Would anyone clear my doubts about whether there is an alkaline or non-alkaline nature of cement/concrete causes corrosion in reinforced concrete? And how?
The concrete media as an electrolyte causes anodic and catodic regions at the embedded steel bars (reinforcements) in the reinforced concrete. This situation leads to a passive corroded layer on the surface of these reinforcements in concrete. Besides, the concrete media is alkaline due to the portland cement hydration product Ca(OH)2 (portlandite). This alkalinity protects the stability of this passive layer. Due to this passive layer, the corrosion of the reinforcements (steel bars) embedded in concrete can not continue. When the amount of portlandite( due to pozzolans or carbonation, etc.) decreases, the alkalinity of the concrete media also decreases. Thus, the corrosion of the reinforcements can develop and the corrosion products expand at the interface region between concrete and the reinforcement bars. These expansions cause to cracking and deterioration of the reinforced concrete. Finally, I can say thay non-alkaline media deactivates the passive layer and leads to corrosion propagation. If you look for detailed information, you can check the books below.
1. A.M. Neville, Properties of Concrete.
2. P.K. Mehta, P.J.M. Monteiro, Concrete: Properties, ......., Microstructure.
The concrete media as an electrolyte causes anodic and catodic regions at the embedded steel bars (reinforcements) in the reinforced concrete. This situation leads to a passive corroded layer on the surface of these reinforcements in concrete. Besides, the concrete media is alkaline due to the portland cement hydration product Ca(OH)2 (portlandite). This alkalinity protects the stability of this passive layer. Due to this passive layer, the corrosion of the reinforcements (steel bars) embedded in concrete can not continue. When the amount of portlandite( due to pozzolans or carbonation, etc.) decreases, the alkalinity of the concrete media also decreases. Thus, the corrosion of the reinforcements can develop and the corrosion products expand at the interface region between concrete and the reinforcement bars. These expansions cause to cracking and deterioration of the reinforced concrete. Finally, I can say thay non-alkaline media deactivates the passive layer and leads to corrosion propagation. If you look for detailed information, you can check the books below.
1. A.M. Neville, Properties of Concrete.
2. P.K. Mehta, P.J.M. Monteiro, Concrete: Properties, ......., Microstructure.
I would like to add that corrosion doesn't necessary need to be happened over a big contacted area. It could be on micro scale as well as micro scale. The bottom line is that corrosion of steel reinforcement in concrete can't practically be avoided for many reasons. the above mentioned books are excellent references, in fact Prof. Neville book is considered as the concrete bible by many.
Dr. Bilir and Dr. Ismail, thanks for your answer.
If we use already less alkaline cement or use some non-alkaline cementitious materials in partial replacement to cement may it effect the corrosion (means it increase or decrease the corrosion)?
As decrease in alkalinity of cement means decrease in early strength. Would it beneficial for late strength and may help in reducing the corrosion?
Also, increased strength means reduced porosity, permeability and compact structure which may reduce the corrosion in reinforced concrete. Am I right? please correct me if I am wrong.
Thanks.
It depends on what type of cement you use..You may give the nrpoperties of the cement or The Books I have mentioned will have exacy knowedgeç
Thank you.
As replied elsewhere to your question, steel rebar used in RCC construction is protected from corrosion due to high pH environment (12.5 - 13.5) where steel becomes passive. However, due to carbonation or use of NaCl for ice-breaking, Cl- ion in coastal region may bring down the pH level significantly affecting passivity of steel and it starts corroding.
Similarly, use of less alkaline mixture/ concrete will decrease the pH value of the environment and steel will corrode at faster rate.
You may find details in the attached paper.
Article Protection of Steel Reinforcement for Concrete - A Review
Please see potential pH diagram for Fe in water .please pitting conditions
two types of corrosion that can be found in the concrete:
- Carbonation, which is a drop in pH of the cementitious paste caused by the interaction of the concrete with carbon dioxide in the atmosphere, causing general corrosion in the steel reinforcement.
- Chloride induced corrosion which is caused by the migration of chloride ions through the cementitious paste until they reach the reinforcement surface. Once a high enough concentration of chlorides is reached, localised corrosion initiates, forming pota on the rebar surface.
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