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, easily repaired . We believe that ultra-light weight concrete is one of the most fundamental bulk building materials of the future.” A Pattern Language
Reprinted from the
US Department of the Interior Bureau of Reclamation
A Manual for the Control of Concrete Construction, Seventh Edition
United States Printing Office: Denver,Colorado: 1963
CHAPTER VIlI-SPECIAL TYPES OF CONCRETE AND MORTAR
142. Definition and Uses. – Lightweight concrete has been used in this country 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.
143. Types of Lightweight 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.
(c) 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.
When obsidian is heated to the temperature of fusion, gases are released which expand the material. The interiors of the expanded particles are vesicular and the surfaces are smooth and quite impervious. Expanded obsidian has been produced experimentally. The raw material was crushed and screened to size and coated with a fine material of higher melting point to prevent agglomeration.
The rock from which perlite lightweight aggregate is manufactured has a structure resembling tiny pearls compacted and bound together. When perlite is heated quickly it expands with disruptive force and breaks into small expanded particles. Usually, expanded perlite is produced only in the sand sizes. Concrete made with expanded perlite has a unit weight ranging from 50 to 80 pounds per cubic foot. It is a very good insulating material.
Vermiculite is an alteration product of biotite and other micas. It is found in California, Colorado, Montana, and North and South Carolina. The color is yellowish to brown. On calcination, vermiculite expands at right angles to the cleavage and becomes a fluffy mass, the volume of which is as much as 30 times that of the material before heating. It is a very good insulating material and is used extensively for that purpose. Concrete made with expanded vermiculite aggregate weighs from 35 to 75 pounds per cubic foot; the strengths range from 50 to 600 pounds per square inch.
144. Properties of Lightweight Aggregates – Properties of various lightweight aggregates, as reflected by those of the resulting concrete, vary greatly. For example, the strength of concrete made with expanded shale and clay is relatively high and compares favorably with that of ordinary concrete. Pumice, scoria, and some expanded slags produce a concrete of intermediate strength; perlite, vermiculite, and diatomite produce a concrete of very low strength.
The insulation properties of the low-strength concretes, however, are better than those of the heavier, stronger concretes. The insulation value of the heaviest material (crushed shale and clay concrete) is about four times that of ordinary concrete.
All the lightweight aggregates, with the exception of expanded shales and clays and scoria, produce concretes subject to high shrinkage. Most of the lightweight concretes have better nailing and sawing properties than do the heavier and stronger conventional concretes. (For information on “nailing concrete, see part B of this chapter.) However, nails, although easily driven, fail to hold in some of these lighter concretes.
145. Construction Control of Lightweight Concrete. – Commercially available lightweight aggregate is usually supplied in three principal sizes depending upon its application. These are fine, medium, and coarse and range in size to ¾-inch maximum. Production of uniform concrete with lightweight aggregate involves all the procedures and precautions that have been discussed elsewhere in this manual in connection with ordinary concrete. However, the problem is more difficult where lightweight aggregates are used because of greater variations in absorption, specific gravity, moisture content, and amount and grading of undersize. If unit weight and slump tests are made frequently and the cement and water content of the mix are adjusted as necessary to compensate for variations in the aggregate properties and condition, reasonably uniform results can be obtained.
Concretes made with many of the lightweight aggregates are difficult to place and finish because of the porosity and angularity of the aggregates. In some of these mixes the cement mortar may separate from the aggregate and the aggregate float toward the surface. When this occurs, the condition can generally be improved by adjusting the grading of the aggregates. This can be done by crushing the larger particles, adding natural sand, or adding filler materials. The placeability can also be improved by adding an air-entraining agent. The amount of fines to be used is governed by the richness of the mix; as the sand content is increased, the optimum amount of fines is reached when the concrete no longer appears harsh at the selected air content. From 4 to 6 percent air is best for adequate workability, and the slump should not exceed 6 inches.
To insure material of uniform moisture content at the mixer, lightweight aggregate should be wetted 24 hours before use. This wetting will also reduce segregation during stockpiling and transportation. Dry lightweight aggregate should not be fed into the mixer; although this will produce a concrete which can be readily placed immediately after being discharged, continuing absorption by the aggregate will cause the concrete to segregate and stiffen before placement is completed.
It is generally necessary to mix lightweight concrete for longer periods than conventional concrete to assure proper mixing. Workability of lightweight concrete with the same slump as conventional concrete may vary more widely because of differences in type, porosity, specific gravity, etc., of the materials. For the same reason, the amount of air-entraining agent required to produce a certain amount of air may also vary widely Continuous water curing, by covering with damp sand or use of a soil- soaker hose, is particularly advantageous where concrete is made with lightweight aggregate.