IS WELDED WIRE FABRIC FOR YOU?
By Dave Sethre, P.E.
A controversy involving the use of welded wire fabric (WWF) for flatwork reinforcing has recently made the news. This seemly age-old standard for keeping cracks from opening is under fire and for good reason. The problem is mostly related to construction issues of location, but any engineer or architect specifying its use must take some responsibility. A quick review of installation needs is hereby in order.
The Welded Wire Reinforcing Institute gives information concerning installation through its product support network. Its instructions specifically restrict placement of WWF at the center point or higher in the concrete slab, preferably higher. The reason for this specification is the negative effects that occur when the mesh falls into the lower part. When placed here, the effect of the mesh is to make the cracks larger at the surface under certain conditions, rather than keep the crack closed.
How many times have you seen flatwork removed and found the WWF barely imbedded in the bottom of the slab or even lying in the granular base beneath the slab? The main issue on larger pours is the need to walk on the WWF to consolidate or screed the concrete. Even with chairs, the WWF in these jobs is just about impossible to control final positioning, whether by lifting as the pour proceeds, or any other means. It is just impossible to lift the fabric when you are standing on it.
Rebar mats for temperature, shrinkage and crack control is a better solution, assuming the chairs are adequate and, again, the construction crew does not displace the mat into the lower part of the slab. Improperly placed rebar will cause the same issues as WWF under this scenario.
Probably the best option for solving crack control issues is with the use of the new steel fiber technology. Steel fibers are routinely used to create a more durable surface on concrete floors, but they also serve a crack control purpose. When used at 25 to 50 pounds per cubic yard of concrete, significant improvements in crack performance is normally noted. Measurement of steel area provided by fibers in comparison to WWF is usually correspondingly close. The distribution of fibers from top to bottom in the slab work as an owner or architect would expect. Considering the labor savings with the fibers, costs of installation are routinely less than using WWF. And this without the liability of improper installation concerns.
For a large application, fiber manufacturers offer design services to select the amount of steel fibers needed for slab characteristics and loading. In some cases, they can recommend a longer joint spacing, also.
Information on steel fiber use is available from most ready mix producers. They should be your source for specifications and design information services, also. Reduce your liability with the use of steel fibers.