11. It is therefore strongly recommended that all structures to be built of reinforced concrete shall be designed by experienced reinforced concrete engineers, and that the drawings to be furnished to the builder shall show clearly and in detail the sizes and dimensions of all parts of the structure, the lengths of all steel rein forcement and its exact position with respect to the con crete members, and that these drawings shall be accom panied by specifications giving the quality of the mate rials to be used, the proportions of the concrete, and the character of the workmanship.
12. It is also strongly recommended that the char acter of the supervision given to reinforced concrete structures be improved. Such supervision as is customary in the building trades is not sufficient to attain either successful or economical construction in reinforced concrete.
13. All reinforced concrete work should be under the constant direction of experienced reinforced concrete superintendents—preferably men having a knowledge of engineering.
14. Cement. Only Portland cement is suitable for reinforced concrete construction, or for any construction that will be subject to shocks or vibrations or stresses other than direct compression.
15. The cement should meet the requirement of the Standard Specifications for cement as adopted by the American Society for Testing Materials.
16. Fine Aggregates. Fine aggregate consists of sand, crushed stone, or gravel screenings passing when dry a screen having diameter holes or a screen having four meshes to the linear inch. It should be preferably of siliceous material, clean, coarse, and free from vegetable loam and other deleterious matter.
17. It is impossible to determine positively the qual ity of a sand by inspection. Frequently a sand containing a very small percentage of silt may have its grains coated with organic or other matter so as dangerously to retard the setting of the concrete; consequently the testing of the sand is fully as important as the testing of the cement.
18. Mortars composed of one part Portland cement and three parts fine aggregate by weight, when made into briquettes, should show a tensile strength of at least 70 per cent of the strength of 1 :3 mortar of the same con sistency made with the same cement and standard Ottawa sand. To avoid the removal of any coating on the grains which may affect the strength, bank sands should not be dried before being made into mortar, but should contain natural moisture. The percentage of moisture may be
determined upon a separate sample for correcting weight. From 10 to 40 per cent more water may be required in mixing bank or artificial sands than for standard Ottawa sand, to produce the same consistency.
19. The relative strength of mortars from different sands is largely affected by the size of the grains. A coarse sand gives a stronger mortar than a fine one, and generally a gradation of grains from fine to coarse is advantageous. If a sand is so fine that more than 10 per cent of the total dry weight passes a No. 100 sieve, or if more than 35 per cent of the total dry weight passes a sieve having 50 meshes per linear inch, it should be rejected or used with a large excess of cement.
20. For the purpose of comparing the quality of different sands a test of the mechanical analysis or granulometric composition is recommended, although this should not be substituted for the strength test. The percentages of the total weight passing each sieve should be recorded. For this test the following sieves are recommended :* is retained on a screen having 1/4-inch diameter holes. The particles should be clean, hard, durable, and free from all deleterious material. Aggregates containing soft, flat, or elongated particles, should be excluded from important structures. A gradation of sizes of the par ticles is generally advantageous.
23. The maximum size of the coarse aggregate should be such that it will not separate from the mortar in layiNg, and will not prevent the concrete fully surround ing the reinforcement or filling all parts of the forms. Where concrete is used in mass, the size of the coarse aggregate may be such as to pass a 3-inch ring. For reinforced concrete, a size to pass a 1-inch ring or a smaller size may be used.
24. Cinder concrete is not suitable for reinforced concrete structures, and may be safely used only in mass for very light loads or for fireproofing.
25. Where cinder concrete is permissible, the cinders used as the coarse aggregate should be composed of hard, clean, vitreous clinker, free from sulphides, unburned coal, or ashes.