BEHAVIOR OF CONCRETE COLUMNS REINFORCED AND CONFINED BY HIGH- STRENGTH STEEL BARS

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1 International Journal of Civil Engineering and Technology (IJCIET) Volume 9, Issue 7, July 2018, pp , Article ID: IJCIET_09_07_132 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed BEHAVIOR OF CONCRETE COLUMNS REINFORCED AND CONFINED BY HIGH- STRENGTH STEEL BARS Agustiar Department of Civil Engineering, Institut Teknologi Sepuluh Nopember (ITS), Surabaya, Indonesia; Department of Civil Engineering, Muhammadiyah University, Banda Aceh, Indonesia Tavio and I G. P. Raka Department of Civil Engineering, Institut Teknologi Sepuluh Nopember (ITS), Surabaya, Indonesia Retno Anggraini Department of Civil Engineering, Institut Teknologi Sepuluh Nopember (ITS), Surabaya, Indonesia; Department of Civil Engineering, Brawijaya University, Malang, Indonesia ABSTRACT With the rapid development of high-rise buildings and mega structures such as long span bridges and mega dams, etc. in recent years, the need of high strength structural materials becomes urgently required. For structural concrete, reinforcement from steel bars is a mandatory. Therefore, this paper investigate the effect of high-strength reinforcing steel bars in concrete columns. According to ACI 318M-14, the yield strength of steel bars used as longitudinal and shear reinforcement in structural concrete is limited up to 420 MPa, particularly for special seismic systems. Furthermore, the characteristics of high-strength steel bars particularly in terms of ductility (elongation) is one of the main issues. There is an assumption that the higher the strength of the material, the more brittle it is. Thus, high-strength steel bars with the grade of 550 MPa (above 420 MPa) were used as both longitudinal and transverse steels to study their effects on column capacity and ductility, respectively. The results show that high-strength steel bars are promising to be introduced for structural members, particularly concrete columns. Key words: Columns, Concrete, Confinement, High-Strength Steel Bars and Seismic- Resistant Concrete Structures editor@iaeme.com

2 Agustiar, Tavio, Raka, I G. P. and Anggraini R Cite this Article: Agustiar, Tavio, Raka, I G. P. and Anggraini, R. Behavior of Concrete Columns Reinforced and Confined by High-Strength Steel Bars. International Journal of Civil Engineering and Technology, 9(7), 2018, pp INTRODUCTION With the rapid increasing of population around the world and the limited area for buildings and infrastructures, the need of high-rise buildings and mega structures have been developing massively in the past decades. The increase in building height has caused the load carried by the structural members also increases significantly. This in turn causes the stresses in the structural members also increase considerably. Recently, the use of concrete as structural material has been increasing popular [1, 2]. Thus, in concrete structures, this issue leads to the much more steel bars required as reinforcement in the structural concrete members. The more reinforcement required to be placed in concrete, the more congested the concrete members, particularly columns. The congestion of reinforcement in concrete will cause the poor concrete quality. Columns are very important structural members that are very vulnerable to the brittle failure since they carry high axial compressive load and more difficult to meet their ductility requirement compared to the beams. When lower grade of steel bars is used as longitudinal reinforcement in concrete columns of high-rise buildings or mega structures, the number of bars required could be extremely high. One of the methods which have been used globally to improve the ductility of concrete columns is by introducing the confinement. Confinement is required particularly for concrete structures in highly seismic regions. The severe the earthquake in the region, the more confinement is required. When very high confinement is required accumulated with very high longitudinal bars and further lower grade of bars used, the congestion of reinforcement in concrete could occurred. This condition can cause the problem in the compacting during concreting process. If this issue is not considered, it can cause honeycombing in concrete, thus produce poor quality of concrete at the end. This will certainly fail the columns in achieving its designed capacity in carrying the designed load, and further resulting in vulnerable structures to brittle collapse due to severe earthquake. Thus, to solve this issue, the use of high-strength steel bars in concrete columns is investigated due to the stereotype that higher grade materials are always more brittle than lower grade materials such as steel bars. ACI 318M-14 [3] limits the strength of steel bars used in structural concrete in special seismic systems. For flexure, axial force, and shrinkage and temperature, the maximum value of fy for design calculations is 420 MPa. For shear, it is also limited up to 420 MPa, whereas for lateral support for longitudinal bars or concrete confinement, the value of fyt (transverse reinforcement) could be up to 700 MPa [3]. The use of higher grade steel bars will have different impact compared to the lower grade ones in concrete columns that needs to be investigated, particularly to the brittleness issue. The impact becomes more crucial when used for special seismic systems in highly seismic regions. The capacity of concrete column in carrying the load and deformation can be increased by providing confinement besides the longitudinal bars. Several studies have been carried out earlier on various columns, beams, and beam-column joints of concrete structures on the effects of longitudinal bars and confinement [4 29]. The effect of confinement in concrete can be improved if: (1) the transverse reinforcement is placed at relatively close spacing; (2) additional supplementary overlapping hoops or cross ties with several legs crossing the section are included; (3) the longitudinal bars are well distributed around the perimeter; (4) the volume of transverse reinforcement to the volume of the concrete core or the yield strength of the editor@iaeme.com

3 Behavior of Concrete Columns Reinforced and Confined by High-Strength Steel Bars transverse reinforcement is increased; and (5) spirals or circular hoops are used instead of rectangular hoops and supplementary cross ties [4 26]. Cusson and Paultre [6] have tested twenty seven square concrete columns to investigate the behavior of large-scale high-strength concrete columns confined by rectangular ties under axial concentric compressive loading. The columns had a cross-sectional dimension of 235 mm and a height of 1400 mm. The test parameters included concrete strength, the yield strength of transverse reinforcement, the distribution of longitudinal reinforcement, the amount and spacing of transverse steel, longitudinal reinforcement ratio, and concrete cover. The concrete strength varied between 53 and 116 MPa. Sheikh and Uzumeri [21] have also tested a total of twenty-four large-size square concrete columns to investigate the rectilinear confinement. It was reported that a reinforcing cage, which was closely placed in both longitudinal and lateral directions, improved the efficiency of confinement. For the conditions of the same number of reinforcements used in concrete columns where a good longitudinal reinforcement distribution and small spacing of bars will increase the strength and ductility of the concrete. 2. EXPERIMENTAL PROGRAM 2.1. Concrete The concrete used to cast the column specimens was designed as normal-strength concrete. The average compressive strength (fc ) was found to be 28.5 MPa at 28 days of age after continuous wet curing process Reinforcement An increase in the strength of steel bars is associated with an increase in the strain of steel bars at yielding, and further, often with a reduction in the fracture elongation, the tensile-to-yield strength ratio, and the length of the yield plateau. To study the impact of the use of steel bars with the strength beyond that limited by the code [3], two different grades of steel bars were used as both longitudinal and transverse reinforcements, namely 420 and 550 MPa. The 420- MPa yield strength steel bars were used to serve as a benchmark against those beyond the maximum value specified by the code [3]. The diameters for longitudinal and transverse bars were 13 and 10 mm, respectively. For the 420-MPa steel bars, the average yield strengths for 10- and 13-mm bar diameter were found to be 447 and 431 MPa, respectively; whereas for the 550-MPa steel bars, they were about 550 and 553 MPa, respectively, for 10- and 13-mm bar diameters. In terms maximum elongations, for the 420-MPa steel bars, the average maximum elongation for 10- and 13-mm bar diameter were found to be 17.5 and 16.5 percent, respectively; whereas for the 550-MPa steel bars, they were about 14.5 and 18 percent, respectively, for 10- and 13-mm bar diameters. The elongations were measured under the gage length of 200 mm as per ACI 318M-14 [3]. It was found that all the reinforcements used in the research have conformed the minimum requirement for elongation of 14 percent [3] Test Specimens Five column specimens were cast to study the impact of the use of high-strength steel bars (Grade 550) in concrete columns against the code-specified strength steel bars (Grade 420). Details of reinforcement used in column specimens are given in Table 1. Figure 1 shows the dimensions of column specimens and their corresponding reinforcing bars. A variation consisted of the use of Grades 420 and 550 for both longitudinal and transverse reinforcements in concrete column specimens was compared and studied to observe the impact of the use high strength steel bars as longitudinal or transverse reinforcement on the behavior of concrete columns against those using the code-specified strength steel bars editor@iaeme.com

4 Agustiar, Tavio, Raka, I G. P. and Anggraini R No. Specimen ID Table 1 Properties of column reinforcement concrete Longitudinal bar Transverse bar f y Diameter ρ g f yh Diameter s (MPa) (mm) (%) (MPa) (mm) (mm) 1 4NN D NH D HN D HH D (a) (b) Figure 1 Typical dimensions of column specimen and details of reinforcement: (a) cross section and (b) elevation 2.4. Test Setup All concrete column specimens were tested under pure axial compressive loading using kn capacity hydraulic universal testing machine with the displacement control capabilities as shown in Figure 2. The load increment was measured using load cell. Two steel collars were placed at the top and bottom of each column specimen to provide additional confinement at the non-test regions at both ends. A monotonic concentric compression load was applied at a very slow rate particularly to capture the post peaks part of the measured load-deformation curve. The load was applied incrementally from zero to failure, which was determined primarily by either rupture of lateral reinforcement or excessive crushing of concrete core together with the buckling of longitudinal bars. The deformations of column specimens were measured using four LVDTs attached at four sides of the column specimens and four strain gauges mounted on the steel reinforcement, two at longitudinal bar and two at transversal bar. All the measured data from load cell, LVDTs, and strain gages was read using data logger and further transferred to the computer for data recording editor@iaeme.com

5 Behavior of Concrete Columns Reinforced and Confined by High-Strength Steel Bars (a) (b) (c) Figure 2 Test setup of column specimen (a) elevation, (b) LVDTs positions and (c) photograph 3. RESULTS AND DISCUSSION 3.1. Effect of longitudinal steel bars The maximum load carried by the 4NN50 column specimen was around kn, which is corresponding to the maximum stress of MPa with the strain of When the stress dropped to 0.85 of the peak stress (25.98 MPa), the corresponding strain was found to be On the other hand, for the 4HN50 column specimen, the maximum load was about kn and the corresponding stress was MPa with the strain of At 0.85 peak stress (26.2 MPa), the strain reached up to Based on the evidence observed from the load capacity carried by the 4NN50 and 4HN50 column specimens, it can be concluded that the presence of longitudinal bars with the yield stress of 550 MPa did not increase the capacity of the reinforced concrete column specimens from those of 420 MPa. The post-peak strain at the stress of 0.85 of the peak stress indicates that column specimens with the yield stress of 420 MPa is greater than those of 550 MPa. This shows that the deformation capability of reinforced concrete columns with the yield stress of 420 MPa is larger than those of 550 MPa as shown in Figure 3. Figure 3 Stress-strain relationships of confined concrete column specimens reinforced with different yield stresses of longitudinal steel bars under axial compressive loading editor@iaeme.com

6 Agustiar, Tavio, Raka, I G. P. and Anggraini R 3.2. Effect of transverse steel bars The 4NN50 and 4NH50 column specimens are compared to determine the effect of different yield stresses of transverse steel bars used in column specimens. The 4NH50 column specimen uses the 550-MPa yield stress of transverse steel bars. The maximum load carried by the 4NH50 column specimen is kn. The corresponding stress at this maximum load is MPa with the strain of The corresponding stress at 0.85 of the peak stress is MPa and the related strain is Figure 4 shows that the difference between the maximum stresses of the 4NN50 and 4NH50 column specimens are negligible. The difference becomes tangible at the post-peak part that is after the column specimens dropped the stresses of 0.85 of the peak stresses. 35 Stress (MPa) Strain(mm/mm) 4NN NH50-30 Figure 4 Stress-strain relationships of reinforced concrete column specimens confined with different yield stresses of transverse steel bars under axial compressive loading 3.3. Effect of yield strength of steel bars The 4NN50 and 4HH50 column specimens were also compared to observe the effect of 550-MPa steel bars when used as longitudinal and transverse reinforcements in column specimens. The 4HH50 column specimen had the maximum load of kn. The maximum corresponding stress to this maximum is MPa with the strain of At the post peak branch, the stress at 0.85 of the peak stress is MPa and the corresponding strain is The increase of column specimen capacity using 550-MPa longitudinal and transverse steel bars is only about 5.6 percent compared to that using 420-MPa steel bars. The comparison of stress-strain relationship between the 4NN50 and 4HH50 column specimens is given in Figure 5. Stress (MPa) Strain (mm/mm) 4NN HH50-30 Figure 5 Stress-strain relationships of concrete column specimens confined and reinforced with different transverse and longitudinal steels, respectively, under axial compressive loading editor@iaeme.com

7 Behavior of Concrete Columns Reinforced and Confined by High-Strength Steel Bars 4. CONCLUSIONS This paper reports the axial compressive load behavior of concrete column specimens reinforced and confined with high-strength steel bars as longitudinal and transverse reinforcements, respectively, and further compared their performances. The following conclusions were made based on the results and visual observations during the tests. The use of 550-MPa steel bars as longitudinal or transverse reinforcements did not significantly contribute to the axial capacity of column specimens compared with that using the 420-MPa steel bars. The use of combination of 550-MPa steel bars as both longitudinal and transverse reinforcements did not significantly increase the axial capacity of column specimen. The deformation of column specimen with the 550-MPa steel bars as either longitudinal or transverse reinforcement or a combination of both decreased when compared to those with the 420-MPa steel bars ACKNOWLEDGEMENTS The authors gratefully acknowledge all the supports received to make this research possible. REFERENCES [1] Ahmad, H. H.; and Tavio, Experimental Study of Cold-Bonded Artificial Lightweight Aggregate Concrete, AIP Conference Proceedings, American Institute of Physics, 1977, 2018, pp [2] Raharjo, D.; Subakti, A.; and Tavio, Mixed Concrete Optimization Using Fly Ash, Silica Fume and Iron Slag on the SCC s Compressive Strength, Procedia Engineering, Elsevier, 54, 2013, pp [3] ACI Committee 318, Building Code Requirements for Structural Concrete (ACI 318M-14) and Commentary (ACI 318RM-14), American Concrete Institute, 2014, 519 pp. [4] Tavio; Pudjisuryadi, P.; and Suprobo, P., Strength and Ductility of External Steel Collared Concrete Columns under Compressive Loading, Journal of Asian Concrete Federation, Asian Concrete Federation, 1(1), 2015, pp [5] Kusuma, B.; Tavio; and Suprobo, P., Behavior of Concentrically Loaded Welded Wire Fabric Reinforced Concrete Columns with Varying Reinforcement Grids and Ratios, International Journal of ICT-aided Architecture and Civil Engineering, SERSC, Tasmania, Australia, 2(1), 2015, pp [6] Cusson, D.; and Paultre, P., High-Strength Concrete Columns Confined by Rectangular Ties, Journal of Structural Engineering, American Society of Structural Engineers, 120(3), 1994, pp [7] Pudjisuryadi, P.; Tavio; and Suprobo, P., Analytical Confining Model of Square Reinforced Concrete Columns using External Steel Collars, International Journal of ICT-aided Architecture and Civil Engineering, SERSC, Tasmania, Australia, 1(1), 2014, pp [8] Tavio; Kusuma, B.; and Suprobo, P., Experimental Behavior of Concrete Columns Confined by Welded Wire Fabric as Transverse Reinforcement under Axial Compression, ACI Structural Journal, American Concrete Institute, 109(3), 2012, pp [9] Cusson, D.; and Paultre, P., Stress-Strain Model for Confined High-Strength Concrete, Journal of Structural Engineering, American Society of Structural Engineers, 121(3), 1995, pp editor@iaeme.com

8 Agustiar, Tavio, Raka, I G. P. and Anggraini R [10] Pudjisuryadi, P.; Tavio; and Suprobo, P., Performance of Square Reinforced Concrete Columns Externally Confined by Steel Angle Collars under Combined Axial and Lateral Load, Procedia Engineering, Elsevier, 125, 2015, pp [11] Tavio; Suprobo, P.; and Kusuma, B., Ductility of Confined Reinforced Concrete Columns with Welded Reinforcement Grids, Excellence in Concrete Construction through Innovation Proceedings of the International Conference on Concrete Construction, CRC Press, Taylor and Francis Group, London, UK, 2009, pp [12] Paultre, P.; and Légeron, F., Confinement Reinforcement Design for Reinforced Concrete Columns, Journal of Structural Engineering, American Society of Structural Engineers, 134(5), 2008, pp [13] Kusuma, B.; Tavio; and Suprobo, P., Axial Load Behavior of Concrete Columns with Welded Wire Fabric as Transverse Reinforcement, Procedia Engineering, Elsevier, 14, 2011, pp [14] Tavio; Kusuma, B.; and Suprobo, P., Investigation of Stress-Strain Models for Confinement of Concrete by Welded Wire Fabric, Procedia Engineering, Elsevier, 14, 2011, pp [15] Sakai, K.; and Sheikh, S. A., What Do We Know about Confinement in Reinforced Concrete Columns? (A Critical Review of Previous Work and Code Provisions), ACI Structural Journal, American Concrete Institute, 86(2), 1989, pp [16] Tavio; and Kusuma, B., Stress-Strain Model for High-Strength Concrete Confined by Welded Wire Fabric, Journal of Materials in Civil Engineering, American Society of Civil Engineers, 21(1), 2009, pp [17] Tavio; Suprobo, P.; and Kusuma, B., Strength and Ductility Enhancement of Reinforced HSC Columns Confined with High-Strength Transverse Steel, Proceedings of the Eleventh East Asia-Pacific Conference on Structural Engineering and Construction (EASEC-11), Taipei, Taiwan, 2008, pp [18] Sheikh, S. A.; and Uzumeri, S. M., Strength and Ductility of Tied Concrete Columns, Journal of Structural Engineering, American Society of Structural Engineers, 106(5), 1980, pp [19] Pudjisuryadi, P.; Tavio; and Suprobo, P., Axial Compressive Behavior of Square Concrete Columns Externally Collared by Light Structural Steel Angle Sections, International Journal of Applied Engineering Research, Research India Publications, 11(7), 2016, pp [20] Sabariman, B.; Soehardjono, A.; Wisnumurti; Wibowo, A.; and Tavio, Stress-Strain Behavior of Steel Fiber-Reinforced Concrete Cylinders Spirally Confined with Steel Bars, Advances in Civil Engineering, Hindawi, 2018, 2018, pp [21] Sheikh, S. A.; and Uzumeri, S. M., Analytical Model for Concrete Confinement in Tied Columns, Journal of Structural Engineering, American Society of Structural Engineers, 108(12), 1982, pp [22] Astawa, M. D.; Raka, I G. P.; and Tavio, Moment Contribution Capacity of Tendon Prestressed Partial on Concrete Beam-Column Joint Interior According to Provisions ACI Chapter (c) Due to Cyclic Lateral Loads, MATEC Web of Conferences, EDP Sciences, 58(04005), 2016, pp [23] Raka, I G. P.; Tavio; and Astawa, M. D., State-of-the-Art Report on Partially-Prestressed Concrete Earthquake-Resistant Building Structures for Highly-Seismic Region, Procedia Engineering, Elsevier, 95, 2014, pp [24] Tavio; and Teng, S., Effective Torsional Rigidity of Reinforced Concrete Members, ACI Structural Journal, American Concrete Institute, 101(2), 2004, pp [25] Tavio, Interactive Mechanical Model for Shear Strength of Deep Beams, Journal of Structural Engineering, American Society of Structural Engineers, 132(5), 2006, pp editor@iaeme.com

9 Behavior of Concrete Columns Reinforced and Confined by High-Strength Steel Bars [26] Astawa, M. D.; Tavio; and Raka, I G. P., Ductile Structure Framework of Earthquake Resistant of High-Rise Building on Exterior Beam-Column Joint with the Partial Prestressed Concrete Beam-Column Reinforced Concrete, Procedia Engineering, Elsevier, 54, 2013, pp [27] Tavio; Anggraini, R.; Raka, I G. P.; and Agustiar, Tensile Strength/Yield Strength (TS/YS) Ratios of High-Strength Steel (HSS) Reinforcing Bars, AIP Conference Proceedings, American Institute of Physics, 1964, 2018, pp [28] Tavio; and Parmo, A Proposed Clamp System for Mechanical Connection of Reinforcing Steel Bars, International Journal of Applied Engineering Research, Research India Publications, 11(11), 2016, pp [29] Anggraini, R.; Tavio; Raka, I G. P.; and Agustiar, Stress-Strain Relationship of High- Strength Steel (HSS) Reinforcing Bars, AIP Conference Proceedings, American Institute of Physics, 1964, 2018, pp editor@iaeme.com