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Pavement Preservation Compendium II
by Katie Zimmerman, P.E., and David Peshkin, P.E.
Today's transportation agencies face challenges that are vastly different than those faced by the previous generation. On the one hand, improvements in technology have resulted in dramatic changes in the tools that are available today for construction, design and program development activities. On the other hand, public agency staffing and budgets are being slashed, there is more competition for available resources and there is growing demand for greater accountability for the decisions that are made.
One of the keys to success in this volatile environment lies in being able to make better use of technology so that decisions are made faster, recommendations are backed up with factual information and trade-offs between investment options can be easily evaluated.
Another key to success is to change the way transportation agencies do business. In today's environment, that means increasing the use of low-cost preventive maintenance treatments early in a pavement's life cycle to slow the rate of pavement deterioration and defer the need for more costly rehabilitation actions.
The selection of an appropriate preventive maintenance treatment for a given set of conditions begins with the identification of the treatments to include in the program. Agencies vary in their selection of treatments, but most agencies include the types of treatments listed in Table 1 in their preventive maintenance program. Some agencies expand the definition of preventive maintenance treatments to include cold in-place recycling or hot in-place recycling for flexible pavements, and various patching and full-depth repair strategies for rigid pavements.
Once feasible treatments are identified, then the following considerations can be used to determine the most appropriate treatment for each situation:
A decision matrix for taking these factors into account in selecting the most appropriate preventive maintenance treatment to match specific project attributes is illustrated in FHWA Publication No. FHWA-IR-00-027, Selecting a Preventive Maintenance Treatment for Flexible Pavements (August 2000) and in the "Participant's Workbook" for National Highway Institute (NHI) Course No. 1311058, Pavement Preservation: Selecting Pavements for Preventive Maintenance.
The matrix is fully customizable so that an agency can consider whatever considerations are most important for the specific project being analyzed. These considerations are included in the table as rating factors. A weight is assigned to each rating factor to represent the relative importance of each in selecting the appropriate treatment.
For example, if an agency decides that traffic disruption is the most important consideration, it is assigned the highest weight among all of the rating factors.
The weights must add up to 100. The final step is to score each of the feasible treatments on each rating factor. The product of the scores and weights are summed for each feasible treatment strategy, and the treatment with the highest total score is identified as the most appropriate treatment for the given conditions.
To ensure the timely application of the selected treatment, many transportation agencies have developed pavement preservation guides that provide agency personnel with useful information for selecting the appropriate treatment to address a given set of conditions. Typically, the guides include information on the most commonly observed distress types and their severities, as well as recommendations for preventive maintenance treatments that address the specific distress. For example, Table 2 provides guidance on the types and amount of distress that can be reasonably addressed by a chip seal. In this example, roads with conditions outside the indicated ranges would not be viewed as good chip seal candidates.
An agency's guidelines should be reviewed regularly to determine whether changes in the agency's practices or policies are influencing the types of treatments being considered or the conditions under which they are considered. Since industry is constantly developing new approaches to pavement preservation, new treatments also should be considered for inclusion in the guidelines.
Agencies that have developed guidelines for the use of preventive maintenance treatments usually base them on observations that have been made by engineers and technicians in the field. But how can an agency develop these guidelines if they haven't used the treatments before or if the treatments have only been applied as bandages on failed pavements? In these situations, an agency must rely on information published by other agencies with similar conditions, or seek other sources of information on treatment performance.
One of the most difficult issues to address is the appropriate timing for applying preventive maintenance treatments. In many cases, applying a treatment to a new pavement surface is probably using preventive maintenance too soon, while waiting until the end of the pavement's life to apply a treatment is too late. However, finding the right time - when the treatment is effective and it is not a waste of money - is a significant challenge.
Some agencies have begun monitoring preventive maintenance test sections as one way of determining the effect of treatment timing on performance.
The Arizona Department of Transportation, for example, has constructed test sections with varying surface thickness to represent pavements at different points in a pavement life cycle (the idea is that the thinner roads will deteriorate faster than thicker sections so they represent a pavement later in its life cycle). Preventive maintenance treatments have been applied to these sections and the performance of the treatments is being monitored so that guidelines on treatment timing can be developed.
Nationally, a study conducted under the National Cooperative Highway Research Program (NCHRP) found that there is little guidance available to agencies regarding the optimal time to apply preventive maintenance treatments. Because of the lack of available information on this topic, the study focused on the development of a tool that could be used to identify the window during which the greatest improvement in pavement condition is realized for the lowest cost.
In concept, the methodology for identifying the optimal timing is relatively simple. However, the application of the methodology can be complex without appropriate tools. A spreadsheet tool was developed to assist agencies with analyzing their preventive maintenance-related data. As part of the study it was tested in several state highway agencies.
One of the agencies included in the study was the Kansas Department of Transportation (KDOT). KDOT is in the process of developing prediction models that reflect the influence of preventive maintenance treatments for its pavement management system. Using KDOT's transverse crack models, the NCHRP tool (referred to as OPTime) was used to determine the optimal time to rout and seal cracks on a flexible pavement. The analysis showed that under the conditions modeled, it was most cost-effective to rout and seal cracks when the pavement is 11 years old.
Another example using data from the North Carolina Department of Transportation found that a seal coat was most cost-effective when applied nine years after resurfacing or reconstruction. It is worth noting that these results are very dependent on the inputs used in the analysis and should not be used by other agencies without modification.
Seeking a management position
Fundamental to applying the right treatment at the right time is the ability to forecast when the conditions are appropriate for applying a preventive maintenance treatment and making sure that the resources are available to construct the treatment then. A pavement management system can assist with these analyses as long as the preventive maintenance treatments are included in the treatments being considered.
Pavement management systems have been used for years to help agencies identify and prioritize pavement improvement needs, determine funding levels needed to meet certain pavement condition objectives and to project network conditions under various funding scenarios. In recent years, some agencies have used their pavement management systems to demonstrate the cost-effectiveness of preventive maintenance programs as a way of securing the funding needed to jump-start their preservation programs. As agencies increasingly consider the use of preventive maintenance treatments as a pavement preservation strategy, challenges are emerging associated with the integration of preventive maintenance treatments into a pavement management system. For the most part, these difficulties are arising because the pavement management system was originally structured to focus primarily on rehabilitation and reconstruction activities.
The inclusion of preventive maintenance treatments requires agencies to re-evaluate their pavement management procedures to ensure that the factors that trigger the use of preventive maintenance treatments are incorporated into the pavement condition survey procedures and are available in the database, that the performance of a preventive maintenance treatment can be effectively modeled and that treatments that have been constructed are reflected in the database and in future analyses. These activities are not always as easy as they appear.
For instance, fog seals may be considered to address pavements that have cracking, low friction numbers or oxidation. Most pavement management condition surveys do not include friction or oxidation, so these treatment triggers are not available in the pavement management database to trigger the use of a fog seal.
Other features that might be important to trigger preventive maintenance treatments include fine cracking or the differentiation between sealed and unsealed cracks.
Perhaps more complicated is the modeling of preventive maintenance treatments in a pavement management system. To appropriately match feasible treatments with pavement conditions, the pavement management system must be able to predict the changes in pavement condition with time. This implies that the performance of pavement sections can be differentiated when preventive maintenance treatments are applied so that the benefit associated with the treatment can be calculated. It also requires that rules exist to identify the appropriate performance curve for each pavement section. If an agency's pavement management system includes only asphalt and concrete surface types, for example, it will be difficult for that agency to identify different performance models for each of its preventive maintenance treatments.
Although the integration of preventive maintenance treatments into an agency's pavement management system may not be as simple as it first appears, there are steps that an agency can take to improve its consideration of preventive maintenance treatments in its pavement management system. An agency can start by establishing a single treatment in its decision trees that represents the broad category of preventive maintenance treatments and is triggered on roads that are in good condition. Over time, more sophisticated models can be developed.
Even in the face of increasing constraints, such as smaller budgets, reductions in staff and greater accountability, there are opportunities to improve our stewardship of our transportation infrastructures. More and more highway agencies are using preventive maintenance as a way to face these constraints head-on.
When properly used - that is, when an appropriate treatment is used on a pavement that is in good condition - preventive maintenance helps to extend the life of the pavement structure, delaying the need for more extensive and costly rehabilitation and improving other desirable characteristics, such as safety and smoothness.
There are several factors that are key to successful preventive maintenance programs. These include identifying the mix of treatments that will provide the desired benefits for your pavements and developing guidelines for when those treatments should be used to obtain the greatest benefit. A recently completed NCHRP project on the optimal timing of preventive maintenance treatments offers guidance on these topics. A spreadsheet tool also is available to help in the analysis of treatment timing.
It's also important to remember that pavement management tools and systems can be extremely valuable in the successful implementation of preventive maintenance practices. The integration of preventive maintenance into a working pavement management system can help to identify pavements which are good preventive maintenance candidates and monitor their performance once the treatments have been applied. Pavement management systems also can be used to identify preventive maintenance funding needs and to illustrate cost-effectiveness.
The use of preventive maintenance treatments is an integral part of a cost-effective pavement preservation program. Employing preventive maintenance treatments effectively enables an agency to do more with less, thereby making cost-effective use of the available resources as part of a coordinated program to improve the serviceability of its road network.
Zimmerman is president of Applied Pavement Technology, Champaign, Ill. Peshkin is vice president of AP Tech, Chicago.
Reprinted from Roads & Bridges, June 2004.