Concrete Mixes & Batch Ratios
For Different Applications
The basic techniques that will enable you to cast a foundation or slab are relatively straightforward. The secret to success lies largely in the correct quantities being mixed together in the concrete mixes you use. Although the principles of mixing concrete and mortar are simple, this can be backbreaking work, especially if you decide to mix the concrete by hand yourself. If there is a reasonably large quantity of concrete to be used, it is advisable to hire a concrete mixer. Whether you are mixing by hand or in a concrete mixer, you will have to measure materials accurately to ensure you achieve concrete of consistent quality.
Batching Generally, materials for smaller jobs are batched by volume. Recommended ratios will enable you to mix concrete to match the function for which it is intended, or the strength of the concrete required.
One 50 kg sack of cement has a volume of 33 litres (0,033 cu m). A builders’ wheelbarrow, filled level to the top, has a volume of 65 litres (0.065 cu m, which is almost double the volume of a sack). When batching by volume, it is safe to assume that one wheelbarrow-load is equivalent to two sacks of cement. Since sand bulks in volume when it is damp, the mix ratio table (below) is based on the use of damp bulked sand. If you are measuring dry sand, reduce the quantity of each batch by 20–25 percent. You will also need to add more water to compensate for the lack of water in the sand. The recommended ratios are based on the use of either 19 mm or 13,2 mm commercial crushed concrete stone. Stone does not bulk in volume when it gets wet and so no correction is necessary.
Large Batches of Concrete Mixes
15 Mpa This is a low-strength concrete mix and is suitable for house foundations that are not reinforced, and for boundary walls and freestanding retaining walls.
25 Mpa This is a medium strength concrete and is suitable for reinforced foundations, light-duty house floors, patio slabs, footpaths, steps, driveways and garage floors.
30 Mpa This is a high strength concrete and is suitable for suspended structural beams, pre-cast beams and flagstones, heavy-duty workshop floors and suspended reinforced floors.
Small Batches of Concrete Mixes
You can use containers such as buckets, drums or tins. It is important that the same size container is used for all materials in a batch.
Moving and placing the concrete
If you have large quantities of concrete to place, it is much more convenient to order it ready mixed. It is then mixed in a factory environment, according to your specifications. You must just ensure that workers are on site to place the concrete as soon as it is poured from the truck.