I found a corroded liner plate.
Concrete(on the back of the plate) test revealed that the chloride and organic carbon contents were very low, but the pH was 8.3.
The gap between the liner plate and the concrete was only 4 mm.
Generally we know that chloride and carbonation cause corrosion.
But how did corrosion and pH decrease in the absence of chlorides and carbon?
Your concrete has carbonated.
Carbonation is the reaction between carbonic acid (formed by the dissolution of CO2 in pore water) and alkaline components of the hardened cement paste (primarily portlandite, but also C-S-H). This reaction leads to a reduction in pH from ~12.5 to ~8.
You question suggests that you have tested for the organic carbon content. However, there is no organic carbon present. Your carbon is bound as carbonate.
Your concrete has carbonated.
Carbonation is the reaction between carbonic acid (formed by the dissolution of CO2 in pore water) and alkaline components of the hardened cement paste (primarily portlandite, but also C-S-H). This reaction leads to a reduction in pH from ~12.5 to ~8.
You question suggests that you have tested for the organic carbon content. However, there is no organic carbon present. Your carbon is bound as carbonate.
To add to Leon's answer; you do not need chlorides for corrosion to occur. Chlorides will help accelerate corrosion, but only the presence of low pH, oxygen and water are required to satisfy the electrochemical conditions for corrosion.
Try to test the surrounding area and depth with a ph-indicator, e.g. water/alcohol mixture with 1% phenolphtaleine, to spray on the surface, maybe this gives you more information about the carbo-progress. Best regards, Andreas
Thank you everybody.!!
Now I have one mere question.
Could the phenomena you described only happen in a local area?
It is only about 20cm in length and 30cm in height.
The pH in all other areas is above 10.
best regards.
When we talk about more specific environment,
it is rainy at the time of concrete plcing, and the place where corrosion occurred is high temperature (40 ~ 60 degrees C).
The scenario I think is:
1. Sulphate leaching due to decomposition of ettringite at high temperature
2. Carbonation progresses due to the combination of pore water and carbon
3. because of carbonation Low pH
4. Sulfate causes corrosion
One question I have here,
Is there an amount of carbon that causes carbonation at a distance of about 4 mm between the liner plate and the concrete?
There are a couple of points to note. Just because the pH is high, it doesn't mean that there is no carbonation. It could be that there is partial carbonation throughout the sample, but with a small quantity of residual portlandite present everywhere apart from this region.
If it is only this region where there has been any carbonation then it could well be due to a localised environment. If the surrounding area is wet then it would lead to reduced carbonation because the pores are saturated.
I don't think that decomposition of ettringite is likely. Ettringite only begins to decompose at 50 degrees C and the formation of gypsum from ettringite decomposition is not likely to lead to corrosion. Besides, if this was a cause then it would be present throughout the sample, rather than localised.
Thank you very much Mr. Leon Black.
I appreciate your kind advice.
Maybe I have to take an additional test to find out exact environmental conditions.
I completely agree with that, carbonation is the reaction between carbonic acid (formed by the dissolution of CO2 in pore water) and alkaline components of the hardened cement paste (primarily portlandite, but also C-S-H). This reaction leads to a reduction in pH from ~12.5 to ~8.
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