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:

• Better predictions of the time when deck repair or rehabilitation will be needed.
  
  
• Better tools to rapidly and accurately quantify the extent and severity of damage to decks without requiring lane closure to support detailed visual surveys or sounding.
  
  
• Better information about which treatments perform the best and the optimum time that each should be applied.

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:

• More effective methods to assess and predict coating system performance under various field conditions.
  
  
• Methods to predict the optimum time for maintenance painting operations so that maintenance is performed before the critical point when the coating system deteriorates beyond repair.
  
  
• Strategies for protection of ends of beams where concentrations of chloride-laden moisture result in a more aggressive environment for corrosion.

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:

• A specific program group actively works to identify scour-susceptible structures and takes preemptive action where possible.
  
  
• Monitoring is in place in a number of locations, with a variety of instrumentation.

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

• Direct, reliable, timely methods of measuring scour.
  
  
• Evaluation of the effectiveness of different scour countermeasures.

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.

• Methods to accurately forecast the remaining life of a bridge.
  
  
• Refined deterioration models.
  
  
• Determination of the lifecycle of a bridge and what repair needs will be encountered during that life.

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

• Value and relative effectiveness of different protective/preservation actions.
  • How much added life results from sealers and overlays?
  • What is the best material for deck patching?
  • How long will bridges with epoxy-coated rebar last?
• Optimum time and/or condition for preservation actions. Examples include the following:
  • Should sealers be applied to new decks?
  • Protection of beam ends.
  • Sealing of joints.
• Accurate data on costs of preservation actions and benefit- cost ratios.
• Balancing of bridge preservation and environmental stewardship.

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:

• Hydraulic.
• Overload.
• Steel Details.
• Collision.
• Concrete Details.
• Seismic.

Best practices for inspection and evaluation of bridges.

• Practical, successful and durable health monitoring technologies.
• Practical, real-time processing of structural health monitoring (SHM) data.
• Tests for proper weathering of weathering steel.
• Data for risk-based inspection frequencies.
• Evaluation of prestressed concrete for early identification of strand corrosion.
• More efficient methods of detecting/monitoring the following:
  • Frozen bearings or excessive movement of bearings.
  • Leaking or failed joints.
  • Fatigue crack initiation and growth.
  • Chloride content and corrosion potential/rate.
  • Concrete deck cracking frequency and severity.
  • Accurate quantities of deck delaminations.
  • Deck condition beneath overlays.
  • Corrosion/breaks in beam prestressing strands.
  • Magnitude and frequency of truck loads, axle weights.
  • Direct, real-time effects of scour.

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.

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:

• Loss of longitudinal post-tensioning over time in precast decks.
• Performance of connection details in prefabricated bridges.
• Distinguishing damage due to live load versus environmental factors in concrete bridges.
• Effect of overloads on bridge durability.
• Durability of repairs done to prestressed concrete girders.
• Fatigue of concrete bridges.
• Increasing truck weights and frequencies.
• Inspection methods (visual and nondestructive evaluation (NDE)/nondestructive testing (NDT)).
• Security.
• Bridge funding.
• Preventative bridge maintenance.
• Vulnerability.
• Deck shear capacity.

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.

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.