No_6_April_1963 — Page 110

Lightweight Aggregate Concretes

2: Uses

CONTINUING the account of lightweight aggregate concretes,

(See Vol. 17, No. 5-February, 1963) this article describes their use in the form of building blocks, screeds and reinforced concrete.

S stated in the earlier article. aggregates at present available in the U.K. for making lightweight concrete include furnace clinker (British Standard 1165: 1957) and four processed aggregates -foamed slag B.S. 877: 1939). expanded

clay. sintered pulverized fuel ash and exfoliated vermiculite.

Clinker concrete is widely used in the form of the well-known clinker blocks and sometimes for floor screeds. It is not recommended for reinforced concrete because of the risk of corrosion.

Vermiculite concrete. although low in strength, is very light and has high thermal insulation value. It can be used either as an insulat- ing screed for roofs and floors or in the form of precast slabs the latter for the fire protection of structural steelwork or prestressed concrete. and for thermal insulation.

Concretes made from foamed slag. expanded clay or pulverized fuel ash can be adapted to a wider range of uses. In lean mixes, where cast in situ without compaction. they provide light insulating creeds: in semi-dry mixes, precast and slightly compacted, they give light blocks of the strength required for load-bearing walls, while in well- graded. fully-compacted mixes they can be made to satisfy the strength requirements of reinforced concrete either pre-cast or cast in situ. while still being lighter than concretes made with dense natural aggregates.

BUILDING BLOCKS

The commonest use of lightweight aggregate concrete in the United Kingdom is still as building blocks. 25 million square yards of which are built annually. The blocks may be either solid or hollow and most makes are available in a range of sizes. The density of the blocks, and consequently their strength and thermal insulating properties. vary according to the aggregate. mix and method of manufacture employed.

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Specification

British Standard 2028:1953 (now under revision) covers both dense and lightweight precast concrete blocks; it specifies dimensions and tolerances for blocks of various sizes. and de- fines the quality in terms of strength. drying shrinkage and moisture movement. Apart from these basic requirements no distinction is made between blocks of different density but provision is made for two types of lightweight concrete blocks, Type B for loadbearing walls and Type C for non-loadbearing walls.

Clinker, foamed slag and expand- ed clay are included in the present standard as suitable aggregates for blocks of either type and sintered pulverised fuel ash is likely to be added in the revision.

The type B blocks are intended for:

(1) External walls when protect-

ed by a rendering or in some other efficient manner

(2) The inner leaf of cavity walls

or as backing to brick work and masonry

(3) Internal walls or partitions (4) Filling panels in steel fram-

ed and reinforced concrete framed buildings.

The principal requirements for Type B blocks are: average compres sive strength of 12 blocks not less than 400 lb/sq. in. (lowest indivi- dual block 300 lb/sq. in.) drying shrinkage not more than 0.06%; moisture movement not more than 0.05%.

The Type C blocks are intended for:

(1) Internal non-loadbearing walls

and partitions

(2) Non-loadbearing panels in steel framed and reinforced concrete framed buildings. The principal requirements for Type C blocks are: tranverse strength minimum breaking load

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ranging from 144 lb to 244 lb ac- cording to the height and thickness of the blocks; drying shrinkage not more than 0.08%.

Using the aggregates specified, there should be no difficulty in meet- ing the strength requirements for either type of block but care is re quired both in the design of the con- crete mix and in the process of manu- facture to bring the drying shrink- age and moisture movement within the specified limits. It is important that these limits should be met in or der to reduce the risk of shrinkage cracking in walls built with the blocks. Even so it is advisable when using blocks to take the general pre- cautions described in the next section.

Shrinkage cracking—precautions

(1) The blocks should be ade- quately dried before they are deliver- ed to the building site; preferably their moisture content should be less than 30% of that when fully satur- ated. (Few manufacturers, however, have facilities as yet for adequate drying of blocks during the winter months). It follows that the blocks should be protected from rain in transit from the works and also on the site. During storage they should be stacked off the ground in such a way as to allow reasonable circulation of air through the stack, and cover- ed, e.g. with a plastic sheet.

(2) The walling should be pro tected as much as possible during construction. If for any reason the blocks are not sufficiently dry when laid, or the walling has become wetted, ample time should be allowed for drying and shrinkage to take place before plastering or rendering.

(3) The blocks should be laid in a weak mortar; suitable mixes for external walls are 1:1:5-6 cement: lime: sand in winter and 1:2:8-9 in summer. For internal work, except in cold weather the mix could be 1:3:10-12.

THE HONG KONG & FAR EAST BUILDER

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VOLUME 17, NUMBER 6

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