DH Equipment To The Rescue: 30 Days
To Coat A Bridge

By Dave Wikoff, Account Manager, Munters Corporation, Moisture Control Services (MCS)

The Onslow Beach Bridge is a 210 foot (64m) long, 40 foot (12.2m) wide bridge near Camp Lejeune, N.C., used by service members, retirees, and employees assigned to Marine Corps Base Camp Lejeune, Marine Corps Air Station New River, and Marine Corps Air Station Cherry Point.

The bridge, which traverses the Atlantic Intracoastal Waterway, was long overdue for a paint job to better protect it from corrosion and increase its longevity. The project scope mandated that the bridge be sandblasted and painted, and have its rivets and bearing pad replaced. The U.S. Marine Corps engineers who oversaw the project stipulated that the work be completed in 30 days due to training and other exercises scheduled a month later that required use of the bridge.

The specification requirements included a near white NACE No.2/SSPC-SP 10 blast with grit abrasive to remove all of the previous coatings, followed by the use of a zinc epoxy primer with a 2-3 mil (50-75µm) paint thickness to serve as the anti-corrosion base. The primer was followed by an intermediate epoxy of 6-8 mils (150-200µm) thickness. Finally, a coat of 2-3 (50-75µm) mil thickness polyurethane to serve as the finish coat. Following the coatings, a test would be conducted to make sure the application thicknesses were accurate. 

In total, 78,000 total square feet (7,246.4m2) of area would be high pressure washed, blasted, and coated inside a 240,000 cubic feet 6,796m3) containment area. Typically, a project of this size and scope requires 60 days to complete. The challenge was to create the optimal conditions inside the containment to fast track completion of the project in 30 days or face penalties for missing the demanding deadline. 

The painting contractor hired a moisture control and water damage recovery firm to handle the temporary humidity control portion of the project during the 30-day timeframe. In addition, they brought in temporary dehumidification (DH) equipment.

Creating An Ideal Environment
The general concept behind the use of temporary DH equipment in a steel coating application is to create an optimum differential between the dew point temperature in the space and the temperature of the actual surface.  If the surface temperature reaches the interior dew point temperature, moisture vapor from the air will condense on the surface. 

This condensation can lead to flash rusting, inter-coat adhesion problems, and related curing issues.  Additionally, the temporary DH system is designed to create a climate that is conducive to the proper curing of the coating.  

The Onslow Beach Bridge project required that desiccant DH equipment maintain a minimum 5○ dew point spread, and 100○F (37.8°C) surface temp.  If the conditions inside the containment went outside the recoat window of 120○F (48.9°C) or closer than a 5○ dew point spread, the contractor would be required to re-blast and recoat.  The environment had to be maintained for eight hours after coating application for proper cure.

A containment area was constructed by the painting contractor and inside conditions were calculated based on a 10 year average of outside temperatures and humidity.  To create the interior space, the contractor used a tarp material to cover the entire bridge. The indoor space was then divided into three areas so work could be completed in three separate shifts.

Desiccant DH Benefits
Desiccant DH is the “work horse” for moisture removal from the air.  This process consists of drawing ambient air through a desiccant substrate which captures moisture from the air within a wheel.  This process is called adsorption.  In desiccant DH systems, the desiccant support structure is a wheel. The desiccant — a titanium-enhanced silica gel — is infused into a corrugated composite material that resembles the honeycombs of a beehive and this is formed into a wheel.

The saturated wheel is then dried by a separate heated air stream to reactivate the wheel.  The air is delivered to the space at extremely low dew points and relative humidity levels.  Depending on the time of the years when the project is occurring, care should be taken to control the temperatures in the interior area.  A direct expansion (DX) package customized air conditioning unit can be combined with the desiccant to optimize the controlled climate.

Utilizing Innovative Equipment For Energy Savings
For the Onslow Beach Bridge, it was determined that use of an energy-efficient and highly-effective desiccant drying system would save on generator fuel costs due to the lower amperage draw and smaller generator requirements. Use of a humidity control unit (HCU)-6000 pre-cooled with a 12.5 ton A/C system allowed the one-third sections of the containment concentrations to be maintained and in specification for the required periods.  In order to document and verify that the conditions written in the specifications were met, a proprietary monitoring system provided by the DH manufacturer was utilized to record and graph conditions both inside and outside the containment.

Highlighting the unpredictability of Mother Nature, on the first day of the project, record heat waves struck the eastern United States.  For two weeks outside temperatures soared into the 100s and still the stringent parameters were maintained and the project was finished on time and on budget. 

For the Onslow Beach Bridge assignment, desiccant DH played a major role in on-time completion of the project. Testing afterward proved that all coating applications had passed for desired thickness and application strength.
The success of a temporary system rides heavily on the specification and how it is written, and significant savings can be achieved through the implementation of energy efficient equipment and the use of monitoring devices.

 

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