Category:503 Bridge Approach Slabs

A bridge approach slab is used to provide a smooth and structurally sound transition from the roadway pavement to the bridge. The area between the roadway embankment and the bridge frequently experiences differential settlement. This can be caused by a different degree of compaction than the roadway fill, or perhaps subsidence of compressible original soil layers under the weight of the new roadway fill while the bridge is founded on a more unyielding foundation of solid rock or piling. In order to mitigate possible settlement and excessive loading on the end bent, a 20 ft. long bridge approach slab is constructed to span/fill the area adjacent to both ends of abridge.

All new and replacement bridge construction shall include bridge approach slabs. Bridge redecking, rehabilitation and widening construction will generally not require repairing, replacing or adding new bridge approach slabs.

Standard details of bridge approach slabs are retained in the Bridge Standard Drawings. The appropriate bridge approach slab will be included on the bridge plan sheets prepared by the Bridge Division. The cost for bridge approach slabs is included in the bridge cost estimate. The concrete approach pavement shown on Standard Plan 504.00, which is the transition pavement between the bridge approach slab and the roadway pavement, is considered a roadway item and shall be included in the roadway cost estimate.

There are two primary classes of bridge approach slabs that follow from MoDOT roadway classification: the Bridge Approach Slab (Major Road) and the Bridge Approach Slab (Minor Road). The Bridge Approach Slab (Major Road) is a reinforced concrete slab. The Bridge Approach Slab (Minor Road) is a contractor-option slab consisting of two subclasses of slabs: either an optional lesser reinforced concrete slab, or an optional asphalt slab.

503.1 New and Replacement Bridges on Major Roads

A reinforced concrete Bridge Approach Slab (Major Road), as well as the concrete approach pavement shown on Standard Plan 504.00, shall be used on all major roads. In site-specific circumstances, the project core team may decide a Bridge Approach Slab (Major Road), and concrete approach pavement, is excessive for the projected traffic volume or loading. In those cases, a design exception is required and the Minor Roads guidance in EPG 503.2 Minor Roads shall be followed.

503.2 New and Replacement Bridges on Minor Roads

The Bridge Approach Slab (Minor Road) shall be used on all minor roads. If the project core team concludes that a concrete Bridge Approach Slab (Major Road) is needed on a minor road, perhaps due to heavy traffic volume, a design exception is required and the major roads guidance in EPG 503.1 Major Roads shall be followed.

The contractor will be given the option of constructing either subclass of slab in accordance with the bridge standard drawings. (Note: asphalt is considered a subclass slab since it is similarly associated with the first 20 ft. adjacent to the bridge ends where it is thickened, crowned and placed on a specifically prepared subgrade for improved drainage).

The concrete approach pavement shown on Standard Plan 504.00 is not to be used with the Bridge Approach Slab (Minor Road).

Bridge Approach Slab (Minor Road) Sub-class Requirements

The Bridge Approach Slab (Minor Road) standard drawing will note the asphalt mix type required for the asphalt option. District Design is responsible for specifying the mix type, which will generally match the adjacent roadway design. The mix type should be noted on the Bridge Memorandum.

Guidance for Estimating Roadway Plans Quantities:

If asphalt is anticipated to be used for constructing the Bridge Approach Slab (Minor Road), and the asphalt approach slab is anticipated to be placed continuously with the adjacent roadway asphalt pavement, the asphalt tonnage for the roadway asphalt pavement will need to be computed for only the roadway pavement leading up to the final 20 ft. of roadway adjacent to the bridge ends. The asphalt tonnage used for the bridge approach slabs is included in the cost of the bridge approach slab pay item.

503.3 Other than New and Replacement Bridges

Bridge approach slabs can be repaired, replaced, and added new in situations determined by the project core team as that requiring a prescribed detailed level of construction. Also, other situations that can require a deeper investigative review by the project core team include using and repairing an existing approach where an bridge approach slab does not exist or a widening project where a bridge approach slab may or may not exist.

Slab or pavement adjacent to an existing bridge end that is not specifically part of the original bridge plans shall be considered a roadway item for repairs, pay items and quantities.

New bridge approach slab class, if used, should follow roadway classification.

Guidelines for bridge redecking, rehabilitation and widening projects, and new or replacement bridge approach slab projects are as follows.

• On Evaluating Existing Bridge Approach Slab and Approach Pavement Conditions:

o Determine cause of deterioration or failure of existing approach slab or approach pavement.

o Is a structural (concrete) slab required?

o Bridge approach slab can be added if settlement is an issue.

o Rehabilitation checklist should include information related to condition of an existing approach slab or conditions that would be cause to add a new approach slab like settlement.

o Determine if existing approach slab has been mudjacked in the past, how often and how much.

o 20 or 25 ft. length may be used if adding a new bridge approach slab.

o A 25-ft. bridge approach slab is preferred length if replacing a 25-ft. bridge approach slab. Using same length replacement bridge approach slab will cost more but may save time. Using shorter replacement will require more roadway pavement replacement. Savings may be partially offset due adding new roadway pavement.

o Standard bridge approach slabs can be lengthened to 25 ft. using same reinforcement and performance should be considered the same.

o Using asphalt adjacent to a bridge introduces drainage issues since asphalt is permeable. For bridges without vertical drains, this could be an issue especially if an asphalt approach slab is replacing a concrete approach or concrete approach slab. Installing vertical drains at bridge end bents is expensive and can be considered if it is a solution to an underlying drainage issue.

• On Bridge Redeckings:

o Redecking a bridge should not be the sole reason to add a new bridge approach slab. It is not practical.

o Minimal amount of roadwork is generally normal and this is a good reason to not include a new bridge approach slab. If there is an existing approach slab, then repairing should be the preferred action depending upon the conditions of the existing approach slab.

o Grade change should not be a sole reason to add new approach slab.

o Replacement of partial existing approach pavement is common practice and 6 ft. is a good minimum. See Special Sheet “Pavement Transition at Bridge Ends”.

• Replace reinforced concrete approach pavement with reinforced concrete.
• Pay Item and quantities for this work will be the responsibility of District Design.
• Contact Bridge Office for structural repair strategy including reinforced concrete repair, reusing steel reinforcement, and anchoring. For example, exposing and reusing existing reinforcement or installing resin anchor systems where partial removal of an existing bridge approach slab or approach pavement is necessary to form a new bridge deck or rehabilitate the bridge end bent cap.

• On Bridge Rehabilitations and Widenings:

o Existing bridge approach slabs can be repaired and/or widened. Constructing new bridge approach slab when widening approach pavement should be evaluated for long term serviceability.

• Pay Item and quantities for this work will be the responsibility of District Design.
• Contact Bridge Office for structural repair strategy including reinforced concrete repair, reusing steel reinforcement, and anchoring. For example, exposing and reusing existing reinforcement or installing resin anchor systems where partial removal of an existing bridge approach slab or approach pavement is necessary to form a new bridge deck or rehabilitate the bridge end bent cap.

• On New and Replacement Bridge Approach Slabs:

o Bridge approach slabs can be added new or replaced depending on criteria “On Evaluating Existing Bridge Approach Slab or Approach Pavement Conditions” (above).

o Project core team should agree that a bridge approach slab is added new or to be replaced.

o If a new or replacement bridge approach slab is required, it’s class should be based on roadway classification.

• On Bridge End Drainage:

o Address if there is visible evidence of a drainage problem from a review of in-service performance or if it is anticipated that new work could create drainage problems like widening a structure where total drainage will increase.

• On Design Exceptions (DE):

o DE is required if a new or replacement bridge approach slab class is used different from roadway classification.

o DE is not required if a new BAS is not used.

o DE is not required if a bridge approach slab replacement length is less than a standard 20 feet bridge approach slab to encourage partial replacement if it is the right solution.

o DE is not required to force a BAS (Minor Road) optional slab sub-class on a bridge on a minor road.

503.4 Bridge End Drainage

Bridge end drainage provisions should be considered on every bridge job.

Bridge end drainage provisions can include concrete spill slope protection, concrete slope aprons, drain basins, curbs, and sideslope drainage provisions which includes rock blanket or drain flumes, or a combination of these. (See EPG 748.7 Bridge Deck and Bridge End Drainage and Standard Plans 609.40 for details.)

503.5 Construction Inspection Guidance for Sec 503

Material and Construction Requirements. For material and construction requirements, refer to Sec 503 of the Standard Specifications.

A concrete bridge approach slab is not subject to the Quality Control/Quality Assurance (QC/QA) requirements of Sec 502 of the Standard Specifications. Concrete bridge approach slabs shall be cured and sealed in accordance with Sec 703 of the Standard Specifications for bridge decks.

Requirements for approach slabs constructed with asphalt will be in accordance with the either Sec 401 or Sec 403, as determined by the mix type to be specified on the plans.

When a Bridge Approach Slab (Minor Road) is constructed with asphalt, it is important to remember that the asphalt tonnage is included in the cost of the bridge approach slab pay item. If the asphalt approach slab is laid continuously with the adjacent roadway asphalt pavement, and the roadway asphalt pavement is paid per ton, the inspector will need to compute the quantity (tons) of asphalt that was needed to construct the 20 ft. approach slab and then deduct that amount from the total asphalt tonnage (i.e. ticket total) placed that day.

503.6 Maintenance Activities

See Mud Jacking Bridge Approach for additional information.