Headed For The End Of The Trail
By Bob Steffes, Materials Research Assistant For The Iowa Department Of Transportation
The beginning of the study of trails from excess vibration and over consolidation in Portland Cement Concrete (PCC) pavements began in 1994 for the Iowa Department of Transportation (IDOT).
Evidence of significant longitudinal, parallel cracks occurring in some of Iowa's Interstate highways put an investigation on the trail.
Most paving projects don't have vibrator trails, but some have trails hidden below the surface. In the case where vibrators are positioned deeper in the concrete, and over vibration occurs, a zone of excessive concrete paste will develop in the vibrator trail above a zone of excessive large aggregate. As that aggregate settles downward from over vibration, it displaces the paste upward (Figure 1). Feeling good about not seeing vibrator trails on the pavement surface may be a feeling of false security. Damage and segregation to a mix can still be occurring below the surface, out of sight, when there is over vibration. (Figure 2).
Over vibration effects can occur for a number of reasons. Some of them are:
* Excessive vibrator frequency.
* Reduced paver forward speed with constant vibrator frequency.
* Concrete mix properties having poor workability.
Evaluation of PCC Cracking
Without significant effort, it was quite clear from the longitudinal, parallel cracks at spacings similar to paver vibrators, that the vibrators were definitely one of the contributors to the cracking problem in Iowa. The next step of the evaluation was to determine in what way and why were the vibrators leaving those "trails to problems ahead," leading to premature pavement failure.
Other Possible Problem Causes
It was known that there may be other forms of PCC deterioration occurring in some Iowa highways, such as distress "D" cracking and also deterioration from aggregate-silica reaction. However, when the cracks and deterioration show up on the surface in parallel lines spaced as paver vibrators, one must accept in some way, the implication of the vibrators along with their operations and actions (Figure 3).
Concrete mix designs and materials also need to be considered when searching for reasons for PCC cracking and premature failures. Since the failures were occurring in vibrator trails and not between vibrator trails, the Iowa DOT standard interstate mix design was not considered to be the source of the problem.
Field Survey of Paver/Vibrator Operations
Initially, an informal gathering of information began in early 1994. Unannounced spot checks to observe operations were made at various PCC pavers working in Iowa. Information was obtained from visual observations, vibrator frequency checks with handheld tachometers (Figure 4), conversations with paver operators, mechanics and supervisors, as well as from state inspectors. Daily field reports were reviewed. The paver vibrator system, including the paver hydraulic system, was studied with specific attention given to individual controls and master controls for vibrator frequency. Vibrator brands, models, sizes, forces, amplitudes, quantity and positioning were also studied. The quantity of vibrators were respectively 15, 17 and 22 vibrators found on the three different pavers, all working on 26 foot (7.92 m) wide interstate projects, all using the same concrete mix.
The primary conclusion of the Field Survey/Vibrator Operations was that vibrator frequencies were often not satisfactorily known, definitely not uniformly controlled and also not clearly recorded. In many cases, frequencies were found out of the specified range. Frequencies found were as high as 12,000 vibrations per minute (VPM) and as low as a dead vibrator with paving construction going forward.
On older equipment, frequency control knob settings often no longer correlated well to actual hydraulic oil flow volume and related vibrator frequency (Figure 5). Frequencies can also change significantly even at a constant control setting as the vibrator's life begins to expire.
The summary of the Field Survey of Paver/Vibrator Operations was that a definite real time monitoring and displaying system of all vibrators' frequencies was essential as a primary step for a quality PCC paving operation. In addition, some improvements to specifications for control of vibrator positioning and spacing were also seen as necessary.
Understanding the problem
The majority of vibrator related problems found were from excessive frequency. A high frequency may make the paving operation appear better because the creamy, smooth cement and sand portion of the mix are vibrated to the top. The creamy mix creates a smooth finish but is detrimental to the concrete life of the pavement. The research results showed the DOT specifications for vibrator frequency of 5000 to 8000 VPM was appropriate. The field problem was the actual operating frequency often was not well known or was simply out of and above the specified frequency. A significant amount of time was spent in presentation of the problem and training while trying to get acceptance of the importance of frequency control. Eventually, understanding led to agreement and then acceptance and support of the following solution.
The Solution to the Problem
Contacts were established with the two largest manufacturers of paver vibrators and with paver manufacturers to discuss the development of a full-time automatic vibrator monitoring and frequency display system. That development, along with field testing of prototypes, has progressed well over the past several years (Figures 6 & 7). Three paving projects in Iowa let for the 1999 paving season and nine projects for the 2000 season were specified to include a vibrator frequency monitoring system on the paver. The Minnesota Department of Transportation is following Iowa's example. They are requiring vibrator monitoring systems on all their major project involving slipform paving this year.
The feedback from the field, so far, is that the frequency monitors are a definite plus for ease of operations, accurate control and also safety. They will contribute to a better quality finished product for the PCC pavement we travel.
Second and third generations of vibrator monitoring systems are already being developed which provide significant amounts of information and control beyond just frequency. This response from the paving industry to help solve a paving operations problem was quick and effective. With the implementation of the vibrator monitoring equipment, we are now headed for the end of the trail.
More to Come...
More research needs to be conducted to find ways to more uniformly consolidate PCC pavements and to reduce the occurrence of excessive vibration and the associated loss of entrained air. The following areas need to be researched in greater detail to determine their impact on pavement consolidation:
* Tilting of the vibrators to increase the area of influence of the vibrator.
* Optimum vibrator spacing to ensure the slab is uniformly consolidated.
* The effect of vibrator diameters and amplitudes on consolidation of PCC for slipform paving.
* The relationship between mix design and the vibratory consolidation of PCC.
Also, research is continuing in the area of consolidation effort and its effect on entrained air contents. The research project involves looking at the impact of using electronic-vibration-monitoring devices to control the entrained air content of PCC in slipform paving.
Research pays off!
This pavement is only 10 years old, but it already needs work because of deterioration caused by longitudinal cracks in vibrator trails. Understanding the over-vibration problem will help prevent this premature deterioration.
A Wyco display system hooked up to a GOMACO paver accurately and constantly measures the vibrator frequency of every vibrator on the paver.