Understanding Concrete for DIY Projects
Concrete is a mixture of cement, sand, gravel (aggregate), and water. When mixed properly and cured, it becomes one of the strongest and most durable building materials available. It's used for foundations, driveways, patios, sidewalks, and much more.
Key terms to know:
- Cement: The binding powder (Portland cement is most common)
- Concrete: The finished mix of cement + aggregates + water
- PSI: Pounds per square inch - measures compressive strength
- Curing: Keeping concrete moist for 7+ days to gain strength
- Rebar: Steel reinforcement for added tensile strength
Concrete Slab Thickness Guide
| Application | Thickness | PSI |
|---|---|---|
| Sidewalks & Pathways | 4 inches | 3,000 |
| Patios | 4 inches | 3,000-3,500 |
| Residential Driveways | 4-5 inches | 3,500-4,000 |
| Heavy Vehicle Driveways | 5-6 inches | 4,000+ |
| Garage Floors | 4-6 inches | 3,500-4,000 |
| Shed/Workshop Foundations | 4 inches | 3,000 |
Pre-Mix Bags vs Ready-Mix Truck
🛒 Pre-Mix Bags (Quikrete/Sakrete)
Best for: Under 1 cubic yard
- 60 lb bag = 0.45 cu ft
- 80 lb bag = 0.60 cu ft
- Cost: $5-8 per bag
Pros: No minimum order, work at your own pace, easy for small repairs
Cons: Labor intensive, more expensive per yard for large projects
🚚 Ready-Mix Truck Delivery
Best for: 1+ cubic yards
- Minimum order: Usually 1-3 yards
- Cost: $120-180 per yard + delivery
- Short load fee for under minimum
Pros: Consistent mix, faster pour, less labor
Cons: Must pour quickly, need helpers, minimum order
How to Calculate Concrete Volume
Cubic Yards = (Length × Width × Thickness) ÷ 27
Where all measurements are in feet. If thickness is in inches:
Cubic Yards = (Length × Width × (Thickness ÷ 12)) ÷ 27
Bags needed:
- 60 lb bags: Cubic Yards × 60
- 80 lb bags: Cubic Yards × 45
- Always add 10% for waste, uneven ground, and spillage
Example: 10×12 ft Patio Slab
Scenario: You want to pour a 10 ft × 12 ft patio with 4 inches of concrete.
- Convert thickness: 4 in ÷ 12 = 0.333 ft
- Calculate volume: 10 × 12 × 0.333 = 40 cubic feet
- Convert to yards: 40 ÷ 27 = 1.48 cubic yards
- Add 10% waste: 1.48 × 1.10 = 1.63 cubic yards
- 80 lb bags: 1.63 × 45 = 74 bags
Recommendation: Order 2 cubic yards from a ready-mix truck (~$300-400 delivered) vs. buying 74 bags (~$450+ plus heavy labor)