Back in January, leak-detection and repair specialist Nathan Weston of Naples, Fla., reached out to AQUA with a simple observation:
"I've been in this business since 1992 and have broken through something in the neighborhood of 3,000 decks. A normal deck is not hard to get through; you create a hole with your jackhammer and then pieces start easily breaking off and it goes pretty fast. But when I have to break through a fiber-reinforced deck, it's at least twice as hard, if not more."
Weston said he finds it easy to tell when he's working on a fiber-reinforced slab because of the way it breaks apart, or more to the point, how it resists his hammer strokes. "When you're jackhammering a fiber-reinforced deck, there are all these little pieces that kind of stay together," he explains. "You'll have a larger piece with tiny satellite pieces stuck to it by all these little hairs, which I've learned are microfibers."
Weston's anecdotal account may not be terribly scientific, but it does speak to one of the main reasons why fiber-reinforced concrete exists in the first place: it adds flexural-tensile strength to concrete. While that characteristic may make life difficult for repair technicians charged with busting through FRC slabs, there are possible advantages for builders that may be well worth considering.
"It's obvious that fiber-reinforced concrete is a tough material," Weston says. "That's why I personally believe it needs to find its way into more pool shells."
NOT SO SIMPLE
Most people intuitively understand the idea of adding fibrous material to concrete to increase strength. The basic idea is that fibers in the mix create a multi-directional, interstitial "mesh" within the concrete matrix that, when used correctly, will make concrete stronger.
Digging deeper, however, reveals a subject that is anything but simple. The types of fibers alone are a hugely complex topic.
For starters, fibers are typically categorized by both size as well as the material they're made of. There are the microfibers, which are very small hair-like filaments made of plastics such as polypropylene, or from minerals, such as glass or basalt. And there are macrofibers, which are almost always steel. Within those categories there are numerous fiber types and sizes that have different performance characteristics.
On the simplest level, microfibers primarily work to prevent micro- or shrinkage cracking, which mostly occurs during the initial curing process, or those critical first 28 days. (There are other benefits we'll discuss below.) By contrast, macrofibers provide load-bearing strength after the concrete cracks.
According to some who use them, the relative complexity of fiber use presents challenges for the industry that may be beyond current levels of expertise.
"Our industry already has an issue with a lack of control when it comes to the shotcrete process," says William Drakeley, managing member of the Drakeley Pool Company (Bethlehem, Conn.). "Fibers can provide benefits, no question, but they are also a very tricky thing. There are a number of control issues that add variables to a process that is already full of variables. I see it as a challenge for the industry."
Although Drakeley's company doesn't use fibers in pool shells, per se, he does have extensive experience working with fibers in applications outside the industry, such as highway and railway tunnels. "We use fibers all the time on those types of projects, but the process is always very controlled. For example, we've been required to create extensive test panels that can cost as much as $100,000 because quality control is so hugely important. It's crucial to make sure you're using the right fiber for the application, in the correct quantities using the proper mixing and application techniques. And you have to understand what those fibers do and what they don't."
INTO THE VOIDS
According to Drakeley and others contacted for this discussion, fibers can provide a handful of significant benefits; chief among them is that they can provide flexural-tensile strength in concrete that has already cracked and they can provide control for micro- or shrinkage cracking during the curing period. The latter benefit helps stop the crack from becoming larger, which in turn helps with preventing moisture intrusion.
"The use of fibers in concrete," says Dr. Marc Jolin, an engineering professor at Laval University (Quebec City, Canada), "can add some tensile strength to the concrete, much the same way as reinforcing bars; I include in this category anything that has to do with controlling crack openings."
"The obvious advantage of fibers, especially in the shell type structure," he adds, "is to offer a three-dimensional distribution of reinforcement. That's something that not-always-perfectly-located reinforcing bars cannot offer."
Adding tensile strength to concrete is important, but in the pool and spa industry, where smooth finish work and crack-free structures are the expectation of pool owners, microfibers and their ability to control shrinkage cracks before they become larger is an especially enticing benefit.
"There's a saying we use in concrete industry: Humans catch cold and concrete cracks," says Alvin Ericson, a technical consultant for ReForce Tech, a fiber supplier based in Bonita Springs, Fla. "It's the nature of the beast. In terms of the challenges facing the fiber-reinforced concrete industry, however, number one is a lack of education and experience. People just don't know how to design using fibers. Part of the issue is it's a little bit of the Wild West out there in terms of all the fibers that are available."
Although use of fibers in pool shells remains relatively unusual, fibers used in decks, in repairs and in decorative concrete work have become relatively commonplace throughout the pool construction industry.
Despite the complexities of fiber use and the inherent challenges of using something new to many, there are some fiber-related issues that remain refreshingly clear. For one, microfibers are not used to replace primary reinforcing steel, i.e. rebar. Macrofibers do replace some structural steel in large applications and in theory could be used to replace rebar in a pool shell, "but there are other issues that make using steel fibers in pools impractical," Drakeley cautions.
Microfibers used in slabs, on the other hand, are widely considered an appropriate replacement for welded-steel mesh.
"Steel mesh is there to control shrinkage cracks, so fibers do basically the same thing," Ericson explains. "The big difference is with steel mesh, you have a piece of steel every four inches, but with fibers that are properly mixed and applied, it's like having tiny secondary reinforcement throughout all of the concrete."
Microfibers have also become used in repairs where for shrinkage-crack prevention and workability are crucial. Steve Swanson, president of The Pool Company (Clayton, Calif.) started using microfibers after working with a concrete subcontractor who uses them in a large number of deck applications.
"I figured if they work in that application, why not in others," he said. "I did my own experimentation and started using them in patches because of the way the fibers reduce slump and add moisture retention. And in some applications we have started using them in gunite, especially on hot days, for the basically same reasons."
Returning to the use of fibers in slabs, Mark Holden, president of Holdenwater (Fullerton, Calif.) notes that in many of the large commercial projects he works on, fibers are used to replace all steel in slabs.
"It's largely a financial issue," he says. "We work on some enormous projects where there are vast expanses of decks, pathways, plazas and other spaces covered in hardscape. Because the slabs are on compacted fill, they don't experience expansive soils or other forces, so even though they don't have steel, those slabs perform beautifully.
"What will happen with micro-cracks is they'll hit the fiber and stop, which happens on a microscopic level," he adds. "It basically stops a small crack from turning into a larger crack."
"There is an interesting distinction to make here," Jolin cautions. "A slab on grade is not per se a structural application; in this case it is relatively easy to replace those reinforcing bars with macrofibers and many manufacturers actually offer tools to easily do the conversion from bars to fibers. But a multi-story building or parking garage is a structural application; this is the Holy Grail! Although it could/would be possible to replace most if not all the reinforcing steel in such an application according to recent R&D efforts, the building codes do not yet go that far."
KNOW THE MATERIAL
In terms of how fiber is selected, added to the mix and put in place, everyone contacted for this discussion stresses the importance of following specific manufacturer instructions and avoiding making assumptions. The question of how much to add per cubic yard is one of those issues that can vary wildly depending on the fiber and desired performance. The same is true of how the fiber is mixed in concrete to achieve even distribution.
When the fibers are supplied in a mixing truck from a batch plant, those and other questions about mixing technique are generally answered for the contractor. When mixed on site, following the instructions and practicing beforehand are crucial.
"Different fibers have their idiosyncrasies," says Ericson. "Plastic fibers, for example, tend to float near the surface and you can get fuzzy concrete where all these filaments are sticking out. Steel fibers, as another example, can do what's known as bird nesting, they interlock with each other and clump together. So when you put them in the ready mix truck, you have to be sure they're broken apart."
"There are interesting rules on batching fibers reinforced concrete in different technical documents on how and when fibers should be introduced," Jolin explains. "The key factors are obviously the dosage and, less obvious, the aspect ratio, which is length divided by a diameter. The dosage and selection of type of fibers is often a discussion you need to have with the supplier and the engineer. If not the engineer, at least the guy who knows why you want to put fibers in the mix."
NOT ENTIRELY UNFAMILIAR
Using fibers as part of a concrete design mix is not unheard of in the pool industry, but it hasn't been terribly common either. There are two applications, however, that are familiar to most.
First, fibers are widely used in artificial rockwork in a product known as glass-reinforced concrete, or GFRC; it's one of the staples of decorative concrete work. An entire subject unto itself, faux rockwork has included fiber since it went into widespread use back in the '50s and '60s, largely because the fibers help create rock panels that are both lightweight and strong.
The other familiar application has a decidedly darker side. While asbestos use is illegal, for years some plaster subcontractors used asbestos fibers to increase workability and ease installations. The problem was that when pools were resurfaced, chipping out the plaster sent the material airborne where it became a significant health hazard to anyone who happened to inhale the asbestos-contaminated dust.
THE STEEL REPLACEMENT QUESTION
In theory, and in some real world applications, macrofibers are used to replace some or all of the steel reinforcement, while in slabs on grade, microfibers commonly replace welded steel mesh. Anytime fibers are used to replace steel, the structure must be properly engineered.
One of the reasons that fibers can be used to replace steel may be surprising to some. According to Drakeley, some large-scale structural applications have so much structural steel in them that it can be extremely difficult to achieve full coverage with shotcrete.
He explains: "A smart engineer might say, 'Let's put in less steel and replace it with steel fibers,' which in turn enables you achieve full coverage of the steel because you don't have to shoot through multiple layers of rebar.
"Ease of application is a huge benefit of using fibers," he says."You still have the same amount of steel via the fibers, but you open up the pathway to fully cover the rebar. It makes for a much better structure."
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