OT - Question about Improperly Poured Concrete Slab ...

Garwood said:

I'm the concrete amatuer.

The successful slabs I've done had good prep, plenty of compacting beforehand.

The cracked slabs had poor compaction. Rebar/mesh/fiberglass doesn't seam to matter.

If you want to learn more, please visit our website.

If you are looking for more details, kindly visit Xingtai Steel.

Click to expand...

It depends on what causes the cracks, and how the rebar is placed. If the cracks are structural, that is, caused by bending stress on the slab that results from the combination of inadequate compaction and excessive load on the slab, then casually placed rebar will be no better than wire or fiberglass.

Structural cracks are the result of its bending failure as a beam, and rebar placed in the middle of a 4" slab adds almost nothing to the slab's ability to resist bending, because it is in the neutral plane of the beam. In other words, rebar in the middle of a slab does not give it real advantage over wire in the middle of a slab if the slab has to act as a beam, which it does if it has to span uncompacted areas.

However, properly engineered and placed rebar will greatly enhance the ability of a slab as a beam, but said rebar has to be near the bottom of the thickness -- the side of the beam in tension. But in practice, this cannot be done on a 4" slab, because it's just not thick enough. Standards require 2" of concrete cover over rebar, so with a 4" slab that means it's in the middle, again at the structurally useless neutral plane. But rebar located 2" from the bottom of an 8" thick slab will greatly enhance the strength as compared to same slab with no rebar.

If a slab fails structurally, that is, has actual cracks going all the way through the thickness, then the presence of wire or rebar will indeed resist the relative dislocation of the slab on either side -- that is, one side won't sink below the other (assuming the loads aren't too great and the steel does not shear). But this is not the design purpose of rebar or mesh. At least from an engineering point of view, the failure is the crack itself, not the dislocation that follows. Put another way, the slab has not "performed" because rebar kept the broken slab from sinking on one side. It might be convenient that it doesn't dislocate, and this might be a desirable performance to the user, but no engineer would ever design with this intent (and code would not allow it.)

If the slab cracks through (fails) it is because the slab is not adequate to handle the spans necessary for the substrate. You accommodate this either by improving the substrate (better, even compaction) of by having a stronger slab. For example, I know someone who recently built a new, 3,000 sq ft slab-on-grade for their house -- the slab is 36" thick. This because the soil cannot be reliably compacted because it's on a geological slide. So the slab is engineered to be able to span its entire length and width.

On the other hand, if the issue is cosmetic shrinkage cracks, then wire, rebar, and fiberglass can help reduce them, but success depends on a complex of factors including concrete mix, placement, curing, etc, etc. Some will say fiberglass (or mesh or whatever) doesn't work because their specific slab had it and still cracked, but that doesn't prove fiberglass (or whatever) doesn't work. It just proves that element alone could not prevent cracking in light of all the other variables present. At the other end of the spectrum, very well prepared, placed, and cured concrete with no steel at all and no fiberglass can be perfectly crack free in quite large spans. But on average, with average quality of all the factors, wire, rebar or fiberglass will reduce shrinkage-induced cracking.