Concrete slab needs proper base material

DEAR TIM: What type of stone would be best for filling a cavity under a planned two-car garage addition? It is approximately 7 feet deep. The local concrete supplier sells recycled concrete for a really good price. Due to the design, compacting will be next to impossible. Whatever is used needs to be “self-compacting.” Do I have options? What else would you do to ensure the concrete slab that will be poured on top of the fill will not crack or settle? – Dave L., Knoxville, Tennessee

DEAR DAVE: I know what you’re thinking. That low price on the recycled concrete is whispering in your ear. It’s a great product for many things, but based on your description, I’d not use it.

I have built custom homes that had attached garages that needed fill around the foundations and under the slabs. I also built garage additions very similar to yours. Because I’d had to go in and repair many a sunken garage slab put in by some builder or remodeler cutting corners, I quickly realized that garage slabs can and will fail horribly if you don’t put in solid, compacted fill under them.

I always use washed pea gravel.

The recycled concrete does compact well using rollers or plate compactors, but as you said, that’s very tough to do in your case. To get full compaction, you need to do it in lifts where the material being compacted is not much more than 4 inches thick.

Some gravel companies have special dump trucks outfitted with conveyor belt delivery systems. This tool swings side to side and up and down much like the digging arm of a backhoe. The driver can shoot tons of pea gravel into your foundation in minutes, saving you hours of back-breaking labor shoveling and hauling.

As for other options, I’d ask local gravel companies what other products they may have that are truly self-compacting. Beware of any products containing sand, as the sand requires compaction. It adds lots of friction to the material.

Here are a few other things I would do to ensure you end up with a fantastic job. For starters, I’d be sure to treat the soil inside this foundation for termites.

I’d also be sure to damp-proof the foundation wall on your home where this addition is connected. You don’t want any water vapor that’s going to rise from the soil in this new garage addition to make its way into your home.

I’d put a cross-laminated vapor barrier on top of the gravel before you pour the slab in the garage. This will keep water vapor from rusting tools and causing havoc in the garage in the winter months.

Because it’s going to have cars and perhaps a pickup on it, I’d make sure the concrete slab had No. 4 steel bars, commonly called half-inch rebar, installed at 2-foot centers in both directions.

This steel will keep shrinkage cracks small. It will ensure the slab will not separate. Steel is concrete’s best friend, as it adds enormous tensile strength to concrete slabs.

If your building code permits it, be sure to include floor drains. Old homes had these and the slab sloped toward the center of the garage. Newer codes tend to favor slabs sloping toward the garage door. All the code officials that voted for this obviously have never parked a car that’s been on snowy roads inside their garage. They’d discover the next day the garage door seal is frozen to the slab!