Civil Engineering Reference
In-Depth Information
28-day compressive strength in excess of 17 MPa (2500
psi). Some job specifications allow air-dry densities up to
1920 kg/m
3
(120 pcf). For comparison, normal-weight
concrete containing regular sand, gravel, or crushed stone
has a dry density in the range of 2080 to 2480 kg/m
3
(130
to 155 pcf). ASTM C 567 provides a test for density of
structural lightweight concrete. Structural lightweight
concrete is used primarily to reduce the dead-load weight
in concrete members, such as floors in high-rise buildings.
Structural Lightweight Aggregates
Structural lightweight aggregates are usually classified
according to their production process because various
processes produce aggregates with somewhat different
properties. Processed structural lightweight aggregates
should meet the requirements of ASTM C 330, which
includes:
• Rotary kiln expanded clays (Fig. 18-1), shales, and
slates
• Sintering grate expanded shales and slates
• Pelletized or extruded fly ash
• Expanded slags
Structural lightweight aggregates can also be pro-
duced by processing other types of material, such as nat-
urally occurring pumice and scoria.
Structural lightweight aggregates have densities sig-
nificantly lower than normal-weight aggregates, ranging
from 560 to 1120 kg/m
3
(35 to 70 pcf), compared to 1200 to
1760 kg/m
3
(75 to 110 pcf) for normal-weight aggregates.
These aggregates may absorb 5% to 20% water by weight
of dry material. To control the uniformity of structural
lightweight concrete mixtures, the aggregates are pre-
wetted (but not saturated) prior to batching.
Fig. 18-1. Expanded clay. (69793)
compressive strengths range from 20 to 35 MPa (3000 to
5000 psi). High-strength concrete can also be made with
structural lightweight aggregates.
In well-proportioned mixtures, the cement content
and strength relationship is fairly constant for a particular
source of lightweight aggregate. However, the relation-
ship will vary from one aggregate source or type to
another. When information on this relationship is not
available from the aggregate manufacturer, trial mixtures
with varying cement contents are required to develop a
range of compressive strengths, including the strength
specified. Fig. 18-2 shows the relationship between cement
content and compressive strength. An example of a
28-MPa (4000-psi) structural lightweight concrete mixture
with an air-dry density of about 1800 kg/m
3
(112 pcf), a
combination of natural sand and gravel, and a lightweight
rotary kiln expanded clay coarse aggregate follows:
Compressive
Strength
Cement content, lb/yd
3
Cement content, lb/yd
3
400
500
600
700
800
400
500
600
700
800
The compressive strength
of structural lightweight
concrete is usually related
to the cement content at a
given slump and air con-
tent, rather than to a water-
to-cement ratio. This is due
to the difficulty in deter-
mining how much of the
total mix water is absorbed
into the aggregate and thus
not available for reaction
with the cement.
ACI 211.2
provides guidance on the
relationship between com-
pressive strength and ce-
ment
6
6
40
40
All low density
Sanded low density
5
5
30
30
4
4
3
3
20
20
200
300
400
500
200
300
400
500
Cement content, kg/m
3
Cement content, kg/m
3
Fig. 18-2. Relationship between compressive strength and cement content of field structural
lightweight concrete using (left) lightweight fine aggregate and coarse aggregate or (right)
lightweight coarse aggregate and normal-weight fine aggregate (data points represent actual
project strength results using a number of cement and aggregate sources) (
ACI 211.2
).
content.
Typical
