or enveloping material, during the process of curing, to prevent the evaporation of the water required for the hydration of the cement. 2. Cement. Portland cement is the product obtained by finely pulverizing the clinker made by calcining to incipient fusion an intimate and properly proportioned mixture of argillaceous and calcareous materials with no additions subsequent to calcination except water and calcined or uncalcined gypsum. Only Portland cement should be used in reinforced-concrete work and every shipment, or car-load, should be TESTED as required by the recommendation of the American Society for Testing Materials, see Appendix. Storage facilities should be provided on the job to PROTECT THE CEMENT from injury by moisture or by the carbon dioxide of the air. This sometimes requires the construction of a storage shed of sufficient capacity to hold from one to three or four car-loads, depending upon the size of the job, rapidity of construction, and frequency of deliveries. Cement is usually shipped in jute sacks. The comparative advantage of bulk shipment is discussed under the subject of Purchasing Materials. In frame sheds, as ordinarily constructed, cement sacks should be piled in such a way as to leave a clear floor-space of one foot or more in width around the outside walls. It is undesirable to pile the sacks more than twelve high, and for long periods a height of six or seven sacks should be the limit, as otherwise the cement on the bottom may become caked. In all cases sufficient room should be left below temporary roofs to enable proper inspection and facilitate the discovery of leaks. As cement is likely to deteriorate when subjected to LONG PERIODS OF STORAGE, under conditions obtaining upon the average job, representative samples should be taken and tests made to determine the condition of the cement when it has been held over for an extended period. Only recently in this country practical use has been made of the so-called "ALUMINA" CEMENT, sold under the name of Lumnite by one of the large cement companies. The particular advantage of this material, as compared to the standard cement, is the fact that it develops at 24 hours a compressive strength equal to that obtained with standard Portland cements at an age of 28 days. As the time of initial set is not appreciably different, this material may be mixed, conveyed, and deposited in the usual manner and offers interesting possibilities in types of work where speed is essential. The use of Alumina cement had its origin in Europe a few years before the war, and since then it has been employed in a number of structures and to a considerable extent for concrete roads. As far as known it has given complete satisfaction and the results apparently warrant its use in this country, especially for incidental work such as machine foundations, miscellaneous minor constructions, and roads where early use is essential. 3. Fine Aggregates. Fine aggregate usually consists of sand, or other inert materials having similar characteristics. As differentiated from coarse aggregate it is generally defined as of such size that practically all particles pass a No. 4 sieve (four meshes to the linear inch). The Joint Committee, 1924, recommends that 85% be required to pass a No. 4 sieve. The same specification places desirable limits of not more than 30% nor less than 10% to pass a No. 50 sieve. Between these limits the particles of a fine aggregate should be WELL GRADED FROM FINE то COARSE, HAVE CLEAN, HARD, JURIOUS AMOUNTS OF DUST, LOAM, to impose a very strict specifica- Sand, wholly or in part composed of particles of QUARTZ, is the most generally used of the fine aggregates. LIMESTONE SCREENINGS, containing crusher-dust, and granulated slag are also sometimes employed, but these materials are not, generally, to be recommended as substitutes for sand. Particularly in the case of limestone screenings, laboratory tests are apparently of little value in determining the wearing quality of the concrete when exposed to the elements. Mixtures containing a high percentage of dust may give very satisfactory com pression tests yet disintegrate under exposure. The CORRECT CHOICE and PROPER PROPORTIONMENT of the sand are two of the most important steps in the determination of a concrete-mixture. By VISUAL INSPECTION and SIEVING the general character of the material can be determined, as well as the size and grading of the particles. The amount of SILT is then gauged by the DECANTATION TEST and the existence of ORGANIC IMPURITIES by the COLORIMETRIC TEST. If the sand under consideration passes this preliminary investigation, it is then tested in MORTAR BRIQUETS, cylinders, or prisms, consisting of one part by weight of Portland cement to three parts by weight of fine aggregate mixed, cured, and broken in accordance with the method recommended by the American Society for Testing Materials and given in the Appendix. Such specimens should develop a tensile or compressive strength at ages of seven and 28 days at least equal to that of a 1 : 3 standard Ottawa sand-mortar of the same plasticity made with the same cement. The results given by the MORTAR-TESTS ARE NOT, HOWEVER, AN INVARIABLE CRITERION OF THE STRENGTH OF THE SAND FOR USE IN THE CONCRETE. This is probably due, at least in part, to the difference in quantity of mixing water required for mortar and concrete of the same workability. In any case where the strength of a mortar is to be used as a criterion of the value of a sand for concrete construction, it would seem advisable to mix the cement with the sand in the same proportions as contemplated for future use in the concrete. That is, to test the value of a sand for use in a 1:24 concrete-mixture, it is recommended that the mortar-tests be made upon a 12 mixture. On important operations it is advisable in all cases to base the final decision, in regard to the acceptance or rejection of a particular sand, or the comparative value of two sands, upon compression tests of concrete cylinders made and cured as described in the Appendix. The coarse aggregate used in the cylinders should be that intended for use upon the prospective job. 4. Coarse Aggregates. Coarse aggregate usually consists of CRUSHED STONE, GRAVEL, BLAST FURNACE SLAG, or other inert materials with similar characteristics and should have CLEAN, HARD, STRONG, DURABLE, UNCOATED PARTICLES FREE FROM INJURIOUS AMOUNTS OF SOFT, FRIABLE, THIN, ELONGATED, OR LAMINATED PIECES, ALKALI, ORGANIC OR OTHER DELETERIOUS MATTER. As differentiated from fine aggregate it is generally defined as of such size that practically all particles are refused upon a No. 4 sieve (four meshes to the linear inch). The Joint Committee, 1924, recommends that not more than 10% of a coarse aggregate pass a No. 4 sieve and not more than 5% a No. 8 sieve. The same specification gives a desirable grading for various aggregates expressed as percentages passing the different sieves. The maximum size of the coarse aggregate depends upon the class of work. In general, the largest particles should preferably be of THE MAXIMUM SIZE THAT CAN BE PROPERLY AND ECONOMICALLY PLACED in the spaces available between the reinforcements. There is ordinarily, however, a tendency to choose too large stones for work heavily reinforced, and in fact the desirability of choosing the largest usable aggregate is apparently less important than at one time considered, and certainly secondary to obtaining a well-graded and easily placed mixture. New York City requires that all aggregate for reinforced-concrete work pass through a 14-in ring, and in general this is the largest size that can be used to advantage in the structural frame of the average reinforced-concrete building. For ribbed construction, where narrow concrete beams of only 4 in, or 5 in, in width contain one, and sometimes two, reinforcing bars, an aggregate of nominal 1⁄2-in or 5%-in size is as large as can be advantageously placed. All sizes of aggregate should be well-graded from fine to coarse not less than 95% passing the sieve of nominal maximum size. Coarse aggregates are obtained by crushing and screening various rocks, among which the most widely used are TRAP ROCK, GRANITE, LIMESTONE, and some varieties of sandstone. Shale, slate, and the softer varieties of sandstone and limestone are not sufficiently hard themselves to give a strong concrete. The differences in mineralogical compositions have no apparent effect on the strength of the concrete except in cases where the materials are not completely inert. A suitable PIT-RUN GRAVEL when properly screened makes an excellent aggregate which, owing to the rounded character of its particles, is more easily placed than corresponding sizes of crushed stone. As the sand-content is usually too high, and the proportion of small to large particles is constantly changing, it is seldom advisable to use pit-run gravel. If the mixture is of uniform proportions it may be possible to make an excellent COMBINED AGGREGATE by adding the required additional amount of coarse aggregate, thus avoiding the labor of complete screening and reproportioning. There is usually considerable economy in using a combined aggregate where such is obtained at a reasonable cost. As most commercial shipments, however, are subject to variation, and also likely to segregate in transportation, constant vigilance is necessary to insure a uniform product reaching the mixer. Since a well-graded mixture takes less cement for a given strength than one which is too high in small particles, or otherwise unsatisfactorily graded, it may be cheaper to expend the necessary money for screening, if a properly graded and uniform aggregate cannot be obtained. Where there is any evidence of DUST, or OBJECTIONABLE ORGANIC MATTER, gravel should be washed before using. As in the case of fine aggregate, the best criterion of the value of a crushed stone, or gravel, which passes a visual inspection and has a satisfactory sieve analysis, is to make COMPRESSION TESTS on cylinders composed of the various materials which it is intended to use, mixed, proportioned, and cured as described in the Appendix. BLAST FURNACE SLAG, when used as a coarse aggregate in combination with sand, makes a light-weight concrete developing a high compressive strength. If the sulphur-content in the slag is not high enough to endanger the life of the reinforcement, a satisfactory and economical concrete may be produced in a location where slag is obtainable at a low price. CINDERS from anthracite coal are widely used in certain localities as a coarse aggregate both for fireproofing and for structural slabs of short-span. Cinders should be composed of HARD, WELL-BURNED, VITREOUS CLINKER, FREE FROM SULPHIDES, FINE ASHES, AND FOREIGN MATTER. The use of gashouse or locomotive cinders, or stove, or heating-furnace ashes should be prohibited. Unburned coal or coke is very harmful, as such serve to introduce sulphur into the concrete, which endangers the life of the reinforcement. An amount of unburned material in excess of 15%* should be cause for rejection even when intended for use in a comparatively rich mixture. The practice of using SOFTCOAL CINDERS in reinforced concrete is not recommended, as such usually contain an excess of sulphide which may very likely cause complete destruction of the reinforcement. 5. Water. The Joint Committee, 1924, specifies that water for concrete should be CLEAN AND FREE FROM INJURIOUS SUBSTANCES. In general, it may be said that all water suitable for drinking is also suitable for concrete, and in fact the serious pollution of sewage, or manufacturing waste, produces hardly any effect unless resulting in an appreciable concentration of some harmful substance. In an exhaustive series of experiments performed at the Lewis Institute,† where cylinders of cement-mortar were mixed with different kinds of water and tested at ages varying from three days to two and one-third years, in spite of the wide variety in the origin and type of the water used, including sea-water, alkali-water, bog-water, mine and mineral-waters, water containing sewage and industrial waste, and solutions of common salt, most of the samples gave good results in the concrete. The only ones giving concrete strength below 80% of that obtained with clear water were the following: Acid waters, lime soak from a tannery, refuse from a paint factory, a mineral-water from Canada, and water containing 5% or more of common salt. 6. Admixtures. By this term is meant "substances other than cement, aggregates, and water which are added to concrete mixtures for the purpose of imparting to the latter certain improved qualities." In this classification fall such materials as HYDRATED LIME, KAOLIN, CELITE, CALCIUM-CHLORIDE, COMMON SALT, and innumerable proprietary compounds containing, as active agents, these and various other similar substances. The use of admixtures for WATERPROOFING PURPOSES is discussed under the subject of waterproofing, and their value as ACCELERATORS and HARDENERS is treated under floor finishes. Where such substances are recommended for use in structural work, otherwise than as integral water-proofing compounds, it is generally with the object of gaining greater protection from the action of frost, either by lowering the freezing-point of the mixing water, or by accelerating the curing process. Admixtures are also sometimes used to increase the so-called WORKABILITY of the plastic concrete. In general, the use of admixtures of unknown composition should be avoided, and standard chemical compounds should be purchased rather than any of the various patented concoctions, some of which have been found to affect seriously the strength of the concrete. CALCIUM-CHLORIDE alone and in combination with other substances is probably the most widely used and most thoroughly accredited of the various admixtures. MAGNESIUM-CHLORIDE is not to be recommended as, except when used in very small percentages, a reduction in strength results. Calciumchloride being highly hygroscopic, that is, having the power of drawing moisture from the air, is often employed as a curing agent and has been accepted by the * Building Code recommended by the National Board of Fire Underwriters. †See Bulletin No. 13, Structural Materials Research Laboratory, Lewis Institute. Bulletin No. 12, Structural Materials Research Laboratory, Lewis Institute. |