delivered to the site in liquid, ready-mixed
form. The mix proportions can, however, be
specified by the building's designer and
concrete is therefore one of the few materials
whose properties can be controlled directly by
the designers of buildings. The properties of
reinforced concrete depend on those of the
constituent concrete and of the reinforcement
and on the location of the reinforcement in the
structural elements.
Reinforced concrete is therefore a complex
material which places certain demands on the
designer who wishes to exploit its potential
fully. The successful use of such a material
must be based on a knowledge of its basic
properties and its behaviour in response to
load. These aspects of concrete technology are
not considered here in detail, although the
principal issues are briefly described.
4.3.2 The terminology of concrete
The constituents of concrete are cement paste,
which acts as a binding agent, and aggregate,
which is an inert filling material. The aggregate
usually consists simply of small pieces of
natural stone and it acts both as a bulking
agent and to impart dimensional stability to
the concrete. Concrete is made by mixing
together appropriate quantities of cement and
aggregate in the dry state and adding sufficient
water to hydrate the cement. After the water is
added, a chemical reaction occurs which
causes the concrete to become solid within a
few hours in what is called the 'initial set'. A
considerable period of time is required before
it develops its full strength, however, and this
latter process is called the 'final setting' or
'hardening' of the concrete. The time required
for final setting varies, depending mainly on
the type of cement which is used, but a typical
concrete will have developed about 80% of its
full strength within three months of the initial
set.
The properties of concrete which are of
principal interest to the building designer are
its liquidity when it is in the 'fresh' state and
its strength when in the hardened state. The
liquidity of fresh concrete is referred to as its
'workability' and this property affects the ease
Fig. 4.21 The principal reinforcement in reinforced
concrete is placed in the locations where tension occurs
due to the bending effect of the load. Secondary reinforce-
ment (not shown) is placed throughout the structural
elements to resist the tensile stress which occurs due to
the shrinkage associated with the curing process and with
thermal movement.with which it can be 'placed' and 'compacted'
to form a dense solid. This is an especially
important consideration where elements of
complex geometry are being constructed or
where the concrete must be compacted around
complicated patterns of reinforcement. High
workability is required in these situations and
this can adversely affect the final strength of
the concrete (see Section 4.3.3.3).
Plain concrete in the hardened state is a
material which has moderate compressive
strength (typically between 20 N/mm^2 and
60 N/mm^2 depending on the mix proportions)
but very low tensile strength (usually about
one tenth of the compressive strength). When
steel bars are incorporated into concrete the
resulting composite material is called
reinforced concrete. This has greatly improved
tensile and bending strength, the precise
strength properties being dependent on the
amount of reinforcement which is used and its
location in element cross-sections. In elements 119Reinforced concrete structures