2. Lightweight Concrete
Definitions:
Lightweight concrete has a maximum density of
1900 kg/m3 and is achieved by using low-density
aggregates.
Intermediate-density concretes, where part or all the
normal-density coarse aggregates are replaced with
structural-grade low-density aggregates, have densities of
1900-2200 kg/m3.
Pioneering applications:
• Building of ships and barges by the US, during the
Second World War.
• High-rise construction (since 1929, in the US)
• Offshore structures (since 1908s)
5. LWC: Structural Applications
• Floor slabs of buildings
• Bridge decks
• Marine structures
• Rehabilitation of bridges and parking decks
• Masonry units
6. Lightweight Concrete (LWC)
Structural-grade low-density aggregate:
• Produced, generally, by heating particles of shale,
clay or slate to about 1200ºC in a rotary kiln. At this
temperature the raw material bloats, forming a
vesicular structure that is retained upon cooling. In
this process, the particle density changes from
about 2.65 to less than 1.55 after cooling. Contains
uniformly distributed pores of 5 to 300 microns
within a crack-free high-strength vitreous phase.
• Could be natural, of volcanic origin.
7. Lightweight Concrete (LWC)
Structural-grade low-density aggregate:
• Has absorption coefficients of 5 to more than
25%. Recognizing and accounting for this effect
is essential in mix design and fabrication of
LWC. The absorbed water is available for long-
term hydration, leading to an internal curing
effect.
• Has lower elastic modulus (e.g., dynamic
moduli of 10-17 GPa).
• Generally, has lower strength than cement
mortar but with good bond between them.
Consequently, cracking in structural LWC
occurs through the coarse aggregates.
8. Lightweight Concrete (LWC)
Pre-wetting or preconditioning of low-density aggregate:
• Before using them in the concrete, the aggregates have
to be partially or completely saturated with water.
• The pre-wetting is done before or during stockpiling.
Alternatively, the aggregates are mixed with about two-
thirds of the mix water for several minutes before the rest
of the components of the concrete are added and mixed.
• This gives better performance during pumping and lower
slump loss during transportation.
9. Lightweight Concrete (LWC)
Strength ceiling imposed by low-density aggregate
Maximum attainable strength is limited by the type of coarse aggregate.
11. LWC: Durability
Performance record:
A ship, the USS Selma, was cast with low-density concrete
in 1919. It is now in the Galveston Harbour.
The 12 to 30 mm cover was effective in preventing the
corrosion of the steel reinforcement.
Original strength of the concrete was 28 MPa, and the
density was 1760 kg/m3. Cores taken in 1980
gave compressive strengths of more
than 55 MPa.
Microscopy has revealed
that the aggregate-mortar interface
has not been penetrated by sea water.
Fractured surface of concrete from the USS Selma, showing
aggregate on left and mortar on right (micrograph width = 550 microns)
12. Ultra-Lightweight Concretes
Concretes with densities of less than 1100 kg/m3 and
compressive strengths of less than about 7 MPa:
• Concretes with expanded or other very low density
aggregates, such as expanded polystyrene, perlite,
blast-furnace slag and vermiculite and rubber
particles.
• Aerated or cellular concrete made with large-diameter
(0.1-1 mm) voids incorporating a foam in cement paste
or mortar, or generating a foam by a chemical reaction
(e.g., by using fine aluminium powder).
Used for insulating fills, conduit linings, fire walls and
non-structural panels.
