Structural Design for Architecture
permits much lighter and more slender
elements to be achieved than is possible with
ordinary reinforced concrete. The quality of
both the concrete itself and the reinforcement
must, however, be higher than in ordinary
reinforced concrete and the components are
generally more sophisticated. Pre-stressed
concrete is usually more expensive to
construct than equivalent reinforced concrete
and, with the exception of the range of pre-
tensioned proprietary components which has
been mentioned above, the technique tends to
be used for architectural structures only where
a special requirement exists, such as the need
to achieve a very long span.4.3.3 The constituents of reinforced
concrete4.3.3.1 Cement
Cement is the binding agent in concrete which
possesses the cohesive strength to hold the
aggregate and reinforcement together into a
solid, composite material. A considerable
number of types of cement are used in building;
most of these are varieties of Portland cement.^9
All of the cements which are used for structural
concrete are dependent on water for the devel-
opment of strength; when water is added to dry
cement a fairly complex series of chemical
reactions takes place in a process which is
called hydration and which causes the resulting
paste to stiffen. The subsequent development of
strength takes place in two stages: initial setting
occurs fairly quickly, usually within a few hours;
a much longer period, of up to a year, is
required for the development of full strength,
although a reasonable proportion of the final
strength will normally have been developed
within seven days. The hardening of the cement
is simply a continuation of the hydration
process which produced the initial set and the
cement must be kept wet if this is to proceed
satisfactorily to compensate for water which
may be lost due to evaporation.One of the principal factors which affects the
strength of hardened cement is the
water-cement ratio, which is the ratio of the
weight of water to the weight of cement in the
wet mix. Only a small quantity of water
(around 25% by weight) is required to bring
about sufficient hydration of the cement to
cause the initial set; if more is present the
extra water remains as a separate phase in the
cement paste and becomes incorporated as a
separate phase into the cement matrix when
setting occurs. The extra water subsequently
evaporates to leave voids in the hardened
cement and this reduces its strength. It is
therefore necessary to ensure that no more
water than the minimum required to produce
initial setting is present in a mix of concrete.^10
Figure 4.26 shows the relationship between
water-cement ratio and final compressive
strength for a typical concrete.
The use of a low water-cement ratio has the
effect of making the concrete very stiff and
difficult to compact. This is a factor which has
to be considered by structural designers when
they specify the water-cement ratio which is to
be used for a concrete, because the entrain-
ment of air into the concrete, due to poor
compaction, will reduce its strength. The
minimum water-cement ratio which is prac-
ticable in a particular case depends on individ-
ual circumstances, such as the complexity of
the element in which the concrete is to be
placed and the type of equipment which will
be available for use in the compaction process.
The practical minimum for water-cement ratio
is usually in the range 0.4 to 0.5 (40-50% by
weight).
Two phenomena which are associated with
the setting and hardening of cement and which
affect the specification of concrete are shrink-
age and heat gain. The cement matrix which is
formed during the initial setting process10 More water must in fact normally be present to
produce sufficient workability (liquidity) to allow the
cement to be compacted effectively. The practical
minimum amount of water is around 40% to 50%,
depending on the system being used for compaction.9 So called because of its supposed resemblance to
122 Portland stone.