Plain and reinforced concrete; hand book

Preface

This book is written as a text book for use of the students taking the course in Plain and Reinforced Concrete at the Massachusetts Charitable Mechanic Association Trade School. The wording has been made as elementary as possible, and only such technical terms have been employed as are generally used on actual construction work.

The subject of Plain and Reinforced Concrete has been divided into chapters, each chapter developing the subject by first taking up in detail how to select and test the materials of which concrete is composed and then taking up the proper ways and means of combining them to produce a first class concrete. Each chapter is followed by a series of questions for the purpose of assisting those students who wish to review the amount of real knowledge which they have derived from a study of the chapter by answering the questions fully without referring to the text.

The author wishes to state that he was assisted in this work by Mr. Frank A. Baker, who figured the tables, made the drawings, took a number of the photographs, and vis6d the manuscript. He wishes to thank the following firms for their kindness in furnishing some of the photographs: — Wilson and Tomlinson, Benjamin Fox, Inc., Edward A. Tucker, and Sanford E. Thompson.

For a more technical treatment of the subject of Plam and Reinforced Concrete the author advises the well-known work of Taylor and Thompson, ''Concrete Plain and Reinforced/' Another splendid book by the same authors on cost of concrete work, titled '' Concrete Costs, " is recommended to those who are interested in estimating, contracting, etc.

THE AUTHOR

CHAPTER I. MATERIALS FOR PLAIN CONCRETE Page

Definition of Plain Concrete 1

Definition of Portland Cement 2

Kinds of Cement 3

Setting of Cement 3

Normal and Abnormal Cements 3

Packing Portland Cement 4

Storing Portland Cement 4

Definition of Fine Aggregate or Sand 5

Kinds of Sand 5

How TO Test for a Clean Sand ^ 7

Method of Using Dirty Sand 7

Definition of Coarse Aggregates 9

Crushed Stone vs. Gravel for Coarse Aggregate 9

Natural Mixtures of Bank Sand and Gravel 9

Screenings from Crushed Stone 10

Water 10

CHAPTER 11. PROPORTIONING OF MATERIALS

Theory of Proportioning 12

Kinds of Mixes 13

Amount of Cement vs. Strength of Concrete 14

Use of Very Fine Sand 14

Screening Bank Gravel 14

Volume of Concrete vs. Volume of Aggregate 15

CHAPTER III. MIXING CONCRETE. Specifications for Hand Mixing 17

Tools and Apparatus 18

Sizes of Gangs 19

Method of Measuring 21

Batch Sizes 22

Consistency of Mix 24

Density 25

V

CONTENTS CHAPTER IV. PLACING OF CONCRETE.

Page

Placing Concrete 27

Placing Concrete in Forms 27

Placing Concrete Under Water 27

Protecting Concrete in Hot Weather 28

Placing Concrete in Freezing Weather 28

CHAPTER V. REINFORCED CONCRETE Definition of Reinforced Concrete 31

Use of Steel in Concrete 31

Kinds of Reinforcing Steel 34

Corrosion of Metal Reinforcement 36

Bond Between Steel and Concrete 36

Quality of Reinforced Steel 36

Fireproofing 36

Joints 37

Precautions 37

CHAPTER VI. FORMS FOR CONCRETE Design of Forms 39

Size of Form Board 40

Thickness of Form Board 40

Width of Form Boards 41

Length OF Form Boards 41

Green vs. Seasoned Lumber 41

Kinds of Lumber 42

Square Edge vs. Tongue and Groove 42

Planed vs. Rough Lumber 42

Dimensions of Cleats, Studs, and Posts 43

Grade of Lumber for Form Work 43

Oiling Surfaces of Forms Coming in Contact with the Concrete . 44

Kinds of Oil 45

Method of Supporting Forms 45

Removing Forms 45

CHAPTER VII. SURFACE FINISH. Rubbing Surface with Brick 47

Picking Surface 47

Plastering Surface 49

Washing Surface with Pure Cement Mortar 49

vi

CONTENTS CHAPTER VIII. CONCRETE DESIGN

Page

Walls ". 53

Cellar and Basement Walls 54

Walls above Cellar and Basement 55

Small Reinforced Concrete Structures 57

Table for Designeng Reinforced Concrete Beams and Slabs . . 59

Sidewalks 61

Foundations for Sidewalks 61

Retaining Walls 62

Dams 63

Fence Posts 65 vu

CONCRETE DATA

Portland Cement weighs per bag, net 94 lb.

'' barrel, net 376 ''

Loose Portland Cement diYtrdiges^^r cubic ioot ?iho\it . 92 ^*

Weight of Concrete per cubic foot

Cinder Concrete averages 112 "

Sandstone Concrete averages 143 ^'

Limestone Concrete averages 148 '^

Gravel Concrete averages 150 ^*

Trap Concrete averages . . 155 '^

-A'derage quantity of concrete mixed, wheeled 50 ft, and placed in 10 hours by 3 men 180 cu. ft.

Add for each additional man in gang . .

Average load of concrete in iron wheelbarrow . . . Average load of sand or gravel in iron wheelbarrow

Average load of a one-horse dump cart

Average load of a two-horse bottom dump wagon . . Amount of cement

Per cu. yd. of concrete . . . . 1 : 1| : 3 mix .

60 2 3

8.0 bags

H ii ii ii ii

1:2 :4 "

. . . 6.3 ii

H ii ii ii ii

. . . 5.2 ii ii ii ii ii ii

1:3 :6 "

. 4.4 ii

Amount of sand

Per cu. yd. of concrete ft ii ii a a a

2:4" .

. . 11.9 "

ii a it ii ii a

2|:5 " .

. . 12.4 "

ii a ii a a a

:3 :6 " .

. '. 12.7 "

ii

Amount of broken stone

Per cu. yd. of concretj .

ft a ii a a a

:2 :4 '■ .

. . 23.8 "

ii ii ii a a a

2|:5 " .

. . 24.6 "

ii a a ii ii ii

3 :6, " .

. . 25.4 "

i(

IX

PLAIN AND REINFORCED

CONCRETE

Chapter I

Definition of Plain Concrete. Plain concrete is artificial stone made by mixing together pieces of broken stone or gravel with clean sand and cement, using enough water to make a mushy mixture.

The cement, mixed with water, is the material which unites all the particles of sand with the broken stone or gravel. This mass of mixed materials begins to stiffen in about half an hour and in from 10 to 24 hours the mass becomes hard enough so that an impression cannot be made readily by pressing on it. In a month's time the entire mass is practically a hard stone.

The broken stone or gravel is known as the Coarse Aggregate and the sand is known as the Fine Aggregate.

Concrete is always indicated on drawings as shown in Fig. 1, page 2. The triangular pieces in the figure indicate the broken stone, the round pieces indicate the sand, and the dots indicate the cement.

The theory of concrete is that the large broken stones or gravel making up the mass of the concrete would have a great number of voids or air spaces between them provided there were no smaller stones. The smaller stones fill up some of the spaces between the larger ones, but still leave numerous voids or spaces unfilled. These in turn are supposed to be filled with the sand. The cement, mixed with water, coats all of these particles from the largest broken stone down to very fine sand grains, and acts as a binding agent. The mass if allowed to remain in this state for several hours begins to stiffen, due to the cement and water undergoing a chemical change which produces the gluing or cementing qualities. In order to produce a very strong concrete every particle, no matter how fine, must be coated with the wet cement. A well made piece of concrete is like two sticks which are properly glued together. It is a fact that a glued stick will split anywhere rather than at the joint if the joint is properly made. The same thing is true with a well made piece of concrete; if hit with a sledge it will break through the stones instead of knocking the stones loose by breaking the bond around them.

Definition of Portland Cement. Portland cement is a manufactured product made by mixing limestone or chalk (which is nearly pure limestone) together with clay or shale. The correct proportions in which to mix these two materials are determined by a chemical analysis. These materials are then ground to a very fine powder and fed, together with powdered coal, into long rotary kilns. The mixture is then burned at a temperature of about 3,000 degrees Fahr. and forms into a dark, porous stone called cement clinker. This cement clinker is cooled, crushed, and ground again to a very fine powder, which changes from the dark color to the very light gray -color which is characteristic of Portland cement.

lo

Fig 1.— Sectional View of Concrete. (See page 1.)

After the cement is ground it is placed in large storage stock houses, where it must remain for a while to season before it is put into bags or barrels for shipment. The cement after being ground is called green cement and is not fit for use, but after remaining in the storage stock house for a short time, it seasons and is ready for use.

To illustrate how fine Portland cement really is: From 75% to 85% of the cement will pass through a screen having 200 openings to the inch each way, or 40,000 openings to the square inch. From 92% to 98% will pass a screen having 100 openings to the inch each way, or 10,000 openings to the square inch. One is not likely to get any brand of standard Portland cement that will not be as fine as just indicated, for all the cement companies comply with the standard specifications for testing cement as issued by the American Society for Testing Materials, the American Portland Cement Manufacturers, the United States Government, etc.

Kinds of Cement. There are a number of different kinds of cement that are used on construction work, but the most common of these, Portland cement, is the only one that need be considered. The layman will not ordinarily have use for the other kinds of cements, such as Natural cement. Slag cement, and Puzzolan cement. Under the head of Concrete Finish, a white finish cement will be described, but this is only used for ornamental purposes.

Setting of Cement. There are two terms which are used to indicate stages of hardening of the Cement after water has been added to it to form a paste. As soon as this paste ceases to be plastic the cement is said to have its Initial Set, This first stage of hardening is the most important, and after cement has reached this stage it must not be disturbed, for if it is the cement will show a loss of strength in the final hardening or even have no strength at all. In good Portland cement the Initial Set will not occur within less than one-half hour, which will give plenty of time to mix and place the concrete before the cement has commenced to harden. The second stage of hardening is called the Final Set, This Final Set has taken place when the cement has acquired such a degree of hardness that only a slight impression will be made on the surface of the concrete when, the thumb nail is pressed lightly upon it. A cement should receive its Final Set in less than ten hours.

Normal and Abnormal Cements. All Portland cements which pass the standard specifications are called ^'normal,'' while any cement which does not pass these specifications in one or several ways is called '' abnormal." Never use abnormal cement.

Abnormal Cements. Causes of abnormal cement. The causes of an abnormal cement may be due to (a) shipment in a leaky railroad car; {b) storage in damp places; {c) storage or shipment during long spells of extremely hot weather. In the case of {a) and (6) the cement becomes lumpy or may even be caked ^ hard so that the lumps cannot be crumbled by pressure between the fingers. The lumpy cement should not be used. A bag of cement which is caked must never be broken up and used. In the case of item {c) the cement may develop a ''flash set." This occurs most frequently in extremely hot weather. Cement has a flash set when it stiffens or hardens in less than a half hour^s time after adding water. It can readily be seen why a flash set cement should not be used, for it would not give time enough to get the concrete mixed, wheeled to the place where it is to be deposited, and spread and tamped, before it had started to stiffen. The stiffening of the soft concrete indicates that the cement has started to get its initial set. If the mass is now disturbed a part of the gluing or sticking property of the cement is destroyed, so that the resulting concrete will not be strong.

When the concrete is machine-mixed sometimes a flash set is not detected, as the cement flashes in the mixer and if the machine is kept running the mass will be broken up after the cement has flashed. If an excess of water is used in the beginning this water will mix with the cement and the cement will be ''regaged. '' Such concrete is very dangerous to use, as it is doubtful whether it will harden properly even after a long interval of time. There is little danger of not noticing the flashing of the cement in the mixer if it is watched by the man at the machine. Besides this, when the mass breaks up more water will have to be added in order to regage the cement and bring the concrete back to a mushy mixture, thus attracting the attention of the man running the machine.

While it is not safe to use a cement which flashes, the cement can be kept for a while, and it may possibly become normal again.

In order to determine whether a cement has a flash set, take a small quantity on a glass plate and having formed it into a crater shape at the top, pour in water. Mix this thoroughly for about 2 minutes, and form the mass into a ball. Leave it exposed to the air and examine it for the next half hour to see whether it hardens or sets in that time. Ordinarily it will take a longer time than this to stiffen, and it is only in extremely hot weather that it will be necessary to make this test.

Packing Portland Cement. Portland cement may be obtained in paper bags, cloth sacks, or wooden barrels. The most convenient form for general use is either the cloth bag or the paper sack, which contains 1 cubic foot of cement, but ordinarily the cloth sacks should be used. These sacks can be returned to the dealer from whom the cement was purchased, and rebate obtained for them if they are kept dry and untorn.

Storing Portland Cement. Portland cement is very susceptible to moisture and weather conditions, and must therefore

MATERL\LS FOR PLAIN CONCRETE 5

be stored in a dry place, such as a barn or shed. The cement will become lumpy or even form a solid mass when kept in a damp place, and when in this condition it should not be used. All lumps that do not crumble at the lightest blow should be thrown out.

Cement stored in a building must not be placed on the bare ground. Make a platform that is at least 6 inches above the ground, and store the cement on this platform. If the cement is to be stored on a concrete floor it is advisable to cover the floor with planking upon which to place the cement.

Fig. 2.— Fine Beach Sand, Greatly Magnified. (See page 5.)

Definition of Fine Aggregate or Sand. The Fine Aggregate is any sand, or broken stone screenings, where the grains vary from % inch in diameter down to a very fine powder.

Kinds of Sand. Sands may be roughly classified as fine, medium, and coarse. Specifications generally call for a clean sharp sand, but very seldom for a fine, medium, graded or coarse sand, which is in reality the most important consideration.

A fine sand is like beach sand, with few particles over Y^ inch in diameter. See Fig. 2, page 5. A medium sand is one which has practically no very fine or very coarse particles. A graded sand is sand having fine as well as coarse particles, neither predominating. A coarse sand is sand having very little fine material in it.

The purpose for which the concrete is to be used and the kind of Coarse Aggregate determine somewhat the kind of Fine Aggregate to be used. See page 13. In concrete for floors it is best to use a very coarse Fine Aggregate, while in concrete which is to be water-tight graded sand may be used which has

Fig 3.— Bank Sand, Greatly Magnified. (See page 6.)

a large percentage of very fine material. Beach sand, on account of its fineness, should not ordinarily be used. Fig. 3, page 6, shows a view (greatly magnified) of a good bank sand. The quality of Fine Aggregate cannot be determined by looking at it in the bank or pit, as the sand may contain vegetable matter which is difl&cult to detect without a special test. The ordinary plan of taking a little sand in the palm of one hand and rubbing it with the other to see whether it discolors the hand is practically of no value, and the other test of dropping sand in water to see the amount of turbidity is also impracticable. The discoloration of one's hand may be due to iron ore, and the turbidity may be due to clay, neither of which has any effect on the quality of the Fine Aggregate. It is the vegetable matter coating the grains, which cannot be detected in either of these ways, that is the harmful element. If the sand grains are coated with this very fine vegetable matter, even so small an amount as a fraction of one per cent, it may prevent the cement which surrounds the grains from setting, and thus make a worthless concrete.

How to Test for Clean Sand. A rough test for clean sand is to pick up a double handful of moist sand from the bank and move the hands in a vertical position, keeping them about ^ inch apart, and allowing the sand to slip quickly between them. Repeat this operation a dozen times and then rub the hands lightly together so as to remove the fine grains of sand which adhere. Examine the hands very carefully to see whether or not a thin film of sticky matter adheres, and if so, the sand probably contains loam or vegetable matter. A further test of this is to scrape some of this matter from the fingers and take a little of it between the teeth. If it does not feel gritty, it is a fair indication that this matter is vegetable loam, and not fine powdered grains of sand. Do not use the sand, if it can be avoided, without further tests or washing. Sand may, however, contain roots or vegetable matter, but if these are in pieces and not in great quantities they will do no harm.

A further test of questionable sand is to make up two 6-inch concrete cubes, using the questionable sand with the gravel or stone which is to be used in the structure. Keep one block in the air out-of-doors for a week and keep the other one in a fairly warm room for the same length of time. The specimens should be hard enough to remove from the molds in 24 hours. The first specimen may take a trifle longer if the weather is cold and damp. At the end of the week, test both blocks by hitting them with an ordinary hammer. If the hammer does not dent thena under light blows, such as would be used in driving tacks, and the blocks ring under the blows and are not broken under medium blows, the sand can be used with safety.

Method of Using Dirty Sand. Sand is ordinarily washed on the mixing board. This is very unsatisfactory, as it generally simply transfers the dirt to the lower part of the pile instead of washing it out of the sand. If this method must be used, the board upon which the sand is washed should be raised from the ground so that the dirty water may flow away from the board and also away from the sand pile. A more satisfactory method of washing sand, provided it is not too fine, is to make an inclined washing trough, as shown in Fig. 4. This trough, which should be lined with tar paper so as to convey the water a safe distance away from the sand pile and washed sand, should be about 16 feet long and 30 inches high, set on a slope of about 5 feet in 12 feet. A screen is inserted in the bottom

Fig. 4. — Screen and Trough for Washing Sand. (See page 8.)

near the lower end, through which the dirty water will pass. For sand, a screen with 30 meshes to the linear inch is necessary to prevent the good particles from passing through. The sand is shoveled on to the upper end of the trough and washed down by a stream from a hose at the upper end. The flow of the water will wash the sand down the incline, and as the mixture passes over the screen, the dirty water will drain through, leaving the clean sand for use. By this arrangement, as shown in

Fig. 4, page 8, the dirt which is washed out is caught in the water trough.

Definition of Coarse Aggregate. Coarse Aggregate is broken stone or gravel whose grains vary from % inch diameter up to y^ inch and sometimes to 2^ inches, according to the class of work.

In thin walls where reinforcement is used the largest size of coarse grains should be limited to ^ inch. For foundations or heavy walls over 12 inches thick, the maximum size of Coarse Aggregate can be 2^ inches. The reason for not using larger Coarse Aggregate in the thin walls is the danger of some of the stones getting caught and leaving a so-called '^pocket'' in the wall. This can be noticed at the time the concrete is being poured, and will not only make a bad appearance when the forms are removed but is liable to lessen the strength of the structure.

In very large foundations it is even advisable to use some large cobble stones in the mass of the concrete, but in no case must these large cobble stones touch each other or extend through the wall.

"Never use a soft sandstone, a soft limestone, shale or slate.

Coarse Aggregate, like Fine Aggregate, may be found to be dirty, especially if the coarse aggregate is gravel. In some cases the gravel grains are coated with a fine powdery matter which may or may not be organic or vegetable matter. Bad results will follow if such gravel is used.

For washing gravel of this character, the same kind of washing trough as shown in Fig. 2 may be used, except that the screen near the bottom of the trough should have openings about }^ inch square.

Crushed Stone vs. Gravel for Coarse Aggregate. The question is often asked, what kind of Coarse Aggregate makes the strongest concrete. The general impression is that crushed stone is the stronger, but tests show that good clean gravel concrete is a trifle better than crushed stone concrete. The crushed stone, however, is more likely to be clean and has therefore an advantage over the ordinary unwashed gravel.

Natural Mixtures of Bank Sand and Gravel. The sand and gravel are generally found mixed in a pit, but they are not usually graded properly for concrete and should therefore be screened over a % inch screen. The expense of screening this material is not warranted on small work, and it therefore is better to use a slightly greater amount of cement than would be used if the sand and gravel were screened and re-mixed. On any work of a large character it would probably more than pay for the extra labor required to screen the materials and re-mix them. For example, using even a very good gravel bank, a mixture of one part cement to four parts of natural gravel must be used, while if screened, one part of cement could be mixed with two parts of sand and four parts of screened gravel. In the above example one bag of cement more in every seven is required with the natural bank or unscreened gravel than with the material when screened and re-mixed in the right proportions.

Fig. 5.— Pieces of Gravel Concrete. (See page 10.)

Screenings from Crushed Stone. The screenings from crushed stone make an excellent substitute for sand, provided there has not been too much fine powder produced. The stone must not be of a soft or shelly character or contain a large percentage of mica.

Water. Ordinarily it is not necessary to test the water for use in concrete. It should not be taken, however, from a polluted steam or pond, and generally sea water should not be used.

The photograph of pieces of gravel concrete in Fig. 5, page 10, shows the difference in the diameter of the stones used in the two cases. The sample on the left of the picture was taken from a wall footing while the concrete was green, thereby breaking the concrete around the stones. The sample of concrete on the right of the picture was cut from a piece of concrete taken from a column which had been cast for several months.

CHAPTER I.

1. What is plain concrete?

2. What kind of cement should be used in plain concrete?

3. What is a Portland cement?

4. What is meant when it is stated that a cement has its initial set?

5. What is meant when it is stated that a cement has its final set?

6. What is an abnormal cement?

7. What causes a cement to become abnormal?

8. In extremely hot weather what test should be performed to see if the

cement is O. K.?

9. What determines whether a cement has a flash set?

10. Should a cement having a flash set be thrown away? If so, why? If

not, why?

11. How should cement be stored when brought to a job?

12. Why is a sand called a "Fine Aggregate''?

13. How can a sand be tested to see whether it is a clean sand?

14. How can a good strong concrete be made with a dirty sand? Is thi s

good practice?

15. Why is a gravel or broken stone called a "Coarse Aggregate'*?

16. What kinds of Coarse Aggregate should never be used?

17. What are the advantages, if any, of screening the sand from the gravel

in a natural bank and remixing them properly?

18. Can screenings from crushed stone be used in concrete?

19. Is it necessary to test the water used in concrete?

Chapter II Proportioning Materials

Proportioning of materials is the proper adjusting of the relative quantities of Cement, Fine Aggregate, and Coarse Aggregate to each other.

Concrete is defined in Chapter I as an artificial stone composed of broken stone or gravel varying from 2 inches down to % inch, and sand graded from % inch down to very fine powder, which is mixed very thoroughly and intimately with cement. The determination of the quantity of each of these materials is called "proportioning."

Theory of Proportioning. The object of Proportioning the aggregate is to make the artificial stone or concrete of maximum density, i. e., to have the fewest possible air spaces or "voids", as they are called, in the concrete. The method of determining the voids in a Coarse Aggregate is as follows :

A Construct a box 12 inches square and 12 inches deep for measuring the aggregate. Weigh the measure empty.

B Fill the measure even full with the Coarse Aggregate and weigh. Deducting the weight of the measure'gives the weight of the Coarse Aggregate, and as the measure holds exactly 1 cubic foot of material, this weight is the weight of 1 cubic foot of the Coarse Aggregate.

C The weight of a cubic foot of the Coarse Aggregate if it were a solid rock varies with the character of the material. The following weights of a solid cubic foot of rock of various materials can be used as shown:

Sandstone, pounds per cubic foot.

Conglomerate, it

Limestone, li

Sand, u u

Granite, li

Trap,

D The percentage of voids, therefore, is figured by dividing the weight of the Coarse Aggregate obtained in {B) by a corresponding weight of the Coarse Aggregate, if solid, and multi-

PROPORTIONING MATERIALS 13

plying the product by 100. Then deduct this result from 100, which will given the percentage of voids.

Illustration: — Assume the Coarse Aggregate to be crushed granite weighing 98J pounds in the measure as given in (B) above. Dividing 98J by 168 obtained from (C) gives 0.585. Multiplying 0.585 by 100 gives 58.5. Now deducting 58.5 from 100 gives 41.5, or 41>^ per cent voids, which is the answer. In other w^ords, if the stone in the box could be made into a solid stone to fit the box, the box would be between 3^ and ^ empty.

Try this same thing with gravel for a Coarse Aggregate instead of broken stone and note that the amount of voids in the gravel is different from that in the broken stone. In order to produce artificial stone these voids must be filled up. To do this, the Coarse Aggregate is mixed with finer material called the Fine Aggregate. Following out the same line of reasoning just used some voids will be found in the mixture, and whether this percentage is small or large depends upon the relation of the amount of Fine Aggregate added to the Coarse Aggregate.