Garage Concrete Quote and a 21-Day Door Deadline
You are the estimator for a 24 ft by 32 ft detached garage with 10 ft walls. The concrete subcontractor's quote is $2,220: 12 cubic yards of ready-mix at $185 per cubic yard for 'floor and footings,' with no other concrete items. The drawings call for a 4 in. slab over 6 mil vapor barrier and 4 in. of compacted gravel; a continuous 12 in. wide by 18 in. deep perimeter footing with two #4 bars; and footings bearing 42 in. below finished grade. The site has a 60 ft narrow driveway; a rear-load mixer cannot reach the slab area, so a pump truck is required. The owner wants the overhead doors installed within 21 calendar days so the garage can be locked. The forecast for pour week is highs of 38°F and lows of 28°F. You must decide whether to accept the quote, what to add, and whether the 21-day schedule is realistic.
1. Compute the concrete volume and evaluate whether the subcontractor's 12 cy quote is adequate. What missing items must be added?
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Compute the slab and footing separately. The slab area is 24 ft x 32 ft = 768 sq ft. At 4 in. thick, the volume is 768 x 4/12 = 256 cu ft. Dividing by 27 gives 9.48 cy.
The footing perimeter is about 2 x (24 + 32) = 112 linear ft. A 12 in. wide by 18 in. deep footing has 1.0 x 1.5 = 1.5 cu ft per linear ft. The footing volume is 112 x 1.5 = 168 cu ft, or 6.22 cy.
The combined volume is 9.48 + 6.22 = 15.7 cy. Add about 5 percent for waste and overpour, and the order should be roughly 16.5 cy. The quote of 12 cy is about 3.7 cy short, or roughly 24 percent low. At $185 per cy, the concrete alone should be about $2,900-$3,050, not $2,220.
The quote is also missing reinforcement, vapor barrier, compacted gravel, forms, labor, pump truck, cold-weather protection, and inspection coordination. Two #4 bars in a continuous footing, 6 mil vapor barrier, and 4 in. of gravel are all specified. A pump truck is required because the driveway will not allow a rear-load mixer to reach the slab. The subcontractor's quote should be rejected or rewritten as a complete scope with quantities, unit prices, and exclusions.
2. Is the 21-day door-installation schedule realistic? Provide a sequence and explain the controlling constraints.
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The 21-day deadline is aggressive and probably unrealistic in cold weather. A workable sequence is: excavate and proof the footing subgrade; footing inspection; pour footings; cure; set forms and place gravel, vapor barrier, and rebar; slab inspection; pour slab; cure; frame walls; frame and sheath roof; dry in; install overhead doors; final inspections. Each inspection depends on the prior one, and the concrete cure is the controlling path.
With highs near 38°F and lows near 28°F, the concrete will cure much more slowly than in summer. The subgrade must not be frozen, the concrete must be protected from freezing, and the slab should not be loaded heavily until it has gained adequate strength. Cold-weather protection, blankets, or heated enclosure add time and cost. A 7-day cure before framing is a practical minimum; a 21-day door installation leaves almost no float for weather, inspection scheduling, or a failed footing inspection.
A realistic schedule is 28-35 calendar days. If the owner needs the garage lockable sooner, propose a temporary secured opening or install the doors after the walls and roof are dry, but do not promise door installation before the slab and framing are ready. Put the cold-weather protection and pump truck in the estimate and schedule as separate line items.