INSTRUCTION PAPERS FOR HOME STUDY INDUSTRIAL EDUCATION SERIES EXTENSION DIVISION OF THE UNIVERSITY OF WISCONSIN CORRESPONDENCE-STUDY . PRACIIC A.L COURSE 430 ASSIGNMENT :^ CcBTRiom, 1916, BY THE UmvsRsmr Extbwsion Division MADISON PUBUSHED BY THE UNIVERSITY FEBRUARY, 1916 UNIVERSITY EXTENSION DlVISiOi^ THE DEPARTMENT OF CORRESPONDENCE-STUDY offers instruction in studies required in each of the following lines, bulletins on which will be mailed free on request: IN THE COLLEGES Eugibeering Basic and technical studies Law Prerequisite liberal arts studies Agriculture Generaj basic studies Letters and Science Studies required for liberal arts degrees Medicine (jCiicraJ basic studies IN COURSES AND DEPARTMENTS Languages Busiuess Accounting, Administration, Cor- respondence, Geography, Law, Management, Retailing Chemistry Commerce Education Engineering Civil, Electrical, ,, , -.^tfiictur^rf* ; • ' * EneiisSi • J * * i Hiatopy ■ * - Home Eoonomics Industry " ^ Joarnalium/^'^ J^* Public Sperfkirtg * Organization, Meciiijuical, French, German, Greek. Italian, Latin, Spanish Mathematics Mechanical Drawing Music Phar macy Philosophy Physical Science AstrqnOnjy, . Bacte'ricHiogy, Bot- * J,; •''.CljMnia&yr • Geology, J . .lETiV^stcs Political Economy Polit^i^ql S«l^nce •feocii^lpjiyl * ' ; Tcachinfs THE DEPARTMENT OF DEBATING AND PUBLIC DIS- CUSSION will mail bulletins on request free within the state, and elsewhere on receipt of price. THE DEPARTMENT OF GENERAL INFORMATION AND WELFARE will mail on request bulletins on Municipal Reference, Civic and Social Centers, Vocational Institutes, Com- munity Institutes, Dependency and its Relation to Industrial Education, Community Music, and Public Health, free within the state, and elsewhere on receipt of price. THE DEPARTMENT OF INSTRUCTION BY LECTURES will mail on request bulletins on the lectures and lecture courses. THE BUREAU- OF VISUAL INSTRUCTION will mail on request information as to the lantern slides and motion picture films lent within the state. PRACTICAL COURSE IN CONCRETE INDUSTRIAL EDUCATION SERIES PRACTICAL COURSE IN CONCRETE PREPARED IN THE EXTENSION DIVISION OF THE UNIVERSITY OF WISCONSIN FIRST EDITION MADISON PUBLISHED BY THE UNIVERSITY FEBRUARY, 1916 Copyright, 1916, by the University Extension Division. THE GETTY CENTEa LIBRARY PRACTICAL COURSE IN CONCRETE CHAPTER I CONCRETE MATERIALS 1. Cement. — Portland cement is made by artificially mixing lime and clay in proper proportions, heating the mixture to the point of fusion, and then finely pulverizing the clinker. The materials from which Portland cement is manufactured vary with the locality, but Portland cement can be made anywhere by burning and grinding a proper mixture of carbonate of lime and a suitable clay. The basic elements of Portland cement are silica, alumina, and lime. Ingredients such as iron, magnesia, alkaUes, sulphuric acid, carbonic acid, and water also occur in vary- ing quantities replacing some of the basic elements. The constituents should approximate the following limits: The essentials of Portland cement — namely: silica, alum- ina, and lime — are obtained from six different sources. (1) Cement rock and limestone. (2) Limestone and clay. (3) Marl and clay. (4) Chalk and clay. (5) Blast-furnace slag and limestone. (6) Alkah waste and clay. Cement rock (argillaceous limestone, low in magnesia) and limestone are chiefly used in the Lehigh district and constitute the raw materials used for about two-thirds of the Portland cement manufactured in the United States. Alumina.... Iron oxide. Lime Magnesia Sulphur trioxide. Silica. Per cent. 20 to 24 5 to 9 2 to 4 60 to 63.5 1 to 2 1.5 6 PRACTICAL COURSE IN CONCRETE In order to insure a satisfactory and uniform grade of cement, shipments for large or important work should be tested according to rules laid down by the American Society of Civil Engineers and the results should conform to the requirements of the standard specifications prepared by the American Society for Testing Materials. Copies of these publications may be obtained from the societies named. Results of tests depend largely upon the method of testing and hence it is important that definite and uniform methods be employed. The kinds of tests usually made are as follows: (1) specific gravity; (2) fineness; (3) time of setting; (4) tensile strength of neat cement and of sand mortar; (5) soundness, or constancy of volume. A chemical analysis is also desirable on large work. The tests for soundness and strength are called primary tests, while the other tests are called secondary since they give only additional informa- tion which is of little value in itself. Cement testing is a difficult process and should be en- trusted only to experienced, well-qualified men. It is a well-known fact that the personal factor has considerable to do with the results obtained in cement testing and, on this account, there is a vast amount of dissatisfaction with the methods specified at the present time. Moreover, results by utterly untrained or careless operators are really worse than nothing and may be positively misleading. The comparative results by any one experienced observer, however, are generally consistent and are of value. From the above it should be recognized that, if cement tests are made, it is worth while to have them made w'ell, even at a possible increase of expense. The cement used in testing should be representative. Generally 1 bag in every 40 is sampled, or, in case of delivery in barrels, 1 barrel out of every ten. These small samples are usually mixed together to make an average sample. The whole is then taken to the testing laboratory, and there submitted to the standard tests. Usually 8 or 10 lb. of cement is taken from each carload for testing purposes. An average sample of a package may be obtained by means of a sampling auger (such as is used by butter or sugar inspectors) inserting it from top to center in bags, and from side to center in barrels. When sampling from barrels, CONCRETE MATERIALS 7 a hole is made for the purpose in the center of one of the barrel staves midway between the heads. Adulterations in Portland cement have generally been looked upon as detrimental to its quality, and the cement specifications in common use are of a character to exclude any- grinding in of cheap material after calcination. Recent investigations have shown, however, that some forms of adulteration are possible without injury to the cement and in many cases may produce a decided improvement in every desired quality. The question at once arises — why should not specifications for Portland cement be changed so as to admit of such a practice, thus materially reducing the cost? The principal reason hes in the fact that with what is known at the present time, specifications permitting adulteration would be difficult to enforce and the results obtained with adulterated material would be uncertain. The specific gravity test of cement is mainly for the purpose of detecting adulteration such as clay, slaked lime, sand, ashes, natural cement, or products of natural rock-Uke ragstone or tufa. The specific gravity test, however, should be taken only as indicative and should be verified by other te^ts before rejecting a material which does not come up to standard. Chemical analysis serves as a valuable means of detecting adulteration, and also shows whether those elements believed to be harmful are present in too large quantities. It is well known that fine grinding has a very great in- fluence on the properties of the cement. The finer the grinding, the better the quality, since fine particles will more easily cover the sand grains, making mortar much stronger and allowing the use of a larger percentage of sand. The time of setting of a cement may vary within wide limits and is no criterion of the quality, but it is important in the fact that it indicates whether or not it can be used advantageously in ordinary construction. A cement may set so quickly that it is worthless for use as a building material (since handling cement after it commences to set weakens it and causes it to disintegrate), or it may set so slowly that it will greatly delay the progress of the work. Long-standing cements absorb moisture from the air and lose their hydrauUc property. A moderate amount of seasoning in weather-tight sheds, however, is often 8 PRACTICAL COURSE IN CONCRETE helpful to secure good results. Fresh cement contains small amounts of free or loosely combined lime which does not slake freely like ordinary hme and causes expansion, endangering the structure in which it is used in this condi- tion. During the time of seasoning the free lime is changed to carbonate of Kme and in this state the cement does, not swell. Well-seasoned cement is generally lumpy, but the lumps are easily broken up. If, however, the cement has been subjected to excessive dampness, or has been wet, lumps will be formed which are hard and difficult to crush. Aside from the consideration of age, the conditions which accelerate the setting are: finely ground and lightly burned material, dry atmosphere, small amount of water used in gaging, and high temperature of both water and air. There are two distinct stages in setting: (1) the initial set; and (2) the hard set. The best cements should be slow in taking the initial set but after that should harden rapidly. Portland cement should acquire the initial set in not less than 30 minutes, and hard set in not less than one hour nor more than 10 hours. The object of testing cement in tension is to obtain some measure of the strength of the material in actual construc- tion. In other words, tests of tensile strength are made primarily to determine whether the ingredients of the cement and the process of its manufacture are such that a continued and uniform hardening may be expected in the work, and whether it will have such strength when placed in mortar or concrete that it can be depended upon to withstand the strain placed upon it. The small shapes made for testing are called briquettes and have a minimum cross-sectional area of 1 square inch, that is, at the place where they will break when tested. Tests are made with briquettes of neat cement and also with briquettes made of cement mortar (one part cement, three parts standard sand from Ottawa, Illinois). The neat cement tests measure directly its quality while the mortar tests are more nearly a true measure of its behavior under actual conditions. It is customary to store the briquettes, immediately after making, in a damp atmosphere for 24 hours. They are then immersed in water until they are tested. This is done to secure uniformity of setting, and to prevent the CONCRETE MATERIALS 9 drying out too quickly of the cement, thereby preventing shrinkage cracks which greatly reduce the strength. Specifications for tensile strength of cement usually stipu- late that the material must pass a minimum strength require- ment at 7 and 28 days. This is required in order to deter- mine the gain in strength between different dates of testing so that some idea may be obtained of the ultimate strength which the cement will obtain. A first-class cement, when tested, should give the values for tensile strength stated in the specifications of the American Society for Testing Materials. W. Purves Taylor in "Practical Cement Testing" gives the following rules for accepting or rejecting material on the results of tensile tests: "At 7 days: Reject on decidedly low sand strength. Hold for 28 days on low or excessively high neat strength, or a sand strength barely failing to pass requirements. "At 28 days: Reject on failure in either neat or sand strengths. Re- ject on retrogression in sand strength, even if passing the 28-day require- ments. "Reject on retrogression in neat strength, if there is any other indica- tion of poor quality, or if the 7-day test is low; otherwise accept. "Accept if failing slightly in either neat or sand at 7 days and passing at 28 days." It must be remembered that the sand test is the true criterion of strength, and no cement faihng to pass this test should be accepted, even though the neat tests are satisfactory. A cement to be of value must be perfectly sound; that is, it must remain constant in volume and not swell, dis- integrate, or crumble. Excess of free or loosely combined lime which has not become sufficiently hydrated, excess of magnesia and alkaHes, and coarse grinding, are all causes of unsoundness. Tests for soundness are among the most important to be made upon cements and should extend over considerable time to fully develop possible inherent defects. The usual way is to form small pats of neat cement about 3 inches in diameter, | inch thick at the center and tapering to a thin edge. These pats should remain a definite time: (1) in air; (2) in water; and (3) in an atmosphere of steam above boiling water. To pass the soundness tests satisfactorily, the pats should remain firm and hard, and show no signs of cracking, distortion, or dis- 10 PRACTICAL COURSE IN CONCRETE integration. The steam tests are what are called accelerated tests and are for the purpose of developing in a short time (five hours) those qualities which tend to destroy the strength and durability of a cement. In the present state of our knowledge it cannot be said that cement should necessarily be condemned simply for failure to pass the accelerated tests; nor can a cement be considered entirely satisfactory simply because it has passed these tests. The air and water tests are called normal tests and extend over a period of 28 days. Full specifications similar to those of the American Society for Testing Materials are advisable for important work whether large or small. When purchasing by such specifi- cations it may often be unnecessary actually to test the cement except for set, soundness and fineness, but the full specifications are recommended so that, if the cement does not work satisfactorily, it may be more carefully examined and unused portions rejected. Some large pur- chasers have apparatus for making complete cement tests in the field, but it is customary to have the tests made at some testing laboratory. In unimportant construction it is usually safe to use a first-class American Portland cement without testing. Cement should be stored within a tight, weather-proof building, at least 8 inches away from the ground and an equal distance from any wall, so that free circulation of air may be obtained. In case the floor of the building is laid directly above the ground, it would be well to give the cement an additional 8-inch elevation by means of a false floor, so as to insure ventilation underneath. The cement should be stored in such a manner as to permit easy access for proper inspection and identification of each shipment. A period of at least 12 days should be allowed by the contractor for the inspection and necessary tests. Where cement in bags is stored in high piles for long periods, there is often a slight tendency in the lower layers to harden, caused by the pressure above; this is known as warehouse set. Cement in this condition is in every way fit for service and can be re-conditioned by letting each sack drop on a solid surface before using the cement con- tained. CONCRETE MATERIALS 11 Cement may be obtained in cloth or paper bags, in bulk, and in barrels. Cloth bags are the containers most generally used since manufacturers will accept the empty bags if returned in good condition. The consumer, however, must prepay the freight when returning the empty bags to the mill. The cloth bag will stand transportation, and its size and shape make it convenient to handle. If properly cared for, it may be used over and over again. The follow- ing is cjuoted from Concrete-Cement Age, May, 1913: "Wooden barrels are out of the question because of increased cost of packing the cement into them; the large initial cost of the barrel itself (35 cents to 40 cents each); the inconvenience in handling a pack- age weighing 400 lb., especially on large work; the increased cost repre- sented by freight on weight of each barrel (approximately 25 lb.); and the increased cost of the cement to the dealer and the user because the barrel cannot be returned. It will be seen, therefore, that the use of the wooden barrel would entail a direct additional tax on the dealer or the user — all items considered — of approximately 40 cents per barrel. "Paper bags are used to a limited extent. They cost the manufac- turer 10 cents per barrel (4 bags to a barrel), and the additional burden to the dealer and the user is represented by this 10 cents per barrel, plus the loss resulting from breakage in transportation and handling, which is large (probably 5 cents per barrel), and very annoying. The shippers will not be responsible for damage to paper bags and, on the whole, paper bags which cannot be saved or returned impose an addi- tional and wholly unnecessary burden of approximately 15 cents per bar- rel — all items considered — on the ultimate consumer. "If a cloth cement sack can be used eight times — as it may be if prop- erly cared for and returned promptly when empty — the loss or burden imi)osed upon the ultimate consumer from the use of cloth sacks is 5 cents per barrel only, which is only § the burden imposed by the use of paper bags, and f the burden imposed by the use of wooden barrels. "Cloth sacks, cheaper than those now used for carrying cement, are out of the question, because they are too frail and weak to stand trans- portation and protect the heavy weight of cement." The following paper on The Proper Care of Empty Cement Sacks was presented before the Conference on Permanent and Sanitary Farm Improvements in August, 1913, by C. S. Fletcher of the Universal Portland Cement Co.: "EVery cement manufacturer in the United States ships most of his product in cloth sacks. This necessarily means a considerable outlay of capital as the sacks are not, by any means, received from a charitable institution. During the last year there were eighty-five million bar- 12 PRACTICAL COURSE IN CONCRETE rels of cement used in the United States. It is only fair to assume that seventy-five miUion barrels were shipped in cloth sacks. This represents 130,000,000 in hard, cold cash and I assure you that no company, cement or other- wise, would show any anxiety to assume such a gigantic loss, "When cement is sold, the price of the sack is includ- ed in the price of the ce- ment. The cement company will re-purchase such sacks as are in good, sound, serviceable condition when delivered to mill, freight prepaid. As a great number of the sacks are so badly abused, the cement company is obliged to reject them. Such rejections are bound to bring about differ- ences of opinion between the Fig. 1- -A sack of cement as it leaves the mill. Fig. 2 — A returned sack made useless through wetting. customer and the manufac- turer. For instance: a man returns 400 sacks, naturally he wants $40.00 for them irre- spective of their condition. When we are obliged to inform the shipper that we can only pay him for the good, sound sacks, which in this case amounts to $30.00, the fur begins to fly. "Not long ago the purchas- ing agent of a company which uses a lot of our cement, complained bitterly of the large number of rejections from their sack shipments. The situation grew so acute that the purchasing agent fm- CONCRETE MATERIALS 13 ally accepted our invitation to visit our plant and watch the count and inspection of his own individual sacks. When the next shipment reached our plant I notified this gentleman and we both started for the scene of action. As the bad sacks were being thrown aside the grieved party picked up a couple of them and asked me why we were throwing such sacks away. I suggested that he try and open some of them. He found this im- possible as the fronts and backs of the sacks were cemented together. I asked him if he thought it possible to put ninety-five pounds of ce- ment in such sacks. Then a great Ught dawned over the purchasing agent — 'These are the sacks that we have been using for covering some newly laid concrete work.' If the men on the job had only realized that they were losing money by such work they would not have done it. "That purchasing agent was made to realize that cement sacks must be taken care of. It's the same way with the deal- er, small or large; he must not expect to receive full credit from the cement company for abused sacks that he has bought back from his customers. It is up to every dealer to stand firmly on that part of the contract which reads 'Cement sacks will be paid for when returned in good, sound, serviceable condition.' Fig. 3 A poorly mended cement sack showing the use of other material in making the patch. This sack was probably opened by the use of a shovel or knife. Fig. 4 — Extreme care is taken in mending sacks at the Universal Plants. Before making the re- pair, the cloth is tested to insure sufficient strength, so that often a mended sack is better than one which has passed inspection without needing repair. 14 PRACTICAL COURSE IN CONCRETE "It is hard to impress upon the ultimate cement consumer the fact that he should stand the sack loss. In the cement business the buyer has the edge; he has the privilege of returning the sacks and receiving the same price that he paid for them. Then, why should that man kick when a few sacks go to pieces on his hands? "In order to show you how carelessly some people handle sacks I have brought along a few actual samples. "The first one we have (Fig. 2) is a sack that has been, allowed to get wet; you can see, the cloth is hard and rotten. Water is, without a doubt, as dangerous to cement sacks as a spider is to a fly. Just as soon as a sack becomes wet and be- gins to dry out, the cement dust in the fibers of the cloth hard- ens and leaves the sack in a weak and worthless condition. "This sack (Fig. 3) has been badly repaired. If this customer had left the sack alone and sent it to our mill, he would be ten cents to the good. We would have repaired this sack, just as we do thousands of others, and paid full value for it. "Here (Fig. 4) you see a sample of our repairing. This sack was badly torn when received by us but the cloth was in good sound shape. The repair was made and the sack is now just as good as the day it left the factory. "The Universal Portland Cement Co. has spent a con- siderable amount of time and money every year educating customers to realize that a cement sack is worth ten cents and ought to be handled accordingly. I am pleased to say that our campaign has been productive of good results, but we still have a lot to do. One of the first fines of educa- tion taken up by us was the handfing of sacks for shipment. "This bundle (Fig. 5) that you see here con- tains fifty sacks tied with three stout ropes, tagged and ready for shipment. We have found from watching thousands of shipments received at our mill that this bundle ^. , „ stands up better than tig- 5 — Bundle of 50 cement sacks tied and +u tx i , tagged ready for shipment. any OthCr. It haS SCVCral CONCRETE MATERIALS 15 Fig. 6 — A bundle of 50 cement sacks laid out flat with 2 ropes 40 inches long under the pile, with longer rope of about 8 ft. resting on top. advantages over a larger bundle, chiefly because it can be handled much easier by freight handlers. It is altogether different with a bundle of one hun- dred sacks; such a bundle will be dragged over a rough floor where nails tear large holes in sacks, and very often this method has a tendency to loosen the bundle considerably. The chances, therefore, of all the sacks in such a bundle reaching destination are very slim. "Some time ago the railroads put through cer- tain regulations govern- ing the shipment of re- turned empty cement sacks. In order to familiarize our trade with these regulations the illus- trated sack placard that you see here was sent broadcast to every cus- tomer on our books. In Fig. 6 you see fifty sacks laid out flat with two ropes 40 inches long under the pile and a longer rope about 8 feet long resting on top of the pile. In Fig. 7 the two short ropes have been brought over the pile of sacks and tied tightly. The last figure shows the bundle turned over and the long rope brought around it and crossed in the middle of the bundle. "We leave very little Fig. 8— After the short ropes have been tied room for a man to say l.'j^^giri'rl?.^Yrn'd7rrssrd^!.\\'e';^id°cll^ that he does not know "Jp^t ''''' ^^""^'^^ Fig. 7 — The first operation in bundling is to bring two of the ropes over the pile, as shown, tying tightly. 16 PRACTICAL COURSE IN CONCRETE how to ship sacks. As already stated, each customer has received one of these sack placards, and a supply of hnen tags is sent out in every car that leaves our mill. Further- more, we have made arrangements with all railroads that a sack placard is to be placed in each and every railroad freight house, throughout the country, so that if a man asks his agent how to ship sacks the agent will know what to tell him." Within the past year or two considerable cement has been shipped in bulk to cement-product factories and to con- struction jobs adjacent to railroad tracks. Much economy has resulted from the saving in labor, and from the elimi- nation of package losses and expense. There seems to be no difficulty in shipping bulk cement in tight box cars and this method will undoubtedly become more popular in the near future. 2. Aggregates. — Present-day success in the use of con- crete is not due to any particular "discovery" but is the result of consistent, scientific study and investigation of the component materials. As ordinarily employed, the term "aggregates" includes not only gravel or stone (the coarse material used) but also the sand, or fine material, which is mixed with the cement to form either mortar or concrete. Fine aggregate is defined as any suitable material that will pass a No. 4 sieve (screen having four meshes to the linear inch) and includes sand, stone screenings, crushed slag, etc. By coarse aggregate is meant any suitable material, such as crushed stone and gravel, that is retained on a No. 4 sieve. Coarse aggregate over 11 inches in largest dimensions is not generally used. The strength of concrete can never be greater than that of the materials used as aggregate. Nothing is -more con- ducive to unsatisfactory results in concrete work than poor aggregates. The quality of the cement, methods of mixing, the proportions used, and the amount of water added, also the method of depositing concrete, all have their effect upon the density and strength of concrete but even with the most careful attention given to these details, good results are impossible without good aggregates. The fact that the aggregates seem of good quality yet may be proved totally unsuitable, shows that study and CONCRETE MATERIALS 17 careful tests are necessary if the best results are to be ob- tained. The idea that the strength of concrete depends entirely upon the cement and that only a superficial exam- ination of aggregates is necessary, is altogether too prev- alent. The man who understands his aggregates, grades them properly, sees that they are washed, if necessary, then mixes them in proportions determined by thorough testing, study, or actual experience, is the one who will make the best concrete. In the selection and use of sand, more precautions are necessary than for the coarser aggregate, due to its physical condition, and a wider variation in properties. A knowl- edge of these properties, and of the method of analysis to determine the suitability of sand for use in mortar and concrete, may be easily applied to an analysis of the coarse aggregate. (Stone screenings, broken stone, and gravel will be discussed only where their properties and the methods of examining them differ from those of sand.) Geologists classify rock in one of two large groups: 1. Igneous. 2. Sedimentary. Igneous rocks are those which have been formed by the cooling of fused material. The original crust of the earth was formed entirely of igneous rock, but it is highly im- probable that any of this original crust is now exposed at the earth's surface. Igneous rocks are classified either as massive or laminated, according to their structure. The massive igneous rocks are those which have been solidified, undisturbed, from a fused state and which have not been subsequently sub- jected to severe external stresses. When the rock was subjected to external pressure during or after cooling, a laminated structure seems to have resulted, with the com- ponent minerals arranged in more or less definite alternating bands. Most granites and all trap rock belong to the first class, while rocks of the second class are termed gneisses. Sedimentary rocks are those derived from the breaking up or disintegration of preexisting strata, the material so obtained being carried, usually in suspension or solution, to some point where it is redeposited as a bed of sand, clay or calcareous material, such as shells, marls, etc. Sub- 18 PRACTICAL COURSE IN CONCRETE sequently, this loosely-deposited material may become con- solidated and compacted by pressure or other agencies, the result being the formation of sandstone, shale, and slate or limestones, dolomites, and marbles. Sedimentary rocks may be classified on a combined chemical and physical basis, distinguished by the material of which they are chiefly composed, as: 1. Silicious sedimentary or sandstone, and conglomerates. 2. Argillaceous or clayey rocks, such as shales and slates. 3. Calcareous rocks, namely marble, compact limestone, granular limestone and magnesian limestone, or dolomites. The materials commonly used as coarse concrete aggre- gate in different places throughout the United States are the sedimentary rocks, which may be grouped into three classes on the basis of origin. 1. Glacial deposits. 2. Costal plain deposits. 3. Stream deposits. All of these deposits contain more or less sill, clay, loam or other finely divided impurities. The gravel beds of the glacial drifts furnish excellent material for concrete. Baker, in Roads and Pavements, says: "Glacial gravel exists in considerable quantities in western Pennslyvania, in the greater part of Ohio, in north- ern Indiana, and in Illinois, and to some extent in several of the states of the southwest. There are large areas of this gravel in Wisconsin, Minnesota and Iowa." Sands differ, not only in chemical and mineralogical com- position but in physical condition. They ofte*!! contain many impurities, and the methods for determining the presence of these impurities, as well as their effects, should be known. Many of these impurities impair the hardening properties of cement, and hence the strength of the resulting concrete. Much has been written relative to the effect of clay upon concrete and many contradictory opinions have been advanced. Engineers are, however, fairly in accord on certain conclusions. When clay exists as a coating on the particles of sand aggregate, it is undoubtedly injurious, as proper adhesion between the cement and the sand surfaces is then prevented. When, however, clay of a silicious nature, in the form of separate particles, exists to a small extent throughout the mass of aggregate, it CONCRETE MATERIALS 19 appears to cause no serious harm in many kinds of concrete work. Although clay in this form acts as an adulterant, some writers have held that from 5 to 10 per cent may be admitted without seriously reducing the strength of the concrete. Their opinions, however, are based largely on the results of tensile-strength tests on relatively dry mixtures. It is doubtful whether under field conditions, or even in large compression-test specimens made up in the laboratory, these results would be obtained. An excess of clay tends to lead one into believing that the concrete contains an excess of cement rather than a shortage. The only advant- age that can be claimed for the presence of clay is, that it increases the density of the concrete by filling some of the voids. The presence of clay in sand may be detected by the well-known method of rubbing the material between the hands. If clean, the sand should not adhere to or discolor the hands. Also a small quantity of the sand may be stir- red or shaken in a tumbler or bottle of water, when the presence of clay will at once be shown by a characteristic cloudiness of the water. Since the clay remains longer in suspension than the sand, it will separate and settle later in a layer on top. A coating of vegetable matter on sand grains appears not only to prevent the cement from adhering but to affect it chemically. Frequently, a quantity of vegetable matter so small that it can not be detected by the eye and is only slightly disclosed in chemical tests, may prevent the mortar from reaching any appreciable strength. Concrete made with such sand usually hardens so slowly that the results are questionable and its use is prohibited. Other impuri- ties such as acids, alkalies, or oils in the sand or mixing water, usually make trouble. The usual way of determining the quality of sand is to make up briquettes in the proportions of one part cement to three parts of the sand to be tested, and compare the results with the strength of a mortar made with the same cement and standard Ottawa sand in hke proportions and of standard consistency. The presence of moisture in sand may make proper mixing with other materials somewhat difficult, as a uniform distribution of cement in the mortar is hard to obtain. 20 PRACTICAL COURSE IN CONCRETE General Requirements The quality of concrete is affected by 1. The hardness, or crushing strength, of the aggregates. 2. Their durability or resistance to weather. 3. Grading, or maximum and relative sizes, of particles. 4. Cleanliness, or freedom from foreign materials. 5. The shape and nature of the surface of the particles. Hardness. — Tlie iiardness of tiie material grows in im- portance with the age of the concrete. Because of the rounded surface of the aggregate, gravel concrete one month old may be weaker than concrete made with comparatively soft broken stones; but when one year old, it may surpass in strength the broken stone concrete, because as the cement becomes harder and the bond firmer, the resistance of the aggregate to stress, becomes a more important factor. The grains should offer at least as high a resistance to crushing as does the cement after attaining its maximum strength. In comparing sands of the same kind, those having the highest specific gravity are likely as a rule to be the strongest. This applies in a general way to the comparison of different kinds of rock also. Value of Different Rocks. — Different rocks of the same class vary so widely in texture and strength that it is im- possible to give definitely their relative values as aggregate. However, a comparison of a large number of tests of con- crete made with broken stone from different kinds of rock material indicates that its value as an aggregate is largely governed by the actual strength of the stone itself, the hardest stone producing the strongest concrete. Comparative tests discussed by various writers indicate that, in the order of tlieir value for concrete, the different materials stand approximately as follows: 1. Granite. 5. Limestone. 2. Trap rock. 6. Slag. 3. Gravel. 7. Sandstone. 4. Marble. The grading — that is, the relative size and quantity of the particles of an aggregate — determines in a large meas- ure the density of the mass, which greatly affects the quality of the concrete. A coarse, well-graded aggregate produces a denser and stronger concrete or mortar. A sufficient CONCRETE MATERIALS 21 quantity of fine grains is valuable in grading the material and reducing the voids, but an excess has a tendency to diminish the compression strength considerably. Weights and Voids. — A high unit weight of material and a corresponding low percentage of voids are indications of coarseness and good grading of particles. However, the impossibility of establishing uniformity of weight and meas- urement due to different percentages of moisture and differ- ent methods of handling, make these results merely general guides that seldom can be taken as positive indications of true relative values. This is especially true of the fme aggregates in which percentages of voids increase and weights decrease with the addition of moisture up to about 6 per cent. Maximum Size. — Within reasonable limits the strength of concrete increases with the size of stones. For mass concrete, the practical maximum size is 2| to 3 inches. In thin reinforced structures, such as floors and walls, the size must be such as can be worked readily about the reinforcing metal, and l|-inch aggregate is generally the maximum. Cleanliness. — As stated, the particles of rock should be free from dirt and dust, and should not be used when even partly covered with clay; such impurities prevent the cement from obtaining a bond on the surface of the particles and often contain materials which retard the hardening of the mortar or concrete and prevent it from acquiring normal strength within a reasonable length of time. An excess of clay or dirt in any form also affects the color of the concrete when hardened, and necessitates more thor- ough mixing. Shape of Particles. — The shape of the rock particles influences the strength of the mortar or concrete. Flat particles pack loosely and generally are inferior to those of cubical fracture. Analysis. The chief value of an analysis of any sand results from the comparison of its various properties with those of other sand tested under similar conditions, and recognized as of a good quality. 22 PRACTICAL COURSE IN CONCRETE Classification. — The sands in common use as aggregate throughout the United States are sedimentary, hence the classification can usually be confined to the degree of con- solidation and the kind of material, on the basis of whether its formation is chiefly silicious or calcareous. Hardness and texture are ready aids in these determinations, which may be conducted in an elementary manner. The natural sands are usually silicious, but they vary in de gree of consolidation, which determines in a large measure the crushing strength and durability of the concrete. Dur- ability is also dependent upon the nature and amount of impurities present, as feldspar, mica metal oxide, etc. Such impurities account largely for the variegated coloring of sand grains. Specific Gravity. — As sands, or rocks of the same kind having the highest specific gravity are likely to be strong- est, a determination of the specific gravity of different sands is valuable, since it is a ready indication of the nature and hardness of the material. As a rule sand having the high- est specific gravity, other things being equal, will give the best results. The specific gravity of a material is determined by divid- ing its weight by the weight of the water which it displaces when immersed. Take a convenient amount of sand, screen it through a |-inch screen, dry, and weigh. Then place some water in a glass graduate, read the height of the water, add the sand, and again read the height of the water. The difference in readings will be the weight of water displaced by the sand. Divide this weight of water into the weight of the sample of sand. The result will be the specific gravity of the sand. (For detailed methods of testing sand and cement-sand mortar. Bulletin No. 33 of the United States Bureau of Standards should be referred to.) Determinations Necessary.— Physical Analysis: The determinations necessary for a good physical analysis of sand are: 1. Strength and density in mortar. 2. Gradation and effective size of grains. 3. Cleanliness, including per cent and nature of silt. 4. Percentage of voids. CONCRETE MATERIALS 23 Density. — In the study of sands, a determination of their density is important as regards both quality and economy. Other physical conditions being equal, the sand which pro- duces the smallest volume of plastic mortar when mixed with cement in the required proportions, makes the strong- est and least permeable mortar, and the densest mortar will be the strongest. This requires that the sand be graded fi'om coarse to fme, the coarser particles predominating. (The question of determining density will be discussed in Chapter II on Proportioning.) Gradation and Effective Size. — Sand made up of coarse grains will produce a greater strength in mortar than that made up of fme grains, because it presents a more compact mass as well as a smaller amount of surface area to cover with cement, and usually a smaller percentage of voids. A fme sand requires more thorough mixing than coarse sand in order to get a proper distribution of cement. The size of sand grains is so important that it is often profitable to ship a coarse sand a considerable distance rather than use a local fme sand. Feret, the French author- ity, computed that it was more economical to use coarse instead of fme sand, even though the cost is several times as great. It does not follow, however, that because coarse particles have the smallest area per unit of volume, the aggregate should all be large. Particles of the same size form a volume having a larger percentage of voids than if graded in size, hence require a larger proportion of cement to produce the maximum strength. Granulometric Composition. — The determination of the granulometric composition or mechanical analysis of sand is made in order to study its properties and to judge of its value compared with other sands, and sometimes to enable regrading its grains so that a denser mass may be secured. That the strength, quality, and value of a sand may be indicated by ascertaining whether the majority of its par- ticles are coarse, medium, or fme, has been generally estab- lished, and it is also important to determine the relative degree of coarseness and fineness. The percentages of different size grains are frequently determined by a mechanical analysis. The sample is first 24 PRACTICAL COURSE IN CONCRETE screened through a number of sieves of successive sizes, and the percentage by weight retained on each, recorded. For this work the following sieves are recommended : A standard sieve is made of woven brass wire, set into a hard brass frame, 8 inches in diameter and 2j inches deep. These sieves are described by numbers corresponding ap- proximately to the number of meshes per linear inch. All material referred to as sand must pass a No. 4 sieve. Not more than twenty (20) per cent should pass a sieve, having fifty (50) meshes per linear inch, and not more than five (5) per cent should pass a sieve having one hundred (100) meshes per linear inch. The tabulated results showing the percentages by weight retained on the different sieves form a valuable basis for a study of the effective sizes of grains, and for comparison with other sands whose value in mortar or concrete has already been determined. Cleanliness. — -The effect of dirty sand is dependent upon the quantity and nature of the impurities and the form and manner in which they are present. The manner in which silt is contained in sand may be determined by in- spection. The silt in a sand is that material which in solu- tion and in suspension is carried away in wash water so applied as not to remove the small grains of sand. This amount may be ascertained by determining either the amount of substance contained in the wash water, or the amount of loss sustained by the sand through washing. The latter method is more generally used. If the silt is vegetable matter in a gelatinous or viscous state, forming a colloidal covering over the surface of the sand grains, its presence may be determined by immersing the material in a dilute solution of sulphuric or hydrochloric acid and comparing the strength of cement mortar made from the sand before and after immersion. The following paper on Presence of Dirt in Concrete Aggre- gates, by Mr. C. D. Franks of Chicago, was presented before Commercial No. of Sieve 4 10 20 30 40 50 80 100 200 CONCRETE MATERIALS 25 the Conference on Pernlanent and Sanitary Farm Improve- ments in August, 1913. "It is well known that screened and well-graded aggre- gate is a necessity for making good concrete. However, no matter how well your material may be graded, if it contains foreign material, such as silt, loam, or clay, the effect of good grading is partially lost. The presence of silt or dirt in sand or gravel materially affects the success of your con- crete work. "Some time ago my attention was called to the construc- tion of a concrete foundation wall. This particular wall had been constructed of a 1:2^:5 mixture — that is, for every sack of cement, 2| cubic feet of sand, passing a \ inch screen, and 5 cubic feet of gravel from \ inch to 2 inches in size were used. The sand had been screened from the gravel and the material properly proportioned, mixed with cement and water and placed in the forms. About 7 days after the wall had been completed, the forms were removed. The out- side surface of the foundation wall appeared very hard, but when the forms had been entirely removed the wall partially collapsed and an examination of the interior of the concrete showed that it was very weak and porous. It was a natural thing for the contractor to place the blame on the material which had least to do with the failure, the cement. How- ever, the engineer who had superintended the construction of the wall became suspicious and began an investigation to determine the true cause of failure. His investigations showed that the thin film coating which appeared on the outside and inside of the foundation wall was silt. In the concrete this same silt had coated the particles of sand and gravel, and as a natural consequence prevented the cement from adhering to the surfaces, resulting in weak concrete and the failure of the wall. "The examination of sand for silt or for foreign material really belongs to the laboratory properly equipped for making analyses. The examination or analysis which is sometimes performed on the job, does not show accurate results. As a rule, the samples of sand or gravel are given a physical analysis and, in the case of the sand, a chemical analysis, in order to give the contractor or engineer a thor- ough knowledge of the quality of the material that he is 26 PRACTICAL COURSE IN CONCRETE to use. Such laboratory methods can not be adopted, however, by the ordinary contractor or farmer who wishes to do good concrete work, and his work is just as important to him as the construction of large buildings. I would suggest, then, when you are selecting material for your concrete work, that you investigate thoroughly the manner in which the material comes from the pit. The strippings of the pit (that is, the clay or loam above the sand and gravel, tree roots and all foreign material) should be removed carefully. Often a close examination of the gravel bank will give a fair idea as to the quality of the material in ques- tion. If you pick up handfuls of sand and gravel from var- ious parts of the pit and allow it to dribble slowly through your fingers, the presence of dirt or silt, evenly divided, will make your fingers and hands feel soapy or shppery. If that condition exists, you can well look upon the quality of the material with suspicion, for it probably contains material which should be removed to insure the success of your concrete work. "If you are doing concrete work, it is not absolutely neces- sary to have extensive laboratory apparatus to determine in a fair way the quality of the sand and gravel. Take a gallon glass jar, for instance, and select representative samples from the pit — that is, a sample that represents the average material as it ordinarily comes out in wagon loads. Put this sample in the jar as carefully as possible and add to the sample an amount of water twice the volume of the material. Shake it up thoroughly three or four times, allowing the material to settle after each shaking. After a short time the dirt or silt which is liable to affect the strength of your concrete will appear in a layer on top of the clean sand and gravel. "You can determine quite accurately the percentage of silt in a certain amount of sand and gravel by the use of a graduated glass cylinder with a capacity of 100 cubic centi- meters. Fill this half full with the material, shaking down well until it has been thoroughly compacted. Then fill the interstices of the sand or gravel with water, shake it well, and then fill again. The shaking will give the air a chance to escape from the sand. Shake the cylinder and fill with water until you have a volume of 100 cubic centi- CONCRETE MATERIALS 27 meters. When the material has been allowed ta settle the amount of fine material may be read. There will be a dis- tinct division line between the dirt or silt and the clean sand and gravel. Multiply the amount read by 2 and you will have the percentage of dirt and silt contained in that sample of sand. Material containing over 2 or 3 per cent of dirt or silt should be washed because we have no way of determining in the field the character of this particular silt or dirt. "It is not always possible for the farmer or the small contractor to equip himself with a screening and washing plant. However, there are methods which he can use eco- nomically and wash his material. For example, I have found, by past experience that an ordinary f-inch screen set at 45 degrees to the horizontal may be used as part of a yvash- ing plant. A stream of water of sufficient strength coming over the top of the screen between two sheets of metal and spreading into a spray, will wash the material thrown on to the screen free from dirt. The gravel will roll off the end of the screen, while the sand will be carried through with the water. If a baffle board is constructed and drainage provided beneath the screen, the water will drain away with a considerable portion of the dirt in suspension. "The ordinary method of determining the percentage of silt in the laboratory is by taking a certain weight of sand, dried at the temperature of the laboratory, placing this material in a glass provided for the admission of water, from the bottom under pressure, the pressure being reduced so that the running water will wash the dirt from the sand but will not carry the finer particles of sand away over the sides of the glass. The water is allowed to pass through the material until it runs clean and clear over the sides of the glass. The sample is removed then from the glass and dried. From the difference in weight of the sample before and after washing the percentage of dirt or silt may be deter- mined. "There are several conditions under which the silt may exist in concrete aggregate, and upon these conditions depend whether or not the silt or foreign material will be detrimental to the concrete when it is mixed and placed in the forms. If the clay for the sample exists in a finely powdered form, 28 PRACTICAL COURSE IN CONCRETE well distributed through the mass, it will act simply as an adulterant, that is, if it does not ball up in the mixer and adhere to the particles of sand and gravel during the mix- ing. If this latter condition prevails then the cement has no chance to adhere to the surface of the particles. On the other hand, if silt or clay exists in layers or chunks through- out the pit, then you will have considerable trouble because from this condition weak places and cracks will in all prob- ability develop in your concrete work. These conditions we desire to avoid in all concrete work. Even though the silt or clay is distributed, it is best to wash the sand and gravel free of this material. "The strength of concrete depends upon the strength of the material you are using. Comparison tests on blocks of concrete have shown, however, that silt or clay, coating the particles of sand and gravel, reduce the compressive strength of the concrete to a great extent. Test pieces made up of good, clean material, at the age of 28 days, when broken under a testing machine, will, as a rule, show a large percentage of broken particles. On the other hand, com- pressive test specimens made of dirty aggregate, using the same amount of cement and tested at the same age, will show the particles pulling out from the bed of mortar around them and less strength. "There is one other thing which should be mentioned in connection with concrete work, that is, the effect of finely powdered dust which is present in limestone screenings. Experiments have shown that this dust, unless removed from the coarser particles of screenings and broken stone, causes approximately the same effect upon the strength of concrete as does the presence of silt or clay. It is essential that this dust be removed by screening and washing prac- tically in the same manner that silt or other foreign material is removed from sand and gravel." Voids. — Voids are air spaces between the grains and are usually referred to as a percentage of the whole. A sand consisting of grains all uniform in size, will present the maxi- mum of voids. This can be illustrated as follows: Perfect spheres of equal size piled in the most compact manner leave, theoretically, but 26 per cent of voids. The only requirement is that the spheres be of equal size. Suppose, CONCRETE MATERIALS 29 now, that the spaces between such a pile of equal sized spheres were filled with other perfect spheres of diameter just sufficient to touch the larger spheres, the voids in the total included mass would be reduced theoretically to 20 per cent; and should this be followed up with smaller spheres, the air spaces or voids could be reduced sufficiently to make the mass water-tight. The shape of the particles also affects the percentage of voids. Round particles compact more readily and firmly, and with less difficulty, than angular particles. Conclusion The scope of concrete work has become so great that it demands a nation-wide study of aggregates. But such study alone will not solve all the problems and insure good work in the future. It will, however, serve to give an idea of the relative merits of the various aggregates available. We now have standard specifications which demand cer- tain requirements from the cement manufacturers. How much more do we need standard specifications for the selec- tion of concrete aggregates. The preceding paragraphs have in a brief way, given you some idea of the properties required in good aggregate, which are, briefly: good grading, cleanUness, and durability. Therefore, with good aggre- gates, standard Portland cement, and careful and efficient workmanship, good concrete can easily be obtained. Examination Questions 1. What is the proper msthod of storing cement? 2. What are the reasons for making tests of cement for set, soundness, and fineness? 3. What does the term agg regale include? 4. How may the presence of clay in sand be detected? 5. Name seven kinds of materials in the order of their values for concrete aggregate. 6. Which has the greater percentage of voids, sand of uniform or of graded sizes? 7. Name the properties required in good aggregate. INSTRUCTION PAPERS FOR HOME STUDY INDUSTRIAL EDUCATION SERIES EXTENSION DIVISION OF THE UNIVERSITY OF WISCONSIN CORRESPONDENCE-STUDY PRACTICAL COURSE IN CONCRETE COURSE 430 ASSIGNMENT Copyright, 1916, by th£ UmvrasrrY Extension Dtviuon MADISON PUBUSHED BY THE UNIVERSITY FEBRUARY. 1916 UNIVERSITY EXTExNSION DIVISION THE DEPARTMENT OF CORRESPONDENCE-STUDY offers instruction in studies required in each of the following lines, bulletins on which will be mailed free on request: Agriculture General basic studies Letters and Science Studies required for liberal arts degrees Medicine General basic studies IN THE COLLEGES Engineering Basic and technical studies Law Prerequisite liberal arts studies IN COURSES AND Business Accounting, Administration, Cor- respondence, Geography, Law, Management, Organization, Retailing Chemistry Commerce Education Engineering Civil, Electrical, Mechanical, Structural English History Home Eeonomios Industry Jour nails m Publle Speaking DEPARTMENTS Languages French, German, Greek. Italian, Latin, Spanish Mathematics Mechanical Drawing Music Pharmacy Philosophy Physical Science Astronomy, Bacteriology. Bot- any, Chemistry, Geology, Physics Political Economy Political Science Sociology Snrveying Teaching THE DEPARTMENT OF DEBATING AND PUBLIC DIS- CUSSION will mail bulletins on request free within the state, and elsewhere on receipt of price. THE DEPARTMENT OF GENERAL INFORMATION AND WELFARE will mail on request bulletins on Municipal Reference, Civic and Social Centers, Vocational Institutes, Com- munity Institutes, Dependency and its Relation to Industrial Education, Community Music, and Public Health, free within the state, and elsewhere on receipt of price. THE DEPARTMENT OF INSTRUCTION BY LECTURES will mail on request bulletins on the lectures and lecture courses. THE BUREAU OF VISUAL INSTRUCTION will mail on request information as to the lantern slides and motion picture films lent within the state. CHAPTER II PROPORTIONING 3. Theory of Proportioning. — In order to comprehend the importance of correctly proportioning the ingredients entering into the composition of concrete we must in the beginning obtain a correct idea of the theory of the material we propose to manufacture. The aggregates consisting of sand and gravel or broken stone are wholly inert, until combined with Portland cement. Consequently it is of prime importance that every piece of coarse aggregate be thoroughly surrounded with sand- cement mortar, and that every grain of sand be enclosed in a film of neat cement. In so far as actual practice departs from this fundamental principle, just so far will the bonding be defective. The second important principle of concrete composition is that voids shall be eliminated by such gradation of materials that the spaces between larger pieces of the coarse aggregate will be occupied by smaller pieces, and the spaces between these will in turn be filled by sand until, in a perfectly proportioned mixture, there will remain only such voids as will be taken up by the cement solution when the concrete is finally compacted in the place of its ultimate use. The absolute ehmination of voids is an ideal condi- tion, hence it is essential to use every means in our power toward approaching the perfection suggested. The more nearly we approximate the theoretical possibility, the more successful we shall be in actual practice. 4. Object of Proportioning. — Both strength and den- sity in finished concrete construction are dependent upon careful proportioning. A very porous concrete may under certain conditions of manufacture be stronger than a very dense concrete which is lacking in cement or in coarse aggregate. Hence, we observe work disintegrate after two or three years, and upon examining a fracture find that the concrete has no large voids but is composed of fine sand 32 PRACTICAL COURSE IN CONCRETE with little or no coarse aggregate. Such material may appear dense, but hardly deserves to be called concrete. On the other hand, remarkable ins^nces of strength developed in porous concrete may be observed where the coarse aggregate was fairly well graded and but little sand used. This practice is not recommended because the work- ing conditions might not be identical, and a concrete possess- ing a large percentage of voids will not be watertight. The point is mentioned merely to emphasize the fact that coarse aggregate and cement give strength to concrete. Sand increases the density. Impermeability, or resistance to the passage of water, is one of the most prominent characteristics of good concrete and is absolutely dependent upon the elimination of voids, which results only from correct proportioning of ingredients. A porous concrete is never watertight. Quite a number of processes for waterproofing have been suggested, some like soap and alum or the "Sylvester Process" are pubhc property, while others are either secret formulas or process patents. Some consist of incorporating compounds in the concrete at the time of mixing, and others of applying com- pounds to the exterior or interior of the work after com- pletion. If the concrete is properly proportioned, there is no reason for using any extraneous waterproofing medium. In reinforced-concrete work a satisfactory bond between the steel and concrete can be obtained only by such careful proportioning as will insure a concrete practically free from voids. This does not mean merely slushing in water enough to fill spaces between aggregate surrounding rods or other reinforcement. Surplus water will disappear by evapora- tion, leaving cavities adjacent to the reinforcement, and when a failure occurs rods will be found pulled out of porous concrete which lacked the bond that saves structures. 5. Methods of Proportioning. — The following meth- ods of proportioning concrete are in general use 1. Arbitrary selection of proportions. 2. Void determination. 3. Proportioning by trial mixtures. Proportioning by Arbitrary Selection. — As many users of concrete do not wish to take the trouble to test their own PROPORTIONING 33 materials, it is customary for them to use the proportions which have been found to produce satisfactory results under average conditions. These are one part of cement, two and one-half parts of sand, and four parts of coarse aggregate (expressed 1:2|:4) for most classes of construction. In the manufacture of products large enough to use aggregate exceeding one inch in greatest dimension, the proportion of coarse aggregate may be increased accordingly. Con- versely, where a fme texture is desired for ornamental pur- poses, the proportion of cement must be increased reaching its maximum in 1:^ trowelled surfaces. The following table gives the proportions recommended for various classes of work: Concrete A 1 :2:3 mixture for: One course concrete highway, street and barnyard pavements. One course floors and walks. Roofs. Fence Posts and for sills and lintels without mortar surface. Water troughs and tanks. A 1 :2 :4 mixture for : Reinforced concrete floors, beams and columns, large engine foundations. Work subject to vibration. A 1:2|:4 mixture for: Building walls above foundation. Silo walls. Base of two course street and highway pavements. Backing of concrete block and similar cement products. A 1:3:5 mixture for: Basement walls and foundations. Small engine foundations. Base of sidewalks and two course floors. Mass concrete footings, etc. Mortar 1 :1| mixture for: Wearing course of two course floors. 1 :2 mixture for: Scratch coat of exterior plaster. Facing blocks and similar cement products. Wearing course of two-course walks, street and highway pavements. 34 PRACTICAL COURSE IN CONCRETE 1:2 1 mixture for: Finish coat of exterior plaster. Fence posts, when coarse aggregate is not used. 1 :3 mixture for: Concrete blocks when coarse aggregate is not used. Cement drain tile, when coarse aggregate is not used. When proportions of the ingredients of a concrete are specified, the specifications should state whether the cement shall be measured loose or as packed in bags and barrels. The reason for this is clear when it is considered that loose cement occupies about 30 per cent more volume than packed cement. The usual method is to specify the barrel of packed cement as the unit, and to assign it some definite volume — the sand and stone to be measured loose. A barrel of Portland cement weighs 376 lb., not including the barrel, and a bag of Portland cement weighs 94 lb.; in other words, there are four bags to a barrel. For con- venience the bag of cement is usually assumed as one cubic foot in proportioning but is actually somewhat less, depend- ing on the brand. When a barrel is assumed to contain 3.8 cu. ft., as is sometimes done, then 100 lb. of cement can be considered as one cubic foot. It should be stated here that the common rule of thumb method of proportioning concrete may cause wide dis- crepancies because of the variation in the character of the ingredients. In fact, arbitrary selection should never be permitted except where the work is not expensive or im- portant enough to warrant special proportioning of the materials and grading of the aggregates. If arbitrary proportioning is decided upon, the best method to follow is to proportion the cement to sand by judgment in accordance with the character of the construc- tion, to adopt (as a trial) twice as much stone as sand by volume, and then to vary this proportion using as much of the coarse aggregate as possible without producing notice- able voids or stone pockets in the concrete. In cases where the coarse material contains a great many small particles, the proportion of sand should be tried at somewhat less than one-half the volume of stone. Experience in the handling of concrete enables one to judge readily whether the mortar is deficient or not. With PROPORTIONING 35 too little sand, stone pockets are apt to occur on the surfaces, and it is difficult to fill all the voids in the stone; with too much sand, a harsh working of the concrete will be noticed or an excess of mortar will rise to the top when placing. Proportioning by Void Determination. — There are four dis- tinct methods of proportioning concrete by the determina- tion of voids: (a) Finding the voids in the stone and in the sand, and then proportioning the materials so that the volume of sand is equivalent to the volume of voids in the stone and so that the volume of cement is slightly in excess of the voids in the sand. (The excess of cement is provided on account of inaccuracies in the void method of porportion- ing.) (b) Finding the voids in the stone, and, after selecting the proportions of cement to sand by test or by judgment, proportioning the mortar to the stone so that the volume of mortar is shghtly in excess of the voids in the stone. (c) Mixing the sand and stone and providing such a proportion of cement that the paste will slightly more than fill the voids in the mixed aggregate. (d) Making trial mixtures of dry materials in different proportions to determine the mixture giving the smallest percentage of voids, and then adding an arbitrary percentage of cement, or one based on the voids in the mixed aggregate. Proportioning by the determination of voids is little (if any) better than the proportioning by arbitrary assign- ment. In the first place, the percentage of voids in sand is greatly affected by even a small percentage of moisture. When sand is moistened, a film of water coats each particle of sand and separates it by surface tension from the grains surrounding it. Fine sand, having a larger number of grains, and consequently, more surface area, is more increased in bulk by the addition of water than coarse sand. Again, the volume of water required in mixing actually occupies space in the resulting concrete and not only every cement but also every sand has a different and definite percentagte of water necessary to bring it to what may be called normal consistency. Voids in absolutely dry sand are certainly no criterion of its qualities for concrete, while a moist sand will give different results on different days. 36 PRACTICAL COURSE IN CONCRETE Inaccuracy in proportioning by void determination is also due in part to the difference in the compactness of the materials under varied methods of handling, and to the fact that many grains of the sand are too coarse to enter the voids of the stone and consequently thrust apart the particles of the large aggregate. Many of the voids in the sand are also too small for the grains of cement to fit into them without expanding the volume. Percentage of voids is usually determined by finding the specific gravity of the solid particles, weighing a known KiG. 9. — -The impossibility of obtaining proper and uniform proportion from this average gravel bank is self evident. volume of the aggregate and computing therefrom the per- centage of voids. The voids in sand will average about 33 per cent, while in stone or gravel the open space averages in the neighborhood of 45 per cent. Proportioning by Trial Mixtures.— The fact that the densest concrete is the strongest and most impermeable makes possible a convenient and accurate method of pro- PROPORTIONING 37 portioning concrete and of determining the relative value of different aggregates. Mr. William B. Fuller describes the method as follows: "Procure a piece or steel pipe 8 to 12 in. in diameter and a foot long and close off one end; also obtain an accurate weighing scale. Weigh out any proportions selected at random, of cement, sand, and stone, and of such quantity as will fill the pipe about three-cjuarters full, and mix thoroughly with water on an impervious platform, such as a sheet of iron; then, standing the pipe on end, put all the concrete in the pipe, tamping it thoroughly, and when all is in, measure and record the depth of the concrete in the pipe. Now throw this concrete away, clean the pipe and tools and make up another batch with the total weight of cement, sand, and stone the same as before but with the proportions of the sand to the stone slightly different. Mix and place as before and measure and record the depth in the pipe, and if the depth in the pipe is less and the concrete still looks nice and works well, this is a better mixture than the first. Continue trying in this way until the proportion has been found which will give the least depth in the pipe. This simply shows that the same amount of materials is being compacted into a smaller space and that, consequently, the concrete is more dense. Of course, exactly similar materials must be used as are to be used on the work, and after having in this way decided on the proportions to be used on the work it is desirable to make such trials several times while the work is in progress, to be sure there is no great change in materials, or, if there is any change, to determine the corresponding change in the proportions. "The above-described method of obtaining proportions does not take very much time, is not difficult, and a little trouble taken in this way will often be productive of very important results over the guess method of deciding proportions so universally prevalent. "A person interested in this method of proportioning will find on trial that other sands and stones available in the vicinity will give other depths in the pipe, and it is probable that by looking around and obtain- ing the best available materials the strength of the concrete obtainable will be very materially increased." 6. Sizing Materials. — Unless sand and gravel are pur- chased separately, it will be necessary to separate them by screening to arbitrary sizes before proportioning. If,, for instance, it is proposed to use bank gravel varying in size from fme sand up to small boulders two screens should be used. The first should reject everything exceeding the maximum size of aggregate suitable for the work, this vary- ing from I inch, for fence posts and block, up to 2 inches for foundations and other work of large cross section. The general rule for walls is that the largest size of aggregate shall not exceed, in its greatest diameter, one-half the thick- ness of the wall. The second screen should in all cases be 38 PRACTICAL COURSE IN CONCRETE of |-inch mesh, the particles retained upon it to be regarded as coarse aggregate, and those passing it as fine aggregate, or sand. The following is taken from a paper on The Selection and Proportioning of Concrete Aggregates by Mr. C. K. Arp of Chicago, presented at the Conference on Permanent and Sanitary Farm Improvements in August, 1913: "Many are of the opinion that when a 1:2:4 mixture is required, it is possible to obtain this by taking one part Fig. 10. — In some parts of the gravel bank the material consists almost entirely of coarse particles. of cement and 6 parts of mixed aggregate or bank-run gravel. As previously stated, bank-run gravel does not occur in the proper proportions of fine and coarse material, but assuming that it did, it can very easily be demonstrated that the 1 :2 :4 mixture is much richer in cement than the 1 :6. "Take, for example, 2 cubic feet of sand to which we add 1 cubic foot or one sack of cement. When mixed together PROPORTIONING 39 the cement is practically contained in the voids or open spaces in the sand, so that the resulting mortar is very little over 2 cubic feet in volume. If, now, we add this mortar, which is the 1:2 portion of our 1:2:4 mixture, to 4 cubic feet of screened gravel or stone, the same condition is repeated and the mortar is practically contained in the voids in the stone. As a result the volume of mixed con- crete is little more than 4 cubic feet. "Likewise in adding one sack of cement to 6 cubic feet of bank-run gravel, the cement is contained in the larger amount of open space between the particles, the concrete resulting being 6 cubic feet, the volume of the bank-run gravel. Thus, it should be clear that in one case we have 4 cubic feet of concrete with one sack of cement, and in Fig. 11. — A slanting screen can be made to separate the sand from the coarse material in bank-run gravel, making possible a great improvement in the quality of the concrete produced. the other 6 cubic feet of concrete with a like amount of cement, which means that the latter is a leaner and weaker mixture. "To make a bad condition worse, bank-run gravel invariably contains an excess of sand, which assists further in weaken- ing the concrete. Therefore, it is always better to separate the sand and gravel by screening and afterward remixing the two in the proper proportions. "For minimum voids and the best concrete, the size of the fine aggregate should grade from \ inch in the largest dimen- sions, down to the finest, with the coarser particles predomi- nating, and in no case should fine aggregate be used, of which more than five per cent passes through a sieve having 100 meshes per linear inch." 40 PRACTICAL COURSE IN CONCRETE 7. Amount of Water. — The consistency will depend upon the use for which the concrete is intended and upon the process of manufacture necessarily associated therewith. Three consistencies or mixtures, determined by the amount of water used, are generally called the dry, the quaky, and the wet. The dry mixture is of the consistency of damp earth, and is used where the concrete is tamped into place, being principally useful in steel molds for making products requiring no reinforcement, such as brick, block, and ornamental vases. The quaky mixture is so named because it is wet enough to quake or shake when tamped. It is used in all molded products requiring reinforcement, such as fence posts, lamp posts, telegraph and telephone poles, drain tile, sewer pipe, ash pit rings and the like; also in engine foundations and the footings of buildings. The wet rnixture contains sufficient water to permit of its flowing from a shovel or wheelbarrow, but not enough to cause a separation of the particles. It is used in building reinforced-concrete structures, such as silos, barns, dwell- ings and other buildings where the concrete is allowed to remain undisturbed in the forms for several weeks. CHAPTER III MIXING 8. Fundamental Principle. — The importance of thor- oughly and carefully mixing the ingredients used in the manufacture of concrete is secondary only to the propor- tioning, because the mixing can not be done until after the proportioning has been accomplished. It is secondary in time but equal in importance. As stated earlier in this assignment, an essential feature of concrete construction is the coating of every grain of sand with a film of neat cement and the coating of every piece of coarse aggregate with sand-cement mortar. This is the fundamental principle of all concrete construction; an earnest effort to accomplish this result will insure success. Assuming that proper proportions have been determined, the result so carefully sought can.be attained only by thor- ough and intelligent mixing. 9. Shovel Mixing. — Let us first consider the rather difficult problem of securing satisfactory results where the volume of work does not warrant the installation of a mix- ing machine. The first requirement will be a water-tight platform large enough for two . men to shovel conveniently from either end as large a batch of concrete as can be used within thirty minutes after water has been added to it. Time is essential, and if on account of meal time, or any emergency, a portion of a batch lies until the cement has become partially har- dened, throw it away rather than jeopardize the work. As proportioning is usually done by volume, one cubic foot is a convenient unit, as it allows full sacks of cement to be used. The required amount of sand should first be spread upon the mixing platform, after which the cement should be spread in a layer on the sand. Two men, using square pointed shovels, will then turn the sand and cement over two or more times until the streaks of brown and gray have merged into a uniform color throughout the mass. The coarse aggregate is then shoveled on and the mixing continued, water being added during the first turning after 42 PRACTICAL COURSE IN CONCRETE adding coarse aggregate. Water should be added gently, as from a hose nozzle or the spout of a watering pot, in order to prevent washing out the cement. Turning should continue until the mortar is of uniform consistency through- out, which will usually require at least three turnings after adding water. * Mixing in the above manner will give satisfactory results, but the labor involved is considerable, and on this account it is too common for those attempting it to slight the work and use the concrete in an imperfectly mixed condition. 10. Machine Mixing. — Mixers have been brought to a high state of efficiency and today there are many on the market designed to produce the best results at minimum cost of labor and power. While it is beyond the scope of this assignment to discuss mixers, we may in passing men- tion one or two of the principles which will assist the con- crete manufacturer in making a selection suited to his needs. The batch mixers, whether cubes, cylinders or truncated cones, allow the material to be introduced in any order desired, provided only that each separate batch contains the proper relative proportions of ingredients. After the batch has been placed in the mixer, it is revolved for a speci- fied time, or a definite number of revolutions until either by the shape of the drum itself or by means of deflectors there- in, the cement, sand and coarse aggregate have been thor- oughly mixed. Most batch mixers are equipped with a small tank from which a pipe leads into' the mixer, and when the materials have been sufficiently mixed in a dry state, water is sprayed on them while the revolutions of the mixer continue. The continuous mixer consists mainly of a number of hop- pers for the several materials, placed over one end of a semi-circular trough containing blades or shovels fixed to a rotating shaft. The motive power is generally supplied by a gasoline engine or an electric motor. The dry materials are , fed automatically from the hoppers into the trough, mixed and carried along by the blades to the discharge end, water being added; meanwhile the concrete is discharged continuously. The batch type of mixer is considered by the majority of engineers to give the best results because the measuring MIXING 43 of the materials can be positively regulated; whereas with the continuous mixer variations in the amount of moisture in the sand or flufTmess of the cement will cause a variation in the relative proportions of these materials in the mixture. On this account, engineers are inclined to favor the batch mixer. Lists of manufacturers of both batch and continuous mixers will be found in the columns of current concrete periodicals. CHAPTER IV PLACING 11. Final Probletn. — But, when all is said and done; when we have selected the best materials, have ascertained the proper proportions of each and the correct amount of water for the consistency required to serve our particular purpose; when by shovel or machine we have combined the different materials required to make concrete, we have produced a mass of material which must forthwith be hon- estly and intelligently deposited, compacted, and made to take some one of the thousand and one shapes which con- crete assumes to serve humanity by increasing the efTiciency of man. This, then is our problem, the placing of the concrete, and we shall fmd three distinct methods of accomplishing this result. 12. Pressure and Tamping. — Whenever a dry mixture is used in steel molds to produce such unreinforced products as ornamental vases, block or brick, concrete is placed by pressing or tamping. If pressure is applied, it will ordi- narily be by means of a press simplifying the process and making it necessary only to see that the molds are ade- quately and evenly filled in order that the product may be uniform in density. If, however, tamping is the method employed, considerable supervision will be found necessary as the quality of the product may vary considerably unless the tamping is uniformly performed. 1 1 is particularly necessary that the mold be tamped while filling, not filled and tamped afterward. The latter method will not only fail to fill the lower corners but will make one-half of the molded product much denser than the other. If tamping is well done by one man (or two if the mold is large) while the mold is being filled by another, there is no reason why the product should not be perfectly satisfactory and as uniform as though made under mechanical or hydrauUc pressure. To secure more uniform density and effect a saving of labor, power tampers are used, the multiple tamp- ers being especially serviceable in making block and brick. PLACING 45 13. Agitation. — Neither tamping nor pressure will be of service in the case of those products requiring the intro- duction of reinforcement, such as tile, pipe, poles and posts. In the manufacture of these and similar products, the steel (in whatever form required for reinforcing) is introduced at the proper place in the mold while it is being filled with a quaky mixture of concrete. The concrete is compacted, forced into corners and around or through the reinforcement, by vigorously stirring the mixture and jarring the mold. 14. Depositing Wet Concrete. — Placing concrete for reinforced-concrete structures, including silos and all sorts of buildings, involves work on a scale warranting the instal- lation of special apparatus to save both time and labor in transporting the concrete from the mixer to the place of use. Elevators, dump cars, and chutes are ordinarily used in the construction of reinforced-concrete buildings. In constructing silos, it is necessary to provide a center hoisting device with derrick and an automatic dumping bucket. The concrete is poured into forms in which reinforcement has previously been placed. It is then necessary to spade it back from the forms in order to prevent large pieces of aggregate from retaining surface positions when the forms are removed. The larger pieces of aggregate should, as far as possible, be forced away both from the reinforcement and the forms so that they may occupy an intermediate position. Though the subject of "Forms" is treated in another assignment, a word of caution relative to their removal may not be amiss at this time. While no definite rule can be given to fit all local conditions and variations of structure, humidity and temperature, good judgment will suggest that too early removal involves danger, while reasonable delay in removing forms is a wise precaution insuring safety. Examination Questions 8. What reason is there for using a waterproofing compound in properly proportioned concrete? 9. Wherein does the density test have its value? 10. State proper proportions for fence posts when coarse aggregate is used; when it is not. 11. What is the fundamental principle of making good concrete? 12. What type of mixer do engineers favor? 13. What is the object of spading? INSTRUCTION PAPERS FOR HOME STUDY INDUSTRIAL EDUCATION SERIES EXTENSION DIVISION OF THE UNIVERSITY OF WISCONSIN CORRESPONDENCE-STUDY PRACTICAL COURSE IN CONCRETE COURSE 430 ASSIGNMENT Copyright, 1016, by the University Extension Division MADISON PUBLISHED BY THE UNIVERSITY FEBRUARY, 1916 UNIVERSITY EXTENSION DIVISION THE DEPARTMENT OF CORRESPONDENCE-STUDY ofers instruction in studies required in each of the following lines, bulletins on which will be mailed free on request; Agrlenltare General basic studies Letters and Science Studies required for liberal arts degrees Medicine General basic studies IN THE COLLEGES Engineering Basic and technical studies Law Prerequisite liberal arts stadias IN COURSES AND Basinesa Accounting, Administration, Cor- respondence, Geography, Law, Management, Retailing Chemistry Cammerce Edueaticn Engineering Civil, Electrical, Structural Eagllah Hiatory Home Economics ladnatry Joaraaliam Pablic Spealcing Organization, Mechanical, DEPARTMENTS Languages French, German, Greek, Italian, Latin, Spanish Matliematios Meclianieal Drawing Music Pharmacy Philosaphy Physical Science Astronomy, Bacteriology, Bot- any, Chemistry, Geology, Physics Political Economy Political Science Sociology Sarveying Teaching THE DEPARTMENT OF DEBATING AND PUBLIC DIS- CUSSION will mail bulletins on request free within the state, and elsewhere on receipt of price. THE DEPARTMENT OF GENERAL INFORMATION AND WELFARE will mail on request bulletins on Municipal Reference, Civic and Social Centers, Vocational Institutes, Com- munity Institutes, Dependency and its Relation to Industrial Education, Community Music, and Public Health, free within the state, and elsewhere on receipt of price. THE DEPARTMENT OF INSTRUCTION BY LECTURES will mail on request btdletins on the lectures and lecture courses. THE BUREAU OF VISUAL INSTRUCTION will mail on request information as to the lantern slides and motion picture films lent within the state. CHAPTER V FORMS The plasticity of concrete and the readiness with which the material can be adapted to all shapes and sizes of con- s,truction have, from the beginning of the more extensive use of concrete, made the production of molds of desired form, a very important consideration in all concrete con- struction work. While iron and steel molds have been used for small members — such as block and brick, and ornamental pieces, in which the same design and size can be indefinitely repeated — larger concrete construction requires individual design determined by local conditions and particular needs. The ease with which concrete may be adapted to such pecuhar requirements of individual use is one of the' chief merits of the material. Consequently, means must be pro- vided for constructing, at or near the place where the con- crete is to be used and from materials easily procured, molds which may be made to fit the circumstances of each in- dividual case. Molds of this diversified character are commonly called forms. 15. Classification. — Forms may be roughly classified as follows: 1. Rectangular forms wholly of lumber. 2. Rectangular forms using metal fastening devices. 3. Rectangular metal forms. 4. Circular forms of wood and sheet metal. 5. Circular forms wholly metal. 6. Miscellaneous. 16. Lumber Forms. — Contrary to the usual practice in building construction, green lumber will keep its shape in all rectangular forms better than lumber that is thoroughly dry. If dry lumber is used, it should be thoroughly wet before the concrete is placed. The use of oil or grease on the inside of forms is recommended as it prevents absorption of water from the concrete by the forms, and makes their removal easier. Where any fine ornamentation is used, the 48 PRACTICAL COURSE IN CONCRETE molding or other device introduced to vary the surface should be painted with equal parts of boiled linseed oil and kerosene. It is, however, essential that forms should be thoroughly cleaned each time they are used, and that no dry concrete be left sticking to the face 'of the forms. Forms may be built from stock-length lumber, requiring very little sawing and permitting of the lumber being used later for other purposes. White pine is considered the best lumber for forms, although spruce, fir, and Norway pine are often used. The face of forms should be free from loose knots, slivers or other irregularities, as concrete will reproduce them all with great faithfulness. Matched lumber may be used to afford a smooth finish and very satisfactory results can be obtained by proper care in the construction of forms. Fig. 12. — -Forms for concrete foundation wall. 17. Rectangular Forms. — In the construction of rec- tangular forms, the first type of construction presenting itself for consideration is foundation work. Where the excavation is made simply for a foundation without cellar or basement, the soil will often be firm enough so that the trench, if carefully excavated, may be used as a form below ground fine. In this case the edges must be protected to keep the dirt out of the concrete. In carrying the founda- tion from the ground line to the level of the first floor, forms must be constructed like those shown in Fig. 12. These FORMS 49 forms may either be constructed in sections and then set in position, or they may be built in place. It should be noted that the forms in Fig. 12 are suspended over the trench and are not allowed to rest upon the new concrete. If the inner and outer parts of the form are built separately, they must, when put into position, be levelled and plumbed carefully. Whether built in sections or built in place, forms must be braced thoroughly and tied together, as the essential duty of any form is to keep rigid until the concrete has hardened. Fig. 12 also shows a convenient method of suspending the bolts which are to be embedded in the concrete for securing the wooden sill or wall plate to the foundation. These bolts are also shown in Fig. 13. They should not pass Fig. 13. — Foundation wall for a small shed, showing bolts for attaching the wooden sill to the concrete foundation. through the cleats but should be carried by a block or small board tacked to the cleats; otherwise it would be difTicult to remove the form without destroying the cleats. Before filling the wooden forms, the surface of concrete previously placed in the trench should be swept off, to remove dirt, leaves or other foreign matter. If it has become dirty, it should be washed and while still wet brushed with a cement and water grout mixed to the consistency of thick cream. Concreting should be resumed before the grout has had time to dry. This treatment will insure a good bond between the old concrete and the new, and should always be used when joining work that has hardened. 50 PRACTICAL COURSE IN CONCRETE Forms for foundation piers and for the foundation of all kinds of machinery are constructed in substantially the same manner as for regular building foundations. The construc- tion of machinery foundations is essentially a problem of securing the necessary mass and weight, consequently the greater part of the foundation will be under ground and all that is required above ground is an open box of sufTicient strength to maintain the concrete in the desired form while hardening (Fig. 14). Fig. 14. — Sketch showing gasoline engine foundation. Where the excavation for a cellar is made by team and scraper, the sides will not be perpendicular and the excava- tion will usually be somewhat larger than the dimensions of the cellar wall. Consequently, it is necessary to use both inner and outer forms. Each form consists of uprights spaced close enough to prevent any spreading or bulging of the sheeting when subjected to the outward pressure of the fresh concrete. The inner form should be securely braced in a perpendicular position by lumber braces from the floor of the excavation. The outer form should be fastened to the inner form by wires running through both near the bottom, and at the same place the forms should be separated by spacing blocks of the width determined upon for the cellar wall. The outer form should, like the inner, be perpendicular unless a slight batter is desired, in which case the spacing blocks should be lengthened to spread the bottom of the forms apart and increase the thickness of the wall at the bottom without interfering with the estab- FORMS 51 lished thickness of the wall at the top. The wires connecting the two forms should be drawn tight by twisting with a large nail or rod until the forms are drawn firmly against the spacing blocks. The top of the uprights should be joined by cleats. The method just described produces a very rigid form. Fig. 15 shows another method of form con- struction for low walls. Externally braced forms. If anf outside entrance to a cellar or basement is desired, the forms should be constructed at the same time that the forms are built for the cellar walls. When in position, these forms will rest upon the floor of the excavation made for the Fig. 16.- — -Ordinary wooden wall form, supported from ground. 52 PRACTICAL COURSE IN CONCRETE steps. If the excavation for the entrance is carefully rnade^ only the inside form will be required until ground line is reached. As the walls will project above the ground where they join the building, and slope from that point to the opposite end of the entrance, an outside form will be required above the ground line. By properly bracing the form, one side wall may be made and, after it has hardened, the form reversed and used for the other side. After both side walls have been made, forms for the steps giving desired height of riser and width of tread may then be securely braced between th€ side walls. Concrete walls above the cellar may be built either as a single wall or as two walls with an air space between them. Fig. 16 shows the necessary form construction for a single wall. The forms may be built in sections or in place. Sectional or unit forms can be used a number of times if carefully removed and assembled, making a large saving in the amount of lumber used. The wire ties shown in Fig. 16 are used to secure the forms against pulling apart, being wound around opposite studs and then twisted with a stick, as a turnbuckle, until the studs are held tight against the spacing blocks. These spacing blocks must be removed as the concreting progresses. Fig. 17 illustrates a method FORMS 53 employed by one construction company for building a wall of considerable height by means of movable forms. In the construction of double walls, such as in ice houses, the intervening air space is not usually wide enough to ac- commodate two sets of forms. Therefore the hollow wall is usually constructed by placing in the forms cores which are later withdrawn. Forms for retaining walls and bridge abutments are erected in a similar manner to building walls, although usually made of much heavier timber. Figs. 18 to 22 inclusive show the form construction employed in the con- struction of the abutments for a railroad bridge a few miles west of Fort Dodge, Iowa, on the Illinois Central Railroad. These views also show the method of handling the concrete materials and the transporting and placing of the concrete. The industrial car and track, and the chutes should be noted. Forms for walks (Fig. 23) and floors should consist of 2-inch lumber in width equal to the desired thickness of the walk or floor, staked in the earth to form slabs of the desired size. The concrete is mixed wetter than for two course work, and where the walk or floor is laid in one course, slabs should be laid alternately, allowing cross forms to remain in place until ready to fill intermediate slabs. This method is also used extensively in two-course work, although many prefer to work consecutively, moving the cross piece each time a slab is completed. If laid continuously, care must be ' exercised to preserve the vertical joints through the entire walk. Horse blocks or carriage steps may be constructed where the walk joins the driveway by the use of simple box forms. The modern farmer is making use of concrete for the con- , struction of various types of tanks, such as the stock watering tank, the hog feeding trough, the dipping vat, and the hog wallow, all of which may be constructed by the use of rectangular lumber forms. The general method of constructing rectangular tanks above ground consists in erecting an outer form, usually of 2-inch lumber, in which the concrete floor of the tank is placed, and the surface finished as desired, after which the bottomless inner form, which must be previously prepared and ready for immediate use before the previously placed Fig. 18.- — Form construction for railroad bridge abutments. Reinforcing rods in place. FORMS 55 PRACTICAL COURSE IN CONCRETE FORMS 59 concrete has hardened, is quickly inserted and securely fastened in place by cleats joining the uprights of the outer and inner forms (Fig. 24), The method of constructing rectangular tanks underground differs only in that the earth usually forms the outer form, and a wood form is required Fig. 23. — A concrete sidewalk on the farm. for the roof. In constructing septic tanks, provision must be made for the several partitions and compartments neces- sary to secure decomposition of the sewage and disposal of the effluent. Two methods are used with equal satisfaction in manu- facturing small troughs which need not necessarily be built in place. One is to use a box mold and finish the interior with a straight batter or a concave surface by striking it 60 PRACTICAL COURSE IN CONCRETE out with a template. The other method is to use a core of firm clay or wood made in shape to correspond with the in- side of the trough. A bottomless box is placed over the inverted core and by filling the box with concrete and striking it off level, the trough is manufactured upside down (Fig. 25). The simplest deviation from home-made molds is to pur- chase clamps for holding forms in place, thus doing away with nailing them to the uprights. There are several systems of clamps on the market, some of which are very Fig. 24. — -The easiest form to construct for building concrete tanks is of the rectangular shape. The inner form is supported on the outer after the concrete tank floor is in position. ingenious, and all of which are designed with two purposes in view, the first being to facilitate the erection and removal of forms, and the second being to save loss of lumber from repeated nailing and tearing down. A still wider departure from the home-made forms brings us to those constructed wholly of metal, which provide a rapid and economical method of concrete construction where a large amount of work is to be done along uniform lines. Only continued repetition, however, will justify the purchase of metal forms. Where the opportunity occurs to rent metal forms for any work of considerable importance, a saving may be effected and the quality of the work somewhat improved on account of greater surface uniformity secured by use of the metal forms. FORMS 61 18. Circular Forms. — Circular forms are extensively used in the construction of tanks because a round tank i& more economical to build and will resist frost action better than a tank of any other shape. The construction of a circular form presents greater difficulty than does that of a rectangular form, and it is usually better for several of those who desire to construct tanks to determine upon a standard size and join in the use of a set of forms, or if this can not be done, a set of forms may be rented if but a single tank is to be made. For a 10-foot circular tank, 2 feet 6 Fig. 25. — Small reinforced-concrete troughs may be made by the use of a wooden or clay inside form in an inverted position, or the shape of the interior may be formed by the use of a wooden template with the side form as a guide. inches in depth, the forms usually cost about |50, while the cost of the tank itself, exclusive of sand and gravel, is only |30. Forms for circular tanks (Fig. 26) consist of an inner and outer wooden frame covered with sheet iron. (Silo forms may be used for the outer forms of large tanks). The height of the inner form is equal to the inside depth of the tank and the height of the outer form is equal to the sum of the inside depth and the floor thickness of the tank. After the inner and outer circles of the form have been laid out, segments are cut from 1-inch lumber and a wooden frame is built up, fence fashion. No. 22 gage galvanized 62 PRACTICAL COURSE IN CONCRETE iron is then attached by screws or nails. The inner form should form an angle with the outer one to give proper batter to the inside of the tank to prevent bursting in case of freezing. The selection of silo forms presents to the modern farmer one of the most important problems in connection with the use of concrete. What are known as homemade silo forms are usually constructed in 3-foot sections, but it is hardly desirable to construct a set of forms for the express purpose of building one silo. It is far better for farmers to unite in the matter, as a set of forms may be used for constructing a large number of silos. However, if one must build his Fig. 26. — A circular form can be made of sheet steel supported on a wooden frame work. The inner form is suspended from the outer as shown. own forms, a most ingenious model is that of Mr. David Imrie, Roberts, Wisconsin, who has introduced his form to hundreds of farmers in connection with the work of the Wisconsin Farmers' Institutes. The inner form consists essentially of hooped sheet metal securely clamped and braced. No. 28 gage galvanized sheet iron is used and the form is assembled in eight segments which are bolted together. The outer form is made of 18 or 20 gage galvan- ized sheet metal 3 feet in width, in two or more pieces joined by heavy band iron riveted to the ends of each piece, which is turned at right angles and drilled to receive the bolts FORMS 63 drawing adjoining sections together. Forms of this type have been built at a cost varying from $25 to $50. Practically all silos now built are roofed. The construc- tion of the roof form is a simple matter, requiring only a box for the cornice and 2-inch by 4-inch rafters radiating from the apex to the roof edge, on which 1-inch by 6-inch sheeting is laid to receive the concrete. Many commercial systems of silo construction are now upon the market. Fortunately, most of them are meri- torious and will result in more satisfactory work than can be obtained from homemade molds unless one is thoroughly familiar with the qualities and method of handling cement and concrete. The various commercial silo systems are operated under different methods. The forms are con- structed wholly of metal and some companies sell them outright to an association of farmers who desire to construct silos; some companies rent their forms for the construction of a single silo; some companies construct a silo for the farmer, acting in the capacity of contractors and guarantee- ing their work in every way. 19. Miscellaneous Forms. — The miscellaneous uses of concrete about the barn, barnyard, and farm in general are innumerable. The preparation of forms for the many uses to which concrete may be put affords pleasant exercise for the ingenuity of any one familiar with the uses of concrete. A few of the possibilities of smaller construction are merely suggested: concrete stalls, mangers, hens' nests, hotbeds, pits for wagon scales, curbing for old wells, pump pits, and waste water receptacles. The form for the last mentioned consists of earth excavation for the outer form and an empty half barrel for the inner form, which indicates how simple concrete construction may be made. The removal of the form is a matter requiring very careful consideration. A great deal of work has been injured and not a little has failed, because of undue haste in removing forms. Two or three days' additional time allowed to new concrete before removing the forms often marks the difference between defective and thoroughly satisfactory work. 64 PRACTICAL COURSE IN CONCRETE Examination Questions 14. Should green or dry lumber be used for building rectangular forms? 15. What are the essential requirements for constructing machinery foundations? 16. What should be the width of lumber used in building forms for walks and floors. 17. Under what conditions are metal forms economical? 18. Of how many segments does the inner silo-form of the Imrie model consist? 19. How may a simple form be constructed for waste-water receptacle? INSTRUCTION PAPERS FOR HOME STUDY INDUSTRIAL EDUCATION SERIES EXTENSION DIVISION OF THE UNIVERSITY OF WISCONSIN CORRESPONDENCE-STUDY PRACTICAL COURSE IN CONCRETE COURSE 430 ASSIGNMENT Copyright, 1916, by the University Extension Division MADISON PUBLISHED BY THE UNIVERSITY FEBRUARY, 1916 UNIVERSITY EXTENSION DIVISION THE DEPARTMENT OF CORRESPONDENCE-STUDY offers instruction in studies required in each of the following lines, bulletins on which will be mailed free on request: IN THE COLLEGES Aicricultui-e Generai basic studies Letters and Science Studies required for liberal arts degrees Engineering Basic and technical studies Law Prerequisite libera! arts studies Medicine General basic studies IN COURSES AND DEPARTMENTS Languages Busineas Accounting, Administration, Cor- respondence, Geography, Law, Management, Retailing Chemistry Co mmerce Education Engineering Civil, Electrical, Structural Engllah History Home Economics Industry Joarnaliam Public Speaking Organization, Mechanical, French, German, Greek, Italian Latin, Spanish Mathematica Meclianical Drawing Music Pharmacy Philosophy Physical Science Astronomy, Bacteriology, Bot- any, Chemistry, Geology. Physics Political Economy Political Science Sociology Surveying Teaching THE DEPARTMENT OF DEBATING AND PUBLIC DIS- CUSSION will mail bulletins on request free within the state, and elsewhere on receipt of price. THE DEPARTMENT OF GENERAL INFORMATION AND WELFARE will mail on request bulletins on Municipal Reference, Civic and Social Centers, Vocational Institutes, Com- munity Institutes, Dependency and its Relation to Industrial Education, Community Music, and Public Health, free within the state, and elsewhere on receipt of price. THE DEPARTMENT OF INSTRUCTION BY LECTURES will mail on request bulletins on the lectures and lecture courses. THE BUREAU OF VISUAL INSTRUCTION will mail on request information as to the lantern slides and motion picture films lent within the state. CHAPTER VI FOUNDATIONS Concrete is especially adapted for use in building founda- tions because of the following characteristic qualities: 1. Compressive strength. 2. Durability. 3. Moderate cost. 4. Ease of Construction. 5. Adaptability to irregular excavations. 6. Capacity for reinforcement. Plain, or unreinforced, concrete shows its greatest strength under direct compression. Carrying capacity is the quality chiefly sought in the selection of material for the foundation of any building. Moreover, concrete lasts forever without repairs, and permanence is a consideration scarcely secondary to strength in determining a choice of foundation material. The cost of a well-built concrete foundation is considerably less than that of one constructed of any other building material of equal strength and durability. Under average conditions, the time required for building a concrete founda- tion is shorter than that required for one of brick or stone. Concrete is the only foundation material which readily adapts itself to slopes, change of grade, or other irregularities in the subgrade on which the foundation is laid. Wherever conditions require a foundation of restricted area, or where a side hill exposes a portion of the foundation wall to danger of accidental injury, or where vibration of engines and other machinery must be withstood — in any of these cases, con- crete has demonstrated its adaptability by permitting the introduction of sufTicient reinforcement to satisfactorily perform the duty demanded. Consequently, concrete is supplanting all other materials for building foundations of every character, irrespective of the character of the superstructure. Some of the principles which must be observed to secure the best results will be here outlined. 66 PRACTICAL COURSE IN CONCRETE 20. Materials. — The proportioning, mixing, and placing of concrete has been thoroughly discussed in Assignment 2, and the practices therein recommended should be rigidly observed. Further, it is often possible in foundation work to increase the size of the largest aggregate up to 2 inches or even 2| inches. Wherever large sizes of hard, durable gravel or broken stone can be used, additional strength is secured; for this purpose field stones may be employed advantageously, 21. Excavation. — In preparing for the erection of any rectangular structure, a base hne should first be determined upon and from the base hne the several corners should be ascertained by accurate measurement at right angles or at such other angles as may be desired in structures of irregular shape. The corners should be staked and definitely fixed by a tack driven in the top of each stake. All measurements and angles should then be checked back to the base line. Several feet outside of the line of stakes other stakes should be set to over-reach the corners, or a frame may be built 10 inches above ground, from which lines are then run to pass exactly over the tacks set in the stakes. These lines show the outside of the proposed excavation, and by measuring the width of the foundation and running parallel lines that far inside of the first lines, the lay-out is ready for excavation. The depth of excavation depends upon the height and character of the building to be erected, but should always go to solid earth, and should at least be lower than frost line. If the ground is filled with surface water at certain seasons of the year, drainage should be provided from the bottom of the foundation trench to a natural outlet. 22. Footings. — As a convenience in setting forms, footings are sometimes provided where ground is firm. Wherever a foundation is to be constructed on filled ground which can not, by rolling and tamping, be made solid enough to guarantee the permanent carrying of the superimposed load without settlement, the weight must be distributed by a layer of concrete wider than the foundation itself. This is known as a footing. It may be twice as wide as the founda- tion but must be thick enough to prevent cracking, and may have either sloping or stepped sides. In extreme cases of very soft earth requiring excessively wide footings, crossbars FOUNDATIONS 67 or reinforcing rods are introduced in the footings to dis- tribute the foundation load without injury to the concrete slab. Kidder (Architects' and Builders' Pocket-Book) gives bearing power of soils as follows: Description Rock — the hardest — in thick layers in native bed. Rock equal to best ashlar masonry Rock equal to best brick masonry Rock equal to poor brick masonry Clay on thick beds, always dry Clay on thick beds, moderately dry Clay, soft Gravel and coarse sand, well cemented Sand, compact and well cemented Sand, clean, dry j Quicksand, alluvial soils, etc Bearing Power in Tons per square foot Minimum Maximum 200 25 30 15 20 5 10 4 6 2 4 1 2 8 10 4 6 2 4 0.5 1 Concrete for footings should be mixed in the proportions of 1 sack of Portland cement, 3 cubic feet of clean, coarse sand, and 5 cubic feet of gravel or broken stone varying in size from I inch up to 2 inches; if reinforced, the proportions should be 1:2:4. Enough water should be used to form a quaky mixture but not enough to cause the cement and aggregate to separate in placing. Concrete foundations and footings may be keyed by partially embedding in the footing vertical rods or horizontal I-beams; in light structures a similar effect may be produced by casting on the footing a central longitudinal projection which will form a tongued- and-grooved joint with the foundation. If the placing of the foundation is delayed until the footing has hardened, the latter should be cleaned, roughened, and wetted, and then grouted with a mixture in the proportion of 1 sack of Port- land cement to 1 cubic foot of sand, mixed to the con- sistency of thick cream. 23. Simple Foundations. — Where there is to be no cellar or basement under a building, and the nature of the ground is such that the excavation can be made for the exact width of the foundation, forms below ground line are unnecessary provided the earth is firm enough to prevent 68 PRACTICAL COURSE IN CONCRETE "caving in" of the sides. It is, however, necessary to pro- tect the edges and sides (especially on the side oppo,site that from which the concrete is poured) by burlap aprons made by tacking a piece of burlap on a piece of lumber 2 by 4 inches, long enough to rest on cross pieces bridging the excavation. When the ground line is almost reached, forms previously constructed of 1-inch boards on 2-inch by 4-inch studding must be placed to receive the concrete from ground line to the top of the foundation wall. (See Fig. 12, Assign- ment 3). No appreciable time should elapse between placing the concrete below and above ground, as an interval of more than 30 minutes will produce a line of cleavage, seriously weakening the wall and lessening its watertightness. 24. Piers and Engine Foundations. — Foundation piers for additional supports under large or heavily-loaded buildings are constructed in the same manner as simple foundations, the size being determined by the estimated load and the character of the ground. The footing is im- portant, as the sole object of such a pier is the distribution of the load. Foundations for gasoline or steam engines and for any machinery subject to considerable vibration are constructed in the same manner as foundation piers. (See Fig. 14, Assignment 3). The size and depth is determined by the amount of vibration to be withstood. The problem is simply to build in the earth, a solid block of concrete of weight sufficient to withstand the action of the engine bolted to its top. Casings for the bolts are made of 2-inch pipe, resting on plates at the lower end of the bolts; they are imbedded in the concrete and provide for any necessary adjustment of the bolts when setting the engine in place. The length of each casing equals the length of the bolt to be imbedded; by tightening the nut on each bolt above the template, the casing fits snugly against the template, and the top of the bolt is brought to proper height. The tem- plates can be made from 1-inch material, and will be sufficient for placing the casings in smaller engine foundations; for larger foundations cross bracing should be added. In setting bolts, first nail the templates securely in place, then mark accurately the position of the bolts and bore holes only slightly larger than the bolts. Be sure the bolt holes are FOUNDATIONS 69 correctly located. Bolts and casings are now set in place, centering casings with bolts by several nails or by wooden strips lightly nailed on the under side of the templates. Proportions of 1:3:5 may be used in foundations for gasoline engines and cream separators. A mixture of 1:2:4, using aggregate up to 2 inches or 2| inches is recommended for steam engines and large machinery. CHAPTER VII WALLS 25. Cellar and Basement Walls. — Wherever the exca- vation is made for a cellar under a building, the problem includes not only the construction of a wall to serve the purpose of a foundation for the superstructure but of one which will also insure a cellar warm in winter, cool in summer and dry at all seasons. Concrete walls of suitable thickness solve the problem of heat transmission and if properly built, the cellar will be always dry. Proper drainage how- ever, should be provided. While a concrete cellar wall may be constructed so impermeable that water standing outside will not penetrate to the interior, drainage Lo natural outlets is a wise precaution and should not be omitted except in soil that is dry throughout the year. When the cellar excavation (often made by team and scraper) has irregular sides and is somewhat larger than the actual dimensions of the wall, it will be necessary to use both outside and inside wall forms. Only in small excava- tions shoveled by hand and left with true sides in firm earth free from indications of caving, can the earth be used for the outer form. In using forms for both the outside and the inside of the wall, quite a large amount of lumber would be required, if forms for the entire work were constructed at one time. To obviate this, forms can be built in sections, each section being of the full height of the cellar wall, and as long as convenient to build and set in place. An entire section should be filled at one operation in order to avoid horizontal joints or hues of cleavage in the concrete. At the end of the section, a piece of 2-inch by 4-inch lumber, with both edges bevelled to permit of easy removal, is fastened to the face of the partition board used as a stop-off at the section's end (Fig. 27). This makes a tongue-and- groove vertical joint. When the forms are ready to fill for the adjoining section, the end of the partially-hardened WALLS 71 section must be cleaned, wetted, and coated with neat cement grout mixed to the consistency of thick cream. Attention is called to the preference in building practice for vertical joints in foundation and cellar walls, whereas horizontal joints are preferable in the upper part of the building. Sectional forms are better and more economically con- structed by building them flat upon the ground than by constructing them in the position in which they are to be used. Care should be exercised to build them true and to have the face as free from irregularities as possible. The sheeting for the inside of the wall should be surfaced on the side next to the concrete to give a smooth interior finish. The outer and inner form should be joined at the top by nailing cleats between the uprights, being careful to separate Fig. 27.— Method of joining foundation walls where it is necessary to leave an expansion joint, or where concreting has been discontinued for any reason. the forms the exact width of the wall. The forms should be united a short distance from the bottom by double wires and should be separated at the same place by wood spacing blocks of a length equal to the thickness of the wall. When the spacing blocks are placed, the double wires are twisted by the use of a large nail, so that the outer and inner forms are firmly fastened together. They are supported by securely bracing the inner form, so that the wall will be plumb. If desired to provide the foundation with greater resistance to lateral pressure, or to afford a firmer base, "batter" in the wall may be secured by lengthening the spacing block which separates the outer and inner forms. Anchor bolts are imbedded in the concrete at suitable intervals for fastening the wall plate to the foundation (Fig. 28). 26. Cellar Floors. — The methods of building concrete walks are fully described in another assignment. The 72 PRACTICAL COURSE IN CONCRETE methods of building cellar floors are similar. To avoid repetition, only the points of dissimilarity will be stated here. Where the ground is firm and well drained, the sub-base may be omitted and the concrete floor laid directly on the ground. Drainage should be provided, preferably toward the center of the floor. The top of the floor should be given grade enough that water accumulating from scrubbing or other causes wiU run ofT through a tile drain laid beneath the floor and communicating with a natural outlet. Where a basement floor is below the level of ground water, the floor should be laid in a single sheet instead of being Fig. 28. — Concrete basement wall under residence showing method of anchoring wall plates to wall. divided into slabs. The concrete should be mixed in the proportions of 1 :2:3 and the floor reinforced in both directions with i-inch rods 8 inches apart, or by wire mesh having an equal cross sectional area of metal. 27. Entrances. — ^Outside entrances to ceflars should be constructed by building (at right angles to the cellar wafl) forms for side walls sloping from the top of the foundation down to the ground and from the cellar floor up to the top of the proposed stairway. If excavation is carefully done, the earth may usually be used for the outer form. By pouring one sidewall at a time, and reversing the form by changing uprights to the other side of the sheeting, one form . WALLS 73 may be used for both sides of the entrance. The form for the steps may be built after the side walls are hard enough to remove the forms. After the desired measurements of tread and riser have been decided upon, the plan should be laid out on the side walls, cross pieces wedged between them and secured by bracing. The concrete used in the con- struction of the base should be as wet as possible without flowing from one step to another. The f-inch wearing course of the risers may be placed either by using a thin metal partition or by plastering the mortar on the inside of the face form before placing the coarse, wet concrete. The wearing course of treads is placed as in sidewalk work and should be finished by wooden float to a surface reasonably smooth but rough enough to afford a good foothold. 28. Window Frames. — Closer joints will be secured under cellar windows if the frames are not placed until the concrete is ready to receive them. However, the frame must be ready and quickly placed in order to avoid one of those delays which will allow concrete to set and make it necessary to clean, roughen, wet, and grout it before a fresh layer can be added without producing a horizontal line of cleavage. The position of the frame should be maintained by nailing wood strips to it or driving nails through and leaving them projecting into the concrete. 29. Finish. — Concrete for cellar walls should be of such consistency that when poured into the forms, it will settle to place by gravity. While the forms are being filled, the coarser aggregate should be spaded away from the face of the wall, bringing the mortar next to the forms. The mix- ture recommended for foundations and basement walls, 1 :3 :5, provides an excess of mortar for this purpose. Spading is equally important on the interior and the exterior. On the interior it gives a more finished surface and on the exterior it increases watertightness. On the outside of the wall above ground line, the plastic appearance which walls will have after forms are removed may be overcome by removing the surface film of mortar by brushing with a wire or a stiff fiber brush and washing the wall with the acid solution mentioned in Assignment 5 on "The Surface Finish of Concrete." 74 PRACTICAL COURSE IN CONCRETE 30. Removal of Forms. — Not only the proportions of ingredients and consistency of concrete itself, but the weather conditions have marked influence upon the time of hardening. Consequently no definite rule can be given for removal of forms. Two to three weeks will suffice under average conditions. Where the earth is utifized for the outer form more time will be required than where both forms are of lumber. Too early removal spells failure, and judgment must be exercised. 31. Block Foundation Walls. — Well-made concrete blocks are extensively used for foundation and cellar walls. For the latter purpose, they possess the advantage of an interior air space which helps to preserve an even temperature in the cellar. Care should be exercised that the blocks are well made of properly-selected and proportioned materials, mixed wet enough so that the percentage of porosity and absorption will be low. For both foundation and cellar walls, blocks must invariably be laid in cement mortar mixed in the proportion of 1 sack of Portland cement to 2 cubic feet of sand, and the joints must be thoroughly filled. For this purpose a template or mortar gage may be obtained from the manufacturers of leading block machines. 32. Walls for Superstructures. — The recent statement that the annual fire loss of American farm buildings equals one-fourth of their total cost should be sufficient argument for concrete — a material that will not burn. There are several methods of using concrete for the main portion of all classes of buildings. The most common forms of its apphcation are concrete block and monohthic walls. The concrete block is fully discussed in another assignment. Monohthic walls may be either plain or reinforced. The principal reason for reinforcing monolithic concrete walls is to prevent cracks from the expansion and contraction of the concrete, caused by changes in temperature. All walls exceeding 12 feet in height should be protected by sufficient horizontal and vertical reinforcement which should be com- puted according to the dimensions and design of the par- ticular structure. It is seldom necessary to reinforce mono- lithic walls over 8 inches in thickness, when less than 12 feet in height, except around window and door openings. One- fourth inch rods should be placed from 1 inch to 2 inches WALLS 75 back from the surface of the wall, and 2 inches from the angles of openings; three rods above and two on each side of the openings; two rods below windows — all projecting 10 inches beyond the point of intersection. Diagonal rods 2| feet long should be placed to pass intersections of horizontal and vertical rods. The monolithic concrete wall lends itself more readily than any other type of building construction to the individual taste of the builder as to variety of design. In this respect it has no limitation except that of the builder's ingenuity in the construction of forms. Forms for walls above ground must necessarily be more carefully constructed than those for cellar work, as more perfect alignment is required and better surface finish desired. In building walls above ground level, continuous forms (Fig. 17, Assignment 3) are sometimes used to avoid vertical joints. Several systems of clamps are now manufactured for con- structing forms of 2-inch plank. They generally provide for courses 24 inches in height, the same form being moved upward as soon as the last course has hardened sufficiently, thus efTecting a great saving in lumber although requiring a little more time in building. Metal forms are now obtainable and are in use by nu- merous contractors. They are serviceable and satisfactory, but too expensive for the individual builder unless he con- templates constructing a large number of buildings. Examination Questions 20. How does the cost of a well-built concrete foundation compare with that of one constructed of any other building material of equal strength and durability? 21. What is ^ footing? 22. What would be the effect upon a foundation wall if an interval of more than thirty minutes between placing concrete below and above ground were allowed to elapse? 23. When the cellar excavation has irregular sides and is larger than the dimensions of the wall, what forms should be used? 24. When may the subbase be eliminated in building a cellar floor? 25. What is the principal reason for reinforcing monolithic concrete walls? INSTRUCTION PAPERS FOR HOME STUDY INDUSTRIAL EDUCATION SERIES EXTENSION DIVISION OF THE UNIVERSITY OF WISCONSIN CORRESPONDENCE-STUDY PRACTICAL COURSE IN CONCRETE COURSE 430 ASSIGNMENT . .. Copyright, 1916. by the University Extension Division MADISON PUBUSHED BY THE UNIVERSITY FEBRUARY, 1916 UNIVERSITY EXTENSION DIVISION THE DEPARTMENT OF CORRESPONDENCE-STUD\ oilers instruction in studies required in each of the following lines, bulletins on which will be mailed free on request: IN THE COLLEGES Agricultiir*^ General basic sludie," Letters and Science Studies required for liberal arts degrees Engineering Basic and tcciinical studies Law Prerequisite liberal lirts studies IVIedicin*! 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