Do Concrete Slabs Really Need Welded Wire Mesh?

Over the past decade, many production builders have turned to synthetic fiber mesh to reinforce concrete slabs to help reduce surface cracking. In the process, many builders have completely eliminated traditional welded wire mesh (WWM).

However, while fiber mesh has advantages, it also has potentially costly disadvantages.

This may sound surprising, because the great appeal of fiber mesh is the time and money it can save. By using it, builders don't have to pay extra for concrete mesh and concrete contractors don't have to take the time to install it properly; in fact, some concrete contractors offer price breaks for fiber mesh.

Unsightly Cracking Problems in Concrete Slabs

While fiber does reduce surface cracking, it will not eliminate it completely. Worse yet, the lack of WWM can be a real weakness when cracks appear.

This is because a properly installed WWM will prevent further separation of the concrete on either side of the crack and keep them in the same plane, i.e., prevent differential settlement. Fiber mesh will not.

Repairs to differential settlement will not leave the greatest impression on the purchaser. You will have to grind the surface on both sides of the crack, fill the gap with epoxy and try to flatten it all out. Even if done properly, this will leave visible scars.

While most of these scars are decorative, they will scream "poor workmanship" to customers, or at least cause many to question the structural integrity of the home's panels. Of course, the builder must pay for the repairs.

We are seeing more and more of these problems on job sites as the use of fiber mesh increases... but we are also seeing more and more builders taking notice. Shortly after switching to fiber mesh, one of our customers found a dozen cracked and settling slabs at any given time. They reintroduced WWM and the problem almost disappeared.

Settled slabs and subsoil specifications

The chance of differential settlement depends heavily on the underlying soil. Where the soil is sandy and stable, settlement is less likely and the use of fibers alone may be a reasonable option.

However, in areas with clay and other expansive soils, correcting the problems caused by WWM elimination may be more cost effective in the long run than the initial cost savings associated with fiber screens.

Concrete Wire Mesh Help Minimize Concrete Slab Cracking and Settlement

In fact, the best way to minimize the chance of cracking and settling is to use both fiber mesh and WWM in the same slab.

As with any structural product, WWM will not work unless it is installed correctly. Unfortunately, this is not always the case.

Proper installation that provides maximum strength requires lifting the mesh off the ground so that when the concrete sets, it sits in the lower third of the slab depth. This means placing the wire on a chair to hold it at the correct height.

Wires not placed on a chair would not be effective, but to get the job done, some crews eliminated the chair and rolled the wire directly onto the dirt-covered plastic sheeting. When installers do use chairs, they must be careful not to knock the wires off the chair during the dumping process. If they do, then they need to reset the concrete screen.

Ensuring that all of this is done correctly can be a training and quality assurance challenge for builders, and avoiding this challenge may be one of the reasons so many people are choosing synthetic fibers for these applications.

But in soils that could lead to settlement, this type of oversight really needs to be a priority.