How much concrete will you need?
The first thing that you must do is to determine how much concrete you are going to need. To calculate the concrete requirement, measure the width, length and depth of the inside of the mold, to the next 1/2 inch (if the length of the mold is 72 1/4 inches then use 72 1/2 inches) and multiply them together and round it up so that there are no decimal places.
In our example we have a width of 26 inches, a length of 84 inches and a depth of 2 1/2 inches:
26 x 84 x 2.5 = 5,460 cubic inches
Then calculate the size of any major holes in the mold. In our example we only have one major hole and that is for the sink; 29 1/2 inches wide, 17 1/2 inches deep and 2 1/2 inches in depth (the depth of the mold) and round it down so that there are no decimal places.
29.5 x 17.5 x 2.5 = 1,290 cubic inches
Subtract the cubic inches of the holes from the cubic inches of the total mold:
5,460 – 1,290 = 4,170 cubic inches
For our example we need 4,170 cubic inches of concrete. We suggest you calculate your concrete requirement in cubic feet. To convert cubic inches to cubic feet, divide by 1,728.
4,170 ÷ 1,728 = 2.4 cubic feet
This tells you how much of each ingredient you will require for the concrete mix and determines the concrete mixer requirement. Remember that a concrete mixer can only mix half of its rated load. In order to mix 2.4 cubic feet of concrete you will need a mixer rated at a minimum of 4.8 cubic feet. It is always best to mix all of the needed concrete in one batch, if possible, as this maintains the overall mix consistency, especially the color.
Note: Always prepare a little more concrete than needed, 5 to 10 percent. It is better to have some left over, than to be short a few cubic inches.
Concrete mixers can be purchased or rented.
Calculating the amount of each of the ingredients
For each cubic foot of concrete that is required to fill the mold you will need:
- 23.5 lbs of Type III portland cement
- 37 lbs of 3/8 inch (maximum size) pea gravel
- 72 lbs of natural sand
- 2.8 ounces of Glenium 3000 water reducer. The Glenium 3000 water reducer allows a maximum of 0.12 ounces per pound of portland cement. If you are using a water reducer made by a different manufacturer read their product label to determine the maximum amount of water reducer you can add for the quantity of portland cement that you are using.
- 6 ounces of Fibermesh 150 polypropylene fibers. As with the water reducer, check the manufacturer’s instructions on the weight of fibers per quantity of concrete.
- 2.3 lbs (maximum) of pigment
- Water: it is impossible to specify the exact amount of water because there are so many variables. The sand and aggregate may have some water content in them and the humidity and temperature will affect the concrete mixture. You want a blended and well mixed concrete that is the consistency of thick porridge which would give you a slump test of 4 inches.
The maximum amount of water you will require, if all the ingredients are truly bone dry, is 9.5 lbs per cubic foot of concrete.
Note: Many manufacturers will specify their ingredients by the cubic yard. To convert from cubic yards to cubic feet, divide by 27. There are 27 cubic feet in a cubic yard.
Many home handymen have worked with concrete and are familiar with the methods of mixing concrete for a sidewalk or patio. One shovel of cement, two shovels of sand and three shovels of gravel and although the shovel used is the same size, one doesn’t always get the same amount of ingredients on each shovel full. While this may work for a patio or sidewalk, it won’t work for building architectural concrete pieces such as countertops.
To make a strong and durable countertop you need to actually measure all of the ingredients that you will be adding to your concrete mix. The best method to accomplish this is to start with a quantity of 5 gallon pails
Place a pail on the scale and set the scale to zero. You do not want to weigh the pail only the ingredients.
Fill a pail or pails with the required amount of Type III portland cement, then pails for the sand and then the aggregate.
Use a kitchen scale to measure the pigment, fibers and water reducer. Again, remembering to zero out the weight of the container.
You should also have an amount of water available. Measure, by weight, pails of water holding half of the maximum amount of water needed per cubic foot of concrete that you will require; 4.75 lbs. (9.5 ÷ 2). If you need 3 cubic feet of concrete, you would measure out 14.25 lbs. of water. Prepare a second set of pails with an identical amount of water.
The first set of pails is the amount of water to start with. The second set of pails is the absolute maximum amount of water you can use.
Note: Have all your ingredients measured and available before you commence mixing. It is also wise to have a couple of helpers. Once the water is added to the other ingredients that make up the concrete mix you will have approximately 40 minutes to complete the pour and that includes the time necessary to vibrate, screed and trowel the concrete in the mold.
Note: If at all possible do not allow the ingredients to sit out in the hot sun. Warm ingredients will set-up quicker than cool ingredients.
Continued - How to mix the concrete ingredients
How to Make Concrete Countertops - Index