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:

GradeMinimum Cement per Cubic Meter28 Day StrengthUses
ST1230kg7.5N/mm2Rough blinding
Infills
ST2255kg10N/mm2Strip foundations
Trench fill
ST3295kg15N/mm2Internal floor slab that will be covered with screed
ST4330kg20N/mm2Garage floor (not suspended)
Internal floor slabs
ST5355kg25N/mm2Suspended 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 cementBags of 25kg all-in ballastNumber of mixes in cubic meterAmount of cement in cubic meter (kg)Grade
17.58.8220FAIL
179.4235.7ST1
16.510.2253.8ST1
1611275ST2
15.512300ST3
1513.2330ST4
14.514.7366.7ST5

#### 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 meterCement in cubic meter (kg)Grade
16.511.3221.5FAIL
1612.2240ST1
15.513.3261.8ST2
1514.7288ST2
14.516.3320ST3
1418.3360ST5

#### 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 cementAll-in ballast (3gal buckets)Number of mixes in cubic meterCement in cubic meter (kg)Grade
189.2229.2FAIL
17.59.8244.4ST1
1710.5261.9ST2
16.511.3282.1ST2
1612.2305.6ST3
15.513.3333.3ST4
1514.7366.7ST5

#### 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

Bluecircle Mastercrete