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Under certain soil conditions, eliminating wires from slabs can be a false economy...at best

Under certain soil conditions, eliminating wires from slabs can be a false economy...at best

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

However, while the fiber web has advantages, it also has the disadvantage of potentially costly.

This may sound surprising, because the great appeal of optical fiber is that it saves time and money. By using it, the builder does not have to pay extra for the wire mesh, nor does the concrete contractor have to spend time to install it correctly; in fact, some concrete contractors offer price concessions for the fiber mesh.

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

This is because a properly installed WWM will prevent the concrete on both sides of the crack from further separating and keep them on the same plane, that is, prevent uneven settlement. The fiber web will not.

Differential settlement repairs will not leave the biggest impression on buyers. You must grind the surfaces on both sides of the crack, fill the gap with epoxy and try to smooth it all (see below). Even if done well, it will leave obvious scars.

Although these scars are mostly decorative, they will scream "poor workmanship" to customers, at least causing many people to doubt the structural integrity of the siding. Of course, the builder must pay for repairs.

With the increase in the use of fiber mesh, we see more and more of these problems on the job site...but we also see more and more builders paying attention to this. Soon after switching to fiber mesh, one of our customers found dozens of cracked and settled slabs at any given time. They reintroduced WWM and the problem almost disappeared.

The chance of differential settlement largely depends on the underlying soil. In places where the soil is sandy and stable, such as most areas in Florida, where settlement is less likely, fiber alone may be a reasonable choice.

However, in areas with clay and other swelling soils, such as Carolina, in the long run, correcting the problems caused by the elimination of WWM may be more costly than the initial cost savings associated with fiber webs.

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

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

Proper installation to provide maximum strength requires lifting the grid off the ground so that when the concrete sets, it is located in the lower third of the depth of the slab. This means putting the wire on the chair to keep it at the correct height (see below).

Wires that were not placed on the chair would not be effective, but in order to get the job done, some staff removed the chair and rolled the wire directly onto the dirt-covered plastic sheet. When the installer does use the chair, they must be careful not to knock the wire off the chair during the dumping process. If they do, then they need to reset the wire.

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

But in soils that can lead to settlement, this type of supervision does need to be a priority. 

As the building performance manager of the IBACOS PERFORM Builder Solutions team, Richard Baker promotes the quality and performance of residential buildings.

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