Concrete Cover for Residential Driveway Slab

[dengebre]

I have a residential project that includes a concrete driveway. The minimum thickness for a slab-on-grade according to the Florida Building Code is 3.50” and most residential slabs are 4” thick. Welded wire fabric (e.g., 6x6-W2.9xW2.9) is often used for crack control. According to ACI 318 (20.6.1.3.1) the required bottom cover is 3” and the top cover 1.50” when exposed to weather. That results in a driveway slab thickness of 5” assuming that the reinforcement takes up 0.50”. The contractor is looking at me like I have two heads and says he has never seen a driveway slab over 4” thick for a residential project. To his point, I have seen 4" driveway slabs predominately on civil drawings. Are there different cover requirements for residential slabs-on-grade that allows a 4” slab?

I realize that fibrous reinforcement can be used in lieu of WWR in order to yield a 4” slab. I still like to include some amount of steel reinforcement for strength so that the slab can span over any soft spots, especially with vehicular traffic (I have been burned in the past).

 

[phamENG]

Question #1: Why are you using 318 on a residential project? ACI 332 is the concrete code for residential. Cover requirements for slabs on grade are laid out separately there.

Question #2: Why are you designing a driveway?

Question #3: If you absolutely must design it, why are you putting reinforcing in it?

 

[XR250]

I realize that fibrous reinforcement can be used in lieu of WWR in order to yield a 4” slab. I still like to include some amount of steel reinforcement for strength so that the slab can span over any soft spots, especially with vehicular traffic (I have been burned in the past).

The only time I would ever "design" a driveway if it needed to span over known soft spots or backill. Otherwise, let the contractor figure it out.

 

[gte447f]

4" driveway slabs in my area (southeast US but not Florida) are typically not reinforced. Control joints are used to, well, control shrinkage cracks.

4" driveway slabs in my area typically aren't designed by a structural engineer either, or anyone else for that matter.

 

[gte447f]

FYI, I did just recently perform a site visit to an existing, high value, single family residential property where some hardscape upgrades were under construction. A new driveway had just been completed and it was at least 6" thick and was reinforced with 2 way mat of rebar. It also had thickened edges and formed depressed areas to accommodate granite stone masonry curb details and inlays. The design was stamped by a licensed landscape architect. My point is, it depends on what type of driveway we are talking about. If the contractor on the job I just described had balked at anything more than a 4" driveway slab, he would have been fired immediately.

 

[dengebre]

ACI 332 it is, thank you.

 

[XR250]

With a solid non-expansive subgrade and enough joints, the slab should not randomly crack.