SURFACES | StonExpo Blog

Most articles about concrete countertops extol the versatility and design potential of the material. After all, when you start with raw materials the possibilities of what you can do with them are vast and seemingly limitless. However, instead of delving into design, I want to discuss something a bit more concrete (pardon the pun): the material itself.

Stone fabricators work with a variety of stone and are familiar with the differences in these materials. Hardness, porosity, brittleness, etc. are all individual characteristics that make one stone different from another (appearances notwithstanding).

Concrete too has characteristics that set it apart from other countertop materials. Some of these physical characteristics are very similar to stone while others are very different. Understanding the similarities and differences between concrete and granite can be a significant factor that results in profits vs. unforeseen costs.

Granite Composition

Let’s start with granite. Differences in mineralogy, grain, origin and other subtleties give each granite a unique personality stone fabricators have learned to recognize. Three characteristics distinguish granite from other rock types.

1. First, granite is an igneous rock.
2. Second, it has large mineral grains.
3. And third, it always has quartz and feldspar as primary mineral components.

Not all of the stone used for granite countertops is geologically classified as granite; black “granite” is really gabbro, which has no quartz in it. However the more relaxed definition of “granite” by the stone industry often includes any stone with large grains and hard minerals. Broadly speaking, most “granites” share similar physical characteristics such as hardness, durability, acid, heat and scratch-resistance.

The hard nature of the mineral grains makes granite difficult to scratch and easy to polish to a very high sheen. Granite is considered strong, but the brittle nature of stone, coupled with natural joints, fissures and veins can make it fragile and vulnerable to unexpected breakage.

Concrete Composition

The basic makeup of most concrete is simple: Hard aggregates of various sizes are mixed with a cementitious binder to create a strong, stone-like composite.

Nearly all concrete uses Portland cement as a binder, but some ultra-high performance concretes use other mineral-based binders that are superior to (and more expensive than) Portland cement.

Photo by Dave Banko, Counterpart, LLC

Concrete can use aggregates with a variety of mineralogies. Crushed granite, quartz, limestone and marble are common coarse aggregates, and silica sand is often the fine aggregate. Concrete can also use recycled materials like crushed glass bottles as aggregate.

Etching Bare Concrete

Acid vulnerability is concrete’s Achilles heel. Bare concrete will etch from acid, but high performance coatings like urethanes solve this issue.

Polishing Ability

Because it is a heterogeneous matrix of various aggregates in a cement binder, concrete polishes differently from most stone. Generally the harder aggregates polish well, which is to be expected from natural stones. Recycled glass aggregates polish well too.

Polished concrete’s sheen generally can’t compete with polished granite, but with the right combination of technique, concrete mix and aging it is possible to get a suitably high shine, if that is what the client wants. Most clients want concrete to have a matte look because the very reason they are choosing concrete is that it is different from granite.

Keep in mind that polished concrete does shed water nicely, but its bare surface is still vulnerable to acid. To prevent acid etching, high performance coatings must be applied.

Strength

Granite like most stones is strong, hard and brittle. Natural fissures, fractures and veins weaken a stone slab, often leading to unexpected breakage. But what does this really mean? When stone is described as being brittle, that means that it cannot tolerate much flexing before it breaks unexpectedly. A seemingly solid slab that’s lifted improperly can shatter in an instant without warning.

The opposite of brittle is flexible. Think of rubber. It doesn’t shatter. Rather it takes considerable stretching and elongation before it breaks. Even though it can break, there’s a clear indication that something is happening to the material long before failure occurs.

Concrete, when it’s properly reinforced and well designed, can be made flexible and strong. True, unreinforced concrete is a brittle material, but adding reinforcement makes concrete a tough, flexible composite. Rodding granite with steel or fiberglass provides a similar benefit, although without nearly the same capacity that reinforced concrete has.

The ability to engineer concrete to be flexible and strong, combined with the fact that slabs are made to order, allow for very long, thin slabs without breakage or even cracking.

GFRC Desk Flexing

Conclusion

Finally, the biggest difference between concrete and granite is that you can tailor concrete to suit your client’s needs and tastes. Aesthetics, durability and structural performance can all be customized to create unique pieces (often seamless and three dimensional) that are prohibitively expensive, difficult or impossible to do with stone. Knowing this, and understanding the material itself, will allow you to be more successful with adding concrete to your repertoire of offerings to satisfy any client’s taste.