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.

What causes cracks in concrete countertops?

Defining Different Kinds of Cracks

They don’t all mean the same thing.

As the adage goes, all concrete cracks. What’s essential to a client is that those cracks are not visible nor do they impact the performance of the countertop. Well-made concrete countertops should not develop structural cracks; however, hairline cracks are possible and not a sign of poor quality.

Hairline cracks and larger structural cracks are signs of stress relief. A crack forms when tensile stress builds up in the concrete and exceeds the material’s capacity to resist those stresses.

Most large, structural cracks in countertops form because of flexing, either because a faucet was tightened too much or, as is the case in this picture, the house settled:

Flexural crack from house settlement:

What causes cracks in concrete countertops?

crack in concrete countertop

Cracks in granite from an over-tightened faucet:

What causes cracks in concrete countertops?

crack in granite countertop due to faucet over-tightening

Multiple flex cracks in an overloaded cantilever beam:

What causes cracks in concrete countertops?

crack in GFRC countertop due to flexing

The Power of Drying and Heat

Both are avoidable. Here are a few examples.

Hairline cracks often occur because of shrinkage, either from drying or heat. These types of cracks are more difficult to control because they generally occur near the surface, so reinforcing doesn’t help prevent them. The best preventative is to use a good mix design that has low shrinkage tendencies. However, hairline cracks can and do occur, and are often located near areas of moisture (sinks and dishwashers), where dry concrete repeatedly absorbs moisture and then dries out. Over time this wetting and drying cycle will cause the concrete to crack, much in the same way a piece of steel will eventually crack if it’s bent back and forth enough times.

Hairline crack at sink:

What causes cracks in concrete countertops?

hairline crack by a sink in a concrete countertop

Heat also can cause hairline cracking. Crock pots are a familiar source of heat-related hairline cracks in countertops. Often it’s not the intensity of the heat, but the length of time the concrete is heated. Crock pots don’t get very hot, but they sit in one spot for many hours. As the concrete heats it expands, and the more concrete that does heat up and grow, the higher the thermal stress that develops. Generally, it’s not just the heating that causes cracking; it’s also the subsequent cooling. As the concrete cools it shrinks, and it’s the shrinkage that causes cracking.

Thermal cracking in solid surface material from a crockpot:

What causes cracks in concrete countertops?

thermal crack in solid surface countertop

If you’re interested in learning more about concrete countertops; how to make them, how to cast, mix, apply sealer and so much more valuable information, check out our online 1-2-3 level classes. They’re always available to you when you need them.

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