MECHANICAL PROPERTIES OF RECYCLED CONCRETE FILLED STEEL TUBES AND DOUBLE SKIN TUBES

Transcription

1 MECHANICAL PROPERTIES OF RECYCLED CONCRETE FILLED STEEL TUBES AND DOUBLE SKIN TUBES X.S. SHI (1), Q.Y. WANG (1), C.C. QIU (1), and X.L. ZHAO (2) (1) College of Architecture and Environment, Sichuan University, Chengdu, , P.R. China (2) Department of Civil Engineering, Monash University, Clayton, VIC 3800, Australia Abstract Nowadays, recycled concrete becomes more popular as a kind of new material which utilizes the waste concrete with the significant advantages of environmental protection and economic benefit. Its members have inferior structural performances compared with normal concrete members. However, their performance can be improved by filling them in steel tubes. In this paper, some experimental testing are described on recycled aggregate concrete filled steel tubes (RACFST) and recycled aggregate concrete filled double skin tubes (RACFDST) subjected to compression. The influence of recycled aggregates replacement ratio on the behavior of such composite structure will be investigated. The load deflection relationships are discussed in terms of strength and ductility. Keywords: recycled aggregate concrete, strength, aggregates replacement ratio, concrete filled tubes, double skin tubes 1. INTRODUCTION Recycled aggregate concrete (RAC) is a kind of material using recycled aggregate (RA) from waste concrete to substitute natural aggregates partly or totally. This is an effective way to solve the construction and demolition waste disposal problem and to gain benefit for environmentally sustainable development. Many basic properties have been studied worldwide mainly focusing on its mechanical properties and behaviors in ordinary structural members. The research [1-7] indicates that, RCA is well performed in view of its low brittleness, low thermal conductivity as well as the low gravity which reduces the self-weight of the structures and good for improving anti-seismic ability. However, the application in structures is restricted to non-load bearing members as infilling walls because of its inferior properties compared with natural concrete, such as lower strength and elastic modulus, weaker workability, higher shrinkage and creep, and etc. In order to fully utilize such valuable material in construction, the composite structure is a good way to overcome its weakness. It is well known that concrete filled steel tube (CFT) performs excellent static and 559

2 earthquake-resistant properties due to its smart combination of advantages of steel members and concrete, with high tensile strength and ductility of steel, as well as high compressive strength and stiffness of concrete. In this case, recycled aggregate concrete filled steel stub (RACFST) has the beneficial qualities of both materials, in which the compressive strength and ductility of RAC could be enhanced by the outer steel tube. At the same time, the encased concrete could avoid early local buckling of the steel tube. It is a prospective way to realize the application of RAC in the main structure. Although there have been many studies conducted on CFT, and only a few researches have been conducted on RACFST. The recent research results [8] show similar behaviors on strength and failure modes of RACFST with that of natural concrete filled steel tub. However, a large number of experimental tests and theoretical analysis should be carried out to learn more specific properties of such structure. Furthermore, considering some new structures of CFT such as thin-walled tube and double skin tubes [9] that have certain advantages compared with traditional CFT columns, recycled aggregate concrete filled double skin tubes (RACFDST) is a new development direction for RAC which is proposed in this paper. This paper describes a series of compression tests carried out on 18 RACFST columns and 18 RACFDST columns with circular and square sections and different RA replacement ratios of 0%, 25%, 50%, 75% and 100%. The influences of RA replacement ratio on the compressive strength and failure modes of these stub columns under axial compression are discussed. The behaviors of such composite structures under axial compressive load are described. 2. EXPERIMENTAL PROGRAM 2.1 Material properties All the steel tubes were produced by hot-rolled process. Standard tensile coupon tests were taken to measure the material properties of the circular and square steel tubes. The average yielding strength (f sy ), tensile strength (f u ), modulus of elasticity (E s ) and Poisson s ratio (µ s ) are listed in Table 1. In the light of the properties of recycled aggregates which were obtained by crushing the waste concrete from Wenchuan earthquake-stricken area, the mix design is specially considered. Five types of concrete mixes were prepared with the expected compression strength of 43.2MPa similar to C35 and slump value about 30-50mm, aiming to ensure that the concrete meets requirements of the specification consistently and uniformly, the concrete mixture proportions with different RA replacement ratios are summarized in Table 2. All specimens were casted from one batch of concrete. Thirty concrete cubes with 150mm 150mm 150mm moulds were casted to determine the 28d compressive strength and the strength on the day of testing. Table 1: The properties of circular and square steel tubes Section type f sy f u E s µ s (Mpa) (MPa) (MPa) Circular Square

3 Table 2: The mix proportions and properties of the concrete Type Norm al concr ete Recyc led concr ete Replacement ratio (%) NA RA Sand Cement Water (kg/m 3 ) (kg/m 3 ) (kg/m 3 ) (kg/m 3 ) (kg/m 3 ) W/C Ratio 28-day f cu (MPa) 0% % % % % Specimen preparations The total of 36 composite columns, including 18 RACFST columns and 18 RACFDST columns were tested. The measured dimensions, confinement factor and experimental ultimate strength are listed in Table 3. The main experimental parameters are: the section type (C = circular, S = square) and filled concrete type (0, 1, 2, 3 and 4 refer to the concrete with 0%, 25%, 50%, 75% and 100% RA replacement ratio respectively). The last letter refers to the first sample (1) or the repeated sample (2). For RACFDST columns, the inner square steel tube is 50mm wide and 2mm thick. Label DC refers to the RACFDST with circular hollow section outer and square hollow section inner, while label DS stands for the RACFDST with square hollow section outer and square hollow section inner. Figure 1 shows the geometry of the specimens. (a) Circular (d) Square (c) CHS+SHS (b) SHS+SHS Figure 1: Geometry of specimens For casting the test specimens, the ends of the steel tubes were cut and machined to the required length. A 14mm thick steel plate was welded on one end of the tubes to ensure the flatness on the bottom, as well as acting as the mould of the concrete. This plate was marked to help obtain a concentric construction of the double skin tubes. The concrete was filled in layers and was vibrated by a poker vibrator. The specimens were air-cured at room temperature. To avoid longitudinal shrinkage of the concrete, the concrete was casted a little higher than the steel tube surface. After curing, the extra concrete was removed to achieve a smooth and flat surface by a grinding wheel with diamond cutters. In order to ensure loading 561

4 on the concrete and the steel tubes simultaneously, a horizontal ruler was used to check the flatness of the cross section. Table 3: Measured properties of specimens Specimen type Circular Square CHS SHS Specimen L D(B) t f' cu A s /A c N ξ ue (KN) No. (mm) (mm) (MPa) (%) Measured Average C C C C C C C C C S S S S S S S S S DC DC DC DC DC DC DC DC DC DS DS DS DS DS DS DS DS DS

5 2.3 Test procedure The experimental study aimed to investigate not only the maximum load-bearing capacity of the composite specimens subjected to axial compression, but also the failure modes up to and beyond the ultimate load. One horizontal strain gauge and one longitudinal strain gauge on every side were stuck on the middle of the column and sealed with epoxy. Three LVDTs were set to measure axial shortening on different corner of the specimen, as shown in figure 2. LVDT Strain gauges Figure 2: Layout of the column test All the specimens were performed on a 5000kN capacity testing machine and seated directly on the rigid steel bed of the machine. The specimens were loaded in several stages. For linear stage, a load interval of one-tenth of the estimated ultimate strength was used. Following yielding, one-fifteenth of estimated ultimate strength was applied. A smaller load interval was used when it was close to failure. Each load interval was maintained for about 2 minutes. The progress of deformation, the mode of failure and the maximum load were recorded during the testing. 3. TEST RESULTS AND DISCUSSIONS Figure 3: The circular specimen after testing 3.1 Circular hollow sections All the circular RACFST columns had no obvious deflection during the initial linear elastic period of the loading process, which was the cooperation of the steel tube and encased concrete. With the load increase, local buckling happened to the specimen gradually. The specimen was not fully symmetric any more, which would lead to some load eccentricity to some extent. The buckling was about 1mm to 3mm heaved along the columns, with cracks appeared on the epoxy, and then the buckling happened on the top and bottom of the columns lead to large longitudinal deformation which resulted in failure. All the specimens with different RA replacement ratios displayed similar failure mode. Buckling happened earlier with higher RA replacement ratios because of weaker strength of the concrete. Figure 3 shows the specimens after testing. 563

6 The ultimate compressive strength of the circular RACFST columns is shown in Figure 4. The ultimate strength decreases with the increase of the RA replacement ratio, but not sharply. However, the greatest difference is about 17% between RACFST with 100% replacement ratio and normal concrete filled steel tube (CFT). For other specimens, the ultimate strength of RACFST is less than 10% lower that of normal CFST. The measured load versus axial deformation curves of circular RACFST with different RA replacement ratios are shown in Figure 5. All the specimens present a similar trend with linear elastic stage and yielding stage. In general, larger deformation was observed for RACFST columns, which is most obvious for 100% replacement RACFST column. In other words, the columns with greater RA replacement ratios achieved more ductility in the post-peak response. 3.2 Square hollow sections The square section RACFST columns acted the similar manner as circular section columns did. Every sides of the square columns appeared roof mechanism after the load up to 60%-70% of the peak load. Once the buckling was initiated at the peak load, the load carrying capacity reduced with increased deflections. Finally, the failure happened due to the collapse of concrete and the steel buckling. Figure 6 shows the ultimate strength of square RACFST columns with various RA replacement ratios. Not great differences exist between different RA replacements columns, except that a 14% lower strength is observed for 100% replacement RACFST column compared with normal CFT column. The strength of square RACFST is about 10% lower than that of circular RACFST. Axial load (kn) Axial load (kn) 564

7 The measured load versus axial deformation curves of square RACFST with different RA replacement ratios are shown in Figure 7, which behaves similarly as those of circular RACFST columns. However, the weak properties are manifested earlier for square RACFST columns, leading to a smaller stiffness before the peak load. The higher the RA replacement ratio is the greater the deformation or ductility is. The specimens were cut open at the local buckling location after the testing. It is seen that the concrete is almost intact in circular column with several cracks across the section, while the concrete in the square column is crushed. This indicates the confinement effect of outer steel tube to encased concrete is greater for circular section than square section, which is also the reason of why higher ultimate strength is achieved for circular section columns. 3.3 Double skin tubes The load-deflection curves of the RACFDST columns with CHS and SHS sections are shown in Figure 8. The ultimate strength of the columns presents the same trend as that of RACFST columns, i.e. more strength reduction with an increased RA replacement ratio. The strength of CHS RACFDST column with 100% replacement ratio is 17% lower than that of the normal CFDST column, and 10% lower as that of SHS RACFDST column. Figure 8: Load-deflection curves for RACFDST columns (a) DC2-1 (b) DS1-1 Figure 10: RACFDST columns after testing 565

8 The linear elastic response is up to 70% of the peak load, where full bond between the filled concrete and the steel tubes is maintained. The load gradually developed to the peak load where the curve is bent indicating non-linear response from yielding of the steel tubes and non-linear behavior of confined concrete. The post peak falling curve is associated with an unloading of the remaining part of the tubes which displays the decreasing load with increasing axial shortening. However, most of them are plotted only up to limited deflection. Fortunately, a much greater deflection up to 30mm was recorded for DC2-2, shown in Figure 9. There are two peak points which is similar to the results of CHS CFDT columns in [10, 11], but without obvious plateau after the first peak. It is estimated that the lock-up of the plastic mechanism of the inner tube at large axial deformations happened when the space between the inner tube and the outer tube is small. That is, the inside surface of any adjacent sides are in contact coming into a kind of hinge which can absorb significant amount of energy. The load started to increase again after the unloading which was also seen in DC1-2 and DC4-2 specimens, but without the second peak, maybe just because no complete curves were recorded. In order to determine the exact reason for this phenomenon, much more tests should be conducted with the various parameters of different section dimensions and full load-deflection curves should be recorded. Figure 10 shows the typical buckling of these two kinds of columns. The buckling appeared mostly on the top side of the CHS RACFDST columns, while concentrated at the middle part of the SHS RACFDST columns. The buckling of CHS section columns was similar to elephant foot mechanism, whereas the roof mechanism was observed on SHS section columns, both resemble normal CFDT columns with the same section types. 4. CONCLUSIONS This study is an attempt to investigate the practicability of using hollow structure steel tube and double skin steel tube filled with recycled aggregate concrete and their behavior under axial compressive loading. Based on the experimental results, the following conclusions can be drawn: The typical failure modes of RACFST columns and RACFDST columns are similar to those of the normal CFT columns and CFDST columns with local buckling resulting in failure. The RACFST columns with circular section have better confinement effect to encased concrete, indicating higher ultimate strength with corresponding square section ones. The ultimate strength of RACFST columns and RACFDST columns decrease with the increasing of the RA replacement ratio. 100% recycled aggregate concrete filled columns have slightly lower (within 20%) ultimate capacities compared with the normal concrete filled columns. Furthermore, RACFST columns displayed larger deformation and better ductility than normal CFST columns especially for larger RA replacement ratio. The load-deflection relationship of RACFDST column acts somewhat different from the previous studies on CFDST columns, presenting as no obvious plateau stage. Meanwhile, the recovery of the load after the first peak in the load-deflection curve should be confirmed by more test results with full load-deflection curve up to larger deformation. More experimental testing and studies should be carried out to understand more the influence of RCA on such composite structures. However, the application of RCA in composite tubular columns seems feasible and has potential. 566

9 ACKNOWLEDGEMENTS The authors are grateful to the support provided by NSFC and State Key Lab of Subtropical Building Science, South China University of Technology through Grant 2009KB22. REFERENCES [1] Hansen T.C., 'Recycled aggregate concrete second state-of-art report developments ', Material and Structure. 19(111) (1986) [2] Tabsh S.W. and Abdelfatah A.S., 'Influence of recycled concrete aggregate on strength properties of concrete', Construction and Building Materials. 23(2) (2009) [3] Tam V.W.Y. and Tam C.M., 'Assessment of durability of recycled aggregate concrete produced by two-stage mixing approach', Journal of Material Science. 42 (2007) [4] Evangelista L. and J. de Brito. 'Mechanical behaviour of concrete made with fine recycled concrete aggregates', Cement & Concrete Composites. 29 (2007) [5] Sagoe-Crentisl K.K., Brown T. and Taylor A.H., 'Performance of concrete made with commercially produced coarse recycled concrete aggregate', Cement and Concrete Research. 31(2001) [6] Limbachiya M.C., Leelawat T. and Dhir R.K., 'Use of recycled concrete aggregate in high-strength concrete ', Materials and Structures. 33(12) (2000) [7] Xiao J.Z., Li J.B. and Zhang Ch., 'Mechanical properties of recycled aggregate concrete under unaxial loading', Cement and Concrete Research. 35(2005) [8] Yang Y.F. and Han L.H., 'Experimental behavior of recycled aggregate concrete filled steel tubular columns', Journal of Construction Steel Research. 62(2006) [9] Zhao X.L. and Han L.H., 'Double skin composite construction', Progress in Structural Engineering and Materials. 8(3) (2006) [10] Elchalakani M., Zhao X.L. and Grzebieta R., 'Tests on concrete filled double-skin (CHS outer and SHS inner) composite short columns under axial compression', Thin-walled Structures. 40(2002) [11] Zhao X.L., Tong L.W. and Wang X.Y., 'CFDST stub columns subjected to large deformation axial loading', Engineering Structures. 32(3) (2010)