Diagnosing Problem Pavements

Preventive maintenance can save money, but first we need to assess pavement problems to see which can benefit from repair and which need replacement.

by Ruth W. Stidger, Editor-in-Chief

Analyzing pavements to determine what to do to keep them in good shape is the first step in any good highway or road maintenance program.

Several new tools can help with the task of analysis. One of the most important is the nonlinear Three-Dimensional-Finite Element Method. It uses computer programs to check current pavement and determine the type of strengthening needed to deal with current and expected traffic loads.

The Mississippi Department of Transportation used 3D-FEM to analyze jointed concrete pavement for dynamic truck loads and for thermal analysis. This let them find the conditions causing rapid deterioration of the agency’s concrete pavement.

Because it considers AASHTO’s pavement design guide (to be replaced this year) outdated, the School of Civil Engineering at Purdue University uses 3D-FEM to analyze various problem pavements, said John H. Anderson, Jr., director, transportation issues, in his earlier report to the Secretary of Transportation.

The method measures strains and deflections from heavy truck traffic, verifying the damage caused by these loads.

The University also used field instrumentation, laboratory testing, and field data collection with subgrade and core sampling to evaluate subdrainage performance and moisture flow through the pavement.

With high moisture conditions, pavement rutting, cracking, and faulting occurred, the study shows. This leads to increased roughness, unsafe conditions, and loss of road serviceability, Anderson says.

Indiana DOT engineers began using 3D-FEM rather than other analysis methods and found that the subdrainage design procedures used on in-place pavement limited moisture outflow. The computer techniques in 3D-FEM recommended use of larger drainage outlet pipes and more efficient filter material to correct the rutting, cracking, and faulting problems discovered.

In Ohio, Battelle researchers and Ohio Transportation Department engineers began using 3D-FEM to address pavement problems, Anderson reports.

Recent 3D-FEM developments

The Federal Aviation Authority has contributed to knowledge of 3D-FEM, using it to analyze pavement problems and to design better-performing pavement.

The computer system can handle greater detail and more complex characterizations of construction materials than can layered elastic analysis, says an FAA report on the method. “It is particularly useful for modeling rigid pavements, since the slab edges that are often critical components in rigid pavements can be modeled — something not possible with LEA. In addition, 3D-FEM can incorporate nonlinear and non-elastic material models not available in LEA.”

The FAA 3D-FEM already has a model for rigid pavements that analyzes slabs, joints, multiple structural layers, and realistic interfaces between layers.

Between now and 2006, the report says, the structural model will be extended to cover flexible as well as rigid pavements and overlays.

Evaluation in Utah

The Utah Department of Transportation maintains a statewide pavement evaluation system that lets them diagnose problem pavements and then decide which state, city, and county roads need to be selected for repair.

A pavement condition report on each road covers:

1. Materials evaluation including subgrade/soil, roadway section, and pavement condition.

2. Expected traffic.

3. Project alternatives.

Identifying distress

Utah DOT engineers begin the pavement condition check by determining road roughness. The pavement’s International Ride Index in inches per mile measures the cumulative movement of the suspension of the quarter-car system divided by the traveled distance. This simulates ride smoothness at 50 miles per hour.

An accompanying table shows the IRI Pavement Condition Scale.

Next, engineers use a visual walk-through of the pavement section. This identifies specific types of distress, severities, and quantities. Engineers classify distress causes as:

1. Load related.

2. Moisture related.

3. Temperature/climate related.

4. Material related.

The Strategic Highway Research Program developed its SHRP Distress Identification Manual, which helps quantify type, severity, and extent of pavement distress. Two accompanying tables show how specific visible distress warnings suggest damage causes on flexible and concrete pavements.

NDT or DT?

Nondestructive testing is used first by UDOT, supplemented with destructive testing when needed.

Deflection surveys form the basis of NDT. These should be conducted at the same time of year each year for best comparison. Coring can verify pavement thickness for analysis.

UDOT uses Falling Weight Deflectomer deflection testing data, which tells engineers the pavement section’s ability to carry projected traffic loads and to determine the pavement overlay needed.

Analysis of pavement sections layer moduli values helps identify pavement layers with structural deficiencies. These could be in bituminous, untreated base, granular, borrow layers, or in the existing subgrade.

Determining distress causes

Distress data provide valuable insight into mechanisms of pavement deterioration. As a first step, the distress can be identified as being either primarily load-associated or primarily climate/material associated, the UDOT report says.

If the distress is primarily load-associated, rehabilitation work should include a structural improvement. If climatic conditions or paving materials are contributing to the deterioration, appropriate measures should be identified to address those deficiencies or to less their impact or effect on pavement performance. If serious climatic or materials problems exist, the best solution may be a total reconstruction of the pavement section.

If pavement performance data is collected on a pavement section at different points in time, then information can be obtained regarding the time that the various distresses began to appear and their relative rates of progression. Such information can be extremely valuable in identifying causes of distress and in programming appropriate rehab actions, for example, determining whether a specific pavement section can wait three years for an overlay or whether it will be too deteriorated.

An overall examination of the data along the project will reveal if there are significantly different areas of pavement conditions along the project, according to the report. For example, a change in subgrade, traffic, or materials may result in a significant change in pavement performance, which will show up in the occurrence of distress. In addition, the inner lanes of multi-lane facilities may exhibit significantly less distress or lower severity levels of distress than the outer lane. By recognizing these trends, rehab designs can be varied along the project and/or across lanes to reduce costs.

The distress data should be summarized in order to provide a clear picture of the existing pavement. Analyze the identified stress and provide a brief explanation of conclusions in report form.

Predict remaining life

The existing pavement condition is an all-inclusive section. In the UDOT system, items to be documented include such topics as location, year constructed, design, available materials and soil properties from reports and surveys, climate, pavement distress history, maintenance, and construction history. Any previous performance data from pavement management studies or other pertinent data should be obtained and included.

A complete traffic evaluation provides information on the estimation of past and current loadings on the structural adequacy of the existing pavement, and on the expected future traffic loadings. The consideration of the future traffic loadings can be an important part of rehabilitation planning and programming and may also influence the ultimate selection of the rehabilitation. The collection of representative traffic data and the correct interpretation and analysis is critical in achieving a proper rehab design, UDOT reports.

Subgrade soils and pavement materials have a major impact on design, construction, structural response, and performance of a pavement. Unstable subgrades present problems in placing and compacting base and subbase materials and in providing adequate support for subsequent paving operations. Without an adequate working platform, critical pavement construction details such as adequate compactive effort may not be accomplished within acceptable tolerances. In many cases, this type of construction deficiency can cause pavement and profile deterioration after exposure to traffic and environment.

Pavement structural responses are also highly dependent on the subgrade support. A larger percentage of the surface deflection of a pavement is a direct result of the support provided by the subgrade.

The desirable properties of a subgrade include adequate shear strength, adequate permeability ease and permanency of compaction, volume stability, and permanency of strength.

Surface and subsurface water is the leading cause of pavement distress. The recognition of the amount, severity, and cause of moisture-related distress plays an important role in the rehab method selection, according to UDOT. Unless moisture-related problems are addressed, effectiveness of the rehab decision will be reduced. A drainage evaluation must be performed in conjunction with the nondestructive testing analysis to identify existing moisture-related problems and to identify the potential for moisture problems to develop within a pavement. This evaluation includes a distress survey and an examination of the external and internal drainage factors that influence the moisture condition in a pavement.

When possible, UDOT reports, identify any material variation on pavement section from construction and maintenance materials testing records. Insight into pavement distress and materials problems can be gained through the analysis of the testing records as well as any records containing field notes concerning construction problems, material variability, and so on. This information can dramatically affect the remaining life calculations for individual pavement sections and should be included as part of an explanation of the cause of distress.

Use NDT and DT to analyze the pavement’s probable deterioration mode to failure.

Use historical traffic data to calculate the remaining structural life of a pavement section.

The year in which a pavement section deteriorates to unacceptable levels can be determined by extrapolating pavement deterioration curves for each section to be rehabilitated.

Charts from Article

Reprinted from Better Roads Magazine
June 2002

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