Shrinkage strain of a Slab on ground supported on a thick EPS core.

[hetgen]

Hi All,

Is there a way to estimate drying shrinkage strain grident of a 7-in [180mm] slab-on-ground built on top of triple layers of High desnity extruded Polyethylene (EPS) with each layer ~ 4in [100mm x 3] thick?

I think the slab is almost floating, would it be unrealistic to ignore the restraint from the EPS core?

Edit: the slab is for a freezer room -4^of uninterrupted, with a heating mat etc to control frost.. so the slab is in a controlled temp and humidity.

floor size 65ft by 50 ft [20m x 15m] without saw cut, slab thickness 200mm

Many thanks!

 

[hetgen]

no comment?

 

[Enable]

I've seen slabs get poured directly on insulation where the insulation clearly restrained the concrete which lead to significant cracking.

Is it possible to install a 6mil poly sheet on-top of the insulation to isolate them from one another? If you can, you get a bonus vapor retarder (in the correct location) out of the deal.

 

[dik]

Sorry... I just saw your post... missed the original somehow. I've done several unheated warehouses, and cooled warehouses with frost penetration. I've always placed the compacted granular fill, with (as much as 6") extruded polystyrene insulation, not polyethylene, and then 6mil PEVB (fixt, I know what I meant, not what I said). Generally DOW Styrofoam Hyload 40 or 60.

If for continuous freezing, you need to have the waste heat from the cooling compressor circulating in coils beneath the insulation to keep the soil from freezing. Even with insulation frost will penetrate into the soil, without the heating coils. Check with your mechanical dude.

Sawcut like a normal slab, and never concerned myself with shrinkage.


I've never poured slabs directly on insulation... but, unless there are steps, the insulation is probably as smooth as your compacted fill.

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

-Dik

 

[hetgen]

@dik you're right, the insulation should read extruded polystyrene not Polyethylene.

.., unless there are steps, the insulation is probably as smooth as your compacted fill.

If the slab is cast directly on a triple layer high-density extruded polystyrene (EPS), with each EPS layer a 100mm thick and disjointed, won't the negligible shear modules of the EPS core and the disjointed layer allow a reduced shrinkage restraint at the bottom of the slab as compare to a compacted subgrade?

In short, will it be too conservative to use the subgrade drag theory to estimate the required shrinkage reinforcement if a joint less floor is desired?

Thanks!

 

[dik]

It might, but I don't think so... I've never paid attention to this and have not experienced any problem with any of them I've done... and I've done a few of them in the last 50 years... Still use temp rfg or better, and sawcuts at the correct time... Because the insulation sheets are overlapped, I don't think there are any steps formed. Your compacted gran fill should be fairly flat to start with. To reiterate, I've never considered shrinkage restraint. I would not reduce reinforcing because the restraint could be less. This, IMHO, is really a false economy. Also use chairs with 'pads' or standees for supporting steel... I usually use about 2" cover and 1-1/2" sawcuts. I find the DOW product to be excellent and I spec it 'or an app'd alternative'. I don't believe in a generic spec... if I find a good product I generally give it 'first kick at the cat'.

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

-Dik

 

[rapt]

Not sure about the restraint, but if you do have to allow for it, the temperature shortening will be far more than shrinkage. While it might be at a constant 4f, it is a lot hotter when it is cast!

 

[Craig_H]

An extra benefit of a poly sheet on top of the insulation is that it keeps individual pieces from floating up during concrete placement. As Dik has mentioned, the contractor will need to do a very good job of levelling the granular base beneath the slab. One local contractor refers to it jokingly as "finish raking".

It's semantics, but note that EPS is expanded polystyrene, and XPS is extruded. EPS is generally blown with steam, and considered open cell, whereas XPS is blown with a hydrocarbon blowing agent and considered closed cell. They have noticeably different performance under wet service conditions. The specific type used may impact the heat transfer calculations that have already been alluded to regarding frost protection for the ground from the refrigerated space.

 

[dik]

Thanks Craig... I always 'spell it out', and don't use EPS, or XPS. I had to check back to see that I'd spec'd the right product... did, just by 'force of habit'. Granular base is not normally an issue. With well graded material the compacted layer is usually quite smooth; you need that for all SOG construction, else the slab 'hangs up'. I wish I could get these comments right the first time. I keep reviewing them and adding 'little bits'.

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

-Dik