How to Make Concrete: Mixing Ratios

Here is some guidance on how to make concrete. There are four basic ingredients in varying proportions that are required for mixing concrete; Portland cement, sand, aggregate and water. The combined mix is measured in cubic feet; 27 cubic feet is equal to one cubic yard of concrete. Mixing can be done manually in a large plastic tub or wheelbarrow, or in a rotating portable cement mixer.

Portland cement is a fine, white powder consisting of lime, iron, silica and alumina and is available as Types I through V. Type I cement is appropriate for most construction applications and can be purchased in standard bags containing one cubic foot of volume and weighing approximately 95 lbs. The ratio of sand to cement is an important factor in determining the concrete’s compressive strength. A ratio of Portland cement to sand at 1:3 will yield a concrete mix with a PSI (pounds per square inch) greater than 3000, sufficient for most minor jobs. Water combined with the cementitious material will form the paste or binder that holds the aggregate in place.

Aggregate, or crushed rock, is classified as course, medium and fine, can be used in combination, and typically makes up ¾ of the volume of the concrete. Course aggregate will reduce the volume of cement needed and will not significantly affect the strength of the concrete, but will result in a rough surface finish. The strength properties of the concrete are inversely proportional to the water/cement ratio (by weight) which should be kept below .60. This means the weight of the water should not be more than 60% that of the cement. A higher ratio will produce a more plastic or fluid mix, but one that is likely to be deficient in strength and surface qualities.

Adding water to the mix will initiate the hydration process, a chemical reaction that causes the concrete to harden or “cure”. Once poured, compacting is done to eliminate trapped air and vibrating can also be done to ensure uniformity within the mix. Excessive vibration however, will cause segregation, where the heavier aggregates settle near the bottom and the fluid paste rises to the top.

Concrete is typically designed to be used at the strength it reaches after 28 days, but the hydration process continues for years. During the first two weeks of hydration, it is imperative that the concrete be kept wet or moist, particularly when pouring slabs, sidewalks or foundations. There are several methods for finishing concrete including “floating”, troweling, brooming and brushing. Also, ingredients known as admixtures are available and often used in larger commercial projects to alter or enhance various properties, such as air-entrained additives (insulation), accelerants and retardants (to change the rate of hydration), plasticizers (workability) and coloring agents.