In recent decades, Alaska DOT&PF has worked with crews and contractors to develop innovations for bridge deck preservation in the face of changing climate, rising materials and labor costs, and increasing pressure to minimize lane closures and related inconveniences to commerce and the travelling public. They also face infrastructure assets approaching the ends of their life cycles. Much of today’s concrete is 40-50 years old, meaning that many of Alaska’s bridges and decking surfaces leave engineers with only two increasingly costly options: repair or rebuild.

At the heart of Alaska DOT&PF’s trials in bridge deck preservation is a decades-long struggle to utilize an alternative surface treatment known as Magnesium Phosphate Cement (MPC). As an experiment this past summer shows, Alaska DOT&PF and its contracting partners are on the verge of an innovation with international applications that could change bridge deck management forever.

The Dalton Decking Experiment

MPC has been an item of interest for Alaska DOT&PF since the 1990s, when it enabled a major accomplishment on the Dalton Highway.  Northern Region M&O crews replaced the standard Ordinary Portland Cement (OPC) with an experimental application of MPC on a series of wooden bridge deck replacements. They were looking to explore the benefits and potential uses of MPC not just as a quick spot fix, but as an entire decking surface.

Fundamentally different than OPC, MPC sets up with the strength of concrete in an hour, and demonstrates low permeability.  Nor is it new. Its use traces back to the Great Wall of China and many historic Indian stupas still visible today. More recently, MPC has been used since the 1970s as a go-to method for quick patches and repairs.  Its fast setting time allows crews to apply it to a hole, crack, or worn surface, and move on to the next spot. In limited doses, MPC proves a durable, cost-effective quick fix. Crews can fix multiple spots on a surface, and then let traffic resume in about an hour.

In 1992 crews completed work on an ambitious project replacing old timber decks on 18 bridges along a 128-mile portion of the haul road, and resurfacing them with MPC. Completed in just eight months, this extensive operation showed engineers MPC’s benefits, but it also exposed some challenges, like the need to apply MPC fast enough to keep pace with its quick set time, and the cumbersome task of applying MPC bucket-by-bucket.[2] Because of the scale of operation, labor and material costs, and other factors, this broad application of MPC was also quite expensive.

Portage Creek

Two decades after the Dalton decking experiment, Alaska DOT&PF conducted another MPC application on the Kenai Peninsula. A Maintenance and Operations crew conducted an informal experiment on the Seward Highway at a bridge near Portage Glacier Road. 243 feet long and just over 34 feet wide, the bridge was like many the department deals with—aging, and in chronic need of repair.

243 feet long and just over 34 feet wide, the Portage Bridge was like many the department deals with—aging, and in chronic need of repair. (Photo: R. Bond)

The area’s climate—defined by its proximity to the ocean, glacier-fed streams, and drastic seasonal variations—brings a constant threat of water. Alkali-Silica Reaction (ASR) refers to a chemical reaction causing concrete expansion and cracking. ASR occurs largely due to water infiltrating concrete and aggregate, and is among the most persistent and common threats to concrete. In the past, typical patch and repair jobs on this bridge were quickly infiltrated by water, which began a freeze-thaw cycle leading to more cracks, holes, and slumping.

A solution to ASR, MPC is unique in that it forms a chemical bond—not a glue or a mechanical bond—that expels, rather than attracts, moisture. This factor makes MPC a very attractive surface treatment because of its extended life cycle, and its ability to prevent the kinds of repeated repair costs associated with OPC. These repeated costly repairs made Alaska DOT&PF wonder if they could somehow apply MPC over an entire bridge deck. One challenge remained: applying MPC quickly, and in bulk.

The first day at Portage Bridge, the crew closed one lane of traffic, prepped the bridge, set laser levels to ¾ inch above the deck, and began rapidly applying MPC. In one treatment, the crew applied 200 feet in three hours with a pour rate of three to four tons per hour. (Photo: R. Bond)

Apun, an Anchorage-based pavement preservation company, had a history of providing products and services to the department. Having recognized the potential uses of a mechanically mixed and placed MPC, Apun had already developed Alaskrete MPC and a trailer mounted machine to dispense it on a small scale for potholes and spalled areas.  In order to accomplish a complete bridge deck overlay, they developed a prototype pump head to fit on the end of a cement mixing truck’s pouring chute. The head allows the team to mix and pump a higher volume of MPC, getting it on the ground quickly before it sets.

As the tourist season slowed on the Seward Highway, crews began their work. On their first day, they closed one lane of traffic, prepped the bridge, set laser levels to ¾ inch above the deck, and began rapidly applying MPC. On the second day, they alternated the lane closure and completed the same process and treatment. In one treatment, the crew applied 200 feet of MPC in three hours, and maintained a pour rate of three to four tons per hour.

For a complete bridge deck overlay, Apun developed a prototype pump head to fit on a cement mixing truck’s pouring chute. It enables the team to mix and pump a higher volume of MPC, getting it on the ground quickly before it sets. (Photo: R. Bond)

Since then, one winter has passed and the team has seen no problems—no cracking, no damage, and no signs of water infiltration.

MPC Benefits

Beyond improving performance and reducing repair costs, MPC can be developed for specific locations, surfaces, and climates—making it an adaptive option for Alaska’s diverse and extreme regional climates. According to the Cold Climate Housing Research Center, MPC demonstrates a much higher compressive strength than OPC. A 2008 CCHRC study found that MPCs show a compressive strength of 8,000-12,000 psi, while the compressive strength of OPC was 3,000-7,000 psi.[3] In this study, MPC also demonstrated improved Bending Tensile Strength (in psi), salt tolerance, self-bonding, and a lower curing temperature range more appropriate to Alaska’s climate.

Minimized lane closures are also a benefit. With current technology, a crew using OPC could spend one month prepping a bridge, then spend one to two weeks of lane closures. Crews using OPC for bridge decking repairs must often grind down the existing surface before applying cement. Using MPC on the other hand, crews can skip this process altogether. They simply blow off the surface to make it clean, and begin applying MPC. Traffic can resume in an hour.

Next Steps

For Alaska DOT&PF, the goal behind MPC is to make concrete repair more like asphalt repair, with immediate return to service, extended multi-year durability, and an ease of use made possible by custom designed equipment.

From Apun’s point of view, several further advancements are on the horizon. It is possible, for example, to double the pace in which crews can apply Alaskrete MPC, requiring a scaled-up pump head prototype that will produce a pour rate of 10-12 tons per hour. Apun has also expressed confidence in backing-up a 10-year warranty on Alaskrete MPC treatments in such a scenario. It is also plausible to expand the use of Alaskrete MPC by allowing other contractors to train and license the use of Apun’s pump head—widening the implementation of this technology.

Minimal lane closures are key. A crew using OPC could spend one month prepping a bridge, then one to two weeks repairing it. Using MPC on the other hand, crews simply blow off the surface, and begin applying MPC. Traffic resumes in an hour. (Photo: R. Bond)

Alaska DOT&PF is now looking for opportunities to continue research and development activities, further reducing the costs of this new innovation. For a one inch overlay, current Alaskrete MPC costs of materials and application are approximately $20 per square foot with a 10-20 year service life.  (For the sake of comparison these costs include only materials and application.)  Compare that with an estimated $10 per square foot materials and application cost for fabric and HMA overlay with a 1-3 year service life.[4] Additionally this 1 inch MPC overlay is half the weight of the 2 inch HMA overlay. With additional research and development into improved application equipment and techniques, Apun estimates these costs will continue to drop. Another cost-reducing factor is the kind of mix used for MPC. Mix designs for magnesium phosphate cements are much like Portland cement in that they can be formulated for the specific job at hand depending on environmental conditions, desired strength, flexibility, set times or using local fillers or aggregates.

So what began as a cost- and time-intensive experiment two decades ago has evolved into an applied innovation that drastically reduces maintenance and repair costs on some of Alaska’s most vital transportation assets.

The group included 12 undergraduate engineering students and was led by Andrew Metzger, Assistant Professor of Civil and Environmental Engineering, and Adjunct Instructor and CEM Alumni Wilhelm Muench. The team was chosen through a competition held within UAF’s Engineering Department as part of an advanced Bridge Design course.

In addition to winning 1^st overall, the team also took the following titles:

• 1^st place in environmental engineering;
• 1^st in bridge efficiency;
• 1^st in bridge stiffness;
• 2^nd in bridge weight;
• 2^nd in bridge economy, and
• 2^nd place in transportation engineering.

The team also took first in a transportation scavenger hunt, and participated in a number of related activities at the annual event. The regional conference also hosted concrete canoe, technical paper, and engineering knowledge competitions.

With the regional title under their belt, the UAF team now prepares to compete in the 2013 National Student Steel Bridge Competition in at the University of Washington in Seattle, May 31-June 1. The event is sponsored by the American Institute of Steel Construction and the American Society of Civil Engineers.

With this title comes the unique opportunity to purchase team t-shirts printed just for this occasion. With very limited quantities, the shirts are available by contacting AUTC Program Assistant Diane Wallace at 907-474-7330, dkwallace@alaska.edu, or on the UAF Campus in Duckering Room  245.

Frank Moolin was the chief engineer on the pipeline portion of the Trans-Alaska Pipeline System. As Moolin left a legacy of ideas for managing very large, or so-called ‘mega-projects,’ this series features a variety of specialists giving tailored presentations on various aspects of this emerging topic. The program will examine aspects of mega-project management like organization, delivery, human resources, regulatory and permitting, economics, communications, and public relations. The program will explore the ways in which today’s ‘mega-projects’ are fundamentally different than stand-alone large projects. (8 PDHs are available for professional development credit.)

To learn more about the lectures, registration, contact, and other details, please view or download an event flyer here.