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December 1, 2023
By Minnich Manufacturing
Precast concrete is an entirely different way of pouring concrete—one that limits the variables found in commercial/vertical applications through a controlled environment. Yet, even with in-house batching controls, material moisture still occurs that alters the mix and magnifies the surface defects caused by higher uncontrolled vibration frequency. Between the completed and pending vibration studies for new consolidation standards, the technical community has offered help in controlling vibrator results.
Throughout the years of the ACI 309 Guide for Consolidation, it has been suggested that the amount of vibration energy should be matched to the mechanical resistance (workability) of the concrete being vibrated. Those researchers had no way to control frequencies. The recent research and new ACI 309 document changes reflect the importance of a Controlled Frequency Vibration (CFV) category that is used to establish vibrator frequencies, and in turn, eliminate some of the surface issues that can occur in concrete pavements.
As the mechanical resistance of the concrete in commercial and precast has been lowered by Water Reducing Agents (WRAs), the vibrator design has not been adapted to match the change in the concrete mix. As a result, the use of vibrators with uncontrolled high frequencies causes the concrete materials to separate, especially water.
Water has a much lower density than rock and absorbs vibrator energy at a greater rate. This forces water to disperse toward the concrete form where it creates surface voids. Researchers now know that the water is being driven away from the vibrator head and looks for an escape path that shows up on the surface in the form of water voids.
Research and field trials have shown that setting a concrete vibrator speed removes vibration as a variable of pre-construction or precast pours. This allows the contractor to focus on other variables in the construction process that may lead to surface defects.
CFV is a type of vibration technology that allows the operator to select from a set of vibrator speeds. The intent is to apply a matching applied energy to the mechanical resistance of the concrete.
As an example, Minnich’s Control Speed Vibrator (CSV) utilizes CFV technology. The electric flex shaft concrete vibrator lets contractors choose between three set speeds—6,000, 8,000 and 10,500 vibrations per minute (VPM)—or use the Open VPM feature. Once the speed is chosen, the CSV maintains that speed as the concrete consolidates and the load changes.
There are many variances in the materials that are introduced during batching, and several other concrete processes that must be controlled when producing good concrete form surfaces. A concrete vibrator that puts out uncontrolled energy does not have to be part of the issue. Many construction firms test the concrete mixture during pre-construction trials by using CFV to determine the fixed frequency for the job. Trials before construction allow users to understand vibration energy and identify mixture variations from batching, transport and pumping operations.
During pre-construction, many of the variances are tested to give a smooth poured wall surface a fighting chance. It is important to approach the type and application instructions of the form release agent that is recommended by the manufacturer. Also, the material that makes up the form panels should be considered for the standards required for the concrete surfaces. These considerations have a great effect on getting the required surfaces during vibration. Improper form material, poor form surface condition, thickness and uniformity of form release can lead to surface imperfections. Non-compliance with ACI 347 recommendations for form joints, ties and shoring can also cause surface imperfections.
The usual choice of a contractor’s quality control department is the construction of several wooden boxes that are built as 24-inch cubes. During the pre-construction stage, the boxes are filled with concrete mix, consolidated at set vibrator frequencies, and then analyzed the day after for evaluation of surface issues.
The boxes are constructed, the joints remain unsealed and the box surfaces are applied with releasing agent. The trial boxes are marked by the VPM that the contractor selects. Normal trial frequencies are 6,000, 8,000 and 10,500 VPM.
Box 1 is filled with concrete mixture that will be used for the intended job. Once filled, the wooden box top is screed off to level. No form of consolidating the concrete is used.
Box 2 is filled with concrete mixture. The concrete vibrator that will be used during actual construction is inserted and vibrated at a predetermined frequency (VPM) for a total of three seconds.
Repeat steps using a different vibrator frequency for box 3.
When the concrete is poured and vibrated for three seconds, a measuring tape can be used to calculate the volume of the air entrapment that occurred during the trial. The trial boxes should be close to the same calculated volume. Beware of the amount of water that is forced through the form’s joints because that also reduces the amount of volume of concrete poured into the trial box.
Allow the boxes to cure, then strip the wooden forms. A contractor can decide which trial frequency was most effective in consolidation based on the visible surface blemishes and bug holes.
Upfront research is the key to achieving a quality finish that minimizes patch work. As a best practice, decide which vibration speed to use on a project by performing a quality control test. Use the same concrete and vibrator you plan to use on the upcoming project, along with 24-inch wooden box “forms” and differing levels of vibration. Once the concrete is cured, evaluate each form to determine which vibration frequency resulted in the best product.
Contact the vibrator manufacturer and ask if and how the vibrator speed can be controlled for pre-construction trial testing.
Ask the vibrator manufacturer to assist with setting up proper trials to reduce variability in the vibration process.
Identify vibrator frequencies that work best for the different proportions and performances of the mixtures in pre-pour trials.
Set up as many constants as possible in the concrete placement process, including repeatable vibration frequency, forming materials, releasing applications and batch uniformity.
Similar to the concrete paving process, using a bigger diameter head at lower set frequencies in the concrete is recommended.
Remember that the more workable the concrete is, the faster an uncontrolled vibrator runs.
No need to change the way you insert the vibrator, just use a lower frequency for better results.
Interested in learning more? Contact Minnich Manufacturing today.
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