Closed-cell foam
Also called Ethafoam, Closed cell foam sheet, Foam underlay, Protection sheet
Closed-cell foam is a foam whose gas cells are sealed from one another, so it does not draw up water, compresses under load and recovers when the load comes off. On site it is used in sheet form as a compressible protective layer — beneath mats and bearings, or between a temporary structure and a finished surface — where a rigid packing would either mark what is underneath or force the component above it to bear on a high point.
From our specifications
We supply Ethafoam closed-cell foam sheets, and we recommend them for two jobs that are worth keeping apart. The first is protection: sheets laid beneath ALIMATS® modules so that a load spread system can be set out over a finished or delicate surface — a completed slab, a podium deck, paving, a plaza — without the mats marking it. The second is structural, and it is the one nobody expects from a sheet of foam. Our published good practice on the ALIMATS® powered access and cost efficiency pages is to place Ethafoam under the mats on hard or slightly uneven ground, because it allows the mats to deflect during operation and so optimises the structural performance of the system. A stiff mat set down on a hard, uneven surface bears on the high spots; a compressible layer beneath it lets the mat take up its own deflected shape and spread the load across its area the way it was designed to.
Source: Brilliant Ideas published ALIMATS® good-practice guidance and the Ethafoam closed-cell foam sheet catalogue entry
Where it matters on site
The counter-intuitive half is that a little softness under a load spread mat can be what makes it work, and that is not the same claim as bedding. The foam is there so that the mat bears evenly; it does not improve the ground beneath it, it does not change the ground bearing capacity, and it does not change the load spread calculation. Nothing in this class of material is a bearing material and none of it should be relied on to carry load in its own right. The other use decides more jobs than it looks like it should, because the objection to setting mats down on a finished surface is rarely the load — it is the finish, and who pays to make it good. Where that is the constraint, get the protection layer onto the temporary works drawing rather than sending it to site on the day and hoping.