Im going to be pouring a 42 by 60 x4 in 5 bag mix floor, i see most people on here space rebar in 12" squares in there buildings . now my buddy say they place rebar in 5 ft squares but i dont see any one else doing this . he works for a big concrete company also . I was thinking of using mesh and rebar around where the lift would be ,but he say no just use rebar 5ft squares is all you need because if it cracks the rebar with hold it from shifting up . Iam in cental wi and will be doing infloor heat an 2in insulation under if that matters. im thinking this is incorrect doing 5ft sq's
The American Concrete Institute has calculations for this, but there are a lot of variables. Going off what other people do, may not be applicable to your situation. Some of the variables to consider:
- How much insurance you want to have against cracking outside of control or expansion joints.
- Grade of rebar/reinforcing
- Cross sectional area of rebar/reinforcing per foot of slab
- Strength of concrete
- Depth of concrete
- Types and spacing of expansion/control joints
What is your plan for expansion/control joints? How thick will your slab be? What strength of concrete are you going to go with?
I just poured a 5" 3000 psi slab, on 2" foam, with radiant tubing and had to have 7.75" squares of #4 rebar on one slab section and 9.5" squares on another. (I called for larger bars and stronger bars initially, but my contractor showed up only with #4 60KSI bar - so I had to whip out my computer and re-run the calculations. Larger, stronger bars would have made for wider spacing, but they were the ones who had to work around it and tie up all that extra rebar.)
The 7.75" was for a slab where I have no control (saw cut) joints. It's a lot of steel, but is what is called for to make the cracks (which will happen) hairline. Those will be filled in/covered by an epoxy coating later.
The 9.5" floor is what I needed for resistance cracking due to bending in my garage slab. I designed this originally for added piece of mind with my lifts and was REALLY glad I did as we went to pour the slab. I found that the 2" 60 psi foam board was so stiff that it bridged imperfections in the compacted base. So, the top of my 2" foam was anywhere from 0" to 1/2" higher than it should have been. Hopefully, this reinforcement will keep things together as the slab settles and can account for any small voids that may be left under the foam.
The 9.5" slab
does have saw cut control joints, but the rebar also enabled me to stretch them out around my lift to 13.5' at the widest point.
Spacing of control/expansion joints is a game of probabilities. Cracking will happen and the trick is to use the joints as a channel to pick where they occur. The variables above impact the likelihood of cracking occurring outside of those control/expansion joints. ACI also gives tables for balancing all of these variables, but the tighter you can make them, the higher the likelihood that cracking will be limited to the joints.
If you are just doing a simple slab with normal (10' saw cut control joints) you may not need any reinforcing, at all. The reinforcing will just help steer the cracking that does happen to those control joints and will help keep any cracking that does happen narrower.
Spacing rebar at 5' seems a bit wide but it may not be as crazy as it sounds. Reinforcing in a slab is based on the cross sectional area of the reinforcing relative to the slab section. I haven't done the math but the cross sectional area of a #4 bar (1/2" dia) at 5' might not be that different from the wire in a 6 x 6 mesh.
It's around twice the steel as 10 ga. mesh, which I found surprising.
Rebar is superior to mesh in that it doesn't sag as much between supports. A #4 bar is rigid enough to span supports that are 4' apart. Mesh would need to be supported every couple feet to keep it from ending up on the ground where it will do no good. The extra supports will coat more than the extra cost for the rebar.
Let me give you a couple additional things to consider;
- Make sure you install a well compacted, granular sub base under your slab
- Install a vapor barrier with taped seams
- Make sure your rebar is properly supported in the middle of your slab
- Keep the mix water to a minimum by limiting slump and/or using a mid range water reducer
- If a random crack will ruin your day, cut some control joints 1/4 the depth of the slab.
- Properly cure your slab for at least seven days
There's obviously more to it but if you hit these items, you'll be well on your way to a good slab. Good luck with your project.
I agree. I did a small, non-critical, section of my slab with mesh on chairs to save some money and it was a total disaster. (It just got trampled and ended up in the bottom of the slab, anyway.) Totally worthless. Fortunately, I planned for this and have enough saw-cut control joints to deal with any cracking. If I was to do it all over again, i would have sprung for the rebar and just spaced it widely.
