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)

20 Properties of structural concrete
the cement particles. For example, BS 12 stipulates that the specific surface
of ordinary Portland cement must not be less than 225 000 mm 2 /g. Cement
particles are angular in shape, and it is usual to state the nominal particle
size in terms of standard sieve numbers. For ordinary Portland cement,
practically all particles will pass a No. 100 sieve (BS 410, 150 ,urn aperture
width) and over 95% will pass a No. 200 sieve (75 .urn).
The fineness to which the raw materials are ground affects the soundness
of the cement. A cement is said to be unsound if excessive expansion of
some of the constituents occurs after the cement has set; such expansion
causes cracking, disruption and disintegration of the mass, and hence
threatens the security of any concrete structure in which such cement has
been used. An important cause of unsoundness is the amount of free lime
(CaO) encased in the cement particles. Such hard-burnt lime hydrates very
slowly and the expansive reactions may continue for months or even years
after the cement has set. Fine grinding of the raw materials brings them
into close contact when burned, thus reducing the chance of free lime
existing in the clinker. In BS 12, an accelerated test for soundness is used;
this consists essentially in measuring the expansion of a cement paste of
standard consistence at prescribed times.
All specifications for cement prescribe some form of tests for its strength
which are usually carried out on mortar or on concrete made with the
cement. According to BS 12, compression tests are carried out on 70.7 mm
mortar cubes or on 100 mm concrete cubes. In the mortar test, cement is
gauged with a standard sand (called Leighton Buzzard sand) in the
proportions by weight of one part cement to three parts sand, and the
water/cement (w/c) ratio is 0.40 by weight. In the concrete test, the
water/cement ratio is 0.60 and the aggregate/cement ratio is to be adjusted
by trial and error such that certain prescribed workability requirements are
met. The cubes are made and stored in a prescribed manner and the
minimum strength requirements for ordinary Portland cement are:
Mortar cubes
3-day compressive strength 2:: 15 N/mm 2
7-day compressive strength 2:: 23 N/mm 2
Concrete cubes
3-day compressive strength 2:: 8 N/mm 2
7-day compressive strength 2:: 14 N/mm 2
Rapid-hardening Portland cement
Rapid-hardening Portland cement often has a higher lime content than
ordinary Portland cement and is more finely ground; in other respects the
two cements are very similar. BS 12 specifies that the specific surface must
not be less than 325 000 mm 2 I g. Practically all particles of this cement will
pass a No. 200 sieve (BS 410, 75 .urn aperture width) and about 99% will
pass a No. 350 sieve (45 ,urn).
Rapid-hardening Portland cement is capable of developing as great
strength in three days as ordinary Portland cement does in seven days. The
strength requirements in BS 12 are: