I'm installing about 600 sq.ft. of concrete pavers in a perfect rectangle. 160 square feet of it will be over 8" thick concrete slab and the rest over very well tamped crushed rock base. I am going to screed about 1.5" of sand and about .5" of sand over the concrete slab so that it is one level area, partially with concrete underneath and partially with crushed rock underneath. I will use concrete to secure the border in place. I am concerned that the pavers, once laid, will sink where there is no concrete underneath them, even if I tamp the soil and crushed rock very well. I am using 2.25" thick pavers for the entire project. My objective is that with the finished result you cannot tell that there is concrete slab under part of it and just crushed rock under the rest of it. Any opinions, ideas, guidance?

Thanks!

 

Yes, differential settlement is a very real possibility. If it were mine, here's what I would do:

1. Tamp the subgrade thoroughly using a Jumping Jack compactor (not a plate compactor; use that only for packing down the pavers and vibrating the sand into the joints). Lay down geofabric over the subgrade.
2. Place your rock base and compact it with a Jumping Jack compactor. Then place your sand over the concrete and rock; I would use the same thickness of sand over both.
3. Set your pavers over the concrete slab so the edge of the paver is in line with (or slightly past) the edge of the concrete. That way, if settlement happens you can pull out just a few pavers, add more sand, and re-set.

There is always a potential for a problem when you have 2 different bases to go over. Compacting the daylights out of it will help minimize any settlement and the geofabric will help spread out the loads and span any minor soft spots. Perhaps someone else will have some additional suggestions. You may also want to contact the paver manufacturer's tech support and see if they have any better ideas.

Bruce

BTW, have fun with that Jumping Jack They're a bugger to use at first, but they do a much, much better job of compacting than the plate compactor.

 

Great advice! I'll go with your advice and rent the equipment this weekend!