No_6_April_1963 — Page 112

The screed should not be less than 1 inches thick. On roofs where it is intended to provide thermal insulation in

in order to minimise thermal movement of the structural roof and to reduce heat loss from the building, greater thicknesses will usually be needed. To illustrate this point Table I com- pares three types of lightweight con- crete used at specified densities and shows the thickness of screed need- ed in each case to produce a U-value of 0.22 in a roof of the specified The thickness requir construction. ed for other concretes, densities or U-values can

be calculated from published data of thermal conduc- tivity.

No great strength is generally re- quired of a roof screed; probably the hardest treatment which it re- ceives is from workmen carrying out the next operation on the roof. Foamed slag. sintered pulverised fuel ash and expanded clay are suit- able aggregates. For producing screeds of 70 lb/ft3 density mixes of 1:8 1:10 are recommend- ed. For screeds of density low- er than 40 lb/ft3, exfoliated miculite or perlite can be used and in this case the mix proportions should be about 1:5 to give ade- quate strength.

ver-

screed

A lightweight concrete contains a large amount of water when first laid; the porous aggre- gate absorbs water in the mixing process and very high water/cement ratios are required to obtain work- able mixes. Such screeds take a long time to dry out. On roofs, the screed is usually given two or more weeks to dry out but if it is not protected from rain, even a few light showers may cause it to gain rather than lose moisture. If the roof covering. when laid, entraps substantial quan- tities of water, serious troubles may ensue. This difficult problem and various ways of meeting it were discussed in

Digest 8 (Second series) in connection wih built-up felt roofs but it applies whatever the roof covering. Particular reference may be made to the method, there described, of employing for the in- sulating screed a concrete consist- ing of cement and no fines light- weight aggregae (e.g. sintered pul- verized fuel ash) followed by a thin topping of 1:4 cement and sand.

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In a recent proprietary develop- ment, relating particularly to asphalt coverings,, bitumen is used in place. of cement as the binder for the lightweight aggregate. and the as- phalt is bonded directly to the screed, thus eliminating any use of

water.

Since floors are subject to much heavier traffic and loads than roofs. the design of a lightweight floor screed must take into account the life of the floor finish to be applied. With floor finishes that are incap- able of spreading point loads for vinyl example, thermoplastic

or

tiles, and sheet vinyl, rubber or lino- leum a topping at least -in. thick of 1:4 cement and sand will be re- quired. With rigid finishes such as wood block, the topping require. ments are not so stringent.

In order to minimise the effects of shrinkage, the topping should be laid monolithically with the light- weight screed, in bays not exceeding 10-12 ft. square.

Since many finishes, or adhesives used to fix them, are susceptible to dampness it is important to allow the screed to dry out before the finish is applied, as well as taking the usual precautions against rising ground moisture.

When a lightweight concrete floor screed is used to provide falls for drainage in a building where wet processes are carried out, care must be taken to prevent water or corro. sive liquids from entering the screed and to protect any structural or other steel embedded in the screed (see Digests 73 and 74 (first series), on corrosion resistant floors).

A limited amount of experience has been gained in the use of light. weight concrete in floors for the pur. pose of sound insulation, the light. weight concrete being sandwiched between the floor and the structural floor slab without a resilient quilt. Such a construction can give Grade I airborne sound insulation, but the impact insulation is usually not bet- ter than that of a dense concrete floor with the same floor finish. To give an overall rating of Grade I. therefore, a resilient floor finish such as thick carpet, cork or rubber is needed. As general requirements. the lightweight screed should have

a density of not more than 70 lb/ft3 and be not less than 2 inches thick; it should have an impervious or airtight layer above it.

REINFORCED AND PRESTRESSED LIGHTWEIGHT CONCRETE

Lightweight aggregate concrete as used in Britain for block mak ing or for cast insitu walls is not usually fully compacted. high strength being less important for these particular

than purposes thermal insulation and production considerations. However, as men- tioned earlier in this and the pre- ceding Digest, high compressive strength can be obtained when re- quired with lightweight concrete as with gravel concrete by suitable ag- gregate mix proportions and full compaction. The increase in strength is accompanied by changes in other physical properties such as density, tensile strength and the modulus of elasticity. Lightweight concretes can, therefore, be used for reinforc ed concrete and even for prestress- ed concrete and may have consider- able technical and economic advan tages. but the special properties of the material need to be taken into account in the design.

The various British Standard Codes of Practice relating to differ- ent types of concrete construction were drawn up with particular re- gard to gravel and crushed rock ag gregate concretes. In applying the terms of these codes to structural lightweight concrete construction, therefore, allowance must be made for the different characteristics of the materials and of the methods of construction. A later Digest will deal with those aspects of design where lightweight concrete differs from normal reinforced concrete or prestressed concrete construction. Lack of authoritative information on this subject has tended to hinder the full development of lightweight concretes for loadbearing structures; in particular the durability of such porous materials has been question. ed. A considerable amount of evi- dence, however, is already available in Britain and

and abroad on the behaviour of lightweight concrete structures, both reinforced and un- reinforced and precast as well as

cast in-situ.

THE HONG KONG & FAR EAST BUILDER

VOLUME 17. NUMBER 6