Compressive Strength and Reinforcing

Supporting this much weight (500 pounds) wouldn’t be
possible if this concrete beam wasn’t reinforced.

Question:
My concrete has a high compressive strength. That means I don’t need reinforcing, right?

Answer:
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 strength of concrete in tension is only about 10% of the strength in compression, but this is a rough average. Tensile strength is highly variable and difficult to predict. It depends not only on the ingredients and their proportions in the mix, but also on the casting technique, curing methods, and the amount, size and distribution of voids and defects (microcracks). Even the amount of pigment or other admixtures in the concrete can significantly affect its tensile strength.

In unreinforced countertops, you must rely completely on the tensile strength of the concrete to hold everything together. Because tensile strength is variable and difficult to predict, you’re taking a big gamble on a finished piece by leaving out the reinforcing steel. Often countertop fabricators will experiment successfully with small slabs, since small pieces don’t develop large tensile stresses when handled. However, success often leads to bolder ventures with larger and longer slabs, and eventually the weight and size of the slab will generate tensile stresses that overcome the concrete. If the countertop contains no reinforcement to resist the tensile load after the concrete cracks, it will suddenly fail and snap in half with little or no warning.

Don’t take these kinds of risks with your concrete countertops. Reinforcing steel adds ductility, allowing the concrete to absorb deflection and overloading without falling apart.