Rehabilitating our nation's bridges; maintaining bridges is no small task. Here's how we make decisions about repair and technology that will help them last longer.
The average age of bridges in the United States is 40 years. Paul Kivisto, the metropolitan region bridge engineer for the Minnesota Department of Transportation (MinDOT) says, "The average life of a bridge is approximately the same as for a person--about 70 years." Whether a bridge makes it that long or exceeds its life expectancy depends on how well it's maintained over its life span--the same as for people.
To ensure that bridges are safe and maintained, the federal government enacted legislation in 1969 mandating inspections by qualified engineers every 2 years for all bridges in the country that are more than 20 feet long. The FHWA has the responsibility to see that bridge owners comply.
The timing and the type of bridge inspection depends on several factors: its age and size, the amount of traffic it carries, the role it plays in relation to cities around it, and the access it provides to emergency facilities. An accident triggers an immediate inspection.
THE EVALUATION PROCESS
Niket Telang, a senior engineer with Construction Testing Laboratories (CTL), Skokie, Ill., explains that there are two types of federally mandated inspections: routine and in-depth. Routine inspections are the most common, involving only a visual inspection. They document areas of noticeable damage and signs of distress. In-depth examinations are much more rigorous and "hands-on." Inspectors literally run their hands over an entire structure and perform more sophisticated testing where it's needed. Bridges with a history of problems are inspected annually, and large important bridges have assigned inspectors who inspect them continuously.
A small bridge inspection requires two inspectors working 3 to 5 days to finish the onsite portion. Large bridges involve larger teams. Not all team members must be engineers, but each member is trained and certified. The leader of a team is always a structural engineer who isn't required to be on the site of an inspection but must sign-off on the inspection. States and counties conduct inspections "in house" if they have enough staff; otherwise they hire consultants. Consultants almost always perform the underwater inspections of piers and footings.
The NBI includes guidelines for rating bridge conditions. Each bridge element receives a rating between 1 and 9. A condition code of 1 means there is a serious problem. (There is a code 0 also. It requires that a bridge must be closed and replaced.)
SETTING A COURSE OF ACTION
"When bridge inspections are complete, the next step is to decide which will be repaired and which won't. Engineering decisions are now interfaced with budgets and political constraints," says Telang. The most important problems get funding. Adrian Ciolko, vice president of CTL, adds that over the past few years many bridges were marked for removal and replacement, so they weren't to be repaired. But in today's downturned economy, there is a new interest in rehabilitating them. And when there is no money to fix a bridge, owners have the option to close it. So, currently there are thousands of closed bridges around the country.
When money is allocated for a project, design teams create plans, predict costs, and recommend techniques for repairs. They finish their task by completing drawings, specifications, quantities, and cost estimates, which become the bid documents for a project. Kivisto adds that in Minnesota, if the expected cost for repairs exceeds 70 percent of the cost of a new bridge, they replace it.
Telang states that it's not always the low bid that gets the work. Contractors must frequently submit their qualifications with their bid, and the low bid may not be qualified. And sometimes bid documents require contractors to supply value engineered alternates either before a bid opening or as an alternate part to the bid package. This can work to the contractors' advantages because they can value-engineer their particular expertise.
TYPES OF REPAIRS
In terms of frequency, bridge decks are the most repaired element, mostly due to chloride penetration from deicing salts that cause spalling and corrosion of the steel reinforcement. Ralph Anderson, director of bridges for the Illinois Department of Transportation (IDOT) states that salt damage is a big concern because IDOT frequently salts bridges in preparation for snow, so they won't become slippery when snow starts to fall.
Chloride-induced corrosion problems in marine settings are also common--the salt source being the ocean rather than deicing salts. Next in line are superstructure issues involving beams, trusses, stringers, and cross frame members. Piers are next, followed by curbs and guard rails. Kivisto says that cold weather presents special problems, sometimes causing bridge bearings and elastomeric joints to contract beyond their limits. Lack of proper air entrainment also causes problems.
CURRENT TRENDS IN REPAIRS
The goal for repairs is to add to the life expectancy of a bridge. But decisions about which methods and products to use also consider the public disruption involved, effectiveness over time, and the extent of the deterioration.
Proactive maintenance is beginning to receive more attention because problems are less expensive to fix when they are first developing. In that light, electronic monitoring holds good promise.
Tom Weinman, manager of the sensors and diagnostic group for CTL, says that there are interesting bridge projects around the country that demonstrate this approach. Monitoring chloride penetration is easy for electronic sensors, which can be installed on new construction or retrofitted.
Sensors also can be embedded in sacrificial overlays to measure chloride penetration through the topping. The readings help owners know when it's time to remove and replace the topping in order to avoid chloride damage to the structure. "And by monitoring corrosion in steel reinforcement, you can add inhibiting admixtures such as calcium nitrite--and then monitor how successful the effort was," he adds.
Ciolko reports that much of the damage to decks first occurs at the expansion joints. Engineers are currently designing more "jointless" deck systems. IDOT wants to eliminate all the joints in their bridge decks.
In terms of concrete technology, Michael Sprinkel, associate director of the Virginia Transportation Research Council, Charlottesville, reports that high performance concrete (HPC) mixes, which are more impermeable and have low water/cement ratios (w/c), are being used for most bridge decks now and should help to eliminate many of the problems that older bridges now face.
He adds that most decks today, either new or repaired, are constructed with hydraulic cement, polymer modified concrete, or 100 percent polymer overlays. The latter consists of epoxy resins and stone aggregate and is usually 1/4 inch in thickness. Sprinkel likes this alternative because it's fast and the cost is low. The moisture content in the deck isn't an issue. "If you can shotblast a deck, the product can go down," he notes. Toppings are considered to be sacrificial protection for structures and can be removed and replaced when needed typically every 15 to 40 years.
Another interesting technology involves electrically removing chlorides from concrete structural members, according to Sprinkel. The process takes 4 to 8 weeks and uses electrical current similar to cathodic protection. When the process is used on bridge substructure, traffic isn't affected during treatment.
The use of sacrificial embedded galvanic anodes to manage the corrosion of steel reinforcement is becoming more common, says Sprinkel. Workers attach small anodes to steel reinforcement in the most endangered areas of a deck. Then, over time, corrosive products gradually consume the anodes, leaving the steel reinforcement in good condition. This is a low-cost way to extend the service life of patch repairs. Embedded galvanic anodes are also used to address potential corrosion problems at the joints and for general substructure repairs. They are also commonly used to protect prestressed concrete elements.
Siloxane and silane sealers are increasingly being used on bridge decks to add up to 5 years per application. Sprinkel prefers them to surface sealers, which can be slippery in severe weather.
In terms of bridge construction, Ciolko states that prestressed and post-tensioning (P-T) reinforcement is the primary reinforcing method today. As a result, bridges are stronger and don't develop the cracks that can transport chlorides to the structural steel. Segmental bridge construction falls under this category and is performing very well in terms of maintenance.
WHERE DO WE GO FROM HERE?
Joey Hartmann, a research structural engineer with the FHWA's Turner-Fairbank Highway Research Center, thinks the most important trend in bridge construction and maintenance is the shift from construction costs to life cycle costs--how much a bridge really costs over time. This holistic approach includes upfront corrosion or deterioration protection strategies, inspection technologies, and maintenance methods.
"We will build bridges in the future that are more resistant to deterioration and easier to both inspect and maintain," says Hartmann. "Nondestructive evaluation technologies will be incorporated into bridge components that provide more detailed, quantitative information than the subjective information provided by current hands-on engineering inspections. Also, the use of alternate forms of reinforcement that are less susceptible to corrosion in concrete bridge decks will increase."
One promising experimental study Hartmann is doing involves eliminating the need for mild reinforcing steel from the concrete bridge decks. Concrete bridge construction will continue to have significant funding, and its use will be increased.
RELATED ARTICLE: Using SCC to Repair Bridge Decks
In St. Louis, the congested Poplar Street Bridge Complex ties Interstates 55, 64, and 70 together. Constructed in the late 1960s, the decks gradually succumbed to the ravages of freeze-thaw cycles and chloride damage.
During the summer of 2003 the Missouri Department of Transportation (MoDOT) contracted 10,000 square feet of bridge deck replacement with St. Louis Bridge Construction, St. Louis, requiring work to be completed in 10-hours shifts, with concrete that would achieve 3200 psi within 4 hours of placement so that traffic could use it during rush hour the following morning. Cleveland-based Master Builders helped the contractor develop a self-consolidating concrete mix design (SCC), including a nonchloride accelerating admixture.
Toward the end of each day ready-mix trucks were loaded with aggregate and water. During the night when concrete was needed, workers added the portland cement and admixtures and placed the concrete. Repair sections averaged 100 to 220 square feet each night, opening the completed section to traffic by 5:00 a.m. Concrete attained the required 3200 psi, and in many cases reached 4000 psi.
--Nasvik is a senior editor with CONCRETE CONSTRUCTION magazine, a Hanley Wood publication. This article originally appeared in the December 2003 issue of CONCRETE CONSTRUCTION.
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|Title Annotation:||Bridge Repair|
|Date:||Feb 1, 2004|
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