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entire fine aggregate may be a lightweight product.
Lightweight aggregates used in structural lightweight concrete
are typically expanded shale, clay or slate materials that have
been fired in a rotary kiln to develop a porous structure. Other
products such as air-cooled blast furnace slag are also used.
There are other classes of non-structural lightweight concretes
with lower density made with other aggregate materials and
higher air voids in the cement paste matrix, such as in cellular
concrete. These are typically used for their insulation
properties. This CIP focuses on structural lightweight
concrete.
WHY Use Structural Lightweight Concrete?
The primary use of structural lightweight concrete is to reduce
the dead load of a concrete structure, which then allows the
structural designer to reduce the size of columns, footings
and other load bearing elements. Structural lightweight
concrete mixtures can be designed to achieve similar strengths
as normalweight concrete. The same is true for other
mechanical and durability performance requirements.
Structural lightweight concrete provides a more efficient
strength-to-weight ratio in structural elements. In most cases,
the marginally higher cost of the lightweight concrete is offset
by size reduction of structural elements, less reinforcing steel
and reduced volume of concrete, resulting in lower overall
cost.
In buildings, structural lightweight concrete provides a higher
fire-rated concrete structure. Structural lightweight concrete
also benefits from energy conservation considerations as it
provides higher R-values of wall elements for improved
insulation properties. The porosity of lightweight aggregate
provides a source of water for internal curing of the concrete
that provides continued enhancement of concrete strength and
durability. This does not preclude the need for external curing.
Structural lightweight concrete has been used for bridge
decks, piers and beams, slabs and wall elements in steel and
concrete frame buildings, parking structures, tilt-up walls,
topping slabs and composite slabs on metal deck.
HOW is Structural Lightweight Concrete Used?
Lightweight concrete can be manufactured with a combination
of fine and coarse lightweight aggregate or coarse lightweight
aggregate and normalweight fine aggregate. Complete
replacement of normalweight fine aggregate with a lightweight
aggregate will decrease the concrete density by approximately
10 lb/ft3 (160 kg/m3).
Designers recognize that structural lightweight concrete will
not typically serve in an oven-dry environment. Therefore,
structural design generally relies on an equilibrium density
(sometimes referred to as air-dry density); the condition in
which some moisture is retained within the lightweight
concrete. Equilibrium density is a standardized value intended
to represent the approximate density of the in-place concrete
when it is in service. Project specifications should indicate
the required equilibrium density of the lightweight concrete.
Equilibrium density is defined in ASTM C 567, and can be
calculated from the concrete mixture proportions. Field
acceptance is based on measured density of fresh concrete in
accordance with ASTM C 138. Equilibrium density will be
approximately 3 to 8 lb/ ft3 (50 to 130 kg/m3) less than the
fresh density and a correlation should be agreed upon prior
to delivery of concrete. The tolerance for acceptance on fresh
density is typically ±3 lb/ ft3 (±50 kg/m3) from the target value.
Lightweight aggregates must comply with the requirements
of ASTM Specification C 330. Due to the cellular nature of
lightweight aggregate particles absorption typically is in the
range of 5% to 20% by weight of dry aggregate. Lightweight
aggregates generally require wetting prior to use to achieve a
high degree of saturation. Some concrete producers may not
have the capability of prewetting lightweight aggregates in
cold weather if temperature controlled storage is not available.
Some lightweight aggregate suppliers furnish vacuum
saturated aggregate.
With the exception of bridges and marine structures,
specifications for structural lightweight concrete do not
typically have a requirement for maximum water-tocementitious
materials (w/cm) ratio. The w/cm ratio of
structural lightweight concrete cannot be precisely determined
because of the difficulty in determining the absorption of
lightweight aggregate.
Air content of structural lightweight concrete must be closely
monitored and controlled to ensure that the density
requirements are being achieved. Testing for air content must
be according to the volumetric method, ASTM C 173, or
calculated using the gravimetric method described in ASTM
C 138. Virtually all lightweight concrete is air-entrained.
Finishing lightweight concrete requires proper attention to
detail. Excessive amounts of water or excessive slump will
cause the lightweight aggregate to segregate from the mortar.
Bullfloating will generally provide an adequate finish. If the
surface for an interior floor is to receive a hard troweled finish,
use precautions to minimize the formation of blisters or
delaminations. See CIPs 13 and 20 for discussions on blisters
and delaminations, respectively.
Due to the inherent higher total moisture content of lightweight
concrete it typically takes a longer time than normalweight
concrete to dry to levels that might be considered adequate
for application of floor covering materials.
The splitting tensile strength of lightweight concrete is used
in structural design criteria. The design engineer may request
the information for a particular source of lightweight
aggregate prior to the design. The splitting tensile strength
corresponding to the specified compressive strength is
determined in laboratory evaluations. Splitting tensile
strength testing is not used as a basis for field acceptance of
concrete.
Ensure that the requirements of the designer relative to fire
resistance or insulation properties of lightweight concrete
building elements are in conformance with applicable
industry standards. For a successful project, information is
available from the supplier of lightweight aggregate and the
ready mixed concrete producer. With proper planning,
structural lightweight concrete can provide an economical
solution to many engineering applications. |