Ready-mix concrete
Concrete batched at a plant to a specified mix design and delivered to the site in a mixer truck, as opposed to being mixed on site or from bags.
What does ready-mix concrete mean?
Ready-mix dominates US construction because plant batching is far more accurate and better documented than site mixing. Ordering means specifying strength, slump, aggregate size, air content, and any admixtures, plus quantity and placement time. The supplier's responsibility for the concrete's properties depends on whether anything was changed at the site, which is why water additions are documented.
In practice
For anything larger than a few wheelbarrows, ready-mix is cheaper, more consistent, and better documented than mixing on site.