Mix design - Concrete Mix Design, Quality Control and Specification - page 205

aggregates in a concrete mix will be between 40% and 70% of the total
aggregate volume.
Once the aggregate blend has been identified, the remaining two key
ingredients to complete the matrix are water and binder. (In this example
we will assume we are using a single Ordinary Portland Cement).
One of the oldest rules of concrete is that the compressive strength of the
final concrete will have a relationship to the water to cement ratio that is
used to make that concrete. It may come as a surprise to many, but when
the solid constituents that make up the vanilla concrete mix design (sand,
coarse aggregate, and cement) are mixed together in the workable propor-
tions noted earlier, the water required to take the matrix to its plastic limit
(the point at where the matrix moves from a semisolid to a plastic state)
doesn't change a great deal from very low cement contents (180 kg/m 3 ) to
very high cement contents (700 kg/m 3 ). The total change in water required
to reach the plastic limits of the matrix of materials will only change by
approximately 10% across the workable range of concrete designs, with a
higher cement content resulting in the slightly higher water demand.
As a quick check, the absolute volume of water per cubic metre or yard
that is used in a batch plant should be within this 10% range. Obviously,
there will be some outlying cases where this is not true, but for most nor-
mal concrete this should be the case. We now know that of the four basic
constituents (sand, coarse aggregate, cement, and water), one constituent
(water) is going to be fairly constant. We also know that the compres-
sive strength should be a function of the w/c ratio. If there is a prescribed
cement content or w/c ratio, and the absolute volume of water required is
known, two of the four components of the mix design have their volumes
determined.
For example, if there is a specification that requires a minimum 350 kg
cement per m 3 , and we know that the volume of water required to make the
given constituent materials plastic is 170 kg, then 520 kg of the mix design
is determined. Alternatively, if the required mix design needs to have a
maximum w/c of 0.50, the amount of cement required to make the matrix
plastic will be X /170 = 0.50, therefore X = 340 kg.
So the question is how to ascertain the amount of water required to get
the matrix of materials plastic. There are two very simple methods. The
first is where the materials are available in a laboratory situation. We would
suggest combining for simplicity the coarse and fine aggregates together
50/50 either by weight or by volume. Add to the mix of aggregates a mid-
range quantity of cement determined by the cement content that is cur-
rently use for a given compressive strength, or if dealing with new materials
pick a cement content of say 300 kg/m3. Add these materials to the mixer
and then add water until the concrete just becomes plastic. (The state at
which the concrete has become plastic if you were measuring by a slump
cone would be just “wet” enough to have some slump, no more than 10 mm