When is a concrete wall air tight? 1

[ThomasH]

I am involved in a project where the client will test equipment. One test means that high altitude will be simulated and that means that the test room shall have a very low air pressure.

That obviously means that walls and roof will get a high "load" due to the external pressure. But for things to work as intended the "room" can't leak air, at least not significantly.

Does anybody know of a definition for "air tight" concrete or something similar. Something like crack width or required thickness for the compressive zone. I have found requirements like that in the Eurocode for liquids but not for gas.

Thanks in advance.

Thomas

 

[BAretired]

Concrete has many small voids, so it is not completely airtight. It may not have "significant" leakage, but that is not a very precise term. You may need to apply a vapor barrier on the high pressure side.

Thanks Enable. I should have typed an air barrier.

 

[ThomasH]

It may not have "significant" leakage, but that is not a very precise term.

Excatly, I am looking for some type of definition. And if that could be accompanied with a requirement it would be helpful .

In Eurocode for Tightness Class 3, the "worst", the requirement is "No leakage permitted". The recommendation is either prestress or liners to ensure watertightness. But would prestress ensure airtightness? I am not entirely convinced about that.

 

[Phil1934]

In the US ACI350 for environmental Engineering Concrete Structures controls. Calls for about twice the reinforcement. If they have a decent vacuum pump may not need a waterproofing membrane.

 

[phamENG]

Here's something from NCMA about air leakage in concrete masonry. Not quite what you're after, but may give you some ideas to look into.

I would suggest building a model of this - maybe a cubic yard of enclosed space? - using whatever detailing you devise and do a leak check on it to make sure it'll work. You may want to force a crack somewhere of equal size to what you think you could get in the full size structure to test the ability of whatever coating/liner to span and fill it.

 

[Enable]

BAretired: For airtightness one needs an air barrier, not a vapor retarder. We sometimes use a vapor retarder as a dual purpose vapor retarder and air barrier (e.g. in Canada poly sheeting on inboard side of studs), but they are distinct control functions and sadly are often confused leading to significant failures. See here for the difference and here for a fun example where someone (not naming names...FEMA) put the vapor retarder on the wrong side of the assembly.

ThomasH: Are you asking for a concrete design guide or for the definition of appropriate air tightness? In terms of definition that's relatively simply and is defined as a maximum flow over an area (or volume) at a given pressure that will allow the building (or assembly) to serve its desired function or meet intended energy targets. See here for a longer description and targets of air tightness for different types of construction (note the material and assembly targets are quite a bit different). I don't know of any design guides for crack widths or the like but if that's what you're after I can shoot an email to Dr.Listiburek and see if he knows of any.

 

[IRstuff]

The chamber does not necessarily need to be vacuum tight; its leak rate needs to be less than the pumping capacity of the vacuum pump. Much depends on how low a pressure you need to sustain; high vacuum, like 10^-6 torr is quite different than low pressure for an airplane at operational altitude.

I would be careful about coatings, since many coatings are hydrophilic, and that would increase pumpdown time, since the trapped water needs to be pumped out. True high vacuum chambers are typically made from smooth metal

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[ThomasH]

First of all, Thanks a lot for your thoughts.

A few clarifications:

The purpose is not the get a wall that is vacuum tight. If we can achieve that it would be good but I don't think it is necessary.

What we are designing is a test facility. The size of the room is something like a very large garage, the equipment size is like a few busses or similar. And the equipment should be tested for different climates and different altitudes. The altitude is from sea level up to 4000 m and that is where the air tightness becomes interesting.

I think the key here is to balance the air leakage through the roof and walls with the capacity for the pumps. The pumps should not only be able to create the low pressure but also to be able to hold it. So we have to try to find some common ground for the pump manufacturer and the guys doing the surrounding concrete structure. That may be a problem and we may end up lining the thing. But I think there may be a way forward in this.

@Enable, thank you for the link regarding air tightness testing. The purpose for the testing may not be the same but it gives an idea how to approach this.

 

[HTURKAK]

Have a look to this .pdf]doc.






Tim was so learned that he could name a
horse in nine languages: so ignorant that he bought a cow to ride on.
(BENJAMIN FRANKLIN )

 

[LittleInch]

I suspect your biggest issue is going to be the doors and openings, not the concrete.

At 4000m you're at the equivalent of 4m water head pressure outside to in. A thin sheet metal skin on the outside would help, but it's all about the doors and holes for wires, pipes etc.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[ThomasH]

@HTURKAK, Thank you for the info

@LitteInch, I have been informed that around doors and openings there are special arrangements. The main concern is the tightness for the concrete walls under 40 kPa loading. But I agree that a thin metal sheet will solve that. However, lining was not the original idea .

 

[JLNJ]

Maybe there an additive out there which will suit your purposes. Xypex claims to densify and decrease porosity.

 

[Ussuri]

A google search for "air permeability of concrete", seems to return quite a few papers on the subject. May be of some use?

 

[Once20036]

Isn't this something that the owner would need to define?
I don't think that there are any research papers that will matter if the owner is expecting something, and you deliver something different.

 

[BridgeSmith]

I agree with Once20036. the owner needs to define the maximum allowed leakage, then you have a target value, and you can determine what you need to do with the concrete mix or coatings, if anything.

I suspect that a vacuum pump able to pull the required vacuum in a reasonable amount of time, will be sufficient hold that vacuum easily with the typical leakage of standard concrete, but that's just a guess.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[IRstuff]

the equipment size is like a few busses or similar.

So, roughly 500 cubic meter volume, and 350 square meter wall area, say. That'll require a literal busload of pumps; given that, it would seem to me that the pumping capacity of the pumps for the initial pumpdown will grossly exceed any conceivable leak rate of the concrete.

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[ThomasH]

I agree that a research paper won't matter if the final installation doesn't work. But I think they will give some help regarding what may work and what definitely won't.

I think that ultimately we (or "somebody" ) will have to make a choice. Prestress, and trust that the concrete will do the job. Or use some type of lining to be sure.

 

[ThomasH]

So, roughly 500 cubic meter volume, and 350 square meter wall area, say. That'll require a literal busload of pumps; given that, it would seem to me that the pumping capacity of the pumps for the initial pumpdown will grossly exceed any conceivable leak rate of the concrete.

I did not see your post because I was writing. But you have an interesting point.