Concrete Carbonation of soffit of first floor slab above below grade garage of condo
Concrete Carbonation of soffit of first floor slab above below grade garage of condo
(OP)
For the case of the first habitable floor above a below-grade unheated parking garage (say the first floor of a condo or apartment building with underground parking) in a cold climate like Canada and the northern U.S., I expect that it would be normal to have insulation on the underside of the floor slab to reduce heat loss from the condo suite to the unheated garage. I also expect that most modern below-grade garages like that would be unheated to save energy.
Questions:
1. Should there be any concern about carbonation (due to vehicle exhaust) of the concrete cover to the bottom reinforcing bars in the first floor of a condo above a parking garage?
2. In a modern properly designed building, would there be a vapour barrier on the top side of the insulation? (i.e. on the underside of the slab, which would be the warmer side of the insulation). If the answer to question 1 is yes, I am wondering whether the vapout carrier would mitigate carbonation f teh concrete
2. Would there also be an air barrier at the slab underside or would an air barrier not be used?
Reason for above questions:
The reason for the questions is to determine if there should be any concern about the carbonization of the bottom 30 mm of the slab due to higher CO2 concentrations that might be expected in a parking garage due to vehicle exhaust. If the bottom 30 mm of concrete cover to the rebar in the slab become "carbonated" then the bottom rebar would be subject to corrosion in the presence of oxygen and water. My opinion is that carbonation in concrete is such a slow process that this is not a real concern even in a parking garage, but I know a structural engineer who perhaps thinks that it is, so answers to the questions that I posed above might help decide the question at least in a qualitative way.






RE: Concrete Carbonation of soffit of first floor slab above below grade garage of condo
That is correct, and to reduce a cold first floor... noticeable on the tootsies.
I also expect that most modern below-grade garages like that would be unheated to save energy.
To save energy and also to reduce corrosion of vehicles parked within. Heated parkades generally have a greater rate of vehicle corrosion. Current energy code requirements applicable in Canada almost force this insulation requirement.
Should there be any concern about carbonation (due to vehicle exhaust) of the concrete cover to the bottom reinforcing bars in the first floor of a condo above a parking garage?
Corrosion in parkades occurs from the slightly increased carbonation as well as the 'salt spray' from the vehicles. You can use a simple phenopthalien (sp?) test to detect the pH reduction due to carbonation.
In a modern properly designed building, would there be a vapour barrier on the top side of the insulation?
Vapour barrier is normally placed on the warm side of the insulation, where warm moister air comes in contact with a colder surface. The vapour barrier and finish would reduce the rate of deterioration due to carbonation as well as salt spray contact. Finishes can be subject to damage. Is the parkade sprinklered? Does it have a floor below?
Would there also be an air barrier at the slab underside or would an air barrier not be used?
The two products have their individual purposes. An air barrier would likely be a bit of an 'overkill' in an enclosed parkade, not subject to significant variations in air pressure.
The reason for the questions is to determine if there should be any concern about the carbonization of the bottom 30 mm of the slab due to higher CO2 concentrations that might be expected in a parking garage due to vehicle exhaust. If the bottom 30 mm of concrete cover to the rebar in the slab become "carbonated" then the bottom rebar would be subject to corrosion in the presence of oxygen and water.
30mm is reasonable concrete cover for corrosion and is often used. A good concrete sealer will help mitigate the problem as well as reduce the effects of salt spray (Silane or Siloxane). If you have enough CO in the air to seriously affect this, you may have more serious problems. Enclosed parkades have to have a minimum number of 'air changes per hour' to keep the CO in check (0.2% of CO can kill you in 30 minutes it bonds to your red blood cells and displaces oxygen). You should also have CO detectors mounted low (CO is heavier than air). I get a chuckle of finding heat and CO detectors sold as a unit.
You should hire an engineer with experience in parkades to confirm this, or to allay any concerns.
Dik
RE: Concrete Carbonation of soffit of first floor slab above below grade garage of condo
I appreciate your time and efforts to answer my questions. I have been heavily involved in parking structure structural analysis, structural design, construction, structural investigation and repair for at least 35 years, so hopefully I know the structural answers.
While I have some data on age of garage concrete vs. depth of carbonation (and concrete properties), it is not extensive and if you have any such data and can share it with me, it would be quite helpful and be just the sort of info I am looking for.
If there is anyone who has such data, or more to the point if there is anyone who who can think of any reason why C1 concrete should be used in such a floor if it is not equired for structural strength reasons, I would be interested in hearing from you. My opinion remains that C1 concrete is not required in such a floor, irrespective of the presence or absence of any vapour barrier.
RE: Concrete Carbonation of soffit of first floor slab above below grade garage of condo
"Structurally reinforced concrete exposed to chlorides with or without freezing and thawing conditions.
Examples: bridge decks, parking decks and ramps, portions of structures exposed to seawater located within the tidal and splash zones, concrete exposed to seawater spray, and salt water pools. For seawater or seawater-spray exposures the requirements for S-3 exposure also have to be met."
Can you take a concrete sample and have it petrographically tested; it's the quickest way to find out what you have (strength, w/c, air, etc.) and degree of carbonation. The phenopthalien test to detect the pH reduction is the common one I've used in the field to get a depth. [Added] The phenopthalien test is easy and quickly shows the depth of carbonation.
I have no data, but there is likely data reflecting the corrosion correlation with chloride content and pH of concrete.
With concrete subjected to chloride penetration it's a matter of periodically checking for signs of corrosion or perhaps doing some NDT work.
You may check with some of the universities to see what research has been done. Maybe someone on the forum can provide this.
[Added] Earlier parkades and related structures were often neglected for the issues of corrosion deterioration. It wasn't known and didn't appear for a decade after construction. Fortunately, I worked with RJC for 10 years, who were one of the foremost parkade planners and designers in Canada. A lot of the work complied with current CSA Parkade requirements, decades before they were implemented.
I've been involved with numerous parkades... analysis, design, and repair over the last several decades, also.
Dik
RE: Concrete Carbonation of soffit of first floor slab above below grade garage of condo
Carbonation is usually limited in depth to about 1/8" to 1/4". Under extreme environments, those depths can increase, but that is not a common issue. It is rare to see carbonation to depths greater than 1/2" or 13mm.
As dik notes, carbonation can be tested by applying a phenolphthalien solution. Make sure it is done on a freshly broken specimen, not the surface of a core. You can core the concrete, but break the specimen for the phenolphthalien application.
RE: Concrete Carbonation of soffit of first floor slab above below grade garage of condo
Dik
RE: Concrete Carbonation of soffit of first floor slab above below grade garage of condo
RE: Concrete Carbonation of soffit of first floor slab above below grade garage of condo
Considering that, I would not worry about carbonation of the slab if insulated. Carbonation is related to air exposure and that is mitigated by either insulation, an air barrier or both.
As for the vapor barrier....treat the slab as if it were a wall and put the vapor barrier where you would for an exterior wall.
RE: Concrete Carbonation of soffit of first floor slab above below grade garage of condo
Dik
RE: Concrete Carbonation of soffit of first floor slab above below grade garage of condo
Do you have any peer reviewed published evidence to support your statement that insulation mitigates carbonation, or do you have your own data that you would like to share, that shows this? I am very doubtful that insulation has any effect on carbonation, although it depends on the type and how impermeable the insulation is.
Your info about the depth of carbonation corresponds with my general findings that it is usually very small (about a few millimetres) although in Europe for some reason carbonation seems to have been a more significant problem. No one seems to know why the difference.
Dik -
So the short answer is you don't have the information that I seek.
I am NOT referring to CSA A23.1. I am referring to CSA S413-2014. I thought that was clear since I am talking about parking garages.
I suggest that you obtain a copy of CSA S413 the 2014 edition and read it, particularly regarding where C1 concrete is required, and note the difference from earlier editions, and then you will better understand why I posed the question.
What has salt spray got to do with my question about corrosion of bottom bars? Have you any evidence that salt spray reaches the soffit of a slab except in microscopic amounts?
I am not sure that your info about carbon monoxide bonding to red blood cells is particularly relevant to my questions, although interesting.
Although it is true that temperature affects the rate of corrosion, I have never heard an architect say that was the reason the garage would not be heated! It is for energy reasons that garages are not heated.
Since silanes and siloxanes are breathable, I am not sure that they are very effective in preventing CO2 from penetrating the concrete.
Of course the underground garage is sprinklered, as per Code, but what has that to do with my questions?
Hokie66 - basically you are correct in that I am not looking for lessons on how to test for carbonation, or corrosion or testing procedures. I agree with you. Thanks.
I was thinking that someone was going to respond with the observation that carbonation is caused by carbon dioxide, and carbon dioxide is heavier than air (molecular weight of 44 g/mol vs 29 g/mol), so it might not collect around the slab soffit...but I expect that is comes out of the exhaust hot and rises, at least until it cools.
No further response to my post is required as I think the subject has been exhausted, unless you have data or research that you can send me that directly responds to my questions.
Thanks all for taking the time and trouble to respond, and the lessons n testing etc. Much appreciated.
RE: Concrete Carbonation of soffit of first floor slab above below grade garage of condo
By reducing exposure to fresh changes of air (CO and CO2) and offering some protection from salt laden spray, it's pretty obvious that the degree of carbonation and corrosion is reduced.
Dik
RE: Concrete Carbonation of soffit of first floor slab above below grade garage of condo
RE: Concrete Carbonation of soffit of first floor slab above below grade garage of condo
[Added] Even with batt insulation, the number of air changes is slowed or reduced and you don't get the same exposure to fresh CO or CO2.
Dik
RE: Concrete Carbonation of soffit of first floor slab above below grade garage of condo
I will give you my data as you are probably interested:
Below grade heated garage built in 1956 (designed by my old firm) and repaired in 1988. Original (1956)specified concrete 28 day strength was 4,000 psi. Repair concrete specified strength: I do not know. I do not know if the samples for carbonation testing came from the original garage concrete or the repair concrete, but when tested in 2016 the depth of carbonation was only 2 mm. So it was either 60 years old or 28 years old. In either case, the depth of carbonation was negligibly small. The slab soffit was exposed (no insulation) and the garage was unusually hot even in winter because a high pressure steam pipe ran thru it.
I am not saying that this is the result that one would always get in garages. I had hoped some others on the forum could contribute their findings of carbonation depth on garage slab soffits and also whether they have found that the soffit of the first floor of habitable space had significant concrete delamination due to rebar corrosion. My opinion is that it would not.
Andy
RE: Concrete Carbonation of soffit of first floor slab above below grade garage of condo
Dik
RE: Concrete Carbonation of soffit of first floor slab above below grade garage of condo
1. Should there be any concern about carbonation (due to vehicle exhaust) of the concrete cover to the bottom reinforcing bars in the first floor of a condo above a parking garage? - No. No heavy truck exhaust and no ramps that would cause a heavier accumulation of CO2. If you had a low ceiling area where busses idle all day long, you'd want to consider carbonation measures (no fly ash in the concrete, low w/c ratio, deeper bottom bar coverage). Of course if you have busses in a parking garage, you have bigger problems (ie you will be doing an extra phase of value engineering because your client is needlessly wasting money).
2. Vapor barrier... - no idea. I don't like the idea of putting an adhered vapor barrier in the way of water.
2-2. Air barrier... - same problem.
If you want a report, do a search. There are all sorts of reports that use words like "passivation" and talk about concrete ph levels. I did learn that the relative humidity needs to be between 50 to 70% for carbonation of concrete to occur so its probably not going to be a problem for you in the cold months of winter where the garage will be warmer than the outside air which will drop your RH to a low level.
RE: Concrete Carbonation of soffit of first floor slab above below grade garage of condo