AGGREGATE/CEMENT RATIO OF CONCRETE MIXES

There are two key considerations when designing concrete mixes for different applications: the ratio of aggregate to cement and the ratio of water to cement.

The aggregate/cement ratio relates to the proportions of dry materials in the mix. The characteristics of finished concrete, including its strength, can be directly affected by the amount of cement in relation to aggregates. Mix proportions are usually specified as a ratio of cement to fine aggregate to coarse aggregate, either by weight or by volume. For example, a 1:3:6 mix contains one unit of cement, three units of fine aggregate and six units of coarse aggregate. It is vital that there is sufficient cement within a mix to bind all of the aggregates together, otherwise weak patches will occur in the finished concrete. On this basis, the higher the cement content in relation to aggregates, the stronger the concrete. For example, a mix of 1:1:2, proportionally having a very high cement content, will produce very strong, impermeable, frost-resistant and durable concrete.

As mentioned previously, fine aggregate and coarse aggregate can be purchased together as all-in ballast but it should not be assumed, when proportioning by volume, that 4m3 of fine aggregate when mixed with 8m3 of coarse aggregate will produce 12m3 of all-in ballast. Volume shrinkage upon mixing of around 30 per cent will occur, caused by the finer particles filling the voids within the coarse aggregate. The resulting volume of all-in ballast will be around 9m3. Accordingly, great care must be taken when specifying the quantity of aggregates.

Typical Concrete Mix Proportions

Cement Fine agg’

Coarse

agg’

Application Cement All-in

ballast

1

1/2

1

Benching in a 1 manhole

1

1

1

2

Chimney flaunching 1

2

1

2

4

General exterior

work, casting

in moulds, concrete 1 3 or 4

patching

1 3 6 Floor slabs and 1 5 or 6

foundations