STRUCTURAL DESIGN FOR ARCHITECTURE

Fig. 4.26 The relationship between the ultimate strength

of concrete and the water/cement ratio. This graph demon-

strates that the more fluid the wet concrete is the lower

will be the ultimate strength. This is because relatively

little water is required to cause the initial set of concrete.

If more is present it forms separate pockets of pure water

in the setting concrete which ultimately evaporate to leave

small voids which weaken the concrete.

shrinks subsequently as the hardening process
proceeds. Shrinkage, by itself, is not necessar-
ily harmful to a concrete component,
especially if the element is not restrained
during the hardening process (a small precast
concrete lintel, for example). Where an
element is restrained, however, such as an in
situ reinforced concrete beam or slab which is
part of a large frame, then the effect of shrink-
age is to generate tensile stress in the element
which may cause it to crack. Shrinkage is there-
fore a phenomenon which must be considered
during the design of a concrete structure.
Normally it is controlled by the incorporation
into the structure of suitable reinforcement;
this simply carries the stress which results
from the shrinkage. Occasionally, movement
joints are used to control shrinkage cracking. A
typical example of this is found in lightly
reinforced non-structural ground floor slabs.
Heat gain is a phenomenon which occurs
because the chemical processes which are
involved in the hydration of cement are

exothermic. The increase in temperature which

results from this tends to accelerate the

process of hydration and this can produce

cracking of elements because it also causes

high rates of shrinkage. The heat of hydration

rarely causes a problem in architectural struc-

tures, however, because the elements are

normally slender enough to allow sufficient

cooling to occur to maintain temperatures at

an acceptable level. Where an element is of

large bulk the use of special 'low-heat' cements

is sometimes required.

4.3.3.2 Aggregate

Aggregate, being cheaper than cement, is used

as a bulking agent in concrete and, typically,

will account for 75% to 80% of its volume. It

also serves to control shrinkage and to

improve dimensional stability. It normally

consists of small pieces of stone, of various

sizes in the form of either naturally occurring

sand or gravel, or crushed rock fragments;

other materials, such as crushed brick, blast

furnace slag or recycled building materials, are

sometimes used. Aggregate must be durable,

of reasonable strength, chemically and phys-

ically stable, and free of constituents which

react unfavourably with cement.

The proportions which are present of the

differently sized particles which occur in an

aggregate are referred to as its grading and if

the aggregate is to be effective as a bulking

agent the grading must be such that a particu-

lar distribution of particle sizes occurs. The

smaller particles should ideally be of such a

size and quantity as to fully occupy the voids

between the larger particles and leave a

minimum volume to be filled by cement (Fig.

4.27). The grading of aggregate also affects the

workability of the concrete, which is better

when a higher proportion of fine particles is

present. This is a factor which indirectly affects

strength because a well-graded aggregate

allows a required workability to be achieved

with a lower water-cement ratio than a poorly

graded aggregate.

If a naturally occurring aggregate is used for

concrete, the designer has no control over the

grading but must ensure that the range and 123

Reinforced concrete structures

Vibration

Hand compaction

Fully compacted concrete

Insufficiently

compacted concrete

Water/cement ratio

Compressive strength