Vibrator trails are streaks of high-paste-content, low-air-content concrete found in the vibrator paths of some slipformed pavements. They are caused by excessive vibration of concrete and are prone to longitudinal cracking. In 1995, a field research project was initiated in Iowa to evaluate the effect of slipform-paver vibrations on the air content of hardened concrete. The study, which involved three separate interstate paving projects, examined three variables: vibrator frequency, paver speed, and vibrator position within the concrete slab.

Based on observations of cores from the pavement test sections, researchers made the following recommendations for how paving contractors can prevent the occurrence of vibrator trails:

  • Set vibrators at 5,000 to 8,000 vpm for paver speeds exceeding 3 feet per minute; for speeds less than 3 feet per minute, use vibrator frequencies near the lower limit.
  • Space vibrators 12 to 18 inches apart; the harsher, drier mixes used in today's paving limit the vibrator radius of action, making the typical 24- to 30-inch vibrator spacing inadequate for removing entrapped air.
  • Place the vibrators near the middle of the slab to provide more uniform consolidation throughout the slab thickness, resulting in more uniform air contents.
  • Frequently check vibrator vpm with a tachometer. Also check vibrator position; don't rely on paver-control readings.