As the industry moves towards more technical concrete, the information about on-site concreting is reducing. This article has my calculations for mixing concrete on-site by weight, volume and both.
BS8500 sets out a minimum amount of cement in each Standard Prescribed Mixes:
| Grade | Minimum Cement per Cubic Meter | 28 Day Strength | Uses |
|---|---|---|---|
| ST1 | 230kg | 7.5N/mm2 | Rough blinding Infills |
| ST2 | 255kg | 10N/mm2 | Strip foundations Trench fill |
| ST3 | 295kg | 15N/mm2 | Internal floor slab that will be covered with screed |
| ST4 | 330kg | 20N/mm2 | Garage floor (not suspended) Internal floor slabs |
| ST5 | 355kg | 25N/mm2 | Suspended floor slabs |
Approved Document A specifies that ST2 is the minimum for foundations.
The mixes I’ve used are with standard strength cement class 32.5. I’ve not gone into the amount of water required or slump classes. As a rule of thumb, you could start with half of the volume cement (half a 3-gallon bucket for 1 bucket of cement), and adjust from there. With this rule, the stronger concrete will be more workable because it has higher water content.
Use my concrete estimator to work out the materials required.
Concrete mixing by weight
Useful for smaller batches where the all-in ballast has been delivered in 25kg bags, or you’ve got scales on site. The absolute maximum amount of aggregate in a mix is 7 bags (175kg) to 1 bag of 25kg cement.
| Bags of 25kg cement | Bags of 25kg all-in ballast | Number of mixes in cubic meter | Amount of cement in cubic meter (kg) | Grade |
|---|---|---|---|---|
| 1 | 7.5 | 8.8 | 220 | FAIL |
| 1 | 7 | 9.4 | 235.7 | ST1 |
| 1 | 6.5 | 10.2 | 253.8 | ST1 |
| 1 | 6 | 11 | 275 | ST2 |
| 1 | 5.5 | 12 | 300 | ST3 |
| 1 | 5 | 13.2 | 330 | ST4 |
| 1 | 4.5 | 14.7 | 366.7 | ST5 |
Concrete mixing by volume
When you’ve larger amounts of concrete to mix up, and your all-in ballast is in a bulk bag. This involves using 3-gallon buckets to measure out the cement and all-in ballast. This method gives you an option for a stronger and more workable ST2 concrete. Note that you can not achieve ST4 using half buckets, you’ll have to use a smaller container.
| Cement (3gal buckets) | All-in ballast (3gal buckets) | Number of mixes in cubic meter | Cement in cubic meter (kg) | Grade |
|---|---|---|---|---|
| 1 | 6.5 | 11.3 | 221.5 | FAIL |
| 1 | 6 | 12.2 | 240 | ST1 |
| 1 | 5.5 | 13.3 | 261.8 | ST2 |
| 1 | 5 | 14.7 | 288 | ST2 |
| 1 | 4.5 | 16.3 | 320 | ST3 |
| 1 | 4 | 18.3 | 360 | ST5 |
Hybrid concrete mixing
A mix of both weight and volume and is quite popular on sites. This method gives you an option for a stronger and more workable ST2 concrete.
Cement goes from bag to mixer and all-in ballast is measured out into 3-gallon buckets.
| Bags of 25kg cement | All-in ballast (3gal buckets) | Number of mixes in cubic meter | Cement in cubic meter (kg) | Grade |
|---|---|---|---|---|
| 1 | 8 | 9.2 | 229.2 | FAIL |
| 1 | 7.5 | 9.8 | 244.4 | ST1 |
| 1 | 7 | 10.5 | 261.9 | ST2 |
| 1 | 6.5 | 11.3 | 282.1 | ST2 |
| 1 | 6 | 12.2 | 305.6 | ST3 |
| 1 | 5.5 | 13.3 | 333.3 | ST4 |
| 1 | 5 | 14.7 | 366.7 | ST5 |
Notes
The numbers I’ve used:
Cement density: 1440kg/m3
All-in ballast density: 1650kg/m3
3 Gallon bucket: 13.638L
3 Gallon bucket of cement: 19.639kg
3 Gallon bucket of all-in ballast: 22.5kg
Sources