MOBILE CONCRETE MIXER PLANT The trailer-mounted mobile concrete mixer plant (fig. 7-2) carries 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 accurately proportioned by weight  or  volume  and  simultaniously  dropped  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. The action of this combined auger  and  paddle  homogenizer  mixes  the  ingredients and water rapidly, thoroughly, and continuously to produce   a   continuous   flow   of   uniformed   quality concrete. The material 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. Operators  assigned  to  the  “crete  mobile”  must thoroughly read and understand the technical manual before  operating  the  plant. The  following  are  the  mobile  concrete  mixer  plant cautions  and  warnings: Follow all preventivc maintenance procedures. Do  not  allow  any  foreign  matter  in  the  cement bin. Do not allow paricles larger than 1 1/2 inch in the aggregate bin. Do not allow the waterlines and flowmeters to freeze with water in them. Do not run the water pump dry. Do not continue to operate the machine if the hydraulic  oil  temperature  exceeds  190°F. Wash out the auger within 20 minutes of the last use. l l l l l l Never attempt to repair the machine while in operation  (always  turn  the  power  source off). Keep your entire body clear from all moving parts. Never attempt to walk on top of the aggregate bin to cross from the cement bin to the water tank (use the ladder). 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). ^- SLUMP TEST The slump test is used to measure the consistency of a concrete mix. The test is normally performed by an Engineering Aid (EA) and is made by using a slump cone.   The   cone   is   12   inches   high   with   an 8-inch-diameter base and a 4-inch-diameter top (fig. 7-3). A bucket is filled with a sample of the concrete from the discharge chute from the mixer. From this sample, the cone is filled in three layers. Each layer should contain approximately one third of the volume of the cone. Each layer is  rodded in the cone with 25 strokes from  a  5/8-inch-diameter  tamping  rod  24  inches  long. The strokes should be distributed uniformly over the cross section of the cone and should penetrate into the underlying layer. The bottom layer should be rodded Never attempt to operate the unit while in motion. Figure 7-3.-Measurement of slumps. 7-7