Kapt said:
My summer project is almost finished except for the floor. When I figure out how to post pics, I will. The inside of my 24x26 garage is excavated to virgin dirt, mostly clay. The walls sit on top of block with the bottom of the walls about 16 inches above the finished floor. I've done everything myself, but I will hire somebody to finish the concrete floor.
I assume I need to put down stone first so my question is what type of stone and how thick? Do I need to rent a compactor?
Should I use rebar or wire mesh? If I go rebar, what kind of spacing?
Some have suggested a grade beam, is this necessary?
Control joints?
TIA
Yes, you need to add stone. You want to find a granular material that is a mix of everything from 3/4" rock down including fines. The granular shape allows the stones to lock together forming a good base. Absolute minimum thickness is 4". 6" is preferred. Stone definitely has to be compacted. Use a vibratory plate compactor and make sure the stone is slightly damp during compaction - it helps the stone settle and lock together.
Make sure you use a quality vapor barrier. BMWPower has info on a really good vapor barrier that he just used in his awesome new garage in New Jersey with great results. I want to say there is info for it in the thread about his garage.
Rebar is probably overkill for a typical garage that is just used for cars and light trucks. If you want something more than the normal 6x6 wire mesh, see what is available in engineered wire meshes in your area. Around Chicago, we can readily get it in 4x4 and 6x6 patterns. Mats work better than rolls since the rolls want to return to the rolled shape when laid out.
Place a 1/2" thick expansion joint at the perimeter of the slab. Use one with a caulk cap or a pre-scored one and if you are concerned about aesthetics. Caulk the top edge.
Whether you use rebar or mesh, specify that it is placed on chairs or 2" solid concrete blocks before the pour. Do not let the contractor pull it up into the fresh concrete as the slab is poured. They walk on the mesh and it ends up at the bottom of the slab. For flatwork, steel has to be in the vertical center (neutral axis) of the concrete to be able to do its job (control tension forces).
Grade beam probably not needed. It sounds like the entire building is up already except for the slab. Grade beams are used to support loads (like walls) and span over questionable soil conditions.
I'd do a saw cut control joint at roughly 10' x 10' intervals but no more than 15' in any one direction.
A couple of other suggestions:
If you're installing a lift (now or in the future) make a 2' x 2' x 1' deep pad and use #5 bar at 6" o.c. each way in the pad at each post location. It will distribute the load way better than it just sitting on a 4" or 6" slab. I would also mark the center of each thickened pad with a piece of steel rebar driven into the base material and set flush with the finished floor height. Pins also act as grade benchmarks during the screeding and floating of the floor.
Do not put a sealer on the concrete if you are planning an epoxy floor system. It will have to be removed before the epoxy can go down and it's not a fun job.
Go with the minimum pitch that local codes will allow and still be functional for your intended use. More pitch equals more shims for a lift install. Less pitch equals ponding water if you want to wash cars inside all day.
Let me know if you want more detail on any of this. Unfortunately, my stuff on concrete is all at work until monday so I'd have to get back to you next week unless I know something off the top of my head.
