ACI 304.4R-95 Placing Concrete with Belt Conveyors.
Plastic concrete is nonhomogemous material; its angle of surcharge is influenced by all of its components. Small aggregates. water, and smooth, rounded and uniform size aggregate tend to reduce the angle of surcharge. Irregular, rough aggregate. cement, and additives which make the mixture more cohesive or reduce the water requirement tend to increase thc angle of surcharge.
The angle of surcharge determines the cross section of the concrete ribbon which can be efficiently carried on the belL It is also an indication of the maximum angle of incline or decline at which concrete can be handled by a belt conveyor. “Angle of incline” and “angle of decline” refer to the angle to the horizontal formed by the load-carrying belt of the conveyer.
The many variables that influence the angle of surcharge of concrete make it difficult to predict the maximum permissible angle of incline or decline. A good role of thumb is that a concrete belt conveyor can operate with less than a 10 percent loss of transverse cross-sectional area at an angle of 20 to25degwhenequippedwithasmootbbehanduptoanan- gle of 30 to 35 deg when the belt is equipped with small straight cornigations or ribs on the load-carrying surface.’ Concrete has been successfully conveyed at greater angles of incline or decline with close control of factors which affect the angle of surcharge.
As the belt passes successively over each belt-supporting idler, the concrete on the belt is disturbed. This tends to work pieces of coarse aggregate to the surface of the concrete and to flatten the concrete ribbon. This is the primary reason that the angle of surcharge is less than the angle of repose. A proper combination of belt tension, belt speed, and idler spacing is necessary to prevent objectionable segregation (see Section 2.5.7). Belt speeds of 300 1pm (92 mlmin) to 600 1pm (183 mlmin) with 3-ft (0.9-rn) idler spacing and belt speed of 600 1pm on idlers spaced about 5 ft (1.5 m) apart have been used successfully on many projects.
2.2.3 Load cross section- The nominal cross section of the ribbon of concrete on a belt conveyor is measured in a vertical plane. All capacity calculations are based on this cross-sectional area and the belt speed. As the angle of the belt (either incline or decline) is in- creased, the ribbon of concrete on the belt becomes shallow- er. This reduction in ribbon size increases the tendency for larger pieces of coarse aggregate to break loose from the rib- bon of concrete and roll away. The size, shape, and surface characteristics of the coarse aggregate have an important ef- fect on this tendency. As the angle of incline is increased, the tendency of the concrete to flow or slide back reduces the belt capacity for a given belt speed. The maximum angle at which a given concrete can be conveyed is determined when one of these factors becomes objectionable (see Section 2.2.1).ACI 304.4R pdf download.