No_5_February_1963 — Page 118

Table 1

PROPERTIES OF CONCRETES MADE WITH LIGHTWEIGHT AGGREGATES

Aggregate

Density of aggregate

lb ft3

Dry density of concrete

Ib/f13

Compressive strength

Thermal conductivity Btu in/f12h' F

lb/in2

Drying shrinkage per cent

Clinker

45-65

65-95

300-1000

0.04-0.08

2.5-4.0

Foamed slag

20-55

60-125

300-3500

0.03-0.07

1.5-3.5

Expanded clay

20-65

45-110

300-9000

0.04 0.07

1.5-4.0

Sintered pulverized-fuel ash

40-60

60--110

400-8000

0.04-0.07

1.5-4.0

Expanded slate

20—60

35-110

200-4000

0.03-0.09

10-3.5

Pumice

30-55

40-00

300—2000

0.04 0.08

1.0-2.5

Exfoliated vermiculite

4-12

25-50

100—500

035-0.35

1.0-1.5

Expanded perlite

5-15

25-70

70-1000

0.20 0.30

0.6—1.5

Table 2

FIRE RESISTANCE OF LIGHTWEIGHT AGGREGATE CONCRETE BLOCK WALLING

Minimum thicknesses (inches), excluding plaster, required to give fire resistance for the periods stated

Hollow blocks"

Unplastered

-in. gypsum plaster

Finish

Solid blocks

Loadbearing

Non-loadbearing

Fire resistance (hours)

Fire resistance (hours)

2

+

2

I

1

4

2

I

14

6

6

6

3

2

6

5

4

21

3

2

3

3

2

N N

2

6

3

2

4

3

2 2

1-in. gypsum/vermiculite plaster

* Single cell across the wall

The above values relate to concrete blocks made with Class 1' aggregates, with include (a) well-burnt clinker, (b) foamed slag, (c) expanded clay, shale or slate, and (d) pumice

richment with cement increases the drying shrinkage; strength resulting from better compaction does not.

The general precautions to be taken to avoid shrinkage cracking in concrete masonry construction were described in Digest 6 (first series) and will be discussed in relation to lightweight aggregate blocks in the next Digest. They include such matters as choosing blocks satisfying the British Standard 1equirements. suitable storage before use. choice of mortars. and the provision of contraction joints and local rein- forcement where necessary.

Corrosion of metals

the

The increasing structural use of reinforced lightweight aggregate con- crete (which will shortly be made the subject of a separate Digest) has drawn special attention to the pos- sible risk of corrosion of the rein- forcement to other embedded steel- work. It was generally recognized that porous concretes might not afford as much protection as dense

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concretes but the degree of risk with different types of lightweight con- crete was uncertain. Current inves- tigations are providing information on this point.

The alkaline environment afforded by concrete provides some protec tion for the steel. and reliance is usually placed on this to prevent cor- rosion. Light-weight concrete, being more porous than dense concrete, offers less resistance to the access of moisture, oxygen and carbon di- oxide. and therefore affords some- what less protection to the steel; moreover, shrinkage cracking is more likely, which further reduces the protection. Nevertheless. exposure tests have shown that the protection afforded by the surrounding con- crete is affected much more by the quality of the concrete and proper- ties such as compaction workability. cement content. concrete cover. and the grading of aggregate. than by the type of aggregate used.

Reinforcement to be embedded in lightweight concrete can be protected

against corrosive influences by simple measures. The surface of the steel bars might be covered with a cement. slurry before casting the concrete. Galvanized steel can be employed, but the zinc coating is slowly attacked by the alkalis present in cement. Where reinforced lightweight concrete is ex- posed to the weather, the concrete cover over the reinforcement should not be less than 2 in.. and the maximum aggregate size should not exceed in.

The only lightweight aggregate; likely to add to the risk by intro- ducing corrosive substances is clinker and, as already mentioned. its use in reinforced concrete is pro hibited for this reason.

Fire resistance

The fue resistance of elements of structure is graded according to the j time during which the element will ! resist a fire of prescribed severity and continue to perform its normal

function.

THE HONG KÔNG & FAR EAST BUILDER -

VOLUME 17. NUMBER

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