Capillary-Free granular base for moisture barrier/retarder 1

[jsrodgers]

Can anyone guide me to a helpful source for a gradation of AB material that is "capillary-free"? I have worked with moisture and vapor barriers in the past, but never one that is made of AB material.

 

[cvg]

I fail to see how AB could be considered to be a moisture barrier or retarder, regardless of the presence of capillary action. Can it provide a good base under a concrete slab - yes. If open graded, it may not promote excessive moisture uptake from the soil (assuming the water table remains below subgrade level)

 

[jsrodgers]

I agree! One of the engineers I'm working with on a project is saying they use this granular base with a high level of success, which I find to be quite unusual.

 

[fattdad]

What is the gradation of "AB" material? Is it a "dense-graded" aggregate or an "open-graded" aggregate? If the former, I'd continue to expect capillary rise. That said, we use dense-graded aggregates beneath slabs on grade almost exclusivelly and my boss is one of the 5 most noted experts on slabs on ground in the country. He is currently re-writing ACI 360 as we speak.

One reason that we like dense-graded aggregates is that they are much better during construction. A subbase (base) of dense-graded aggregate will shed off rainfall, which in turn allows for better site control following rain. Open-graded aggregates, while less likely to facilitiate capillary rise, allow for 100 percent infiltraiton of rainwater. If the roof is not on the building and you have a gully washer, where does all this infiltrating water end up - on subgrade. This perched water within open-graded aggregates has led to numerous subgrade failures, rutting from trucks and highly variable slabs on grade.

Just some things to consider.

p.s., I did a bench test on a dense-graded aggregate (VDOT No. 21A) and recorded well in excess of 2 ft of capillary rise. Textbook calculation, based on the D10 (or is it D20) size would suggest a dense-graded could have up to 40 inches of capillary rise. If you are worried about this use a vapor barrier.

f-d

¡papá gordo ain’t no madre flaca!

 

[DIRTyEngineer]

I have spent some time with these problems for small to medium retail construction.
Here are my random thoughts on the subject.
I think that owners are getting conflicting info and current specs sometimes show this.
Owners who have dealt with there floor coverings not sticking are willing to spend money on vapor barriers. As a result I have seen upgraded vapor barrier to 10-15 mil taped together and taped off penetrations placed above 6 “ of crushed 57 stone (I worry that the vapor barrier will be cut or ripped during concrete placement). Open graded natural moves too much during placement. The other increasing problem I see is the (clay) pad is built then by the time the plumber, electrician, computer cables etc are placed more than 50% of the pad is disturbed and maybe replaced with sand, stone? each contractor uses a different material? Wouldn’t 2 feet of medium clean sand be a good replacement for the 6” of crushed stone? Better protection of the vapor barrier can be reused in the trench backfill, gives some resistance to capillary rise. Medium clean sand seem to be hard to write specs to be used in multiple states.
My last random thought!
I still like the vapor barrier buried in 4 to 6 inches of sand.

 

[cvg]

the problem with sand is the same as with open graded gravel - it moves when you are setting reinforcement and placing concrete. As a result, concrete slab thickness will vary. Unless confined, the sand will spread and 2 feet would be very expensive. Personally, I think it would be better to use ordinary dense graded aggregate which is commonly used for road base. It is relatively cheap, it compacts very well and drains quite well also. Does have capillary rise, but who cares? Trades can trench through it and put it back easily. Or, then can backfill with sand or open graded material if they want. Place the plastic vapor barrier, (tape it if you want) and pour the concrete.

 

[DIRTyEngineer]

cvg I think we agree. The fine clean sand that moves I would call beach sand in the field. Our local road gravel spec allows up to 60% sand (finer than #4 screen) so I call it sand.

Anyway back to the original question about the AB vapor barrier. Didn't the old BOCA code allow AB instead of visqueen with approval form an engineer?

 

[theCorkster]

DIRTyEngineer;

I assume by AB you mean aggregate base. In general I believe that aggregate base describes a specific material associated with asphalt or concrete pavements that has specific strength and quality properties.

Given that, if a site is free-draining, such that moisture cannot migrate upward beneath the slab from either internal (broken pipes) or external (landscape, groundwater, etc.), then the gradation of the material supporting the pad shouldn't really be restricted, other than by the ability to construct on top of it without creating depressions resulting in thin and thick spots in the slab.

If there is no potential for water development beneath the slab, then the vapor barrier actually refered to by ACI 302.2R-06 and ACI 360R-06 as a "vapor barrier/retarder") would in my opinion not be necessary.

Check out ACI 302.2R-06 Guide for Concrete Slabs that Receive Moisture-Sensitive Flooring Materials for more information and guidance.

 

[civilperson]

The gap graded gravel, (with a paucity of fines), is used to mitigate the possibility of frost heave when used beneath a foundation along with horizontal insulation. The theory being that by breaking the capillary flow using gravel, the formation of ice lenses will be stopped. Works good in climates where frost depth is reported at 10-12 feet and water table is below the frost penetration.

 

[fattdad]

Just to add, if the concrete flooring will be covered by any type of flooring, you need a plastic sheet beneath the slab.

f-d

¡papá gordo ain’t no madre flaca!

 

[DIRTyEngineer]

I would call AB slang; if it doesn't have an ASTM or local DOT it means different thing to different people has been my experience.

Remember water vapor can travel upward nicely through free draining material.

I have done most of my work in the Midwest so I have never seen a site that has no potential for water development under the slab.

The new flooring glues are much more environmentally friendly but a little bit of water vapor and the don’t stick.

I would say that ACI has been behind the times on this issue over the last 10 years or so.