I assume that the ceiling slab of an indoor parking structure is not at greater risk to carbonation than any other slab because any carbon dioxide that causes carbonation will descend to the floor and any exhaust that may inlcude CO2 will not rise to the ceiling...do you agree?
While CO2 is denser, the Brownian motion of molecules keeps CO2 homogenously mixed. Otherwise, the CO2 would kill you if you were on the floor or in the basement. There are even heavily molecules that manage to stay afloat in air.
All very interesting comments and thoughts. Much appreciated. But I have spent a majority of my 5 decades engineering career in parking garage related work - design, investigation and repair - and have never seen soffit delamination caused by corrosion of bottom steel that can be attribuatble to carbonation - always due to chloride laden water leakage from top surface of slab. Work by Bickley showed that carbonation of concrete in the atmosphere with a medium w/cm concrete would take over 150 years to reach 25 mm into the concrete. Our own investigation of carbonation depth in 85 year old concrete (which I expect had a high w/c concrete in the order of 0.6) in the heavy traffic downtown Toronto area, showed the carbonation front in the bridge soffit had advanced only to about 5 mm in the worst case of the samples tested.
Questions
1) does anyone know of any Code or Standard anywhere that specifies minimum depth to protect against carbonation?
2) the soffit of the floor of the apartment (which forms the ceiling of the garage) is insulated and has a vapour barrier. For the sake of argument, let us assume that I am wrong (a;waysa prudent assumption) and the carbon dioxide from exhaust is a threat to the soffit rebar and will cause the carbonation front to penetrate 25 mm to the steel within the say 100 year design service life of the building,wouldn't the vapour barrier protect it?
3) Does anyone have any data on how much the carbon dioxide content increases in the atmosphere of a normal apartemnt building parking garage which meets Code requirements for ventialtion? And how much does it avry between floor and ceiling level or os it equal?
4) The web litrature says that an "external" source of moisture is required for carboantion to occur. Anyone diagree with that?
1. Don't know of a code provision for carbonation protection. Generally taken that "non-contact" cover requirements are adequate.
2. Any inhibition of moisture and/or air migration into the concrete will delay carbonation; as will high density concrete, protective coatings, etc.
3. Check with ASHRAE on carbon dioxide levels. They have a lot of data on indoor air quality issues, including levels of carbon dioxide.
4. All that is required for moisture is that the relative humidity within the concrete and at its surface be in the range of about 60 or 70 percent for carbonation to occur. This condition is prevalent in most climates.
A few other observations from my experience....
Your concrete is often designed to resist chloride permeability, which also makes it more resistive to carbonation.
Carbonation occurs faster and to greater depths in my area (southeast US) as compared to yours. I believe it has to do with a couple of things....
A. Our temperatures are higher for longer periods, thus increasing the rate of reaction for essentially any chemical reaction, carbonation included
B. We have inherently higher ambient humidity, which typically increases carbonation potential
C. Many of our concrete mixes are designed and placed with water-cement ratios of 0.55 or higher, thus making less dense, more permeable concrete (as well as keeping the relative humidity in the concrete at higher levels)
D. We tend to get more instances of wet-dry cycling in conjunction with higher temperatures
In normal exposures, such as a parking garage, the rate of permeation of CO2/carbonic acid is very slow. Add to that the amount of CO2 required to neutralize and then acidify concrete, and you get a very long time before corrosion initiates. Yes, moisture is required, since the reaction is not really a function of gas diffusion but one of acidification - CO2 is dissolved in water, forming weakly acidic H2CO3. Condensation on the bottom of your apartment floor slab would be sufficient, if it was not insulated.
Check out this document on carbonation rates, including a list of references:
Vapor barriers do not stop gas diffusion, but may slow it down. Assuming you are talking about a foam plastic insulation (board or spray), you would get a significant reduction of air flow (hence less CO2) through that layer. Oxygen and CO2 will diffuse through polyethylene sheet. Obviously, the rates will depend upon concentrations/partial pressures and sheet thickness. But, absent another moisture source, the poly should significantly restrict the availability of outside moisture. Are you sure there wouldn't be an internal moisture source that you would be trapping against/in the slab?
We are speaking of CO2, but remember that CO monitors are installed at or near the floor of residences because lethal concentrations of CO are heavier than "air". The critical word here is "concentrations".
However, in the mixed, not concentrated state, I agree.
I think that you will find that any variation in concentration will be the result of traffic movement, emission patterns, and airflow, rather than molecular weight and gas density.
While you should not expect perfect mixing, there is not a significantly greater concentration of CO2 closer to the ground generally, which is what one would expect if the mixing and diffusion of a gas were gravity-dependent. This is attributed to the generally turbulent nature of the atmosphere below 100 km altitude.
I would expect the same phenomenon to dominate inside a parking garage, on a smaller scale, but still enough to maintain fairly even mixing once a source has moved away.
Slight tangent from a non-structural engineer, but:
* At the scales we are interested in, mixed gases stay mixed. No gradient with height. Heavy molecules do not spontaneously separate.
* Emissions of cold or higher molecular weight gases will sink, but only until they mix with air. That propane tank leak into a cellar, petrol vapours into an inspection pit or Lake Monoun/Lake Nyos on a grander scale.
* CO is actually lighter than 'air' (air ~29 kg/kmol, nitrogen and CO both 28 kg/kmol and oxygen 32 kg/kmol, all close enough to ideal gases as makes no difference). Process plant etc, we put CO monitors at face height.
A few comments based upon my experience in parking garage and bridge investigations in your area. I worked on a project where a set of lift out precast hollowcore slabs at the entrance to an apartment building's parking garage where the entire soffit was delaminated. The concrete we tested was severely carbonated and the slabs were located directly above a very old, very poorly maintained furnace/boiler room, which was believed to be the source of the problem. I did hear about a TTC suspended slab some years ago that was severely affected by carbonation, however, the carbonation occurred during winter construction as a result of the heaters not being vented properly.
Chloride intrusion is normally much more likely to result in deterioration than carbonation. We often tested for depth of carbonation on core samples, however, rarely was it found to be to the depth of the steel. It was interesting to test depth of carbonation at crack locations, where locally the depth of carbonation could be much deeper.
I believe the depth of carbonation is most influenced by the porosity of the concrete. The vintage Bickley/Emery/MacDonald report I have refers to an estimation by Forrester of carbonation depth based on time, concrete quality and exposure conditions.
CSA A23.1 has several somewhat vague references to carbonation and concrete cover (low to moderate cover can be problematic), however, most of the focus is on the permeability of the concrete. MTO usually has some excellent research in such matters as well, although I don't have any specific examples come to mind.
