No_5_February_1963 — Page 116

preferably contain

mineral con- stituents that will give off gases at that temperature: alternatively, gas- forming substances may be added.

a

Expanded aggregates of this type may be produced either as rounded pellets in a rotary kiln, or as frag. mented particles from the breaking of the porous cake formed on sinter-strand. Both forms are manu- factured in Britain, using clay as the raw material. The aggregates are used for block-making, for cast in situ work, precast reinforced con- crete structural units and. recently. in pre-stressed concrete units.

Expanded slate

Certain types of slate-waste. when heated quickly, expand and bloat in much the same way as clays and shales, to give a porous lightweight material suitable as an aggregate. The process has not been exploited in Britain although it has been shown to be technically practicable with British slates, but there has been some commercial production America and in Eastern Germany.

Sintered pulverized-fuel ash

in

Pulverized-fuel ash or 'fly-ash' is the residue from the burning of pow- dered coal. It is a grey powder. of fineness close to that of Portland cement. When the ash is moistened and made into pellets, and these are fired at about 1200°C. hard porous nodules or pellets are formed which are crushed and graded give the lightweight aggregate. Supplies of raw material are very plentiful. for about 6 million tons of pulverized- fuel ash are produced anually and the output is increasing.

The aggregate is produced and marked in Britain and is suit- able for block-making, lightweight screeds and reinforced concrete. When fully compacted, the concrete is capable or giving high strength at a moderate density.

Pumice

Pumice is a spongy lava which in its natural state makes a good light- weight aggregate, needing only to be crushed, washed to remove dust and clay and graded. It density varies from 30 to 55 lb/ft3 according to its origin and grading.

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Although formerly used in Great Britain, it is no longer readily avail- able there, but is still widely used in many countries where it occurs.

Expanded vermiculite

a

Vermiculite, a mineral resembling mica which is mined chiefly in America and Africa, is flaky material, each flake being formed of several thin layers. When heated rapidly to between 650' and 1000°C. depending on the particular material. the flakes open out or exfoliate, to give particles resembling concertinas. The product is very light, weighing only some 4 to 12 lb/ft.3 Expanded or exfoliated vermiculite has been used in Britain for some years. both as an aggregate for plaster and for very lightweight concrete. The concrete has verv low structural strength but very high resistance to heat flow; it is used almost entirely as in situ screeds for the insulation of flat roofs.

Perlite

Perlite is a glassy volcanic rock occurring in Western America, Italy. Northern Ireland and other areas. When it is heated rapidly to the point of incipient fusion it expands to form a very light cellular material having a density of about 5 to 15 lb/ft3. Like exfoliated vermiculite, it pro- vides a concrete of high thermal in- sulation and low strength. Perlite concrete is used to a limited although increasing extent in Britain.

Organic aggregates

Sawdust is used as the principal aggregatet in at least one proprietary type of 'composition' building block, and wood-shavings are used in wood wool cement slabs and similar pro- ducts, but such materials do not fall within the scope of this Digest.

PROPERTIES OF

LIGHTWEIGHT AGGREGATE CONCRETES

Lightweight aggregate concretes are used primarily for the sake of the saving in weight and the degree of thermal insulation that they afford. The wide range of aggregates avail- able makes it possible to prepare concretes with a correspondingly wide variation in density, thermal insulation and strength to suit dif

ferent purposes. The range of pro- perties obtainable is shown in Table 1.

As a very broad generalization, it may be said that lightweight aggre. gate concretes are not as light as aerated concretes, nor is their insula tion value as high, but they are capable of giving much higher strength. However, the lightest of the aggregates -- exfoliated vermicu lite give weak concretes that are comparable in lightness and thermal insulation with low-density aerated

concrete.

The properties of the concrete depend not only on the nature of the aggregate, but also on factors such as the grading of the aggregate, the proportion of cement. the water cement ratio and the degree of com paction. The density. thermal in. sulation and strength of the concrete are largely inter-dependent; general- ly speaking, the denser the concrete. the less its thermal insulation. but the greater its strength.

With many lightweight aggregates. the angularity and rough surface of the particles leads to poor workabil ity of the concrete mix. To obtain a sufficient degree of compaction in cast in situ construction it may be necessary to improve the workability by using a richer mix, or by adding sand or other fine aggregate; foam- ed slag coarse aggregate. for exam- ple, is often used in conjunction with sand. Workability can also be improved without adding to the weight. by the use of air entraining agents.

Drying shrinkage and moisture movement

Concretes made from lightweight aggregates usually have a higher drying shrinkage and moisture move- ment than heavy concretes, and they are more prone to shrinkage crack ing. The drying shrinkage of precast units can be reduced by drying in the course of manufacture. but this is not applicable to in situ work. Precast units are sometimes steam. cured in order to reduce the length of curing time; if the steam is used at high pressure, both the drying shrinkage and the moisture expan sion are reduced. and the strength is enhanced. In connection with the composition of the mix, it may be noted the strength obtained by en-

THE HONG KONG & FAR EAST BUILDER

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VOLUME 17, NUMBER 5 :