Concrete mix is a combination of cement, sand (fine aggregate), gravel (coarse aggregate), and water. The ratio and PSI strength rating determine the mix's durability and load-bearing capacity.
PSI Strength Ratings
- 2,500 PSI: Residential sidewalks and patios
- 3,000 PSI: Driveways and garage floors
- 4,000 PSI: Commercial floors, exterior slabs
- 5,000+ PSI: Structural applications, bridges
Calculating Volume
Concrete is measured in cubic yards. For a rectangular slab: Volume = Length (ft) × Width (ft) × Thickness (ft) ÷ 27. Always add 5-10% for waste.
Real-World Example
A 20' × 10' patio, 4 inches thick: Volume = 20 × 10 × (4/12) ÷ 27 = 2.47 cubic yards. With 10% waste: 2.72 cy. At $150/cy delivered: approximately $408. In 80-lb bags (0.6 cf each): need about 112 bags.
Frequently Asked Questions
How thick should a concrete slab be?
Sidewalks: 4 inches. Driveways: 4-6 inches. Garage floors: 4-6 inches. Structural foundations: 8-12 inches. Thicker slabs with rebar are needed for heavy loads like RVs or commercial vehicles.
What is the standard concrete mix ratio?
A common ratio is 1:2:3 — one part cement, two parts sand, three parts gravel. The water-to-cement ratio is typically 0.45-0.55. Too much water weakens the concrete; too little makes it unworkable.
How long does concrete take to cure?
Concrete reaches about 70% strength in 7 days and full strength (design PSI) in 28 days. It should be kept moist for the first 7 days. You can walk on it after 24-48 hours and drive on it after 7 days.