770.7 Maintenance of Bridge Superstructure
The bridge superstructure consists of that portion of the bridge above the bearings. Maintenance consists of repairs to the deck, handrail, curbs, floor system and structural members. Maintenance of structural members is normally minor unless members are damaged by collision or fire. Major damage to structural members which may affect the load carrying capacity must be reported immediately to Central Office Bridge Maintenance. If any doubt exists as to safety, the bridge should be closed until an inspection can be made and the damages determined.
Girder ends under open expansion joints of all steel bridges should be flushed regularly to remove snow and ice control chemicals, dirt and debris. Truss members in the splash zone and lower chords should also be regularly flushed. Bridges with ASTM A709 weathering steel do not retain the protective oxide layer in the presence of snow and ice control chemicals. Therefore, it is important to regularly clean and flush these bridges under open joints.
|Rebar, Stainless Steel|
|Bridge Deck Condition, Ground Penetrating Radar|
|Bridge Deck, Replacement, Precast RC|
|See also: Research Publications|
770.7.1 General Bridge Deck Maintenance
The maintenance of bridge decks must be timely and performed to a high standard to prevent further deterioration and to protect the investment. Repair of holes or deterioration must be done immediately and not delayed for approval at the time of the annual bridge inspection. Bridge decks that present unusual problems may require contract repair.
Bridge deck maintenance is generally in the following categories:
1. Cleaning and flushing dirt, debris, and snow and ice control chemicals which retain moisture and accelerate deck deterioration.
2. Surface and crack sealers, chip seals and other wearing surfaces to minimize deck wear and assist in preventing further deterioration or possible damage from ice control chemicals.
|Penetradar. They use ground penetrating radar to evaluate pavement, as in this image from their website. It also efficiently analyzes bridge decks similar to a giant CAT scan or MRI. Information gathered helps to identify problems within the bridge before a single cut is made in the pavement. This helps the repair contractor know what is to be faced before opening the surface. Fewer surprises make for less stressful, more profitable work. It also helps MoDOT. Fewer surprises make for fewer last-minute (often expensive) fixes and keep work zone time to a minimum. Perhaps most of all, it helps motorists. When contractors know what they will face, they can work and get out of drivers' way more quickly.|
3. Bituminous mat overlays are used to level uneven decks. This work may be performed by district routine maintenance crews or by contract. Since the overlay is adding dead load to the structure, the bridge must be evaluated and approved for the overlay by the Bridge Division. Repairs to these mats must be made with materials similar to the original material, that is, asphaltic concrete mix must be used to patch or repair asphaltic concrete mats except for temporary repairs in an emergency. A seal coat or double seal coat to waterproof the deck should be placed before placing a mat overlay.
4. Resurfacing with concrete or replacement or patching of concrete floors is performed by district special crews or by contract. Asphaltic materials should not be used for making permanent repairs in concrete decks.
5. Repair or replacement of timber decks may be performed by district special or regional bridge maintenance crews.
6. Epoxies may be used for sealing cracks. Accelerated concrete, epoxy mortar and micro-cements may be used for patching holes or spalled areas. This work should be performed by district special or regional bridge maintenance crews trained in the use of these materials in accordance to the manufacturers recommendations.
770.7.2 Concrete Deck Maintenance
Concrete deck repairs range from crack sealing to full depth repairs. Emergency partial depth repairs may temporarily be made with bituminous materials. Permanent repairs should be scheduled as soon as possible. Full depth failure should be repaired immediately. In an extreme emergency, where traffic or weather condition make it impossible to do this, the short term use of steel plates is acceptable. For steel plate and back filled work, review EPG 616.6.46 Steel Plate Ahead Signs and Typical Applications 616.8.47a and 616.8.47b.
Failures less than 1 in. deep are called spalls. Failures over 1 in. deep but that do not go through the deck are classed as partial depth.
Partial and full depth repairs are made with concrete, or approved accelerated mortars.
Concrete repairs of full or partial depth failures may be made with high early strength cement or approved quick setting cement concrete, such as duracrete.
Spalls are repaired using partial depth methods or they may be filled with approved polymers or latex modified asphalt emulsion mixes.
Regardless of the permanent patching material to be used, the procedures are basically the same. The area around the failure should be sounded to determine the limits of the failure. The area should be marked and sawed with a concrete saw. Care must be taken not to cut the reinforcing steel. On a deck with an asphaltic overlay, it should be removed wider than the deck area to be patched. The patch can then be finished smooth and flush with the deck and the overlay properly compacted.
Void tubes of voided slab bridges which are opened during concrete removal shall be formed to prevent filling the void tube with patching material. The Central Office bridge maintenance may be contacted for assistance in forming methods.
The total deck repairs made to bridges with concrete superstructures (main supporting element for span) such as voided slabs, concrete box girders, concrete girders, etc., shall be limited to 100 sq. ft. per span per day. This shall also apply to culverts. This limit does not apply to bridges with steel superstructures. If extensive repairs are required, a special repair plan should be requested from Central Office bridge maintenance.
A 65 pound class breaking hammer is to be used to remove the broken and deteriorated concrete to a depth below the top reinforcing steel. Loose concrete and pack rust shall be cleaned from exposed steel. All loose material is to be removed from the hole and fine particles blown out with compressed air. Additional reinforcing steel may be added where needed.
The patching material is mixed, placed and finished according to accepted procedures or manufacturer's recommendations.
Care must be taken to assure the patch has gained sufficient strength before traffic is allowed on it. Pavon should be used to seal patch edges. The asphalt overlay is replaced after the repaired deck is primed and waterproofed with liquid asphalt. SS-1, CSS-1, EA90P, CRS-2P, or pavon is recommended for this waterproofing.
Full depth repairs in the decks of precast concrete slab spans shall be made in the same manner as the decks of other span types. Only the deck is to be repaired. The deck shall be that concrete above the plane of the bottom of the slab. The repair shall extend the full width of the panel over both stems as shown below.
If slab reinforcement in precast concrete slab panels must be replaced, use #3 or #4 bars at the same spacing as existing reinforcement. If it is necessary to splice longitudinal slab reinforcement, use a 12 in. lap splice.
Should the stems of precast concrete slab panels have shear cracks at the ends of panels, as shown above, Central Office Bridge Maintenance should be contacted before repairs are attempted.
770.7.3 Seal Coat Resurfacing
If the condition of the deck is such that deterioration is not advanced and the deck is not uneven, an asphaltic seal coat or double seal coat may be applied. See Chip Sealing Deck for additional information. Bridge deck seal coats should be performed during the summer months and be completed by September 1.
The deck should be repaired prior to sealing in accordance with the procedures for concrete repairs.
The entire deck should be cleaned by sweeping or by cleaning and flushing thoroughly and allowed to dry completely prior to sealing.
The bituminous material shall be EA90P or CRS-2P liquid asphalt for bridges on all routes. It is permissible to use No-Strip with these asphalts. The asphalt shall be applied to the entire deck at the necessary application rate which is normally 0.3 to 0.4 gallon per square yard. The interiors of curb outlets should be sprayed thoroughly with asphalt and it is not objectionable to apply asphalt to the vertical faces of the curbs. Care must be taken not to spray asphalt on any steel members. All expansion joints should be covered prior to sealing. Tar paper should be used to provide a neat beginning and ending of the seal.
The cover material shall be Iron Mountain Trap Rock, Joplin chat or similar material approved by the Maintenance Division. The gradation shall be: 100% passing 1/2" sieve, 95-100% passing 3/8" sieve, 0-10% passing #4 sieve, and 0-1% passing #200 sieve.
The cover material should be applied immediately after the asphalt and rolled. The application rate should provide a single rock thickness. It is not to be applied in the curb outlet, or for a distance of two feet from the vertical face of the curb.
Traffic is to be maintained at all times. The seal coat area is to be kept barricaded to traffic for a sufficient period to allow proper curing.
Before opening the lane to traffic, all excess cover material should be removed from the deck. A day or two later, it will be necessary to again remove the excess aggregate from the deck, chords and substructure caps. Any excess oil (bleeding) should be blotted with sand or cinders immediately.
If the approaches have been resurfaced with mat, patched or are in a deteriorated condition, they should be sealed at the same time as the bridge deck to present a good appearance. Concrete or mat approaches should also be sealed at the same time when doing so will reduce traffic impact to the bridge.
770.7.4 Bituminous Mat Resurfacing
If the bridge deck is uneven or the deterioration is advanced, a bituminous mat may be specified to temporarily correct the condition. Decks in this condition would normally be candidates for programming. All mat added to bridges must be approved by the Maintenance Division office.
The length of the mat should be the length of the bridge plus an additional length past each end to provide for transition sections to reduce the impact on the bridge.
The deck and the approaches should be repaired and cleaned as outlined under Seal Coat Resurfacing.
The minimum thickness of the mat is determined by allowing for sufficient cover over the high points in the deck to ensure a satisfactory bond and to prevent raveling. It should also be sufficient thickness to provide a smooth riding surface without overloading the structure.
Expansion devices may be adjusted using the methods shown on standard drawing 712.40.
The deck shall be coated with a seal coat or double seal coat application as described under Seal Coat Resurfacing prior to asphaltic mat application. The seal chips should extend to the curblines for asphaltic mat overlays.
Type A asphaltic concrete is preferred for use on bridges on heavy traffic routes. It is preferred for use on all bridges, but a Type B mix may be used for bridges on medium or low traffic routes. The maximum aggregate size should be 1/2 in. The mat material should not be too rich or this will cause the mat to shove under traffic.
The asphaltic concrete shall be placed with asphalt pavers. Other mixes may be placed with pavers or motorgraders; however, asphalt pavers are preferred. The mat shall be rolled as soon as possible with special attention being given to the area along the curbs and around the curb outlets as traffic will not assist in rolling this area. Curb outlets shall be cleaned of mat after rolling and before the asphalt has set. It is extremely important to obtain a smooth riding surface and one which will drain properly.
Any material deposited on the chords or substructure caps should be removed before the operation is completed.
770.7.5 Timber Decks
Some bridges in the highway system that have timber decks as the supporting members have not been designed to support a concrete deck. The type most commonly found are laminated timber decks which consist of treated rough sawn 2 in. x 4 in. lumber placed transversely and on edge.
When placing a 2 in. x 4 in. laminated timber deck, a 4 in. x 12 in. is bolted to the I-beam stringers at each end of the bridge to provide a solid header. A 2 in. x 4 in. is spiked to the header and each board is spiked to the preceding board with 40d spikes. To secure the flooring to the stringers, a floor clip is fitted over the top flange of each stringer approximately every foot (12 in.) and spiked to the flooring.
Laminated timber decks shall be surfaced with approximately 1 in. of bituminous mat wearing surface to smooth out the unevenness of the rough sawn flooring and to provide a crown in the roadway for drainage.
This type of decking is flexible and may, after a period of time, work loose. To tighten the decking, the recommended procedure is to drill countersunk holes through the deck adjacent to the stringers and, using 5/8 in. carriage bolts with O.G. washers (special thick washers used to distribute force; typically used in timber bolted connections) under the top flanges of the stringers, draw the deck down tight to the stringer.
770.7.6 Steel Decks
Steel grid and structural plate decks normally require little maintenance. If the floor sections separate from the stringers, they can be repaired by jacking or loading the deck down to the stringers and bolting or re-welding. Central Office bridge maintenance concurrence is required prior to non-emergency repairs to steel grid or structural plate decks.
770.7.7 Expansion Joint Maintenance
Expansion joints are designed to provide for the movement of the structure caused by temperature changes. Some are constructed with added feature of sealing and surface water in order to keep other members below the joint dry.
The useful service life of many expansion joints can be extended by periodic cleaning with compressed air or high pressure water. Normal sweeping and flushing operations will usually not remove the incompressible materials which damage all types of expansion devices.
Many expansion device units can be saved from complete failure if problems are detected early and corrective measures taken immediately. This is especially true for elastomeric units where anchor failures are common. Failure to act quickly could result in extensive damage to the device and to the surrounding bridge deck. Some accelerated mortar materials will not perform well in areas of high stress such as at expansion devices. Central Office bridge maintenance is able to recommend suitable materials for extreme service areas.
Expansion devices occasionally become loose. Loose joints can be detected by observing their sound under traffic and/or observing the absence of dirt in the countersunk areas around the bolts.
Flat plate devices can be repaired by tightening or replacing the bolts, or by cutting slotted holes and welding along the edges of the slots.
Elastomeric expansion joints can be tightened by replacing anchor bolts or broken sections of the anchor blocks. In no case should these anchor blocks be allowed to flop loosely, creating a traffic hazard.
All dirt and debris should be removed from any expansion device or open joint between spans to permit normal expansion movements.
Each Fall, maintenance personnel should check the riding surfaces adjacent to the expansion devices to ensure that they or any bar dams welded to them will not be damaged by snowplows, or vice versa.
It is suggested that a fine mix bituminous material be applied to raise the grade sufficiently to ensure that snowplows will not damage the expansion devices. Care should be used not to create speed bumps.
Tabs may be used to assist the snow plow across the opening of flat plate devices or when bar dams are placed over existing devices. This is useful when the skew of the expansion device is close to the skew of the snow plows. Tabs from 1/4 in. x 2 in. to 1/4 in. x 4 in. and of sufficient length to span the maximum opening should be spaced on 4 to 6 ft. centers. The tabs are to be welded on the approach side of the device only and parallel to traffic. Avoid placing tabs close to the centerline of roadway where oncoming snowplows might remove them. The tabs should be rounded or tapered on the approach side to minimize snow plows catching them.
770.7.8 Handrails, Curbs, Medians and Sidewalks
Damaged or deteriorated handrail should be replaced immediately and a stock of handrail materials should be kept on hand for this purpose.
The high cost of aluminum products and the high theft rate for aluminum necessitates the following repair guidelines for aluminum railing. See the figure below for determining if railing is ornamental or high strength structural railing. Contact Central Office bridge maintenance if additional information is required.
1. Single Ornamental Type Rail
- A. Minor damage - repair for appearance.
- B. Major damage - confirm that curb is 9 in. wide or less, remove remainder of handrail, cut off bolts below surface and fill with epoxy mortar or cut off flush and paint with zinc rich paint to prevent rust staining. If curb is wider than 9 in., repair for appearance and pedestrians.
2. Double or Single High Strength Rail
- A. Minor Damage - repair for appearance.
- B. Major Damage - submit a report immediately with details of damage or theft. Include the width of the curb, height of railing and any other information which might affect the decision to replace or remove the remainder of the handrail. Bridge division and Central Office bridge maintenance will review and compare the standards. No handrail will be removed to create a substandard height.
3. Combination Traffic and Bicycling Railing or Pedestrian Fence
- A. Repair as built
Products such as PBC 516 (from Stahl Specialty Company), Viscotene (from Wynn Oil Company), and Tread Eze (from National Chemsearch Company) may be used when replacing aluminum handrails on bridges. A thin coating of material may be applied to the stainless steel cap screws and the stainless steel fillister head machine screws to prevent locking to the aluminum post or tube.
Deteriorated or damaged curbs, medians and sidewalks should be repaired to acceptable standards.
Various types of bearing plates, neoprene pads, rollers and rockers are presently being used. Regional bridge maintenance crews can reset bearing plates which have slipped out of place by jacking up the girder slightly, removing, cleaning, lubricating with graphite, and returning the plates to their proper position. Under no circumstances are sledge hammers to be used in an attempt to drive the bearing plates back into position. It cannot be done without burring the plates, thereby adding to the work when the job is performed correctly. All bearings are to be kept free of dirt and pack rust in order that they may function properly.
Access plates of roller nests under long spans should be taken off, all dirt and debris removed, the roller nest lubricated and the access plates repositioned by regional bridge maintenance crews when necessary.