towers. A chute is supported by the cable. The other type, see Fig. 24, consists of a SUSPENSION-BOOM hung from the hoisting tower and supporting the chute, often with the assistance of tripods placed upon the floor. Combinations of the two types are also sometimes employed, as the suspended-boom design has a maximum economical radius of about 100 ft. In both cases the concrete is discharged directly from the mixer into a bucket at the foot of the hoisting-tower, conveyed to a RECEIVING-HOPPER at the top, or at any intermediate point, and distributed by means of INCLINED TROUGHS.

These latter

are usually of about No. 14 gauge steel, connected by bolted flanges, or by swivel joints, the last section delivering directly into the forms, or into a charging hopper, from which buggies are filled for further distribution. The MINIMUM INCLINATION for the troughs is given by the Joint Committee, 1924, as 27° to the horizontal, that is, one vertical to two horizontal. Above this limit the slope may be varied as required by conditions but should be such as to insure, in conjunction with a correctly designed plant, a PRACTICALLY CONTINUOUS FLOW. Neither should the concrete be permitted to fall any great distance from the end of the chute into forms, or hopper, or other practices allowed

which result in the separation of the ingredients. Both before and after each day's run the troughs should be thoroughly FLUSHED WITH WATER which should be discharged outside of the forms.

As a general thing the PLANT-COST is considerably higher for the chuting system of distribution than when wheeling is employed. It must also be remembered that the plant-cost increases with the increase in distance between the hoist and point of deposit, as it is necessary to make a proportionate increase in the height of the tower for the greater distance which the material has to be chuted. As the attendance at the mixer is the same for any given output of concrete, a comparison must be made between this increased cost of plant and the expense of wheeled transportation which it supplants. The SHAPE OF THE

BUILDING also has a very important bearing upon the choice of means for distributing the concrete. Obviously, one in which all points of deposit fall within about a hundred-foot radius of a centrally located plant is well adapted to distribution by chutes of the suspended-boom type, while a long, narrow building requires two distributing plants of the suspended-boom type, one adjacent to either end, or a continuous-line installation suspended between two towers. The latter is often combined with a rehoist tower and a boom distribution on one or both of the towers. In any case, a CHUTE-SYSTEM is not economical unless a CONSIDERABLE YARDAGE is involved. At least one authority states that, in general, a gravity plant (distribution by chutes) is not economical for work involving less than 2 000 cu yd. This is probably a very fair estimate, but each job presents an individual problem in which a decision can be formed

only after investigation of probable costs. Figs. 25 and 26 show two typical installations where chutes were employed, the first for the foundations of the Grant Park Stadium and the second for the general construction of a large Cincinnati warehouse. HAND-WHEELING, usually by buggies, see Fig. 27, will then be the probable choice for small and medium-size jobs, or where the plan of the building or local conditions make a chute-installation either awkward or excessively expensive. However, hand-wheeling is ordinarily economical for only short distances, usually limited to 200 ft parallel to the longer axis of the building, and 150 ft across the building. Beyond these limits additional plants, or other means of distribution, should be installed.

To effect EFFICIENT DISTRIBUTION by hand-wheeling the run-ways must be

laid out for a continuous and uninterrupted handling of the concrete. These are preferably made up in sections about 10 ft long and of sufficient width to permit FREE WHEELING without any danger of running off. They should, in all cases, be substantially supported on LOW TRESTLES, OR HORSES, a sufficient distance above the floor to be entirely free of the reinforcement. Occasionally, for long hauls, it is desirable to give an incline in favor of the loaded carts, but this is not usually done on the short hauls typical of building construction. Two different methods are employed in hoisting the concrete intended for wheeled distribution. It can either be discharged from the mixer directly into the buggy, in which case the latter is raised upon the hoist and wheeled out upon the floor, or the buggy can be filled from a hopper, placed at the proper level above the floor that is being poured, and supplied, as in the case of gravity

distribution, by the hoist-bucket. STEEL TOWERS, which are generally desirable in gravity-systems owing to the height requirement, are ordinarily less appropriate for systems of wheeled distribution than those built of wood.

15. Depositing. Before depositing concrete all debris must be removed from the forms. Small chips of wood and shavings invariably accumulate beneath the steel in the bottoms of beam-troughs and at the base of columnforms. CLEAN-OUTS should always be left at the bases of the latter for final cleaning and inspection prior to pouring the concrete. Except in freezing weather wooden forms are often wet down immediately ahead of the concrete. If this is not done the forms should be oiled. This operation, however, must be carried on before the steel is in place to avoid oiling the latter, thereby decreasing the bond between reinforcement and concrete. As previously mentioned, concrete should not be permitted to fall when being deposited, as this may cause a separation of the ingredients. The deposition should also be carried on so as constantly to maintain, as far as possible, a HORIZONTAL SURFACE. Over large areas, such as floors, the plastic material should be placed in layers, gradually working up to the desired height, fixed by gauges arranged as may be required by the character of the work. It is not good practice to accumulate masses of concrete at the base of the chute and then accomplish the spreading by means of shovels. In general, it should be deposited as near as practicable in its final position, avoiding REHANDLING. Where large masses of concrete of considerable depth are deposited at one time, LAITANCE, a whitish, chalk-like substance of very little strength, invariably forms upon the upper surface. This should be removed by chipping before casting contiguous sections.

When pouring THIN WALLS AND COLUMNS great care should be exercised to cause the plastic concrete to surround the reinforcement properly. A moderately stiff mixture, or one containing comparatively large particles, will often be impeded by contact with the steel, which may result in a serious amount of honeycomb. It is also necessary to prevent the accumulation of HARDENED CONCRETE on the insides of forms or upon the steel. Spiraled columns are poured from the center rather than attempting to place concrete outside of the spiral reinforcement. During the operation of pouring continuous tamping is absolutely necessary to insure solid concrete without air-holes. For both columns and thin walls a 5%-in reinforcing bar makes a very good tamper. For difficult work the pounding of forms by means of an AIR-HAMMER is of considerable value in agitating the concrete and avoiding air-pockets. Except when COLD WEATHER makes a different procedure imperative, it is desirable to pour the columns and walls up to the bottoms of beams or, in girderless construction, up to the bottoms of drops of floor-slabs the day before the remainder of the floor-system is cast.* The Joint Committee, 1924, requires a minimum of two hours to elapse after depositing concrete in the columns, or walls, before depositing in beams, girders, or slabs. If this is impracticable very thorough rodding

*The Joint Committee, 1924, requires column-capitals to be poured at the same time as the floor-construction, but as the concrete mixture used for the entire column, both shaft and capital, is often different from that of the floor-construction, the procedure given in the text is followed by most contractors.