Sand above or below insulation for exposed slab on grade poured in hot dry weather? 2

[AaronMcD]

All over the internet I get different opinions. Contractors on site have different opinions.

The concern with pouring directly on the insulation is cracks along rebar during curing. This slab will be exposed, heated, with no control joints. The architect wants cracking to look natural rather than in straight lines. So we have 6" slab, low w/c ratio, 0.5% rebar ratio.

I see a lot of websites opposed to pouring the slab on sand due to the sand holding moisture which can travel through the slab into floor finishes over time. But is this a concern for exposed slabs? For a slab like this, I wonder if it may be better to put the sand over the insulation to mitigate cracking. What about the sand getting all lumpy while they tie all the radiant tubes and rebar in place?

Thoughts?

http://www.hellodwell.design

 

[JAE]

Insulation??? Or vapor barrier???

 

[AaronMcD]

Insulation and vapor barrier

http://www.hellodwell.design

 

[dik]

I'd use sand, insul, 6mil PEVB, and concrete in that order... You need to sawcut it at the right time if you want to 'create' a nice sawcut pattern... maybe 6 hours after finishing or earlier with a soff-cut saw.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[AaronMcD]

As mentioned, they will not be cutting it, they do not want a straight pattern. They want natural looking cracks, but not cracks along rebar.

http://www.hellodwell.design

 

[jhnblgr]

Typically what I have found is stone works best for a level finish. Plumbing must be run deep and not in the slab. Corners should be rounded. Look into type k cement concrete. Foam under slab should not be an issue. The finish of the slab I would recommend that they use a pan then trowel this should give the flat smooth finish. Then do a wet cure. You could use ice in the batch to reduce the heat from hydration of cement. Applying a densifyer during finishing could also help.

 

[dik]

inject coloured epoxy...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[JAE]

The concern with pouring directly on the insulation is cracks along rebar during curing.

I'm not sure why you think insulation would affect rebar-aligned cracking. Never heard of that and have never had it be a problem.

The architect wants cracking to look natural rather than in straight lines.

Well OK - the nice thing about straight control joint cracking is that, for exterior slab joints, you can fill the joints with a sealant and minimize water infiltration (and salt infiltration) into the slab below and thereby minimize rusting of the bars. With irregular and random cracks, no sealant is possible so you are then OK with allowing all the salt water into your slab?

If an exterior slab - why the vapor barrer? Exterior slabs rarely have vapor barriers under them.

So - I'd just place the slab on the insulation. Sand is a bad thing to use under slabs in my opinion. (as you suggested, the foot traffic when placing the concrete can move the sand all over and create a very irregular slab-bottom surface with variations in slab thickness.)

 

[dik]

Missed that... usually use cmpt Gran A...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[GeorgeTheCivilEngineer]

I don't understand the role of sand directly below the concrete. I would expect a polythene sheet above the insulation to minimise water seeping out of the concrete before it has cured.

Sand here is used to protect the damp proof membrane from stones and to trim up any unevenness (although it's been a while since I've seen contractors using it).

 

[dik]

The PEVB is adjacent to the concrete and provides a relatively flat smooth surface and contains the insulation a bit. If adjacent to the soil, it prevents moisture from the soil 'wicking' into the concrete, and to a lesser extent keeps water in the plastic concrete from soaking into the soil. In these environs, well compacted Gran "A" will nearly pond water; it doesn't readily allow water to flow into it. Also well compacted Gran "A" is relatively smooth and there are no 'sharp' surfaces. Often 'chairs' are supported on the PEVB. Here, the PEVB is used to prevent moisture coming into the slab, not leaving.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Eng16080]

This might be unconventional, but if cracks along rebar are a concern, why not pour the slab without reinforcement? What benefit does reinforcement provide especially considering the desire for natural cracks? There would also be no risk of rebar corrosion that way.

I often design slabs on grade without reinforcement, although I do provide closely spaced control joints, which obviously differs from your scenario.

 

[Enable]

Don't put sand directly beneath your slab. Please don't. And if you do end up doing that because - reasons - ensure you do something intelligent with the finishes such as a ditra underlaying to equilibrate the vapor drive.

EDIT - Noticed you said exposed slab so no finish. Still don't do it as the vapor drive will still occur and your conditioned space will be the recipient of all that goodness for a very long time. Mitigation of cracking is best handled by a proper wet cure rather than wetted sand below.

 

[GeorgeTheCivilEngineer]

The PEVB is adjacent to the concrete and provides a relatively flat smooth surface and contains the insulation a bit. If adjacent to the soil, it prevents moisture from the soil 'wicking' into the concrete, and to a lesser extent keeps water in the plastic concrete from soaking into the soil. In these environs, well compacted Gran "A" will nearly pond water; it doesn't readily allow water to flow into it. Also well compacted Gran "A" is relatively smooth and there are no 'sharp' surfaces. Often 'chairs' are supported on the PEVB. Here, the PEVB is used to prevent moisture coming into the slab, not leaving.

Just the same here. The polythene adjacent to the soil is our 'damp proof membrane' to keep water out. The upper polythene is just an extra to prevent excess water losses and reaction with aluminium foil insulation, if applicable. Agreed that compacted well graded stone is fine (I wonder if the decrease in sand blinding is due to better availablity of easy to use levels).

It was the idea of sand-in-contact with concrete which confounded me.

 

[dik]

bad practice... at least in our neighbourhood...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[AaronMcD]

@Eng16080, rebar is to reduce crack width. And I don't think I've ever seen or specced a slab w/o rebar here. They don't desire cracks, they prefer natural looking cracks to regular or straight cracks. So we went with the typ recommendation of 0.5% rebar.

@JAE, it's an interior slab. The rebar aligned cracking is something the contractor has seen on other projects, albeit with worse concrete.

http://www.hellodwell.design

 

[JAE]

OK - interior slab.

Then why the heating? To heat the space?

Will there be auto traffic on the slab bringing in snow/ice/salt?

 

[Eng16080]

Regarding no rebar, ACI has a document (which I can't happen to find right now) which recommends not using any rebar for certain slab on grade conditions. The conclusion was that closely spaced control joints are adequate alone to control cracking and that introducing rebar creates a corrosion risk. I think the document may have been specific to grade level parking where road salts pose an increased corrosion risk. FWIW, I've designed several slabs in this manner with no known issues.

 

[AaronMcD]

Heating for the space. It is a house so no traffic. I thought the heating made it obvious it was interior, totally forgot there are places with snow and ice!
There is also a similar slab outside at the patio areas, not insulated or heated.

http://www.hellodwell.design

 

[Eng16080]

I understand that the design that I mentioned above doesn't make sense in your case since you don't want control joints.