Slab Shrinkage Reinforcement Spacing 1

[XR250]

Why are we limited to 18” O.C. spacing for this? If it was installed 36” O.C., is the concrete really going to crack between bars? Seems with Gr 60, #4 bars and a 4” slab, you could place 'em 36” O.C. and meet the minimum area requirements and save a bunch or labor and materials.

 

[Settingsun]

Pulling up and then what? What keeps it up?

 

[GeorgeTheCivilEngineer]

It's a bit crazy, isn't it? It's a clear misuse of WWF which is actually really useful, both for designers and contractors.

 

[dauwerda]

Pulling up and then what? What keeps it up?

The idea is that the concrete itself holds it in place. As the concrete is placed, the placement crew will reach into the concrete and pull the WWF up into the middle of the slab. This typically isn't very effective, not because it won't stay in the middle once pulled, but because it then gets walked on by the guys mucking and running the screed rod and pushed right back down into the bottom. For it to work, the WWF needs to be constantly getting pulled up right in front of the screed rod - which rarely ever happens. Out of site, out of mind, right?


For a 4", non-structural slab on grade the idea isn't to use reinforcing to control cracks is it? That is why we cut control joints in, so it cracks where we want, which (when properly spaced) relieves enough of the internal stress it won't crack in other spots. Unless you are going to put lots of reinforcement in, the best way to control cracks is to have a properly prepared subgrade (well compacted, free draining material) and a good mix design (large aggregates are your friend as you can get a stronger mix with less cement, less cement = less shrinkage, low water/cement ratios also reduce shrinkage).

The rebar (or WWF) in these slabs isn't there to prevent cracks, it is there so that when it does crack there is something to help keep the crack from separating, and possibly to help with some bridging of a poorly prepared subgrade. If a good mix is used, the subgrade is prepare properly, and control joints are cut in as soon as possible after placement and spaced correctly there is no reason a slab w/o any reinforcement wouldn't perform just as well as a slab with limited reinforcement.

 

[dik]

I was once 'chastised' for using sawcuts on a residential basement slab with a re-entrant corner... the absence of the sawcut and the 6" slump concrete did exactly what I thought it would... In these environs, sawcuts are almost unheard of in residential construction.

Dik

 

[dik]

If done properly, concrete slabs can be relatively crack free... using that line is only an excuse for not doing the work properly.

Dik

 

[dik]

That's why I prefer properly chaired rebar... it's easier to step through the holes... and unless you have little 'hoofies' you cannot step through the mesh, pulling over any accessories and causing the mesh to end up on the bottom...

Dik

 

[dauwerda]

It will work, but with tradeoffs.

For shrinkage cracks:
[ul]
[li]If the slab has continuous reinforcement and has control joints cut in it, the reinforcement will make the control joints less effective, unless the reinforcement is also cut at the control joint, (either by the concrete saw or originally placed that way).[/li]
[/ul]

- If the control joints are installed properly, it shouldn't matter what the reinforcement is, the slab will crack at the control joints and that is it.​

[ul]
[li]If the slab has continuous reinforcement and there are no control joints, or the control joints are too far apart or don't interrupt the reinforcing then you will get "random" cracking.[/li]
[/ul]

- If smaller reinforcing spaced closer together is used you will have more cracks develop that stay tighter together.​

- If larger reinforcing spaced further apart is used you will have less cracks develop that open up a bit more.​

For cracks that are caused by loading (poor subgrade, slab needs to bridge) I would say it depends on the loading. If it is a point load (tire of a car), I would expect the closer spaced reinforcing to perform better as the point load could easily fall in the middle of the larger spaced reinforcing, making it less effective.

 

[dauwerda]

I was once 'chastised' for using sawcuts on a residential basement slab with a re-entrant corner...

dik, I'm assuming this is because they would need a saw? I'm curious if this was long ago or recently? In my experience saw cutting in the residential world (as opposed to hand tooling joints) became much more prevalent in the early 2000's. Before that, I saw more hand tooled joints, or another popular method (for basements especially) was to use a margin trowel to cut all the way through the slab then re-seal the top when doing the hard trowel finish.

 

[dik]

by sawcut... not the old 'Goldblatt groover'.

Dik

 

[Settingsun]

I've never come across this lifting of mesh as the pour is happening, but it sounds as though they're constantly wriggling the mesh sheet around after compacting the concrete around part of the mesh. Sounds poor at best.

On the original question, and for same total reinforcement area, larger bars have a longer bond length for the same force transfer back into the concrete so don't restrain cracking as well as smaller bars at closer spacing: elongation of bar is greater; and crack spacing is larger so the cracks are wider. Mesh would have the added advantage of the cross wires providing some anchorage. This assumes that the reinforcement doesn't yield which it could well do for the small quantity of reinforcement being suggested. Since the reo quantity is small and has limited effect on crack control anyway, 70% of limited effect (due to doubling the bar spacing) may be good enough.

 

[XR250]

Steveh49,

A lot of that makes sense. I am curious why there will be more elongation in the bars if the area is the same? Another issue with mesh is making sure it is lapped sufficiently - something I usually Do not see.

 

[Settingsun]

At each crack, the reinforcement carries 100% of the residual restraint force, so the bar stress is the same for equal bar area. Away from the crack, the concrete carries the tension after bond re-establishes, but the concrete must crack sufficiently (number of cracks * crack width) that the tensile stress is less than the concrete's strength. Larger bars require a greater bond length to re-establish the concrete tension, so a longer length of bar is stretched locally at each crack, giving greater elongation and greater crack width. As a result though, each (wider) crack relieves more of the restraint force so fewer cracks are needed to reduce the stress to less than the concrete's tensile capacity.

 

[dik]

Maybe 15 or 20 years back... for normal (5" or 6")slabs, I typically sawcut 1-1/2" deep and have 1-1/2" cover to rebar so the saw just misses it. With sawcutting, timing of the cut is critical... as short as possible.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik