A ready-mix plant network map looks like a scatter of dots on a metropolitan area. It is better read as a map of drive times, because the constraint that placed every one of those dots is the number of minutes the concrete has between the batch plant and the chute.
The discharge window
Mixed concrete has a limited useful life. The standard purchase specification defines the limit two ways: discharge should be complete within ninety minutes of the water first contacting the cement, or within three hundred revolutions of the drum, whichever comes first. Both limits may be waived where the concrete still has satisfactory workability at discharge and where a retarding or hydration-stabilising admixture has been used deliberately to extend the window.
The revolution limit deserves attention because it is the one people forget. A truck that spends an hour circling a congested site at agitating speed accumulates revolutions without covering distance, and drum revolutions abrade the aggregate and generate fines. Time and revolutions are separate clocks.
Temperature compresses both. On a hot day, slump loss and stiffening run fast enough that the practical window is well short of ninety minutes unless the mix is designed for it. On a cold day the window is longer, but the concrete arrives cold and gains strength slowly at the other end.
Turning the window into a radius
The delivery window has to cover the whole cycle, not just the drive out: loading, travel, waiting at the site, discharging, and returning. A useful way to think about it is to subtract from the window everything that is not travel.
Suppose the window is ninety minutes. Loading takes five to fifteen depending on plant type. Site waiting — the truck queued behind others, waiting for a pump to be repositioned, waiting for the crew to catch up — is the least predictable term and is routinely fifteen to thirty minutes on a busy job. Discharge takes five to fifteen for a chute placement, longer through a pump. What remains for the outbound drive is often thirty minutes or less, and on a poorly organised site it can approach zero.
Thirty minutes of driving is not a fixed distance. On a suburban arterial at ten in the morning it might be fifteen miles. On a congested urban freeway at four in the afternoon it might be six. This is why an effective service area is an irregular blob shaped by the road network and the time of day, not a circle, and why producers plan around isochrones rather than radii.
Why networks are dense rather than large
Three consequences follow, and together they explain the observed pattern of many modest plants rather than a few large ones.
Overlap is deliberate. A large placement needs more trucks per hour than one plant can batch and dispatch, so it is fed from two or three plants at once. That only works if their service areas overlap, which requires them to be closer together than their individual radii would suggest.
Redundancy is operational. A mixer failure, a silo run-out or a power interruption at a single plant cannot be allowed to strand a pour in progress. Overlapping coverage is the insurance.
The frontier moves. Plants near the growth edge of a metropolitan area eventually find themselves surrounded by the housing they supplied, and industrial operations rarely survive that transition. New plants open further out. Over decades, the network migrates outward, and its density at any moment maps where construction was recently active.
What a customer controls
The site controls the least predictable term in the whole calculation, and can therefore do more to protect the delivery window than the supplier can. In descending order of impact:
- Placement rate. Order trucks at the rate the crew can actually place. Ordering faster produces a queue of trucks aging in the sun; ordering slower produces cold joints. An honest cubic-yards-per-hour figure is the single most useful number a supplier can be given.
- Access. A firm, drained, unobstructed route in and out, adequate turning space, and a clear washout location. A truck that must reverse two hundred feet down a muddy track adds minutes to every cycle.
- Readiness. Forms complete, reinforcement inspected, embeds set, pump primed and crew present before the first truck arrives. Waiting time consumed at the front of a pour is subtracted from every load behind it.
- Timing. Scheduling a large placement outside peak traffic buys travel minutes directly, and in hot climates it buys temperature margin as well.
Pumping changes the arithmetic
A pump decouples the truck from the placement point, which helps access enormously, but it introduces a bottleneck of its own: the pump's throughput, not the truck fleet, now sets the delivery rate. It also changes the concrete — pumping reduces air content and demands a mix with adequate fines and pumpability, which is a mix-design decision rather than an afterthought. The proportioning considerations are covered under mix design.
Related reading
Plant configurations and what they imply for cycle time are under plant types. The information a supplier needs in order to schedule is under ordering, and the commercial treatment of waiting time and short loads is under procurement and terms. The purchase specification that defines the discharge window is published by ASTM International, and industry guidance on delivery practice by the National Ready Mixed Concrete Association.