No abstract

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Concrete possessing high-passing ability needs to be flowable and cohesive. Hence, passing ability cannot be improved by solely adding superplasticizer, which increases both flowability and segregation of concrete simultaneously. Decreasing the maximum size of aggregates so that concrete segregates at lower cohesiveness is a possible but undesirable way as it narrows the aggregates' grading and decrease dimensional stability of concrete. With the same maximum size of aggregates, passing ability can be improved by raising the concurrent flowability-segregation envelope of concrete. In this paper, fly ash and silica fume (cementitious fillers) and limestone (inert filler) were selected to replace cement partially and subsequently the passing ability of concrete was studied. From the results, it was evident that when either type of fillers were used, the passing ability and maximum limits of flowability and segregation achieved simultaneously increase. It is because these fillers are finer than cement that provides better filling effect to increase packing density and excess water leading to better flowability. Concurrently, the cohesiveness of concrete also increases as the content of fine particles increases. These allow concrete to hold the coarse aggregates more firmly when passing through narrow gaps, after which the concrete will keep flowing rapidly. K E Y W O R D S

SUMMARY Concrete‐filled‐steel‐tube (CFST) columns have been widely adopted for column construction of tall buildings due to its superior strength and ductility performance contributed by the composite action. However, this beneficial composite action cannot be fully developed at early elastic stage as steel dilates more than concrete and thereby causing imperfect interface bonding. Hence, it reduces the elastic strength and stiffness of the CFST columns. To resolve the problem, external confinement in the form of steel rings is proposed in this study to restrict the lateral dilation of concrete and steel at initial elastic stage. In this paper, CFST columns of various dimensions cast with normal‐strength or high‐strength concrete and installed with external steel rings were tested under uni‐axial compression. From the results, it was evident that (a) the external steel rings could restrict the lateral dilation of CFST columns and improve the interface bonding condition and (b) externally confined CFST columns had uni‐axial strength and stiffness larger than those of unconfined CFST columns. With the experimental results, an analytical model taking into account the confining effects of steel tube and rings has been developed to predict the uni‐axial strength of ring‐confined CFST columns. Copyright © 2012 John Wiley & Sons, Ltd.