Calculation on Bond Strength of High-Strength Concrete Filled Steel Tube

Transcription

1 4th International Conference on Sensors, Measrement and Intelligent Materials (ICSMIM 215) Calclation on Bond Strength of High-Strength Concrete Filled Steel Tbe Xiao jn Ke1, a *, Hai yang Sn2,b and Zhen Yang,c 1 College of Civil Engineering and Architectre, Gangxi University, Nanning, China 2 College of Civil Engineering and Architectre, Gangxi University, Nanning, China College of Civil Engineering and Architectre, Gangxi University, Nanning, China a xj-ke@16.com, bhaiyangsngx@16.com, c79668@qq.com Keywords: concrete filled steel tbe; high-strength concrete; psh-ot test; bond strength; slip Abstract. This paper describes a test stdy of the interface mechanical behavior of high-strength concrete filled steel tbe. According to psh-ot tests of for specimens of high-strength concrete filled steel tbe, the failre pattern and load-slip crves were analyzed. Test reslts show that the load-slip crves at the load end and free end are both a similarity for the stress-strain crves of steel with no apparent yield point. A reasonable calclation method for predicting the bond strength of high-strength concrete filled steel tbe is presented based on the related theories and test reslts. Introdction With the improvement of concrete strength grade, the brittleness and deformation ability of high strength concrete redces, which can affect the application scope in civil engineering. Ths, the concept of high-strength concrete filled steel tbe is pt forward, which is sed in the lateral restraint of steel tbe to overcome the brittleness of high strength concrete. Several investigations by Cai (2), Zhang (24), Ellobody (25), Zhong (26), Han (27), Li (29), Hoang (211), Ke (214), et al. have been performed to qantify the mechanical properties and bearing capacity of high-strength concrete filled steel tbe at home and abroad. Some researches by Cai (2), Yang (26), Kang (28), Gorley (28), X(21),et al. have been carried on the calclation of bond strength between steel and normal or recycled aggregate concrete. However, few stdies were done on the interface bond properties and bond strength of high-strength concrete filled steel tbe. Up till now, the bond strength calclation theory was established by the basis of normal strength concrete filled steel tbe mostly, and this theory verified by more tests. Therefore, this paper designed for specimens of high-strength concrete filled steel tbe for psh-ot test. It can be help for theoretical analysis, nmerical simlation and application of high-strength concrete filled steel tbe Smmary of Test Design and Manfactre of Specimens. For specimens of high-strength concrete filled steel tbe, considered the mainly parameters of concrete strength and steel tbe bond length, were designed for psh-ot test to reveal the interface mechanical behavior between steel tbe and high-strength concrete. The design parameters of specimens are shown in Table 1. Table 1 Design parameters of specimens. Labels concrete strength L/mm d/mm t/mm Le/mm CST-1 C CST-2 C CST- C CST-4 C7 11 Notes: L is specimen length; d is the diameter of steel tbe; t is the thickness of steel tbe; Le is bond length of steel tbe. High strength concrete was spplied by a commercial ready-mixed plant, i.e R brand ordinary Portland cement, sand, gravels with continos gradation whose particle size ranged from 5 mm to 2 mm, tap water and AF-C poly hydroxy acid high efficiency water redcing agent. The design mix 216. The athors - Pblished by Atlantis Press 995

2 proportion and concrete cbe compressive strength (f c ) measred by Chinese Standard are shown in Table 2. Circle steel tbe se the straight welded steel tbe with a diameter of 11 mm. The yield strength (f y ), ltimate tensile strength (f ) and elastic modls (E s ) are 98 MPa, 458 MPa, and MPa respectively. All empty steel tbes were accrately ct to the reqired length, and the bottom of steel tbes was sealed with plastic film for avoiding the leakage of any slrry. mm distance withot poring concrete on the top of steel tbes was reserved to control bond length dring poring concrete. All specimens were vibrated by a vibration table, and placed indoor to 28 days' natral cring after poring concrete. Table 2 Design mix proportion and performance index of concrete. Material /(kg. m-) water redcing concrete strength slag /% silica fme /% f agent /% c /MPa cement Sand gravel water C C C Test Loading and the Arrangement of Measring Points All specimens were performed on RMT-21 rocked and concrete compression machine, prodced by Institte of Rock and Soil Mechanics Chinese Academic of Science and SIMENS Company. Test set-p is shown as in Figre1. In this test, It shold be need that a mm thickness steel plate, whose diameter is slightly smaller than the inner diameter of steel tbe, was ptted on the load end to ensre that loading can be transfer from core concrete to steel tbe, and make the bond failre. Three dial indicators were distribted on the load end and free end to measre the relative slip between steel tbe and core concrete. Loading was condcted by displacement, and the displacement speed was.2 mm/s. When the slip vale at the load end was abot 6 mm, loading stopped. Figre 1 Test set-p Figre 2 Failre pattern Test Reslts and Analysis. Failre Pattern When loading to a certain load, it appeared a tiny slip at the load end, accompanying with slight noise. The noise may be released from the chemical bond force failre between steel tbe and concrete. With the increasing of loading, the slip vale at the load end increased continosly, and the noise became freqently. Before bond failre, a small amont of concrete debris fell off at the load end, and steel tbe did not appear bending failre. The typical failre pattern is shown in Figre 2. Load-slip Crve The load-slip crves at the load end and free end are shown in Fig.. It can be seen that the crve shape of all specimens is similar to the stress-strain crves of steel with no obvios yield point, and it generally posses the following characteristics: a) The interface slip between steel tbe and concrete can be divided into no slip, local slip and whole slid, which is a process of damage accmlation gradally. b) The slip at the load end is the very early appearing, that is, the development of interface bond failre is from load end to free end. 996

3 c) After the peak load, the load-slip crve falls slowly, and the bearing capacity is more stable with the increasing of concrete strength and steel tbe bond length (a) Specimen CST (b) Specimen CST (c) Specimen CST- (d) Specimen CST-4 Figre Load-slip crves (P-s) Nominal bond strength On the basis of the assmption that the bond stress is evenly distribted along the interface between steel tbe and concrete, the measred nominal bond strength of concrete filled steel tbe can be calclated by sing Eqation 1. P = A (1) Where is nominal bond strength; P is the bond failre load corresponding to the peak point or inflection point of load-slip crve; A is the effective contact area between steel tbe and concrete. Table shows the measred nominal bond strength of all specimens. It can be seen that the bond strength improves with the increasing of concrete strength and steel tbe bond length on the whole. Calclation Method of Bond Strength On the basis of frther researches abot the interface bond performance of normal or recycled aggregate concrete filled steel tbe at home and abroad, different methods for calclating bond strength between steel tbe and concrete were pt forward. To verify whether the existing theories are sitable for high-strength concrete filled steel tbe, several practical calclation methods are discssed in this paper. Calclation Method by Cai Shaohai A typical calclation formla for bond strength of concrete filled steel tbe in China, applied to concrete strength grade ranging from C4 to C8, is proposed by Cai Shaohai..4 =.1fc (2) Where f c is the cbic compressive strength of concrete Calclation Method by Kang Xiliang Throgh the regressive analysis of experimental data, Kang Xiliang pt forward a calclation formla for bond strength of concrete filled steel tbe, shown as follows. 1 = kft[.28(4 Le / d) d / t α +.49θ 7.67] () γ 997

4 Where: γ is the correction coefficient with the inflence of ncertain factors, and γ=.96 is sggested; f t is the tensile strength of concrete; k is the inflence coefficient of steel tbe's srface condition, and k =1. is sggested withot rst; θ is the confinement coefficient (θ=αf y /f c ), in which the ratio of steel tbe α=a s /A c, f y is the yield strength of steel tbe, f c is the axial compressive strength of concrete, A s, A c are the area of steel tbe and concrete, respectively. Calclation Method by Gorley Gorley holds that the diameter-thick ratio is the key factor which inflences the bond strength of concrete filled steel tbe, and it shold be considered for calclating bond strength of concrete filled steel tbe. According to the regressive analysis of test reslts, the bond strength between concrete and steel tbe can be calclated by sing Eqation 5. = ( d / t) (4) Where: d is the oter diameter of steel tbe; t is the wall thickness of steel tbe. Calclation Method by Chen Zongping On the basic of psh-ot test, Chen Zongping proposes the following calclation formla, which can consider simltaneosly the inflence of replacement rate of recycled coarse aggregate, concrete strength grade and length-diameter of ratio on bond strength of recycled aggregate concrete filled steel tbe. = [ δ.28( Le / d)] fc (5) Where: δ is the replacement rate of recycled coarse aggregate; L e /d is the length-diameter ratio. Predicted bond strength of high-strength concrete filled steel tbe sing different methods is compared with test reslts ( ) in Table. Reslts in Table 4 show that bond strength calclated by Eqation is abot 1/ of test reslts, and calclation reslts by Eqation 4 and 5 are abot 2/ of test reslts. Therefore, the first three calclation methods are conservative for predicted bond strength, and give a safe prediction. The reslts calclated by Eqation 6 are in a good agreement with test reslts, and the mean vale of their ratio is 1.. This represents that the calclation method proposed by Chen is the best predictor to predict the bond strength of high-strength concrete filled steel tbe. Table Bond strength calclated by different calclation methods. Labels P /kn /MPa.2 /MPa.2 /. /MPa /.4 /MPa.4 /.5 /MPa 5 / CST CST CST CST Notes: is the measred nominal bond strength;.2,., 4,.5 are corresponding to the reslts calclated by the Eqation 2~5. Smmary 1) The load-slip crves at the load end and free end are both similar to the stress-strain crves of steel with no obvios yield point, the slip at the load end is the very early appearing. 2) The bond strength of high-strength concrete filled steel tbe improves with the increasing of concrete strength and steel tbe bond length on the whole. ) Comparing with the specification reglated data, the measred bond strength is far greater. According to the existing theories, a reasonable calclation method is pt forward to predict the interface bond strength of high-strength concrete filled steel tbe. Acknowledgments The research reported in the paper is part of a research program spported by National Natral Science Fondation of China (No.58112) and Gangxi Natral Science Fondation (No.215GXNSFBA1921). 998

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