Concrete is created using several ingredients. Coarse aggregate or “ballast” is a primary component. This is added to cement that binds the aggregate particles together. Water and any additional additives are also used to create the final mixture ready for pouring.

The properties of concrete can vary widely depending on the amount of ingredients used and how they are mixed together. Changing the amount of ballast added affects the concrete’s strength; the addition of lightweight ballast produces low-strength concrete. As the amount and consistency of ballast is altered, the concrete produced varies in strength.

The imperial strength of concrete is measured in pounds per square inch (psi) or megapascals (MPa). Low-strength concrete typically has a strength of 2,000 psi (14 MPa) while the strongest concretes, used in superstructures and bridges, would typically top 10,000 psi (70 MPa). Many specific civil building and structural applications use 5,000 psi (35 MPa) concrete. The strength of concrete mixed is generally related to the application needed.

Equipment required to mix concrete should effectively blend large aggregate into a mixture of even consistency – this will aid workability and provide consistent physical and chemical properties. The process usually begins by mixing the water and cement into a paste before adding the aggregate and any other additives required to affect the concrete strength and properties. This initial paste has been shown to increase compressive strength of the resultant concrete.

What Is Ballast?

Aggregate or ballast is typically composed of chunks of rock such as limestone or coarse gravel together with other ingredients such as sand. Recycled aggregates are often employed, while the material can also be pre-manufactured in a blast furnace. Some applications require a finish or specific look, and to achieve this it is possible to add glass or decorative stone into the mixture, this is known as exposed aggregate.

To create concrete of specific hardness, the following general rules are often applied. High strength concrete would be mixed using 1 part cement to 5 parts ballast – this affords the sort of water-tight concrete used for building slabs or concrete panels. Mixing 1 part cement to 6 parts ballast creates concrete suitable for driveways, paths and patios, and increasing the ratio to 1:8 gives foundation-strength concrete. It is of course possible to add other materials to increase the strength of the concrete – these often include steel reinforcing bars (“rebar”).

Concrete Mix Ratio Using Ballast

For typical domestic use, 1 cubic metre of concrete at 3500 psi (25 MPa) can be readily created by mixing seven 50 kg bags of cement with 0.7 cubic metres of sand and 0.7 cubic metres of stone aggregate. Concrete of this strength is ideal for patio slabs, footpaths and steps. By increasing the cement:aggregate ratio, stronger concrete can be produced, suitable for heavy-duty flagstones or structural beams. Smaller batches can easily be created by scaling down the ingredients and using a bucket or drum for mixing, the concrete typically should be used within 45 minutes of mixing for optimum workability.