Ongoing research into limiting concrete form face surface defects with controlled frequency vibration (CFV) technology, which has been around since the mid-1990s, is showing great promise. With modern tweaks and an age-old approach, field investigations into limiting high-frequency vibration to control the amount of concrete material separation in commercial applications are successfully limiting the surface distresses in concrete pavements.
Through the application of common sense and solid clinical analysis, researchers have made pavement surfaces more durable.Because of these advancements, the development of on-site, nondestructive tests for concrete pavement mixes is being increasingly adopted by state departments of transportation (DOTs) as a viable prediction of the compatibility between vibration variability and concrete mixture variabilities.
As a result, the usual confusion that takes place when attempting to quantify vibration’s effects has been identified and controlled by applying paving vibrator behavior constants coupled with data collected from CFV systems to apply to vibrator frequencies in commercial concrete vibrator applications.
Destructive Nature of Vibrator Frequency
On a paving machine, vibrators are mounted on centers no greater than 16 inches to ensure enough gravitational forces (g-forces) for proper consolidation away from the vibrator head. The amplitude of the vibrator remains close to a constant value, regardless of vibrator speed, and the off-center weights produced by different vibrator manufacturers are closely comparable. Vibrator frequency is isolated as a variable, and its effects are studied both clinically and empirically.
The results of studies from state DOTs, federal highway reports and the Portland Cement Association’s technical bulletins identify elevated vibrator frequencies as the culprit in reducing necessary air entrainment and causing aggregate separation. Researchers at the National Concrete Pavement Technology Center at Iowa State University and the Bert Cooper Engineering Laboratory at Oklahoma State University have developed testing methods that use CFVs in the evaluation of mix/vibrator compatibility.
Limited Vibrator Frequencies for Commercial Applications
For the past 10 years, specifiers and agencies have adopted reduced vibrator frequency ranges of 5,000 vibrations per minute (vpm) to 8,000 vpm for concrete pavements as a standard practice. If it makes sense to limit vibrator frequencies to control issues with 0- to 1-inch slump pavement mixes, then applying lower frequencies to 8-inch slump pavement mixes in commercial applications is an elementary approach.
The original vibrator frequency investigation that was used to write the American Concrete Institute (ACI) 309 Consolidation standards in 1970 suggested a 10,000 vpm frequency limit before the vibration separated 3.5-inch structural mixes. It is not entirely clear how the ACI guidance standards strayed from vibrator frequency control as the use of water reducers for pumpable mixes took slump values to 6 inches and beyond.
The laborer using a concrete vibrator usually may not realize that vibrator motor horsepower and head diameter vary greatly from one manufacturer to another. The same vibrator model can vary from 11,000 vpm to 17,000 vpm and will change frequencies dependent on the concrete consistency, frustrating operators who are trying to vibrate variable concrete mixes with a variable vibrating tool. Adding insult to injury, after stripping the forms, concrete structures that were pumped may end up needing to be patched, chipped and patched, or torn out. And, most of the time, a vibrator operator is blamed for poor consolidation results, when they have no control over the tool.
Controlled Vibrator Application at Pre-construction Trials
Upon the stripping of forms, what may look to a contractor like air pockets from under-vibrated concrete are usually pockets of separated available water from over vibration of pumpable concrete mixes. Studies on the effects of water movement from high frequency vibration (above 10,000 vpm) are in the initial stages but show that available water is moved by vibrator frequency. The question is what effect frequency has on the concrete structure’s surfaces, both in strength and in permeability.
There are several CFVs in the commercial space that are being used for both in-house and field trials. At preconstruction trials for constructability, vibrators that are controlled and range from 10,000 vpm down to 6,000 vpm are being used to find better vibrator compatibility. CFVs that can be manually set at a controlled frequency or operated by a cellphone or tablet are setting compatible speeds for construction. Researchers can look at load curves through the Bluetooth feature of one manufacturer’s CFV flex shaft models and evaluate the concrete mix.
Much like in concrete paving, because the dynamics of the CFV are controlled, there are inconsistencies in batching behaviors. In early trials, a significant distinction between batch uniformity and vibration has been observed, and the amount of water in retempered concrete, or in concrete with added water and chemical reducers, alters the vibrator/mix compatibility target.
In pavement concrete mixes, batching practices and vibration are regulated. Although perfection is not guaranteed 100% of the time, when a CFV operator knows what the vibrator delivers every time, they can begin to look elsewhere in attempts to limit the effects of vibration on concrete form surfaces.
One thing is certain in the research that has come from preconstruction trials: Vibrator operators can’t use the same vibrator frequencies for 8-inch slump pavement mixes that they can on those that are 4 inches. More importantly, they certainly can’t solve surface defect issues with a tool that behaves differently every time the concrete mix changes. With consideration of these findings, operators have the chance to gain better control and better outcomes.