Manufacturers rate the size of tilting-drum concrete mixers by the volume of the mixer drum. A 6-cubic-foot cement mixer holds 6 cubic feet of material if positioned upright and filled to the brim. The maximum working capacity of the mixer falls far short of that. Six-cubic-foot mixers can mix only 3.5 cubic feet of material or less. Mixing less than the maximum load puts less strain on the mixer's motor.
How It Works
Tilting-drum mixers rest at an angle while mixing concrete, reducing the amount of material the drum holds without spilling. A motor spins the drum slowly through a geared drive that fits to gear teeth cast into the base of the mixer drum. The motor constantly rotates the drum while operators load ingredients and discharge concrete. A lever allows tipping the drum back to pour the wet concrete or mortar into a wheelbarrow for transport to the actual project. The mixer's working limits depend on the power of the motor as well as the drum capacity at mixing position.
Calculating a mixer's capacity by the maximum load weight gives a clearer idea of how much concrete the machine can safely produce per load. The maximum load weight can vary with the model and brand of 6-cubic-foot mixers. If a 6-cubic-foot machine holds a maximum of 420 pounds, for example, that includes the weight of water added to the dry mix. Loading 420 pounds of dry mix overloads the mixer even though the machine might hold that amount without spilling. Operators calculate mixture amounts either by bag count or by shovel count, if using discrete cement, sand and gravel.
Each 80-pound bag of ready-mix concrete requires about 6 pints of water, for a total weight per bag of 86.25 pounds. Five bags of ready-mix produces 3 cubic feet of wet mix that weighs approximately 431.25 pounds. Check the mixer operation manual for maximum weight limits to be sure you're working within the safe limits of the machine. When working with discrete ingredients, 15 No. 3 shovels of sand and gravel aggregate plus three shovels of Portland cement and 2 gallons of water makes a reasonable load for a 6-cubic-foot mixer, according to Northern Industrial Tools, Inc.
Concrete mixers working within design limits can operate continuously. Each batch of concrete stays workable for about 20 minutes. Mixers produce usable wet mix in only a few minutes after loading the ingredients. In home projects, consider the time needed to place the concrete before mixing a full load. More than one wheelbarrow load mixed at a time could cause problems if owners work alone. To pour a 24-square-foot section of sidewalk 6 inches thick requires 20 80-pound bags of ready-mix, about five safe mixer loads. How quickly workers can place and finish the wet mix sets the real limit to total production.