Bond Strength and Development Length Models for Straight Plain Longitudinal Reinforcement in Tension

Abstract

Previous experimental and analytical studies have shown that state-of-practice provisions for evaluating the bond strength and development length of plain bars are not appropriate. Existing provisions tend to provide overconservative estimates of the required development length of plain bars. Using a database of 518 development and 35 splice test specimens, this study proposes simple models for evaluating the bond strength and development length of plain round and square bars. The study demonstrates that the bond strength of a plain bar depends on the casting position, concrete strength, and the ratio of concrete cover to bar diameter. The study also shows that the influence of the loading rate and stirrup confinement level on the bond strength of plain bars may be insignificant. Furthermore, it is shown that the bar size factor in ACI 318 for deformed bars is not justified for plain bars. Using the proposed model, it is concluded that the required development length of a bottom-cast plain bar is 1.33 times that of a bottom-cast deformed bar. Also, the required development length of a top-cast plain bar is two times that of a top-cast deformed bar. The proposed model in this paper is recommended for incorporation into assessment standards.