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REPORT
This report is an archived publication and may contain dated technical, contact, and link information
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Publication Number:  FHWA-HRT-14-052    Date:  October 2014
Publication Number: FHWA-HRT-14-052
Date: October 2014

 

Long-Term Bridge Performance High Priority Bridge Performance Issues

 

CHAPTER 4. SUMMARY OF FINDINGS

COMMON PERFORMANCE ISSUES THAT NEED RESEARCH

The LTBP Program is to be a long-term program to collect comprehensive information on bridge performance issues that will support improvements to bridge management practice and lead to improvements in bridge performance. However, a clear message is being received from many stakeholders that the program must provide early results and clear benefits in the near term. The issues most often identified by the focus groups and other stakeholders were related to preservation of bridges and performance improvements that would show great promise for service life extension, increase in safety, or reduction of user and State transportation department costs relative to maintenance and operation of the structures. Focusing efforts early on these issues can pay swift dividends and pave a clear path toward longer term gains. The performance issues most frequently cited by the State transportation departments during the focus group meetings are described below. It is recommended that these items become the key topics that the LTBP Program addresses.

Decks

In middle and northern latitudes of the United States, degradation of reinforced concrete bridge decks is a widespread problem. Cracking, spalling, and delaminations were common results requiring maintenance and rehabilitation. Indeed, all of the focus groups, with the exception of Florida Department of Transportation (FDOT), noted that repair and rehabilitation of decks accounts for more than half of the maintenance expenditures on their bridges and represent the highest priority issues for performance. Hence, significant benefits can be gained from better characterization of deck performance and prevention of deterioration of decks. Performance concerns cover both bare (untreated) concrete decks and concrete decks treated with sealers and overlays. These State transportation departments are seeking methods to diagnose problems early, accurately, and with minimal traffic impact. Further, these departments expressed a need for the following:

Joints

Almost hand-in-hand with decks is the problem with joints; some State transportation departments report very limited service life of most available joint systems and difficulty in monitoring joint conditions. Often, joints have completely failed and remained so for a period of time before they can be resealed or replaced. Joint failures can lead to corrosion-induced deterioration of bearings, abutment bridge seats, pier caps, and the ends of beams, especially in regions with significant de-icing operations. A practice that is becoming more common is to design and build bridges that do not have joints in the deck, thereby eliminating the need for joint maintenance to protect bearings, bridge seats, girder ends, etc.

Jointless Bridges

As noted above, when bridge joints fail and potentially corrosive materials are allowed to contact other elements of the bridge, significant corrosion-induced deterioration of other critical elements of the bridge can result. A good practice that can serve as an alternative to open or sealed joints on a bridge is to design and build a bridge without joints in the deck. Some of these jointless bridges are designed to have a fully integral abutment that allows thermally induced changes in the bridge superstructure to be taken up by movement of the abutment. Other jointless bridges are designed with semi-integral abutments, i.e., with integral superstructure/backwall connections that move according to the thermal demands but are independent of the vertical load support system. This practice eliminates the necessity for maintenance of the joint material or assembly and helps prevent corrosion-induced deterioration of bearings, abutment bridge seats, pier caps, and the ends of beams.

Bearings

Types of bridge bearings range from fixed to those that allow rotation and longitudinal movement, such as simple steel rollers and rockers, to unreinforced and reinforced elastomeric pads, to sophisticated high load multirotational bearings. Some are intended to allow expansion and rotation of the girders under live loading and as the ambient temperature changes. Bearing performance can degrade, as corrosion or other forms of deterioration occur, deleterious materials build up at the bearings, unanticipated movements occur, etc. These conditions may lead to unanticipated stresses in superstructure and substructure components. Long-term studies are needed to investigate and evaluate how different types of bearings perform in the field under various conditions and over an extended time. Changes in structural behavior of the bridge that may be caused by bearings not performing as designed may also be investigated.

Coatings for Steel Superstructure Elements

Protective coatings have served as the primary corrosion protection system for steel bridges for many decades. Coating technology has progressed from the lead-based paint era to the current era of environmentally compliant, high-performance coating materials. However, many of the focus groups identified performance of coatings for steel bridges as an area where further improvements would be beneficial. Some needs expressed included the following:

Identification of the Condition of Embedded Prestressing Strands and Post-tensioning Tendons

Bridges that use prestressed concrete girders to support the bridge deck are more commonly used in today’s infrastructure. In the early 1950s, prestressed girder bridges made up roughly 2.5percent of the total bridge inventory, whereas in2007, 40percent of the newly constructed bridges were made with prestressed concrete girders. A critical factor influencing the long-term performance of these types of bridge superstructure systems is the performance of prestressing strands and tendons both in pretensioned (embedded) and post-tensioned (ducted) applications. It is important to verify the condition of prestressing strands to determine whether they are performing well. Visual inspection is the most common approach for inspecting prestressed concrete girders but signs of early corrosion activity and damage to the prestressing are difficult to detect. Long-term field studies are needed to investigate and examine the performance of embedded or ducted prestressing wires and tendons in bridges in various service environments and under various traffic loadings and service conditions. An important element of the studies will be investigating and evaluating methods for early assessment of the condition of pretensioned and post-tensioned strands and tendons in prestressed concrete.

The Bump at the End of the Bridge

A common issue at many bridges occurs when differential settlement at the roadway/bridge interface causes a change in elevation between pavement and bridge. This difference in elevation can be a hazard to driver safety and can cause undue impact loads on the deck and superstructure when heavy truck vehicles cross the bump. Most bridges are designed to have a concrete “approach slab” that is intended to minimize the differential settlement. In many cases, however, the final magnitude of settlement exceeds the capacity of the approach slab, and costly and sometimes repetitive repairs are necessary. Thus many State transportation departments regard the settlement of bridge approach slabs as a substantial maintenance problem.

Scour

Most of the focus groups identified scour as a significant safety concern and ranked it high among substructure performance issues. For example, the California Department of Transportation (Caltrans) takes a proactive approach to scour, including the following:

Critical needs identified by several State transportation departments included the following:

DESIRED OUTCOMES OF THE LTBP PROGRAM

Each State transportation department was asked what it would most desire as an outcome or multiple outcomes from the LTBP Program. The numerous suggestions covered many different aspects of bridge engineering. As an illustration of the different desired outcomes, suggestions included the following:

Better forecasting of bridge conditions and need for actions.

Best practices for preserving bridges and bridge components in satisfactory condition.

An effective risk-based prioritization or ranking process or a better classification system to rank structures in an inventory according to overall risk. Currently, New York State Department of Transportation (NYSDOT) has defined six basic failure modes of vulnerability:

Best practices for inspection and evaluation of bridges.

RECOMMENDATIONS ON LTBP PROGRAM PRIORITIES

The identification of the high priority bridge performance issues to be studied under the LTBP Program has been an ongoing process that began at the earliest stages of the program. As early as February 2008, the LTBP research team had identified a list of high priority research topics related to various aspects of bridge performance. The list was compiled and supplemented with some background information and the general scope of possible research study. The team then proceeded to rate each topic on a scale from 0 to 3 with respect to perceived importance and urgency relative. A rating of 0 meant least important or least urgent, and a rating of 3 meant most important or most urgent. The team discussed each topic and arrived at consensus on the perceived importance and relative urgency of the topic. The two ratings were then added to arrive at an overall rating for each topic that could range from 0 to 6. Table 9 shows the ratings assigned by the LTBP team.

The list—without any ratings—was then shared with a select working group of experts in government, academia, and industry who were thoroughly familiar with bridge design, construction, management, and maintenance practices. The list was also shared with an internal steering group of FHWA bridge experts. Each of the members of both groups was asked to individually rate the topics according to urgency and importance. Group members were also invited to submit comments regarding the topics or scope of the program and to suggest additional topics that might have been addressed in the presented list. Table 9 also presents a summary of ratings of each topic by the external stakeholders and by the FHWA internal steering group. In both of these cases, the ratings for each topic were calculated by averaging the ratings of all the members in each group. Again, the two ratings were then added to arrive at an overall rating for each topic that could range from 0 to 6. Appendix C provides a list of the members of the select working group and FHWA internal steering group.

Table 9 is arranged so that the topics are listed in order, from high to low, by the overall rating from the external stakeholders.

Table 9. Ratings and rankings of proposed study topics by stakeholders.

Proposed Study Topic External Technical
Working Group
LTBP Team FHWA Steering Committee
Imp. Urg. Total Rank Imp. Urg. Total Rank Imp. Urg. Total Rank
Performance of
Untreated Concrete
Bridge Decks
2.9 2.8 5.6 1 (Tie) 3 3 6 1 (Tie) 3.0 3.0 6.0 1
Performance of
Bridge Deck
Treatments
2.8 2.9 5.6 1 (Tie) 3 3 6 1 (Tie) 2.8 2.3 5.2 3
Performance,
Maintenance and
Repair of Bridge
Deck Joints
2.8 2.5 5.3 3 3 3 6 1 (Tie) 2.7 2.0 4.7 4 (Tie)
Performance of
Coatings for Steel
Superstructure
Elements
2.4 2.1 4.5 4 (Tie) 3 2 5 5 (Tie) 2.3 2.0 4.3 6 (Tie)
Performance of
Bare/Coated
Concrete Super-
and Substructures
2.5 2.0 4.5 4 (Tie) 2 1 3 13 (Tie) 2.3 2.0 4.3 6 (Tie)
Performance of
Innovative Bridge
Designs and
Materials
2.3 2.1 4.4 6 1 1 2 17 (Tie) 1.8 1.6 3.4 15
Performance of
Embedded
Prestressing
Wires and
Tendons
2.1 2.1 4.3 7 2 3 5 5 (Tie) 2.3 2.0 4.3 6 (Tie)
Performance of
Bridge Bearings
2.1 2.0 4.1 8 3 2 5 5 (Tie) 2.7 2.0 4.7 4 (Tie)
Performance of
Precast
Reinforced
Concrete Deck
Systems
2.1 1.9 4.0 9 (Tie) 2 1 3 13 (Tie) 2.0 2.0 4.0 13
Performance of
Jointless
Structures
2.1 1.9 4.0 9 (Tie) 2 1 3 13 (Tie) 1.3 1.0 2.3 19 (Tie)
Performance of
Alternative
Reinforcing
Steels
2.1 1.8 3.9 11 2 2 4 9 (Tie) 2.3 2.0 4.3 6 (Tie)
Direct, Reliable,
Timely Methods
to Measure Scour
1.8 1.8 3.5 12 (Tie) 3 3 6 1(Tie) 1.8 1.8 3.5 14
Performance of
Weathering
Steels
1.8 1.8 3.5 12 (Tie) 1 1 2 17 (Tie) 1.7 1.7 3.3 16 (Tie)
Serviceability of
Cracked HPC
Decks
1.9 1.5 3.4 14 2 2 4 9 (Tie) 3.0 2.7 5.7 2
Performance of
Scour
Countermeasures
1.7 1.6 3.3 15 (Tie) 2 1 3 13 (Tie) 2.5 1.8 4.3 6 (Tie)
Risk and
Reliability
Evaluation for
Structural Safety
Performance
1.7 1.6 3.3 15 (Tie) 3 2 5 5 (Tie) 1.7 1.7 3.3 16 (Tie)
Performance of
Prestressed
Concrete Girders
1.6 1.6 3.3 15 (Tie) 2 2 4 9 (Tie) 1.7 1.7 3.3 16 (Tie)
Unknown
Foundation Types
1.6 1.6 3.1 18 - - - 20 2.3 2.0 4.3 6 (Tie)
Performance of
Structure
Foundation Types
1.5 1.5 3.0 19 1 1 2 17 (Tie) 2.3 2.0 4.3 6 (Tie)
Criteria to
Classify
Functional
Performance
1.1 1.1 2.3 20 2 2 4 9 1.3 1.0 2.3 19
Imp. = Important
Urg. = Urgent
HPC = High performance concrete

 

The members of the external review group individually offered additional topics for consideration under the program. These are summarized as bullets, in no particular order, below:

Many of these suggested topics overlap or complement topics within the ranked list, while others may deserve consideration for future implementation within the program. Table 10 presents the comprehensive list of topics after considering all input from stakeholders. This list was derived from a sequence of steps as follows:

  1. Develop preliminary list of performance topics.

  2. Gather input from internal and external stakeholders.

  3. Evaluate the list of potential topics and prepare a brief literature review and background summary for each.

  4. Revise this list based on input derived from the focus group meetings and periodic feedback from discussions with other stakeholders as previously described.

 

Table 10. Long-term bridge performance suggested study topics.

Category LTBP Bridge Performance Topic

Decks

Performance of Untreated Concrete Bridge Decks
Performance of Bridge Deck Treatments
Performance of Precast Reinforced Concrete Deck Systems
Performance of Alternative Reinforcing Steels
Influence of Cracking on the Performance of High-Performance Concrete Decks

Joints

Performance of Bridge Deck Joints
Performance of Jointless Structures
Bearings Performance of Bridge Bearings

Concrete Bridges

Performance of Bare/Coated Concrete Super- and Substructures
Performance of Embedded Prestressing Wires and Tendons
Performance of Prestressed Concrete Girders
Performance of Impact-Damaged Concrete and Prestressed Concrete Beams

Steel Bridges

Performance of Coatings for Steel Superstructure Elements
Performance of Weathering Steels
Performance of Impact-Damaged Steel Beams
New Construction Performance of Innovative Bridge Designs and Materials

Foundations and Scour

Performance of Scour Countermeasures
Performance Issues at the Bridge Approach-Abutment Interface
Performance of Substructure Components
Performance of MSE Walls
Risk Risk and Reliability Evaluation for Structural Safety Performance
Functional Performance of Functionally Obsolete Bridges
MSE = mechanically stabilized earth

 

After discussions among the research team, FHWA, State coordinators, and the TRB LTBP Committee, it was recognized that all topics could not be addressed immediately, and the number of topics to be addressed would likely be constrained by program resources. Therefore, the research team, in conjunction with the FHWA LTBP staff, developed a short list of top priority topics for immediate consideration. Table11 presents the six top priority topics recommended for immediate study in the execution phase of the program. This list was refined based on discussions with the TRB LTBP Committee. As time and resources permit, additional topics may be incorporated into the program.

 

Table 11. Initial study topics for LTBP Program.

Category Issue
Decks Untreated Concrete Bridge Decks
Decks Treated Concrete Bridge Decks
Joints Bridge Deck Joints
Bearings Bridge Bearings
Steel Bridges Coatings for Steel Superstructure Elements
Prestressed Concrete Bridges Detection of Condition of Embedded Pretensioned Strands and Post-Tensioning Tendons

 

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