Safety

Like most construction equipment, transit mixers, when not operated safely, can injure or kill personnel and damage property. As a TM operator, keep in mind the following guidelines: l Transit mixers have a high center of gravity.

Their stability is further decreased by the weight of the load. Use extreme caution when traveling over uneven terrain.

. Always use caution and a signalman when backing on a jobsite.

l Remember to secure the discharge chute properly with the chute locked to the right side of the truck for travel.

. Make sure the mixer is stopped before making adjustments.

l Observe environmental regulations concerning disposal of waste and wash water from mixers.

. Avoid prolonged skin contact with concrete or cement.

MOBILE CONCRETE MIXER PLANT

The mobile concrete mixer plant, sometimes called a crete mobile, is a combination material transporter and mobile concrete mixing plant (fig. 14-3). In the NCF, the unit is mounted on a trailer that carries sufficient unmixed materials, such as cement, sand, coarse aggregates, water, and any chemicals required, for special mix specifications to the jobsite.

OPERATION

The trailer-mounted crete mobile carries the cement, sand, and coarse aggregates in divided bins, mounted on the unit. The cement is carried in a separate bin, located across the rear of the unit, and the sand and aggregate are carried on each side of the unit. Water is carried in a single tank, mounted in front of the aggregate bins, and is pumped to the mix auger.

Sand and aggregates are proportioned accurately by weight or volume and dropped simultaneously with a mixture of cement from the material feed system into the charging end of the mix auger/conveyor at the rear of the unit. At this point, a predetermined amount of water enters the mix auger. This action of the combined auger and paddle homogenizer mixes the ingredients and water rapidly, thoroughly, and continuously to produce a continuous flow of uniformed quality concrete.

The mixing action is a continuous process that can proceed until the aggregate bins are empty. On the other hand, mixing and delivery may be stopped at any time and then started again at the will of the operator. This permits production to be balanced to the demands of the placing and finishing crews and other job requirements.

NOTE: When cleaning the crete mobile, you must cycle the auger to remove all sand, aggregate, and cement. Excess material in the bin and auger will harden and freeze the components.

Figure 14-3.-Mobile concrete mixer plant.

SAFETY

Operators, assigned to the crete mobile, must read thoroughly and understand the technical manual before operating the plant. A few safety precautions when operating the crete mobile areas follows: l Follow all preventive maintenance procedures. l Do NOT allow any foreign matter in the cement bin.

l Do NOT allow particles larger than 1 1/2 inches in the aggregate bin.

l Do NOT allow the waterlines and flowmeters to freeze with water in them.

l Do NOT run the water pump dry.

. Do NOT continue to operate the machine if the hydraulic oil temperature exceeds 190F.

. Wash out the auger within 20 minutes of the last use.

. Never attempt to repair the machine while in operation (always turn the power source off).

l Keep your entire body clear from all moving parts.

l Never attempt to walk on top of the aggregate bin to cross from the cement bin to the water tank (use the ladder).

l Never walk or stand under the auger.

. Never climb inside the aggregate bin (use a small pole to dislodge any aggregate that has bridged).

. Never enter the cement bin while in operation (there are moving parts inside the bin).