In Decorative Problem Clinic (October), the response to the question “Which is more important for a concrete countertop: reinforcement or a strong concrete mix design?” brought up excellent points. Understanding the forces that cause cracking or curling helps concrete countertop fabricators produce a better product. I would like to point out a few more details.
Tensile strength through proper reinforcement becomes more important when the countertop has long spans or cantilevers. The increase in tensile stresses is nonlinear as the length of the countertop span increases. In other words, tensile strength becomes real important real fast for long spans.
The best way to impart tensile strength is to place steel reinforcement where the tensile forces occur, which is normally on along the bottom of the counter-top, not in the middle of the cross section. In the middle of what is essentially a beam, tensile and compressive forces cancel each other out. For cantilevers, tensile forces are greatest on the top of the slab, so reinforcement is needed there. Of course, all reinforcement must have some amount of concrete cover.
Precast concrete countertop slabs should be carried and transported on edge, like glass. This minimizes extreme tensile forces on the slabs. Subjecting the slabs to some tensile forces during installation is unavoidable, though, so proper reinforcement is key to preventing failure during installation.
For preventing shrinkage, the answer mentions low water-cement ratio and low cementitious content, both very important factors. A third factor is aggregate gradation. Proper aggregate gradation minimizes the voids between the aggregates. Because cement paste is what fills the voids between aggregate particles and is also what shrinks, minimizing the voids minimizes the amount of cement paste resulting in less shrinkage. (All-sand mixes are sometimes used for precast countertops, but these mixes rely on carefully controlled curing conditions to prevent shrinkage.)
Concrete countertops are a challenging product that requires the contractor to understand mix design and structural design, in addition to actually making the product. Thank you for including discussion about these important topics in your magazine, so that contractors can learn to make better products. — Jeffrey Girard, P.E. President, The Concrete Countertop Institute