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.

Introduction to GFRC (Glass Fiber Reinforced Concrete)

If you aren’t yet familiar with glass fiber reinforced concrete (GFRC) you should be. GFRC is a specialized form of concrete with many applications. It can be effectively used to create façade wall panels, fireplace surrounds, vanity tops and concrete countertops due to its unique properties and tensile strength. One of the best ways to truly understand the benefits of GFRC is to take a deeper look into this unique compound.

What is GFRC?

GFRC is similar to chopped fiberglass (the kind used to form boat hulls and other complex three-dimensional shapes), although much weaker. It’s made by combining a mixture of fine sand, cement, polymer (usually an acrylic polymer), water, other admixtures and alkali-resistant (AR) glass fibers. Many mix designs are available online, but you’ll find that all share similarities in the ingredients and proportions used.

Some of the many benefits of GFRC include:

  • Ability to Construct Lightweight Panels– Although the relative density is similar to concrete, GFRC panels can be much thinner than traditional concrete panels, making them lighter.
  • High Compressive, Flexural and Tensile Strength– The high dose of glass fibers leads to high tensile strength while the high polymer content makes the concrete flexible and resistant to cracking. Proper reinforcing using scrim will further increase the strength of objects and is critical in projects where visible cracks are not tolerable.

GFRC is strong. Check out this YouTube video to see just how strong it can be:

 

The Fibers in GFRC- How They Work

The glass fibers used in GFRC help give this unique compound its strength. Alkali resistant fibers act as the principle tensile load carrying member while the polymer and concrete matrix binds the fibers together and helps transfer loads from one fiber to another. Without fibers GFRC would not possess its strength and would be more prone to breakage and cracking.

Understanding the complex fiber network in GFRC is a topic in and of itself. Stay tuned, I’ll post a more in-depth article on GFRC fibers next week.

Casting GFRC

Commercial GFRC commonly uses two different methods for casting GFRC: spray up and premix. Let’s take a quick look at both as well as a more cost effective hybrid method.

Spray-Up

The application process for Spray-up GFRC is very similar to shortcrete in that the fluid concrete mixture is sprayed into the forms. The process uses a specialized spray gun to apply the fluid concrete mixture and to cut and spray long glass fibers from a continuous spool at the same time. Spray-up creates very strong GFRC due to the high fiber load and long fiber length, but purchasing the equipment can be very expensive ($20,000 or more).

Premix

Premix mixes shorter fibers into the fluid concrete mixture which is then poured into molds or sprayed. Spray guns for premix don’t need a fiber chopper, but they can still be very costly. Premix also tends to possess less strength than spray-up since the fibers and shorter and placed more randomly throughout the mix.

Hybrid

One final option for creating GFRC is using a hybrid method that uses an inexpensive hopper gun to apply the face coat and a handpacked or poured backer mix. A thin face (without fibers) is sprayed into the molds and the backer mix is then packed in by hand or poured in much like ordinary concrete. This is an affordable way to get started, but it is critical to carefully create both the face mix and backer mix to ensure similar consistency and makeup. This is the method that most concrete countertop makers use.

Spray-up GFRC Fibers

Coming soon: A more in depth look at GFRC mix designs, casting, thickness, curing and processing.

Quick Facts About GFRC

  • GFRC was first created in the 1940s in Russia, but it wasn’t until the 1970’s that the current form came into widespread use.
  • GFRC tends to run about $2.50-$3.00 per square foot for ¾” thick material. The cost increases to about $3.50-$3.75 per square foot for 1” thick material when accounting for the prices of sand, cement, admixtures, fibers and polymer.
  • Just like regular concrete, GFRC can accommodate a variety of artistic embellishments including acid staining, dying, integral pigmentation, decorative aggregates, veining and more. It can also be etched, polished, sandblasted and stenciled. If you can imagine it, you can do it, making GFRC a great option for creating concrete countertops and especially three-dimensional concrete elements.

To learn more about GFRC check out these great articles on our blog:

Concrete technology from early 1900s still applies today

Scientific American magazine has a section called “50, 100 & 150 Years Ago” that features blurbs from the magazine in each of those years. Here is a recent entry for March 1911:

“Concrete for Construction”

“About fifteen years ago serious attempts were made to combine steel and concrete by moulding one into the other in such a way that the resulting product would possess a high resistance not merely to compressive but to bending and tensional stresses. A vast amount of experimental work was done, out of which has sprung our modern reinforced concrete. Not only is concrete found to be available for practically every form of construction which hitherto has been built in brick and stone, but it has now invaded the field which was supposed to be peculiarly reserved for iron and steel.”

If you’ve seen my free video about reinforcing concrete countertops, you know exactly what these compressive, bending (flexural) and tensional stresses are! And you know that your concrete countertop construction methods are based on sound scientific principles. What’s surprising is how recently reinforced concrete was invented, seeing as concrete has been used since Roman times.

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

Rebar size in Concrete Countertops: It can be too big.

The size of reinforcing steel in a countertop is an important consideration. Concrete countertops that are made with rebar that is too large in scale for the slab thickness are susceptible to telegraph cracking.

When concrete shrinks due to drying out, high tensile stress levels develop around proportionally oversized reinforcing. These stresses cause cracks that run along the length of the steel. Smaller diameter reinforcing has less of an affect on the concrete, so the same amount of concrete shrinkage develops much less tensile forces, thus the likelihood of cracks caused by the steel itself drops dramatically. Because all concrete shrinks to some degree, and shrinkage occurs over a long period of time, the occurrence of telegraph cracking may not appear until long after a countertop is installed.

In addition, oversized reinforcing occupies so much space inside a thin countertop slab that there is very little cover between the steel and the slab surface. Generally, reinforcing is fabricated in a grid arrangement, with strands running along the length of the slab and overlapped strands running across the width of the slab. When stacked, larger rebar can take up half of the total slab thickness, while smaller rebar takes up much less space.

 Oversized rebar can cause telegraph cracking

The sheer size of rebar that is too big places a significant amount of the steel closer to the visible surface of the countertop instead of down near the bottom of the slab. For example, a grid made from 3/8” diameter rebar held only 1/4” away from the bottom (the bare minimum cover for such a size) would place the top of the rebar at the midpoint of the slab, leaving only 1/2” of concrete cover between the steel and the visible surface. Contrast this with a grid made from 3/16” diameter wire held ¼” away from the bottom of the slab, and now there is 7/8” of concrete between the surface and the steel, a 75% increase in cover.

In addition to causing large stress concentrations in the concrete (and therefore increasing the likelihood of telegraph cracking), using oversized reinforcing actually decreases the load capacity of the concrete.

In the drawing above, the same concrete is reinforced with equal amounts of steel reinforcing (based on cross sectional area). The top drawing shows one piece of 3/8” (#3) diameter reinforcing steel while the bottom drawing shows four pieces of 3/16” structural reinforcing wire. The cross sectional areas are the same, so the tensile capacity of the steel is the same.

But because the four pieces of wire can be located lower in the slab, the load capacity of the wire-reinforced concrete is now 13% (lower reinforcing layer) to 78% (upper reinforcing layer) greater than the slab with the single #3 rebar, even though there is the same amount of steel in the concrete. In actuality, the structural wire has a higher strength than the rebar, so the difference in capacity is even greater.

Are concrete countertops still at risk of cracking if they have high compressive strength?

“My concrete has a high compressive strength. That means I don’t need reinforcing to protect against, right?”

Not necessarily, and not in all circumstances. All concrete, regardless of the mix design or the magnitude of the compressive strength, is much weaker in tension than in compression. And cracks are caused by tensile failures of the concrete.

The tensile strength of concrete is often only about 10% that of the compressive strength, but this is a rough average. Tensile strength is highly variable, difficult to predict, and is dependent not only on the ingredients and their proportions, but on the casting technique, the curing history, the amount, size and distribution of voids and defects (microcracks), etc. Even the amount of pigment or other admixtures can significantly reduce the tensile strength of the concrete.

Unreinforced countertops rely completely on the tensile strength of the concrete itself to hold everything together. Since it is already known that the tensile strength is variable and difficult to predict, relying solely on it is tantamount to gambling with a finished piece.

Often experimenters will initially try out small pieces with success, because small pieces don’t develop large tensile stresses when handled. However, success leads to bolder ventures, and longer slabs result. Eventually the sheer weight and size of the slab generate tensile stresses that overcome the concrete. Because there is no reinforcement to resist the tensile load after the concrete cracks, the slab suddenly fails and snaps in half with little or no warning.

Reinforcing steel adds ductility, the ability for a material to absorb deflection and overloading without falling apart. Unreinforced concrete fails in a brittle fashion. That is, it fails suddenly and without warning; one moment it’s fine, the next it’s broken. Don’t take these kinds of risks with your concrete countertops.

Compression/Tension: Concrete countertops are beams

A beam is a horizontal structural member that spans some open space and is supported near the ends. The beam can then support some weight placed on top of it somewhere between the end supports. A floor joist is a beam. Concrete countertops are also beams.

When a beam has weight placed on top of it, that weight causes the beam to deflect (bend). Small weights on stiff beams cause almost no deflection, while large weights on flexible beams cause significant deflection. The deflection in the beam causes two things to happen: The top surface of the beam is compressed and tries to get shorter, and the bottom surface is in tension and tries to get longer.

Between the two something important occurs. Compression is the opposite of tension, so as one progresses down the beam from the top surface to the bottom, the compression stress gradually decreases to zero and then the stresses reverse, go into tension and gradually increase towards the bottom of the beam. If an unreinforced beam has a symmetrical cross-section (like a rectangle), the stress switch occurs at the midpoint between the upper and lower faces. This is important because given that there is no tension or compression stress at the midpoint of a countertop, placing reinforcing steel there does absolutely no good. The point at which this switch occurs is called the neutral axis, and can be thought of as an imaginary line that runs parallel to the length of the beam.

compression-tension in beam

If a countertop is made out of concrete (with no reinforcement), any significant weight placed on top of it will cause it to fail at the bottom of the countertop because the tension stresses in the bottom of the countertop will exceed the tensile strength of the concrete. A crack will form at the bottom and progress upward literally at the speed of sound.

Some argue that because concrete countertops usually actually span only the width of a cabinet box (usually a maximum of 36″), they are rather short beams, and therefore the stresses involved are not that high. This is true, but what about when an 8 foot long precast slab is picked up in the shop and loaded onto a truck for transportation? The largest stresses and biggest risk of cracking occur in the shop. Once the slabs are installed, only settling of the cabinets or building would impart much stress.

This video goes much more in depth on the subject of compression and tension forces in concrete countertops.

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