Civil Engineering Reference
In-Depth Information
would be obtained at 7 days with high-early-strength
cement. The relationships shown do not apply to auto-
claved products.
in.) form. The material is then pressure steam cured (auto-
claved) over a period of 6 to 12 hours using a temperature
of 190ºC (374ºF) and a pressure of 1.2 MPa (174 psi). This
forms a hardened mortar matrix, which essentially con-
sists of calcium silicate hydrates.
This porous mineral building material has densities
between 300 and 1000 kg/m
3
(19 and 63 lb/ft
3
) and
compressive strengths between 2.5 and 10 MPa (300 and
1500 lb/in
2
). Due to the high macropore content—up to
80 percent by volume—autoclaved cellular concrete has a
thermal conductivity of only 0.15 to 0.20 W/(m
•
K) (1 to
1.4 Btu
•
in./[h
•
ft
2
•
ºF]).
Autoclaved cellular concrete is produced in block or
panel form for construction of residential or commercial
buildings (Fig. 18-6).
Additional information can be found in
ACI 523.2R
,
Guide for Precast Cellular Concrete.
Resistance to Freezing and Thawing
Insulating and moderate-strength lightweight concretes
normally are not required to withstand freeze-thaw expo-
sure in a saturated condition. In service they are normally
protected from the weather; thus little research has been
done on their resistance to freezing and thawing.
Drying Shrinkage
The drying shrinkage of insulating or moderate-strength
lightweight concrete is not usually critical when it is used
for insulation or fill; however, excessive shrinkage can
cause curling. In structural use, shrinkage should be con-
sidered. Moist-cured cellular concretes made without
aggregates have high drying shrinkage. Moist-cured cel-
lular concretes made with sand may shrink from 0.1% to
0.6%, depending on the amount of sand used. Autoclaved
cellular concretes shrink very little on drying. Insulating
concretes made with perlite or pumice aggregates may
shrink 0.1% to 0.3% in six months of drying in air at 50%
relative humidity; vermiculite concretes may shrink 0.2%
to 0.45% during the same period. Drying shrinkage of insu-
lating concretes made with expanded slag or expanded
shale ranges from about 0.06% to 0.1% in six months.
Expansion Joints
Where insulating concrete is used on roof decks, a 25-mm
(1-in.) expansion joint at the parapets and all roof projec-
tions is often specified. Its purpose is to accommodate
expansion caused by the heat of the sun so that the insu-
lating concrete can expand independently of the roof
deck. Transverse expansion joints should be placed at a
maximum of 30 m (100 ft) in any direction for a thermal
expansion of 1 mm per meter (1 in. per 100 lin ft). A fiber-
glass material that will compress to one-half its thickness
under a stress of 0.17 MPa (25 psi) is generally used to
form these joints.
Fig. 18-6. (top) Residential build-
ing constructed with autoclaved
cellular concrete blocks. (bottom)
Autoclaved cellular concrete
block floating in water. (70025,
44109)
HIGH-DENSITY CONCRETE
High-density (heavyweight) concrete and has a density of
up to about 6400 kg/m
3
(400 pcf). Heavyweight concrete is
used principally for radiation shielding but is also used
for counterweights and other applications where high-
density is important. As a shielding material, heavy-
weight concrete protects against the harmful effects of
X-rays, gamma rays, and neutron radiation. Selection of
concrete for radiation shielding is based on space require-
ments and on the type and intensity of radiation. Where
space requirements are not important, normal-weight
concrete will generally produce the most economical
shield; where space is limited, heavyweight concrete will
allow for reductions in shield thickness without sacri-
ficing shielding effectiveness.
AUTOCLAVED CELLULAR CONCRETE
Autoclaved cellular concrete (also called autoclaved aer-
ated concrete) is a special type of lightweight building
material. It is manufactured from a mortar consisting of
pulverized siliceous material (sand, slag, or fly ash),
cement and/or lime, and water; to this a gas forming ad-
mixture, for example aluminum powder, is added. The
chemical reaction of aluminum with the alkaline water
forms hydrogen, which expands the mortar as macro-
pores with a diameter of 0.5 mm to 1.5 mm (0.02 in. to 0.06
