Working with Versatile Lightweight Concrete
While still not light and resilient enough for a vehicle tire, properties of lightweight concrete lend creative architectural design possibilities to the realm of structural engineering. The properties of lightweight concrete have some remarkable advantages. While the term “lightweight concrete” sounds strange, the material itself is anything but contradictory. Regular Portland cement concrete production consists of loading aggregates (gravel and sand), cement, and water, then mixing to a thick slurry consistency. The mixing process usually incorporates a small amount of air into the concrete as well. Once the mix has cured it weighs in at around 145 pounds per cubic foot, give or take 5 pounds or so.
Most of this variation is due to the amount of air present and minor differences in the weight of the aggregates. Clearly, to make concrete lighter, more air and/or lighter aggregates need to be used. When more air or gas is incorporated, the result is referred to as lightweight aerated or foamed concrete. When lighter aggregates are loaded, the material is generically referred to as lightweight aggregate concrete. Lightweight aerated concrete typically weighs in between 3 pounds per cubic foot up to around 70 pounds per cubic foot. While this is still not suitable for a vehicle tire, it is tremendously useful for a wide variety of architectural applications. Due to the entrapped gases, it also has good insulating properties.
Roofing, walls, fences, ornaments, vaulted ceilings, sculptures, stairs, doors, even canoes are good examples of the usefulness of this material. It can be cast into blocks or decorative panels to create the impression of stone without the massive weight. And of course it still retains much of the durability of regular weight concrete. The major departure from regular concrete is lower structural and compressive strength.
Lightweight aggregates can consist of naturally occurring gas expanded materials such as vermiculite, pumice, certain volcanic rocks, and some diatomaceousearths. They can also be manufactured by heating some types of clay, shale, slate, and obsidian in a rotary kiln, or by using air quenched cinders and blast-furnace slag. These expanded aggregates incorporate air into a porous structure which retains some degree of structural strength. When used in structural concrete the mix has a density ranging from 90 pounds per cubic foot up to about 115 pounds per cubic foot.
However with a suitable mix design strengths similar to regular weight concrete can be attained. The major advantages of this material over regular concrete are lower densities, better fire resistance and greater insulating properties. Less reinforcing steel is required and a lower dead load of the structure can be achieved. This in turn yields smaller supporting columns, footings, and other engineering elements. It also results in improved seismic characteristics, longer achievable spans, thinner cross sections, and generally smaller sized structural members.
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