STRUCTURAL DESIGN FOR ARCHITECTURE

Structural Design for Architecture

Fig. 4.27 If aggregate is to form an effective bulking

agent the relative sizes of the particles present must be

well matched. This has occurred in the sample on the left

here, where only small voids are left to be filled with

cement, but not in that on the right.

distribution of particle sizes is satisfactory.

This is determined by carrying out a sieve

analysis, which involves passing a sample of

the aggregate through a series of progressively

smaller sieves and noting the fraction which is

retained on each (Fig. 4.28). The percentage of

the whole sample which passes each sieve is

then plotted to give a grading curve (Fig. 4.29).

The relevant British Standard (BS 882) sub-

divides aggregates into coarse aggregate, in

which particles larger than 5 mm in diameter

predominate, and fine aggregate or sand, in

which particles smaller than 5 mm in diameter

predominate. The grading limits for each type

which are considered to be suitable for making

concrete are specified in the Standard by defin-

ing zones within which the grading curves of

the fine and coarse aggregates must fall.

Where a naturally occurring aggregate with a

suitable grading is not available, an aggregate

with a specific grading can be produced artifi-

cially by mixing together appropriate quan-

tities of screened, single-sized batches of

crushed rock fragments.

Aggregate particles are classified into three

categories according to their shape and are

said to be either rounded, irregular or angular.

Fig. 4.28 The grading of a

naturally occurring aggre-

gate is determined by a

sieve analysis. In the nest

of sieves shown the mesh

size diminishes down the

stack. To carry out the

analysis the sample is

introduced at the top and

the weight of particles

which are retained at each

level determined. This

allows grading curves to be

plotted.

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