O/T, Concrete slab

Was wondering what size rebar would be best to use and how far to space it out...was thinking the 3/8 would be fine at about 2' spacing.

Part of this addition will be used as a woodshop and tool storage area, so won't have any excessive load.

Also, am planning a 40'x40' shop next to this one for my tractor work etc. Will a 4" slab be ok here too, or should I do something different.

Thanks,
Tim

More babble, have some downtime at the moment.

We take note of anyone saying no cracks, because up here regardless who pours it there are cracks. Those who say no cracks need to take some dust and spread it on the floor and jar the floor, It many times will show the cracks that they have never seen before.

The real issue is to make it crack where we want it to crack (saw joints) and to hold the random unwanted cracks together so you can't see them---thus rebar---up here wire mesh with the 6 x 6 pattern no.10 wire is a feel good for the lower budget folks. A total waiste in my opinion.

I do totaly agree on the compaction issue raised preveously, however in our area a uniform fill is very important. It most always cracks where the fill meets the solid ground. We live in rough hillsides, and insist on digging out the hard fill so the building gets the same depth of fill overall. Very expensive but if they use the words (guarentee no cracks) it is a must. Compaction in uniform 4-6 inch lifts is also a must. You can drive your pickup over a sheet of thin plywood if the fill is uniform and solid under it.

Rebar in our area and my mind is money well spent-1/2 inch no grade is the most bang for the buck--again in our area.

If you have a drain dug across--guess what -- the trench can settle, thus a known place for a potentual crack, This may not related to you without frost, but expansion or control joints are a must.

You will have to know where your building site is, but if you have good drainage, we do NOT use a visqueen as we want the sand to suck up the moisture from the concrete floor. Precautions are needed to avoid this during the pour. If in a damp location you would of course want to stop it from drawing into the slab with visqueen.

When using rebar, it costs you some major money so why not make it do its job by having it in its proper place--in a slab the bottom 1/3 is prefered. There are plactic chairs that will clip over your rebar and hold them in place, they have a circular foot to give it surface to avoid it setteling down. Rebar can be tied together by the kids with a twisty tool and 6 inch twisty wires, not very costly at all. Now you get all the bang for the buck. If you use the pull up the rod as you pour option, or random rocks to hold it up, a rebar to high (surfacing rebar) while pouring you have major problems when you sure as heck do not need them. If the rod are on top of the fill they are expensive and worthless.

We use no less than a 4000lb mix and many times use a reverse mix. (more sand less gravel), easy to get a good finish. We also use a roller bug that perhaps you can rent (PRICELESS), it rolls out and pushes the rock 1/2 - 1/4" towards the bottom---thus you have a top mix with no rocks to mess with you, and provides a super hard top surface that is easy to finish. Do not pour on a hot dry windy day as the top dries to fast. A cure and seal will help--extra expence probably not maybe worth it.

Fiber mesh is a secondary reinforcment - not primary, was never intended to do so. It is used mainly in our crew for the precast and thin over old cement pours. Not money waisted just not cost effective. We would rather spend that money on rebar issues.

This no doubt is extreem, Never can afford to do it right the first time, but can wish you had for the rest of your life. Actualy this is more of a labor issue that can be done by the home owner other than a little profesional help may be a good idea during the pour. Dave NE IA

If you get over 16ft in any dimension you are basically doomed to have a crack with unreinforced concrete.

You can sawcut to make the crack look pretty but it is still a crack!

Concrete shrinks as it cures. Rebar or mesh or fiber keep the inevitable cracks from growing apart, and generally keep the separe pieces of slab in the same plane.

Look at the floor of any big box store. You will see crack control joints on 10 foot centers in each direction. This allows the shrinkage stress to be relieved in a controlled, aesthetic way. That is what you should use in your shop. The crack control joints can be tooled into the concrete during finishing, or sawn once it sets up. There is also a finishing accessory called a "quick strip" or some such you can buy. It is basically a metal "tee" you can embed in the concrete during placement, that forms a crack control joint along its length. Try to avoid a floor plan with "re-entrant corners". Anywhere that there is a notch or 90 degree "inside" corner you have a stress concentration that tends to crack, despite crack control joints and reinforcement.

3/8 rebar on 18 inch centers is a pretty good recipe. Use plastic rebar chairs or concrete dobies to hold them up. Don't use chunks of brick -- the clay brick soaks up water and weakens the concrete around the brick. The rebar is superior to mesh because the placement crew can step between the squares and keep the rebar in the center of the slab. With mesh, the crew stomps the mesh to the bottom of the slab, and it has to be pulled up with hooks, which usually doesn't work, leaving the mesh near useless at the bottom.

Use a 6 sack mix, and use as little water as possible. Add a mid-range water reducer or super-plasticizer to the mix to increase slump and improve workability. Keep the truck driver away from his water hose. A low water/cement ratio will have less tendency towards slab curl and plastic shrinkage cracking.