GOMACO World Index --- GOMACO World 30.3 - November 2002
By Dennis Clausen, GOMACO University Director of Training
Back the ready-mix truck or the dump truck into position in front of the paver, dump the load and move on. That may have been the way concrete was placed in front of the paver before the advent of the profilograph. But with today's smoothness specifications, placement of the concrete becomes a critical issue. No longer can we just dump it and go and expect excellent rideability.
Dumping the concrete in piles in front of the paver (Photo #1) causes the head of concrete the paver is pushing to constantly change. If the paver encounters a large amount of concrete for a short distance and then runs out of concrete and then loads up again, this varying load will make it difficult to maintain an accurate grade or a smooth slab. Equipping a paver with an auger/strike-off system will help, but it won't eliminate the problem entirely. A head of concrete in front of an auger/strike-off system that is constantly changing a large amount will still affect the loading of the paver.
When dumping concrete on grade in front of the paver, the concrete should be spread as uniformly as possible to avoid overloading the paver. If dumping from a ready-mix truck, swing the discharge chute on the truck back and forth to evenly spread the mix. If dumping on grade from dump trucks, control the dumping by limiting the amount the tailgate can open. As the truck is raised, move it forward to keep from creating large piles. When placing concrete on grade from dump trucks, some paving contractors use an end loader to spread the concrete in front of the paver (Photo #2).
When dowel baskets are preset on the grade, it is necessary to use a placer or a placer/spreader to get the concrete on the grade (Photo #3). When using a belt placer, swing the belt back and forth to maintain a uniform head of concrete in front of the paver. Using a belt placer in front of the paver has other advantages. When starting the paving operation, the belt placer can be positioned close enough to the paver to place enough concrete in front of the machine to get started. Also, if the paver is getting low on concrete, the placer can be backed up to place more material where needed. Contractors even use their belt placer to deliver concrete to their barrier machine or when doing form work.
When using a placer/spreader to place the concrete, it is important that the operator of the placer/spreader keep a close watch on the head of concrete in front of the paver (Photo #4). If the level of concrete is getting too low, the strike-off level must be raised a small amount to allow more concrete to the paver. If the head of concrete is getting too high, the level of the strike-off must be lowered so the paver is not overloaded. It is also important that the placer/spreader not get more than 25 feet (7.62 m) ahead of the paver. If the placer/spreader gets too far in front of the paver, it will be difficult to change the amount of concrete at the paver if it gets too low or too high. If the concrete level at the paver gets too low, it will be necessary to back the placer/spreader up and place additional concrete. If the level of concrete gets too high, it will be necessary to back the placer/spreader up to remove the excess material.
Equipping the paver with an auger/strike-off system will add to the ability to attain smoothness specifications, however it will not do it on its own. The paver operator must control the level of concrete in the grout box by raising or lowering the strike-off blade when required. It is normally recommended to maintain the concrete level in the grout box near the center of the vibrator mount tube (Photo #5). It's just like using a placer/spreader, if the level of concrete begins to get too low, raise the blade slightly to allow more concrete into the box. If the concrete level in the grout box is getting too high, lower the blade slightly. The blade can be raised or lowered across the entire width, on one side or the other, or in the center. Adjust the blade in small increments to avoid making rapid changes in the concrete level. Once the blade is set, it normally doesn't require a lot of adjustment if the concrete supply to the paver is maintained at a uniform level.
Proper placement of the concrete in front of the paver goes a long way towards achieving a smooth slab. If the placed concrete is leveled off, the final slab has a much better chance of meeting profilograph specs. Other factors such as concrete mix design, subgrade preparation, paver set-up and operation and finishing operations behind the paver also have an effect on smoothness. We will discuss these items at a later date.