Effect of reduced concrete cover on strength of lapped joints

Abstract

The paper reports on an experimental investigation into the performance of lapped joints in structural concrete. The investigation focuses primarily on concrete covers close to or below the minimum set for structural reasons in design codes such as Eurocode 2 or ACI 318. Two other parameters are also varied in the tests reported here, namely the proportion of bars lapped at a section and the relative size of the bars within a lapped pair. It is found that a reduction in minimum cover from 1·2 to 0·6 times bar diameter would require an increase in lap length of 21% to develop the same stress in a lapped bar. It is verified that provisions for lapped joints in the fib Model Code 2010 are valid for minimum covers down to 0·5 times the diameter of a lapped bar. It is also found that lap strength does not change significantly when laps are staggered. The finding raises questions over the validity of the α6 coefficient in Eurocode 2, which is dependent on the proportion of bars lapped at a section. Results demonstrate that, where lapped bars differ in diameter, lap length may safely be based on the diameter and covers to the smaller diameter bar.