hydraulic  system  for  the  truck-bed  hoist  should  be frequently  inspected  to  guard  against  hydraulic  fluid leakage.  Such  leakage  on  the  roadway  surface  will prevent good bonding between the roadway and the new mat. When enough oil is spilled that the mix can absorb it, the mix can become unstable at that spot. For this reason,  leaking  trucks  should  not  be  used. Tarpaulins should be pulled over the mixture during hauling in cool weather or on long hauls to protect the mixture from excessive cooling. A cool mix forms lumps and a crust over its surface. When a tarpaulin is used, care must be taken to be sure it is securely fastened to the top of the truck bed so that cold air cannot funnel under it. During delivery, the driver must direct the truck squarely against the paver and should stop the truck a few inches from the paver before the truck tires make contact with the paver roller bar. Backing the truck against the paver can force the screed back into the mat, leaving a bump in the pavement even after the mat is rolled. The truck bed must be raised slowly. When the mix is dumped too rapidly, segregations occur, because the coarser aggregates will roll down the sides of the load. BOTTOM-DUMP  TRUCKS.—   Bottom-dump trucks can be used when a grader is spreading the mix or when a pickup device is used to feed the windrow left by the truck into the paver hopper. Two common methods for unloading bottom-dump trucks are in use. The first method involves the use of a spreader box, designed to be operated under the gates of the truck. The amount of material, placed in the windrow, is governed by the width of the spreader box opening.  The  disadvantage  of  this  method  is  that  the spreader box can restrict the amount of material to less than the required amount. The second method, which is used more often than the first, is to use chains to control the  dump  gate  opening. NOTE: Automatic devices are also available for controlling  gate  openings. Variations in the size of the windrow, deposited by the bottom-dump truck for pickup by the paver, and irregularities in the surface on which the material is to be placed will cause variations in the amount of material fed to the paver hopper. This often causes variations in the finished surface. It is, therefore, essential that the windrow,  deposited  by  the  truck  be  as  uniform  as possible. When  the  windrow  is  deficient  in  size, material can be added to it to keep the paver from starving. When the windrow contains too much mix, a short  gap  in  depositing  with  the  next  truck  will compensate  for  the  excess.  The  windrow  length  must also   be   controlled   particularly   in   cool   weather. Windrowed material will cool below spreading and compaction  temperatures  in  cool  weather,  particularly when  delay  occurs  because  of  paver  malfunction.  The limit of the windrow should be no more than one truck load ahead of the pickup machine to prevent excessive cooling of the mix in cold weather. When the loader and paver are directly coupled, vibration  of  the  pickup  device  maybe  transmitted  into  the paver, causing ripples and roughness in the mat surface. These  vibrations  generally  result  from  worn  and  defective parts  or  from  improper  mounting  or  adjustment. Truck Hitches The purpose of a truck hitch on the front of the paver hopper is to keep the truck dumping hot mix into the hopper in contact with the paver. If, during dumping, the truck and the paver separate and hot mix spills, it must be cleaned up before the paver passes over it. Two types of truck hitches are in common use. One type uses an extension that reaches under the truck and hooks onto the rear axle of the truck. The other type of hitch has retractable rollers that are attached to the truck push bar and grip the outer side of the rear wheels of the truck. The rollers revolve with the wheels while the truck dumps its load into the hopper. Pivoted Truck Push Rollers The pivoted push roller is a device, mounted on the front of the paver, that adjusts when alignment between the truck and paver is uneven. This device reduces the uneven force exerted on the paver by the misaligned truck, minimizing interference in the steering of both vehicles. ASPHALT  FINISHERS  (PAVERS) Various  makes  and  models  of  asphalt  finishers  are used  by  the  Naval  Construction  Force  (NCF).  Two types  are  shown  in  figure  16-38.  Even  though  the finishers may operate differently, their primary jobs are all the same: receiving asphalt and spreading it in a predetermined,  uniform  length,  width,  thickness,  and shape. The finisher also provides initial compaction of the mat (layer of mixture in place.) Because  asphalt  finishers  are  different,  you  must always read the operator’s manual for the unit you are operating.  It  is  also  good  to  have  a  practice  sand laydown before actually using a hot mix. This is to familiarize  yourself  and  others  with  the  machine  and also ensure that the machine is working properly. Figure 16-39  shows  a  practice  laydown,  using  just  the aggregate mix without the asphalt. 16-34