STRUCTURAL DESIGN FOR ARCHITECTURE

particles is larger than if the same bulk is pro-

vided with a smaller number of larger particles.

A final consideration in relation to the

aggregate which is used in concrete is the

maximum particle size of the coarse fraction.

This must usually be restricted, depending on

the size of the elements in which the concrete

will be cast and on the complexity of the

reinforcement pattern. The maximum size of

aggregate is normally no greater than 25% of

the minimum thickness of elements and at

least 5 mm less than the clear distance

between reinforcing bars.

4.3.3.3 Specification of concrete

The specification of concrete has two aspects,

which are the selection of the constituents,

that is of the type of cement and type of aggre-

gate, and the determination of the mix propor-

tions. The latter process is called the 'mix

design'.

So far as the selection of materials is

concerned the type of cement is determined by

the requirements in respect of rate of harden-

ing, heat gain and resistance to chemical

attack; all Portland cements achieve approxi-

mately the same final strength for a given set

of mix proportions. It is usual to select

naturally occurring aggregates for concrete, if

these are available; they are usually specified

in two batches, fine aggregate (sand) and

coarse aggregate, and the grading of these

must normally conform to the limits which are

specified in the relevant standard (e.g. BS 882).

Where a crushed rock type is used this too

must be correctly graded. Crushed rock aggre-

gate will normally require a higher water

content to achieve a given workability and will

therefore produce a weaker concrete than a

naturally occurring aggregate. The workability

of a concrete which is made with crushed rock

can be improved by increasing the proportion

of fine particles present. This allows the use of

a lower water content, which improves the

strength, but it results in a higher cement

content being required, which increases the

cost of the concrete.

The mix proportions of a concrete are speci-

fied in terms of the weights of materials which 125

Reinforced concrete structures

Fig. 4.29 Aggregate grading curves. Following sieve
analysis a sample of aggregate will produce a grading
curve similar to those depicted. Its suitability as a bulking
agent in combination with other aggregates can then be
judged.

Naturally occurring aggregates tend to be
rounded while crushed rock types are predom-
inantly angular. The significance of this factor
is that it affects the workability of the concrete:
aggregates with rounded particles produce a
more workable mix for a given water-cement
ratio than do those with angular particles. It is
therefore possible to achieve a particular
workability with a lower water-cement ratio if
the aggregate is rounded than if it is angular,
and this in turn produces a stronger concrete.
Aggregate shape, like grading, therefore has an
indirect effect on concrete strength and, if
other things are equal, naturally occurring
aggregates will produce stronger concrete than
artificially produced aggregates.
The grading of an aggregate can affect the
cost of concrete because it affects the quantity
of cement which must be provided to produce
a given volume of concrete. If a large number
of fine particles are present, the total surface
area of the aggregate is high and the quantity
of cement which is required to coat all of the

BS sieve sizes

Percentage passing