Concrete is a heavy construction material which requires substantial investment in transport and handling. However, it has many benefits and is very useful in reducing dead loads, particularly in areas of high groundwater, poor soils or for structural support in existing buildings. It also helps to save on foundations, improve thermal properties and guard against steel corrosion. In addition it can reduce the amount of formwork and propping required.
Lightweight aggregate concrete (LWC) is a type of concrete where the aggregates used are lighter in density than standard concrete. This is achieved by replacing part or all of the normal weight coarse and fine aggregates with lightweight materials. Structural LWC has a density in-place of about 90 to 115 lb/ft3. This means it is only slightly heavier than the equivalent strength normal weight concrete, but considerably stiffer due to its reduced self-weight.
The strength of LWC is determined by the grading, porosity and water-cement ratio. Usually pozzolanic material is added to the mix along with plasticizer admixtures in order to improve its workability, but care must be taken not to overdo this as excess moisture can lead to segregation of the concrete and its subsequent performance.
In general, the design of LWC is similar to that of conventional concrete, but specific rules apply regarding the maximum air content within the mix and a specific set of tests for checking this. The cylinder and cube test methods described in Eurocode 2 Part 1-1 should be followed for this. Air entrainment of concrete is achieved by the inclusion of porous aggregates with low apparent specific gravity. These are generally expanded shale, clay or slate materials that have been fired in a rotary kiln to develop a porous structure. Other natural materials such as pumice, scoria, volcanic tuff and diatomite are also used, and industrial cinders or blast furnace slag that has been air-cooled can be included too.
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