Overview of Research to Improve the Development Characteristics of Reinforcing Bars

Abstract

The results of the first major reevaluation of reinforcing bar geometry in the United states in nearly 50 years is described. The study involves experimental and analytical efforts designed to broaden the understanding of factors that control bond strength, improve the development characteristics of reinforcing bars, and develop practical design expressions that more accurately represent development and splice strength than exisiting expressions. The research has established that deformation pattern has little effect on the bond strength of uncoated bars that are not confined by transverse reinforcement. Deformation pattern, however, as represented by the relative rib area, does have a major effect on the bond strength of bars that are confined by transverse reinforcement. Increases in relative rib area, obtained with either higher ribs, closer ribs, or a combination thereof, result in an improved bond strength for confined bars. The study has also established limits on how closely ribs can be placed without resulting in pullout failure. High relative rib area bars provide a reduction in development/splice length of 20 percent for all coated bars, independent of the presence or absence of confining transverse reinforcement. Based on the experimental work, expressions are developed that accurately characterize development/splice strength. The resulting design expressions are accurate for compressive strengths between 2500 and 16,000 psi (17 and 110 MPa). The most accurate representation of the effect of transverse reinforcement on bond strength include parameters that account for the number of transverse reinforcing bars that cross the developed/spliced bar, the area of transverse reinforcement, the number of bars developed or spliced at one location and the relative rib area, and size of the developed/spliced bar. The yield strength of the transverse reinforcement does not play a measurable role. Practical comparisons illustrate reductions in splice lengths of 20 percent for conventional bars and 30 percent for high relative rib area bars to current requirements in ACI 318-95.