Calculate the exact volume of concrete needed for slabs, footings, columns and steps — in cubic yards, cubic metres and bags of premix.
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Multiply length × width × depth to get the volume, then convert to cubic yards (divide by 27, since 1 cubic yard = 27 cubic feet). For a 10 ft × 10 ft slab at 4 inches thick: (10 × 10 × 0.333) / 27 = 1.23 cubic yards. Always add 5–10% for spillage and overfill.
It depends on bag size: sixty 80 lb bags, ninety 60 lb bags, or forty-five 90 lb bags yield approximately one cubic yard of mixed concrete. For small jobs under 1 cubic yard, premix bags are convenient. For larger jobs, ordering ready-mix by the cubic yard is more economical.
A standard structural mix is 1 part cement : 2 parts sand : 3 parts aggregate (1:2:3) with a water-to-cement ratio of about 0.5. For driveways and slabs, a 4,000 PSI mix is common (roughly 1:1.5:3). Premix bags already contain the correct proportions — just add water as directed.
Typical thicknesses: 4 inches for residential driveways and pathways, 5–6 inches for heavy vehicle driveways or workshop floors, 3.5 inches for residential patios, and 8–12 inches for foundation footings. Always check local building codes for structural slabs — they specify minimum thickness and rebar requirements.
Concrete reaches about 70% of its design strength in 7 days and full design strength (100%) in 28 days. You can walk on it after 24–48 hours and drive on it after 7 days, but it continues to strengthen for months. Curing conditions matter: keep it moist and above 50°F (10°C) for the first 7 days for best results.
A standard ready-mix concrete truck holds 8–10 cubic yards (a full load). Most suppliers have a minimum order of 1 cubic yard, with short-load fees for small orders. For projects under 1 cubic yard, consider volumetric mixer trucks (which mix on-site) or purchasing premix bags from a hardware store.
Fresh concrete shrinks slightly as it cures — typically 0.04–0.06% by volume. On a 20 ft slab this is about 1/8 inch. Control joints (saw cuts) are placed every 8–12 feet to direct where shrinkage cracks form, keeping them straight and manageable. This is why you see those grid lines cut into concrete driveways.
Ready-mix concrete typically costs $120–$200 per cubic yard in the US, depending on region, mix design and delivery distance. Short-load fees of $15–$25 per cubic yard under the minimum apply on small orders. Premix 80 lb bags cost $5–$7 each — more expensive than ready-mix per cubic yard for large projects.
For structural slabs (driveways, footings, foundations) yes — rebar or wire mesh is typically required. Residential driveways commonly use #4 rebar (1/2 inch diameter) on 18-inch centres. Decorative patios and pathways may use only wire mesh. Always check local building codes. Rebar significantly increases tensile strength and crack resistance.
Concrete should not be poured when temperatures are below 40°F (4°C) or when freezing is expected within 24 hours without protective measures. In cold weather, use heated water or aggregates, hot-mix concrete, insulating blankets, and cold-weather additives. Concrete that freezes before setting loses strength permanently.
