concrete corrosion

huangyhg

concrete corrosion
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?
rc
all that is necessary for the triumph of evil is that good men do nothing.
    edmund burke

here's a couple possible causes to consider.  
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.
  
lobstaeata,
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

hi rc
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.
lobstaeata,
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

rc
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.
lobstaeata,
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

my humble opinion is that the spray-on insulation concern would be a non-starter.  if the concrete is chloride laden, then atmospheric humidity and/or rain would be all that would be necessary to activate the corrosion cell.  there has been a lot of study regarding the subject of steel reinforcement corrosion in concrete structures over the years.  i understand not wanting to drop money on the textbook, if this is only a small project, but you'll need to perform reasonable due diligence on this assignment regardless.
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.
i find the chloride content throughout a bit of a puzzle.  any chance the original mix had cacl2? or used salt laden water?
dik
the slab was poured in 1968 or so. it is possible the original mix contained cacl2. it was waterproofed on top and did not show a real sign of water infiltration. we were wondering if the insulation on the underside of the slab could move the dew point to in the slab on cold/hot days, where the interior temperature and exterior temperature can vary by 30-40 degrees.
  
rc
all that is necessary for the triumph of evil is that good men do nothing.
    edmund burke

i would give odds on that the concrete had calcium chloride in it if it was cast in winter.  replacing the slab is the right decision.