The past decade has seen many production builders switch to synthetic fiber mesh reinforcement for concrete slabs to help reduce surface cracking. In the process, many of these builders have completely eliminated traditional welded wire mesh (WWM).
But while fiber mesh has advantages, it also comes with potentially costly drawbacks.
That may sound surprising, given that fiber’s big appeal is its time and money savings. By using it, builders don’t have to pay a premium for wire mesh, and concrete contractors don’t have to take the time to correctly install it; in fact, some concrete contractors offer a price break for fiber mesh.
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The Problem of Ugly Cracks in Concrete Slabs
While fiber does reduce surface cracking, it won’t eliminate cracks completely. Worse, when a crack does develop, the lack of WWM can be a real weakness.
That’s because properly installed WWM will keep the concrete on both sides of a crack from separating further and will keep them on the same plane—that is, prevent differential settling. Fiber mesh won’t.
Repairs to differential settling don’t leave the greatest impression on homebuyers. You have to grind down the surface on either side of the crack, fill the gap with epoxy and try to smooth it all out (see below). Even when done well, this leaves a visible scar.
While such scars are mostly cosmetic, they scream “poor workmanship” to customers, leading many to doubt the structural integrity of the home’s slab, at least. And of course, the builder has to pay for the repair.
As use of fiber mesh use has grown, we’ve seen more and more of these problems on job sites ... but we’re also seeing more builders take notice. Soon after switching to fiber mesh, one of our clients found a dozen cracking and settling slabs at any given time. They reintroduced WWM and the problems virtually disappeared.
Settling Slabs and Underlying Soil Specs
The chance of differential settling depends largely on the underlying soil. Where the soil is sandy and stable, as in much of Florida, settling is less likely and fiber alone can be a reasonable choice.
However, in areas with clay and other expansive soils, such as the Carolinas, correcting problems caused by the elimination of WWM can cost more in the long run than the initial cost savings associated with fiber mesh.
How to Minimize Concrete Slab Cracking and Settling
In fact, the best way to minimize the chance of cracking and settling is to use fiber mesh and WWM in the same slab.
Like any structural product, WWM won’t do its job unless it’s installed correctly. Unfortunately, that’s not always the case.
Proper installation that provides maximum strength requires the mesh to be raised off the ground so that when the concrete sets, it’s in the lower third of the slab depth. That means placing the wire on chairs to hold it at the correct height (see below).
Wire that’s not placed on chairs will not be effective, but in the rush to get jobs done, some crews eliminate the chairs and roll the wire directly out over the plastic sheeting that covers the dirt. And when installers do use chairs, they must take care not to knock the wire off the chairs during the pour. If they do, then they need to reset the wire.
Making sure all of this gets done right can be a training and quality assurance challenge for the builder, and avoiding that challenge may be one reason why so many opt for synthetic fiber for these applications.
But in soils that make settling likely, this type of oversight really needs to be a priority.
Richard Baker drives quality and performance in homebuilding as the building performance manager on the PERFORM Builder Solutions team at IBACOS.