Cheap concrete uses ballast mix
Cheaper volumetric concrete suppliers don’t use premium aggregates, and don’t separate them. They use a random mix of recycled sand, stone and crushed old concrete, known as ballast. This is all loaded into the volumetric mixer’s hoppers together. Because of this, the operator has far less control of the mix and does not know the ratios of sand to stone being mixed.
The operator has very little control over the concrete, making it difficult to achieve an accurate mix design. The only control they have is choosing more or less cement or water.
In such cases, the concrete is not tested for quality, as the suppliers do not need to comply with any trading standards. It is uncertain what quality of concrete the customer will get.