Replies

1. 
   user-5946022 | Feb 12, 2021 05:09pm | #1

   For strength, resteel.
   To reduce shrinkage cracks, fiber mesh.

2. Expert Member
   Michael Maines | Feb 12, 2021 05:27pm | #2

   Peter, I'm guessing you might have a typo--did you mean to ask about #4s at 24" oc and #3s at 12" oc? That would be a closer comparison. #3s have about half the cross-sectional area of #4s and about half the tensile strength, so using twice as many as you would with #4s will result in about the same tensile strength.

   There is more to the strength, though--the deformed surface of the rebar mechanically bonds to the concrete (along with a weaker chemical bond) to prevent the rebar from slipping inside the concrete. The surface area of #3s is 75% that of #4s, so using twice as many #3s will grip the concrete better.

   If this is a slab exposed to weathering agents (water or salt) such as a garage or driveway there is another concern; in case the rebar corrodes, larger dimension steel will last longer.

   Finally, there is the question of what kind of strength is important. In most residential slabs-on-grade or basement slabs, rebar in the field is primarily there to reduce cracking due to shrinkage, while rebar at the perimeter or in footings is there to support loads. There is essentially no risk of rebar in the field failing. Welded wire mesh serves a similar role with even smaller diameter wire. Its main drawback is that it's difficult to keep in position when placing the concrete.

   There are other things you can do to reduce cracking, such as using only as much water as necessary for hydration and placement, letting it damp-cure slowly, and various additives.

   I don't have experience with macro-fiber but often spec micro-fibers to limit fine cracking. If the slab is to be a finished floor, be very careful with what fibers they use, or use a different method, or you'll have a hairy floor. There are also structural fiber additives that can replace some rebar .

1. 
   T_Barker | Feb 13, 2021 10:00am | #3

   Good summary!

   The only thing I would add is when you specify too tight a pattern (e.g. 12") on residential projects it's hard for the workers to step between the rebar while doing the pour. As a result, especially with a careless contractor, there's more chance they'll put the rebar on the ground in certain places, and it becomes worthless. This is why I NEVER use wire mesh. It NEVER ends up at the right height in the concrete floor.

   From a structural design perspective, in many jurisdictions non-load bearing residential floors (i.e. NOT engineered slab) very rarely require any rebar. However, I personally would always insist on rebar to reduce crack separation, upheaval, etc.. When you do need rebar, it's mostly about the total cross section of rebar per cross section of concrete.

3. 
   johnwtaylor | Feb 13, 2021 08:35pm | #4

   Another option is #3 Rebar on chairs with 1 or 2 (staggered) layers of screen that is wired to the rebar.

   Fiber is not added for strength

4. 
   jackofalltrades777 | Feb 16, 2021 12:48am | #5

   On my current house they used #3 rebar at 24" oc and a 4,000 psi hydro-mix and I have ZERO cracks in my garage (house floor is covered with tile but I assume it's like the garage floor).

   Of course the sub base was 4" of compacted AB (fines/sand and 3/4" or smaller rock).

5. 
   Zdesign | Feb 16, 2021 08:21am | #6

   #4 or #3 rebar is completely overkill and unnecessary for a garage slab. 6x6x10 Welded wire mesh is all that is needed with 3000 psi concrete. If you have a heavy truck bump it 4000 psi or a 5" or 6" slab. Leave the steel for walls and reinforced point load areas.