Whether you use a bagged mix specifically designed for concrete countertops, or mix your own, mix design is an important consideration 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 this 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.
Note also that even if you use the best mix in the world, if you use bad concreting practices such as using adding too much water, not reinforcing properly or not curing properly, then your mix won’t matter. Or if you don’t know how to manipulate the mix to get the look you want, that best mix won’t help you produce beautiful concrete.
Types of Concrete Countertop Mixes
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
Requirements of All Concrete Countertop Mixes
Regardless of the style of mix, the following basic principles apply.
|What you need from any concrete countertop mix:
|Our Precast Mix Calculator contains two excellent mix designs. Get it here.
1. 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. This is 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.
2. High Flexural Strength
For traditional precast concrete, 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 in precast concrete countertops effectively boosts flexural strength values many times that of unreinforced concrete. GFRC concrete countertops use a special mix design and high glass fiber loads that create high flexural strength.
3. 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.
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.
Proper curing also combats shrinkage. For example, 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. It 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.
4. Suitability to the Project Needs
Every project has aesthetic, economic, structural and other practical needs. The choice of a particular concrete mix must take all of those into account.
For example, I could never, ever make anything as thin and strong (like a lounge chair) from a generic bag of Quikrete concrete mix from home depot without radically changing the thickness of the piece, how it’s cast, or most importantly how it’s reinforced.
The mix you choose has inherent capabilities and limitations: cost, availability, aesthetics, versatility, compressive and flexural strength, shrinkage, etc. Choosing one mix vs another involves far more than simply knowing the ingredients in the mix, or finding reassurance from an online testimonial that “it’s the best!”.
Knowing what that mix can do, how it can be manipulated, what its limitations and drawbacks are, and the overall suitability to a specific application are the principle keys to knowing how to choose the right mix and how to be successful with it.
Only knowledge can lead to success. Knowledge imparts confidence, versatility, and flexibility. If you have a deeper grasp on what your concrete can and can’t do, you can manipulate it within its limits to achieve something that a novice can’t or isn’t aware is possible. Ignorance leads to blind dependency and unexpected issues when the wrong mix is used, or the right mix is used for the wrong reason.
The following video illustrates why “mix doesn’t matter”. (Well, it does matter, but specialized mixes aren’t necessary for different aesthetics.)
For free training on exactly how all of the ingredients of concrete work (sand, cement, water, admixtures), click here.
For more information about concrete countertop mix designs, click here.
To learn about GFRC, click here.