Corrosion pits in steel bars embedded in cracked concrete

Here is a short update on what I've found on inspecting the bars after extracting them from the specimens. The first image shows the size of pits found in a specimen with a 'smaller' crack width'.

The second image shows what happens when the crack width in increased. Both specimens were subject to the same amount of salt-dry cycles.

In both cases, the original image dimensions were 5600x4200 microns approx. However, these images have been resized for posting purposes.

There are very interesting findings that will be published in the near future.

Until next time,

Corrosion of steel in concrete

Figuring out what is really going on during corrosion of steel in concrete is no easy task That's why we use advanced experimental techniques to characterize the process. Here's an example:

 

On the left, we see the concrete at the steel concrete interface (dark), in the middle, corrosion products are visible (gray), and on the right, we see the sound steel (white). Characterizing their mechanical properties is our task! Stay tuned for updates! 

Update 1:

How corrosion induced cracking takes place:

Also, different cross sections of the same beam can be observed (top - with PVA fibers; bottom - no fibers):

Visual analysis of corroded bars

I've been working on the analysis of corroded reinforcing steel embedded in cracked concrete. The cubes were mechanically split so the reinforcement can be assessed. All images are taken with the stereomicroscope.

A layer of orange rust can be observed on the steel surface. However, there are still some corrosion free spots. The original image size was aprox. 17000 x 1300 microns*.

This last image shows the thickness of the oxide layer on the steel surface. 

Next step will involve the removal of the rust layers and assessing the size if pits, if any.

Until next time,

* Note: Images were resized for this post.

REPOST: I've just added two images of bars embedded on concrete samples with different crack widths. See the effect of the crack width on the deterioration.

Cheers

When the levee (or concrete) breaks

This week finished with a set of concrete specimens being cracked for chloride penetration and reinforcement corrosion tests. Overall, it was a successful experiment.
Cheers

Cracked concrete infrastructure

Betondag 2012 is coming next Thursday November 15th to Rotterdam. A brief, but interesting presentation regarding the influence of cracks on concrete durability will be given by two PhD candidates of the M3C4 project. If interested, don't hesitate to attend the presentations given by PhDs during Betondag.

Until Thursday!

Corrosion rates of steel in cracked concrete

Cracks in concrete cover allow fast penetration of aggressive substances such as chlorides, leading to reinforcement corrosion. In the Figure 1, corrosion rates of embedded reinforcing bars in concrete with transverse cracks with widths ranging from 135 to 375 microns are shown. It seems that crack width has an effect on the  evolution of corrosion rate in laboratory conditions.

Figure 1. 

In Figure 2 corrosion potentials are shown, providing indication of the thermodynamical condition of the steel reinforcement. One interesting effect is found on crack width of 217 microns. Although potentials are significantly less noble than the rest, corrosion rates were found to be the lowest of all specimens. It is clear that the experimentation must go on, but so far interesting results have been obtained!

Figure 2. 

More results to be posted soon!

Corrosion induced crack patterns are influenced by many things. Rebar diameter, spacing, cover depth, and boundary conditions all play a role. In the videos, two different scenarios are compared: both specimens have 2 rebars of 10mm diameter, but one has a cover depth of 10 mm, while the other one has a cover depth of 20 mm. Crack growth in both specimens is shown. The difference is obvious.

However, in reality, corrosion rarely causes uniform expansion of the reinforcing steel. Therefore, it is possible that real pressure on the surrounding concrete acts in a different way (Ohtsu and Uddin, "Mechanisms of corrosion-induced cracks in concrete at meso- and macro-scales", Journal of Advanced Concrete Technology Vol.6 , No. 3, 419-429, October 2008):

The effect of different scenarios on the crack patterns in our model is visible in the following snapshots (at 5000 steps- yellow for visibility):

(top to bottom- radial, horizontal and vertical loading scheme, respectively)

It is clear that the loading condition makes a difference in the cracking behavior of the cover. Hope everyone is convinced! Enjoy the holidays!

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!