Part 1

Produced by Tom Cosmas compiled from images made available by The Internet Archive.

Transcriber Notes

Text emphasis is denoted as _Italics_ and =Bold=. Whole numbers and fractional parts denoted as: 33-3/4.

U. S. DEPARTMENT OF AGRICULTURE

FARMERS' BULLETIN No. 1279

PLAIN

CONCRETE

for

FARM USE

The successful and economical use of concrete involves the selection of suitable materials, the correct proportioning of mixtures in the development of qualities to meet specific requirements, the proper placing and the care of the green concrete.

A concrete of great strength is uneconomical if a weaker mixture will serve and a cheap or weak concrete is costly if it does not fulfill all requirements. The cost of concrete depends not only upon the price of the materials and labor but also upon the judicious use of the two. Lack of foresight in locating the mixing plant, in the design of forms, and in planning the successive operations may cause unnecessary expense, while neglect of any one of the precautions which should be observed is likely to result in unsatisfactory work.

The bulletin discusses the requirements of good concrete and describes the making and placing of plain concrete according to the best practice.

Washington, D. C.

Issued October, 1922

PLAIN CONCRETE FOR FARM USE.

T. A. H. Miller, _Agricultural Engineer, Division of Agricultural Engineering, Bureau of Public Roads_.

CONTENTS.

Page.

Introduction 1 Materials 1 Proportioning the materials 6 Quantities of materials required 7 Consistency 8 Estimating 9 Forms 10 Mixing 13 Placing 18 Care of concrete 21 Protection from freezing weather 21 Contraction and expansion joints 23 Lintels 23 Surface finish 24 Concrete exposed to fire 25 Water-tight concrete 26

INTRODUCTION.

Portland cement concrete is the mass formed by mixing Portland cement, sand, gravel (or particles of other suitable materials), and water.

The quality of concrete may be made to conform to certain requirements which vary with the purpose of the structure in which the material is to be used; economy, strength, water-tightness, fire resistance, or resistance to wear and shock may be the chief requisite. The character of the constituent materials, the proportions in which they are used, the consistency, the method of mixing, and the placing and curing of the concrete are important factors in securing the desired qualities of the finished product.

Total failure or a product which does not give the service expected is often the result of the nonobservance of practices recognized as necessary in the preparation and use of concrete. This bulletin is intended to assist the inexperienced in making and using concrete suitable for general farm construction and is confined to a discussion of the rudiments of plain (not reinforced) concrete work.

MATERIALS.

CEMENT.

Portland cement is used because it is the only kind adapted to general construction. Other cements are manufactured but they possess individual characteristics that restrict their use. The word Portland is not a trade name, but signifies the kind and distinguishes it from the slag, natural, and other cements.

A number of brands of Portland cement are manufactured, most of which are made to meet the requirements of a fixed standard adopted by the United States Government and the American Society for Testing Materials. Cement always should be tested for use in important work, but this is impractical for the user of small amounts and it is generally safe practice to omit the test if a reliable brand of Portland cement of American manufacture is selected, especially if the dealer's or manufacturer's guaranty that it meets the standard is secured.

The following simple test for soundness is easily made and is on the side of caution. Make a ball, about 1-1/2 inches in diameter, if neat cement and water; place it under a wet cloth and keep it moist for 24 hours, then put the ball in a vessel of water; allow the water to come to the boiling point slowly and to boil for 3 hours. A good cement will not be affected, but an inferior one will check, crack, or go to pieces entirely.

Portland cement is shipped in paper bags, cloth sacks, and wooden barrels (sometimes in bulk). For the average user the cloth sack is the best container, as it is easier to handle; and while the manufacturers charge more for this kind of package, they allow a rebate for the return of the sacks in good condition. A sack of Portland cement weighs 94 pounds and a barrel contains the equivalent of four sacks.

STORING.

As cement readily absorbs moisture from the atmosphere, it should be stored in a dry place; if exposed to dampness it soon becomes lumpy, or even a solid mass, and in this condition it is useless and should be thrown away. The lumps caused by pressure in piling the sacks are not injurious. They can be pulverized easily, thus distinguishing them from those due to dampness.

Cement never should be stored on the ground. Build a raised platform for it and keep it away from the sides of the shelter. As it is heavy, care should be taken not to overload the supporting floor.

FINE AGGREGATE (SAND).

All grains, small pebbles, or particles of broken stone are considered as sand if they will pass through a wire screen with one-fourth inch meshes. The particles or grains should be hard and well graded and should vary in size, as a stronger concrete is thus obtained than when the size of the grains is nearly uniform. If a large proportion of the sand is very fine an extra quantity of cement should be used and if exceptionally fine it is advisable to use 25 per cent more cement.

The sand should be clean; that is, free from vegetable matter, loam, or any considerable amount of clay. If the hands are soiled when a small quantity of sand is rubbed between them the following test should be made: Put 4 inches of sand into a pint preserving jar, fill with clear water to within an inch of the top, fasten the lid, and shake the jar vigorously until the whole is thoroughly mixed. Set the jar aside and allow the contents to settle. The sand will settle to the bottom with the clay and loam on top of it. If more than three-eighths of an inch of clay or loam shows, the sand should be rejected or washed. The difference in fineness and color shows clearly the line of division between the clay or loam and the sand.

Should sand require washing the simplest way for small quantities is to build a loose board platform from 10 to 15 feet long, with one end higher than the other. On the lower end and sides nail 2 by 6 inch boards. Spread the sand over the platform in a layer 3 or 4 inches thick and wash with water. The water may be supplied by any means which will cause agitation of the sand and allow the lighter material to run off with the water. When pressure or a head is obtainable the water is most easily applied by means of a garden hose. The washing should be started at the higher end and the water allowed to run through the sand and over the 2 by 6 inch piece at the bottom. Figure 1 illustrates a convenient trough for washing larger quantities.

A small amount of clay, provided it is not in lumps, does not injure sand, but amounts over 10 per cent should be washed out.

COARSE AGGREGATE (STONE, GRAVEL, ETC.).

The larger particles used in concrete may be gravel, broken stone, air-cooled blast-furnace slag, or other suitable materials. The coarse aggregate should be sound and clean, that is, free from disintegrated or soft particles, loam, clay, or vegetable matter. Air-cooled blast-furnace slag should weigh at least 70 pounds per cubic foot. The best results are obtained from a mixture of sizes graded from those retained on a one-fourth inch screen to those passing a three-fourths to 2 inch ring, depending upon the work. Ordinarily the greatest dimension of any particle should not be over one-fourth of the thickness of the concrete work.

GRAVEL.

Gravel which is too dirty for use usually can be detected by observation. It may be washed in the same manner as sand. Lumps of clay should be eliminated and care should be taken to see that the gravel is not coated with a film of clay or loam which will prevent the bonding of the cement.

BROKEN STONE.

Broken stone should be clean, hard, and of a size suited to the character of the work, and the same care in grading should be exercised as in the case of gravel. Trap, granite, hard limestone, and hard sandstone are commonly used. The composition and physical character of the stones should be considered, as some possess qualities that limit their use under certain conditions (see Substitutes for gravel).

Field stones are common in many localities and their use, when crushed, may be economical. The finer particles, after the dust is removed, can be used as sand. Small stone crushers, operated by three or four horsepower gasoline engines, can be purchased at a relatively low price and may prove profitable if a large quantity of stone is needed.

BANK-RUN GRAVEL.

Bank or creek gravel, which will answer the purpose of sand and gravel combined, sometimes can be obtained, and frequently it is used in small jobs of concrete work just as it comes from the pit or creek. Although such gravel occasionally contains nearly the right proportions of sand and gravel, in the majority of sand pits and gravel banks there is a great variation in the sizes of the grains and pebbles or gravel and in the relative quantity of each. It is advisable to screen the sand and gravel and to remix them in the correct proportions, as well-graded aggregates make stronger concrete and, ordinarily, enough cement will be saved to pay for the cost of screening.

Experience has shown that it is advisable to screen bank gravel twice; first over a screen with large meshes to eliminate particles too large for use. The size of the mesh will depend upon the nature of the work involved (see Coarse aggregate); then the material which has passed through this screen should be sifted again over a screen with one-fourth inch meshes. All material which passes the latter screen may be considered sand and should conform to the characteristics discussed under "Fine aggregate."

SUBSTITUTES FOR GRAVEL OR STONE.

For general work gravel or broken stone always is preferred to other coarse aggregate. Other materials at times are easier to obtain and, when used with discretion, will provide a satisfactory concrete.

Broken terra cotta, brick, and old concrete, if hard and strong, may be used for unimportant work where no great strength is required, but special care should be taken that the particles do not show on the finished surface.

The maxim that a chain is only as strong as its weakest link applies to concrete. If the coarse aggregate is weaker than the cement mortar, as in the case of some sandstones, it should be used with caution. The aggregate may have properties that render it unsuitable for use under certain conditions; for instance, cinders should not be used if water-tightness or strength is expected, but they are useful for fireproofing. Material that disintegrates or flakes when heated is undesirable in places exposed to high temperature; thus marble and some limestones should not be used in fireplaces. Some aggregates when exposed at the surface of concrete are apt to cause discolorations, and when this would be objectionable aggregates of this type should be avoided. Flat or elongated slab-like fragments should be avoided, as particles of this shape do not bond well; slate and shale are examples.

CINDERS.

Cinders should be composed of hard, clean, vitreous clinkers, free from sulphides, soot, and unburned coal or ashes. As a precaution against the presence of small amounts of detrimental substances, cinders should be soaked thoroughly with water 24 hours before being used. If clean they will not discolor the hands when a small quantity is rubbed between the palms.

Cinder concrete, on account of its light weight, commonly is used for filling between sleepers of floors and grading roofs, and frequently for fireproofing, for which it is very effective. Cinders should never be used when the concrete is to be subjected to heavy loads or abrasion.

LAVA ROCK.

Lava rock varies widely in chemical composition and physical qualities. In some instances lavas are so light and frothy or contain so large a proportion of easily oxidizable material that they are wholly unsuited for concrete work. In general, the lava rock found in the Northwestern States is a suitable substitute for gravel. Rhyolite, a light colored volcanic rock, and many of the darker colored basaltic lavas can well be used for concrete for building purposes.

WATER.

Water should be clean and free from strong acid and alkali. Sea or brackish water should not be used if fresh water can be obtained.

PROPORTIONING THE MATERIALS.

In mixing concrete various proportions of cement, sand, gravel, and water are employed, depending upon the purpose for which the concrete is to be used. The ideal mixture is one in which all the spaces or voids between the grains of sand are filled with the cement and all the voids in the gravel are filled with the cement-sand mortar. This perfection is seldom attained, because the voids in each lot of gravel and sand vary slightly, and in order to be absolutely safe a little more sand and cement than will just fill the voids are used.

The strongest concrete is not required in every structure, and, in many instances, the cost of it would be unwarranted. For important work involving large quantities of materials of unknown qualities, tests should be made to determine the best proportions. Such tests, being rather complicated, are made usually in a laboratory, and are not practical for the user of small quantities of concrete. Various proportions have been tested by experienced engineers to determine which, under average conditions, will develop the greatest strength, best resist wear, and assure greatest impermeability or water-tightness. The mixtures given below have been found to meet the requirements indicated, and having been adopted as arbitrary standards, are recommended for use in farm concrete work. The amount of water required is discussed under "Consistency."

ARBITRARY MIXTURES.

=Rich mixture.=--Used for concrete subject to high stresses or where exceptional water-tightness and resistance to abrasion are desired: 1:1-1/2:3; i. e., 1 part cement, 1-1/2 parts sand, and 3 parts gravel.

=Standard mixture.=--Used generally for reinforced concrete and water-tight work: 1:2:4; i. e., 1 part cement, 2 parts sand, and 4 parts gravel.

=Medium mixture.=--Used for plain concrete of moderate strength: 1:3:5; i. e., 1 part cement, 3 parts sand, and 5 parts gravel.

Leaner mixtures are sometimes used after a test has proved them to be suitable for the work at hand.

It will be noticed that always in indicating the proportions the first number refers to the cement, the second to the sand, and the third to the gravel. The three materials must be measured by volume, using the same unit. The cubic foot is a convenient measure, because a sack of cement, weighing 94 pounds, is considered to contain 1 cubic foot.

When the coarse aggregate (gravel, etc.) is omitted the mixture is generally spoken of as mortar and the proportions are indicated thus, 1:2, meaning 1 part cement and 2 parts sand. Mortar is used for plastering, stucco, top coats of floors, and for laying masonry.

QUANTITIES OF MATERIALS REQUIRED.

More concrete can be made from given volumes of aggregates if the gravel used is graded from fine to coarse than if the particles are too nearly of one size, because the small stones help to fill the voids between the larger ones and less sand-cement mortar is required. The extra mortar thus adds to the volume of the concrete.

A common mistake to be guarded against is to assume that the volume of concrete produced is equal to the quantity of sand plus the gravel as indicated in the proportion. For instance a 1:2:4 mixture will not produce 6 cubic yards of concrete, if 2 yards of sand and 4 yards of gravel are used, because the sand will lodge in the voids between the pebbles. If 6 cubic yards of concrete are desired it will be necessary to use 2.7 cubic yards of sand and 5.34 cubic yards of gravel.

Table 1 shows the quantity of cement, sand, and gravel required under average conditions for the indicated proportions.

Table 1.--_Materials for 1 cubic yard of rammed concrete._

Proportions. | | | | | | Cement.| Sand. | Gravel.| Cement. | Sand. | Gravel. -------+-------+--------+----------+----------+----------- | | | _Sacks._ |_Cu. yds._| _Cu. yds._ 1 | 1 | --- | 19.20 | 0.74 | --- 1 | 2 | --- | 13.48 | 1.00 | --- 1 | 2-1/2| --- | 11.00 | 1.01 | --- 1 | 3 | --- | 10.40 | 1.16 | --- 1 | 1 | 2 | 10.52 | .39 | 0.78 1 | 1-1/2| 3 | 7.64 | .42 | .85 1 | 2 | 4 | 6.04 | .45 | .89 1 | 2-1/2| 5 | 4.96 | .46 | .92 1 | 3 | 5 | 4.64 | .52 | .86 1 | 3 | 6 | 4.24 | .47 | .94 -------+-------+--------+----------+----------+----------

CONSISTENCY.

The quantity of water used in mixing has a very great influence on the strength of the concrete. An excess of water weakens the concrete, while an insufficient amount prevents thorough mixing.

Therefore, only sufficient water should be used to produce a workable or plastic mixture.

Recent tests have proved that to secure the greatest strength the concrete should be mixed considerably drier than has heretofore been customary. Of course, for thin walls containing closely placed reinforcement, or for water-tightness, a fairly wet mix is necessary. A little experience will show the proper amount of water to use.

A very rough estimate of the quantity of water required in mixing for general work is 4 to 5 gallons to each sack of cement.

Three degrees of consistency (corresponding to different proportions of water) are used in general practice, namely, wet, medium, and dry. In the light of recent investigations it is thought the wet mixture of present-day practice contains too much water. The following definitions are therefore recommended:

=Wet mixture.= One that does not flow readily and yet can not be piled up. It is recommended for thin sections when reinforcement is closely placed.

=Medium mixture.= One that is between the wet and dry mixture. This consistency is recommended for general work.

=Dry mixture.= One about like damp earth. If a handful is squeezed it will retain its shape. This consistency requires thorough ramming to eliminate voids and is used when forms are to be removed immediately, but should not be used where a water-tight job is expected. The porous structure of the concrete in Figure 2 is due to the fact that it was placed as a dry mixture.

ESTIMATING.

ESTIMATING CONCRETE.

In estimating the amount of concrete in a given piece of work and the quantities of materials required, the unit of measurement is usually the cubic yard (27 cubic feet). The following examples will explain best the method of determining the quantities required:

=Example 1.=--A wall 9 inches thick, 12 feet high, and 30 feet long has a door opening 3 feet wide and 6 feet high, also a footing 18 inches wide and 9 inches deep. The concrete is to be mixed in the proportions of 1:2:4.

The volume of the footing is found by multiplying together the dimensions expressed in feet, thus, 1-1/2 × 3/4 × 30 = 33-3/4 cubic feet. Similarly, the volume in the wall is 3/4 × 12 × 30, less the door opening 3/4 × 3 × 6 = 256-1/2 cubic feet.

The total volume in footing and wall is 290-1/4 cubic feet = 10-3/4 cubic yards.

To find the quantity of cement, sand, and gravel, multiply the amounts for 1 cubic yard, indicated in line 7 of Table 1, by 10-3/4, and it will be found that 65 sacks of cement, 4.83 cubic yards of sand, and 9.56 cubic yards of gravel are necessary to build the wall.

=Example 2.=--A pavement 27 feet long, 4 feet wide, and 6 inches thick has a 5-inch base mixed in the proportions of 1:3:5 and a 1-inch surface mixed in the proportions of 1:2.

The volume in the base is 27 × 4 × 5/12 = 45 cubic feet = 1-2/3 cubic yards.

The volume in the top is 27 × 4 × 1/12 = 9 cubic feet = 1/3 cubic yard.

Multiplying the quantities in line 9 of Table 1 by 1-2/3 and those in line 2 by it is found that the base requires 7.74 sacks cement; 0.86 cubic yard sand; 1.43 cubic yards gravel; and the top requires 4.49 sacks cement; 0.33 cubic yard sand.

=Example 3.=[1]--A tank 9 feet inside diameter has walls 6 inches thick and 4 feet high (above the floor). The floor is 6 inches thick, the concrete is to be 1:2:4.

The volume in the floor is 10/2 × 10/2 × 22/7 × 1/2 = 39-2/7 Cubic feet.

The area of the larger circle is 5 × 5 × 22/7 = 78-4/7 cubic feet.

The area of the smaller circle is 4-1/2 × 4-1/2 × 22/7 = 63-4/7 cubic feet.

The area of the wall, therefore, is 15 cubic feet and the volume is 15 × 4 = 60 cubic feet.

The total volume in the structure is 99-2/7 cubic feet or 3-2/3 cubic yards. Multiplying the quantities in line 7 of Table 1 by 3-2/3, it is found that the following material is needed: 22.14 sacks of cement; 1.65 cubic yards of sand; 3.27 cubic yards of gravel.

[1] A practical rule in finding the area of a circle is to multiply one-half the diameter (radius) by itself and the product by 22/7. In finding the volume in the wall of a circular structure, such as a silo or tank, the area of the circle formed by the inside circumference is deducted from the area of the circle formed by the outside circumference and the remainder is multiplied by the height.

FORMS.

Forms are required to hold the concrete in place until it has attained sufficient strength to sustain itself and the initial loads to which it may be subjected. Concrete is plastic and will assume the shape of the form, thus any imperfection or impression on the face of the forms will be reproduced.

Wood is commonly used for forms, as it can be easily worked into different shapes, though various other materials sometimes are better adapted to special conditions. Cast iron, for instance, is suitable for casting small objects that are to be reproduced in quantities, such as concrete block or tile; plaster of Paris, glue, or moist sand are employed for casting ornaments or to produce a fine, smooth surface; sheet metal is suitable when the forms can be used repeatedly or for such circular structures as silos. When the sides of an excavation are not likely to cave in the earth may serve as a form.

WOOD FORMS.