Getting your concrete mix right is critical for strength and durability. Too much water weakens the mix; too little makes it unworkable. This guide covers the standard ratios and how to calculate quantities for common projects.
Standard Mix Ratios
Concrete is specified as a ratio of cement : sand : aggregate by volume:
| Mix | Cement:Sand:Aggregate | Use case |
|---|---|---|
| C10 (1:3:6) | 1 : 3 : 6 | Blinding, fill |
| C15 (1:2:4) | 1 : 2 : 4 | Bases, paths |
| C20 (1:1.5:3) | 1 : 1.5 : 3 | General, driveways |
| C25 (1:1:2) | 1 : 1 : 2 | Structural, foundations |
| C30 (1:0.75:1.5) | 1 : 0.75 : 1.5 | Heavy structural |
Calculating Quantities
Step 1: Find the volume needed
Volume (m³) = Length × Width × Depth
Example: 3m × 2m slab, 100mm (0.1m) deep:
Volume = 3 × 2 × 0.1 = 0.6 m³
Add 10% for waste: 0.6 × 1.1 = 0.66 m³
Step 2: Calculate dry materials
For a C20 mix (1:1.5:3), total ratio parts = 1+1.5+3 = 5.5
Dry volume (fresh concrete has a bulking factor of ~1.54):
Dry volume = 0.66 × 1.54 = 1.02 m³
| Material | Fraction | Volume | Density | Weight |
|---|---|---|---|---|
| Cement | 1/5.5 | 0.185 m³ | 1,440 kg/m³ | 267 kg ≈ 5.3 bags |
| Sand | 1.5/5.5 | 0.279 m³ | 1,600 kg/m³ | 446 kg |
| Aggregate | 3/5.5 | 0.557 m³ | 1,450 kg/m³ | 808 kg |
(Cement bags = 50 kg each)
Water-Cement Ratio
The most critical factor for strength:
| W/C ratio | 28-day compressive strength |
|---|---|
| 0.4 | Very strong (C30+) |
| 0.5 | Strong (C25) |
| 0.6 | Standard (C20) |
| 0.7 | Weak |
Water needed for our example (W/C = 0.5):
Water = 0.5 × 267 kg cement = 133.5 litres
Ready-Mix vs On-Site Mixing
| Volume | Recommendation |
|---|---|
| < 0.5 m³ | Bag mix (premixed) or on-site |
| 0.5–2 m³ | Mixer hire and on-site batching |
| > 2 m³ | Ready-mix truck delivery |
Setting Times
| Condition | Initial set | Final set |
|---|---|---|
| Standard (20°C) | 2–4 hours | 24 hours |
| Hot weather | 1–2 hours | 12–18 hours |
| Cold weather | 4–8 hours | 48+ hours |
Do not allow fresh concrete to freeze — protect with insulating blankets below 5°C.