is quoted from Mr. Albert Moyer's article on this subject, to which he has given a great deal of study: "It has been known for some time that a very wet mixture of concrete is more apt to craze and show these undesirable hair cracks than a medium dry mixture of concrete. "Neat cement, or the richer mortars, are found to be much more liable to hair cracks and crazing than mortars containing a larger proportion of sand or finely crushed stone. This is particularly true in the manufacture of cement stone by the use of sand moulds in which the mixture is poured very wet. It has also been noted that, when the stone is properly seasoned by keeping the surface covered with a thick layer of very wet sand, or when the stone is immersed entirely and for some time in water, the trouble has been overcome almost entirely. "In the past this trouble has been partially overcome by brushing off the surface of the concrete or cement stone with a stiff steel brush; or by scrubbing the surface with a cement brick and wet sand or carborundum stone, thus partially removing what might be termed a neat cement face. It has been found, however, that this does not entirely overcome the trouble, the remedy proving but temporary, the cracks appearing several months afterward. The brushing or scrubbing is merely an assistance; the real remedy lies in keeping the surface thoroughly and continuously wet as long as possible. "It is desirable to have the surface of the concrete or cement stone as near the same texture as the body of the concrete. The exterior should then be kept wet by the application of wet sand, clean sawdust, hay, etc., sprinkled from time to time with water or hanging wet cloths over the perpendicular surfaces, keeping the exterior wet and the cloths wet by sprinkling, or by any other method which will accomplish this result and supply similar or same conditions as when hardened under water. By so doing not only is crazing avoided, but a stronger, tougher, and harder concrete is obtained. It is reasonable to conclude that if so treated the surface will slightly expand, but not to a greater extent than the body of the concrete which is already wet. "Hair cracks may be avoided by the addition of mineral oil to the wet mixed concrete. "Mineral oils added to wet mixed concrete and the concrete immediately remixed has the effect of emulsifying the oils. The proportion of oil used should be 10 to 15 per cent of oil to the weight of the cement. Oil weighs from 7 1/2 to 8 pounds per gallon. "This oil-mixed concrete, when hard, appears to be non-evaporative, indicating that the emulsifying oils held all the excess water in the mortar or concrete, keeping the cement particles moist until the water had been taken up in crystallization and ultimate strength reached. Thus similar conditions are supplied as apply to concrete set under water." In this chapter the readier methods that can be employed in producing artistic effects have been considered. "This humble material, so replete with possibilities, but as yet so little understood, is manifestly destined to take an important place in the construction of our buildings and must therefore strongly influence their design." Our leading architects are beginning to find in concrete a new and useful friend, and with its help will evolve a new architecture that will be full of life and character, strength and dignity and all else that goes to make up a living style." "The Artistic Treatment of Concrete," by A. O. Elzner, in Proceedings of the National Association of Cement Users, 1907. SECTION III THE MAKING OF CONCRETE CHAPTER XIII CONCRETE BUILDING BLOCKS Advantages and Disadvantages of Concrete Blocks.-Materials for Concrete Blocks.Types of Blocks.—Block Machines.—Making the Blocks.—Coloring the Blocks.— Waterproofing the Blocks.-Building Details.-Cost of Blocks.-Objections to Concrete Blocks and Remedies for Same.-Table of Concrete Block Data.Concrete Tiles, etc.—Specifications for Concrete Blocks. THE concrete products manufacturing industry has had a very phenomenal growth, and in fact, the growth has been too rapid for the good of the business, as it has caused a large volume of poor products to be placed upon the market and the disrepute into which much of the industry had fallen on account of this, has not yet been fully removed; the tendency now, however, is toward better products and with renewed confidence due to wider experience and the law of the survival of the fittest, we expect to see an accelerated increase in all lines of manufactured concrete. The use of concrete blocks as a substitute for wood, brick, and stone has become very extensive. Concrete blocks, when properly made and used, form an excellent material for building construction. They commend themselves for their cheapness when compared with brick and stone, and their greater durability when compared with wood; they also possess the advantage over the latter of being fireproof. When concrete was first applied to building construction, it was used to build monolithic walls. The idea of making a wall hollow for the sake of economy, or for prevention of moisture or frost working through the wall was a later development. At the present time practically the whole concrete-block industry aims to produce a wall made of hollow blocks, with continuous air chambers, or of blocks which, though not themselves hollow, can be laid so as to produce a hollow wall. Advantages of Concrete Blocks.-Briefly enumerated, the following advantages are claimed for concrete blocks: 1. A properly constructed concrete-block wall is as strong or stronger than a brick wall of equal thickness. 2. The hollow form results in a saving of materials over brick walls, amounting to from 20 to 50 per cent. 3. It costs less to build a concrete-block wall than one of brick. This is due to the much larger dimensions of the concrete block. 4. The hollow chambers in the concrete walls tend to prevent moisture from penetrating to the interior face of the wall; lathing can often, therefore, be dispensed with, and the plastering done directly on the wall, particularly when the blocks or the wall has received a waterproofing treatment. 5. The hollow chambers form an air cushion that prevents sudden changes of temperature, and tends to keep the building cool in summer and easily heated in winter. 6. The fireproofing qualities of concrete blocks are superior or at least equal to those of brick. 7. Pipes and wires can be run through the hollows of the blocks, resulting in a saving of space and labor and avoiding ugly appear ances. 8. Concrete blocks can be manufactured near the building site. This will save breakage, also part of the cost of transportation as compared with brick, as cement in bags requires less handling than brick. Materials for Concrete Building Blocks.-Building blocks are made of cement, sand, and water mixed in proper proportions, in which case they are properly called "mortar" blocks; or, the above materials can be combined with either broken stone, gravel, or cinders, in which cases a concrete block is produced. Sand and gravel will usually be found the cheapest and most available materials to employ. Since the space for concrete in the mould is very small, the blockbuilder is limited to the use of gravel and stone not exceeding 1/2 to 3/4 inch in size. A 1:5 mixture containing such gravel or screen ings will produce a block as strong and as durable as a 1 : 3 mixture with sand only. Cement.-Only Portland cement should be used in the manufacture of concrete blocks, as, owing to its present cheapness, nothing is gained by using substitutes. Natural and slag cements are sometimes used for blocks that are supposed to remain constantly wet, but such blocks rapidly deteriorate when dry. No cement is as fully reliable as Portland cement, and only the latter should be considered for concrete blocks. Broken Stone.-This should be small enough to pass through a 1-inch mesh screen. If there is much dust present it must be removed by means of a small mesh screen. Another way is to wash out the dust. A barrel having a wire-sieve bottom is filled with the broken stone, and water is run through the stone; the water, as it runs out, will carry with it all the dust. Gravel. If gravel is used it should be screened through a 1-inch screen. If it contains much clay or earth, they must be removed in the manner described for broken stone. The strength of the concrete will not be impaired if the quantity of clay and earth present does not exceed 3 per cent. Cinders.-Cinders are sometimes used for concrete blocks with fair results. Such blocks are inferior in strength to those made with broken stone and gravel because the cinders are very porous and are easily crushed. For these reasons they should be used only where great strength is not required; for instance, in interior walls carrying light loads. Lime is sometimes mixed with cement mortar to improve its qualities. The dry-slaked or hydrated lime is the most convenient form to use, and it is mixed in the proportion of one-quarter to onehalf of the cement employed. As lime is about as expensive as Portland cement there is no saving in its use. It will, however, cause the blocks to set more rapidly, will make them lighter in color, and the concrete will be denser and will resist better the penetration of moisture. In 14 and 15 sand mixtures at least one-third of the cement can be replaced with lime without appreciable loss of strength. Proportion of Water. The quality of concrete blocks will depend greatly upon the amount of water used. A dry mixture is necessary |