Civil Engineering Reference
In-Depth Information
and troweling should be done; magnesium finishing tools
are preferred.
The same curing practices should be used for light-
weight concrete as for normal-weight concrete. The two
methods commonly used in the field are water curing
(ponding, sprinkling, or using wet coverings) and pre-
venting loss of moisture from the exposed surfaces (cov-
ering with waterproof paper, plastic sheets, or sealing
with liquid membrane-forming compounds). Generally,
7 days of curing are adequate for ambient air tempera-
tures above 10°C (50°F).
in both moderate-strength and structural lightweight con-
crete (up to about 1900 kg/m
3
or 120 pcf air-dry).
Group III
concretes are made by incorporating into a
cement paste or cement-sand mortar a uniform cellular
structure of air voids that is obtained with preformed
foam (ASTM C 869), formed-in-place foam, or special
foaming agents. Ovendry densities ranging between 240
to 1900 kg/m
3
(15 to 120 pcf) are obtained by substitution
of air voids for some or all of the aggregate particles; air
voids can consist of up to 80% of the volume. Cellular con-
crete can be made to meet the requirements of both insu-
lating and moderate strength lightweight concrete.
Aggregates used in Groups I and II should meet the
requirements of ASTM C 332,
Standard Specification for
Lightweight Aggregates for Insulating Concrete.
These aggre-
gates have dry densities in the range of from 96 to 1120
kg/m
3
(6 to 70 pcf) down to 16 kg/m
3
(1 pcf) for expanded
polystyrene beads.
INSULATING AND MODERATE-
STRENGTH LIGHTWEIGHT CONCRETES
Insulating concrete is a lightweight concrete with an
ovendry density of 800 kg/m
3
(50 pcf) or less. It is made
with cementing materials, water, air, and with or without
aggregate and chemical admixtures. The ovendry density
ranges from 240 to 800 kg/m
3
(15 to 50 pcf) and the 28-day
compressive strength is generally between 0.7 and 7 MPa
(100 and 1000 psi). Cast-in-place insulating concrete is
used primarily for thermal and sound insulation, roof
decks, fill for slab-on-grade subbases, leveling courses for
floors or roofs, firewalls, and underground thermal con-
duit linings.
Moderate-strength lightweight concrete has a density
of 800 to 1900 kg/m
3
(50 to 120 pcf) ovendry and has a
compressive strength of approximately 7 to 15 MPa (1000
to 2500 psi). It is made with cementing materials, water,
air, and with or without aggregate and chemical admix-
tures. At lower densities, it is used as fill for thermal and
sound insulation of floors, walls, and roofs and is referred
to as fill concrete. At higher densities it is used in cast-in-
place walls, floors and roofs, and precast wall and floor
panels. See
ACI documents
for more information.
For discussion purposes, insulating and moderate-
strength lightweight concretes can be grouped as follows:
Group I
is made with expanded aggregates such as per-
lite, vermiculite, or expanded polystyrene beads. Ovendry
concrete densities using these aggregates generally range
between 240 to 800 kg/m
3
(15 and 50 pcf). This group is
used primarily in insulating concrete. Some moderate-
strength concretes can also be made from aggregates in
this group.
Group II
is made with aggregates manufactured by ex-
panding, calcining, or sintering materials such as blast-
furnace slag, clay, diatomite, fly ash, shale, or slate, or by
processing natural materials such as pumice, scoria, or
tuff. Ovendry concrete densities using these aggregates
can range between 720 to 1440 kg/m
3
(45 and 90 pcf).
Aggregates in this group are used in moderate-strength
lightweight concrete and some of these materials
(expanded slag, clay, fly ash, shale, and slate) are also used
Mixture Proportions
Examples of mixture proportions for Group I and III con-
cretes appear in Table 18-2. In Group I, air contents may be
as high as 25% to 35%. The air-entraining agent can be
prepackaged with the aggregate or added at the mixer.
Because of the absorptive nature of the aggregate, the vol-
umetric method (ASTM C 173 or AASHTO T 196) should
be used to measure air content.
Water requirements for insulating and fill concretes
vary considerably, depending on aggregate characteris-
tics, entrained air, and mixture proportions. An effort
should be made to avoid excessive amounts of water in
insulating concrete used in roof fills. Excessive water
causes high drying shrinkage and cracks that may
damage the waterproofing membrane. Accelerators con-
taining calcium chloride should not be used where galva-
nized steel will remain in permanent contact with the
concrete because of possible corrosion problems.
Mixture proportions for Group II concretes usually
are based on volumes of dry, loose materials, even when
aggregates are moist as batched. Satisfactory proportions
can vary considerably for different aggregates or combi-
nations of aggregates. Mixture proportions ranging from
0.24 to 0.90 cubic meters of aggregate per 100 kg (4 to 14
cu ft per 100 lb) of cement can be used in lightweight con-
cretes that are made with pumice, expanded shale, and
expanded slag. Some mixtures, such as those for no-fines
concretes, are made without fine aggregate but with total
void contents of 20% to 35%. Cement contents for Group
II concretes range between 120 to 360 kg per cubic meter
(200 and 600 lb per cubic yard) depending on air content,
aggregate gradation, and mixture proportions.
No-fines concretes containing pumice, expanded slag,
or expanded shale can be made with 150 to 170 kg of water
per cubic meter (250 to 290 lb of water per cubic yard), total
