"The next section of vertical reinforcement is tied to these short lengths, and they will not interfere with the setting of the concrete forms." A roof should be made of 2" X 6" rafters set at a good pitch, and covered with 1" sheeting; this in turn may be covered with galvanized iron, tin, or shingles. A hollow-wall silo is constructed in the same way, except that the forms are placed one foot apart and circular boxes used to form the air space as the concrete is placed. TABLE XXXV.-DATA FOR REINFORCED-CONCRETE SILOS. (Including 6-inch Floor.) Place vertical rods same size as horizontal, 2 1⁄2 feet apart. Concrete is also found in many other useful forms upon the farm, such as: well curbs, ice-houses, root and mushroom cellars, hen houses, green houses, flower boxes, cold frames, wind mill foundations, lawn rollers, porch steps and lattice, and chimney caps. Convenient uses for concrete in such domestic construction will occur to the builder's mind as necessity arises. Useful Hints for the Farmer.-1. Always use the best Portland Cement obtainable. 2. Store your supply of cement in a dry place until ready to use. 3. Use sand that is both clean and well-graded. A large proportion of the grains should measure from 1/32 to 1/4 of an inch in diameter. If fine sand must be used, increase the amount of cement; that is, use a richer mixture. 4. If the sand is dirty, wash it. 5. If the gravel is dirty, wash it. 6. Before using the product of a gravel bank, screen through a 1/4-inch sieve and remix, using about twice as much stone as sand. 7. Use gravel or broken stone up to 2 1/2 inches in diameter for foundations and thick walls but limit the size to 3/4 inch diameter when reinforcement is to be used. 8. Avoid the use of soft stones in the aggregate. 9. Use clean water, free from alkalis. 10. Use enough water to give the concrete the consistency of heavy cream. 11. For ordinary work use a 1: 2: 4 mix. 12. For forms, use white pine, fir, yellow pine, or spruce and green timber if possible. 13. Grease the inside of the forms with soap, linseed oil, lard, and kerosene, or petroleum. 14. Omit the greasing if the surface of the concrete is to be plastered, in which case, wet the forms just before placing the concrete. 15. Lay sheathing or form boards horizontally. Place studs 2 ft. apart for 1 in. sheathing and 5 ft. apart for 2 in. sheathing. 16. Brace the forms securely. 17. Do not drive the nails all the way home, but let the heads project so that they may easily be withdrawn. 18. Keep forms from bulging or separating by the use of bolts or wire. 19. Place concrete in forms in layers from 6 to 12 inches thick. Spade and tamp. 20. After removing the forms, concrete which is exposed to the sun should be soaked with water cach day for a couple of weeks. 21. In laying the concrete in hot or freezing weather, use the precautions outlined in Chapter VII. SECTION VI IMPORTANT MISCELLANEOUS DATA ON CONCRETE CONSTRUCTION CHAPTER XXX THE WATERPROOFING OF CONCRETE STRUCTURES The Necessity for Waterproofing.-Modern Methods of Waterproofing.—General Conditions of the Work.-Principles to be Followed.-The Membrane Method in Detail. The Integral Method in Detail.-Waterproofing by Means of Surface Coatings.-Tabular Outline of Modern Waterproofing Processes. The Necessity for Waterproofing. In many of the forms of construction work to which concrete is so admirably adapted, its use brings with it one inherent fault-a fault for which remedies have long been sought, but which, until recent years, have not been found in a practical form suited to all the varied needs of modern construction. This striking fault of concrete work is its great thirst for water, a fault which varies in its gravity according to the proportioning and mixing of materials and to the nature of the structure, it frequently being the cause of extremely serious difficulty. Of all the opposing forces which constructors have had to combat from time immemorial, none has exceeded in its power for evil the unwelcome intrusion of water, and building materials which in their nature favor such intrusion must suffer in value to the extent of their permeability or absorptive power. The fact that in practice, concrete is frequently found to be porous and permeable has been one of the leading checks in its rapid development. Volumes have been written on how the ingredients might be mixed to produce a watertight concrete, but we might as well seek to solve the problem of perpetual motion as to try to mix cement, sand, and stone so as not to absorb water. If we could examine a section of concrete under a powerful microscope, it would appear to us like an immense sieve through which fine particles of water flow with more or less freedom. We have seen water rise up through concrete walls for many feet, and it will rise until the weight of the water absorbed is equal to the capillary attracting force. As already stated in Chapter VII, if concrete is mixed rich and mixed wet, a high degree of impermeability can be secured. Mixing rich imposes greater barriers to the passage of water; mixing wet minimizes the formation of blowholes by displacing much of the extrained air, but neither mixing rich nor mixing wet destroys the "capillary positive" property of the concrete mass. Its absorptive capacity has been largely decreased, but its attraction for moisture has, however, not been eliminated; thus the water-tightness secured by rich and wet mixtures, however theoretically correct the proportions might be, is one of degree only, a degree sometimes approaching ideal but never reaching it. We cannot expect that a mixture made of cement and stone, each of which is in itself "capillary positive," or water-attracting, can become absolutely proof against the absorption of water by the mere act of mixing, unless, indeed, the operation had produced some phenomenal change in the very nature of the constituent materials. By care and diligence, a mixture may be produced which is sufficiently close-grained to prevent the free transmission of water, prevent it sufficiently, in fact, to be all that is required in many forms of construction work. But where water absorption, besides water penetration, is to be absolutely prevented, no degree of mixing, no richness of mixture, will altogether answer the purpose; and yet in many of the forms in which concrete enters our modern buildings, it is resistance to water absorption that is required. Not merely water-tightness in the ordinary sense of the word, but resistance to the ceaseless endeavors of atmospheric moisture to find its way by capillarity through porous bodies. Some counteracting influence to this tendency of ordinary concrete to take up water by capillarity, is, therefore what is required when dampness is to be eliminated. It is true that concrete exposed to the free passage of water becomes after a time so clogged up by fine silt present in the water that the permeability is greatly reduced; and Hagloch states that concrete-block buildings exposed to the weather become watertight in from three to twelve years, a fact which we must likewise ascribe to the clogging of the surface of the blocks by atmospheric dust deposited by rain, and which remains after evaporation. Modern engineering or architectural practice should certainly not sanction a practice of waiting for the erratic and uncertain hand of time where it is essential to secure water-tightness and dampproofness in concrete structures, and in the meantime to incur the annoying consequences that always accompany damp and leaky structures; and yet this is precisely what is being done in numberless instances by those who refuse to realize the importance of watertightness in concrete work, or while realizing it, are willing through motives of false economy, to gamble with the future-nearly always at their loss. The number of mistakes made by inadequate provision for waterproofing, and their costly consequences, running into thousands of dollars, should serve as object-lessons to those who have the design of concrete work in hand and the same degree of attention and study should be given the subject of water-tightness as that given to other details of construction. The importance of the subject and the scarcity of literature concerning it has induced the author to cover the subject in greater detail than would otherwise be necessary.* Method of Conducting the Work.-Work Under Contract.Waterproofing work should be done, if possible, under contract by a specially skilled waterproofer, or by the concern making or supplying the material. In a large proportion of cases, the actual construction is left largely to a contractor, sometimes under a more or less loose guarantee; often under no guarantee at all, and frequently without the least supervision being exercised on the part of the owner. In case of trouble after the completion of the work, the owner may consider himself fortunate if he happens to have a guarantee from a responsible contractor who values his reputation for good work as much as he does the cost of remedying the trouble. It is usually not a difficult * Much of this chapter has already appeared under authorship of Myron H. Lewis in Cement Era for 1909-1910, at whose special request the material was prepared and is here rearranged with their permission. |