In the first two parts of this series, we discussed concrete finishing and concrete curing. The third topic of our discussion is usually the first one you deal with in a project — the concrete mix design. Ready-mix suppliers create mix designs to meet specification requirements. Some mixes have higher compressive strengths while others may be more liquid to accommodate pumping to an elevated pour. There are endless options when it comes to mix designs.
Mix designs are always a little tricky. As most Concrete Decor readers are not architects or engineers, there is a lot of risk associated with modifying a designed and engineered concrete mix. You don’t want to alter the structural strength of the slab as other, more serious issues can occur if you do. Alterations are best left to the ready-mix supplier, the architect or the structural engineer.
As a contractor, there are a few things you can look for in mix designs that are “red flags.” These issues can be potentially detrimental to the decorative or polished concrete installation.
It’s easy to determine the percentage of these cement substitutions. Take the pounds of fly ash or slag listed in the mix, add them together and then divide that number by the total weight of the mix. The resulting number gives you a total percentage.
For explaining purposes, let’s say you have a mix with 800 pounds of portland cement and 200 pounds of fly ash and/or blast slag. Added together we get a total weight of 1,000 pounds of material. When you divide the 200 pounds by 1,000 pounds you get .20 or 20 percent ash and/or slag.