The interval is the number that runs the pour
Truck count is the easy half: yardage over load size, rounded up. The half that decides whether the day works is the interval between arrivals, and it is set by how long a truck spends on site — the time to discharge its load plus the time to back in, set up and rinse out. Space them at that interval and each truck arrives as the one in front pulls away. Space them tighter and you are paying for trucks to queue. Space them wider and you have opened a gap in the pour.
Discharge time uses the slower of two rates, and knowing which one it is changes what you would do about a slow day. If the truck is slower than the crew, the crew stands waiting and adding people changes nothing; the fix is on the delivery side — a pump, a second chute position, a shorter buggy run. If the crew is slower, the truck sits turning and a faster pump changes nothing; the fix is placing and finishing capacity. The page names which one it is, from the two rates you entered.
The rotation count is the supplier problem, shown anyway
A truck that spends an hour on the round trip and thirty minutes on site is unavailable for ninety minutes. If you want a truck arriving every thirty minutes, the plant needs three of them in rotation on your job. That is their arithmetic and their fleet, but it is worth having the number before you ring them, because "can you give me one every half hour" and "can you put three trucks on this" are the same request and only one of them gets a straight answer.
It is also the figure that explains why a long haul costs more than the extra driving. The plant is not selling you distance, it is committing rolling stock, and a site an hour away consumes twice the trucks of a site half an hour away for the same interval.
The crew rate is the box people fill in wrong
Discharge rates are reasonably knowable — a chute into a form is fast, a bucket on a crane is slow, a pump is somewhere between depending on the line. The placing rate is the one that gets estimated optimistically, because people time the part that looks like work and leave out moving the screed, chasing the edges, fetching what nobody put out, and the pause where the whole crew waits for one person.
The only reliable way to know it is to time a real pour with the real crew, once. Everything downstream of that number in this page — the interval, the duration, the finish time, whether the last truck lands on stiff concrete — is proportional to it, so an hour of measurement is worth more than any amount of adjustment afterwards.
What is deliberately absent
| Question | Where it belongs |
|---|---|
| How long can concrete wait between loads | Your supplier, for the mix you ordered and the weather that day |
| Whether a gap has made a cold joint | The engineer of record and whoever placed it |
| What placing method suits this pour | The people doing it and the site conditions |
| Whether the mix is still workable | The supplier and the specification, at the truck |
The maximum acceptable gap on this page is a field rather than a figure for exactly that reason. Mix design, admixtures, ambient temperature, wind and direct sun all move it, and the range between a hot windy afternoon and a cool overcast morning is wide enough that a single number would be wrong most of the time.
One hazard worth naming rather than explaining: wet concrete is strongly alkaline and burns skin chemically, and the burn develops over hours rather than at contact, so people working in it often do not feel the injury being done. Pump lines under pressure and buckets on cranes are their own hazards with their own trained operators. None of that is procedure here.
Questions people ask
How many concrete trucks do I need?
Total yardage divided by what your supplier actually loads, rounded up — 46 cubic yards at 9 yards a truck is six trucks, the last one carrying a part load. The load size is worth asking about rather than assuming, because it is often below the drum rating and it varies with the haul, the mix and local weight limits on the road to your site.
How far apart should I order the trucks?
At the time one truck spends on site: its discharge time plus positioning and washout. At an effective 16 cubic yards an hour, a 9 yard load takes about 34 minutes to discharge, and with 8 minutes of positioning and rinsing that is roughly 42 minutes per truck. Order them at that interval and each arrives as the last one leaves, with no queue and no gap.
Is the truck or the crew the limit on how fast a pour goes?
Whichever rate you entered is lower, and the answer changes what is worth doing about it. When discharge is slower, the crew waits and more people do not help; the change has to be on the delivery side. When placing is slower, the truck waits with the drum turning, which is where waiting charges come from, and a faster pump changes nothing. The calculator names which case you are in.
How long can concrete sit before the next truck arrives?
That is a question for your concrete supplier about the specific mix, on the specific day, and this page supplies no figure for it. It depends on the mix design, the admixtures in it, the concrete temperature, the air temperature, wind and direct sun, and the range is wide. The calculator has a field for the gap you have been told to work to and prints it beside the interval the schedule produces, without judging either.
Why does the calculator ask for the round trip haul time?
Only to work out how many trucks the plant has to have in rotation to hold your interval. A truck that is away for 45 minutes and on site for 42 is committed for about an hour and a half, so an interval of 42 minutes needs three trucks cycling. That is the supplier fleet question rather than yours, but it is useful to have the number before you book the pour, because a long haul consumes trucks in proportion to distance.