Concrete Order, Truck Access, and a Mix That Sets Fast
A crew is pouring a small addition: a 24 ft × 16 ft slab, 5 inches thick, with a 12-inch-wide × 12-inch-deep grade beam around the perimeter (measured on the beam centerline). The ready-mix plant quotes 27 cubic feet per cubic yard and will add a 5% waste allowance if asked. The only truck access is a residential driveway the homeowner needs back by 1:00 p.m. The crew can place and finish roughly 3 cubic yards per hour. The forecast is 92°F, and the mix is a 4-inch-slump air-entrained mix with no retarder. The finisher has one bull float and no second finisher on site. The foreman wants one 10-cubic-yard truck so there is "one pour and one joint."
1. Compute the concrete order in cubic yards, including 5% waste, rounded up to the nearest quarter yard. Show each step.
Read the worked answer
Slab volume
- Area = 24 ft × 16 ft = 384 ft²
- Thickness = 5 in = 5/12 ft = 0.4167 ft
- Volume = 384 × 0.4167 = 160 ft³
Grade beam volume
- The beam is 12 in wide, so the centerline rectangle is 1 ft inside the slab edges on each side: 23 ft × 15 ft
- Centerline perimeter = 2(23 + 15) = 76 ft
- Volume = 76 ft × 1 ft × 1 ft = 76 ft³
Total
- 160 + 76 = 236 ft³
- 236 ÷ 27 = 8.74 yd³
- With 5% waste: 8.74 × 1.05 = 9.18 yd³
- Rounded up to the nearest quarter yard: 9.25 yd³
Two traps: measuring the beam to the outside face double-counts the corners, and forgetting to convert inches to feet before dividing by 27 produces a number that is wrong by a factor of 12. Always state the units at each line so the error is visible before the truck rolls.
2. Should the foreman order one 10-yard truck or two split loads? Give the reasoning, including what the schedule and the weather do to the answer.
Read the worked answer
Split the loads. The single truck fails on the clock, not on capacity.
- 9.25 yards at 3 yd³/hour is roughly 3 hours of placing and finishing.
- A 4-inch-slump mix with no retarder in 92°F weather reaches initial set in roughly 90 minutes or less. The back half of a single 9.25-yard pour would be unworkable before the crew reached it.
- The driveway window (back by 1:00 p.m.) makes a long pour worse: a truck that cannot leave until the pour is finished is a truck that blocks the access it needs.
What to order instead:
- Two loads of about 4.5 to 4.75 yd³ each, spaced so each is placed and floated inside the working window, with a construction joint or a planned cold joint at a sensible location.
- Or keep one pour but change the mix (retarder, higher slump with plasticizer) and add a second finisher and a second float - but the crew has one finisher, so the mix alone does not solve it.
Compare the cost of a second truck or a short-load charge against the cost of chipping out a cold joint. The split load is cheaper almost every time.
3. The plant says it can only send one truck and cannot add a retarder. What would you change before the pour starts?
Read the worked answer
Changing the crew and the timing is cheaper than changing the concrete.
- Start earlier. A 6:30 to 7:00 a.m. start buys cooler air and hours of working time before the heat peaks; by 92°F afternoon the same mix behaves very differently.
- Add a second finisher and a second float. The bottleneck is finishing capacity at 3 yd³/hour, not discharge. Matching the crew to the pour is the direct fix.
- Pre-stage everything - forms, vapor barrier, chairs, screed, edger, curing compound, and a shaded water source - so no clock time is spent hunting for tools once the mud is on the ground.
- Plan the joint location deliberately rather than letting the mix choose it - saw-cut or construction joint at a third point or at a control joint line, agreed with the owner before the pour.
- Confirm curing. At 92°F and wind, curing compound or wet cover goes on immediately after finishing; a slab that dries fast is a slab that cracks.
- Protect the driveway window. Stage the truck, wheelbarrows, and washout so the drive is clear by 1:00 p.m.
If none of that is possible, reschedule - a hot-weather pour with one finisher and one truck is a known bad outcome, not bad luck.