Author: Lorella Angelini, Angelini Consulting Services, LLC
Second from the right: Rod Thornton with MDOT-SHA
Rod Thornton is the Maryland Department of Transportation – State Highway Administration’s (MDOT-SHA) Deputy Director of Structure Inspection and Remedial Engineering Division. In addition to his responsibilities with MDOT-SHA, he is active with TSP2 being the Vice-Chair of the Northeast Bridge Preservation Partnership Committee. I had the opportunity to ask Rod a few questions about bridge preservation strategies at Maryland DOT.
What does bridge preservation entail at MDOT-SHA?
It is comprised of a number of programs that are all directed to preserving the integrity of the original bridges and eliminating emergency repairs.
A key program concerns bridge painting. The program, which advertises $12 million worth of painting projects every year, does not simply entail painting steel structural elements, but it also includes repairing deteriorated critical areas of the girders by plating, replacing leaking joint seals, and installing troughs under roadway joints. This type of preservation work is included into the paint contract since paint scaffolding provides easy access to the areas to be repaired. By performing preservation work in parallel with painting, we eliminate the need to return to do these repairs in a following stage, which results in less of an impact to traffic and cost savings. Funds above $12 million are added to the paint program in order to perform preservation activities.
Plating over severely deteriorated areas of a structural element, such as a beam, preserves the structural integrity of the element but also improves how long the paint system will last. The new “retrofit” plate provides a smooth surface that can be prepped to have the correct profile for proper paint adhesion. It is well-known that the long-term adhesion of a paint primer may be prevented if rust and other imperfections are not completely removed during the blast cleaning process and a proper profile of the blasted steel is not achieved. A good adhesion of the primer is essential for the intermediate coat and final coat to work. Plating over the pitted areas of a beam with new steel allows for the creation of a good paintable surface profile, which in turn increases the longevity of the paint system. In areas that are subjected to a lot of moisture, like finger joints, the non-structural “retrofit” plates provide a layer of sacrificial steel while improving the adhesion of the paint.
Our paint program performs well, beyond the expected 20 years of steel protection. We are getting anywhere from 25 to 30 years out of the program. The color of the top coat may fade, but the paint system still provides the steel protection that is needed to prevent corrosion.
Another important bridge preservation program at MD-SHA entails the use of latex-modified concrete overlays in order to preserve the deck thus avoiding or delaying its replacement. The process starts with an evaluation of the chloride levels at the various depths of the deck. If a deck is in fair condition with concentration of chlorides near or just beneath the top mat of the deck reinforcement, we remove chloride-contaminated areas through hydrodemolition and we then cast a new latex-modified concrete overlay. Since the early ‘70s, MDOT-SHA has required both the bottom and top rebar mats to be epoxy coated. Epoxy protects bars from corrosion induced by chlorides thus making it possible to reconstruct parts of the deck rather than replacing it.
MDOT-SHA is currently working to determine the best preservation actions for the old deck parapets that have high chloride concentration. We are looking into new types of sealers that contain chloride inhibitors to treat these parapets when the deck is partially reconstructed using latex-modified concrete thus preventing their replacement.
Another top preservation program at MD-SHA entails the installation of troughs under roadway joints where the seal is in NBI condition state 4 and 3. We have set up a dedicated “open-ended” contract that allows us to assign a number of bridges to a contractor and get the trough installed once a bridge is identified. We also have a dedicated “open-ended” contract that allows us to wrap and strengthen the concrete structural elements of the substructure, such as caps and columns, with E-glass fiber reinforcement and coat these surfaces with an aliphatic polyurethane coating. Typically, we only wrap columns that are close, or adjacent, to roadways, thus creating a permanent water barrier from the salt spray generated by vehicles traveling past.
Since roadway joints are a source of deterioration for many elements of the substructure, such as bearings and beam ends, our newly designed bridges are built with no roadway joints or in a very limited number.
The last preservation program I would like to mention entails improving the protection of the steel tendons placed at the bottom of pre-stressed girders. We require a 4” clearance from the outside face of the girder instead of just meeting the 3” concrete cover required by AASHTO. The additional clearance ensures that the tendons get adequately covered with concrete. There are known tolerances in casting of pre-stressed members that could end up reducing the cover of steel tendons that are close to the exterior surface. We also include a design modification that increases the slope of the bottom flanges of AASHTO concrete girders so as to prevent accumulation of debris, bird droppings or bird nests.
Could you outline the key points of the bridge preservation strategy at Maryland SHA?
I would summarize the strategy in three points. The first one involves the evaluation of Structurally Deficient (SD) and Fair rated bridges on the entire highway system with the goal to properly intervene on SD bridges and to prevent Fair rated bridges from becoming SD. Every year two managers, one from the design and the other from the maintenance side of the bridge office, visit all bridges that are rated SD and also those bridges that had been rated 5 for one or more items (deck, superstructure, or the substructure) for more than 10 years. Once all these bridges have been looked at, we determine whether they are candidates for maintenance actions, preservation activities, major rehabilitation, or replacement. In doing so, we monitor all bridges that are in bad condition and determine actions to be implemented and scheduled.
A second element of the strategy entails examining common design or construction details that cause maintenance issues or are linked to potential deterioration problems. If in the inspection reports there are details that show the presence of a recurring bridge defect, then proper recommendations are made to the designers so as to eliminate it. As an example, in the 1960s they used to weld stiffeners halfway from the top to the bottom of steel girders’ diaphragms. Once we recognized that these stiffeners induced fatigue cracking over 10 – 15 years, we informed the design team so as to find a more effective solution. A recurrent issue entails the accelerated deterioration that we experience with galvanized corrugated metal pipes used to carry water runoff with high concentrations of chlorides. There are many more examples of issues found in the field resulting in design changes, such as poorly performing roadway joint systems, multi plank bridge deck details, and parapet details. Through the constant communication with the design office we were able to improve what is being designed for our new bridges.
The third element of our strategy calls for an actual and effective communication between the bridge design team and the office I am in charge of, which entails inspection, construction, and engineering design for maintenance. We are in constant communication and we work together to decide whether bridges should be preserved thorough minor rehabilitation, major rehabilitation or replacement. In principle, we can save and keep any bridge if enough money is available, but there are cases where it is not economical to rehabilitate a bridge. When we decide to make a major investment to rehabilitate a bridge, the design team should agree that no future plans are going to be made to replace or enhance that bridge. On the opposite, if the design team programs a bridge for replacement, then the maintenance team will focus on safety repairs only, since there is no reason to spend money on a bridge that will not be around for much longer. Maintenance preservation actions are a priority for those bridges that are intended to be kept in service for a long time. A lot of engineering judgement and experience is requested to know when to intervene and perform repairs and where we may have capacity and safety issues, such as beams not capable to carry the load or pieces of concrete falling on vehicles, if we do not intervene. This mix of engineering knowledge and experience is an essential component to ensure public safety, which is our most important goal.
What are your bridge preservation goals, both short and long term?
Our short term bridge preservation goal is to comply with the Governor’s mandate to reduce the number of Structurally Deficient bridges to zero. We are going to achieve this goal by combining repair, rehabilitation, and design actions. (Ed. Note: Out of the 2567 bridges managed by Maryland SHA, 67 are classified as Structurally Deficient, corresponding to 3% of the total).
Extending the service life of bridges is our long-term goal. If we can get 20 more years from a bridge by doing preservation actions, it makes sense to perform them. Preservation actions can vary. If a bridge has a deck that is in fair condition with 20 more years of life in it, we focus on the key bridge elements that may need preservation, such as beams and substructure. If we prolong the service life of these elements, we eliminate the need to replace the bridge in the short term and can wait until the deck needs to be replaced. By bringing bridges to Fair or better condition, we extend their service life and have time to focus on those bridges that are in critical conditions and cannot be expected to last long without a large capital investments. We perform holding actions to ensure the safety of the traveling public.
Which challenges have Maryland SHA encountered in the implementation of the bridge preservation strategy?
We have an issue with environmental permitting, which is an impediment for pressure washing.
Other States have been able to get an agreement with their environmental people, for example scheduling power washing during heavy rain. In Maryland we can power wash only a limited number of bridges provided that we collect water. Grime, grit, debris, and oils pollute water during power washing thus making water a hazardous material that is cost-prohibited to collect. As a consequence, it is difficult for us to maintain clean critical structural elements, such as trusses, that collect a lot of debris.
Do you have a success story that you would like to highlight?
In addition to the paint program, of which I have already spoken, I would like to mention the paved pipe invert contract.
We have restored over 300 galvanized corrugated metal pipes by installing a new reinforced grout invert. The hoop design of the pipe is re-established by this structural design methodology which essentially consists of welding steel reinforcing bars in a pattern longitudinally and transversely along the inside length of the pipe and then pouring a 4” thick concrete grout invert over top. By adopting this methodology, we have repaired pipes anywhere from 3 ft to 20 ft in diameter located under major Interstates or throughout our highway system. By not having to replace these pipes, we have saved a lot of money. The typical cost of the grout repair method is $40/SF, which is typically 20+ times less costly than replacement. We started by paving the worst pipes and we will continue until we have paved 1300 + pipes.
In my opinion a big bridge preservation success story in Maryland is arriving to the point of having sustained funding. We have always had funding for maintenance and repair, but it was a one-shot approach. We now have funding that allows for broadening the use of preservation actions and setting up systematic programs. The recent gas tax imposed by the State has been a tremendous windfall for the highway program in Maryland. It has allowed us to focus on the Governor’s goals to achieve zero SD bridges and to perform all improvements that are going on in the entire highway system here in Maryland.
How can TSP2 assist you with reaching your bridge preservation goals?
TSP2 is giving me a unique opportunity to develop personal relationships in the bridge preservation community. I can connect with representatives from other States to find out what their programs are like, what processes they follow, and what products they use.
All States are trying to achieve common bridge preservation goals as cost effectively as possible and with the least impact to the travelling public. TSP2 provides the means to achieve these goals by learning and replicating what the different States have done successfully.
LINKS
NCHRP Project 20 68A, Scan 15-03
Successful Preservation Practices For Steel Bridge Coatings
http://onlinepubs.trb.org/onlinepubs/nchrp/docs/NCHRP20-68A_15-03.pdf
SHA Paved Invert Specifications