field, will require from two to four times as long to set as the neat-cement paste mixed with little water in the laboratory. Cement, therefore, showing an initial set at the expiration of 20 min. with the Vicat needle, will rarely begin to set on the actual work in less than hr., which gives ample time for mixing and placing the materials, and cement setting in less than 10 hr.,' will usually have hardened completely in the work in 24 or at least in 36 hr. Specifications usually stipulate that Portland cement shall show initial set in not less than 20 minutes and shall develop hard set in not less than 1 hr. nor more than 10 hr. Cement reaching initial set in less than 12 or 15 min. should never be used for any work. Tests for Fineness. The fineness of cement is important, because it affects both the strength and the soundness of the product. The fineness of cement is determined by passing it through a series of sieves of different mesh and then measuring the amount retained on each. Three sieves are commonly employed, namely, those having 50, 100, and 200 wires to the linear inch. Sieves for cement testing should never be used until they have been carefully examined and found to conform to the following standard specifications: 1. Cloth for cement sieves shall be of woven brass wire of the following diameters: No. 50, .0090 in.; No. 100, .0045 in.; and No. 200, .00235 in. 2. Mesh to count on any part of the sieve as follows: No. 50, not less than 48 nor more than 50 per lin. in.; No. 100, not less than 96 nor more than 100 per lin. in.; and No. 200, not less than 188 nor more than 200 per lin. in. 3. Cloth to be mounted squarely and to show no irregularities of spacing. The method of using the sieves in the fineness test is to weigh out 50 g. of cement on a scale sensible at least to g. and to place it on the No. 200 sieve, on which it is shaken until not more than g. passes the sieve at the end of 1 min. of shaking. The arrival of this stage of completion can be watched either by using a pan under the sieve or by shaking over a piece of paper. The residue remaining on the sieve is weighed, placed on the No. 100 sieve and the operation repeated, again weighing the residue. The amount remaining on the No. 50 sieve is then determined similarly. The process of sifting can be accelerated by placing a small quantity of coarse shot or pebbles on the sieves with the cement during the shaking. These may be separated from the cement by passing the residue with the shot through a coarse sieve, such as the No. 20. Portland cement should be ground to such a fineness that it will leave a residue of not more than 25%, by weight, on the No. 200 sieve, and not more than 8% on the No. 100 sieve. Of these two requirements, the first is the more important, because it is only that part of the cement passing the finest sieve that is active in the setting and hardening of the material. The amount remaining on the No. 100 sieve is also important, because this part is most liable to cause unsoundness in the cement, and although specifications do not call for tests with the No. 50 sieve, it is usually employed for the same reason as the No. 100 sieve. Any appreciable residue on this sieve indicates that the material is much more liable to unsoundness. Any cement failing to pass the fineness test should be watched more carefully in the soundness and strength tests, but if these tests show good results up to 28 da., the cement can, as a rule, be used safely. It must be remembered, however, that only that part passing the No. 200 sieve is really cement, so that a coarsely ground shipment is practically equivalent to one adulterated with weak and unsound material. Tests for Specific Gravity.-The object of the specific gravity test is to furnish indications of the degree of burning, the presence or absence of adulteration, and the amount of seasoning that the cement has received. When Portland cement is burned, the separate ingredients are in close contact and gradually combine by a process of diffusion. The greater the amount of this burning the more thoroughly are the elements combined. Thus, by their contraction they give, in volume, a higher density or specific gravity. Since, to secure good cement the burning must have been made within definite limits, it follows that the specific gravity must also lie within fixed limits if the cement has been properly manufactured. An The common adulterants of Portland cement, namely, limestone, natural cement, sand, slag, cinder, etc., all have specific gravities ranging from 2.6 to 2.75, while the specific gravity of Portland cement averages about 3.15. appreciable amount of adulteration, therefore, is at once indicated in the results of the test. Seasoning is indicated because the cement on standing gradually absorbs water and carbonic acid from the air. These ultimately combine with it and thus lower the specific gravity. Of the many forms of apparatus employed for the specific-gravity test, the Le Chatelier flask, shown in Fig. 2, is the one most commonly used. It is also the one adopted by the technical societies as standard. It consists of a glass flask about 30 cm. high. The lower part up to mark a contains 120 cc., and the bulb between the marks a and b contains exactly 20 cc. The neck of the flask above the mark b is graduated into cc. The funnel c inserted in the neck is to facilitate the introduction of the cement. FIG. 2 30 cm. +20 cu.cm✈ 30 The method of conducting the specific-gravity test is as follows: 64 g. of cement is carefully weighed on scales that should have a sensibility of at least .005 g. The flask is filled to the lower mark a with benzine or kerosene, which has no action on the cement, and is carefully adjusted precisely to the mark by adding the liquid a drop at a time. The funnel is then placed in the neck of the flask and the weighed cement introduced slowly through it, the last traces of the cement being brushed through with a camel's-hair brush. The funnel is then removed and the height of the benzine read from the graduations, estimating to .01 cc. The displaced volume is then 20 plus the reading, in cubic centimeters, and the specific gravity of the cement is 64 divided by that quantity. For example, suppose that the reading on the flask is .54, then the displaced volume will be 20+.54-20.54 and the specific gravity will be 64 20.543.116. The apparatus must be protected from changes in temperature while in use; even touching the flask with the fingers will change the volume of the liquid noticeably. The flask is sometimes immersed in water during the tests to prevent these changes of temperature, but this precaution is unncessary if proper care is exercised. The specific gravity of well-burned Portland cement averages about 3.15 and should not fall below 3.1. If it falls below 3.1, tests should also be made on dried and on ignited samples to ascertain whether or not this condition has been produced by reason of excessive seasoning. As a rule, low specific gravity merely indicates well-seasoned cement, and if sound and sufficiently strong, such cement is the best sort of material for use, as its durability is scarcely open to question. Tests of Natural and Slag Cements.-The methods of conducting tests of natural and slag_cements are, in all important particulars, identical with those employed for Portland cement, although the results obtained and the interpretation to be put on them are often radically different. In the testing, the only essential difference is in the amount of water required by these cements to produce normal consistency; natural cement requires from 23 to 35% and slag cement takes about 18%. or an average of 2 or 3% less than Portland. Tests of natural cement for tensile strength are also frequently made on 1-1 and 1-2 mortars, but recent practice is to test mortars of all kinds of cement in 1-3 mixtures. For these cements, moreover, the specific-gravity test has practically no significance, except in determining the uniformity with which the different brands are made. CEMENT SPECIFICATIONS A good example of a complete modern specification for Portland cement is here given. SPECIFICATIONS FOR PORTLAND CEMENT Kind. All cement shall be Portland of the best quality, dry and free from lumps. By Portland cement is meant the finely pulverized product resulting from the calcination to incipient fusion of an intimate mixture of properly proportioned argillaceous and calcareous materials to which no addition greater than 3% has been made subsequent to calcination. Packages.-Cement shall be packed in strong cloth or canvas bags, or in sound barrels lined with paper, which shall be plainly marked with the brand and the name of the manufacturer. shall contain 376 lb. net. Bags shall contain 94 lb. net and barrels Inspection. All cement must be inspected, and may be reinspected at any time. The contractor must submit the cement, and afford every facility for inspection and testing, at least 12 da. before desiring to use it. The chief engineer shall be notified at once on receipt of each shipment at the work. No cement will be inspected or allowed to be used unless delivered in suitable packages properly branded. Rejected cement must be immediately removed from the work. Protection. The cement must be protected in a suitable building having a wooden floor raised from the ground, or on a wooden platform properly protected with canvas. It shall be stored so that each shipment will be separate, and each lot shall be tagged with identifying number and date of receipt. Quality. The acceptance or rejection of a cement to be used will be based on the following requirements: Specific gravity: Not less than 3.1. Ultimate tensile strength per square inch: POUNDS 7 da. (1 da. in air, 6 da. in water). 500 28 da. (1 da. in air, 27 da. in water). 600 7 da. (1 da. in air, 6 da. in water), 1 part cement to 3 parts of 170 28 da. (1 da. in air, 27 da. in water), 1 part of cement to 3 parts of 240 Fineness: Residue on No. 100 sieve not over 8%, by weight; residue on No. 200 sieve not over 25%, by weight. Set: It shall require at least 20 min. to develop initial set, and shall develop hard set in not less than 1 hr. nor more than 10 hr. These requirements may be modified where the conditions of use make it desirable. Constancy of Volume: Pats of cement 3 in. in diameter, in. thick at center, tapering to thin edge, immersed in water after 24 hr. in moist air, shall show no signs of cracking, distortion, or disintegration. Similar pats in air shall also remain sound and hard. The cement shall pass such accelerated tests as the chief engineer may determine. Analysis: Sulphuric anhydride, SO3, not more than 1.75%; magnesia, MgO, not more than 4%. The common requirements for high-grade Portland, natural, and slag cements are given in the following table. REQUIREMENTS FOR HIGH-GRADE CEMENTS PLAIN CONCRETE DEFINITIONS AND TERMS Concrete is usually made of cement, sand, and broken stone. The cement in a plastic state, either by itself or with the sand that is generally mixed with it, is called the matrix, and the broken stone, gravel, or other material used as a filler is called the aggregate. The sand is correctly classed as a part of the aggregate, although some engineers include it with the matrix. The aggregate is used to cheapen concrete. Pure, or neat, cement, when wet with water, would in a way fulfil all the physical requirements of concrete, but it would be too expensive. In the concrete of today, hydraulic cement is used almost exclusively. For this reason, the term concrete, as commonly used, refers only to that variety. In specifying any other kind of concrete, the usual custom is to mention it by its full name, as bituminous concrete, lime concrete, etc. Such varieties, however, are of comparatively little importance.. The term concrete, besides being restricted to hydraulic-cement concrete, has another restriction: the aggregate must not be sand alone, although it may be partly sand. A mixture of hydraulic cement, sand, and water is called by the special name of mortar. Concrete is usually named from the kind of aggregate used. For example, stone concrete embodies the use of broken stone or coarse pebbles, while in cinder concrete, the aggregate consists of cinders or broken slag. The proportion of cement and sand to the broken stone depends on the spaces between the stones, which are known as voids. In all instances, there must be sufficient mortar to fill the voids entirely and to cover all surfaces of the separate stones. AGGREGATES OTHER THAN SAND The aggregates or broken stone used in concrete work should possess three qualities: (1) They should be hard and strong, so as to resist crushing and shearing or transverse stresses; (2) they should have surface texture that will permit the cement mortar to adhere to their surfaces; and (3) where the concrete is to be used for building construction, such as in reinforced-concrete work, and for fireproofing, they should possess refractory, or fire-resisting, qualities. Usually, aggregates that break in such a way as to allow the smallest spaces, or interstices, between the particles, will make the strongest concrete for construction purposes because the voids can be most economically filled with cement mortar. Size of Aggregates.-When measuring broken stone, the size of the stone is determined by the size of the ring through which it will pass. For instance, a 2-in. stone is one that will pass through a ring, or hole, that is 2 in. in diameter. The broken stone used in concrete work varies in size with the nature of the work. For foundation and mass construction, it is the custom to use broken stone of a size that will pass through a 2- or 21-in. ring. For filling the spandrels of bridges or the spaces between walls, where mere bulk is desired, broken stone of a much larger size is used. In reinforced-concrete work, the broken stone must be small, owing to the narrow spaces in the forms. For columns and wall work, stone that will pass through a 1- or 3-in. ring is suitable, while for filling beam and girder forms, where numerous reinforcing rods occur, the broken stone is sometimes so small as to pass through a 1-in. ring. The latest practice in making concrete is to use stone as it comes from the crusher, without screening it. While such stone, termed the run of crusher, contains broken stone of a size specified, it also has smaller particles of stone and such stone dust as is carried along with the broken stone from the crusher. Where the run of crusher is used, the proportion of the cement and sand must be changed, because the stone dust takes the place of some of the sand. In using run of crusher the very finest dust should be washed or screened out as it tends to coat the large pieces and to prevent the cement from adhering to them. The size of the aggregates has much to do with the quality and strength of the concrete. It can, however, be stated as a general proposition, that the larger the stones the stronger will be the concrete. This fact was well proved in a series of tests made at the Watertown Arsenal in 1898. These tests also showed that the concrete becomes heavier per cubic foot, or, in other words, more dense, the larger the stone used. All these tests were made with concrete manufactured in the proportion of 1 part of cement, 1 part of sand, and 3 parts of broken stones, or a 1-1-3 (1 to 1 to 3) mixture, as it is usually expressed. The figures on cinder concrete in the table are added simply to give a comparison of weights, for it will be noted that the cinder concrete is older than the other concretes, and therefore stronger in proportion. Aggregates that consist of stone of varying sizes are best for making concrete, owing to the fact that they pack closer. It is well, however, to screen all the fine particles, such as -in. sizes, and use them with the sand, as otherwise they will not mix properly with the cement. Selection of Aggregates.-Usually the character of the aggregates used in mixing concrete depends on the availability of the supply. Where there is much choice in the selection of the aggregates those that are hardest and break with a cubical fracture will make the best concrete, although rounded pebbles are considered by some engineers to possess great advantages. Some years ago the American Society of Civil Engineers, American Society for Testing Materials, American Railway Engineering and Maintenance of Way Association, and the Association of American Portland Cement Manufacturers appointed committees to obtain information concerning the practice in and properties of concrete and reinforced concrete and recommend formulas for design, etc. This general committee is commonly known as the Joint Committee and references will be made to its report in the following pages. These references are taken from the Proceedings of the American Society of Civil Engineers, Vol. XXXIX, No. 2, pp. 117 to 168, where the Progress Report of the Special Committee of that Society on concrete and reinforced concrete will be found. It was presented to that society by its committee on Jan. 15, 1913. The relative merit of various aggregates for concrete cannot of course be defined accurately, because in any one aggregate the quality may vary considerably. The working stresses for concrete have been discussed by the Joint Committee. This committee_recommends in its report certain tests for the ultimate strength of concrete. In the absence of such tests there are given certain values for the strength of concrete that should be obtained under certain conditions. The values given vary among other things with the kind of aggregate used. The aggregates are arranged into four groups in so far as they govern the strength of the concrete. These groups are as follows: First group, granite, trap rock. Second group, gravel, hard limestone, and hard sandstone. Third group, soft limestone and sandstone. Fourth group, cinders. The difference in quality between any two adjacent groups is not constant. Elsewhere in the report it is stated: "Cinder concrete should not be used for reinforced-concrete structures. It may be allowable in mass for very light loads or for fire-protection purposes. The cinders should be composed of hard, clean, vitreous clinker, free from sulphides, unburned coal, or ashes." PROPORTIONING OF INGREDIENTS Effect on Strength and Imperviousness. The strength of concrete depends on the strength of the cement and the thoroughness with which the cement binds together the various pieces of aggregate. The more completely the voids are filled, the more completely will the aggregate be held together. Therefore, the more solid and condensed the concrete is, the less voids it will have, and the stronger it will be. The same is true with regard to making concrete water-proof; the more dense the concrete is, the more nearly water-proof it is. A mixture of 1 part of cement, 1 parts of sand, and 3 parts of stone, which would be considered extravagantly rich for a dry place, is probably as dense a concrete, and as good for waterproofing qualities, as can be made. When a concrete is made of cement, sand, and stone, and the stone is of such a size that it will pass through a 3-in. ring, but will not pass through a 24-in. ring, the concrete is weaker and requires more cement than one made with graded stone from 3-in. down. When the stone is graded in size, the stones of smaller size fill the voids between the larger stones and thus reduce the quantity of cement and sand required. Proportioning by Weights. The ingredients for a sample batch of concrete are weighed out in known proportions and mixed to the desired consistency on a sheet of steel. They are then tamped in a piece of 10-in. pipe capped at one end. The pipe thus partly filled is weighed, and subtracting the weight of the receptacle a check is obtained. The height of the concrete in the pipe is then |