Fibers in Precast, GFRC, and ECC Concrete Countertops

In the last article, I discussed how PVA fibers are used in engineered cementitious composite concrete (ECC). Today I’d like to review some of the many types of fibers that you can use when making concrete countertops and some of their purposes, as well as explain which types of fibers are appropriate for which method/mix:

  • Traditional precast
  • GFRC
  • ECC

3D Fibers

What Fibers Can Help With

Fibers are used in concrete for a variety of reasons, but not all fibers do the same thing or have the same effect. They can be used for reinforcing or can be used to prevent shrinkage and cracking.

  • When fibers are used for improving the flexural/tensile properties of the concrete, this is known as primary reinforcing.
  • When fibers are used for plastic shrinkage control, and to prevent crack creation and propagation in the cement matrix by bridging the microcracks, it is known as secondary reinforcing.

The choice of what fiber to use depends upon a variety of factors. In commercial construction, cost is often the primary factor, as most fibers are used for secondary reinforcement reasons. For concrete countertops and other creative concrete applications, the cost of the fiber is often less important than the effect of the fiber on the concrete’s performance and on its appearance.

Additionally, different mixes are best suited for different fibers. GFRC fibers must be used in high volumes, so the mix is built around a specific fiber used in a specific dose. ECC is the same way, but with different mix proportions and very different fibers.

The takeaway here is that fibers are not generic ingredients that can be plugged into any mix without considering the benefits those fibers can bring and their effect on the concrete’s workability.

Fibers as Primary Reinforcing

Most fibers used in most types of concrete don’t provide any benefit to boosting the tensile strength of the concrete. These tend to include cellulose, polypropylene, nylon, and other common types of “stealthy” fibers. Only certain types of fibers can, and these must be carefully matched with a tailored mix design built around the type and amount of fiber used in the mix. GFRC and ECC are two examples of specialized mixes that use specialized fibers.

Fibers as Secondary Reinforcing

As the concrete sets and transforms from a workable paste into a hard solid, plastic shrinkage can occur. This is especially true in concrete slabs exposed to heat or wind. The matrix of fibers helps to stabilize the wet concrete and distribute the shrinkage stresses so that large cracks are minimized or eliminated.

Fibers can help reduce shrinkage and cracking.

Fibers can also help combat shrinkage by spreading the tensile loads across the concrete. The fibers act as a net, in this case holding small cracks together and transferring stresses across cracks into adjacent concrete. This helps keep any cracks that appear small, often too small to even see. Rather than having one or two large, highly visible cracks, you’re left with a series of small, hard to see cracks spread across the slab.

The Different Types of Fibers used for Traditional Precast

Fibers in precast concrete countertops can play a valuable role in both boosting primary reinforcing and providing secondary reinforcing. However, the type of fibers and the methods used will vary depending on which type of reinforcing you’re after.

Most commonly used fibers are synthetic, either polypropylene or nylon, but some are natural, like cellulose fibers. Let’s dive into a few of your fiber options. Remember, this is not an exhaustive list!

PVA Fiber for Concrete Countertops

Polypropylene or Nylon Fibers – Polypropylene and nylon fibers are used for shrinkage control; they add no structural tensile strength to the concrete. These fibers play a valuable role during the curing process but provide no benefit after. They simply stretch too much to provide any resistance to tensile stresses.

Polyvinyl Alcohol (PVA) Fibers – PVA fibers have some structural strength and can also be used for shrinkage control. While they cannot replace reinforcing steel, they improve the mechanical properties of cured concrete, boosting its strength. These are the best choice when the fibers cannot show at all. However, caution must be used because these fibers are so fine, very small volumes can choke the mix. They are an important component of ECC (see below).

Alkali Resistant Glass Fibers – AR glass fibers are the type of fiber primarily used with GFRC (see below). They can also be used to provide primary and secondary reinforcing in traditional steel reinforced concrete countertops. These fibers are special glass fibers that won’t break down, even when in contact with alkaline concrete. They will show in traditional precast, however, and are not commonly used.

Other Fiber Choices for Precast

Some fibers are strong and can provide adequate structural strength, but the material they’re made of doesn’t make them a good choice for concrete countertops. They are primarily in large industrial concrete projects.

hooked steel fibers

Hooked Steel Fibers

Hooked Steel Fibers – Hooked steel fibers possess structural strength. They can help to distribute tensile stresses across the countertop. However, they are large, ugly and will show.

Chopped Carbon Fibers – Chopped carbon fibers have stiffness and strengths equal to or greater than steel. Reinforcing is still needed, but the fibers provide a helpful boost of strength and minimize shrinkage during the curing process. But because they are black, carbon fibers will show definitely show in most concrete that’s not black or very dark.

Fibers used for GFRC

Properly Aligned Fibers in GFRC, Resulting from Thin Layer and Rolling

GFRC utilizes both specialized concrete and strong AR glass fibers. Both possess benefits on their own, but when combined they become something amazing. The tensile strength helps GFRC to resist pulling apart forces while the flexural strength helps it to resist bending. The glass fibers and the high polymer content of GFRC provide these unique properties that are essential to a long lasting concrete countertop.

Rather than using steel for reinforcement, GFRC relies on these glass fibers to prevent cracking and breakage. Making GFRC isn’t as simple as just adding some fibers to your concrete mix design. The size, shape, material, and amount of fibers used has a significant effect on the concrete. Using the wrong type of fiber, or not using enough, can lead to disappointment and a failed concrete countertop.

Because AR glass fibers are used in such high volumes in GFRC in order to provide the necessary strength, they would be extremely visible if a veneer coat without fibers were not used. This fiber-free veneer coat is called a mist coat or face coat. It has no strength and is solely for aesthetic purposes.

For more information about GFRC, please see this index of GFRC articles and videos.

Fibers Used in ECC

As explained in the last article, PVA fibers are used in ECC to provide both structural strength and shrinkage control. The combination of well-dispersed, micro PVA fibers and the strong, fine-grained homogeneous matrix is what results in the amazing ability of ECC to bend and crack without losing strength. Because of the highly dispersed microfibers, cracks tend to be small, and sometimes even invisible.

Recommendations

There are many different types of fibers used for different reasons in the various methods of creating concrete countertops. My recommended usage is:

  • Precast: For the type of precast concrete we make, you may optionally use acrylic, nylon and PVA fibers, at a dose of 0% to 0.5% of total mix weight. This is only for secondary reinforcement and plastic shrinkage cracking. It does not replace primary steel reinforcing. Higher doses can choke the mix – essentially turning it into a big hairball!
  • GFRC: Use AR glass fibers, 3% of total mix weight, properly compacted. GFRC is not GFRC without AR glass fibers as a replacement for primary steel reinforcing. (See this article.) To purchase AR glass fibers, click here.
  • ECC: ECC is a complex composite of PVA fibers, 1% to 2% of total mix weight, in a properly engineered mix utilizing very fine aggregates. It is impractical to prescribe a DIY recipe for ECC due to the complexity of the mix design.

The Versatility of ECC Concrete

Conventional concrete versus bendable concrete made with ECC

ECC (Engineered Cementitious Composite) is a specialized form of concrete developed in the early 1990’s by Dr. Victor Li, a professor of Structural and Materials Engineering at the University of Michigan. Dr. Li wanted to produce a form of concrete that wasn’t brittle, and that retained its strength even after it cracked.

ECC has properties and characteristics not shared by other forms of fiber reinforced concrete (FRC). These include:

  1. Tensile properties greater than conventional FRC
  2. Easy mixing and casting
  3. Low fiber volume (compared to GFRC)
  4. No weak planes in the cast concrete

Not Your Average Concrete

ECC is a mix that provides flexibility and high bending (flexural) strength.

ECC is not just ordinary concrete with fibers in it. It is a carefully tailored marriage of:

  • cementitious binder
  • very fine aggregates
  • PVA fibers

The types and amount of fiber play an important role, but so too does the formulation of the concrete itself.

ECC depends upon the micromechanical interaction between a strong, fine-grained homogeneous matrix, and well-dispersed structural microfibers. This engineered composite is carefully tailored to create a form of concrete that can bend and crack without losing strength. And because of the highly dispersed microfibers, cracks tend to be small, and sometimes even invisible.

ECC bends without developing large cracks, but it can have visible cracks.

The basic ingredients, composition, and proportions of ECC are similar to, but not the same as, the base mix used for GFRC. ECC mixes have less sand than GFRC, and the sand gradation itself is more important in ECC. The greatest difference between the two is in the fibers.

ECC is a mix that provides flexibility and high bending (flexural) strength, similar to what GFRC provides. While GFRC uses high volumes of large AR glass fibers, ECC uses comparatively low volumes of small synthetic PVA (polyvinyl alcohol) fibers.

Like AR glass fibers, PVA fibers have excellent structural properties that are ideal for ECC and set them apart from the ordinary fibers used in flatwork.

Ordinary fibers like nylon, polypropylene, and cellulose are either too stretchy, weak, or both, and these ordinary fibers function as stabilizing agents and are there only to help control plastic shrinkage. None of these add any strength after the concrete hardens.

In contrast, PVA fibers have some structural strength and can also be used for shrinkage control. They improve the mechanical properties of cured concrete, boosting its strength.

Comparing ECC with GFRC

Casting is faster and more efficient.

GFRC uses bundled alkali-resistant (AR) glass fibers in high doses (typically 3% for premix GFRC, and 5% and higher for spray-up applications). AR glass fibers are typically 19mm long, and the large, 200 filament bundles can be very visible if the fibers become exposed. This aesthetic drawback is why GFRC usually has a face coat, which is a decorative veneer layer that often is fiber-free.

GFRC is a material originally designed and optimized for the efficient casting of single-sided pieces, where one face of the casting is decorative, and the opposite side remains unseen.

While wet casting GFRC with only a flowable backer is possible (it’s called direct casting, and foregoes a face coat), the large AR glass fibers just below the surface are sometimes noticeable, and, the presence of the highly visible fibers in the concrete preclude grinding and polishing. Thus, direct casting is limited to pieces where an as-cast surface is desired, and the visible presence of large fibers is acceptable.

PVA Fiber for Concrete Countertops

PVA Fiber for Concrete Countertops

What makes ECC different from GFRC is that the PVA fibers are mixed into the whole mass of the concrete, instead of just in the backer layer. PVA fibers are nearly invisible when properly dispersed during mixing, unlike GFRC fibers which need to stay in large, very visible bundles.

Since PVA fibers are transparent, short (only 6-8mm long), and their diameter is a fraction of the diameter of a human hair, they disappear in the mix. This greatly simplifies mixing and casting, as there is no need for a separate face coat.

ECC can be made stiff and hand-packable, or, it can be made fluid and vibrated. Casting is faster and more efficient, as molds can be filled in one continuous pour, rather than in individual layers. This versatility makes ECC a smart choice for both precast and cast-in-place applications.

However, the complexity of ECC makes it impractical to prescribe a from-scratch mix formula. (In contrast, GFRC is a very simple mix easily made from scratch.) For this reason, The Concrete Countertop Institute does not have a from-scratch mix calculator for ECC and recommends that you buy a preblended ECC mix such as the Buddy Rhodes ECC Blended Mix product.

Q: Can I use monofilament fibers in GFRC?

A: No.

First, let’s understand what “monofilament” means. Fibers can be configured in single strands (monofilament) or bundles (fibrillated). Many types of fibers that are sold in fibrillated configuration have fiber bundles that are designed to disperse in the mix – break up into single fiber strands.

GFRC fibers are different. The fiber bundles are meant to stay bundled (about 200 filaments per bundle with 19mm “fibers”). Each bundle is bonded together so that the filaments remain grouped during mixing. Here’s why:

With monofilament fibers, each fiber is exposed to the concrete’s cement paste and is well bonded to the matrix. While this seems ideal, it creates a composite that results in stiffer, less elastic material.

monofilament-fibers-for-concrete

Monofilament Fibers (image courtesy of www.dmireadymix.com/products/view/reinforcing-fiber)

GFRC fibers, on the other hand, remain in a bundle. This results in a stiffer fiber grouping that maintains the fiber’s orientation even when the overall length gets very long compared to the diameter of the individual fiber filament. Long, stiff fibers (or fiber groupings) are far more effective than long, thin, floppy fibers that fold over and ball up.

fibrillated-fibers-for-concrete

Fibrillated Fibers (not GFRC fibers, image courtesy of www.dmireadymix.com/products/view/reinforcing-fiber)

Fiber Bundle Properties

In addition, the fiber bundles themselves are engineered to create a strong, yet flexible concrete. The spacing between each filament is about 0.1 microns, or 1/10,000,000 of a meter. That’s 1000 times smaller than the average diameter of a human hair! The spacing between the filaments is too small to let cement particles penetrate, but it is large enough to let the polymer enter.

Capillary action draws water and polymer into the dry fibers as they’re mixed into the concrete. By filling the spaces between the filaments with polymer, two things are achieved:

  1. The polymer blocks any mineral growth from entering the fiber bundle.
  2. The stretchy polymer bonds the inner filaments to the filaments at the outside of the bundle.

Only the outside filaments are in contact with the concrete matrix, but the inner filaments are not. This is important because:

  • If the spaces between the filaments were to become filled with rigid crystalline mineral deposits (which occurs during concrete hydration over time), all of the filaments would be rigidly bonded together and the fiber would act as a single, thick fiber bar.
  • Rather, the stretchy polymer allows the inner filaments to move relative to the outer filaments, yet still bear some of the load.

In a way, the polymer and glass fiber filaments create mini bungee cords within the concrete, giving GFRC great flexibility.

The Importance of Surface Area

As explained above, the surface area of fiber which is exposed to the cement paste is important. This is true for a number of concrete ingredients. For example, the surface area of pigment and how well it disperses in the mix are important factors in creating the final color. Surface area is an important concept that plays a role in mix design and adjustments.

Other Considerations about Fibers

Please note, this article discusses only the difference between monofilament and fibrillated fibers. There are many other important characteristics relevant to fibers. For example, depending on the material the fiber is made of, the fiber may be hydrophobic (repel water and therefore have no effect on mix water requirements) or hydrophilic (attract water and therefore affect mix water requirements).

Here are some good articles by Concrete Construction magazine about the broader topic of various fiber chemistries and configurations. While they don’t cover GFRC, they give a good overview of other types of fibers.

 

The Many Uses of Fibers in Concrete Countertops

Lately we’ve been talking a lot about GFRC and the alkali resistant glass fibers used in making this unique type of concrete. But, AR glass fibers aren’t the only type of fibers you can use in concrete mixes. Today I’d like to discuss some of the other types of fibers that you can use when making concrete countertops and some of their purposes. When used properly, fibers have many benefits in countertops. Fibers aren’t just for GFRC.

Fibers – Primary vs. Secondary Reinforcing

Fibers have many purposes in concrete countertops. They can be used for reinforcing (think GFRC) or can be used to prevent shrinkage and cracking. When fibers are used for structural reinforcing this is known as primary reinforcing. When they are used for shrinkage control it is known as secondary reinforcing.

Fibers can play a valuable role in both primary and secondary reinforcing. However, the type of fibers and the methods used will vary depending on which type of reinforcing you’re after. Many of the fibers listed below can provide both primary and secondary reinforcing benefits. While most fibers won’t replace steel reinforcing (like AR glass fibers do in GFRC), they can still make concrete stronger and help your concrete countertops to last longer and look better.

The Many Types of Fibers Used in Concrete Countertops

When I say there’s a lot of different fibers to choose from, I’m not kidding. In this post we’ll look at several of your options, but just briefly. I could easily create lengthy posts on the uses and benefits of each type of fiber listed here.

  • Polyvinyl Alcohol (PVA) Fibers – PVA fibers have some structural strength and can also be used for shrinkage control. While they cannot replace reinforcing steel, they improve the mechanical properties of cured concrete, boosting its strength. These are the best choice when the fibers cannot show at all.

PVA Fiber for Concrete Countertops

  • Alkali Resistant Glass Fibers – AR glass fibers are the type of fiber primarily used with GFRC. They can also be used to provide primary and secondary reinforcing in steel reinforced concrete countertops. These fibers are special glass fibers that won’t break down, even when in contact with alkaline concrete. The alkali resistance is achieved by using zirconia. AR glass fibers come in different sizes; the ones used for GFRC are long (13 and 19mm) and large, so they will show if they’re simply mixed in to ordinary concrete. Smaller fiber bundles of AR glass are less visible, but will still show if they happen to be right on the surface.
  • Polypropylene or Nylon Fibers – Polypropylene and nylon fibers are used for shrinkage control; they have no structural strength. These fibers play a valuable role during the curing process, but provide no benefit after. They simply stretch too much to provide any resistance to tensile stresses.
  • Cellulose Fibers – Most of the time synthetic fibers are used for secondary reinforcement, but cellulose fibers are a naturally occurring alternative.

Some fibers are strong and can provide adequate structural strength, but the material they’re made of doesn’t make them a good choice for concrete countertops.

  • Hooked Steel Fibers – Hooked steel fibers possess structural strength. They can help to distribute tensile stresses across the countertop. However they are large, ugly and will show.

hooked steel fibers

  • Chopped Carbon Fibers – Like hooked steel, PVA and AR glass fibers, chopped carbon fibers have stiffness and strengths equal to or greater than steel. Reinforcing is still needed, but the fibers provide a helpful boost of strength and minimize shrinkage during the curing process. But because they are black, carbon fibers will show definitely show in most concrete that’s not black or very dark.

Fibers and Shrinkage Control: How Does it Work?

Fibers thicken and stabilize the cement paste, giving it body. Fibers act like an internal three dimensional net, supporting the aggregate and minimizing settlement.

Aggregate settlement and consolidation are what drive the generation of bleedwater. For bleedwater to reach the surface, it has to form microchannels in the concrete. These microchannels form weak zones in the concrete. In addition, the walls of the microchannels can have much higher water/cement ratios than the concrete itself because of the dilution effect of the bleedwater.

All this results in weaker, more porous concrete that shows greater tendencies for cracking and shrinkage. Adding fibers boosts the properties of the concrete, mostly by preventing the detrimental effects of plastic shrinkage and bleedwater segregation.

Fibers can help reduce shrinkage and cracking.

Fibers can help reduce shrinkage and cracking.

Fibers can also help combat shrinkage by spreading the tensile loads across the concrete. Like I mentioned previously the fibers act as a net, in this case holding small cracks together and transferring stresses across cracks into adjacent concrete. This helps keep any cracks that appear small, often too small to even see. Rather than having one or two large, highly visible cracks, you’re left with a series of small, hard to see cracks spread across the slab. Cracking might be inevitable, but an almost invisible crack is always better than a big one.

One last point: the amount of fibers needed to provide good secondary reinforcement to control micro-cracks is rather low. Typical volume fraction doses range from 0.1% to 0.5%; this translates to 4 lb to 20 lbs of fibers in a cubic yard (4000 lbs) of concrete.

The fiber dose necessary to make GFRC strong is much higher. Primary reinforcing has to resist much more stress and deflection, which is why so many more fibers are needed. The typical (and minimum) GFRC volume fraction dose is 3%, equivalent to 120 lbs of fibers in a cubic yard (4000 lbs) of concrete

Creating the perfect mix for each concrete countertop requires careful consideration of the type of countertop you need to create. I hope this article has helped you to see some of the ways fibers can enhance your mix design.

Glass Fibers – An Essential Component of GFRC Concrete Countertops

If you’re wondering about the importance of glass fibers in GFRC just think about the name for a minute.  Glass fiber reinforced concrete – without the fibers all you have is concrete. These alkali resistant glass fibers give GFRC its strength and make it an ideal choice for a variety of applications including concrete countertops.

According to Wikipedia.com, “[g]lass fiber reinforced composite materials consist of high strength glass fiber embedded in a cementitious matrix. In this form, both fibers and matrix retain their physical and chemical identities, yet they produce a combination of properties that can not be achieved with either of the components acting alone. In general fibers are the principal load-carrying members, while the surrounding matrix keeps them in the desired locations and orientation, acting as a load transfer medium between them, and protects them from environmental damage.”

GFRC utilizes both concrete and strong AR glass fibers. Both possess benefits on their own, but when combined they become something amazing. Let’s take a look at the important role fibers play in GFRC.

Why Fibers?

One of the benefits of GFRC is its tensile and flexural strength. The tensile strength helps GFRC to resist pulling apart forces while the flexural strength helps it to resist bending. The glass fibers and the high polymer content of GFRC provide these unique properties that are essential to a long lasting concrete countertop. Rather than using steel for reinforcement, GFRC relies on these glass fibers to prevent cracking and breakage. Reinforcement is essential any time you create a concrete countertop, and GFRC uses fibers to create this reinforcement.

This nine minute video, while it addresses steel reinforcing, will help you better understand the importance of reinforcement in general when constructing a concrete countertop:

Tips for Using Fibers in GFRC

Making GFRC isn’t as simple as just adding some fibers to your concrete mix design. There are many important considerations to remember. Here are a few:

  • Amount of Fiber Present– GFRC relies on a high load of glass fibers. Without sufficient fiber the concrete will be unable to resist cracking and breakage when faced with a high tensile load. Fiber content varies, but is at least 3% of the total mix weight. Some mixes go as high as 10% fiber content. The more fiber present the stronger the GFRC, but increased fiber does lead to decreased workability and even to compromised compaction.

However, decreased fiber leads to the worse problem of less strength. Some concrete countertop teachers recommend only 2% fibers. I’m not sure what the motivation behind this is, but 2% is not sufficient. 3% is the minimum.

  • Orientation of Fibers– Orientation of the fibers in the mix is also important. Truly random fiber orientation means more fiber is needed since many of the fibers will be pointing in the wrong direction. See below for

Some concrete countertop teachers recommend creating a fluid backer mix and pouring it into the forms, effectively an “SCC” backer mix. This should not be done, as it results in random fiber orientation. See below and read this article for more information about why this is problematic.

  • Method of Reinforcement Used– There are three different levels of reinforcement used in general concrete and GFRC. Each type carries different benefits.

Level 1: Random 3-D Reinforcing

This type of reinforcement occurs when fibers are mixed into the concrete and the concrete is poured into forms. The fibers are evenly distributed throughout the concrete and point in every direction. Typically only 15% of the fibers are oriented in the proper direction requiring very high fiber loads. This level of reinforcing is very inefficient requiring large amounts of fiber for lower levels of reinforcement. This should not be used for GFRC.

Random 3-D Fibers

Fiber Orientation with Random 3-D Reinforcing

Level 2: Random 2-D Reinforcing

In this level of reinforcing concrete is sprayed onto a form using special equipment that chops and adds the fiber during the spraying process. Spray-Up GFRC is an excellent example of this type of reinforcing. Typically 30% to 50% of the fibers are optimally oriented. This can also be achieved by placing thin layers of backer and compaction rolling each layer. This method is more effective than 3-D reinforcing, and is the recommended method for either hand-placed or sprayed-on GFRC backer coat.

Spray-up GFRC

Spray-up GFRC

Level 3: 1-D Reinforcing

The final level of reinforcing, one-dimensional reinforcing, is the most effective method available because it uses the least amount of reinforcing material to resist tensile loads. All reinforcing is placed in the tensile zone, or the area that needs the extra strength, reducing the overall amount of reinforcement needed. This method is used to create structural concrete beams with steel reinforcing. When creating a concrete countertop slab, the bottom of the slab is the tensile zone, as you saw in the video. Steel in precast concrete is an example of 1-D reinforcing.

Scrim in GFRC is another example of 1-D reinforcing. Scrim is a glass fiber mesh used to give extra strength to GFRC, in addition to the fibers. Although the scrim does provide targeted 1-D reinforcing in critical areas, you still need fibers throughout the backer coat to provide tensile and flexural strength throughout.

Reinforced Beam

1-D Reinforcing

When it comes to GFRC glass fibers are essential, but as this article clearly illustrates there is more than one way to add those fibers in. The method you select will determine how much fiber is needed and how strong your finished concrete countertop will be.

Fibers as Secondary Reinforcement in concrete countertops

Fibers are used in concrete for a variety of reasons, but not all fibers do the same thing or have the same effect. The size, shape, material and amount of fibers used has a significant effect on the concrete. Using the wrong type of fiber, or not using enough, can lead to disappointment and a failed concrete countertop.

Fibers are generally added to concrete as shrinkage control (also known as secondary reinforcement; structural reinforcement is primary reinforcement). As the concrete sets and transforms from a workable paste into a hard solid, plastic shrinkage can occur. This is especially true in concrete slabs exposed to heat or wind. The matrix of fibers helps to stabilize the wet concrete and distribute the shrinkage stresses so that large cracks are minimized or eliminated.

Fibers are often advertised as capable of replacing welded wire mesh. This is true, but only when the welded wire mesh is used only as plastic shrinkage control. The confusion stems from the fact that welded wire mesh can also be used as structural (primary) reinforcement, while synthetic fibers cannot.

Most commonly used fibers are synthetic, either polypropylene or nylon, but some are natural, like cellulose fibers. None of these fiber materials are stiff or strong enough to provide any significant tensile reinforcement to uncracked concrete. And they simply stretch too much to do any good once the concrete cracks. After the concrete hardens, these fibers don’t contribute anything (nor can they) to resisting external structural tensile stresses.

There is a class of fibers that provides some resistance to external structural tensile stresses, but these are more esoteric and not generally necessary if you use proper steel primary reinforcing.

** This information applies to precast concrete countertops, not to GFRC concrete countertops (glass fiber reinforced concrete). In the case of GFRC, fibers do provide the primary reinforcing because there are so many of them, they are aligned two-dimensionally by rolling, and there is enough polymer in the concrete to provide a great deal more flexibility than normal concrete. Be aware though, GFRC is a system. You cannot take one element of GFRC and add it to regular precast concrete countertops and expect it to convey the properties of GFRC to the countertops. If you want to use glass fibers to reinforce your concrete countertops, you have to follow the GFRC system from start to finish; don’t just add glass fibers to your mix.

To learn more about using fibers as reinforcing, check out these articles on our blog:

Structural AR glass fibers in GFRC:Random 3D Fibers

Properly Aligned Fibers Resulting from Thin Layer and Rolling

Last updated by at .