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,

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

Calcareous aggregates on BSE mode

Studying the composition of concrete samples can be performed easily under the ESEM. Sometimes, the origin of the aggregates and their main composition must be determined. In this case is the presence of calcareous (CaCO3) aggregates in the concrete mix. In some countries, Aggregates are obtained from crushed rocks. Calcium carbonates have a similiar intensity under the BSE mode when compared to CSH. This quite be uncommon in places where aggregates are commonly from siliceous origin.

 Sample w/b ratio 0.45

 

Sample w/b ratio 0.65

A quick look on the previous images confirms the presence of calcium carbonate as the main aggregates in concrete samples. In the coming future, more explanations regarding the EDS analysis on this type of samples will be shown.

Until next time,

Bending cracks in reinforced concrete cubes

During the course of the M3C4 research, a Modified Wedge Splitting Test (MWST) was employed as the main source of getting controlled cracks in reinforced concrete specimens. For this purpose, we used concrete cubes that contained a recess, where the action of the wedges is applied; and two reinforcing bars embedded in concrete.

After cracking, one can only measure the COD (Crack Opening Displacement) at the concrete surface by means of LVDTs. After some other procedures, we're now able to visualise the shape of the crack inside the concrete cubes.

More results are about to come.
Until next,

Big city, big Conference

The view of Chicago's skyline from Jon Hancocks's Observatory.

 The 35th International Conference on Cement Microscopy was held from April 29th to May 1st in Chicago, Ill. The M3C4 had the opportunity to attend and show some of the research lines we've been currently looking into. The conferece had a lot of interesting presentations and extensive discussions regarding cement microscopy.

We also had the opportunity of visiting the headquarters of PCA (Portland Cement Association) based in Skokie, Ill. The facilities were excellent for research on cementitious materials.

From the M3C4 here's a big shout-out to the organizing team of ICMA and hopefully, we'll see you again next year in Italy.

Headquarters of PCA, Skokie, Ill.

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

Of concrete and microscopes

The use of the scanning electron microscope (SEM) in concrete research is not new. One of the main advantages of using the SEM is the characterisation of elements in cement phases, hydration products (CSH) or deleterious reactions (ASR). Energy-Dispersive X-ray Spectrometry (EDS) is capable of detecting elements heavier than carbon (C), but in order to provide fully-quantitative analysis, a reference mineral must be employed.

Why is this reference mineral so important? Well, let me provide an analogy to explain it.

Imagine that your car's speedometer is not properly calibrated. It tells you that your speed is 60 mph, yet your driving speed is 100 mph. It would be hard to convince the police officer that you didn't intend to overspeed.

Going back to microscopes, the reference mineral will calibrate your EDS detector to acceptable values.  This will provide your research with strong scientific proof that the elements that you've encountered, and their atomic proportions, are correct. The selection of the reference mineral will depend on the elements you are interested in, their atomic ratios, etc. While it is not a simple task, it will improve your results dramatically.

Until next time,

SEM training at WHD Microanalysis

The first half of this week I had the opportunity of visiting Aldeburgh (see above) and Woodbridge, UK; working place of Nick B Winter of WHD Microanalysis Consultants. For two days, Dr. Oguzhan Copuroglu and myself could talk to Nick, an expert on SEM microanalysis, and engage in great discussions regarding the possibilities of SEM for concrete and cementitious materials. Nick is an expert on this field with more than 30 years of experience; he even had the opportunity of working with Hal Taylor, known author of Cement Chemistry book. On top of that, Nick owns a SEM (see below) which is probably one of the best to have fun with. Turning knobs and clicking buttons does make a difference when learning this stuff. He was so kind to let me use it for a while and do some EDS analysis. Hopefully our collaboration will continue in the future.

Concrete cracking Pt1.

Cracks in concrete cover are definitely a concern for concrete infrastructure. The figure shows cracks propagate not only vertically, i.e. towards the reinforcement, but also horizontally once it has reached the steel bar position.  It is clear that the internal cracked concrete area is far larger the surface crack width. Hopefully, this will be taken into account in future durability design approach.

Until next time,

Jose

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!

Other concrete blogs

There seems to be an increasing number of blogs dealing with concrete research on the internet. We don't know if we (Microlab) started it, but we like the trend!

For more info, check out the following:

1. www.cementscience.com  - This is a website on research of cement science, a world of cementitious materials by Tan Zhijun.
2. http://www.csla-project.blogspot.com - Concrete Service Life Assessment (The VTT/Tekes Research Project CSL)  will study the characteristics of chloride ion transport in concrete subject to frost attack (surface scaling and internal cracking), and develop models for more reliable service life design approach taking into account the interacted effect of degradation mechanisms. This blog was started by Dr. Miguel Ferreira of VTT, Finland.

Cheers!

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!