The Path to a Specification Update: Oscillatory Rollers for HMA Bridge Deck Paving

A Western/Pacific Smooth Ride Sentinel 1st time exclusive!
by David Gent, P.E., Executive Director
Washington Asphalt Pavement Association (WAPA)

The Washington State Department of Transportation (WSDOT) has long used relatively thin (typically less than two-inch depth) hot mix asphalt (HMA) overlays, often in combination with waterproofing membranes, to protect concrete bridge decks from surface wear and moisture damage.

For the HMA to provide an impervious “seal” and a good wearing course service life, the pavement must be well compacted (typically at least 92% of maximum theoretical density).  The combination of thin paving depths and ambient surface temperature conditions on bridge decks makes achieving the targeted densities challenging, even with favorable conditions.  Add to this challenge the fact that WSDOT currently restricts the use of traditional vibratory rollers when compacting HMA on bridge decks.  WSDOT’s current standard specifications now read:

On bridge decks and on the five feet of roadway approach immediately adjacent to the end of the bridge/ back of pavement seat, operate rollers in static mode only.”  (WSDOT standard specification 5-04.3(10)A, final sentence).

This “static mode only” restriction is due to the fact that traditional vibratory rollers apply large vertical impact forces that can be transferred into the bridge deck and the bridge structure, potentially causing structural damage.  The concern is amplified by the fact that many of the bridges that incorporate an HMA wearing course are aging structures, increasing the potential damage from vertical impact vibrations.

The combination of challenging factors encountered when paving a bridge deck has led to paving contractors necessarily using extra tools, time and resources in their efforts to achieve compaction without access to their predominant HMA compaction tool, the traditional vibratory roller.  Most paving contractors find it necessary to add rollers to achieve more coverage in the static mode or to add a pneumatic roller that they would otherwise not typically use in their roller train.  Because bridge decks are generally a small portion of a larger paving project, the inefficiency of the altered rolling train adds to the overall cost of the project.

In an attempt to add a new tool to the paving contractor’s HMA roller train, the Washington Asphalt Pavement Association (WAPA) began to engage with WSDOT to study the possibility allowing the use of oscillatory rollers on bridge structures.

Oscillatory rollers impart compactive energy with a different dynamic.  The roller drum oscillates “back and forth” rather than vibrating “up and down”, transferring forces largely along the horizontal plane (instead of vertically).  The resulting compactive energy is much “softer” and more localized than is typically experienced with a traditional vibratory roller.  Both the vertical dynamic force and the surface area influenced by the oscillation dynamic is greatly reduced.  While the combination of vibratory and oscillatory compaction is generally ideal for maximizing compaction efficiency (most oscillatory rollers incorporate both oscillatory and traditional vibratory modes), good success can be obtained using the roller’s oscillatory mode alone.

WAPA conversations with WSDOT began in earnest in mid-2016 when WSDOT committed to updating its HMA paving on bridge decks construction standards in an attempt to increase the service life on bridge deck HMA overlays (see WSDOT Construction Bulletin #2016-05).  WAPA and WSDOT representatives, from both the Construction and Bridge/ Structures divisions, began a dialog on what was needed to achieve extended service life for bridge deck HMA overlays given the restrictions on the use of vibratory rollers.

The most obvious need was for better compaction options and oscillatory rolling was quickly identified as the most likely readily available technology/ solution, but WSDOT needed solid to consider any change in its “static rolling only” bridge paving specification.

As an “action item” resulting from the initial conversations with WSDOT, WAPA reached out to roller manufacturers and its sister State Asphalt Pavement Associations (SAPAs – FYI: there are 39 SAPAs in the U.S.) in an attempt to collect compelling evidence regarding the appropriate use of oscillatory rollers on bridge decks.

The roller manufacturers were asked to provide documentary evidence attesting to the appropriateness of using oscillation on bridge decks and the SAPAs were asked to provide the state DOT specifications from their state allowing for the use of oscillatory rollers and, if possible, any documentary evidence of how the specification was developed.

It quickly became apparent that, while oscillatory rolling has been widely allowed on several high-profile bridge paving projects, neither the roller manufacturers nor the SAPAs in the states that already allow oscillatory roller use on bridges had readily attainable documentation of the oscillatory roller’s dynamic impact on bridge structures.  Because of this lack of supporting data, it fell to WSDOT and WAPA to cooperate to independently develop baseline evidence that could be used to validate the acceptability of oscillatory rollers for paving bridge decks.

The first step was in this process was to demonstrate the vibration dynamics of an oscillatory roller and to do so in a safe (non-bridge deck) paving application.

With the knowledge that WAPA and WSDOT were anxious to develop baseline data to support oscillatory roller use, Lakeside Industries (a WAPA Regular Member paving contractor) expedited a demonstration project on August 2, 2018 in Lacey WA, using a HAMM HD⁺70i roller in an “on-grade” setting (i.e. paving a new parking lot).  The HAMM roller was supplied by Modern Machinery (a WAPA Associate Member equipment dealer).

Additionally, Dave Bell of Lakeside Industries commissioned a vibration monitoring firm to record various roller modes/ types and to thereby document the wide variance in roller dynamics.  The vibration monitoring eventually recorded pre-rolling, static (steel wheel roller), traditional vibratory (steel wheel roller), oscillatory vibration (steel wheel roller), static pneumatic rolling and vibratory pneumatic modes.   In a show of support for this effort, WSDOT sent four representatives to observe and discuss the demonstration with WAPA and Lakeside.

The field demonstration quickly illustrated the vastly different energy signatures of the various rolling modes.  The WSDOT team could easily validate from simple observations “through the soles of their boots” that the HAMM roller in oscillatory mode (and, incidentally, the vibratory pneumatic roller) had a considerably less aggressive vibration signature than the same roller in traditional vibratory mode.  It was also noted how quickly the oscillatory vibrations dissipated from the source, becoming nearly imperceptible within 10 feet of roller.

After observing the on-grade demonstration, WSDOT’s project lead for this effort, Anthony Mizumori, P.E., S.E. stated “…we (WSDOT) think there is definite potential to use this type of equipment on some of our bridge paving projects.”  After subsequently reviewing the vibration monitoring data, Mr. Mizumori authorized a “next step” demonstration project for compacting with an oscillatory roller on a WSDOT bridge paving project.  Integral to this effort was performing additional vibration monitoring during construction to evaluate the bridge structure’s reaction to the oscillatory rolling mode.

The end of the traditional paving season in Washington state was quickly approaching so WSDOT’s leads officials for this effort (Mr. Mizumori and Bob Dyer, P.E., one of WSDOT’s Assistant State Construction Engineers) worked with WAPA to quickly identify a bridge project that was still scheduled to be paved before season’s end.  In an admirable display of “time is of the essence” cooperation, WSDOT rapidly identified an ideal candidate, Bridge No. 82/102S (Selah Creek Bridge) near Selah WA.

Not only was the Selah Creek Bridge scheduled to be paved in mid-September 2018, but the approved paving roller train already included a HAMM 120 HD roller, which is one of HAMM’s oscillatory models.  Columbia Paving (also a WAPA Regular Member) was very cooperative in working with the WSDOT team to execute the proposed demonstration project.  WAPA, Columbia and WSDOT representatives met on the evening of September 14, 2018 to observe oscillatory rolling on a WSDOT bridge.  The original vibration monitoring company, Pacific Geo Engineering (PGE), was contracted by WAPA to provide vibration measurement and a detailed report.

Mr. Mizumori personally traveled the 350-mile (round trip) to oversee strategic placement of the monitoring equipment and to supervise the data collection that would be critical in WSDOT’s evaluation.  The monitoring equipment was located mid-way between the first two structural piers on the bridge, a location chosen to magnify any possible structural resonance to the oscillatory vibration.

Knowing full well that the oscillatory vibrations were highly unlikely to cause any concerns (due to the on-grade rolling demonstration results), the bride deck paving demonstration and data collection progressed as planned and WAPA forwarded the vibration monitoring data to WSDOT as soon the PGE report was received.

There were many encouraging cooperative engagements in the process of evaluating oscillatory rolling for WSDOT, but the most notable result was the personal dedication of Mr. Mizumori to produce a detailed report that serves as the basis of WSDOT Bridge/ Structure’s decision to update the standard specification to generally allow the use of oscillatory rollers.

Mr. Mizumori authored a 69-page report titled Oscillatory Drum Rollers for HMA Paving on Bridge Decks that now likely stands as the definitive publication on this subject.  The report chronicles the process summarized above and includes all of the relevant data that Mr. Mizumori’s team considered in evaluating oscillatory roller vibration.  As a result of his efforts, the WSDOT specification will be updated on April 1, 2019 to read:

On bridge decks and on roadway approaches within five feet of a bridge/back of pavement seat, rollers shall not be operated in a vibratory mode, defined as a mode in which the drum vibrates vertically. However, unless otherwise noted on the plans, rollers may be operated in an oscillatory mode, defined as a mode in which the drum vibrates in the horizontal direction only.” (Bold emphasis added).

This specification update is a very satisfying result and it shows how cooperation in working toward the same goal (maximizing HMA effectiveness and service life for bridge deck pavements) can be successful when the right team is assembled.  WSDOT and Industry came together to maximize the quality of future bridge deck HMA pavements, to the benefits of all the parties involved and to the benefit of the traveling public.

Now…what about that vibratory pneumatic roller?

Authors Note:  While HAMM rollers were used in the WSDOT/WAPA demonstrations, many other manufacturers build oscillatory rollers, including Sakai, Caterpillar, Bomag, Dynapac and Volvo.