Behavior of concentrically loaded high performance concrete tied columns Mahesh Awati a,⇑,1 , R.B. Khadiranaikar b,1,2 a Civil Engineering Department, B.L.D.E.A’s V.P. Dr. P.G. Halakatti College of Engineering and Technology, Bijapur, Karnataka 586 103, India b Department of Civil Engineering, Basaveshwar Engineering College, Bagalkot, Karnataka 587 102, India article info Article history: Received 13 November 2010 Revised 23 December 2011 Accepted 24 December 2011 Available online 2 February 2012 Keywords: Column Concentric Ductility High performance concrete Post-peak response abstract The paper presents an experimental study of behavior of high performance concrete (HPC) columns under monotonic concentric loading. The test variables include strength of concrete, amount and spacing of transverse reinforcement, configuration of ties and amount and yield strength of longitudinal rein- forcement. Concentrically loaded columns of HPC are characterized by early cover spalling due to the plain of separation formed between the cover and core. The equation in codes and literature to predict the axial load capacity of columns were based on tests of normal strength concrete (NSC). Hence, use of existing code equation leads to overestimation of concentric load capacities. The failure pattern is sequenced by the buckling of longitudinal bars, yielding of lateral ties and rupture of ties. The test results indicate decrease in the spacing of lateral ties and increase in volumetric ratio of transverse steel, increases marginally the strength but largely the ductility. Also an equation is proposed such that the pre- diction of axial load capacity is neither overestimated nor too conservative. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction The technology of high performance concrete (HPC) has greatly improved over the last decade. Higher compressive strength, great- er modulus of elasticity and substantial savings resulting from sec- tion reduction are all properties of high performance concrete that greatly influence the structural designers. However, most of the current design specifications, such as ACI [1], IS 456 [2], AASHTO LRFD bridge specifications [3], are still based on tests conducted using normal strength concrete (NSC). The Indian standard specifications (IS: 456-2000 [2]), published recently limits its applicability to maximum concrete strength of 55 MPa, unless physical tests are made to establish the relationship between concrete strength and its other properties. These limita- tions reflected the lack of research data at the time, rather than the inability of the material to perform its intended function [4]. Therefore, it is necessary to examine the behavior of structural members in order to extend its use for HPC. Confinement of NSC with lateral ties is not posing a serious problem as in higher strength concrete. This phenomenon is due to poor ductility of higher strength concrete. Since, higher strength concrete assumes large loads without any crack until peak is reached, then they tend to suddenly fail in the absence of proper confinement. At peak, cover concrete suddenly looses the stability and peels off reducing the load carrying capacity of column. Once the cover is broken the longitudinal bars and transverse steel becomes active in resisting the side pressure. In poorly confined columns an explosive failure occurs where as in highly confined columns the ultimate failure is delayed due to yielding of closely spaced ties and ultimately crushing of core concrete. It is also ob- served that the ductility of columns can be improved by proper confinement. Under the absence of strain gradient, the peak strains achieved are of very low magnitude compared to columns tested in flexure. The few studies carried out on the structural behavior of con- fined concentric columns in the recent past include the studies made by Bjerkli et al. [5], Li et al. [6], Cusson and Paultre [7], Saatcioglu and Razvi [8,9], Foster and Attard [10], Sharma et al. [11], Ho and Pam [12], Kim and Lee [13] and Sungjoong [14]. There is also lack of regulations in the codes on the behavior of higher strength concrete columns under concentric loading. The compar- ison of tested columns with HPC indicates overestimation of load capacities by ACI, due to early cover spalling. Hence, the present study with the variables such as strength of concrete (60– 120 MPa), amount and spacing of lateral ties, amount and yield strength of longitudinal reinforcement has good relevance. 2. Research significance In today’s construction NSC is replaced by HPC due to its greater advantages especially in columns of high rise buildings. Many 0141-0296/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.engstruct.2011.12.040 ⇑ Corresponding author. Tel.: +91 099029 88138; fax: +91 08352 263945. E-mail addresses: mahe_awati@yahoo.co.in (M. Awati), rbknaikar@rediffmail. com, dr.rbknaikar@gmail.com (R.B. Khadiranaikar). 1 Affiliated to Visvesvaraya Technological University, Belgaum, State-Karnataka, India. 2 Tel.: +91 094489 86877; fax: +91 08354 234204/234121. Engineering Structures 37 (2012) 76–87 Contents lists available at SciVerse ScienceDirect Engineering Structures journal homepage: www.elsevier.com/locate/engstruct