ACI 304.4R-2020 Guide to Placing Concrete with Belt Conveyors

ACI 304.4R-2020 Guide to Placing Concrete with Belt Conveyors. 3.4—Belt discharge Plastic concrete is traveling at the same speed as the belt when it is discharged from a belt conveyor. Plastic concrete would leave the belt as a cohesive mass except that the inertia and impetus of the larger pieces of coarse aggregate tend to separate from the strewn and some mortar will cling to the belt. The energy contained in the concrete mass should be dissipated or redirected by a discharge hopper to prevent segregation. The turbulence created by this dissipation of energy produces a remixing action in properly designed hoppers. As the angle of incline or decline of a concrete belt conveyor is changed, the angle of the discharge hopper with respect to the horizontal is also changed. While most discharge hoppers function properly if slightly tilted, they may plug and delay operation. Fach end-discharge concrete belt conveyor should be equipped with a belt wiper or scraper to limit mortar loss (Panarese 1972). The wiper or scraper should be positioned so that the mortar is directed into the discharge hopper for remixing. Belt wipers depend on moisture in the concrete for lubrication and cooling. Dry belts should not be operated unless the belt wipers are removed, Properly designed discharge hoppers. chutes, drop- chutes, or “elephant trunks” will eliminate concrete segregation problems at transfer and discharge locations. Job conditions frequenily limit the size of such accessories, so numerous designs have been developed through trial and error that produce satisfactory results with the specific concrete mixture being placed, AC’l 304 and The U.S. Bureau of Reclamation (19X8) provide for a 24 in. (610 mm) minimum length drop-chute or elephant trunk. This length has been found to produce acceptable results in most cases: however, this should not preclude the use of other designs that demonstrate satisfactory performance under job conditions (6.2). 3.5—Belt conveyor design principles 3.5.1 Belt mt#eria/,v—Recenl improvements in conveyor belt carcass material and construction have greatly reduced the restrictions on capacity that were made necessary by conveyor belt limitations in the past. Substitution of stronger synthetic materials such as nylon for cotton fabric have made it possible to use higher horsepower drives and to increase capacity by deep troughing or cupping of the belt. New cover compounds have extreme resistance to abrasion from the concrete and the belt wipers. The synthetic materials have also eliminated the mildew problems that...