ACIStructuralJournal/March-April2000 331 ACI Structural Journal, V. 97, No. 2, March-April 2000. MS No. 99-034 received March 9, 1999, and reviewed under Institute publication policies. Copyright  2000, American Concrete Institute. All rights reserved, includ- ing the making of copies unless permission is obtained from the copyright proprietors. Pertinent discussion will be published in the January-February 2000 ACI Structural Journal if received by September 1, 2000. ACI STRUCTURAL JOURNAL TECHNICAL PAPER The load-slip behavior of high relative rib area and conven- tional reinforcing bars subjected to reversed cyclic loading is compared. No. 8 (No. 25) production reinforcing bars with rela- tive rib areas of 0.119 and 0.085 are subjected to reversed ten- sion-tension cyclic loading to evaluate the effect of relative rib area on slip and bond deterioration. The tests demonstrate that the high relative rib area bars exhibit 50 to 70% less unloaded end slip, and 30 to 40% less loaded end slip than the conven- tional bars under multiple cycles of loading. The results sug- gest that high relative rib area bars could be used to improve the behavior of reinforced concrete members and frame joints that are affected by bond deterioration, such as caused by seis- mic loading. Keywords: bond (concrete to reinforcement); cyclic loads; deformed rein- forcement; reinforcing steels; seismic loading; structural engineering. INTRODUCTION Reversed cyclic loading can result in severe deterioration in the bond between reinforcing steel and concrete (Ciampi et al. 1982; Balazs and Koch 1991; ACI Committee 408 1992). This deterioration is of particular concern for beam-column joints in reinforced concrete frames subjected to earthquake loads (Meinheit and Jirsa 1977; Briss, Paulay, and Park 1978; Ehsani and Wight 1985; Durrani and Wight 1985; Leon 1989), since the slip of beam and column bars through joints contributes sig- nificantly to the loss of frame stiffness (Durrani and Wight 1985; Ehsani and Wight 1985; Shu and Jirsa 1983). Darwin and Graham (1993) used ASTM A 944 beam-end specimens to evaluate the bond strength of reinforcing bars with a wide range of relative rib areas R r (ratio of projected rib area normal to bar axis to the product of the nominal bar perimeter and center-to-center rib spacing) under monotonic loading. They observed that, under all conditions of confinement, the ini- tial stiffness of load-slip curves increases with an increase in rel- ative rib area. Their results suggest that a similar improvement can be obtained for high relative rib area bars under cyclic load- ing. This paper presents the results of the first study designed to compare the load-slip behavior of high R r and conventional bars under reversed cyclic loading. The study is significant because stiffer load-slip response with high R r in bars could be used to improve the response of beam-column joints in reinforced concrete structures subjected to earthquake forces. Full details of the study are presented by Zuo and Darwin (1998). EXPERIMENTAL PROGRAM Test specimens High R r and conventional No. 8 (No. 25) reinforcing bars were embedded in concrete, as shown in Fig. 1. The concrete specimens were 16 ft (4.88 m) long, 16 in. (406 mm) high, and 12 in. (305 mm) wide. The bars were placed horizontally through the middle of the specimens at a spacing of 30 in. (762 mm), with bonded lengths of 10 in. (254 mm). The high R r and con- ventional bars were alternated within each specimen. Short pieces of polyvinyl chloride (PVC) pipe were used to control the bonded lengths and to prevent a cone-type failure at the concrete surfaces. Two No. 5 (No. 16) bars were placed at the top and bottom along the length of the specimens to provide flexural strength. No stirrup or tie reinforcement was used. Materials Reinforcing steel —The bars met the requirements of ASTM A 615. The high R r bars, designated 8N3, had a relative rib area of 0.119, and a yield strength of 80.57 ksi (555 MPa). The con- ventional bars, designated 8C0A, had a relative rib area of 0.085 (near the upper limit for most conventional bars [Darwin et al. 1996]) and a yield strength of 69.50 ksi (479 MPa). The yield strengths were determined from tests of three samples of each bar. Bar properties are given in Table 1. Title no. 97-S37 Bond Slip of High Relative Rib Area Bars under Cyclic Loading by Jun Zuo and David Darwin Fig. 1—Schematic of reversed cyclic loading specimen (1 in. = 25.4