Mix design - Concrete Mix Design, Quality Control and Specification - page 204

to have the fastest flow time in a sand voids test? Should these materials be
proportioned for some other characteristic, like surface texture, absorp-
tion, density, or price?
In reality, there are many properties of aggregates that influence either
the plastic or hardened properties of concrete. In truth, it is difficult to
point to any change in concrete performance, whether in the concrete plas-
tic or hardened state, where that change can be directly correlated to a
change in an aggregate characteristic (e.g., grading, shape, texture, surface
texture, LA abrasion, Micro-Deval, sulfate soundness) other than dura-
bility issues due to contamination or reactive components. There will no
doubt be a change in concrete performance when one or a combination of
these characteristics change, but it will not be possible to predict exactly
what the magnitude of the change will be, especially over a wide range of
aggregates and concrete mix designs.
Having decided on the blend of the aggregates, the basic deviations
that may change the ideal blend must be considered. For example, varia-
tion in aggregate quality, which can be extreme. Of course, it is necessary
to take into account the accuracy and capability of the concrete batch
operation. Therefore, in proportioning these coarse and fine aggregates
and the other materials in the concrete, the material consistency and the
capability of the production process combined must ensure the robust-
ness to be able to deliver a consistent product to the placer and the speci-
fier. The changes in aggregate or cementitious quality, combined with
production variances, need to be absorbed by a mix design so that each
change does not produce a noticeable change in the concrete properties
for the placer or finisher, and wide variances are not experienced in the
concrete quality control data as covered in Chapter 10.
8.4 MATERIALS PROPORTIONING
As mentioned earlier there is a narrow workable range of concrete constitu-
ent materials when the materials are proportioned. In general, the workable
percentages for the amount of sand in the total aggregate matrix (the com-
bination of all sands and coarse aggregates) are between 30% and 60%.
These percentages assume that you are working with usable quality con-
crete sands and aggregates.
Typically anything less than 30% of sand in the aggregates blend will
yield a concrete that is very harsh or boney; it will probably have segre-
gation issues and will not be cohesive. Running the concrete through a
pump would be difficult. Likewise concrete where the sand percentage is
over 60% typically requires a lot of water to attain a given workability;
the concrete will be prone to bleeding and often a high degree of shrinkage
occurs as a result. From these rules of thumb, the workable range of coarse