Mix design - Concrete Mix Design, Quality Control and Specification - page 209

gaps in the particle distribution will give a higher packing density than
that of evenly graded.
To really understand the aggregates, sand, and powders that you are
using in concrete, the grading should be expressed as individual percent-
ages retained on a given sieve or size measurement. This is particularly
important when it comes to the finer particles. This is important because if
you have too many particles that are in the same size range, their packing
efficiency will be very low (similar sized particles pack together inefficiently
leaving large void spaces that need to be filled by even finer particles) and
this will have a detrimental impact on the quality of the concrete, or the
concrete mix will require a higher water to cement ratio or more cement to
reach a desired workability. This will obviously have a negative impact on
cost, shrinkage, heat generation, and so on.
When considering volumetrics in concrete, the intention is to fit the finer
particles between the voids left by the larger particles. Typically, the larger
the particle in a concrete mix, the lower the cost per ton of that material.
The amount of paste required to make a plastic concrete mixture (in
this case paste being cementitious materials, sand particles less than
100 micron, and water) equals the total volume of voids space in the aggre-
gate matrix plus 3% or 4% more absolute volume to allow the system to
“float”, or become hydraulic.
In general, when you have the option to use more than one coarse aggre-
gate in a concrete design, it will yield better results than using a single-
sized aggregate. It is very difficult to try to get a single coarse aggregate to
have an even grading (a grading that will give a low voids content) and not
be prone to segregation. In the United States, the most common sized con-
crete coarse aggregate is a “57”, which in metric units is a 20 mm-5 mm
material.
The 57 grading covers too many sieve sizes and is prone to segregation,
as well as being very sensitive to the inconsistencies in aggregate produc-
tion. The result is a very inconsistent grading that can go from very fine to
very coarse. Obviously, as the grading changes, so do many of the other
properties of the aggregate. One such major change is the voids content
of the coarse aggregate. Because the voids content on the coarse aggregate
fluctuates, the volume of finer materials that are required to fill those voids
(sand, cementitious, and water) will also change.
In the majority of cases where the mix design methodology being
described here has been employed, and acceptable quality natural con-
crete sands and coarse aggregates (both natural and crushed) are used,
the rough percentage of sand to that of coarse aggregate for everyday
concrete (10-35 MPa, 50-200 mm slump) yields an ideal sand percentage
(by volume) of around 55%, with that percentage decreasing to around
45% when you get into the high powder factor concretes that are typi-
cally deemed high-performance concrete (HPC) in excess of 500 kg/m 3 .