Ongoing research into limiting concrete form face surface defects with controlled frequency vibrators (CFVs) has shown great promise. Field investigations into limiting high frequency vibration to control the amount of concrete material separation in commercial concrete applications are using the same approach to limit the surface distresses in concrete pavements. Researchers have applied some common sense and solid clinical analysis to make pavement surfaces more durable.
The development of on-site, non-destructive 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. The normal confusion in quantifying 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 vibrator applications.
Destructive Nature of Vibrator Frequency
On a paving machine, vibrators are mounted on centers no greater than 16 in. 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 Portland Cement 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.
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.
Limited Vibrator Frequencies for Commercial Applications
If it makes sense to limit vibrator frequencies to control issues with 0- to 1-in. slump pavement mixes, then applying lower frequencies to 8-in. slump in commercial applications is an elementary approach. The original vibrator frequency investigation that was used to write the ACI 309 Consolidation standards in 1970 suggested a 10,000 vpm frequency limit before the vibration separated 3.5-in. structural mixes.
It is not totally 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-in. slump and beyond. The laborer of a concrete vibrator doesn’t realize that the same vibrator motor horsepower motor and head diameter that is about to be used will vary greatly from one manufacturer to another. The same vibrator model can vary from 11,000-17,000 vpm and will change frequencies, dependent on the concrete consistency. The frustration of vibrator operators is trying to vibrate variable concrete mixes with a variable vibrating tool. After stripping the forms, concrete structures that were pumped may end up needing to be patched, chipped and patched or torn out .
Most of the time a vibrator operator is blamed for poor consolidation results, when he has no control over the tool.
Controlled Vibrator Application at Pre-Construction Trials
What look like air pockets from under-vibrated concrete to a contractor upon the stripping of forms is usually pockets of separated available water from over-vibration of pumpable concrete mixes. The effects of water movement from high frequency vibration (above 10,000 vpm) is in the initial stages but shows that available water is moved by vibrator frequency. The question is, what effect does frequency have on the concrete structure’s surfaces both in strength and in permeability.
There are several CFVs in the commercial space that are being used both in-house and in field trials. At pre-construction trial for constructability, vibrators that are controlled and range from 10,000 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 cell phone/tablet are setting compatible speeds for construction. Researchers can look at load curves through the Bluetooth feature of one manufacturer’s CFV flex shaft model 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 re-tempered concrete, or in concrete with added water and chemical reducers, alters the vibrator/mix compatibility target.
In pavements, concrete mixes, batching practices and vibration are regulated. Although perfection is not guaranteed 100% of the time, when a CFV operator knows what a vibrator delivers every time, he can begin to look elsewhere in limiting the effects of vibration on concrete form surfaces.
One thing is certain in the research that has come from preconstruction trials: an operator can’t use the same vibrator frequencies for 8-in. slump that he can on 4-in. slump concrete. More importantly, he certainly can’t solve surface face defect issues with a tool that behaves differently every time the concrete mix changes.
There are smarter and smoother ways to address form face defects.
Written by Paul Jaworski, product manager at Minnich.