Concrete Floor Question

John B.

Well-known Member
I'm wanting to put concrete in a small shed of mine. Can I just spread sand down and level it off rather than using 3/4" rock under the concrete slab?
 
I'd highly recommend putting visqueen (plastic sheeting) down between the sand and the cement too. Stops a lot of the sweating.
 
Compacted sand with a vapor barrier is fine. It needs to be compacted well though. You also need to have the slab back filled well after you pour the sand can easily wash out around the edges.

I prefer 3/4 crusher run/road stone myself. The fines make it compact well. Think about most concrete highways. They use crushed stone over sand.
 
It will work, but as others said, put some plastic down. Compared to the concrete, the plastic is next to nothing expense wise.
 
Sand will compact itself if you flood it with water.

As JD Seller said - make sure it is contained under the slab.
 
I would disagree that few engineers specify gravel sub base under concrete. Roads for example, that have concrete pavements, are not built on sand sub bases. Sand is one sieve size and although it will pack in when flooded and retained. It has no structural qualities. Crusher run or appropriate sub base types of gravel have many sieve sizes of aggregate and with optimum moisture will compact 95% or better, making a suitable sub-base to provide structural support. The sub base must compact with a certain density to provide support for a slab, pier, wall, etc. For slab work a well compacting and draining material that will compact is ideal. Add your vapor barrier and you have a good sub base. Washed 3/4 stone, with no other fines or sieve sizes will not compact either, you have to have a composite mix of several sieve sizes of aggregate to achieve compaction for structural loading. I've never seen a detail on any set of plans in my entire career calling for sand. Footings on undisturbed subsoils yes, but never just sand.
 
Bank sand/sand pit compacted is what is under my factory building slabs. Very few cracks in over 40 years in the 100 X 70 molding room. Those are where we had over three feet of fill at the back where the truck docks are on grade. The bank sand has some fine clay in it that washed sand don't.
 
I would say proper site prep is far more important that the particular fill you use. If you don't have good drainage, you'll have problems.

For my 30x40' post frame shop, I hauled in sandy fill, packed it with my tractor and did the final compaction with a walk-behind plate compactor. I then poured a six inch floor, with an inch of foam insulation underneath. 15 years later there are no cracks.

For the slab in front of my garage, I was concerned the fill might wash away. So I brought in crushed limestone, leveling and spreading it with a box scraper. Six inch slab over that. Eight years later it's still in good shape.

If you're concerned about stability, crushed rock is very good but harder to work with than sand. For a small shed, sand fill will be just fine; just make sure there's no risk it will wash out due to poor drainage.
 
i would think you would be ok just make sure to get sand a uniform depth all the way across if at all possible and I would pack it down good
 
(quoted from post at 19:01:34 12/14/17) Would sand hold moisture, and make the concrete efflorescence more?

Sand is used a lot in my area under roads to DRAIN the moisture. crushed rock gravel goes over it for strength. I have never heard of sand holding moisture, unless it had a lot of silt or clay in it and then it would not be sand. Sand is also very difficult to compact. You could run a compactor over it all day and a truck would still get stuck in it. Trying to drive in sand is the best way known to break an axle in a truck. You go real easy because it is so easy to spin. As soon as it starts to spin it will hop. It will not hop more than a couple times without breaking an axle.
 
Around here it's referred to as "cushion sand". Nothing cushioning about it really, just a name. What it does do is to provide a means to level out irregular terrain so that you can do a better job of ordering the right amount of concrete.

If you have a fill requirement for a "pad" or such, "Select Fill" is the product which is a mixture of sandy loam and clay. It gets watered and packed and supports good weights without moving around like sand. Course with any fill material, it's totally dependent on the soil under it as to whether or not it does what's expected.
 

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