Mix design - Concrete Mix Design, Quality Control and Specification - page 210

HPCs will often contain fly ash or slag, and silica fume. To accommodate
this extra volume of powder, the optimum volume of sand will decrease.
This may sound completely the opposite or counterintuitive compared
with current practice in a mix design. But this point should be considered in
the context of the method of mix design. Water is to be kept to a minimum,
and the amount of water required to design and manufacture concrete with
this methodology is based on the water content being limited to getting the
concrete to a workability that is described as plastic.
Getting the desired workability from this stage onward should be
achieved by the addition of mid- or high-range water reducers. Following
the result is that, in the majority of situations, this procedure for concrete
will have the following differences to “conventional” concretes:
1. The total cost of materials will be lower.
2. The volume of cementitious material required to achieve a given
compressive strength will be lower.
3. The amount of admixture used will be higher (this is offset by the
reduction in cementitious material).
4. Sand contents are higher.
5. The concrete will be preferred by pumpers, placers, and finishers.
6. Changes in workability, from 50 mm through to genuine self-
consolidating concrete can be achieved from the same mix design,
just altering the admixture dosage.
The potential disadvantages that can come from this methodology are
when
1. The sand cost is prohibitively high.
2. The sand is of very low quality.
3. There is an imbalance in the admixture and cementitious material
delivered costs.
When proportioning concrete this way, it is important to follow the admix-
ture addition recommendations. Typically, the later the water reducers or
plasticisers are added, the more effective they are or the less required. This
is critical when considering cost of the materials to make the concrete.
Users may be pleasantly surprised by the performance of the concretes
that are proportioned using this method, the same mix design with the
exception of the admixture dosage can cover a wide range of workabilities.
In some cases, designing mixes this way has allowed some “conventional”
concrete designs to behave as high-quality self-compacting concretes just
with the addition of extra admixture, a real bonus when aggregates or
material storage or bin numbers is an issue at the plants.