Concrete Corrosion
Concrete Corrosion
(OP)
On a site visit for a renovation job we found a slab that had deteriorated bottom steel but no real signs of water intrusion. There was a spray on fireproofing from 30+ years ago applied to the underside of the slab, and the top of the slab was a terrace area. Could this fireproofing cause the temperature to vary enough in the thickness of the slab to cause moisture to accumulate on the bottom steel?
Has anyone run into this type of problem before?
Has anyone run into this type of problem before?
RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke






RE: Concrete Corrosion
1. Alkali Silica Reaction (ASR) is found when the cement is too alkaline, due to a reaction of the silica in the aggregates with the alkali. The silica (SiO2) reacts with the alkali to form a silicate in the Alkali silica reaction (ASR), this causes localized swelling which causes cracking. The conditions for alkali silica reaction are: aggregate containing an alkali reactive constituent, sufficiently high alkalinity, and sufficient moisture, above 75%RH within the concrete. Once the concrete is conpromised, exposure of reinforcement to atmospheric conditions, could explain the corrosion.
2. Chlorides, including sodium chloride can establish a corrosion cell in the concrete and lead to corrosion of steel rebar (typically the bottom layer in structural slabs). Reasons for this? Mixing concrete with chloride contaminated water, use of cement and aggregates having a high chloride content, and use of salts for deicing. Additionally, in some regions use of calcium chloride as an admixture to promote rapid set-up can lead to higher reinforcement corrosion rates. A simple low cost test for chloride content can be employed to determine whether this might be a cause for concern.
Hope this has been helpful.
RE: Concrete Corrosion
Thanks for the information.
We have done some testing and found high chlorides throughout the slab, not just concentrated at the top (salt, environment, etc.) So I think it is more item #2. However, we are trying to determine how the moisture got in the slab because there is no indication of reaction in the top steel in the same areas that the bottom steel is in bad shape. This is pretty consistent around the building in areas like this.
Has anyone run into problems with the spray-on fireproofing?
RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke
RE: Concrete Corrosion
The reason you have observed no degradation at the top of slab is that in an electro-chemical corrosion cell, the top layer serves as the cathode toward which the electrons flow. The cathode doesn't corrode. The bottom layer of rebar is the anode, from which electrons flow. This results in corrosion of the bottom layer along with subsequent concrete cracks and spalling.
If testing has shown chloride counts to be high, the medium for the corrosion cell to be established is already present. Also, experience has shown that trying to simply "patch" the bottom of slab will not have long term success since the root cause, the corrosion cell, still remains.
RE: Concrete Corrosion
Thanks for the information. We are planning on simply replacing the entire slab due to the extent of the corrosion. I will try to find some information regarding the above reaction flow, do you have any articles or locations to find this? I'm going to search ACI and ICRI.
Thanks again!
RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke
RE: Concrete Corrosion
Try pasting the following link into your browser. This book would be a good purchase and should offer the information you are seeking. Good Luck.
http:/
RE: Concrete Corrosion
With the spray-on insulation on the underside of the slab, and the top-side exposed to the elements, do you think the dew point could fall somewhere in the concrete, allowing a small amount of condensation to be in the slab, or is the chloride sufficient by itself?
I found some articles stating that exposure to oxygen is enough to kick start the creation of the cell and corrosion.
Is this true?
(I don't have $125 to throw around right now on that book)
RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke
RE: Concrete Corrosion
It might be wise to visit your municipal or university library. I'm sure you would be able to find plenty of information on this subject there in order to make an informed decision.
RE: Concrete Corrosion
Dik
RE: Concrete Corrosion
RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke
RE: Concrete Corrosion