Mix Design for Concrete Countertops
Concrete countertops have seen significant improvements due to precise calibrations of mixing the concrete. Using accurate mixture ratios has eliminated many of the problems that resulted from earlier concrete countertops, namely cracking and warping. The proper mix design allows for customizing your countertops with your choice of color by using pigments and with your choice of texture by carefully adding the correct amount of finishing component to the mix design.
Concrete is formed by mixing certain amounts of sand, water, gravel, aggregate (crushed rock), and portland cement. The most important ingredient is Portland cement because different types of it are intended for different projects; the one you want to look for is Type I, which is calibrated for residential concrete projects such as countertops. Components called admixtures are added to the portland cement to change some of its properties.
When deciding on your concrete mix design, you first want to look for concrete that has had a plasticizer added to its Portland cement. Once poured, this will make smoothing the surface of your countertop with a hand tool called a trowel much easier while the concrete is still wet. Many concrete countertops have turned out to have surfaces that are not completely level once the concrete has cured, and a plasticizer will help you avoid this problem.
A second admixture you will need to consider is the pigment if you plan to have a finished countertop in a color other than grey, buff, or white. There has been a considerable increase in the variety of shades, tones, and hues available, and keep in mind that the more powered pigment is added, the more saturated the color of the end result.
An additional ingredient in concrete is a mixture of fine and/or coarse aggregates. The exact ratio of fine to coarse determines the texture of the surface of your countertop. Coarse and fine aggregates are determined by the size of the particles that make up the crushed stone; the larger the particles, the coarser the texture will turn out. For those who want a smoother surfaced countertop, concrete sand is an alternate aggregate that can be added. Make sure you obtain your concrete sand from a supplier who has filtered out any contaminants and silt.
The amount of concrete you will need to mix depends on the desired size of your concrete countertop. Measure the form you will be using to pour the concrete and calculate the volume by multiplying the length, width, and height. Divide the result by 27 to get the number of standard cubic yards of concrete mixture you will need. When you are ready to mix the dry ingredients, measure each carefully in a clean container. The standard rule of thumb for this mix design combines three parts aggregate, two parts sand, and one part Portland cement.
Gradually add water (usually about 5 gallons on average) and make sure that the mixture does not stay stuck to the sides of the container, as this will create uneven proportions of water and dry material. Concrete that has been mixed correctly will hold its shape when you form ridges in it with a trowel. Test this with a small amount of your mixture on a clean piece of wood.