International Journal of Civil Engineering and Technology (IJCIET), ISSN (Print), INTERNATIONAL JOURNAL OF CIVIL ENGINEERING

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1 INTERNATIONAL JOURNAL OF CIVIL ENGINEERING AND TECHNOLOGY (IJCIET) ISSN (Print) ISSN (Online) Volume 5, Issue 8, August (2014), pp IAEME: Journal Impact Factor (2014): (Calculated by GISI) IJCIET IAEME EXPERIMENTAL STUDY ON RETROFITTING OF SQUARE RC SHORT COLUMN SUBJECTED TO CONCENTRIC AXIAL LOADING BY JACKETING 1 Bishnu Gupt Gautam, 1 M.Lakshmipathy, 2 G.Senthil Kumaran* 1 Department of Civil Engineering, SRM University, Chennai, India 2 Department of Civil Engineering & Environmental Technology, University of Rwanda, Kigali, Rwanda ABSTRACT Strengthening of existing reinforced columns in old structures is becoming a major issue during repair and maintenance. Reinforced columns in a structure get distressed due to various reasons like aging, deterioration of materials, corrosion of steel and abnormal loading conditions like earthquake, fire, higher wind loads, shock loads and blasts, etc. Retrofitting of existing buildings to meet safety requirements in seismic areas where older constructions were not designed for earthquake actions. Other typical application of strengthening techniques can be found where the bearing capacity has to be increased because of the change in the use of structure, retrofitting reduces further destruction of historical structures, industrial structures, urban transport structures, marine structures and earth retaining structures. There are different methods for strengthening of existing structures. Jacketing is one of the most popular method for columns. Steel jacket, reinforced concrete jacket, fiber reinforced polymer composite jacket, etc are most common examples of jacketing. Strengthening of concrete member with externally applied steel and Ferro cement is an accepted option for repair and rehabilitation of structures, which will improve the load carrying capacity together with ductile characteristics, required in the case of seismic retrofit. In this study, two methods of retrofitting of short reinforced concrete square columns were attempted. Wire mesh mortar jacketing (WMM) and Steel Cage Mortar jacketing (SCM) were given to the reinforced concrete column. For analysis of test results, a plain reinforced column (CS) was tested. A total of 9 column specimens were tested after 28 days and results were analysed. The design and the testing was done as per the Indian Standard. Results were analysed for SCM and WMM. Theoretical capacities of columns were calculated and compared. The stiffness variation for various columns are presented. The energy absorption was obtained from load versus axial shortening graphs. 140

2 From the result analysis, it was observed that when compared to control specimen CS, WMM was 1.75 times greater, SCM was 2.28 times greater. The stiffness of WMM was the highest value initially, 1.2 times higher that CS. The energy absorption was 5.01 times higher for WMM than CS. Finally, from the strength consideration SCM is preferred. From the stiffness and energy absorption consideration WMM is preferred. Keywords: Concrete-Restoration-Steel Jacket. INTRODUCTION Various seismic retrofitting techniques are available depending on the type and condition of the structure. Selection of the most appropriate method(s) and material(s) is based on results of structural assessment and detailed structural analysis. The retrofitting of column is one of the methods to enhance the characteristics of its behaviour. There are different methods of jacketing used for the strengthening of column, some are FRP composite jacketing, steel plate jacketing, spiral rebar jacketing, welded wire fabric, tubed RC column, etc. These methods were tried in both in buildings and bridges. Kenji Sakino et.al [1] conducted research on square RC column with steel jacketing and reported that the ultimate bending strength of the retrofitted RC column can be accurately evaluated by using ultimate strain, εcm and the stress block parameters α and β. The shear strength of the retrofitted RC columns can be evaluated by Aakawa s equation and the design formula for evaluating the limit rotation angle, R u, defined and detailed, predicted the experimental deformation capacity of the retrofitted RC column with a reasonable accuracy. Stephen Pessiki et.al [2] studied the behaviour of circular and square RC column jacketing with FRP and reported that improvements in the axial load-carrying and deformation capacities of FRP jacketed concrete members over unjacketed concrete members and the factors influencing the axial stress-strain behaviour of FRP confined concretes are identified. Halil Sezen et.al [3,6] performed tests on circular column using concrete jackets reinforced with spiral rebar, welded wire fabric and a new reinforcement termed as PCS under different axial load conditions, concluded that the behaviour of specimens with spiral rebar and PCS reinforced concrete jackets were somewhat similar, however a large variation in the postcracking behaviour of concrete jackets with spiral rebar was observed. Sittichai Seangatith et.al [4] studied the behaviour of square steel section RC column subjected to concentrically axial load applied directly onto the RC core. The compressive strength of concrete and the wall thickness of the steel tube are the major factors influencing the behaviour with respect to axial compressive capacity and modes of failure. Pedro A.Calderon et.al [5] conducted tests on axially loaded RC column strengthened by steel caging, is being easy to apply and relatively inexpensive and concluded by suggesting a design proposal for these columns and verified by experimental and numerical studies. From the above literature review, various methods were adopted for strengthening an RC column. These methods were tried in both buildings and bridges. Still there is a need for further data and investigation on RC column retrofitted with steel jacketing and concrete jacketing at an affordable cost. The proposed experimental programmes is to study the behaviour of Reinforced Concrete Columns retrofitted with steel caging and ferrocement laminates tested to failure to find out the load carrying capacity. EXPERIMENTAL PROGRAMME The test programme included both aspects i.e., the properties of constituent materials namely cement, sand, coarse aggregate and steel bars as per relevant Indian Standard Specification and the behaviour of retrofitted column. The size of the columns are 110mmx110mmx510mm using M

3 grade Concrete, 4 numbers of 8 mm dia longitudinal bars with 6mm dia ties at 100mm c/c. The test specimens were designated as Control Specimen (CS), Wire Mesh Mortar (WMM) and Steel Cage Mortar (SCM). Three samples were cast for each type of specimen. Table.1 shows the properties of constituent materials used in the preparation of concrete. Table 1: Properties Materials Sl.No Description of Test Test Results Standard Value Reference Code 1 Specific Gravity of Cement IS: Normal Consistency of Cement 31% Not less than 30% IS: Initial Setting Time of Cement 54 mins Greater than 30 mins IS: Fineness of Cement 6% Specific Gravity of Fine Aggregate IS: Fineness Modulus Specific Gravity of Coarse Aggregate IS: The average compressive strength of cement cubes for 3 days and 7 days are N/mm 2 and 37.1 N/mm 2. The mix design of M 20 grade concrete by weight [7] is arrived as 1:1.44:2.5:0.5 as Cement, FA, CA and water respectively. 100mm cubes were used to test the compressive strength of concrete mix for 7 days and 28 days as 14 N/mm 2 and 21N/mm 2 respectively. The steel angle section of ISA 30mm x 30mm x 3mm and steel plate of 30mm wide and 3mm thick were chosen for jacketing. The steel cage has been made by using the angled sections in all four corners together with steel plates which were welded with angle section each at top, middle and bottom as shown in Fig.1 and designated ad SCM. The gap between the test specimen and the steel cage was filled with 1:3 cement mortar with w/c Now after wrapping the specimen were cured for 28 days. Fig.1 Specimen made of Steel Cage Fig.2 Specimen made of Wire Mesh Mortar The column with wire mesh mortar (WMM) specimen were made by M 20 grade concrete with 4 numbers of 8mm dia. Longitudinal bars and 6 mm dia. Ties at 100mm c/c. Wire mesh was wrapped in single layer of all four sides. The steel wire mesh of 1 mm diameter and 10mm x 10mm square opening used for jacketing. Both sides of wire mesh 5mm thickness of 1:3 cement mortar 142

4 with w/c of 0.35 were applied and equally done for all four sides as shown in Fig.2. Now after wrapping the specimen were cured for 28 days. TESTING PROGRAMME All the test specimens were placed in water for curing of 28 days. Once removed from water, all the specimens were air dried before the testing. The dimensions of each specimen shall be measured before testing. Preparation of the surface is done using Plaster of Paris (POP) and were white washed an hour before testing to note down the cracking pattern. A 100T SERVO Computerised Universal Testing Machine was used with a rate of loading as 5mm per minute. Before placing of specimen in the testing machine, the Demec points along the length of the specimens at the centre was fixed. The axis of the specimen shall be carefully aligned with the axis of the loading device. During the observation, in all the three specimens, columns were shortened due to axial compressive load and vertical cracks were developed and this was recorded upto maximum load. The load versus shortening and the load versus strain is plotted using Demec Gauge for CS, WMM and SCM as shown in Figs.3 to 8. Fig.3 Load vs Axial shortening of CS Fig.4 Load vs Axial shortening of WMM Fig.5 Load vs Axial shortening of SCM Fig.6 Load vs Strain of CS Fig.7 Load vs Strain of WMM Fig.8 Load vs Strain of SCM 143

5 ANALYSIS OF RESULTS AND DISCUSSION Ultimate Load The experimental load carrying capacity of the column with concrete of CS, WMM and SCM were compared with theoretical values determined from Strength of Materials approach and by using IS: [8] design method. The following formulas were used to calculate the ultimate load by Strength of Material and IS: approaches: Strength of materials approach, P u (str) =f ck. A c + f y. A s IS: design code approach, P u (IS) = 0.46f ck. A c f y. A s Where f ck is the characteristic compressive strength of concrete, fy is yield stress of the grade steel, A c is the area of concrete and A s is the area of steel bars. Fig.9 and Table.2 shows the comparison of ultimate loads for CS, WMM and SCM under different approaches. Fig. 9: Comparison of Ultimate Loads Group CS WMM SCM Specimen ID CS CS CS WMM WMM WMM SCM SCM SCM Table 2: Comparison of Ultimate Loads Average of Ultimate Observed Observed load by Experimental Experimental Strength of Ultimate Load Ultimate Material (kn) P u(exp) Load (kn) Approach P u(exp) (kn) P u(str) Ultimate load by IS code Approach (kn) P u(is) The observed average ultimate load for column CS is the lowest and equal to kN; for the column WMM is 580 kn which is 1.75 times greater than CS; for the column SCM is 760 kn 144

6 International Journal of Civil Engineering and Technology (IJCIET), ISSN (Print), ISSN (Online), Volume 5, Issue 8, August (2014), pp IAEME which is 2.28 times greater than CS. This shows that the column with steel cage retrofitting is better than that of the column with wire mesh jacketing. The strength of Material approach gives ultimate load for CS as kn which is 1.24 times greater than observed load. The IS code method predicts the load for CS as kn which is 0.58 times lesser than the observed load. Therefore the IS code is conservative. The strength of material approach gives ultimate load for WMM as kn which is 0.95 times lesser than the observed load. The IS code method predicts the load for WMM as kn which is 0.43 times lesser than the observed load. Therefore the IS code is conservative. The strength of material approach gives ultimate load for SCM as kn which is 0.77 times lesser than the observed load. The IS code method predicts the load for SCM as kn which is 0.40 times lesser than the observed load. Therefore the IS code is conservative. Stiffness From the load versus axial shortening graphs for various specimens, the stiffness versus load ratio and the energy absorbed were obtained and these were compared with control specimens. Stiffness was calculated as µ= P where P= load and = axial shortening. Using this equation, the stiffness of CS, WMM and SCM were calculated and compared as shown in Fig.10. Fig.10: Stiffness vs Load ratio From Fig.6, it was inferred that the stiffness of the columns of CS was 73k N/mm, WMM was kn/mm and SCM was kn/mm. This reveals that for stiffness purposes, WMM, i.e column with Wire Mesh Mortar jacketing is to be preferred. Energy Absorption: The energy absorption was calculated as the area under the load versuss deformation graphs as in Fig.11. Fig.11: Average Energy absorption of Columns 145

7 The average energy absorption of columns CS, WWM and SCM were shown in Fig.11 and its performance with CS were given in Table 3. Table 3: Energy Absorption of Column Specimens Sl. Energy Absorption Ratio with Column ID No in kn.mm (U) respect to CS 1 CS WMM SCM With reference to Fig.11 and Table 3, it was seen that the energy absorption for column CS is kn.mm comparing to this with column WMM was 4568 kn.mm which was 5.01 times higher than CS. The SCM column showed the energy absorption as kn.mm which is 4.56 times higher than CS. From the observation it is inferred that WMM withstands higher energy. From energy absorption purposes, WMM column may be preferred. CONCLUSIONS 1. The observed ultimate load for various columns were compared and it was found that the column WMM was 1.75 greater than CS and that of SCM was 2.28 times greater than the column CS. 2. The Indian Standard (IS) code is conservative and it gives lower value for prediction with respect to WMM and SCM. 3. The strength of materials approach gives higher value for SCM and lower value for CS. 4. The initial stiffness of the SCM column was kn/mm 5. The initial stiffness of the WMM column was kn/mm 6. The initial stiffness of the CS column was 73 kn/mm 7. The energy absorption of the column WMM was the highest and equal to 4568 kn.mm which was 5.01 times higher than the column CS 8. From the energy absorption consideration column WMM is preferable. 9. From the strength consideration, column with steel cage jacketing (SCM) is preferred. FURTHER RESEARCH By increasing the number of layers wrapping, the compressive strength of the column will increase; this may be explored. The FRP jacketing is another option for increasing the compressive strength. It is having the main advantage that it occupies less volume and space. Also it increases the load carrying capacity. Exploring the energy absorption, stiffness, ductility and buckling effects can be determined. REFERENCES [1] Kenji Sakino and Yuping Sun (2000), Steel Jacketing for improvement of column strength and ductility, 12 WCEE, New Zealand, pp 2525 [2] Stephen Pessiki, Kent A.Harries, Justin T.Kestner, Richard Sause and James M.Ricles (2001), Axial behaviour of reinforced concrete columns confined with FRP jackets, Journal of Composites for Construction, Vol.5,No.4,pp [3] Halil Sezen and Eric Miller (2007), Retrofit of Circular Reinforced Concrete Columns using FRP, Steel and Concrete Jackets, Ohio State University, Columbus, Ohio, ASCE pp

8 [4] Sittichai Seangatith and Jaksada Thumrongvut (2009), Experimental Investigation on square steel tubed RC columns under axial compression, Journal of Science and Technology, July- September, Vol.16, No.3, pp [5] Pedro A.Calderon, Jose M.Adam, Salvador Ivorra, Franscisco J.Pallares and Ester Gimenez (2009), Design Strength of axially loaded RC columns strengthened by steel caging, Materials and Design, Vol 30, pp: [6] Halil Sezen and Eric A.Miller (2011), Experimental Evaluation of axial behaviour of strengthened circular Reinforced Concrete Columns, Journal of Bridge Engineering, ASCE, vol.16, No.2, March 1, pp [7] IS:10262 (1982), Recommended Guidelines for Concrete Mix Design, Bureau of Indian Standards, New Delhi. [8] IS: 456 (2000), Plain and Reinforced Concrete code of Practice, Bureau of Indian Standards, New Delhi. Readings [9] Miller AE (2006), Experimental Research of Reinforced Concrete Column Retrofit Methods, Master Thesis, Ohio State University. [10] Neelam Sharma (2010), RCC Design and Drawing, S.K.Kataria and Sons Publisher, New Delhi. 147