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)

24 Properties of structural concrete
Strength of aggregates
As stated earlier, within fairly wide limits the crushing strength of the
aggregate has little effect on the compressive strength of the concrete.
Thus, for aggregates complying with BS 882, their strengths are not an
important consideration in general structural concrete construction.
However, if a concrete of high characteristic strength is required, say
above 60 N/mm 2 , it may be necessary to use crushed-rock coarse aggregate
with a natural-sand or crushed-sand fine aggregate. Broadly speaking, the
compressive strength of a practical structural concrete cannot exce·ed that
of the aggregate used in making it, but the 'ceiling' strength of the concrete
using a particular type of aggregate depends not only on the strength but
also on the surface characteristics of the aggregate. Indeed the crushing
strengths of aggregates complying with BS 882 are generally well above
100 N/mm 2 ; strengths of above 200 N/mm 2 are not uncommon.
Unit weight of aggregates
The specific gravity of aggregate particles depends on the mineral contents;
for sand and gravel, it is usually taken to be 2.60. The bulk density, that is
the unit weight per cubic metre, further depends on the moisture content
and the degree of compaction. As a rough guide, the bulk density of sand
and gravel is about 1 700 kg/m 3 .
2.4 Water
BS 3148 gives the requirements for the testing of water for its suitability for
use in concrete making.
If the water is suitable for drinking it is generally suitable for concrete
making. If the water is suspected to be unsuitable, two series of test cubes
may be made: one series with the suspected water and another with
drinking water. The strengths and general appearances at 7 and 28 days
will provide useful information.
2.5 Properties of concrete
Strength and durability are generally considered the most important
qualities of concrete. Other important properties include creep and
shrinkage characteristics, and the elastic modulus. Fire resistance,
resistance to abrasion and thermal conductivity are sometimes important
considerations, but these will not be discussed here and the reader is
referred to specialist texts [ 5].
Unit weight of concrete
For structural design purposes, the unit weight of concrete made with
normal aggregates covered by BS 882 is usually taken as 24 kN/m 3 . This
unit weight includes an allowance for the weight of reinforcement bars.