When thixotropic materials like fresh concrete are vibrated, they move. This property of concrete that overcomes the friction associated with viscosity not only assists in concrete levelling but also helps drive entrapped air to the surface.
However, how much vibration is needed for how long? What is the optimal spacing between vibrators? Although we know vibration amplitude and frequency, aggregate type, particle size distribution, viscosity, water-reducing admixtures, and air-entraining agents all impact a vibrator’s radius of influence and how the concrete responds, we don’t know the extent of this influence nor whether the relationships are linear or logarithmic or something else—so we continue relying on vibration rules of thumb, such as the following:
- “3 seconds per cubic foot”
- “Watch for bubbles”
- “Listen for the sound change”
- “Vibrator every 16 inches”
To consistently avoid the all-too-often negative impacts of vibration on concrete durability (such as segregation or water accumulation at the forms), we must learn how much vibration in various concrete mixtures is enough to move entrapped—but not entrained—air and how much is too much.
A step toward this goal was reported at the August 2019 Mid-Continent Transportation Research Symposium where Dr. Yifeng Ling, Postdoc Research Associate-Concrete Pavement Technology Center (CP Tech Center), presented preliminary research seeking to understand these relationships.
The contribution of Paul Jaworski, Team Member-Minnich Manufacturing’s Research & Development, to the work of the CP Tech Center has been invaluable. People or entities interested in the vibration research goal of helping DOTs, contractors, and manufacturers better optimize concrete construction practice are encouraged to contact:
Peter Taylor, Director-CP Tech Center | E-mail: ptaylor@iastate.edu.