Alberta’s 6 Best Bridge Practices
Alberta, Canada has developed guidelines for the best practices for assessments, corrosion control, design, deck rehab, abutments, and footings.

by Ruth W. Stidger, Editor-in-Chief

States, countries, and in some cases cities and counties are putting together best-practice guidelines for bridge maintenance.

In the United States, several studies are underway, including one in Virginia that is partially sponsored by the Federal Highway Administration.

In Canada, the Alberta Transportation Department already has numerous best-practice guidelines in place and plans to add more.

Bridge assessments

Looking at each bridge and making a plan for it lies at the heart of the Alberta Transportation guideline. Assessments

determine the best long-term solution whether it results in maintenance, rehabilitation, or replacement. The goal is to maximize service life and obtain the lowest lifetime-cost solution.

The first step is to determine which bridges need maintenance, rehab, or replacement in a short period of time. The assessment factors include condition deficiencies, functional deficiencies, and proposed highway improvements. Three basic types of assessments are used.

The bridge assessment may be started if a bridge has an existing condition that creates a problem or if the highway on either side of the bridge will be improved. Within this step, there is a summary of the bridge condition; proposed highway improvements where applicable; functional deficiencies such as width, strength, or vertical clearance; environmental issues and impacts; hydrotechnical adequacy and replacement possibilities; alternatives such as actions, timing, and life-cycle cost analysis.

If the bridge will need rehab or maintenance within the next three to five years, a bridge rehab assessment is scheduled.

The bridge rehabilitation assessment includes a summary of the bridge condition, bridge rail analysis, environmental issues and impacts, alternatives, and a recommendation for the best action.

If bridges need replacement, a complex assessment is completed. This includes an assessment of the need for river protection works, detailed structural analysis, and site-specific analysis.

Metal culverts

Corrosion surveys keep metal culverts used as bridges in action following this best-practice guideline. Historically, the guideline states, metal culverts were evaluated on initial cost.

Now, the agency uses corrosion surveys to determine whether use of the culverts is the most cost-effective solution, especially on crossings where the culvert will cost $200,000 or more.

The agency considers that water and ground conditions, combined with the type of corrosion protection on the metal culvert, should result in a lifetime of 45 years or more.

Corrosion surveys include making a number of measurements:

• pH and resistivity values of the soil on the road side slope on both sides of the road and in the upstream and downstream banks. The time to first perforation for soil side corrosion is calculated for different plate thicknesses using average pH and resistivity measurements.

• pH and resistivity values of the water at the upstream and downstream ends. The time to first perforation for waterside corrosion is calculated for different plate thickness using average pH and resistivity measurements.

• Check for sulphides, sulphates, and chlorides when recommended by the corrosion specialist.

• Take static potential readings between the soil and culvert at 3, 6, 9, and 12 o’clock positions at the upstream and downstream ends of a current culvert to establish the condition of the soil side galvanizing and the rate at which the existing galvanizing has been consumed. Average readings can be used to estimate life expectancy of the new culvert.

The final report should provide data summarizing pH, resistivity, sulphide, sulphate, chloride, and potential readings along with average values; brief details of testing methods used; theoretical life expectancy of the culvert plate thickness evaluated; and recommendations in regard to the material to use and measures to control corrosion.

Alternate design

When a bridge is past repair, replacement is the next step. Alberta uses a best-practice guideline to  consider alternate design for long life and lowest lifetime cost.

Alternate designs are generally considered only for some structures of more than 11,000 square feet. In such a case, the alternate design provided should have an estimated cost difference of at least 5%. If the cost difference is 10% or more, the agency goes ahead with the most cost-effective alternative. If the difference is 6 to 9%, engineering judgment is used in making the decision. Factors considered are new product development, schedules, cost trends, and aesthetics.

If the bridge is 21,000 square feet or more, it should definitely be considered for an alternate design (using the same percentage criteria given above) unless it is a standard SC girder or composite SCC girder-type structure.

Concrete deck rehab

With thousands of concrete bridge decks carrying heavy loads daily, this best-practice guideline is useful anywhere. The department uses the guidelines for various typical bridge deck rehab measures. Selection is based on life-cycle costs.

The standard deck waterproofing system uses a 0.7-inch asphalt membrane, 0.7-inch protection board, and two 1.5-inch lifts of hot-mix asphalt concrete pavement. Initial rehabilitation of this system can be replacement of the top lift of asphalt concrete as condition warrants. Subsequent rehab may require removal of the waterproofing and installation of a new deck overlay.

Concrete overlays used have Class SF concrete with steel gibers and a minimum thickness of 2.5 inches. This approach is used on decks that originally had a 2-inch asphalt concrete pavement wearing surface with no membrane, or had concrete cast to grade. Concrete deck delamination or other defects signal the need for rehab.

Polymer overlays are thin, flexible, multi-layered polymer-aggregate wearing surfaces intended to bridge narrow cracks in the concrete and prevent moisture seeping into the deck. This system is normally used with existing concrete cast to grade or an existing concrete overlay where the surface is in reasonably good condition.

Reinforced concrete overlays are Class SF concrete reinforced with steel fibers. The overlay has a minimum thickness of 6 inches with 3 inches covering the top of the slab. The slab is reinforced with one layer of epoxy-coated  reinforcing steel. This overlay is used on decks with concrete girders placed side by side without grout keys or short-span concrete girders where grout key breakdown exists.

Integral abutments

Short bridges work best for the use of integral abutments. According to the guideline, for composite concrete girder bridges with a total length of up to 55 yards, integral abutments should be used. For steel-girder bridges with a total length of up to 44 yards, integral abutments should be used.

For longer bridges, integral abutments can be considered. However, the design must consider cyclic thermal movements of the structure.

Use of integral abutments eliminates abutment joints and reduces maintenance.

Spread footings

Spread footing foundation designs are not always appropriate. Alberta’s best-practice guideline separates appropriate from inappropriate use.

Assuming adequate bearing capacity and slope stability, the guideline states, spread footings are viable alternatives for grade separations, abutments, and land-based piers.

Because of the additional risk, the need to inspect and possibly underpin, spread footings should not be considered for land-based piers at the outside edge of river bends, on the banks of highly mobile streams, or within stream beds.

Except for bridges in the foothills with rock outcrops in the stream bed, cost differences between bored or driven piles and spread footings shouldn’t be substantial.

Best Practices for Covered Bridges

Practices that work well on conventional bridges are not necessarily right for covered bridges, says Arnold M. Graton, Jr. Graton, who took part in the University of Vermont’s Historic Preservation Program, says that accepting state and federal funding often creates problems because the monies carry standards that aren’t appropriate for historic bridges. Graton recommends funding in other ways, including grants.

Best practices include maintaining the original clearance, load, width, and wind load.

Increasing load capacity turns the bridge into a shelter sitting on a steel bridge rather than a functioning historic covered bridge, Graton says.

Increasing clearances result in the loss of the floor system and encourage larger vehicles, which can overload the structure.

Increasing the width is too costly for the benefits gained, he says, and can destroy the appearance of the bridge.

“When we set out to restore a covered bridge, our goal should not be to change the design of the bridge,” Graton says, “rather it should be to return the bridge to its own optimal state.”

Use a restorationist or covered bridge specialist from the planning stage onward, he says, and don’t change the design.

Best Approach Practices in Iowa

In a project to research the best practices for design, construction, and repair of bridge approaches, Iowa State University’s Partnership for Geotechnical Advancement at the Center for Transportation Research issued its final report this year. David White, assistant professor in the Department of Civil, Construction, and Environmental Engineering, was the principal investigator.

New guidelines were needed to best deal with bridge approach problems, which include poorly performing backfill material, foundation settlement, void development under the approach, inadequate drainage, and joint movement.

New bridge approach construction guidelines include better compaction of granular backfill, removal of more moisture from backfill before compaction, and keeping abutment subdrains open.

Bridge maintenance practices highlighted included use of injection of expansive polyurethane under the approach slab to lift the pavement back to its original profile.

International Roughness Index and Bridge Approach Performance Index data were used to develop maintenance rating criteria.

Use of steel reinforcement can prevent pavement notch failures and the subsequent approach settlement.

Timber Bridges — Restoration or Preservation?

Selecting the best practices for preserving any single historic covered bridge can easily evolve into an intense focus on a narrow range of alternatives, each of which poses some undesirable result, says Professor Robert McCullough, University of Vermont,  Burlington.

Vermont’s covered bridge preservation plan, for example, begins with two principal goals: first, to continue using covered bridges as part of the state’s network of highways; and second, to preserve historic integrity (both structural and material) to the maximum extent possible.

Those who devised this plan clearly understood that for many bridges, service to one goal will often compromise the other. Concerns about preserving the Mill of Spring Road Bridge in Tunbridge, built in 1883, were justified when it was destroyed by ice in 1999.

Reprinted from Better Roads Magazine
August 2005

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