INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume 3, No 1, 2012

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1 INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING Volume, No 1, 01 Copyright by the authors - Licensee IPA- Under Creative Commons license.0 Research article ISSN Bond stress characteristics on circular concrete filled steel tubular columns using mineral admixture Radhika. K.S 1, Baskar. K 1- Research Scholar, National Institute of Technology, Trichy, India - Associate Professor, National Institute of Technology, Trichy, India radhika.sridhar10@gmail.com doi: /ijcser ABSTRACT Concrete filled steel tubular columns are becoming widely used in engineering. This paper studies the bond stress characteristics in concrete filled steel tubular columns using mineral admixture (). And also this paper presents the effects of change in length of the tube, diameter of the tube, strength of the infill concrete and percentage variation of in concrete. CFT applications in buildings and the importance of bond stress are studied. This paper presents the bond carrying capacity of twelve conventional concrete and thirty six in concrete filled steel tubular columns. The effect of in concrete was also investigated. The bond carrying capacity is interrelated with slip between steel and concrete interface. From the experimental study it was found that the bond stress decreases with increasing in percentage of. An experimental study is described and evaluated. Keywords: Bond Strength, Composite column, CFT, in filled tubes, Metakaoline, Push out test. 1. Introduction Composite steel concrete columns have been widely used in recent decades. The use of concrete filled steel tubular columns (CFTs) in high rise buildings has become popular in recent years as they provide several advantages over reinforced concrete or pure steel columns. Two types of composite columns, those with steel sections encased in concrete and those with steel sections in-filled with concrete are commonly used in building (Shanmugam and Lakshmi 001). Steel members have the advantages of high tensile strength and ductility, while concrete members have the advantages of high compressive strength and stiffness. (Ahmed Elr y and Atorod Azizinamini, 00) focused on the behaviour and strength of circular concrete filled steel tube columns with the diameter-to-thickness ratio of to 1, filled with the high-strength concrete of ( to 10Mpa). The results of these tests were exhibit very high levels of energy dissipation and ductility. Push out test is the common method to evaluate the bond carrying capacity of the CFT columns. Compression strength of the concrete core is one of the important parameter to affect the bond strength of CFT columns using expansive cement (Chang Xu, 007). Though researches made push out test of CFTs have been carried out for many years. Those include the work of (Roader Charles and Cameron Brad, 1999). Experimentally (Dung and Tsong Yen, 00) were studied rectangular CFT columns with high strength concrete of 9 to Mpa (Gengying, 00). Strength of concrete filled steel tubular columns by the use of fly ash was studied (Gengying, 00). From these researches bond strength and compressive strength of CFTs can be improved by adding fly ash. (Mouli and Khelafi. 007) studied the strength of short composite rectangular Received on May, 01 Published on July 01 1

2 columns with light weight concrete. The test results reported that light weight aggregate concrete offered higher bond strength and compressive strength than normal concrete, but load-slip behaviour of all specimens is similar for both type of concrete. It is well known that the strength of ordinary Portland cement concrete is significantly influenced by the replacement of cement with mineral admixture (Wild, 1996). In the last years, a new material is being studied because of its high pozzolanic properties. Owing to its fineness and chemical composition, shows closer behaviour to silica fume (Duval, 1998). Many investigations have focused on the behaviour and strength of CFT columns using fly ash and expansive agent. In this study use of to the concrete of CFTs to improve the bond strength and compressive strength of the concrete was experimentally investigated.. Experimental work Push out test is emerging as an important experimental tool for characterizing the interfacial behavior of the steel tube and concrete in concrete filled steel tube columns. The main objective of this study is to improve the bond carrying capacity of the concrete filled steel tubular columns using the mineral admixture..1 Material properties Concrete and steel are the two important materials used to carry out the experimental work..1.1 Concrete Concrete and steel are the two important materials used to carry out the whole experimental work. The mix design of concrete grade was carried out in accordance to the Indian standard code. In this paper the addition of was also studied. The chemical analysis of cement and are presented in table 1. The coarse aggregate was well graded with a maximum size of 1mm; the fine aggregate was river sand with a fineness modulus of.7. Mix proportion of cement, sand, aggregates of 1::.7 and a water cement ratio of 0. are used in this paper. All specimens were cast from the same delivery of materials (sand, aggregates and cement) and similar casting and curing procedures were adopted throughout the test program. For each mix standard cube tests were used to determine the compressive strength of the concrete. A total of 0 cubes were prepared by adding different percentage of in concrete and tested on a compression testing machine of 000kN capacity after 7 days and 8 days of curing. Table 1: Chemical analysis of cement and Fe Type Sio Al O Ca Mg SO Na C K O O O O O T i O C C S S A A F Cement Metakaol ine C Thick ness Widt h Cross sectiona l area (mm ) Yield Load (kn) Table : Properties of steel tube Yield Stress (N/mm ) Tensi le Load (kn) Tensile Stress (N/mm ) Initial gauge length Final gauge length % of Elongati on

3 .1. Steel All circular tubes were fabricated with high yield strength of Y st 10 according to the code of Indian standard All stiffeners were fabricated with mild steel flat of width and thickness of 0mm and 6mm respectively. Standard coupon test were conducted to carry out the properties of steel. Test coupons were cut from the steel tube section, and the testing was done based on ASTM A70 procedure. Properties of steel tube are shown in table.. Test specimen preparation In preparing the tube of a composite specimen, circular steel tube was fabricated. If stiffening was specified, longitudinal stiffeners were welded on the circular steel tube with tack welds. Each tubular specimen was tack welded by a mild steel flat with a thickness of 6mm and with width of 10mm. After that concrete mix was filled in multiple layers for all specimens and was vibrated by a vibrator machine. These specimens were then naturally cured in the indoor climate of laboratory. Prior to testing, the top surfaces of the concrete filled steel tube columns were smoothened in order to avoid the eccentricity of loading. It was thus expected that the stiffeners could not share the axial load. The columns were tested as pin-ended supported and subjected to axial loading. Loading was applied through edges of each specimen to the concrete only.. Experimental setup All experiments were done in 100T capacity loading frame machine. To measure the applied load, a strain gauge base load cell is used and to measure the axial displacement (slip), two linearly variable displacement transducer (LVDT) is placed diametrically opposite to each other. Figure 1: Typical view of experimental setup

4 All data was scanned every second and the data is stored using AI channel data logger with a help of computer. Also at required point of loading a separate tag file is stored for analysis of data with a help of PRO-sof software. Experimental setup is shown in figure 1.. Testing procedures To determine the strength of the concrete, other mix design variables like quality of ingredients, mix proportions including the dosage of super plasticizers, mix procedures, curing conditions and testing procedures were kept as constant. Hence the change in concrete properties occurred primarily due to the replacement of cement by adding percentage of to the concrete (, 10, 1 and 0%). To investigate the compressive strength of the concrete, 0 cubes were casted and their strength was experimentally found out. In order to study the bond carrying capacity of the concrete filled steel tubular columns, totally forty eight specimens were prepared for the strength test. In that twelve of them were prepared for conventional concrete and thirty six of them were prepared for replacement in concrete for bond strength. Before casting concrete, specimen with a gap of about 0mm was left at the bottom of each specimen to enable the movement of concrete relative to the steel during loading. Although nominal dimensions and wall thickness are measured at several locations. All steel tube specimens are hot rolled with specific yield strength of 19Mpa. These tubes are seam welded and the edges of the tubes are machine finished after cutting to avoid eccentricity while loading. The curing of CFT specimens was done by sealing the top surface with jute, after wetting the top surface in order to avoid the shrinkage of concrete and also evaporation loss of the concrete filled steel tubular columns.. Testing of CFTs Total of forty eight specimens were tested after 8 days of curing on the loading frame machine of 100T capacity. The test specimen and location of instrumentation as shown in figure 1. At every regular interval the axial displacement of the steel infill concrete was noted from linearly variable displacement transducer. The displacement was allowed up to mm where the ultimate load was observed and the readings were noted up to the failure of load after the attainment of peak load. The parameters for different specimens in the test with their maximum peak experimental compressive load obtained with testing. During the tests, all the specimens having 0mm gap at the bottom were collapsed by axial displacement while giving load only to the concrete. The variation of load vs. slip curves are compared for different lengths to diameter ratio of the concrete filled steel tubular columns. It was observed that the initial gradient of the curves remains 80% unchanged for all the specimens. Table shows the summary of test specimens and results.. Test results During the tests, it was observed that welding stiffeners had no apparent influence on the failure modes. For all specimens, an axial displacement failure mode was observed. The bond test result of concrete filled tubular columns modified by and conventional concrete with different lengths to diameter ratio is shown in figure. "Equation (1) is used for finding out the bond stress of all specimens from their failure load." f b =P/ (πdl) (1)

5 Table : Summary of tests specimens and results Details of concrete Control concrete Metakaolin e % Metakaolin e 10% Metakaolin e 1% Metakaolin e 0% Specimen designatio n CCFT1 CCFT CCFT M CFT1 M CFT M CFT M 10 CFT1 M 10 CFT M 10 CFT M 1 CFT1 M 1 CFT M 1 CFT M 0 CFT1 M 0 CFT M 0 CFT Diamete r D Lengt h L Thicknes s T L/D rati o Ultimate compressio n load f u (kn) Concrete compressio n strength f c (N/mm ) Bond stress f b (N/mm ) Where P is the applied load at which initial rigid body slip of the concrete core relative to the steel occurs, and in this paper P is called failure load. D and L are the inside diameter of the steel tube and the length of the concrete steel interface respectively. Figure, and shows the load vs. slip relationships of conventional concrete and concrete for three different lengths to diameter ratio. According to the test results, the load slip responses of all specimens seem to exhibit a similar pattern. It can be observed from these curves that the strength of the concrete and load carrying capacity of the specimen reduces with the percentage increase of adding in concrete up to 0% but at 1% of the strength was seem to be higher than that of 0% of in concrete. Each curve consists of a linear relationship up to about 80 to 90% of the failure load, a non linear pattern after failure. Each curve consists of a linear relationship up to about 80 to 90% of the failure load, a non linear pattern after failure. For most of these specimens, loads drop to a certain level and almost remain constant after failure load until the concrete core body moves to the support foundation. A big sound can be heard just when failure load reaches and then rigid body motion between the concrete core and the steel tube begins with increasing slip displacements. The typical bond failure is shown in figure 6.. Discussions As mentioned above, the specimens of conventional concrete filled steel tubular columns have same diameter of steel tube, wall thickness of the tube and same compression strength of the concrete but slight difference in length to diameter ratio of the steel tube. However, it is clear that the bond carrying capacity of conventional concrete filled steel tubular columns specimen is lower than that of concrete filled steel tubular columns specimens. The failure loads for specimen CCFT1, CCFT and CCFT are 1.70kN, 16.7kN and 6.7kN and the bond stress are 1.78, 1.96 and.1mpa. The difference in the characteristics of the load slip curves of CCFT and MCFT specimens are obvious. In the linear part of the curves, CCFT specimens seem to slip at a relatively higher rate and have longer slip displacement before failure load than do MCFTs specimens.

6 The test results indicate characteristics of the concrete core have an important influence on the bond strength as well as the slip behavior. Figure : Bond stress of CCFTs and MCFTs Figure : Load vs. Slip behavior for L/D= Figure : Load vs. Slip behavior for L/D= Figure : Load vs. Slip behavior for L/D= For the specimens of MCFT all the parameters are same except the compression strength of the concrete. The failure loads of specimens M CFT1, M 10 CFT1, M 1 CFT and M 0 CFT1 are 7.8kN, 0.6kN, 16.kN and 71.6kN and their bond stress are 1.9,.16,.8 and 1.9Mpa. Obviously, bond strength increases with increasing in length to diameter ratio of the steel tube. The compression strength of the concrete core for all specimens mentioned above varies from 0 to 7Mpa. In order to investigate, how the compression strength of the concrete core affects the bond strength, compression strength of the concrete core is designated as to 70Mpa. The bond strength of MCFT specimens obviously increases with the increase of the compression strength of the specimen. 6

7 . Conclusion Figure 6: Typical bond failure of CFT column This paper presents an experimental study on circular concrete filled steel tubular columns. Parameters for this study included the length to diameter ratio of the steel tube, grade of concrete and the effect of addition of in concrete. The influence of these parameters on the confinement of the concrete core, bond carrying capacity of the CFTs was investigated. Based on the results of these investigations, the following conclusion is obtained. Adding mineral admixtures like to the concrete is effective in increasing the member ductility and also effective in ultimate strength of the CFT columns. It was observed that the bond carrying capacity decreases with increasing in percentage of but increases up to 1% of in concrete. Compression strength of the concrete core is other important parameter to affect the bond strength of MCFT specimens. The results from push out tests in this experiment indicate that bond strength of MCFTs specimens is greater than that of CFTs specimens. In the short columns with the load applied to the concrete section, the concrete core exhibited greater compressive stresses than predicted, due to the confinement of the steel tube. The effect was most pronounced for the stub column with bond strength between the concrete core and the steel tube, when the load applied only to the concrete section. The stiffness was also influenced by the changed bond strength for this loading situation. Increased bond strength resulted in a greater contribution from the steel tube, i.e. stiffness of the column increased. Finally, even though the efficiency of the steel tube in confining the concrete core is greater when the load is applied only to the concrete section, it seems not reliable to trust just the natural bond strength to get full composite. At the ultimate load performance level, this bond stress is distributed evenly around the periphery of the interface and along a length of the steel concrete interface of the CFT columns. Finally, even though the efficiency of the steel tube in confining the concrete core is greater when the load is applied only to the concrete section, it seems not reliable to trust just the natural bond strength to get full composite. At the ultimate load performance level, this bond stress is distributed evenly around the periphery of the interface and along a length of the steel concrete interface of the CFT columns. 6. References 1. N. E. Shanmugam, and B. Lakshmi, (001), State of the art report on steel-concrete composite columns, Journal of constructional steel research, pp

8 . Ahmed Elr y, and Atorod Azizinamini., (00), Behaviour and strength of circular concrete filled steel tube columns, Journal of constructional steel research, pp Chang Xu et al, (007), Push out test of pre-stressing concrete filled circular steel tube columns by means of expansive cement. Construction and building materials, pp Roeder Charles et al, (1999), Composite action in concrete filled tubes. Journal of structural engineering, pp Gengying Li et al, (00), Improve the strength of concrete filled steel tubular columns by the use of fly ash, Cement and concrete research, pp Gengying Li et al, (00), Behaviour of concrete filled steel tubular columns incorporating fly ash. Cement and concrete research, pp M. Mouli, and H. Khelafi., (006), Strength of short composite rectangular hollow sections columns filled with light weight aggregate concrete. Engineering structures. pp N. J. Coleman, and C. L. Page., (1997), Aspects of the pure solution chemistry of hydrated cement paste containing. Cement and concrete research, pp S. Wild et al, (1996), Relative strength pozzolanic activity and cement hydration in super plasticised concrete. Cement and concrete research, pp S. Wild et al, (1999), Factors influencing strength development of concrete containing silica fume. Cement and concrete research, pp R. Duval, and E.H. Kadri, (1998), Influence of silica fume on the workability and the compressive strength of high performance concrete. Cement and concrete research, pp IS 106 (198), Indian standard recommended guidelines for concrete mix design. Bureau of Indian Standards, New Delhi, India. 1. IS 1161 (1998), Indian standard steel tubes for structural purposes. Bureau of Indian standards, New Delhi, India. 1. Nagarnaik and Pande., (011), Inflence of silica fume in enchancement of compressive strength, flexural strength of steel fibre concrete ant their relationship. International Journal of Civil and Structural engineering,, pp