PCC Consolidation

Consolidation is the process of making the freshly placed PCC into a more uniform and compact mass by eliminating undesirable air voids and causing it to move around potential obstructions (such as reinforcing steel).  Consolidation is usually accomplished using long, slender vibration rods called vibrators.  Vibrators work by rotating an eccentric weight which causes the entire vibrator to move back and forth.  This movement excites particles within the PCC mass, causing them to move closer together and better flow around obstructions.

Vibrators

Vibrators can be defined by the amount of energy the impart to the surrounding PCC mass.  This energy transmission is defined by two processes.  First, the amount of energy generated by the vibrator is proportional to the size and speed of the rotating weight.  Usually, the size is fixed and the speed is variable.  Second, the energy transmitted from the vibrator to the surrounding PCC mass is related to paver speed (the faster the paver runs, the less time the vibrator has in a particular volume of PCC) and vibrator location within the PCC mass.  All of these factors together comprise and control the size and shape of an “influence zone” – the volume of PCC mass around a vibrator that receives its energy (see Figure 1).  This influence zone is usually conical in shape and varies in size depending on the previously mentioned factors.

 

Figure 1. Vibrator influence zone.

Proper consolidation by vibration is critical to rigid pavement performance.  In particular:

  • Too much vibration, either by allowing vibrators to operate too long in one area or by using to high a vibration rate, can result in (1) non-uniform distribution of coarse aggregate particles, (2) loss of entrained air, and (3) bleeding (water accumulation on the surface).  All of these results can greatly reduce PCC durability.
  • Too little vibration, either by not allowing vibrators enough time to operate in one area or by using to low a vibration rate, can result in (1) non-uniform distribution of coarse aggregate particles, and/or (2) large air voids within the PCC mass.  Again, either result can greatly reduce PCC durability.
  • Vibrator static head (amount of PCC above the vibrator) influences efficiency.  Higher static heads will help push coarse aggregate particles together behind the vibrator as it travels along.