collection  or  diversion  of  wash  water  from mixer  equipment. At the plant, flush a minimum of 150 to 250 gallons of water, depending on the size of the mixer, into the drum. With the flushed water in the drum, rotate the drum in the mixing direction for a few minutes, then discharge the flushed water at the maximum drum rpm. Complete the cleaning of the mixer, particularly around the  discharge  end. Never  pound  the  bottom  of  the  drum  to  loosen materials, since this procedure may cause dents and bumps in which concrete and cement can stick. During cold weather, the water tank, pump, and lines must be drained to prevent possible damage from freezing. The mixing blades inside the drum must be kept clean  and  free  of  built-up  concrete.  If  not  cleaned properly, the blades in the drum will wear down and this can result in improper mixing. If this occurs, the blades should be either changed or built up by using hardfacing procedures. If a minor repair is required on a loaded transit mixer, take the TM to the shop for a quick fix. If the downtime is going to be for more than an hour, mix in 5 pounds of sugar or concrete retarder to keep the concrete  from  setting  up  inside  the  truck. NOTE: A small amount of sugar (5 pounds) acts as a retarder; however, a large amount will act as an accelerator. If a quick fix is not possible, the concrete must be removed  as  quickly  as  possible.  Either  check  for  a hydraulic adapter, which can be hooked up to another TM to operate the drum to discharge the concrete, or remove the access hatch from the drum, roll the drum until the access hatch is facing down, and washout the concrete  mix,  if  possible. 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. 14-3