How the concrete vibrator’s design has changed since the 1960s and how the transformation has impacted the concrete industry today.
Over the last 60 years, concrete vibrators have evolved into a necessary machine for concrete jobs. Industry studies have highlighted separation issues, vibration energy, surface defects, and mix incompatibilities—leading to the development of a more predictive concrete placement experience.
Concrete vibration dates to the late 1960s when Thomas Reading, an engineer from the U.S. Army Corps of Engineers, set vibration placement recommendations through vibration tests. At the time, the normal slump of structural concrete was three to four inches, had a “peanut butter” like consistency, and placed in forms by a concrete bucket.
Reading used a larger horsepower motor to maintain a maximum vibrator speed that ran the vibrator heads for the current consistency of mixes. Reading concluded that the vibrator frequency should never exceed 10,000 vibrations per minute (vpm) due to his observation of concrete material separation. At that time, the American Concrete Institute (ACI) 309 Consolidation Guidance Specification reflected Reading’s research and limited the vibrator frequency to that maximum frequency.
Ten years later, mix designs were being transformed by a chemical additive called a water-reducing agent (WRA) to allow for a more workable concrete mix for the future of economical concrete placements by pumping instead of concrete buckets. By the end of the next several decades, the volume of pumped commercial concrete reached 80%. The increased use and type of WRAs (plasticizers) allows for more possibilities of bleeding.
With the increased bleeding in pumpable mixes, present concrete mix designs started to take on a “soup-like” consistency. While the vibrator design remained the same, manufacturers began to increase the amount of vibrator frequency. As a result of this vibrator design frequency disconnect, surface defects began appearing on the form face of freshly poured concrete surfaces. These surface voids were mistaken for entrapped air and even more vibration energy was added by contractors to try to alleviate the problem.
Through today’s research, we’ve come to understand that surface blemishes come from vibration-frequency-forced bleed water.
The available water is a product of the WRA-induced hydration delay that makes pumping easier. In these WRA mixtures, the higher the vibrator frequency, the more water is moved to the form faces.
Then, Everything Started to Change
In the late 1980s, the Federal Highway Administration (FHWA) and pavement industry representatives evaluated the long-life road design that was constructed in Europe. From the implementation of European road designs in the U.S., the pavement vibration approach had to be changed to meet new pavement mixes. By developing a controlled frequency vibration (CFV) system in coordination with an FHWA study, many vibrator design specifications were changed along with the product.
The vibrator off-center weights were increased.
Vibrator frequency was limited to 4,000 to 8,000 vpm.
Vibrator centers were lowered from 24 to 16 in.
The study used data that came from the vibration monitoring and control system to set pavement construction specifications to limit the material separations, which were causing surface bubbling and scaling, aggregate displacement, and uneven patches of surface texture. The developed standards are in use today by individual state department of transportations (DOT), airport construction, and several other infrastructure placement practices.
The History the Internal Concrete Vibrator
A CFV system showing how set speeds can be chosen via Bluetooth
Minnich Manufacturing
The Effect on Vibrator Designs from Concrete Mix Formulas
Learnings from resolving concrete paving vibration issues have been applied to commercial-use concrete that has been over-vibrated since the mid-1980s. The lack of vibration industry controls, field research, standards, or innovative designs to deal with the rising level of over-vibration was becoming an issue with the increase of form face defects. These surface defects are originated from the available water movement caused by high vibrator frequency.
While FHWA studies were conducted for roads, commercial concrete studies were not funded to evaluate the origins of having to patch freshly poured concrete.
Even without a funding source, the concrete material separations of water, aggregates and micro air by vibration frequencies in commercial construction still needed to be addressed. The industry vibrators that were used to vibrate concrete in the U.S. were brought in for evaluation as part of a study. An identical vibrator in horsepower, shaft size and head diameter was obtained from five leading manufacturers and evaluated. The study concluded that all evaluated vibrators ran higher frequencies than the original ACI specification, and all ran differently from manufacturer to manufacturer, meaning the vibrator user had a very little chance of solving vibration patching issues.
The chosen method of evaluating the issues with commercial concrete placement by vibration was to control the energy that came out of a vibrator, predict the effects of that energy, and evaluate the energy against a changing concrete variability in batching, transport, and placement.
A Vibrator Energy Workability Curve
Pavement control technologies and field study procedures have been adopted and helped achieve empirical behaviors for today’s types of commercial concrete mixtures. From these pre-construction trials, vibrator frequency was lowered from the standard 13,000 to 17,000 vpm range to a compatible range of 6,000 to 10,500 vpm range in field trials. This change reduced the phenomenon of driving available mix water of pumpable mixes to the surface of concrete forms.
The History the Internal Concrete Vibrator
A concrete form showing how 12,500 VPM caused different levels of surface voids.
Minnich Manufacturing
The History the Internal Concrete Vibrator
A concrete form showing how 6,000 VPM caused different levels of surface voids.
Minnich Manufacturing
These unwanted blemishes become visible on the form surface of freshly placed concrete and are routinely misidentified by vibrator operators as air voids from under-vibration when instead, they could be easily avoided.
Although most types of material separation can be traced to vibrator frequency, it is often difficult to understand concrete viscosity versus vibrator frequency compatibility. Now, simplified and easily digestible vibrator energy wave concepts are used in clinical analysis. These concepts will also be used in the future to help users understand the predictive behavior of both vibrator and concrete variability tendencies upon truck delivery.
The wave concept of a vibrator examines the two waves that propagate from the same vibrating head source. In vibrator wave behavior, the elevated pulsating pressure waves (p-waves) are better in the consolidation effort than the elevated shear waves (s-waves) that contribute to material separation.
Vibration research has used CFV to study forces at wave values and report performance back to the user. The CFV senses a concrete load and reports the data in the form of a curve via Bluetooth to an iOS or Android device. The load curve shows the resistance against the workability of the concrete being vibrated. A workability log can be studied to bridge the gap between predictive vibration energy versus changes due to inconsistent quality in batching, transporting and pumping. In understanding vibration wave energy behaviors, higher p-wave values can be applied for consolidation while limiting s-wave values to limit material separation.
With science-backed research, mapping the concrete vibrator’s behavior and the changes in the concrete workability curve can be understood and better managed. The adjustment to the vibration frequency or wave manipulation creates a more predictive consolidation result.
The History the Internal Concrete Vibrator
Example of a load impedance curve
Minnich Manufacturing
Concrete Control Tests
In the past two years, there has been a large increase in vibration research and funding for future investigations into the issues caused by vibrator frequency and mix incompatibilities. There are several field trials that are conducted annually to look at using the right amount of vibration energy at pre-construction trials to make the construction outcome more predictive. This is done when the contractor prepares several wooden boxes built into 24-in. cubes to test the concrete when pouring their mock-up forms prior to actual construction. The trial boxes are marked by the vibrator frequency that the contractor selects—normal trial frequencies are 10,500; 8,000; and 6,000 vpm. During the pre-construction delivery to the mock-up forms, the boxes are filled and analyzed the day after for evaluation of surface issues.
With science-backed research, mapping the concrete vibrator’s behavior and the changes in the concrete workability curve can be understood and better managed.
Researchers are working on quality control tests that can add more data for contractors on the mixture’s bleed tendency or slump variance from the original mix design. The work has helped develop an easy, accurate and repeatable workability meter to dial in on concrete variances and how they will affect concrete construction.
Using CFV and evaluating vibration curves as a source of controlling batching or transporting variances, contractors can receive help in understanding the data that’s being logged by the manufacturer. In collecting job electronic data, the CFV will report a curve that can be compared to the curve that is reported at pre-construction trials.
The variety of industry concrete vibrators all act differently against concrete loads, and to limit patching needs, vibration control and evaluation are the first steps in a predictive outcome. Vibration compatibility testing should be done in pre-construction trials and contractors should always get help from the manufacturer of the trial vibrator.
Concrete vibrators are a vital part of the workability system. Through years of research and trials, the concrete industry is addressing the issues caused by over-vibration and concrete material separation. These efforts are allowing CFVs to become a useful tool in limiting the potential patching costs once associated with the vibrator model designs from the 1960s.
