9. Lightweight Concrete

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Lightweight Concrete

Lightweight Aggregate Concrete

CE 342 Concrete TechnologyProf. Ravindra Gettu

IIT Madras



Lightweight Concrete

Definitions:

Lightweight concrete has a maximum density of1900 kg/m3 and is achieved by using low-densityaggregates.

Intermediate-density concretes, where part or all thenormal-density coarse aggregates are replaced withstructural-grade low-density aggregates, have densities of1900-2200 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)



LWC – Types





LWC: Structural Applications

• Floor slabs of buildings

• Bridge decks

• Marine structures

• Rehabilitation of bridges and parking decks

• Masonry units



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 thistemperature the raw material bloats, forming avesicular structure that is retained upon cooling. Inthis process, the particle density changes fromabout 2.65 to less than 1.55 after cooling. Containsuniformly distributed pores of 5 to 300 micronswithin a crack-free high-strength vitreous phase.

• Could be natural, of volcanic origin.




Structural-grade low-density aggregate:

• Has absorption coefficients of 5 to more than25%. Recognizing and accounting for this effectis essential in mix design and fabrication ofLWC. The absorbed water is available for long-term hydration, leading to an internal curingeffect.

• 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.




Pre-wetting or preconditioning of low-density aggregate:

• Before using them in the concrete, the aggregates haveto 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 restof the components of the concrete are added and mixed.

• This gives better performance during pumping and lower

slump loss during transportation.




Strength ceiling imposed by low-density aggregate

Maximum attainable strength is limited by the type of coarse aggregate.



LWC

Properties



LWC: DurabilityPerformance record:

A ship, the USS Selma, was cast with low-density concretein 1919. It is now in the Galveston Harbour.

The 12 to 30 mm cover was effective in preventing thecorrosion of the steel reinforcement.

Original strength of the concrete was 28 MPa, and thedensity was 1760 kg/m3. Cores taken in 1980gave compressive strengths of morethan 55 MPa.

Microscopy has revealedthat the aggregate-mortar interfacehas not been penetrated by sea water.

Fractured surface of concrete from the USS Selma, showingaggregate on left and mortar on right (micrograph width = 550 microns)



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 densityaggregates, such as expanded polystyrene, perlite,

blast-furnace slag and vermiculite and rubberparticles.

• 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 andnon-structural panels.



References

• http://www.escsi.org

• http://www.pumice.ca

• Concrete , S. Mindess & J.F. Young, Prentice-Hall,

Englewood Cliffs, New Jersey, USA, 1981

•

High-Performance Concrete , P.-C. Aïtcin, E&FN Spon,London, 1998

• Properties of Concrete , A.M. Neville, Pearson Education,

Delhi, 2004

• Concrete Technology , A.M. Neville & J.J. Brooks, PearsonEducation, Delhi, 2004

• ACI Materials Journal

• Concrete International Journal, ACI

9. Lightweight Concrete

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