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.

Floral Edging for the “Tuscan” look in your concrete countertop

Concrete countertops allow decorative details that are not possible with any other type of countertop. Decorative edges are an exciting way to add pizazz to your countertops.

The following photo shows a floral edge that offers stunning detail and looks particularly good when paired with a veined or “pressed” look in an Old World color scheme such as Tusan Creme.

Floral edge on Tuscan Creme countertop

 

The following photo shows a tile, a knob and a cabinet door sample on top of an Umbria colored countertop. You can see the deep ochre color of the countertop with the rich brown veining.

Umbria countertop sample with tile

 

The following photo shows an amber acid-stained countertop with floral edging on top of dark cherry cabinets. The amber and cherry provide a warmer look than the previous color scheme of ochre and brown.

Floral edge with amber acid stain

 

Imagine the possibilities when you can incorporate fancy edging with Old World colors, pulling together your cabinet trim, flooring, wall colors and tile! With concrete, any color scheme or shape is possible. Visit The Concrete Countertop Institute’s “Find a Contractor” listing to find a skilled craftsperson to make this for you. Be sure to read our Helpful Info for Consumers first.

Portland Cement Type I, II, III: Which to use in a concrete countertop mix?

Portland cement comes in a variety of different types. In the United States, these types are classified as Type I, II, III, IV and V. Only Types I and III are necessary for consideration by concrete countertop fabricators; the benefits of Type II cement are generally irrelevant to the concrete countertop industry.

Type I is ordinary Portland cement, and it is available in white or gray.

Type II is a moderate sulfate resistant cement, important when concrete is cast against soil that has moderate sulfate levels.

Type III is a high early strength cement. It is ground finer and reacts faster than Type I, so the early strength gains are greater. However, the ultimate strength is not higher than Type I. Concrete made with Type III will have slightly higher 28 day strengths than concrete made with Type I, all else being equal. Type III is available in white or gray, but white Type III is difficult to find in small (less than pallet) quantities; it often has to be special ordered. Given this, it is best to stick with Type I cement.

Type IV and V are often used in special construction applications where high sulfate resistance is required or a low heat of hydration is important. Neither of these types are practical choices for countertops.

Good concrete countertop mix design for cast in place

Key characteristics that define a good cast-in-place concrete countertop mix are finishability and shrinkage resistance. However, these two characteristics are at odds with each other and must be carefully balanced in order to produce a good cast in place concrete countertop mix that is a joy to finish and does not curl or crack.

Other beneficial characteristics worth mentioning include workability and flexural strength. While high compressive strength is not necessary (though it is impressive), high quality concrete, often a byproduct of creating a high compressive strength mix, is also beneficial and desireable. Additionally, adequate work time, high early strength and a good appearance add to the list of desireable characteristics.

The two key characteristics that are very important, finishability and shrinkage resistance, are often determined by the aggregate gradation and the cement to aggregate proportioning.

Finishability, that is, the ease of trowelling the concrete into a smooth, even, high-quality surface relies on a sufficient amount of cement paste and very fine aggregate to create enough cream to trowel. Cream is the fine portion of concrete that is floated to the surface early in the casting process and is worked and reworked during trowelling.

Shrinkage resistance is also influenced by the water-cement ratio, by the cement paste content and by the amount of fine aggregate. Whereas finishability benefits from more cement paste and fine aggregate, shrinkage resistance benefits from less cement paste that has a lower water-cement ratio, since that is what actually shrinks. Minimizing the fine aggregate preserves workability when the cement paste volume is reduced, because fine aggregate (sand) has much more surface area than coarse aggregate, so more cement paste is needed to coat and separate fine sand than is required for a coarse blend of aggregates.

A poor cast in place concrete countertop mix would have large aggregate of one size, say 3/8″, mixed with fine sand. This is a case of “gap grading”. A good mix will have well-graded aggregate.

aggregate gradation in concrete countertop mix

Click here for a free seminar including a specific cast in place mix design.

What causes cracks in concrete countertops?

As the old adage goes, all concrete cracks. What’s important 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:

crack in concrete countertop

Cracks in granite from over-tightened faucet:

crack in granite countertop due to faucet over-tightening

Multiple flex cracks in a overloaded cantilever beam:

crack in GFRC countertop due to flexing

Hairline cracks often occur because of shrinkage, either from drying or from 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:

hairline crack by sink in concrete countertop

Heat also can cause hairline cracking. Crock pots are a common 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 expand, the greater 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:

thermal crack in solid surface countertop

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