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. Lightweight Concrete
Why use lightweight concrete? • The primary use of lightweight concrete is to reduce the dead load of a concrete structure. • Which then allows the structural designer to reduce the size of column, footings and other load bearing elements • Lightweight concrete mixtures can be designed to achieve similar strength as normal weight concrete.
Lightweight concrete provides a more efficient strength-to-weight ratio in structural elements. • The cost of lightweight concrete is offset by size reduction of structural elements, less reinforcing steel and reduced volume of concrete, resulting in lower overall cost. • 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)
Trench reinstatement • Is an ideal material for trench reinstatement (the filling of trenches dug in roads when pipes are laid or repairs are carried out). The traditional methods of filling trenches in the roads, i.e. The use of granular fill materials, result in settlement and damage to the road and potentially, to the pipes. With foamed concrete there is no settlement; and because the foamed concrete is very fluid, it will fill any voids and cavities in the trench sides
Bridge Abutment • Is particularly suitable for bridge abutments because it does not impose the large lateral loads, which can be a problem when using traditional granular materials. With traditional abutments, there is a lot of sideways pressure against the bridge walls caused by the materials used and their compaction.
Void Filling • Is also very useful for void filling. As it is very fluid it will pour into even the most inaccessible places. It can be used for planned work, but also in emergencies to provide stability and support very quickly.
Roofing Insulation • Has been supplied for roofing insulating. A low density mix is chosen and the resulting air content gives the material excellent thermal insulation properties. The low density also has the advantage that it does not significantly add to the overall weight of the roof.
Road Sub-Base • An be used to make road structures less heavy. This helps solve the problem where the traditionally heavy road structures cause severe settlement of the road, particularly in areas of soft ground. By constructing the road sub-base from a lightweight material, the overall weight of the structure can be greatly reduced
Floor Construction • Provides very good material for floor construction. It is ideal for building sub-floors quickly and cheaply and can be used for levelling terrain and raising floor levels as well as for insulation purposes
Fire Breaks • The excellent fire resistant properties makes it an ideal material for fire breaks in buildings where there are large undivided spaces. It is used to prevent flame penetration through the services void between floor and ceiling in modern construction, and also to protect timber floors in old houses
Sound Insulation • Reduces the passage of sound, both from background noise and due to impact. It is, therefore, an ideal material for internal walls and suspended floors in multi-storey buildings, especially ones with communal use
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 Lightweight Concrete (LWC) Maximum attainable strength is limited by the type of coarse aggregate.
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. LWC: Durability Fractured surface of concrete from the USS Selma, showing aggregate on left and mortar on right (micrograph width = 550 microns)
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. Ultra-Lightweight Concretes