Understanding Cement and Concrete
Cement is the binding ingredient in concrete, not concrete itself. Concrete is made by mixing cement (Portland cement), sand (fine aggregate), gravel or crushed stone (coarse aggregate), and water. The cement reacts with water in a chemical process called hydration to form a hard, stone-like material that binds the aggregates together.
The mix ratio determines the strength and workability of the concrete. A 1:2:3 mix (1 part cement, 2 parts sand, 3 parts gravel) is standard for most construction applications, producing concrete with approximately 3,000 psi compressive strength after 28 days of curing.
Concrete Volume Formula
Common Mix Ratios
| Mix Ratio | Strength (psi) | Application |
|---|---|---|
| 1:1:2 | 5,000+ | Structural, columns, high-load |
| 1:1.5:3 | 4,000 | Foundations, driveways |
| 1:2:3 | 3,000 | General purpose, sidewalks |
| 1:2:4 | 2,500 | Footings, non-structural |
| 1:3:6 | 1,500 | Mass concrete, fill |
Mixing Tips
- Use clean, potable water for mixing; impurities weaken concrete.
- The water-to-cement ratio should be 0.45-0.55 for optimal strength.
- Too much water makes concrete easier to pour but much weaker.
- Cure concrete for at least 7 days by keeping it moist; 28 days for full strength.
- Do not pour concrete when temperatures are below 40°F (4°C) without cold weather precautions.
Frequently Asked Questions
How many bags of cement for 1 cubic yard?
For a standard 1:2:3 mix, you need approximately 5-6 bags of 94-lb Portland cement per cubic yard of concrete. Pre-mixed bags (like Quikrete 80 lb) require about 45 bags to make 1 cubic yard.
What is the difference between cement and concrete?
Cement is the powder (Portland cement) that acts as the binding agent. Concrete is the finished product made from cement, sand, gravel, and water. Using "cement" to mean "concrete" is a common misnomer. You drive on concrete, not cement.
When should I use ready-mix vs. mixing my own?
For projects over 1 cubic yard (27 cu ft), ready-mix concrete delivered by truck is more economical and provides more consistent quality. For small projects under 1/2 cubic yard, bagged premix is convenient. Site-mixed concrete from raw materials is best for very large projects or when ready-mix is unavailable.