Question 1

How is concrete volume calculated for a slab, wall, or footing?

Accepted Answer

Concrete volume = length × width × depth (thickness), with all dimensions in the same unit. For a slab: length_ft × width_ft × thickness_ft. For a wall or footing: length × height × thickness. Convert to cubic yards for ordering — U.S. ready-mix suppliers deliver by the cubic yard (1 cu yd = 27 cu ft = ~4,050 lb of standard 4,000-psi mix). A 10' × 12' × 4'' slab = 10 × 12 × (4/12) = 40 cu ft = 40/27 = 1.48 cu yd. Round-slab volume = π × r² × depth. For irregular shapes, break into rectangles and cylinders, sum, and add waste factor.

Question 2

How much waste should I add to my concrete order?

Accepted Answer

Standard construction practice per American Concrete Institute (ACI) and NRMCA guidance: add 10% waste for slabs and footings under 10 cu yd, 5-7% for larger pours (10+ cu yd amortize spillage better). Waste covers subgrade over-excavation, spillage during placement, form deflection, edge losses, and finishing takeoff. Under-ordering costs more than over-ordering: short-load minimum charges at U.S. ready-mix plants run $150-$300 for orders under 3-4 cu yd (the 'short-load fee'), and a same-day return truck usually is not available. For a 2 cu yd job, order 2.25 cu yd (2 × 1.125) minimum. Never order less than the calculated volume.

Question 3

Should I use bagged concrete or order ready-mix?

Accepted Answer

Bagged concrete (60-lb or 80-lb Quikrete/Sakrete): yields 0.45 cu ft per 60-lb bag, 0.60 cu ft per 80-lb bag. Break-even is ~1 cu yd (45× 60-lb bags at $6 each = $270, vs ~$180-$220 ready-mix plus short-load fee $200 = $400). Bagged wins for volumes under 0.5 cu yd (fence-post footings, small pads, repairs) and where a truck cannot reach the pour. Ready-mix wins above 1-1.5 cu yd on cost and above 3-4 cu yd on convenience because you don't want to hand-mix 100+ bags in a single day. Ready-mix must be placed within ~90 minutes of batching (ASTM C94), so plan crew, forms, and finishing tools first.

Question 4

What PSI strength do I need for my project?

Accepted Answer

Standard mix strengths per ACI 318 and residential building code (2021 IRC): sidewalks and residential slabs 2,500-3,000 psi; driveways, garage floors 3,500-4,000 psi (higher for freeze-thaw regions); structural footings and foundation walls 3,000 psi minimum (2018+ IRC), 4,000 psi in seismic design categories D+E; commercial slabs and heavy-traffic areas 4,500-5,000+ psi; freeze-thaw-exposed slabs require air-entrainment (ASTM C260) 5-8% by volume. Higher-psi mixes cost 5-15% more per cubic yard but pour and finish similarly. Specify air-entrained mix and 4,000+ psi for all exterior work north of the Mason-Dixon line.

Question 5

How long does concrete take to cure, and when can I use the surface?

Accepted Answer

Portland cement concrete gains strength on a well-documented curve per ACI 308R: 24 hours ~15% of design strength (walk gently, no vehicles), 3 days ~35-40% (foot traffic OK), 7 days ~65-70% (form removal for slabs, light equipment), 14 days ~80-85%, 28 days 100% (design strength — the standard 28-day test cylinder strength). Full hydration continues for months in the presence of moisture. Keep concrete moist for the first 7 days (wet burlap, plastic sheeting, or spray-on curing compound per ASTM C309) — this improves final strength 15-25% and prevents surface crazing. Don't drive vehicles onto a residential slab until 7 days minimum, and heavy loads (dump truck, RV) until 28 days.

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