RE: LIGHTWEIGHT CONCRETE
Lightweight concrete.docx (Size: 49.65 KB / Downloads: 443)
Concrete is a plastic medium and has incredible potential for creating fluid, sculptural forms. It should be admitted that some of the dullest structures around us are made of concrete but dullness isn't a limitation inherent in the material.
Light weight concrete differs from heavy concrete by it's use of naturally light weight materials (aggregates) such as pumice (volcanic stone) in place of the sand and gravel used in ordinary structural concrete mixes. It only weighs half as much. Not all concrete is ugly, hard, cold and difficult to work with. There exists a whole range of light weight concretes "which have a density and compressive strength very similar to wood. They are easy to work with, can be nailed with ordinary nails, cut with a saw, drilled with woodworking tools, and easily repaired. We believe that ultra-light weight concrete is one of the most fundamental bulk building materials of the future."
Lightweight concrete has been used since the eighteen centuries by the Romans. The application on the ‘The Pantheon’ where it uses pumice aggregate in the construction of cast in-situ concrete is the proof of its usage. In USA and England in the late nineteenth century, clinker was used in their construction for example the ‘British Museum’ and other low cost housing. The lightweight concrete was also used in construction during the First World War. The United States used mainly for shipbuilding and concrete blocks. The foamed blast furnace-slag and pumice aggregate for block making were introduced in England and Sweden around 1930s. Nowadays with the advancement of technology, lightweight concrete expands its uses. For example, in the form of perlite with its outstanding insulating characteristics. It is widely used as loose-fill insulation in masonry construction where it enhances fire ratings, reduces noise transmission, does not rot and termite resistant. It is also used for vessels, roof decks and other applications
Light weight concrete is about one half the weight of hard structural concrete. It can be mixed from a variety of light weight aggregates including vermiculite, perlite, scoria, and pumice. Some form of suitable aggregate is available most everywhere in the world. Our locally available aggregate here in San Miguel is a type of pumice (espumilla or arenilla) which we typically mix 8:1 or 10:1 (by volume) with cement for walls, and 5:1 for roofs. Most lightweight concrete has a good R-value and is a good insulator of heat and sound. It is used as soundproofing in subway stations. It has tremendous sculptural possibilities and is ideal for monolithic, wall-roof construction.
Lightweight concrete, weighing from 35 to 115 pound per cubic foot, has been used in the United States for more than 50 years. The compressive strength is not as great as ordinary concrete, but it weathers just as well. Among its advantages are less need for structural steel reinforcement, smaller foundation requirements, better fire resistance and most importantly, the fact that it can serve as an insulation material! It can cost more that sand and gravel concrete, and it may shrink more upon drying.
WHAT IS LIGHTWEIGHT CONCRETE
Lightweight concrete can be defined as a type of concrete which includes an expanding agent in that it increases the volume of the mixture while giving additional qualities such as nailability and lessened the dead weight. It is lighter than the conventional concrete with a dry density of 300 kg/m3 up to 1840 kg/m3; 87 to 23%lighter۔
Lightweight concrete has been used in USA for more than 50 years. Its strength is roughly proportional to its weight and its resistance to weathering is about the same as that of ordinary concrete. As compared with the usual sand and gravel concrete it has certain advantages and disadvantages. Among the former are the savings in structural steel supports and decreased foundation sizes because of decreased loads, and better fire resistance and insulation against heat and sound. Its disadvantages include greater cost (30 to 50 percent), need for more care in placing, greater porosity, and more drying shrinkage.
The principal use of lightweight concrete in Bureau work is in construction of underbeds for floors and roof slabs, where substantial savings can be effected by decreasing dead load. It is also used in some insulated sections of floors and walls.
Lightweight concrete may be obtained through use of lightweight aggregates, as discussed in the following sections, or by special methods of production. These methods include the use of foaming agents, such as aluminum powder, which produces concrete of low unit weight through generation of gas while the concrete is still plastic. Lightweight concrete may weigh from 35 to 115 pounds per cubic foot, depending on the type of lightweight aggregate used or the method of production. In Bureau construction, lightweight concretes have been limited to those whose lightness depends on inorganic aggregates which are light in weight.
Lightweight concrete may be made by using lightweight aggregates, or by the use of foaming agents, such as aluminum powder, which generates gas while the concrete is still plastic. Natural lightweight aggregates include pumice, scoria, volcanic cinders, tuff, and diatomite. Lightweight aggregate can also be produced by heating clay, shale, slate, diatomaceous shale, perlite, obsidian, and vermiculite. Industrial cinders and blast-furnace slag that has been specially cooled can also be used
TYPES OF LIGHT WEIGHT AGGREGATE
Lightweight aggregates are produced by expanding clay, shale, slate, diatomaceous shale, perlite, obsidian, and vermiculite through application of heat; by expanding blast-furnace slag through special cooling processes; from natural deposits of pumice, scoria, volcanic cinders, tuff, and diatomite; and from industrial cinders. Lightweight aggregates are sold under various trade names.
(a) Cinders - Cinders used as aggregates are residues from high-temperature combustion of coal or coke in industrial furnaces. Cinders from other sources are not considered suitable. The Underwriters Laboratories limit the average combustible content of mixed fine and coarse cinders for manufacturing precast blocks to not more than 35 percent by weight of the dry, mixed aggregates. Sulfides in the cinders should be less than 0.45 percent and sulfate should be less than 1 percent. Stockpiling of cinders to permit washing away of undesirable sulphur compounds is recommended. Cinders have been used in concrete construction with satisfactory results for more than 50 years. Cinder concrete weighs about 85 pounds per cubic foot, but when natural sand is used to increase workability in monolithic construction the weight is from 110 to 115 pounds per cubic foot.
(b) Expanded Slag - Expanded slag aggregates are produced by treating blast-furnace slag with water. The molten slag is run into pits containing controlled quantities of water or is broken up by mechanical devices and subjected to sprays or streams of water. The products are fragments that have been vesiculated by steam. The amount of water used has a pronounced influence on the products, which may vary over wide ranges in strength and weight. Concrete in which the aggregate is expanded slag only has unit weights ranging from 75 to 110 pounds per cubic foot.
© Expanded Shale and Clay - All expanded shale and clay aggregates are made by heating prepared materials to the fusion point where they become soft and expand because of entrapped expanding gases. With the exception of one product made from shale, the raw material is processed to the desired size before it is heated. In some cases the particles are coated with a material of higher fusion point to prevent agglomeration during heating. In general, concrete made with expanded shale or clay aggregates ranges in weight from 90 to 110 pounds per cubic foot.
(d) Natural Aggregate - Pumice, scoria, volcanic cinders, tuff, and diatomite are rocks that are light and strong enough to be used as lightweight aggregate without processing other than crushing and screening to size. Of these, diatomite is the only one which is not of volcanic origin.
Pumice is the most widely used of the natural lightweight aggregates. It is a porous, froth-like volcanic glass which is usually white-gray to yellow in color, but may be red, brown, or even black. It is found in large beds in the Western United States and is produced as a lightweight aggregate in several States, among which are California, Oregon, and New Mexico. Concrete made with sound pumice aggregate weighs from 90 to 100 pounds per cubic foot. Structurally weak pumice having high absorption characteristics may be improved in quality by calcining at temperatures near the point of fusion.
Scoria is a vesicular glassy volcanic rock. Deposits are found in New Mexico, Idaho, and other Western States. Scoria resembles industrial cinders and is usually red to black in color. Very satisfactory lightweight concrete, weighing from 90 to 110 pounds per cubic foot, can be made from scoria.