GRADING OF AGGREGATES
‘Grading’ refers to the size distribution of particles within a batch or sample of aggregate. The particle size (and hence the surface area) of an aggregate is a very important factor in designing the mix proportions for concrete. In order for concrete to attain its maximum strength, all surfaces of the aggregates must be coated with cement paste. Since the surface area of one grain of sand is less than that of two grains of half the size, it follows that a concrete mix should not contain too many fine particles. Particles that are too fine will contribute to a weaker mix due to the increased surface area needing to be coated with cement paste. Alternatively, an aggregate such as uncrushed stone that is more uniform, containing particles of similar size and lacking finer particles, will require more cement paste to fill the voids that would have been filled by fine particles found in better-graded aggregates. The use of uniformly graded aggregates will result in a mix that lacks workability, and weak, porous finished concrete.
Clearly, when well-graded aggregates are used, containing the full range of particle sizes, with the smaller aggregates filling the voids between the larger stones, a strong, workable concrete mix can be produced with the minimum of cement.
Fig. 8 Grading of aggregates.