Whether you use a bagged mix specifically designed for concrete countertops, or mix your own, mix design is critical for concrete countertops. Unlike sidewalks or foundations which are slabs on grade, concrete countertops are generally long, slender, thin beams that not only behave very differently structurally from slabs on grade but also have very different aesthetic requirements. For example, color is not an important consideration in structural concrete mix design, but it is in concrete countertop mix design.
Before we get into mix design considerations, note that you can either use a bagged mix specifically designed for concrete countertops, or mix your own from scratch. Our preference at CCI is for mixing your own, since Jeff is an engineer as well as a thrifty Yankee. However, there are pros and cons to both approaches. (More on that in another article.) We do believe that even if you choose to use a bagged mix in your business, you should fully understand how concrete really works, so that you are better able to troubleshoot or adjust for your climate and working conditions.
|What you need from a concrete countertop mix:|
High Early (Compressive) Strength
Jeff is always saying, “Compressive strength is not as important as you think.” However, high early (compressive) strength is important early on when you need to get the concrete out of the forms, flip it over and start grinding it as soon as possible to get it into your client’s home. Concrete that develops high compressive strength quickly is going to be harder than concrete that develops strength more slowly. This means that the cement paste between the hard sand grains and aggregate will be harder, and the concrete can be ground and polished sooner.
High early strength is accomplished by using a low water to cement ratio, proper pozzolan loading and cement contents higher than construction grade concrete.
High Flexural Strength
Steel reinforcing is still essential, since the flexural strength of concrete is always much, much lower than the compressive strength. For example, the predicted value of flexural strength for ordinary construction concrete that has a very high compressive strength of 12,000 psi is only about 900 psi! But, if the flexural strength of your concrete is as high as possible, it is going to better withstand bending (flexural) forces along with the steel reinforcement, and show less cracking.
High flexural strength is achieved through both mix design and proper reinforcement. Steel reinforcing effectively boosts flexural strength values many times that of unreinforced concrete. GFRC uses a special mix design and high glass fiber loads that create high flexural strength.
Low Shrinkage Potential
Shrinkage can cause either cracking for restrained slabs or curling for unrestrained slabs. Shrinkage occurs when the cement paste dries out. Moisture evaporating from inside the concrete causes strong capillary suction forces in the cement paste that cause it to shrink. If the shrinkage forces are high enough, the concrete cracks. The underlying causes of this can be poor curing practices (allowing the concrete to dry out too soon before it’s strong enough to resist the suction forces), too much mix water, too much cement in the mix, or poor aggregate gradation that requires too much cement paste to achieve good workability.
Curling occurs when one face of a countertop shrinks more than the other side, and the result is that the countertop curls towards the side that shrank more. Curling can occur if one side of the slab remains wet and the other side is dry. Curling is a symptom of shrinkage. Concrete mixes that don’t exhibit significant amounts of shrinkage don’t curl much or at all.
Shrinkage reducing admixtures (SRA’s) are chemicals that reduce the suction forces generated during evaporation. This helps reduce the root cause of cracking and curling: the suction forces in the cement paste.
There are many different styles of concrete countertop mixes:
- all-sand mixes designed to be stiff and hand packed
- aggregate-based mixes designed for vibration or cast in place
- polymer-based mixes that flow like pancake batter
- GFRC mixes
Regardless of the style of mix, the basic principles discussed above apply.
Want to know more? Both of the above from-scratch mix designs, as well as a handy mix calculator, are included in our Precast Mix Design 101 self-study course. Questions about this article? Submit a comment below. And happy concrete mixing!