Concrete vs. stone for leveling mat over rock

[firststatewelder]

Why is concrete typically specified in lieu of crushed stone (#57)as a leveling pad under a concrete footing poured on uneven rock? When pouring a concrete footing on soil #57 is typically specified under the slab. Is there any engineering reason for the difference?

 

[Mad Mike]

Think about the column loads acting on the footing vs. the (usually) nominal loads distributed across the slab.

Keep in mind that one side of your footing would be seated on hard rock, the other side on 1,2,3, maybe 4m of #57 crushed stone. Also keep in mind that immediately adjacent that #57 stone would be a soil; there would be some interaction between it and the stone.

So it depends on the situation- in some cases your #57 stone may be perfectly adequate, in other cases, could lead to failure. The other thing is, if you're going to cast blinding anyway, might as well use the low strength concrete to level off the rock somewhat.

All the best,
Mike

 

[BridgeSmith]

If I'm understanding the descriptions I've read for the #57 stone, it's good for drainage, but not necessarily the best for compacting, since it doesn't contain fines. It might not be a suitable material to interface with a hard rock surface, especially if shear resistance is required.

Anyway, depending on the site, pouring concrete might be more economical, since it wouldn't require any compaction or other work that placing rock might require. For block-faced MSE retaining walls, we give the contractor the option to use crushed base or concrete for the leveling pad, and more often than not, they choose concrete.

 

[fattdad]

I worked for a company that designed very large industrial buildings, some in excess of 1M square feet! We never used open-graded aggregate directly below industrial floor slabs! It ruts too deep for the concrete trucks and their tailgating. Slabs never got uniform thickness. Also, if the roof wasn't on the building and it rained, all that water just went straight to subgrade!

I don't really know how to address your design concern.

I'm currently working on a project that has a culvert in proximity to bedrock. The design stipulates that the culvert will be set on an engineered subbase of at least 5 ft of dense-graded aggregate. That's to control locations where there would be zero-inches of settlement (i.e., rock at the subgrade) adjacent to more measurable settlement.

Good luck in your efforts.

f-d

ípapß gordo ainÆt no madre flaca!