Mixing by Machine
The size of a concrete mixer is designated by its rated capacity. As mentioned earlier, the capacity is expressed in terms of the volume of mixed concrete, not of dry ingredients the machine can mix in a single batch. Rated capacities run from as small as 2 cubic feet to as large as 7 cubic yards (189 cubic feet.) In the Naval Construction Forces (NCFs), the most commonly used mixer is the self-contained Model 11-S, shown in Figure 12, with a capacity of 16 cubic feet (plus a 10 percent overload.)
The production capacity of the 11-S mixer varies from 5 to 10 cubic yards per hour, depending on the efficiency of the personnel. Aggregate larger than 3 inches will damage the mixer. The mixer consists of a frame equipped with wheels and towing tongue (for easy movement), an engine, a power loader skip, mixing drum, water tank, and an auxiliary water pump. The mixer may be used as a central mixing plant.
Charging the Mixer
Concrete mixers may be charged by hand or with the mechanical skip. Before loading the mechanical skip, remove the towing tongue. Then load cement, sand, and gravel and dump them into the mixer together while the water runs into the mixing drum on the side opposite the skip. A storage tank on top of the mixer measures the mixing water into the drum a few seconds before the skip dumps. This discharge also washes down the mixer between batches. Place the coarse aggregate in the skip first, the cement next, and the sand on top to prevent excessive loss of cement as the batch enters the mixer.
Mixing Time
It takes a mixing machine with a capacity of 27 cubic feet or larger 1 1/2 minutes to mix a 1 cubic yard batch. Allow another 15 seconds for each 1/2 cubic yard or fraction thereof. Start the water into the drum for a few seconds before the skip begins to dump, so that the inside of the drum gets a washout before the batched ingredients go in. Measure the mixing period from the time all the batched ingredients are in, provided that all the water is in before one fourth of the mixing time has elapsed. The time elapsing between the introduction of the mixing water to the cement and aggregates and the placing of the concrete in the forms should not exceed 1 1/2 hours.
Discharging the Mixer
When the material is ready for discharge from the mixer, move the discharge into place to receive the concrete from the drum of the mixer. In some cases, stiff concrete has a tendency to carry up to the top of the drum and not drop down in time to be deposited on the chute. Very wet concrete may not carry up high enough to be caught by the chute. Correct this condition by adjusting the speed of the mixer. For very wet concrete, increase the speed of the drum. For stiff concrete, slow down the drum.
Cleaning and Maintaining the Mixer
The mixer should be cleaned daily when it is in continuous operation or following each period of use if it is in operation less than a day. If the outside of the mixer is kept coated with oil, the cleaning process can be speeded up. Wash the outside of the mixer with a hose, and knock off all accumulated concrete. If the blades of the mixer become worn or coated with hardened concrete, the mixing action will be less efficient. Replace badly worn blades. Do not allow hardened concrete to accumulate in the mixer drum. The mixer drum must be cleaned out whenever it is necessary to shut down for more than 1 1/2 hours. Place a volume of coarse aggregate in the drum equal to half the capacity of the mixer and allow it to revolve for about 5 minutes. Discharge the aggregate and flush out the drum with water. Do not pound the discharge chute, drum shell, or the skip to remove aggregate or hardened concrete. Concrete will readily adhere to the dents and bumps created by such pounding. For complete instructions on the operation, adjustment, and maintenance of the mixer, study the manufacturer’s manual.
All gears, chains, and rollers of mixers should be properly guarded. All moving parts should be cleaned and properly serviced to permit safe performance of the equipment. When the mixer drum is being cleaned, the switches must be open, the throttles closed, and the control mechanism locked in the OFF position. The area around the mixer must be kept clear.
Skip loader cables and brakes must be inspected frequently to prevent injuries caused by falling skips. When work under an elevated skip is unavoidable, shore up the skip to prevent it form falling in the event the brake fails or is accidentally released. The mixer operator must never lower the skip without first making sure that there is no one underneath.
Dust protection equipment must be issued to the crew engaged in handling cement, and the crew must wear the equipment when so engaged. Crewmembers should stand with their backs to the wind, whenever possible. This helps prevent cement and sand from being blown into their eyes and faces.