×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

carbon dioxide

carbon dioxide

carbon dioxide

(OP)
Is carbon dioxide heavier or lighter than air, or the same weight? Will it rise to the ceiling or fall to the floor?

RE: carbon dioxide

(OP)
Thanks.

RE: carbon dioxide

Definitely heavier.  Just look at the atomic weights of the compounds.

Mike McCann
MMC Engineering

 

RE: carbon dioxide

(OP)
thanks

RE: carbon dioxide

(OP)

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?

RE: carbon dioxide

ajk1...I would not agree with that.  Carbonation will still occur on the underside of the slabs.

One example is efflorescence.  You will often see efflorescence on the underside of slabs, overhangs, etc.  It is also a product of carbonation.

Keep in mind that carbonation can be present in slabs, absent any evidence of efflorescence.

RE: carbon dioxide

IRstuff is correct.  CO2 is a component of air, not separate from it.

RE: carbon dioxide

(OP)
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?

RE: carbon dioxide

ajk1....a few comments to your questions

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

Just a few thoughts!

 

RE: carbon dioxide

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:
http://www.gemite.com/tech_bulletin/Tech_Bulletin_Concrete_Corrosion_due_to_Carbonation_04_05.pdf

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?

RE: carbon dioxide

(OP)
Thanks Ron.  

RE: carbon dioxide

Thanks TX...

Dik

RE: carbon dioxide

Have to disagree somewhat here guys.  

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.

Mike McCann
MMC Engineering

 

RE: carbon dioxide

Even with Brownian movement, the heavier molecules will have a slightly greater concentration at the bottom.

RE: carbon dioxide

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.

RE: carbon dioxide

All,

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.

Regards,

Matt

RE: carbon dioxide

Thanks, Matt

Dik

RE: carbon dioxide

ajk1,

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.

 

RE: carbon dioxide

Just a clarification...most of you probably know.  Carbonation in itself does not cause deterioration, but the decrease in alkalinity due to carbonation renders the structural element incapable of resisting attack from chlorides, etc.

RE: carbon dioxide

no kiddin'... <G> I still feel the pain from a 'too hasty' response.

Dik

RE: carbon dioxide

This string's a gas.

Mike McCann
MMC Engineering

 

RE: carbon dioxide

Be careful of the assumed differences  between "science lab" definitions of gas motion, and the real world.

"Air" is a well-stirred mix of O2 (32 AW)and N2 (28 AW), with less than 390 ppm of CO2:  390/1,000,000 or 0.39 percent.   CO2 is heavier than the average molecular weight of "air", and Brownian motion will tend to mix it up (even in very, very, very still air) but ... in the real world, the wind and ventilation MUST mix up ALL air in a parking garage to such an extent (meaning forced movement plus convection of hotter exhaust fumes) than the CO poisons are diluted enough to be safe.  

CO2 release from curing concrete will be concentrated at the surfaces (top and bottom and walls) of the concrete, because it is going to come out of every surface.  An enclosed underground room would build up CO2.  But that CO2 rate is small compared to other gasses.

CO (12 + 16) molecular weight is lighter than CO2 (12 + 16 + 16) but it will be present in exhaust at rates 1000 to 5000 greater than CO2.   Since CO (if not diluted) will kill quickly and with no odors, your first concern must be forced ventilation (or natural wind flow) to prevent CO deaths/exposure.  That forced movement will combined with but completely overwhelm any Brownian motion (the molecule-to-molecule vibration by uneven numbers of collisions of a gas by nearby gas molecules) exchange of CO2 in air.       

RE: carbon dioxide

Who asked about CO?  I don't think CO is deleterious to concrete.

RE: carbon dioxide

No: CO is hazardous to people occupying the garage (later) when it is in use: Thus, the absolute need for ventilation (to remove the CO) in the garage during use is going to provide more than enough ventilation to remove the CO2 from the concrete during construction ...

RE: carbon dioxide

Not the issue.  Carbonation of concrete is long term, not affected by ventilation.

RE: carbon dioxide

Can carbon monoxide react with water to produce carbonic acid?  If so, then it could contribute...

Dik

RE: carbon dioxide

(OP)
Sorry, I have been away from this site a few days. All very interesting and fascinating comments. Seems that there are many factors to condider.  Thank you all. Thanks particularly "Skis and Bikes".  Although some comments strayed from my original question, they were still of interest. My original question was related to carbon dioxide and carbonation to the underside of an insulated concrete slab which is the floor of a heated condo building above the garage.  Perhaps I can rephrase my question as follows:

Given:
 a) the insulation that will keep the slab soffit warm,
 b) the Code required ventilation in a garage,
 c) the CSA exposure Class C-1 concrete (w/cm = 0.40 maximum),
 d) 25 mm clear concrete cover to the bottom bar

Question:
 What is the likelihood of corrosion damage to the slab soffit due to carbonation in the 100 year design service life of the building? My opinion is that it is negligible.  Anyone think it is significant?  

RE: carbon dioxide

You stated that the parking garage is indoors.  Carbonation is likely in that time frame, but it is unlikely to lead to corrosion damage.

RE: carbon dioxide

(OP)
Thnaks Hokie66.

RE: carbon dioxide

Quote (ajk1):

Anyone think it is significant?  

Not me.

RE: carbon dioxide

100 years is a long time to project .... however, given your parameters, I would also not expect carbonation to lead to any widespread deterioration. I would not be surprised if during 100 years there were some small, localized soffit delaminations due to carbonation at crack locations. Likely insignificant. I would expect a waterpfoofed podium slab subjected to chlorides to deteriorate significantly more over 100 years.

Keep in mind that if the slab is indoors, heated and not subjected to chlorides or exterior exposure, it is unlikely a C-1 concrete will be used for the slab unless you specifically specify.

RE: carbon dioxide

(OP)
to Skis amd Bikes - it was specified as C-1 exposure - maybe because it becomes exterior for part of its area. Not my design so don't knw all the details.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources