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)

Aggregates 21
Mortar cubes
3-day compressive strength ~ 21 N/mm 2
7-day compressive strength ~ 28 N/mm 2
Concrete cubes
3-day compressive strength ~ 12 N/mm 2
Though rapid-hardening Portland cement has a higher rate of hardening
and a higher rate of heat development than ordinary Portland cement, the
setting times of the two cements are similar. In BS 12, the setting-time
requirements are the same for both; similarly, the requirements for
soundness are the same.
Unit weight of cement
The specific gravity of the particles of cement is generally within the range
of 3.1-3.2; for most calculations, it is taken as 3.15. The unit weight of
bulk cement depends, obviously, on the degree of compactness; as a rough
guide, it may be taken as 1450 kg/m 3 . In the UK, one bag of cement weighs
about 50 kg.
2.3 Aggregates
Aggregates used in concrete making are divided into two categories: (a)
the coarse aggregate such as crushed stone, crushed gravel or uncrushed
gravel, which consist of particles that are mainly retained on a 5 mm sieve,
and (b) the fine aggregate such as natural sand, or crushed stone sand or
crushed gravel sand, which consist of particles mainly passing a 5 mm sieve.
It is a most important requirement that the aggregate should be durable
and chemically inert under the conditions to which it will be exposed.
Other important requirements concern the size, the shape, the surface
texture, and the grading, which are discussed below. Surprisingly enough,
the compressive strength of concrete is not much affected by the strength
of the aggregates unless they are very weak; the flexural strength of
concrete, however, is more affected by weak aggregates.
In this section we shall restrict our discussions to aggregates from natural
sources such as those referred to above; these aggregates are covered by
BS 882. However, it is worth noting that, despite the present-day
engineer's preference for natural aggregates, he will find these in
increasingly short supply in the not too distant future. To quote an ACI
report: 'In many areas, supplies and reserves of naturally occurring
aggregates will become depleted and increased emphasis will therefore be
placed on manufactured aggregates, many of which are lightweight' [4]. In
the UK, lightweight aggregates for concrete should comply with BS 3797.
See also BS 8110: Part 2: Clause 5.
Size of aggregates
In reinforced and prestressed concrete construction, nominal maximum
sizes of the coarse aggregates are usually 40, 20, 14 or 10 mm. The nominal