Ready-mix concrete

Ready-mix concrete is produced at a central batch plant to match a customer’s exact specifications, then delivered to the construction site in a finished, ready-to-pour state. It consists of Portland or other cements, water, and aggregates like sand, gravel, or crushed stone—all of which should be washed and low in fines, dirt, or clay. Admixtures are frequently added to improve workability or, with retarders, to extend the setting time so the concrete remains usable during transit. Delivery comes in two main forms: barrel trucks (in-transit mixers) that carry the concrete in a plastic, wet state, and volumetric mixers that haul dry ingredients and mix them on site. Some classifications split the material into three types: Transit Mix, Central Mix, and Shrink Mix.

The history of ready-mix concrete includes some disagreement over the first delivery and factory. One source points to 1913 in Baltimore. By 1929, more than 100 plants operated in the United States, but the industry did not see major expansion until the 1960s, with growth continuing since then.

At the batch plant, precise amounts of gravel, sand, water, and cement are combined by weight according to a mix design approved by the structural engineer or architect. This allows specialty concrete mixtures to be developed and used on site. Ready-mix is often chosen over other materials for its cost and versatility, especially in large projects like high-rise buildings and bridges. It also offers a longer lifespan—about 30 years under heavy traffic, compared to 10 to 12 years for asphalt concrete under similar conditions.

Ready-mix concrete is used when a construction site either cannot or chooses not to mix its own concrete. For small to medium projects, the expense and hassle of hiring mixing equipment, labor, and storing ingredients—plus health concerns from cement dust—often make ready-mix more economical, since customers pay only for what they use. For large projects, outsourcing to a ready-mix supplier hands over quality control, testing, material logistics, and mix design to specialists, though it introduces an external contractor who needs a profit margin and reduces the control and immediacy of on-site mixing.

The product is bought and sold by volume, typically in cubic meters (or cubic yards in the US). Batching and mixing happen under controlled conditions. In the UK, ready-mix concrete is specified informally by weight or volume ratios (such as 1-2-4 or 1-3-6) or formally using European standard EN 206+A1, supplemented by BS 8500. This lets customers specify required resistance to ground conditions, exposure, and strength, while the manufacturer designs a mix using locally available materials. Verification comes from lab tests (like cube tests for compressive strength and flexural tests) and field tests (such as slump tests for plasticity).

Admixtures can alter a mix’s performance—reducing water needs, entraining air, improving surface durability, or superplasticizing concrete to make it self-leveling (self-consolidating concrete). Their precise dosing and mix design require the equipment and lab support of a batch plant, making them hard to use outside ready-mix. Concrete has a limited working life between batching and curing: in the US, it should be placed within 30 to 45 minutes to maintain slump and specifications; in the UK, environmental factors and in-transit mixing allow up to two hours. Modern admixtures and water reducers can adjust this window.

Transport and placement methods vary. The most common is the chute on the back of a transit mixer truck, suitable where the truck can back in. Dumper trucks, crane hoppers, truck-mounted conveyors, and even wheelbarrows can be used for less direct access. Some mixes are pumpable with a concrete pump. In 2011, the United States had 2,223 companies employing 72,925 workers in ready-mix concrete production.