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Can I inbed foam in concrete?

DoRight's picture

I have a column or post.  It has a footing and an eight by eight concrete block post rising about 32 inches to about 6 inches above grade. On top is a steel bracket and then an 8 by 8 wooden post.  I wish to dig down to the footing and pour a 24 by 24 inch by 32 inch high pier around the 8 by 8 block.  I wish to do this so as to frame a wooden pier above the poured pier and attach stone veneer to the framed portion of the pier.  And becuase I don't like the look of stone veener "HANGING" in mid-air.  I like the look of stone resting on a "BRICK LEDGE". 


Now the 24 by 24 by 32 pier is a lot of concrete and does not need alot of structural strength.  so can I rap the existing 8 by 8 post in foam board and then pour concrete over this and thuse reduce the number of cubic feet of concrete from 9 cubic to 4 or 5?  Will the foam absorb moisture expand and screw up my concrete?  Or am I just silly, I should just mix the concrete and pour it (all hand mixed I might add)?  Other suggestions?  I suppose I could find some 5 or 6 inch diameter rocks and imbed that in teh concrete as I pour is to save on concrete.


Thanks as aways 


or should i embed rocks? (post #216001, reply #1 of 6)

Or should I finve a bunch of 4 to 6 inch rocks and just drop them in as I pour the concrete?  mix a bag, dump it in, place a few rocks up against teh 8 x8 blocks, add more concrete, more rocks, etc.  Again I dont nee a lot of structural strength, it is more for teh asethics of having the stone veneer to appear to be set on the cement base rather than appear to float.  And I suppose to a degree it would make adding the wood, boxed out pier above grade easier as well. 


maybe... (post #216001, reply #2 of 6)

It's pretty common practice hereabouts to place 1 or 2" xps foam under slabs and to insulate footings. It won't screw up the concrete. There may be other flaws in the way you propose though so I would, were it me, go with the extra rocks idea.  


Foam vs rock (post #216001, reply #3 of 6)

Thanks oldhand.

I am sure foam under a slab is not a problem since even if the foam expands due to moisture it would just lift the entire slab a fraction of an inch.  But if I were to wrap my 8 x8 post with foam and then pour a 3 or 4 inches of concrete around that, and then the foam were to expand then it might crack teh concrete and let more water in and cause more trouble.

So I think I will find a bunch of rock,  I just hate the idea of hand mixing 9 or 10 cubic feet of concrete for no real purpose.


Closed cell foam should not (post #216001, reply #5 of 6)

Closed cell foam should not expand at all. I have poured plenty of concrete over foam.

don't waste foam (post #216001, reply #4 of 6)

Just form around the existing block with scraps of dry wall or backer board, anything but wood. Fill this with sand or dirt or rocks or whatevery you have. Pour around this. 

On the large commercial (post #216001, reply #6 of 6)

On the large commercial projects I work on, we use large foam slabs as backers for monolithic concrete pourings on a regular basis - both to save concrete and to lighten the overall mass. However, foam is not structural and all structural requirements must still be met. The best way to approach it is to consider the end result as if the foam was not there - will the finished pour meet the requirements?

In order to satisfy your concerns about cracking, you might consider adding rebar or mesh (which, in the long run, could make the job bigger than just going with a solid pour.) I'd be cautious about using a large proportion of large rocks - these could interfere with the integrity of the concrete matrix.

As far as the foam expanding, it's probably not an issue, considering the weight of the concrete, the nature of the foam itself, and the fact that the freshly-poured concrete will probably be introducing more moisture than the foam is ever likely to see afterwards. If you're concerned about freezing expansion, it wouldn't hurt to build on a reasonable drainage bed. The work always expands, doesn't it?