Getting the rigid foam flat before concrete pour

[RLW]

Hello fellow Garage Enthusiast,

I am prepping my substrate, (what we call Crush and Run) so I can place the 2" XPS over it before we pour a 4" concrete slab. It has been compacted.

There are voids that can be felt (walking) and seen on the edges where the XPS is not being supported and not sitting flat. I can shoot pics tomorrow.

The concrete contractor is attempting to tell me that some variation is okay. That the weight of the poured concrete will compensate for those low spots under the XPS. That the insulation will lay down with the concrete on top of it. I'm not buying it.

Is it possible to get the substrate completely flat? Would some pea gravel be a good material to fill the voids.

Any replies are appreciated.

 

[RivennHewn]

Some voids are going to happen. You'll never get it perfectly flat.

Concrete/bar will span the small stuff just fine. Larger voids have the potential to be a problem.

If you have questionable areas, just add more bar.

Vibrating the mud does a good job of filling the voids.

 

[SunsetsAndFriends]

...

 

[Nivekdodge]

Sprinkle fine sand over your bedding material to fill in the voids.


Sent from my iPhone using Tapatalk

I'm having the same issue and a pile of sand is ten feet away. Should I use it to get it flat? No one has replied after they laid their concrete as to whether or not close was good enough?

 

[like2wheel]

Ill be in the same situation soon, & I'm not the expert here, but technically I think you're supposed to have a layer of some sort of clean washed stone to break any capillary action that would let moisture accumulate against the foam.

Interested to see what the experts say here

 

[ConCretin]

While a 4" concrete slab can span short distances, you obviously want to minimize voids under your slab. The only way to achieve this is careful compaction and fine grading. Obviously a finer material makes this easier to achieve.

Insulation will conform to gentle undulations but isolated high or low spots will result in voids. 4" of wet concrete weighs about 50 lbs/sf, which will help press the boards down.

As an aside, rebar won't make a difference other than to hold cracks together if they occur.

While it's counterintuitive to install a product under your slab that is virtually certain to create voids, it's done every day and problems seem to be rare. Do the best you can and move on.

 

[small&rusty]

The last 3 building jobs I've been on we put 4" of rigid foam under the slab and there was voids all over and it poured fine. The bigger variations we would add or subtract material to help even it out. The rule of thumb we go by is its better to be low than high.

 

[PWC Repair]

I shoveled in and raked around 3/4" minus using a 2x4 as a screed to "flatten everything out before I layed out my foam board. I wasn't taking any chances. Some relatively easy labor, no biggie.