No_1_June_1963 — Page 112

Lightweight Aggregate Concretes-3:

Structural Application

THIS is the third of a group of Digests on lightweight-aggregate

concretes. It is intended to assist in their application to load- bearing structures.

block or

High-grade lightweight aggregates have been developed in this coun- try and abroad during the post-war years, capable of producing con- crete which combines high strength (cube strength of up to 7000-8000 lb/in2) with a density of 30-40% less than that of ordinary gravel concretes. These materials are be- ing used in non-compacted porous concretes for masonry partially compacted cast in situ loadbearing walls. or in compacted structural concrete for reinforced and pre-stressed members, replacing natural aggregates in many where reduced weight. better heat insulation or greater fire resistance are required. Their material pro- perties and the concretes made with them have been described in detail in Digest 25 (second series).

STRUCTURAL USES

cases

In some case lightweight aggre- gate concrete is merely used to re- place gravel concrete where con- venient. Often, however, it is used not as a substitute material but one that is especially suited in its own right for particular purposes, for example where reduced density. great thermal insulation or fire pro- tection are required. In selecting a lightweight aggregate, it should be remembered that certain aggregates may be more suitable for some pur- poses than others. For example. pumice aggregate, which has in the past been imported occasionally in- to this country from Germany, is not generally used for cast in situ construction because of its tendency to float to the surface. causing se- gregation of the mix (although in some instances pumice concrete has been used successfully in cast in situ floor and roof slabs). Nor are very light aggregates such as vermiculite or perlite generally used structural- ly, partly because of their high cost

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and partly because they are rarely suitable for this purpose: again. however, it is known that complicat- ed shell structures have been erect- ed in Britain with this type of material with crushing strengths of 1000 lb/in2 and even more. Where it is important to keep the density low it may be necessary to take in- to account the strength/density ratios. In general, fully compacted foamed slag concrete has a lower strength density ratio than light- weight concrete made with one the newer sintered or expanded types of aggregate. Nevertheless. by using suitable mixes it is possible. to attain high strengths with foamed slag concrete and to use it econo- mically for structural purposes. if a somewhat greater density (115-120 lb/ft3 as against 85-105 lb/ft3) can be accepted. Most lightweight ag gregates at present produced in this country are equally suitable for por- ous, no-fines or partially compacted mixes used in loadbearing or non- loadbearing in situ wall construc- tion.

a

The first type of lightweight con- crete loadbearing elements used in this country

were masonry block walls. These were discussed in Digests 25 and 26 (second series). More recently. larger unreinforced solid masonry blocks made of lightweight porous type of concrete have been introduced for multi- storey loadbearing wall construc- tion, mostly in Eastern Europe. These blocks are

of half-storey height, about 3 ft. wide and about 1 ft. thick, and are precast on the site in large quantities. rapidly and simply. A further development is the introduction of large, storey- height precast panels of reinforced lightweight concrete. These have gained ground not only in the So- viet Union and other Eastern Euro- pean countries but also in France,

Denmark and, more recently, in the United Kingdom. They are usually interconnected by welding the rein- forcing bars which are allowed to protrude at the edges of the panels, and cast in situ dense concrete wall beams are usually employed to pro- vide adequate stiffness for tall. multi-storey buildings. Some sys- tems of construction assemble com- plete 'box' units in the factory into one or more rooms of a flat; these elements are transported to the site in large trailers and erected with special cranes, With such large units, the use of lightweight concrete leads to considerable savings in weight; it can also eliminate the need for additional thermal insula. tion. The structural design of rein- forced large-panel structures has not yet been fully explored; it is governed, in the main. by C.P.114: 1957.

Lightweight concrete has also been found to be suitable for in situ work. Cast in situ loadbearing cross-wall construction has been in- troduced with considerable success for multi-storey buildings in West- Ger- ern Europe. particularly in

of many, where a saving in cost more than 30% is being achieved in the erection of the structure com. pared with conventional framed re- inforced concrete or steel structures. No-fines concrete loadbearing walls using gravel aggregate have been used for some time for this type of construction in the United King- dom, and more recently, expanded clay and sintered pulverized fuel ash well as foamed slag aggregate as have been used, for two-storey__and for multi-storey structures. Their structural design is generally based on C.P.111.

Smaller precast concrete building units are an important application of lightweight aggregate concrete. Reinforced lightweight concrete floor and roof slabs, as well as wall slabs of limited size and span, have been produced throughout Western Europe, and in some coun tries the production and design of

THE HONG KONG & FAR EAST BUILDER - VOLUME 18, NUMBER 1