After 24 hours the wooden shutters were demoulded, then finishing was done and kept for air curing. The brick masonry with a total thickness of 115mm using 1:4 PPC mortar mix was reconstructed with 1450 x 450mm door opening, instead of 80mm thick brick masonry without door opening as shown in Fig: 3. The compressive strength of brick infill masonry is 7.5 N/mm². 241

3 Fig: 2 Column Chipped at Cracked Portions for Local Retrofitting Fig: 3The X- Plate Damper Fig: 3 Retrofitted Building B. Global Retrofitting Using X Plate elasto-plastic Damper device XPDs are made of thin metallic plates that dissipate energy through their flexural yielding deformation. X-plate damper (XPD) is a metallic damper that is capable of sustaining many cycles of stable yielding deformation resulting in a high level of energy dissipation. XPD is also a hysteretic device because its energy dissipation depends primarily on relative displacement within the device and not on its relative velocities. XPD facilitates a constant strain over the height of device, thus ensuring that yielding occurs simultaneously and uniformly over full height of damper. The idea behind adding this device to structure is to enhance its energy dissipation capacity against environmental loads, providing an alternative to wind and conventional earthquake resistant design and retrofit. The properties like yield stress, elastic modulus, and hardening rate for the element material were evaluated as 235 MPa, 1.94 x 10^5 MPa and 5.00 x 10^3MPa, respectively. The XPD used for this experiment is shown in Fig:3. The thickness of X- plate is 12mm and total height of X plate is 80mm. In each bay of the ground storey 2 such X- plates is used so that the stiffness becomes equal to the brick masonry of upper floors only in one direction. The X plate is connected to the building using the angle bracings. The end conditions of XPD are both ends are fixed. The effectiveness of a XPD depends on the percentage of energy dissipated by the XPD. Based on the various study conducted, as the thickness of XPD increases the energy dissipation increases, also as the height decreases and width increases for an X plate the percentage of energy dissipation increases. Two XPD with thickness of 12mm, height 80mm with 6mm neck are used in one bay of the building. The X plate is connected to the building using angle bracings. Two equal angles of 50 x 50 x 6mm are connected back to back at 65 degree inclination with horizontal as shown in Fig: 4.The connections of the angles are provided using welding with a size of 6mm. The XPD is connected using 10mm size bolt. The ratio of ground storey stiffness to upper storey stiffness is 0.87, thus soft storey condition of the building is removed. 242

4 Fig: 4 XPD Connections in Ground Storey V. REACTION MASS SHAKER TEST In this experiment a three storey retrofitted building is excited using a reaction mass shaker to find out the fundamental natural frequencies and mode shapes. The RCC building specimen is retrofitted with geopolymer concrete (GPC) on ground storey columns and has door open infill in second and third stories, while the ground storey is stiffened with X plate dampers. The results of retrofitted building with and without X plate are compared. A. Instruments Used The instrumentation setup is shown in Fig: 5.The instruments used for free vibration test are: Reaction mass shaker Power amplifier Accelerometer Conditioning amplifier FFT analyzer Low distortion audio generator B. Test Procedure Fig: 5 Instrumentation Setup The audio generator and power amplifier are input instruments to the reaction shaker and the conditioning amplifier and FFT analyzer are output devices. The input frequency values from 1 to 25 were applied using audio generator and power amplifier and the corresponding acceleration of reference A accelerometer, B accelerometer for each floor, The vibration measurement was carried out using Accelerometer. The response in the form of acceleration was measured using accelerometer. Frequency response function (FRF) value and frequency response phase is obtained from FFT analyzer. Then corresponding to each fundamental natural frequency of building and mode shapes isplotted. 243

5 VI. TEST RESULTS A graph is plotted between FRF value and corresponding output frequencies from FFT analyzer. The frequency response function (FRF) is actually the ratio of output acceleration and input acceleration. The frequencies corresponding to peak acceleration is taken as natural frequencies of translational mode. A. Mode Shapes The mode shape of building for first mode is given in Fig: 6, based on the acceleration from B1, B2, B3 accelerometers. Similarly the second mode shape is plotted in Fig: 7. The results obtained by shake table test before damage are also compared with the experimental results as shown in Table: I Mode First mode, Second mode, Table: I Natural Frequencies Experimentally Original building before seismic damage Locally Retrofitted building with Geopolymer concrete Globally Retrofitted Building with XPD B. Mode Shapes of Building Experimentally Fig: 6 First Mode Shape VII. Fig: 7 Second Mode Shape FINITE ELEMENT METHOD The frequency of the building is directly proportional to the stiffness of the building; f =, so by using the frequency of the building the stiffness of the building can be compared. The natural frequency of the building at different modes is determined analytically by finite element method. The Finite Element analysis done for original building and for locally retrofitted building. The update version of finite element software Ansys is used to analyze the building before and after retrofitting. For modeling the beams and columns of the building 2 nodded beam 188 element is used and for brick masonry also 2 nodded beam element is used. The brick masonry is modeled as diagonal element having same thickness of brick masonry and width equals one third of its diagonal length. The 4 nodded shell 63 element is used for modelling slab, with 100mm thickness. In order to have good bond between infill and RCC frame, nodes are merged after meshing. The building is modeled with the same dimensions of the test specimen. Concrete generally exhibits large number of micro cracks, especially at the interface between coarse aggregate and mortar, even before it is subjected to any load. The input parameters in ANSYS for different materials are: 1) Portland pozzolana cement concrete: modulus of elasticity (E) = 25e9 N/m², Poisson s ratio (µ) = 0.2 Density (ρ) =2500 Kg/m² 244

6 2) Geopolymer concrete: modulus of elasticity (E) = 27.9e9 N/m², Poisson s ratio (µ) = 0.27 Density (ρ) = 2500 Kg/m² 3) Masonry infill: modulus of elasticity (E) =4.125e9N/m², Poisson s ratio (µ) = 0.17 Density (ρ) = 1800 Kg/m² To provide the raft foundation to the building, the bottom nodes of all ground storey columns was provided with fixed support i.e. degree of freedom restrained in all direction. The young s modulus value and Poisson s ratio for modeling the building was obtained from the experimental test. The natural frequencies of building before and after retrofitting were compared. The modeling of the non-retrofitted building with door opening infill in Ansys is shown in Fig: 9 and Fig: 11. Fig: 10 and Fig: 12show its translational mode shapes after modal analysis. A. Results The Finite Element analysis is done for original building, before seismic damaged and locally retrofitted building. The natural frequencies as in table II and modes shapes of the building are obtained. Fig: 10 Mode Shapes of Original Building from FEM Fig: 11 Finite Element Model of Locally Retrofitted Building Table: II Natural Frequencies Analytically Translational modes Original building Frequency Retrofitted building frequency First mode Second mode Third mode Fig: 9 Finite Element Model of Original Building Fig: 12 Mode Shapes of Locally Retrofitted Building from FEM VIII. CONCLUSIONS From the above experimental study we can conclude that: The Geopolymer concrete (GPC) can be easily replaced in the local cracks, since the geopolymer has good bond strength with PPC concrete, only 1 day air curing at room temperature and thermally stable. Since GPC is having similar stress-strain response and Poisson s ratio, there is match in tensile strength and stiffness of GPC and PPC concrete. 245

7 Since GPC replaces the cracked PPC there is no change in size and weight as in the case concrete jacketing. The building dimensions remain same even after retrofitting. GPC retrofitting cannot improve tensile strength as that provided by steel jacketing. And GPC have all the disadvantages of PPC concrete expect that it is environmental friendly. Practically for local retrofitting purposes, compared to jacketing methods and epoxy injection methods, GPC casting is easy, durable and structurally and aesthetically convenient. Natural frequencies of retrofitted and original open ground storey building are found to match experimentally well. Thus geopolymer concrete replaces the PPC. The comparison of reaction mass shaker test on locally retrofitted building and both locally as well as globally retrofitted building is carried out. From the test results it is found that the natural frequencies increased, when the building retrofitted globally. Thus from the experimental results, it is concluded that the X shaped elasto-plastic passive energy dissipater increases the global response of the building by increasing the natural frequency from 4 to 6. A sophisticated 3D finite element model of the open ground storey building is developed using ANSYS. The model has proved to be effective in terms of predicting the natural frequencies and mode shapes. Finite element analysis provides a better insight into realistic behaviour. Natural frequencies of retrofitted and original open storey building are found to be predicted exactly by finite element analysis. The experimental and analytical results shows good agreement in natural frequencies and mode shapes of original and locally retrofitted building. Thus geopolymer concrete can be effectively used for retrofitting local damages and XPD increases the global response of building under earthquakes. Acknowledgement This paper is being published with the permission of the Director, CSIR-SERC, Chennai. The cooperation and guidance received from the technical staff of Advanced Seismic Testing and Research Laboratory of CSIR-SERC are gratefully acknowledged. REFERENCES [1] Amarnath, C., Menon D., and Sengupta, A.K., Handbook on Seismic Retrofit of Buildings (2008), Narosa Publishing House Pvt.Ltd, New Delhi. [2] Bansal,P.P., Kumar, M, and Kaushik, S.K., Effect of type of wire mesh on strength of beams retrofitted using Ferrocement Laminates, National Building Materials and Construction, July 2008,pp [3] Costantino Menna et al, Use of geopolymers for composite external reinforcement of RC members, Composites: part B 45 (2013) [4] C.V.R Murthy, Diptesh Das, Brick masonry infills in seismic design of RC framed buildings: part 1- cost implications, The Indian Concrete Journal, july 2004, pg [5] F. Pacheco-Torgal, Z.Abdollahnejad, S.Miraldo, S.Baklouti, Y.Ding, An overview on the potential of geopolymers for concrete infrastructure rehabilitation, Construction and Building Materials 36 (2012) [6] H.R.Ronagh, A.Eslami, flexural retrofitting of RC buildings using GFRP/CFRP- A comparative study,composites:part-b, 46 (2013) [7] IS456(2000), Indian code of practice for plain and reinforced concrete, Bureau of Indian standards, NewDelhi. [8] K.Sathish Kumar, K. Muthumani, N. Gopalakrishnan and B. SivaramaSarma, Reduction of large seismic deformations using Elasto plastic Passive Energy Dissipaters, Defence Science Journal, Vol.53, No.1, January 2003, pp [9] K. Sathish Kumar, K.Muthumani, N.Gopalakrishnan, B.SivaramaSarm, G.R.Reddy and Y.M.Parulekar, Seismic response reduction of structures using elasto-plastic passive energy dissipation, ISET Journal of earthquake technology, paper No.421, Vol.39, No.3, September 2002, pp [10] Madheswaran C K, Ambily P S, Rajamane N P and Arun G, Studies on Flexural behavior of reinforced geopolymer concrete beams with lightweight aggregates, International Journal of Civil and Structural Engineering, vol:4, No:3(2014), ISSN [11] R.santhakumar, E.Chandrasekaran, R.Dhanaraj, Analysis of retrofitted reinforced concrete shear beams using carbon fiber composites, Electronic Journal of Structural Engineering, 4(2004). [12] Rangan. B. V, Hardjito, D, 2005, Development and properties of low calcium fly ash based geopolymer concrete. Research report GC-1, Faculty of Engineering, Curtin University of Technology, Perth, Australia. [13] Uma K, Anuradha R, Venkatasubramani R, Experimental Investigation and Analytical Modeling of Reinforced Geopolymer Concrete Beam, International Journal of Civil and Structural Engineering, vol 2, [14] Warner, R.F, B.V.Rangan, A.S Hall, K.A. Faulkes (1998), Concrete Structures, Melbourne, Addison Wesley Longman Australia Ltd. [15] YousefShirnejad, PanamZarfam, Reza Tajalifard, Improvement the Seismic Behaviour of Existing Steel Structures in Iran By TADAS Damper Modern Technology, 5 th Symposium on Advances in Science & Technology (2011). 246