Mixing the concrete is as precise as making the vanity form. To figure out how much material you'll need for the project, you need to first figure out the total weight of the countertop and then measure out parts by dividing.
A two-inch thick countertop weighs about 25 pounds per square foot. Calculate the total weight of your project by multiplying the vanity square footage by 25 pounds. Round up so you have a little extra concrete to work with.
For instance, a 4 foot by 3 foot vanity is 12 square feet with a weight of 300 pounds. Round up to 315 pounds so you have a little extra material to work with.
Divide the concrete mix into nine equal parts: three parts coarse sand (Image 1), three parts white playground sand (Image 2), two parts white Type II Portland cement (Image 3) and one part water.
All measurements are calculated by weight, so to figure the weight of each part, divide the project's total weight by the number of parts (nine parts total).
For example, each part of the 4 foot by 3 foot vanity (rounded up to 315 pounds) is 35 pounds. So you need to measure out 105 pounds (3 x 35 = 105) of coarse sand, 105 pounds of white playground sand, 90 pounds of Type II Portland cement and 35 pounds of water.
Note: Water weighs eight pounds per gallon, so in our project, we just a little over four one-gallon containers of water.
Measure out each ingredient ahead of time in clean five-gallon buckets, using the bathroom scale. To make the job easier, measure out the water in small containers to add a little water at a time.
Wearing a mask, add the dry ingredients to the wheelbarrow and mix them thoroughly with the perforated hoe. Then slowly add water, mixing until the cement texture turns from a crumbly mixture to an oatmeal consistency (Image 4)
Gauge your water amount carefully. To create a strong mix, the water amount should always be less than half the weight of the cement.