When the reinforcing steel bar inside concrete corrodes, it starts expanding, since the volume of the oxides that form during the process is higher than the volume of the original steel. It, therefore, exerts pressure on the surrounding concrete, which eventually cracks. To study the mechanism, the lattice model can be used. Here is the result of the first simulation of corrosion induced cracking. Enjoy!

The "rebar effect"

It has been reported in the literature that the buildup of chloride ions occurs at the top of the rebar. This means that the higher chloride concentration can be observed at the top of the reinforcing steel, than at the same depth to the side. Since reinforcement is impermeable to chloride ions, this actually makes perfect sense. Therefore, sampling the chloride ions at the side of the rebar to study, for example, the chloride threshold concentration, can lead to serious errors. This has be proven experimentally by Yu et al. (2007) in their study. So, what is a simple way to check this hypothesis? Well, numerical simulation, of course!

First figure shows the setup. Part of the mesh was was made impermeable to simulate the effect of reinforcing steel, while the chloride can freely penetrate the other side of the sample. The specimen was then subjected to chloride penetration for a certain period.


And here is the result:

Clearly, the phenomenon does occur. And what are the implications for the Round Robin test of the CTC Rilem committee? I guess we'll just have to wait and see!

Reference: H. Yu, W.H. Hartt, Y.P. Virmani, "Effects of reinforcement and coarse aggregates on chloride ingress into concrete and time-to-corrosion: Part 1-Spatial Chloride Distribution and Implications", NACE Corrosion 2007 Conference & Expo