EXPERIMENTAL STUDY ON CONFINED CONCRETE OF THIN COLUMN SECTIONS (070S)

Transcription

1 EXPERIMENTAL STUDY ON CONFINED CONCRETE OF THIN COLUMN SECTIONS (070S) Ketut Sudarsana 1 1 Civil Engineering Dept., Udayana University, Bali, Indonesia civil2001ca@yahoo.com; ksudarsana@civil.unud.ac.id ABSTRACT The use of thin sections for columns in reinforced concrete structures of low rise residential buildings increases as design required to meet its aesthetic. On the other hand, code of practices require that the ratio between short and long column sides is limited to be not less than 0.3 for ductile structure in high seismic risk regions. This paper presents experimental results on confined effects of hoops on uniaxial strength and ductility of thin column sections subjected to compressive uniaxial loads by varying the hoops volumetric ratio.there are 18 specimens of short columns having thin sections with size of 100 x 400 x 300 mm tested to failure. The variations of the hoops volumetric ratio (ρ sv ) considered are 0% (no hoops), 1.377%, 2.404%, 3.247%, 4.749%. There are 3 specimens for each combination of ρ sv. Axial deformations are recorded for each load increments until the specimen failure and the specimens maximum load. The specimens are casted with concrete strength of MPa obtained from the average of concrete cylinder tests at 28 days. The yield strength of rebars are obtained from tensile test which are 437 MPa; 284 MPa; 326 MPa; 327 and 270 MPa for rebar diameter of D13mm (deformed), Ø 5.6mm, Ø 7.4mm, Ø8.6mm and Ø10.4mm (plain), respectively. The hoops are fabricated to meet the Indonesian Concrete Codes (SNI ) requirements.the results show that increasing the volumetric ratio of hoops can increase in axial capacity of thin section columns. The axial capacity of the columns are 910,0 kn, 795,0 kn, 920 kn, 920,9 kn, 990 kn dan 1010 kn, respectively, for hoops volumetric ratio of 0% (no hoops), 1,38%; 2,40%; 3,25%; dan 4,75%. The axial ductility of the sections also increases with an increase in the volumetric ratio of hoops. For volumetric ratio of 0% (no hoops), 1,38%; 2,40%; 3,25%; dan 4,75%., the axial ductilities are 1,460; 1,625; 1,667; 2,260; 2,250 dan 2,292, respectively. The experimental results are also compared with confined model proposed in literature for normal strength concrete. Keywords: Confined concrete, volumetric ratio, thin sections, ductility, reinforced concrete. 1. INTRODUCTION 1.1 Background In Earthquake resistant design of structures, ductility plays an important role to keep buildings survive during earthquake events. Ductility of the structures is closely related to the ductile nature of the structural material used (Sudarsana, 2010). In reinforced concrete members such as columns, ductility can be obtained by providing a good confinement of the lateral reinforcement or hoops. Many factors influence the degree of hoops confinement in the columns, such as the magnitude of the axial force, columns sectional shape, confining area of concrete, concrete compressive strength, yield strength of steel reinforcement, concrete cover thickness and volumetric ratio of hoops. The hoop volumetric ratio is the ratio between the volumes of hoops and confining core concrete calculated from center to center of rebars (Rasvi & Saatcioglu, 1999). Several studies have been done on the effect of confinement, however, most of them base on square and circular column sections (Valenas et. al, 1977; Sheikh and Uzumeri, 1980; Scott et.al, 1982; Rasvi & Saatcioglu, 1999; Sudarsana et.al., 2004; Kristiadi, 2008; Tavio et.al., 2008 ). In practice, the use of thin column sections to be flashed with the thickness of partition walls becomes popular due to architecture requirement for aesthetics especially for two or three residential buildings in high risk seismic region. ACI Code (ACI ) and SNI 2002 (SNI ) require for a column of a ductile moment resisting frame should have a minimum dimension of column section is 250 mm with a ratio between short and long column side at least 0.3. S - 99

2 Since the columns are the most important members in preventing total collapse of the whole structure and the use of thin column cross-sections increases, the experiment study on the effect of the volumetric ratio of hoops on the strength and ductility of reinforced concrete columns having thin section subjected to axial load is needed. 1.2 Research Significance Many RC low rise residential buildings are built in high seismic risk region with thin column sections. The strength and ductility of thin column sections are not yet well known; therefore, this experimental study are expected to give information on behavior of thin column sections in RC structure subjected to axial compressive loads. 2. EXPERIMENTAL PROGRAM 2.1 Column Specimens There are 18 specimens of reinforced concrete column of thin section with size of 100 x 400 x 300 mm as shown in Figure 1 were made and tested to failure under uniaxial loads. All specimens have longitudinal reinforcement of 10D13mm ( l = 3.32%). The hoop diameters are varied to have a certain value of the hoop volumetric ratio ( ρ ). sv Figure 1. Typical specimen reinforcement The variations of the hoops volumetric ratio ( ρ ) considered are 0% (no hoops), 1.38%, 2.40%, 3.25%, 4.75% as sv listed in Table 1. There are 3 (three) specimens for each combination of ρ sv. The yield strength of rebars are obtained from tensile test which are MPa; MPa; MPa; and MPa for rebar diameter of D13mm (deformed rebar), Ø5.6mm, Ø7.4mm, Ø8.6mm and Ø10.4mm (plain rebars), respectively. The volumetric ratio is calculated using the following expression. ρ sv A st.2( p l) x100% p. l. s Whereρ sv = volumetric ratio of hoops; A st = area of hoops; s = center to center hoop spacing (mm); p = length of confined core; l= width of confined core. Specimen ID (Hoops Diameter) Table 1. Tested specimens Number of Specimens Volumetric Ratio A B C S1 (no hoops) AS1 BS1 CS S2 (5,6 mm) AS2 BS2 CS2 S3 (7,4 mm) AS3 BS3 CS3 S4 (8,6 mm) AS4 BS4 CS4 S5 (10,4 mm) AS5 BS5 CS5 (1) S - 100

3 All specimens were casted at the same day and moist curing for 7 days. The specimens were stored in a room temperature until testing time. The concrete strength is obtained from the average of concrete cylinder test at 28 days of MPa. 2.2 Test Setup and Instrumentations The specimens are tested using uniaxial compression machine with capacity of 2000 kn as shown in Figure 2. Loads are applied continuously up to specimen failure. In order to obtain the applied stress to be distributed uniformly on column section, steel plates of 200x400x35mm are placed on the top and bottom of the specimens. Axial deformations are recorded during the test using three dial gauges installed at short column side (2 gauges) and at long column side (1 gauge). Initially, the data are recorded every 20 kn. 3. RESULTS AND DISCUSSIONS Figure 2. Test setup Testing the specimens was conducted after the age of specimens 28 days. The average compressive strength of the concrete is MPa. All data and discussions are made within the same concrete strength. The corresponding test results are as follows. 3.1 Failure Modes and Capacity Failure modes of all specimens are similar in which hair cracks occur at average stress of range 40.16% to 58.28% of the peak stress and strain of 0,001 to 0,004 as shown in Table 2. Table 2. Summary of the average rasio between cracking and stresses. Column Specimens Volumetric ratio Cracking stress f crack =P cr /A (MPa) ε crack =DL/Lo Max. stress f max =P max /A (MPa) Ratio f crack /f max S S S S S At this stage, concrete is in unstable condition, therefore, a volume changing due to lateral expansion causing concrete cover breaks. Once the concrete cover spalls, only core section will effectively resist the loads. Initially, micro cracks occur on long section side up to a certain stress and then micro cracks occur on short section side. The cracks growth and opening are faster on the increase in the applied loads. Condition of the specimens for each group after testing can be seen in Figure 3. From specimen S4 and S5, it is evident that buckling of longitudinal rebars at S - 101

4 long side of column occurred due to high applied axial load and lack of rebar restraint whichh has to be provided by hoops. Therefore, cross ties have to be added to improve this behavior. (i) Specimen S1 (ii) Specimen S2 (iii) Specimen S3 (iv) Specimen S6 (v) Specimen S5 Figure 3. Failure of specimens S1, S2, S3, S4, and S5 after testing 3.2 Stress Strain Relationships The stress-strain relationships for each specimen are developed for every load increments. The curves are plotted together as shown in Figure 4. For comparison purposes, only the average curves of each variable are plotted. These curves are obtained by calculating the average stress of the three specimens using f r = (f 1 +f 2 +f 3 )/3 on the same strain value. In General, the curve shows that up to 30% to 50% of maximum stress, the concrete is still elastic that shows by the curve with lines relatively straight. The concrete strain in this condition is in range of 0.00 to mm/mm. After these points, the curves start to bend into a parabolic curve until the maximum stress within a range of and 0.006, and then drop as significant cracks occur. The existence of hoops result in the curve after peak stress is flatter. Increasing the volumetric ratio of hoops can increase load capacity and ductility of the specimens as shown by Specimen S3, S4 and S5. Figure 4. Axial compressive stress strain relationship. S - 102

5 3.3 Effect of Volumetric Ratio on Axial Capacity The effect of the hoops volumetric ratio in thin column sections on axial capacity of alll specimens is presented in Table 3 and Figure 5 to show the trend of the data. Table 3 shows that the axial capacity for the volumetric ratio of 0% (without stirrups), 1.38%, 2.40% %, 3.25%, and 4.75%, is respectively for 795 kn, kn, 920 kn, 990 kn and 1010 kn. Figure 5 can also be seen that by increasing the volumetric ratio of 4.75% can increase in axial capacity increased by 27.04%. Table 3. Axial capacity of specimens Specimens Volumetri c Ratio Axial compressive Capacity (kn) Compare to S1 S S S S S Figure 5. Correlation between hoop volumetric ratio and axial capacity 3.4 Effect of Volumetric Ratio on Axial Ductility of the Specimens Rasvi and Saatcioglu (1989) suggest that ductility of columns subjected to axial load can be estimated based on strain ductility which is the ratio between the strain at the stress of 85% peak stress and the strain at the peak stress (ε 85 / ε maks ). The same method was also used by Sudarsana et.al. (2004) and Tavio et.al.(2008). Calculation of strain ductility can be seen in Table 4. The stresses and strains are obtained from the stress-strain relationship shown in Figure 4. It is shown that significant increase in strain ductility is obtained from S1,S2 and S3. From specimen S3, S4 and S5, increase in hoops does not improve significantly the ductility. It may due to lack of ties on long side of column section so that confinement hoop in this side is ineffective. Table 4. Axial ductility of specimens Specimen ID Volumetric ratio Max. Stress (f'c maks) MPa Max. Strain (ε maks ) mm/mm 85% Max.Stress (f'c 85 ) MPa Strain at 85% Stress (ε 85 ) mm/mm S1 0,000 19,600 0,004 16,660 0,007 S2 1,380 20,000 0,008 17,000 0,013 S3 2,400 22,900 0,005 19,465 0,011 S4 3,250 24,750 0,005 21,038 0,011 S5 4,750 25,500 0,006 21,675 0,014 Strain ductility (ε 85 /ε maks ) Comparison 1, , , , , Effect of Hoop Volumetric Ratio on concrete core strength The increase in concrete core strength due to confinement can be calculated based on the ratio between the maximum strength of concrete (P cm max) and the compressive strength of concrete core of the experimental results (P 0core ) without considering the reinforcement (Rasvi and Saatcioglu, 1989). The value of P cmax and P 0core are obtained from Equation 2 and 3. P α. f ' c.( A A 0 core core s P P A. f c max test s y ) (2) (3) S - 103

6 Where A core = area of concrete core, A s = area of longitudinal reinforcement, A g = gross crosss section area, = ratio between unconfined concrete strength of structural elements and concrete strength of cylinder test, P test = axial capacity of the specimens, f c = cylinder concrete strength at 28 days, f y = yield strength of longitudinal reinforcement. Rasvi and Saatcioglu (1989) also suggest that the value of = 1 for dimension of specimens is the same as cylinder test, and = 0.85 to for specimens with a bigger scale. Table 4 shows the strength of concrete core due to confinement effect provided by hoop volumetric ratio. The data are plotted in Figure 6 to show their trend. It is shown from Figure 6 that strength of concrete core increases almost linearly as the volumetric ratio of hoop increases. Comparing the results of specimen S1 and S5, it shows that by increasing the hoops volumetric ratio (ρ sv ) of 4.75%, the increase in concrete core strength is only 17.3%. This shows that confinement in thin section is less effective especially when no ties are provided on long section side. This condition can also be seen from Specimen S4 and S5, in which the increase in core concrete strength is very small although the volumetric ratio of hoops for these specimens is quite high. Table 5. Strength of the concrete core. Specimen P ρ s sv O core P Cmax (kn) (kn) P C max /P O core S S S S S Figure 6. Relationship between ρ sv and strength of the concrete core 3. 5 Comparison with proposed model. In order to show the shape of confined concrete stress-strain curve for a rectangular thin section, comparisons are made with an analytical model proposed by Rasvi and Saatcioglu (1999). This model is chosen due to its simplicity and suitability for rectangular sections. In Literature, there are many proposed analytical model available, however, most of them are applied for square and circular sections. In addition, this paper does not compare those models but only to show the shape and behavior of the test specimen. Figure 7 (a) to (d) plots the experimental result together with Rasvi and Saatcioglu model (1999) for each hoops volumetric rasio. It show that comparing the stress-strain curve of the specimen with Rasvi and Saatcioglu Model (1999), all data show a good trend correlation as indicated by ascending and descending branches of the curve, except Specimen S2. Rasvi and Saatcioglu Model (1999) underestimate the peak stress and descending branch of the test data. This may due to the model was not proposed for thin column sections. (a) Specimen S2 (b) Specimen S3 S - 104

7 (c) Specimen S3 (d) Specimen S5 Figure 6. Comparison between test data and model Rasvi and Saatcioglu (1999) for specimen S2, S3, S4 and S5 4. CONCLUSIONS The experimental results show that increasing the volumetric ratio of hoops can increase in axial capacity of thin section columns. The axial capacity of the columns are 910,0 kn, 795,0 kn, 920 kn, 920,9 kn, 990 kn dan 1010 kn, respectively, for hoops volumetric ratio of 0% (no hoops), 1,38%; 2,40%; 3,25%; dan 4,75%. The axial ductility of the sections also increases with an increase in the volumetric ratio of hoops. For volumetric ratio of 0% (no hoops), 1,38%; 2,40%; 3,25%; dan 4,75%., the axial ductility are 1,460; 1,625; 1,667; 2,260; 2,250 dan 2,292, respectively. Increase in volumetric ratio by 4,75%, the ductility increases by 17.3%. Crossties at long section side of the column should be provided to improve confining effect of the hoops. Comparing the stress-strain curve of the specimen with Rasvi and Saatcioglu Model (1999), all data show a good trend correlation as indicated by ascending and descending branches of the curve. Rasvi and Saatcioglu Model (1999) under estimated the peak stress and descending branch of the test data. ACKNOWLEDGEMENTS This research has been possible with financial supports from Department of Civil Engineering at University of Udayana. Special thanks are due to colleagues at the department for their input and comments during various stages of the experimental program. REFERENCES Badan Standar Nasional Tata Cara Perencanaan Struktur Beton Bertulang Untuk Bangunan Gedung (SNI ). Jakarta. Bousalem and Chick Development of Confined Model for Rectangular Ordinary Reinforced Concrete Columns, Materials and Structures Journal., DOI,10.167/s Wight, J.K. and MacGregor, J. G Reinforced Concrete: Mechanics and Design. Sixth Edition, Pearson Education Inc., New Jersey, USA. Razvi, S.R. and Saatcioglu, M., 1989, Behavior of Reinforced Concrete Columns Confined with Welded Wire Fabric and/or Rectilinear Ties, Research Report No. 8902, Department of Civil Engineering, University of Ottawa, Ottawa, Ontario, 68 pp. Razvi, R. and Saatcioglu, M., 1999, Confinement Model for High-Strength Concrete, ASCE Journal of Structural Engineering, Vol. 125, No. 3, March, pp Sudarsana, I K., 2001, A Comparative Study On Stress-Strain Confined Concrete Models (Literature review and Analytics), Jurnal Ilmiah Teknik Sipil, Vol. 5, No. 9, Jurusan Teknik Sipil, Universitas Udayana, Juli, Hal Sudarsana, IK., Salain, I M.A.K., Binawaty, V Uniaxial Compressive Strength and Ductility of Confined R/C Circular Column, Jurnal Ilimiah Teknik Sipil, Vol. 8No. 15, Jurusan Teknik Sipil, Universitas Udayana, Januari. Sudarsana, I K Analisis Pengaruh Konfigurasi Tulangan Terhadap Kekuatan Dan Daktilitas Kolom Beton Bertulang, Jurnal Ilmiah Teknik Sipil. Vol. 14, No. 1, Januari Tavio, R. Purwono and M. L. Ashari Confinement of Circular RC Columns with Fine Mesh. Proceeding International Conference on Earthquake and Disaster Mitigation, Jakarta, April. S - 105