ACI 304.4R-20 Guide to Placing Concrete with Belt Conveyors

ACI 304.4R-20 Guide to Placing Concrete with Belt Conveyors. 3.2—Concrete ribbon parameters The characteristics of the ribbon of concrete on a conveyor belt are determined by the angle of surcharge of the concrete, the required minimum edge distance, and the load cross section. 3.2.1 Angle of surcharge—Each plastic concrete mixture has its own angle of repose. This is the angle that the surface of a normal, freely formed pile makes to the horizontal. The angle of repose for 2 to 6 in. (50 to 150 mm) slump concrete will usually range from 20 to 30 degrees. The angle of surcharge is the angle to the horizontal that the surface of the same concrete assumes while it is being carried on a moving (horizontal) belt conveyor. The angle of surcharge for most concrete falls in a range from 0 to 10 degrees (Conveyor Equipment Manufacturer Association 1997). A lower angle of surcharge results in a shallower ribbon of concrete. The angle of surcharge is infuenced by slump/slump fow, aggregate characteristics, and mixture proportions, meaning that the ability of the concrete to fow by its own weight (rheology) properties will also afect the angle of surcharge. Such properties include: a) Size and shape of the aggregate b) Surface texture of the coarse aggregate c) Ratio of fne aggregate to coarse aggregate (FA/CA) d) Ratio of aggregate to cementitious materials (a/cm) e) Ratio of water to cementitious materials (w/cm) f) Additives such as fy ash, slag, metakaolin, and others, which afect cohesiveness 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 stream 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. Each end-discharge concrete belt conveyor should be equipped with a belt wiper or scraper to...