Mixing concrete is a basic skill that every DIYer should know how to do. Concrete is one of the most common and inexpensive building materials on the planet and is primarily made up of sand, gravel, and cement.
Mixing concrete is similar to baking in that to achieve consistent results, it’s best to follow a specific recipe. There are a multitude of different concrete mixes for different applications depending on the strength, workability, and application of the concrete.
Mixing concrete is largely a science for professionals, but for the average backyard enthusiast, learning the basics of concrete is enough to get you started.
When mixing concrete, you’ll need to mix aggregates like sand and gravel along with powdered cement and water as a catalyst to begin the chemical process of binding the ingredients together. The concrete mix is a function of proportions where you will measure the volume of aggregate used to the volume of cement used.
The most common concrete mix used around the world is the 3-2-1 mix that uses three parts gravel, two parts sand, and one part cement. The water is added with the idea that you should use as little water as possible to make the concrete workable for what you need it for. Concrete viscosity is measured by “slump,” which refers to a specific test in which the amount that a concrete-filled cone will lose its shape once the cone is removed is measured in inches or mm. A zero slump concrete would be very stiff and dry and sloping to hold its shape, while a concrete with a slump of six inches or more would be extremely wet and not sloping to hold its shape.
The 3-2-1 mix is widely used because it can fill large areas with concrete for a relatively low price. Potentially, you could use just sand and cement, with no gravel, to create what’s called mortar. In the 3-2-1 mix, the gravel serves as filler, taking up a large amount of space while retaining a relatively high overall strength. The disadvantage of the 3-2-1 mix is that the larger gravel aggregate will often float to the surface during finishing and is not suitable for fine or detailed concrete applications.
The best general mix for a DIY enthusiast to memorize and use is a simple 3:1 mortar mix. By using three parts sand and one part mortar, you can create the strongest possible concrete, as well as concrete that is easy to finish and detail with designs, stencils, or stamps. By not using the gravel, the mortar will have a smoother overall consistency, but will ultimately take up less volume than concrete made with gravel as well. If you’re pouring large volumes of concrete, gravel might pay off; however, for most DIY projects, a 3:1 mortar mix is the way to go.
The sand you need to use for a 3:1 mortar mix should be sharps sand, masonry sand, or joint sand. Every place you buy sand from will use a different term, so it can be confusing when shopping. All you need to remember is to avoid playing with sand. Play sand is sand that has been washed and sterilized (this is good) and then tumbled to smooth all the edges of the individual sand grains (this is bad).
The sharp edges of the sand help to hold the concrete mix together better than the rounded edges of the tumbled sand. Ideally, you’d like to find sand at landscaping supply stores that will sell you by the cubic yard. With a strong trailer you can pick up all the sand you may need for next to nothing, plus the quality of sand for making cement will be very high.
The applications for a DIY enthusiast using a 3:1 mortar mix are vast. Everything from repairing broken outdoor steps and stairs, driveways, patios, decks, ponds, waterfalls, artificial rock, statues, and much more is possible with a simple mixture of sand, cement, and water.
Start with small concrete projects to get a better idea and understanding of how to work with concrete. Since concrete is so popular around the world and is a very advanced science, you will never run out of interesting concrete applications to learn about or try. Combined with being the most affordable building material on the planet and readily available everywhere, learning how to mix basic concrete recipes is a fundamental DIY project.