rebar vs. wire mesh

[fiataccompli]

I have a combination of rebar & wire mesh laid out in a slab ready to pour (probably wednesday). This is all on a plastic vapor barrier. The rebar was sort of an afterthought on the part of the builder, but was left over from the helluva wall we built for the back wall of the garage. Right now the rebar (>10' pieces) is in a grid, perhaps at 2.5' centers, but we didn't have enough to cover all of the floor. It's focused on the main entrances to the garage bays, but I have only the mesh farther back. (36' wide, 30' deep). Is there enough greater value strength-wise with rebar for the floor to be worth using the same grid throughout the garage? The area where my 2 post lift is likely to be located is covered & I expect a 4 post lift to be somewhere (maybe multiple locations) in the garage, but probably no other really substantial point loads.

 

[ConCretin]

Keep in mind that the rebar isn't making your slab stronger. It's only function is to keep cracks from opening up. From this standpoint, there isn't any practical difference between rebar and mesh assuming both are properly supported. Mesh is less rigid and harder to support.

Make sure you cut some properly spaced control joints with an early entry or 'soffcut' saw to avoid ugly random cracks. Plan your sawcuts to create panels approximately 10 x 10 and as square as possible. Also plan the cuts to keep them clear of your planned lift per the mfg instructions.

Give my Guide to Floor Slabs a read for some additional thoughts.

 

[strutaeng]

Either, as long as it is located in the right location (not in the dirt!)

But at your 2.5' spacing with rebar, mesh should be better with a closer wire spacing. 30" spacing? That seems too far apart!

 

[ConCretin]

But at your 2.5' spacing with rebar, mesh should be better with a closer wire spacing. 30" spacing? That seems too far apart!

Good point struteng. It's a bit farther apart than I'd recommend but its probably fine.

 

[matt_i]

If we agree that a slab also reacts to "plate bending", then its my opinion that rebar placed in the lower third also helps the concrete to react to tensile forces that can crack it in new places.

One might say that it would be best on the bottom and I would say yes if it could just get a grip.

That's why I say the lower 1/3 is best place for it. Believing this concept, I poured a 6" slab and placed my bar on 2" chairs.

Bottom line is the people placing the concrete have to be able to walk *all over* your bar or mesh and not permanently displace it, if you care about the elevation, otherwise you might as well have them hook it into place.

 

[brownbagg]

rebar and wire mesh does two different functions and they are not interchangeable, first a typical slab, needs neither.

wire mesh is for the shrinkage cracking in the first 48 hours, after that, it does nothing, they no strength factor in wire mesh

rebar is for load factors its design to keep the concrete from curling when a load is applied, as in weight of building on footers. If no load, then the rebar does nothing but lay there

a typical motor vehicle is a non load dead load, not enough weight to make a slab curl.
the bending of the slab is what causes the structural cracking

 

[ConCretin]

Hmmm. If my math is correct matt i, I think you actually placed you bars in the middle third.

 

[ConCretin]

rebar and wire mesh does two different functions and they are not interchangeable, first a typical slab, needs neither.

wire mesh is for the shrinkage cracking in the first 48 hours, after that, it does nothing, they no strength factor in wire mesh

rebar is for load factors its design to keep the concrete from curling when a load is applied, as in weight of building on footers. If no load, then the rebar does nothing but lay there

a typical motor vehicle is a non load dead load, not enough weight to make a slab curl.
the bending of the slab is what causes the structural cracking

I've got to respectably disagree brownbagg. Rebar and mesh perform exactly the same function in a slab on grade. Neither does anything to prevent cracks. In fact they increase the likelihood of cracking by restraining shrinkage. Both simply hold cracks together after they occur.

Concrete curling is a factor of differential shrinkage on the top and bottom of a slab and has nothing at all do with reinforcing.

 

[fiataccompli]

Wow, obviously a hot topic. It sounds like perhaps for my hobby auto shop use the very short answer is that it won’t matter enough to lose sleep over. I rather expect that with even the beat of planning & thought, I’ll still have cracks . I’d rather not, but I am more concerned that it stays in place (which is rather certain I think with concrete filled blocks pinned on the footings), and being smooth & level. Of course this said, I’ll check out the referenced document (thanks!) & I may make some small changes. The work is being done by folks who do this kind of stuff all the time, but I guess I don’t always assume that means they always do it right.

Thank you all!