Tag Archives: fiber mesh

Wire Mesh in Slabs on Concrete

Many concrete finishers have switched to synthetic fiber mesh reinforcement for concrete slabs to help reduce surface cracking. In doing so, many of these finishers have completely eliminated traditional welded wire mesh (WWM).

But while fiber mesh has advantages, it also comes with potentially costly drawbacks.

This may sound surprising, given fiber mesh’s big appeal is its time and money savings. By using it, finishers don’t have to pay a premium for WWM, and don’t have to take time to correctly install it. Some concrete finishers actually offer a price break for fiber mesh.

While fiber does reduce surface cracking, it won’t eliminate cracks completely. Worse, when a crack does develop, lack of WWM can be a real weakness.

rebarThis is because properly installed WWM will keep concrete on both sides of a crack from separating further and will keep them on the same plane— preventing differential settling. Fiber mesh won’t.

Repairs to differential settling don’t leave a great impression on new post frame building owners. 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. Even when done well, this leaves a visible scar.

While such scars are mostly cosmetic, they scream “poor workmanship” to clients, leading many to doubt the slab’s structural integrity. 

As the use of fiber mesh has grown, more and more of these problems have been seen on job sites, however finishers are beginning to take notice. Soon after switching to fiber mesh, one found a dozen cracking and settling slabs at a given time. They reintroduced WWM and these problems virtually disappeared.

Chances of differential settling depends largely on underlying soils. Where soil is sandy and stable, settling is less likely and fiber alone can be a reasonable choice.

However, in areas with clay and other expansive soils, correcting problems caused by elimination of WWM can cost more in the long run than initial cost savings associated with fiber mesh.

In fact, the best way to minimize chances of cracking and settling is to use fiber mesh and WWM together.

Like any structural product, WWM won’t do its job unless it’s installed correctly. Unfortunately, this is not always done.

Proper installation to provide maximum strength requires mesh to be raised off the ground so when concrete sets, it is in the slab’s depth lower third. This means placing WWM on chairs to hold it at the correct height.

It is crucial to ensure WWM be placed on proper height chairs. Otherwise, WWM will not effectively hold the slab together. 

Wire not placed on chairs will not be effective. But in a rush to get jobs done, some crews eliminate chairs and roll WWM directly out over Code required under slab plastic sheeting covering underlying properly compacted fill. And when installers do use chairs, they must take care not to knock WWM off the chairs during pour. If they do, then they need to reset WWM.

Making sure all of this gets done right can be a training and quality assurance challenge for finishers, and avoiding this challenge may be one reason why so many opt for synthetic fiber for these applications.

However, in soils or poorly prepared sites making settling likely, this type of oversight really needs to be a priority.

Concrete Slab-on-grade Reinforcement

Long time readers will recall concrete finishing does not rank amongst my favorite building tasks. It is an art form with a gene I was not blessed with. Most pole barns, post frame buildings and barndominiums utilize slabs poured on grade.

Reader KYLE in KAPLAN writes:

“In your pole barns, do you typically use wire mesh, or rebar when doing a slab?”

For areas where heavy vehicles or equipment may be placed, our independent third-party engineers will specify rebar reinforcement for concrete slabs on grade. Much of your need for reinforcement will be dictated by how well your site was prepared. Properly compacted sub-grades can reduce needs for reinforcement – as well compacted site is less prone to adverse effects from uneven weight distributions. Sadly, most clients and builders just do not spend enough time and effort to arrive at good site preparation.

Choosing concrete for a residential or commercial construction project is a great way to ensure you are using a strong, durable material. There are several ways to make sure your concrete has proper strength for your building. Concrete changes density when it sets, making it vulnerable to cracking. Concrete also can crack due to changes in temperature or unevenly distributed weight or stress. When pouring concrete for driveways, foundations, or floors, three common ways to reinforce concrete are to use wire mesh, rebar or fibers.

Using wire mesh is a common method to reinforce poured concrete. Wire mesh makes a square grid pattern and is laid down before concrete gets poured. Wire mesh is usually one layer of a two-dimensional grid running along length and width of poured concrete, but not height. During concrete pouring processes, you or your workers will raise previously laid down wire so it runs along concrete height (thickness) middle. This reinforcing material inside helps to prevent cracking during temperature changes and while concrete is setting.

Instead of laying down a wire mesh before concrete is poured, using fiber mesh involves mixing in different fibers such as glass, steel, synthetic fibers, or natural fibers. Fiber mesh reinforces concrete throughout its entire structure rather than just one plane. This comprehensive reinforcement protects against not just cracking due to fluctuating temperatures and changing densities from setting, but also helps prevent water from bleeding out of concrete and gives concrete’s surface a higher impact resistance.

In addition to providing a more thorough protection for your concrete pour, fiber mesh typically takes less time than wire mesh to use. This is because wire mesh has to be carefully measured to fit the pour site and needs to be held up at a certain level during pouring processes. Conversely, fiber mesh can be added straight to mix, removing the need for an extra step while pouring. Fiber mesh is also more cost-effective since there is less time involved in pouring and material is used more efficiently. There has been concern among some as this fiber mesh method can create a “hairy” finish due to some fibers protruding from the slab’s surface. However, this is only temporary since they are often laid down flat when trowels flatten concrete’s surface, and any fibers still protruding are quickly worn down or burned off by the sun if outside, or a torch if indoors.

American Concrete Institute (ACI) lists factors playing a role in how thick covering concrete must be to support rebar.

Cast-in-place concrete requires placement of wet concrete around rebar, then holding it in place as it sets and dries around it. This is usually done with rebar supports helping hold it at correct depths, but this does open it up to a certain level of operator error.

For cast-in-place concrete in contact with ground permanently, recommended covering concrete thickness is three inches.

This means for cast-in-place slabs less than 5 inches thick, in most circumstances, there should be no rebar involved. Slabs at this thickness are simply too thin to adequately cover and protect rebar while still exploiting its reinforcing nature.

One thing certainly helping is to check your local regulations, as they take into consideration local environment for optimal construction.