Replies

1. Expert Member
   Michael Maines | Jun 17, 2023 12:40pm | #1

   Comparing the welded wire mesh (WWM) or welded wire fabric (WWF) meant for concrete reinforcement with hog panels, the main issues I see are: gauge of wire, strength of steel, strength of weld and potential for galvanizing to cause problems.

   The wire gauge varies. I have a variety of hog panels and other livestock panels and the wire appears to be 9 gauge, but it's probably available in other gauges. WWF is available in a variety of gauges. The larger the size, the stronger it is, and also the better the concrete will grip the wire.

   Steel strength I'm not sure about for either one, as I normally don't use WWF, but it matters because it affects the tensile strength. I know that rebar is available in both 40,000 psi and 60,000 psi versions, but that 60,000 psi is standard.

   The strength of the weld where the wires cross matters because if the steel is not deformed, the welds help the concrete grip the steel. WWF is also available with deformed wire, similar to rebar, which will better grip the concrete.

   Galvanizing is a good thing; if the steel ends up too close to a weathering surface it can rust and swell, spalling the concrete, but galvanizing adds protection against rust and I'm not aware of any chemical incompatibilities between galvanized steel and concrete. (Epoxy-coated rebar is a different matter; the epoxy coating is good as long as it remains in perfect condition, but if it gets nicked, the rebar rusts worse that it would without epoxy.)

   The reasons I typically don't specify WWF are that it rarely ends up where it's supposed to in the depth of the slab, so it ends up being a waste of money and leaves a weak slab; it's not as good at structural support as a rebar grid; it's not as good at preventing micro-cracks as fine fibers.

2. 
   joenorm | Jun 17, 2023 03:43pm | #2

   Thank you for the reply,

   What would you recommend for a small 4" thick slab poured over 1.5" pink foam that is intended for a small shop(not likely to see the weight of a car or tractor).

   Rebar grid? Wire mesh? Fiber additive? None of the above? or combo?

   thanks

1. Expert Member
   Michael Maines | Jun 18, 2023 10:30am | #3

   Joe, there are a lot of different ways to do slabs. In most cases you don't have to add any reinforcement at all; if the slab is kept damp for a week or two after it's poured, and if the sub-slab (and sub-foam) conditions are good, your concrete may never crack.

   My clients usually want to avoid cracking, so I might spec #3 or #4 rebar on 24" centers in both directions, min. 2" cover (and preferably 2 1/2" cover) at the top to avoid ghosting; diagonal rebar at any inside corners or penetrations; extra-fine micro-fibers to control fine cracking. And I would still keep the slab damp for a week to allow the curing that creates strength.

   I can't recall ever having slabs crack with that spec, though you might hear from others that "concrete always cracks."

1. Expert Member
   MALCOLM TAYLOR | Jun 18, 2023 10:47am | #4

   Michael,

   My 30 year old house has an exposed slab on the ground floor. It has grouted saw cuts @ 3 ft oc, with no reinforcing or fiber - and no cracks.