Building Materials, Third Edition

ƒ™—2g



2—2g

2g™

 RVI

Items from gas concrete are manufactured in the manner described below. A mixture of
ground sand and water is fed to the stirrer and mixed with cement, aluminium powder, water
and un-ground sand, after which the mix is cast into moulds. After 4–5 h of hardening, gas
concrete is cut into slabs and loaded into autoclaves where the items finally harden at a
temperature of 175°C and at a pressure of 8 atm. Autoclaving enhances the strength of gas
concrete and, in addition, substantially reduces the consumption of cement which can thus be
fully or partially replaced by lime.
Gas concrete is similar to foam concrete in properties and is used for the same purpose.
However, it is simpler to manufacture, and items form it have more stable qualities than from
foam concrete: in particular, this applies to their bulk densities. These are the chief advantages
of gas concrete over foam concrete. Among the main shortcomings of cellular concrete are high
tendency to deformation, shrinkage, etc.

g——™™

hThe density of LWC varies from 300–1200 kg/m^2

‡—˜ Due to low density and the characteristic texture of porous aggregate especially in
the crushed state, the workability of concrete needs special attention. In general, placing
compacting, and finishing lightweight aggregate concrete requires relatively less effort; therefore,
even 50 to 75 mm slum may be sufficient to obtain workability of the type that is shown by 100
to 125 mm slump of normal-weight concrete.

…2‡Unit weight and strength are the two properties generally sought from lightweight
concrete. With given materials, it is generally desired to have the highest possible strength/
unit weight ratio with the lowest cost of concrete. The air-dried unit weight of concrete is
limited to a maximum of 18.40 kN/m^3. The use of normal sand to control the properties of
hardened concrete tends to increase the unit weight, although this tendency is partially offset
from the balancing effect of entrained air, which is invariably prescribed for improving the
workability. Most structural lightweight concretes weigh between 16.00 to 17.60 kN/m^3 ;
however, job specifications in special cases may allow higher than 18.40kN/m^3.

ƒDesign strengths of 20 to 35 MPa, 28 day compressive strengths are common, although
by using a high cement content and good quality light weight aggregate of small size (9 to 13
mm maximum) has made it possible, in some precast and prestressing plants, to produce 40 to
48 MPa concrete. Lightweight aggregates with controlled micro porosity have been developed
to produce 70 to 75 MPa lightweight concrete which generally weigh 18.40 to 20.00 kN/m^3.
The ratio between the splitting tensile strength and compressive strength decreases significantly
with increasing strength of lightweight concrete.

„—2s—is about 3–4 times more than that of bricks and about 10 times than that of
concrete.

p2—™ is excellent.

ƒ2—is poor.

h—˜Aerated concrete is slightly alkaline. Due to its porosity and low alkalinity the
reinforcement may be subjected to corrosion and as such, require special treatments.