Integrating an Asset Management Approach with Bridges

Back to basics: Identifying key data points

One of the first steps MnDOT took to transition to an asset management strategy was to transform the way bridge inspections are conducted. Through rigorous training, detailed manuals, demanding quality control standards and strict quality assurance methods, Minnesota has confidence that the collected data is accurate and consistent. This resulting data feeds research, deterioration models and decision-making tools. With accurate data, MnDOT can properly understand needs and therefore prescribe appropriate treatments. Making better decisions increases the service life of our bridges while lowering life cycle costs. The heart of all these benefits is collecting and analyzing the right data.

MnDOT collects bridge inspection data in a wide range of methods:

• Visual Inspections: Engineers assess the bridge’s condition by observing structural elements for signs of wear, damage or deterioration. This includes checking for cracks, corrosion and deformation.
• Structural Health Monitoring: Sensors and measuring devices detect deformation, corrosion or damage to the bridge structure. This includes strain gauges, accelerometers and corrosion sensors.
• Non-Destructive Testing: Ultrasound, thermography and radiography detect internal defects without causing damage to the bridge. These can identify issues like internal cracking, voids and material degradation.
• Environmental Data: Information on environmental conditions (such as temperature, humidity and application rates of road salt) is collected to understand their impacts on the bridge materials and structure.

MnDOT can create a comprehensive picture of each bridge’s condition by integrating these diverse data sources. This approach allows us to prioritize a variety of repair activities based on actual needs rather than just age or visible damage. MnDOT can then identify potential issues and correct them in an efficient and effective manner.

Predictive modeling leads to preventative action

Armed with the right data, MnDOT uses predictive deterioration modeling to more accurately determine when to take preventative action.

Predictive deterioration modeling is a sophisticated analytical network technique used to estimate and forecast the future condition of bridge structures. It leverages historical data, material science, environmental factors and advanced algorithms (such as machine learning) to predict how a bridge’s structural components, such as decks, beams and supports, will degrade over time.

For example, let’s say we analyzed the life cycles of 100 girder bridges and discovered that the average expansion joint life is about 30 years. Therefore, MnDOT now knows that when girder bridges reach their 25th year of service life, efforts to plan a joint replacement project should take place in the next 5 years.

If the bridge is older, we may eliminate the joints, which was the case for MnDOT’s 106-year-old Third Avenue Bridge over the Mississippi River. This open-spandrel arch bridge had 45 joints. We rearticulated the deck and reduced the number of joints to 11 for less future maintenance and to extend the life of this signature and beloved structure.

With predictive deterioration modeling, we can better manage those key bridge elements to improve the longevity of the asset.

Standardizing design life decisions:

Predictive modeling helped MnDOT create the standards and policies in our Service Design Life Manual, one of the first in the industry. The manual breaks target design life into three categories and provides specifications for achieving each one. It also provides information on the renewal elements of an existing asset that can easily be repaired or replaced to extend its useful life.

By tailoring design choices to meet the needs of normal, enhanced or maximum service life, MnDOT has developed a clear framework for extending bridge longevity, minimizing future disruptions to the traveling public and optimizing costs. The service life approach applies to all elements, including deck, superstructure and substructure.

• Normal service life is 75 years. Determined to be a sufficient and cost-effective selection, most bridges in Minnesota fall into this category. For the bridge deck on a normal service life structure, MnDOT would use high-performance deck concrete, epoxy-coated rebar and a 3-inch cover to the top rebar. That material combination with a seven-day wet cure is standard for new and redecked bridges.
• Enhanced service life designs the bridge deck to last more than 75 years or require less maintenance during a 75-year duration. Candidates include:
  • Superstructure types that would make staged deck replacement problematic
  • Decks of bridge with a construction cost exceeding $20 million
  • Decks of bridges with annual average daily traffic volumes greater than 60,000

For the bridge deck on an enhanced service life structure, MnDOT would consider a few options, including the use of high-performance deck concrete, epoxy coated ASTM 1035 with 4% chromium or GFRP (Glass Fiber Reinforcement Polymer) for deck and barrier reinforcement, 3-inch cover to the top rebar and 14-day wet cure.

• Maximum service life ensures all nonrenewable structure elements will last 100 years. MnDOT consider maximum service life for:
  • Concrete box girder decks that prohibit replacement
  • Decks of bridges with construction costs of more than $35 million
  • Decks of bridges where minimizing user delays is crucial

Ideally, maximum service would be our choice for every bridge, but because of its high initial cost, MnDOT must be selective about where we apply it. For example, using corrosion-resistant stainless-steel rebar on high-volume traffic routes, which can’t be closed for repairs, often justifies the material’s higher cost. For maximum service life structures with monolithic decks, MnDOT uses high-performance concrete deck mix design, stainless steel deck and barrier reinforcement, 2.5-inch cover to the top rebar and 14-day wet cure.

Modern designs strive to reduce the amount of maintenance a bridge should expect over its life. One example is to reduce or eliminate bridge expansion joints. This can be advantageous as joints not only create a maintenance expenditure, but also are a pathway for water and road salts to deteriorate underlying structural components. One example of MnDOT using modern design approach is on the 106-year-old Third Avenue Bridge over the Mississippi River. This open-spandrel arch bridge had 45 deck joints. MnDOT rearticulated the deck and reduced the number of joints to 11, which is expected to greatly extend the life of this signature and beloved structure.

For large, important, historic or complex structures, MnDOT develops individual Bridge Management Plans (BMP). These identify a matrix of repair options for scoping engineers to help prioritize, budget and align with other corridor needs.

MnDOT also has developed a proactive High Priority Bridge Preservation program for 24 large and expensive structures. This fund dedicates $10 million per year toward preservation strategies (guided by the BMP) to keep these bridges in good condition for as long as possible.

Optimizing resources for the future

Shifting to a predictive bridge asset management approach is the right strategy for MnDOT. By using accurate and consistent data, combined with predictive deterioration modeling, we can proactively maintain and extend the life of our bridges, reducing long-term costs. Our new approach optimizes resources, enhances safety and ensures Minnesota’s bridges meet the needs of the public now and in the future.

ABOUT THE AUTHOR

Ed Lutgen, P.E.
State Bridge Engineer
Minnesota Department of Transportation

Ed Lutgen is the State Bridge Engineer for the Minnesota Department of Transportation (MnDOT), with nearly 30 years of experience in bridge design, inspection and management. He holds a Bachelor of Civil Engineering from the University of Minnesota (1995). Ed has contributed to numerous AASHTO and NCHRP committees and research projects. He currently chairs the AASHTO COBS Safety and Evaluation Committee and leads efforts to innovate bridge asset management and safety.