Replies

1. 
   Matt | Jun 16, 2010 05:40pm | #1

   It's somewhat subjective...

   Some people swear by fiber - some like wire.  I've used a lot of both.  Here, fiber fell out of favor about 8 years ago so we use mainly wire.  Personally I believe that fiber is more to prevent minor surface cracks where as wire prevents cracks from becomming large.  If you can afford it I'd use both.  Before I did that though I'd put some rebar in the footings.   The other thing is to make sure they really do give you a full 4" of concrete.  What you do is string line the forms prior to the pour.  It seems that nearly every concrete guy thinks a 2x4 is 4" wide which isn't so bad, but when it starts getting down to 3" or even 2.5 that just plain isn't enough.  If you use wire, in order for it to be effective it either has to be pulled up in the concrete as the concrete is placed (that is wiat the hook on the back of a come-along tool is for) or set up on charis so that it doesn't end up on the bottom of the slab.  Rebar needs chairs for the same reason.  Having a uniform substraight below the slab is probably even more important than the reinforcment.  Stone (gravel) is normally used for that.

2. 
   fingers | Jun 16, 2010 06:11pm | #2

   Fiber is used as secondary reinfocement.  WW mesh or rebar is more of a primary reinforcement.  Nylon fibers were first developed for use in the concrete of ICBM silos but they also had lots of steel.  I'd say for your job go with the WW mesh as a minimum and fiber if you can afford it.  What's most important is what's under the slab. . .stable and well compacted.

3. 
   davidmeiland | Jun 16, 2010 11:06pm | #3

   I generally go with

   a mat of #3 rebar on 12" centers, on 1-1/2" dobies, fiber in the concrete, and a low-water mix with a plasticizer in it.  This last piece is critical, you will get a lot less shrinkage if less water is used, and there won't be any incentive for the finishers to try to water your mix if it already seems sloppy. Good finishers already know about plasticizer (AKA water reducer).

   I think WWM is pretty much worthless, as it usually ends up on grade instead of up near the middle of the slab. With rebar you can easily control the elevation even if people walk all over it during the pour.

   After the pour, keep the slab damp by misting it with a hose for several days.

4. 
   Piffin | Jun 17, 2010 07:03am | #4

   Depends what you want it to do....

   Forget either of them until you get rebar first. That gives you the structural strength and holds it together IF it does crack.. Fibre and wire mesh will not provide strength.

   fibre helps prevent and controls shrinkage cracks, but the better way to do that is to not use too much water, the cause of shrinkage cracking. The #1 mistake DIYsand hacks make is ading too much water to make pouring it easier. You said you are HAVING this garage built. Does the builder not know how to do the crete work?

   I use a rebar mat #4 grid tied at 18" to 24" oc depending...and I use a thickened edge slab with extra rebar there.

   And before that, the two most important things are drainage and soil structure. The base needs to be well compacted mineral soil, not topsoil or clay, and water must be led away from under the slab.