What is minimum Rebar Cover in a Concrete Slab
I am preparing the rebar for the floor of my personal workshop. The concrete will be at least six inches thick – the shallowest part is six inches while some spots are 7 inches. Underneath the concrete, I have two inches of XPS foam board and two layers of 10 mil plastic film between the foam and the concrete. There is 12 inches of 3/4 inch crushed stone below the foam board and the subsoil is well drained. The horizontal dimensions of the slab are 24 feet by 26 feet.
The intended loading on the floor will be for a two column automobile lift – 10,000 lb capacity and heavy-machine-tools – ie lathe and milling machine.
The rebar is composed of a #4 grid on 12 inch centers except for the areas that support the lift towers and machine tools. These areas have #4 rebar on 6 inch centers.
I would like to set the rebar so that I have a 20 mm (13/16) cover between the bottom rebar strands and the plastic film. As far as I can tell, this is standard practice in Europe when the concrete is protected from moisture.
Given the above, what specifications should I give to the concrete supplier regarding aggregate size – should I ask for 1/2 inch crushed stone or something smaller?
Also, has anyone had experience using a less-than-one-inch cover?
Replies
3/4 inch aggregate should be fine. Your rebar grid should be supported on dobies which should place it in the middle of your 4" slab. 13/16" is far to little,
OP stated a 6" minimum slab, not 4".
I agree that the bars should be close to mid-depth of slab.
Your reinforcing spacing is a bit too tight. Over reinforcing is just a waste of material. If it were an elevated slab it would actually be dangerous.
The goal with concrete reinforcing is to have the reinforcing yield (stretch, providing notice before failure) before the concrete fails (very abrupt and without any advance indication of a problem).
yes
I would recommend the following:
1. Concrete mix design: 3000 psi, 5 1/2 sack mix with air entraining admixture (3%-6% entrained air.) Don't pour the slab too loose. Unless your mix has a water-reducing admixture added to it, the concrete slump as it comes off the chute should be in the range of 4" to not more than 6". Reject any concrete that has a higher slump.
2. Except where your pads are for the lift supports, the rebar is overkill. Thicken the slab to 12" where the lift supports will be; reinforce the pads with the reinforcing you mentioned. Unless the lift manufacturer has specific size requirements for the thickened pads at the lift supports, I would make them at least double the size of the base plates, with the edges tapered up to the main slab at a 45 degree angle. Use welded wire mesh everywhere else in the slab. All the rebar & wire mesh should be supported with not less than 1" bar chairs or runners. These are placed directly on top of your vapor barrier. With a vapor barrier, you don't need as much concrete cover as you would if you were pouring the slab directly on the ground. 3" would be the minimum concrete cover in that case. I recommend a reinforced vapor barrier not less than 10 mils thick with all lap joints and penetrations sealed with the vapor barrier manufacturer's tape (not duct tape). Fortifiber and StegoWrap are good products in my experience. I avoid using poly for multiple reasons. You mentioned that you have 12" of crushed stone under your XPS insulation. The strength and stability of the concrete slab depends in large amount on how well this subgrade was installed. Did you compact it with appropriate compacting tools? i.e., a jumping jack with 4" maximum loose lifts of the stone or a vibratory roller/compactor (up to a 6" loose lift)? Tracked vehicles, like a skid steer, won't adequately compact the stone.
using a 13/16" bar chair is fine for VERTICAL formed surfaces, like the sides of a grade beam or a column. I wouldn't use anything less than a 1" chair for a formed HORIZONTAL surface. for an un-formed concrete surface in contact with the ground, you need 3" of concrete cover. Don't let your concrete sub "hook" the wire mesh up off the vapor barrier when they pour the concrete- it doesn't work and is, at best, inconsistently located. You would be better off eliminating the wire mesh alltogether and using fibermesh added to the concrete mix. The sub's finishers will love the hairy concrete. Personally, for this application, I would insist on a steel-trowled finish on the slab, followed by a densifier/hardener.