F. K. Kong MA, MSc, PhD, CEng, FICE, FIStructE, R. H. Evans CBE, DSc, D ès Sc, DTech, PhD, CEng, FICE, FIMechE, FIStructE (auth.)-Reinforced and Prestressed Concrete-Springer US (1987)

22 Properties of structural concrete
maximum size actually used in a job depends on the dimensions of the
concrete member. Particles too large in relation to these dimensions may
affect the strength adversely, particularly the flexural strength; a good
practice is to ensure that the maximum size does not exceed 25% of the
minimum thickness of the member nor exceed the concrete cover to the
reinforcement (i.e. the clear distance between the reinforcement bar and
the formwork). There is also the important requirement that the aggregate
should be small enough for the concrete to flow around the reinforcement
bars so that it can be adequately compacted. For example, BS 8110 [3]
recommends that
(a) the nominal maximum size of the coarse aggregate should be less than
the horizontal clear distance between the reinforcement bars by at
least 5 mm; and
(b) it should not exceed 1! times the clear vertical distance between the
bars.
Apart from the above considerations, it is advantageous to use the
higher maximum sizes because, in general, as the maximum size of the
aggregate increases, a lower water/cement ratio can be used for a given
workability, and a higher strength is obtained. This applies for a nominal
maximum size of up to 40 mm; above this, the gain in strength due to the
reduced water/cement ratio is offset by the adverse effects of the lower
bond area between the cement paste and the aggregate and of the
discontinuities caused by the large particles.
Shape and surface texture of aggregates
In practice, the shapes of aggregates are usually described by terms such as
rounded, irregular or angular, which are necessarily imprecise; similarly
terms such as smooth or rough are used to describe surface textures.
The particle shapes affect the strength of the concrete mainly by
affecting the cement-paste content required for a given workability. If the
cement content is the same, then an angular aggregate would require a
higher w/c ratio than an irregular one, which in turn will require a higher
w/c ratio than a rounded one.
The surface texture affects concrete strength in two ways. First, it affects
the bond between the cement paste and the aggregate particles; second, it
affects the cement-paste content required to achieve a given workability.
On balance a rough surface results in a higher concrete strength,
particularly flexural strength.
The above remarks apply to both coarse and fine aggregates.
Grading of aggregates
For a concrete to be durable, it has to be dense and, when fresh, it should
be sufficiently workable for it to be properly compacted. The mortar, i.e.
the mixture of cement, water and fine aggregate, should be slightly more
than sufficient to fill the voids in the coarse aggregate; in turn the cement
paste should be slightly more than sufficient to fill the voids in the fine
aggregate. The voids in an aggregate depend on its particle-size distribution,
or grading. The grading of the aggregates affects the strength