Load bearing capacity of existing concrete half-joints

Abstract

Concrete half-joints are a specific support detail in reinforced concrete structures, which reduce the construction height of the total structure. The use of concrete half-joints became popular around 1950s, but decreased its interest as a result of new insights on structural behaviour and collapses of these structures. Typical issues regarding concrete half-joints are either due to inadequate reinforcement detailing or due to deterioration mechanisms. The most critical deterioration is when a crack at the re-entrant corner allows for water ingress and thus corrosion of the rebars. This thesis focuses on the negative influence of this corrosion on the load bearing capacity and proposes an assessment method, which includes the reinforcement detailing. Firstly, problematic half-joints have been categorised and studied for reinforcement detailing. Subsequently, they have been analysed using an analytical tool, in which corrosion was implemented on the rebars. The outcomes were validated numerically.

For this thesis research, a series of Dutch concrete bridges has been studied to identify general reinforcement issues and categorise concrete half-joints. It has been observed that the majority showed short transfer- and/or anchorage lengths of the rebars and that all of them showed no shear stirrups as hanger-reinforcement. In stead, only a horizontal- and hanger rebar are present, which can be accompanied with a diagonal rebar, prestressing at the top or nib (or combinations in between).

An analytical tool is designed to calculate the load bearing capacity of (un)corroded concrete half-joints. The analysis is based on a lower-bound approximation using a strut-and-tie approach and an upper-bound approximation using a kinematic approach. The analytical tool is used to determine the load bearing capacity of the series of investigated Dutch concrete half-joints. Both approximations are comparable when rebar failure is the governing failure mechanism. The strut-and-tie approach also incorporates detailing checks, which are not considered in the kinematic approach. Therefore large differences occur when detailing governs the capacity.

The nodes in the strut-and-tie model, in which two ties are connected to one concrete strut, appear to be critical in the lower-bound solutions. The capacity depends on the concrete strength and dimensions of the node. The dimensions are influenced by the mandrel diameter of the hanger-rebar and anchorage length of the horizontal rebar. In order to study the influence of corrosion on the load bearing capacity, the effect of a reduced rebar capacity due to an increasing corrosion rate was implemented in the analytical tool. The kinematic approach appears to be more sensitive to load bearing capacity loss, as this calculation depends mainly on the strength of the rebars. The strut-and-tie approach is able to redistribute forces over the struts and ties and is less sensitive.

In order to verify the analytical results, a numerical study is performed. The specimens are modelled in such a way that rupture of one of the rebars at the re-entrant corner is governing. Both analytical solutions appear to be conservative compared to the numerical results, in which the lower-bound solutions are very conservative. Different crack’s angles have been found between the upper-bound calculation and numerical results. If the same angle is applied in the analytical tool, the difference reduces from 7% to 2% for an uncorroded concrete half-joint without diagonal. The differences can be explained by the simplification in the kinematic approach, in which the concrete compression zone is not able to transfer shear stresses.

Based on the conclusions of the analytical tool and numerical verification, an assessment method is proposed in which the upper-bound solution is combined with the lower-bound solution. If the load on the concrete half-joint is lower than the calculated lower-bound solution, the concrete half-joint is safe. However, questions arise if the load is between the lower- and upper-bound solution, in which structural safety cannot be guaranteed. The analytical tool is still a useful tool to understand the behaviour and vulnerabilities of the concrete half-joint. The analytical tool is even more useful if the strut-and-tie approach is governed by rupture of the horizontal-, diagonal- or hanger-rebar. The kinematic approach can be extended by implementing the same crack’s angle, which occurs in the existing concrete half-joint.