Lightweight Concrete Lightweight Aggregate Concrete CE 342 Concrete Technology Prof. Ravindra Gettu IIT Madras
Lightweight Concrete Definitions: Pioneering applications: 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)
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 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.
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.
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.
Lightweight Concrete (LWC) Strength ceiling imposed by low-density aggregate Maximum attainable strength is limited by the type of coarse aggregate.
LWC Properties
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)
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.
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, Pearson Education, Delhi, 2004 ACI Materials Journal Concrete International Journal, ACI