Subgrade beneath Concrete Slabs 2

[medeek]

In my concrete notes I typically include a note stating: All concrete slabs shall be underlain by 4" of compacted, free draining granular material such as 3/4-1 in. minus clean gravel.

I also like to show in any of my details a 6 mil vapor barrier underneath any slabs to prevent the rise of water vapor through the slab into the garage or living space.

A lot of residential work I've been recently encountering though is built on beach sand, which levels and compacts quite nicely especially when wetted for a few days.

I am wondering what others experience is regarding sand as a subgrade and also your thoughts on using a vapor barrier.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

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[JAE]

Try to avoid sand in my area - seems to pump and move when concrete puddlers get walking on it.
Also will erode out if water flows occur.
Beach sand will not work as well as a capillary break for moisture coming up from below.

We use a 10 mil to 15 mil VB to avoid tears - used to use 6 mil a lot but recently moved up in thickness

We don't use a vapor barrier in cases where the concrete will be left exposed in more industrial situations to avoid curling and cracking.


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[medeek]

Thanks JAE for some excellent insight. My thinking with gravel underlayment is that it not only provides a good means of leveling and compacting the surface in prep. for the concrete but it also helps prevent the capillary action and water wicking up into the concrete. However, with 3/4" minus gravel with all of its fines maybe this theory doesn't really work so well.

The local contractors here do not like to use a vapor barrier as I found out today after some rather lengthy conversations. There assertion is that the beach sand does not have any significant capillary action and remains dry under the house provided there is adequate drainage from the roof away from the house, I'm not sure I completely buy that.

On a personal note my own house is a slab-on-grade foundation and we constantly have significant moisture / high humidity to the point that I had to purchase two dehumidifiers for the first floor. Granted our native soil beneath the house is mostly clay and the water table in the wet months is probably about 3 feet deep.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

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[bigmig]

The attached reference is a good one in terms of vapor barriers and retarders. Another helpful note on this subject, I found this verbage on a soils report once and researched it and found it to be correct...

Polyethylene (“poly”) sheeting (even if 15 mils in thickness which polyethylene sheeting commonly is not) does not meet the ASTM E-1745 criteria and is not
recommended for use as vapor barrier material. It can be easily torn and/or punctured, does not possess necessary tensile strength, gets brittle, tends to
decompose over time, and has a relatively high permeance.

 

[medeek]

None of these reports seem to have any values listed for 6 mil visqeen (polyethylene plastic sheeting) which is what is used typically for residential applications. I don't think I would ever use any of these other proprietary products, I'm sure the cost would be prohibitive.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[thaidavid40]

That is the nature of residential work - reducing the first-cost is the prime consideration, and preventing future problems is in a distant second place (or worse). Only a well-educated customer - or a wealthy one - will likely opt for a proper vapor barrier, as most will never appreciate the value it adds. The most difficult type of customer is the uneducated one, and many residential customers (and contractors) fit that bill to a tee. The first homeowner will likely be gone before ten years' time, and the contractor will likely have started and failed three companies in that same time. That is the true status of today's residential building market. This is a sad state of affairs, but it is real

Thaidavid

 

[Ron]

Agree with JAE with one exception....sand can be a capillary barrier if the fineness modulus exceeds about 2.75. Beach sand, in many areas, has a fineness modulus of less than 2.5, so not so good as a capillary barrier. Medium sands (SW) are quite capable to serve as a capillary barrier. Fine sands....not so much.

A 6-mil Visquene or similar polyethylene sheet is marginal at best. Agree again with JAE that thicker is better. Placement of the vapor barrier and placement of the concrete over it must be done with reasonable care to protect the integrity of the vapor barrier.

 

[DBronson]

Medeek:

You might want to look at this short paper on SOG's. Joe is one of the world's experts on moisture in buildings.

Link

There are several other papers on this topic there as well.

Regards,

DB

 

[JAE]

I don't know for certain that 10 mil or even 15 mil works over long periods of time - the deterioration rate.

I've had more than one geotechnical engineer tell me that moisture quantities in soils under buildings grows over time - higher than the surrounding soils outside the building footprint.
So beach sand would probably not simply drain away any moisture but rather slowly attract it over time - unless the larger gradation as Ron suggests.



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[medeek]

After speaking with a couple more builders today I actually did find one who does put down a 6 mil polyethylene vapor barrier beneath his garage slabs, I was surprised. Every other contractor didn't see the point unless it was a living space with a slab-on-grade. None of them had ever heard of any of the more premium products, again not surprising.

His reasoning for doing so was less about moisture control and more about controlling his concrete pour as he felt that pouring onto sand dried the concrete out too fast and made it weaker and harder to work (set up too quickly). He also stated that pouring concrete on top of beach sand was too tricky since the flowing concrete tends to disturb the sand bed and create pockets of sand within the slab.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

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[JAE]

His reasoning for doing so was less about moisture control and more about controlling his concrete pour

Ya, so his slabs will curl more and crack later once the car hits the slab near the corner.

...since the flowing concrete tends to disturb the sand bed and create pockets of sand within the slab

...and thus the reason not to use sand.

But having said that - in your particular area the sand may be the most available material.

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[medeek]

From the paper by Joe:

[tt] If a slab dries only from the top—the top becomes drier than the bottom—duh…The top shrinks relative to the bottom and the slab edges curl upwards. This is not a big movement and is not a big deal except where you need ultra flat floor systems—like in chip manufacturing plants. Slab curl is very easy to control, use a low water-to-cement ratio (less than 0.5) and wet cure the top (wetted burlap works—Photograph 5). Or just use a low water-to-cement ratio and forget about doing anything else.[/tt]

Seems like a reasonable solution to the slab curl problem but there are always trade offs in engineering.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[JAE]

You won't get a total non-curl slab in most cases.
So even a small amount of lift, especially with thinner slabs, will result in a small diagonal crack across the corner of the panels once a wheel load is applied.

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[TXStructural]

Curling is controlled by proper concrete mix design and curing. It is an issue for all slabs, not just high precision, ultraflat slabs. Very few residential and light commercial contractors understand curing and even fewer do it properly.

Do not use granular fill where the underlying soil will create a "bathtub" of retained water. This is a common condition that results in wet slabs and heave in expansive soils. I worked on repair of a church where the sanctuary was constructed on 18" of sand fill that was placed into an 18" excavation of native clay on a flat lot. When we dug to the bottom of the grade beam to see what was going on there was a rush of water from under the slab despite a prolonged drought. The center of the sanctuary had risen more than 6" due to expansion of underlying soil because of the retained water. The accumulation was due to gravity and relative impermeability of the surrounding soil.

There is dispute on whether moisture barriers should be in contact with concrete or under a layer of sand. I can't give any insight into that topic but the only reason to have sand above a barrier is to reduce the likelihood of puncture during subsequent activity. I would also keep the membrane above the level of the surrounding soil unless the soil is self-draining and well above the water table. One concern is placement of reinforcement on top of barrier membranes. Contractors should use the right supports for reinforcement (plastic or metal sand chairs or precast "dobie" blocks over soft subgrade or barriers. More specifics can be found in the CRSI RB4.1 standard.)

 

[medeek]

According to Joe's paper above a layer of sand above the vapor barrier is a serious no-no. The layer of sand creates that "bathtub" of retained water.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com