15psi vs. 25psi under slab insulation

Concrete is a rigid pavement so you can assume load distribution.

Click to expand...

Being rigid is why you want to understand how its loaded, it will try and distribute a load, but its stress level may be higher than its design rating (a nice way of getting crack in your slab).


If anyone is interested in the right way to figure out foam strength for under a slab....

http://www.foamular.com/assets/0/144/172/174/4e330789-b09e-4cf0-b2d9-e16ed89ae5c3.pdf

And a thread about it....

http://www.garagejournal.com/forum/showthread.php?p=3110464#post3110464

The modulus of elasticity for the foam and the concrete are dramatically different, you cant accurately calculating loading assuming they are the same.

What happens? As the concrete is loaded, it's stresses go up very quickly (since its more rigid). The foam, tends to give a bit as its loaded and does not see nearly as much loading.

Basically, the give of the foam increases the localized loading of the concrete ( All materials deflect as they are loaded, even the concrete floor).

The foam failing is not the issue, localized loading of the slab is. The foam with the higher PSI rating is also stiffer (increased modulus of elasticity). The increase in foam stiffness provides better support for the concrete, reducing deflection and therefore strain (the foam board starts to behave more like soil, but still has a long way to go.

The main reason to specify stronger foam is that as its strength goes up, it becomes stiffer. That's one reason why DOW recommends calculating using a percentage of the foams rated strength (not its full advertised strength).

Basically, you need a thick enough slab to distribute your load forces to keep the stress in the slab within its design limits. The more your foam gives the thicker your concrete should be.

The thicker the slab, the lower the better it can distribute forces, that reduces localized loading on the foam.

If you ever want to put a lift in, the calculations should be a requirement. For any other usage the worst that happens is your nice floor cracks.

Being rigid is why you want to understand how its loaded, it will try and distribute a load, but its stress level may be higher than its design rating (a nice way of getting crack in your slab).If anyone is interested in the right way to figure out foam strength for under a slab....And a thread about it....The modulus of elasticity for the foam and the concrete are dramatically different, you cant accurately calculating loading assuming they are the same.What happens? As the concrete is loaded, it's stresses go up very quickly (since its more rigid). The foam, tends to give a bit as its loaded and does not see nearly as much loading.Basically, the give of the foam increases the localized loading of the concrete ( All materials deflect as they are loaded, even the concrete floor).The foam failing is not the issue, localized loading of the slab is. The foam with the higher PSI rating is also stiffer (increased modulus of elasticity). The increase in foam stiffness provides better support for the concrete, reducing deflection and therefore strain (the foam board starts to behave more like soil, but still has a long way to go.The main reason to specify stronger foam is that as its strength goes up, it becomes stiffer. That's one reason why DOW recommends calculating using a percentage of the foams rated strength (not its full advertised strength).Basically, you need a thick enough slab to distribute your load forces to keep the stress in the slab within its design limits. The more your foam gives the thicker your concrete should be.The thicker the slab, the lower the better it can distribute forces, that reduces localized loading on the foam.