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:
Cracks in granite from over-tightened faucet:
Multiple flex cracks in a overloaded cantilever beam:
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:
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: