HIGH-STRENGTH CONCRETE COLUMNS CONFINED BY RECTANGULAR TIES By Daniel Cusson I and Patrick Paultre, 2 Member, ASCE ABSTRACT: This paper presents an experimental study of the behavior of large- scale high-strengthconcrete columns confined by rectangular ties under concentric loading. Effects of key variables such as the concrete compressivestrength, the tie yield strength, the tie configuration, the transverse reinforcement ratio, the tie spacing, the longitudinalreinforcementratio, and the spallingof the concrete cover are studied in this research program. The behavior of high-strength concrete col- umns is characterized by the sudden separation of the concrete cover, leading to a loss of axial capacitybefore the lateral confinementbecomes effective. After the concrete is completely spalled, important gains in strength, toughness, and ductility are recorded for the concrete core of well-confined columns. INTRODUCTION The technology of high-strength concrete (HSC) has greatly improved over the last decade. Higher compressive strength, greater modulus of elas- ticity, and substantial savings resulting from the section reduction are all properties of high-strength concrete that appeal to designers. Unfortunately, present code provisions are often used without any additional precautions for safety in construction projects involving high-strength concrete. More- over, the parameters defining the requirements for lateral confining rein- forcement in current codes are the results of tests done on reinforced con- crete members with concrete compressive strengths lower than 40 MPa (ACI Committee 318, 1989). Therefore, current code requirements may not be adequate nor safe for HSC members. Confinement of normal-strength concrete by rectangular ties has been extensively studied in recent years (Vallenas et al. 1977; Sheikh and Uzumeri 1980; Mander et al. 1988). It has been observed that columns with a good distribution of longitudinal and lateral reinforcement provided in sufficient quantity, as shown in Fig. l(b), possess great ductility and present significant strength gain. Refined stress-strain models for confined concrete have also been proposed. These models, based on test results from normal-strength concrete columns, may not be adequate for concrete of much higher strength. Published data on the performance of large scale (smallest dimension larger than 200 ram) HSC tied columns are scarce (Bjerkeli 1992; Itakura et al. 1992; Nagashima et aL 1992). It has been observed that ductility of HSC columns can be improved by lateral confinement, although in a lesser degree than normal-strength concrete columns. ACI Committee 363 (1987) concluded that more data on the confinement of HSC tied columns are needed. 1Ph.D. Candidate, Dept. of Civ. Engrg., Univ. of Sherbrooke, Sherbrooke, Que- bec, Canada, JIK 2R1. 2prof., Dept. of Civ. Engrg., Univ. of Sherbrooke, Sherbrooke, Quebec, Canada. Note. Discussion open until August 1, 1994. To extend the closing date one month, a written request must be filed with the ASCE Manager of Journals. The manuscript for this paper was submitted for review and possible publication on December 23, 1992. ~This paper, is part of the Journal of Structural Engineering, Vol. 120, No. 3, March, 1994. ISSN 0733-9445/94/0003-0783/$2.00 + $.25 per page. Paper No. 5189. 783