Official Report on Reinforced Concrete

38. Concrete for reinforced concrete structures should not be mixed or deposited at a freezing temperature, unless special precautions are taken to avoid the use of materials containing frost, and to provide means for preventing the concrete from freezing after being placed in position and until it has thoroughly hardened.

39. Placing Concrete. Concrete must be placed in the work immediately after mixing, and deposited and rammed or agitated by suitable tools in such a manner as to produce thoroughly compact concrete of maximum density. No concrete should be placed until the rein forcing steel has been placed and firmly secured by wiring or other methods to prevent displacement. Concrete should be frequently wet for several days to prevent too rapid drying out.

40. Concrete should not be placed in water, unless unavoidable. Where concrete must be placed under water, unusual care must be taken to prevent the cement from being floated away. This usually can be accom plished in still water by placing the concrete through a large pipe or tube, or, in large work, by means of a bottom-dump concrete basket. Before placing concrete, all shavings and debris of every nature must be removed, and the concrete surface thoroughly cleaned and drenched with water.

41. Forms. Forms should be substantially built and secured to prevent movement or deflection during con creting, and tight to prevent leakage of mortar.

42. Before the removal of forms, the concrete shoula be carefully inspected and its strength ascertained. Much care should be given to this portion of the work, which is fraught with danger under incompetent direction. No exact time for the removal of forms can be safely pre scribed, because of the varying character of the work, the variations in the setting of different cements, and the influence of atmospheric conditions.

43. Joints. Temperature changes and shrinkage dur ing setting necessitate joints at frequent intervals or else effective reinforcement, depending upon the range in temperature and the design of the structure.

44. In massive work, such as retaining walls, abut ments, etc., built without reinforcement, joints should be provided approximately every 30 to 50 feet throughout the length of the structure.

45. Girders should never be constructed over freshly formed columns without allowing a period of at least two hours to elapse to permit settlement in the columns. Before resuming work the top of the column should be thoroughly cleansed of foreign matter and laitance.

46. To obtain tight joints between new and old con crete, the old surface should be roughened, thoroughly cleaned of all foreign material and laitance or scum, drenched, and slushed with neat cement or with a mortar not leaner than one part Portland cement to two parts fine aggregate.

47. Joints in reinforced concrete should be avoided when possible by casting the entire structure at one opera tion. In building construction, joints may be made in the columns flush with the lower side of the girders; and joints in members of a floor system in general should be made at or near the center of the span. In all cases joints should be at right angles to their surfaces.

48. Design of Reinforced Concrete. By the use of metal reinforcement to resist the principal tensile stresses, concrete becomes available for general use in a great variety of structures and structural forms. This com bination of concrete and steel is particularly advan tageous in the beam where both compression and tension exist; it is also advantageous in the column where the main stresses are compressive, but where cross-bending may exist. The theory of design will therefore relate mainly to the analysis of beams and columns.

49. Loads. The loads or forces to be resisted con sist of : (a) The dead load, which includes the weight of the structure and fixed loads and forces.

(b) The live load, or the loads and forces which are variable. The dynamic effect of the live load will often require consideration. Any allowance for the dynamic effect is preferably taken into account by adding the desired amount to the live load or to the live-load stresses.

The working stresses hereinafter recommended are in tended to apply to the equivalent static stresses so determined.

50. In the case of high buildings, the live load on columns may be reduced in accordance with the usual practice.