An investigation of the strain profile over the cover in reinforced concrete elements subjected to tension

The strain profile over the cover in reinforced concrete ties subjected to tension is investigated in this paper. This is normally neglected in the crack width calculation methods recommended by Eurocode 2 and fib Model Code 2010, meaning that it is assumed uniformly distributed over the cover. However, this assumption is questionable in the case of large covers. A pragmatic approach of accounting for the non-uniform strain profile over the cover in an analytical crack width calculation model can be by using the concept of the strain variation parameter ψ, which relates mean concrete strains over the cover to concrete strains at the steel bar surface at an arbitrary section over the bar length, x. Virtual experiments on six cylindrical concentrically reinforced concrete specimens were thus conducted by using nonlinear finite element analysis to establish a better understanding of the strain profile over the cover and, if possible, to obtain a proper value for ψ(x). The results show that ψ more or less remains constant over the bar length, except for the region close to the loaded end. This means that assuming a constant value for ψ seems reasonable. The practical significance of this finding is that the non-uniform strain profile over the cover can be properly accounted for in deriving and solving the second order differential equation for the slip between concrete and steel. This would ultimately yield an analytical crack width calculation model that predicts crack widths consistently, also in cases for large covers and bar diameters.