Table 3
THERMAL CONDUCTIVITY OF AERATED AND DENSE CONCRETES
Material (nominal density grade)
Aerated concrete 20 lb/ft3
*
H
30
40
"
50
бо
"
Dense concrete
The thermal transmittance, ΟΙ U- value, of a composite structure such as a wall, roof or floor, which is the value used for calculating the heat loss through the structure, is generally obtained by calculation from measured k-values, using conventional values for the thermal resistances of the exposed surfaces and of the cavity, if any (see the I.H.V.E. "Guide to Current Prac- tice, 1959'). Some measured U-values are also available for north-facing walls, naturally exposed with a moderate to sheltered exposure. They may be re- garded as practical
values for the conditions specified. Test results show that the insulation value of the 11 in. cavity brick wall may be increased by about 23% when the inner leaf of clay brick is replaced with aerated concrete of similar thickness and
50 lb/ft3 density. With concretes of lower density the improvement will be greater.
Fire resistance
important
Fire resistance is an property of building structures. Fire grading is a scale of assessment of fire resistance and is measured by the time in hours during which an element of structure will resist a fire of prescribed severity and continue to perform its
normal function.
Tests have shown that a 4 in. load- bearing wall of aerated concrete with- out surface finishes will have a fire resistance grading of two hours. A non-loadbearing wall will achieve four hours with a 4 in. thickness and six hours with 6 in. thickness. These performances may be improved, or the thickness for the same performance reduced, by applying suitable surface finishes such as gypsum-vermiculite plaster
a
Sound insulation and
sound absorption
The degree of insulation against air- borne sound afforded by any structural element such as a partition depends very largely on the weight of material in it (always provided that it is airtight
66
Thermal conductivity, k (Btu in/ft2 h F}
0.58
0.75
1.00
1.40
1.80
8-12
and not 'porous' to sound). The sound insulation of an aerated concrete parti- tion therefore depends on the density and thickness of the units, and is thus much the same as that given by other lightweight concretes of similar density. Flastering the wal! on both sides im- proves the insulation by increasing the weight and sealing the surface.
The sound absorption of aerated concrete, like that of most other light- weight concretes, is higher than that of dense concrete but the difference is of little practical significance if the surface is plastered.
Water absorption
The water absorption of a material can be expressed either as a percentage by weight or as a percentage by volume. In comparing the absorption of materials of roughly similar density, e.g. different bricks, it does not make
Material
periods exceeding six hours, without showing water staining on the back face. This tends to confirm field ex- perience abroad, where aerated concrete (even in some instances without surface protection) has shown satisfactory re- sistance to rain penetration, except in the most exposed conditions when many other materials would likewise fail.
Apart from
rain any question of penetration to the inner face, however, it is always advisable to keep the aerated concrete itself as dry as pos- sible by means of a rendering or some equally effective coating, in order to maintain its thermal insulating value. Although, in reinforced units, the reinforcement is specially protected against corrosion, these too should be kept dry as a further safeguard,
Durability
Aerated concrete is much softer than dense concrete and therefore has less resistance to abrasion. This is of little importance in a country where external rendering
plastering is normal Where, however, plastering practice.
and
is omitted care is needed to avoid surface damage by impact or abrasion such as might result from the moving of heavy furniture.
experience of Britain, but in
frost Sweden
There is no damage in where building in aerated concrete was in some cases interrupted for several years during the last war, it is reported that unrendered aerated concrete did not suffer from exposure to several Scandinavian winters.
Table 4
WATER-ABSORPTION
Aerated concrete, density 51.5 lb/ft3
"
43.5
+
35
31
very much
difference which way is chosen. But, with materials of widely different density, such as dense and lightweight concrete, the two methods of expression give quite different com- parisons, as is shown in Table 4.
Resistance to rain penetration
In recent tests aerated concrete blocks, rendered externally, and re- inforced wall panels sprayed with a protective coating have been subjected to the standard artificial rain test for
Water absorption (%)
wt/wt basis
vollvol basis
7
16
27
22
30
ΤΙ
46
54
Cutting and nailing
26
27
Aerated concretes can easily be sawn, planed and chiselled, and quite com- plex shapes can be produced with simple tools. Chases can be cut either before ог after the erection of the blocks. The blocks can be nailed, but the nailholding capacity varies with the density grade of the concrete. Cut nails hold better than wire nails.
The uses of aerated concrete will be described in a later Digest.
THE HONG KONG & FAR EAST BUILDER—VOLUME 16, NUMBER 5
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