Chip seal and microsurfacing are two of the main preventive maintenance treatments used in Ohio for the preservation of asphalt-surfaced pavements.
The primary intent in using these two treatments is to slow pavement deterioration and defer costly rehabilitation. In Ohio, chip seal is a sprayed application of a polymer-modified asphalt binder covered immediately by washed limestone or dolomite aggregate and rolled with a pneumatic-tire roller to seat the aggregate in the binder. Chip seals are used to provide a new wearing surface on low-volume roadways that is intended to eliminate raveling, retard oxidation, reduce the intrusion of water, improve skid resistance and seal cracks. Microsurfacing is a cold-applied paving mixture composed of polymer-modified asphalt emulsion, crushed aggregate, mineral filler, water and a hardening-controlling additive. A traveling pug mill is used to proportion, mix and apply a thin layer of the mixture to the pavement. No rolling is required, and the finished surface can generally be opened to traffic soon after placement. Like a chip seal, microsurfacing can be used as a blanket cover on pavements suffering from loss of skid resistance, oxidation, raveling and surface permeability. In addition, microsurfacing can be used to fill ruts and improve rideability by removing minor surface irregularities.
Of late, many highway agencies, including the Ohio Department of Transportation (ODOT), are increasing their investment in chip seals and microsurfacing as a means of preserving the system and postponing more costly rehabilitation efforts. Underlying this shift in focus is the widely accepted assumption that these efforts are consistently cost-effective. Nationally, it is estimated that a total of some 950 million sq yd of chip seals and about 1 million tons of microsurfacing are placed each year. In fact, despite the widespread use of chip seals and microsurfacing nationally, very little performance monitoring has been performed to quantify their cost-effectiveness on pavements of different levels of distress.
Thorough understanding of how well these treatments are performing is critical to the nature and extent of their continued use in the future. Currently, there is a lack of objective information on fundamental issues such as the expected improvement in pavement condition resulting from the use of chip seal and microsurfacing, the extent to which the treatments slow the deterioration of the pavement and the optimum timing of the treatment. As a result, present guidelines are based on anecdotal observations and experience. This study was initiated to systematically evaluate and quantify the performance and cost-effectiveness of ODOT’s current chip-sealing and microsurfacing practices using the data from completed and in-service projects. The study addressed three basic issues:
1. Treatment effectiveness: How well do chip seals and microsurfacing improve the condition of treated pavements?
2. Extension of pavement service life: To what extent does each of the treatments delay the pavement deterioration process?
3. Influence of treatment time: What is the optimal time or pavement condition when the treatment can be most effectively applied?
This study resulted in a critical review and comprehensive understanding of the chip-seal and microsurfacing program in Ohio and provided the basic data needed to determine when and where such preventive-maintenance treatments are appropriate from the standpoint of both economics and performance.
In the 200s
A total of 225 chip-seal and 214 microsurfacing treatments were applied in Ohio between the years 1999 and 2006. ODOT’s Pavement Management Information System (PMIS) consisted of project location, dates of treatment, Pavement Condition Rating (PCR) (before the treatment and for every year after the treatment), pavement type and functional classification. An experiment was designed to utilize this data to evaluate the effectiveness of the treatments. In addition, control sections with similar attributes were identified and used as “do-nothing” sections. An important aspect of the project was to clearly define the performance indicators that would adequately describe the performance and cost-effectiveness of chip-seal and microsurfacing treatments. After reviewing the data gathered, the following performance indicators were derived:
1. Service life of treatments based on actual number of years in service
The data for this task was obtained directly from ODOT’s PMIS. A pavement section is deemed to have completed its service life when a maintenance and/or rehabilitation activity is reported in the database following the treatment installation. The service life of chip-seal and microsurfacing treatments were calculated as the time from the period of treatment installation until the time another activity was reported in the database.
2. Average performance gain
Performance gain for each year was calculated as the difference in PCR between the treated and control sections. The performance gain varies with time, with maximum gain achieved soon after the treatment and the difference in PCR becoming narrower with time. Performance data was most often available for the treated sections for three to five years. As a result, a minimum of three and up to five years of data was used to calculate the average performance gain.
3. Service life of treatments using performance models
As a precursor to the analysis, PCR groups were created on a five-point scale beginning from 51, such as 51-55, 56-60 and so on. All the treated pavement sections included in the study were placed in one of these groups based on their prior PCR values. Performance-prediction models were developed for each group of treated pavements with PCR as a function of age. Various types of models, namely linear and nonlinear, were attempted. It was determined in most instances that the linear models either provided the best fit or another shape was marginally better. For the sake of uniformity, only the linear models are presented. The number of years required for any group of pavements to reach a threshold PCR value was read from the graph and this value was reported as the life of the treated pavements. The threshold PCR values used in the analysis are 60 for general-system and 65 for priority-system roads. It should again be recognized here that all chip-seal installations were made on general-system roads while microsurfacing installations were made on both general- and priority-system roads.
Cost-effectiveness was a method of comparing the relative efficiency (in monetary terms) of two or more alternatives, which allows the decision maker to consider whether one preventive-maintenance treatment is better than the other. Such comparisons are made between two competing materials to determine the relative cost-effectiveness. In 2008, ODOT investigated the effectiveness of thin overlays as a cost-effective maintenance alternative. In using the historical data, the study concluded that thin overlays provide cost-effective maintenance solutions.
In order to determine the cost-effectiveness of chip-seal and microsurfacing treatments, the cost-effectiveness of these treatments were compared with that of thin AC overlays. To do this, it was necessary to generate benefit-cost ratios of chip-seal and microsurfacing treatments. The area of the performance curve was calculated and reported as the benefit. Cost of chip seal and microsurfacing was obtained from construction records. Benefit cost was computed for various groups of pavements, depending on the prior PCR values. In the next step, the ratio of the above two ratios, i.e., benefit-cost ratio of chip seal divided by the benefit-cost ratio of thin AC overlay, was determined. Terming this ratio Relative Benefit Ratio, it is expressed as: If the ratio is greater than 1.0, it can be deduced that chip-seal or microsurfacing treatments provide more cost-effective performance than thin AC overlays, otherwise thin AC overlays would be a more cost-effective treatment.
5. Life-cycle costs
Life-cycle cost analysis entailed an analysis period and selection of various possible maintenance and rehabilitation scenarios during that period. As shown, three different scenarios were considered:
Three successive chip-seal treatments;
Chip seal followed by thin overlay; and
Two successive treatments of thin AC overlay.
For microsurfacing jobs, similar scenarios were used. However, the service life of the treatments varied. Primary data used for this analysis was the life of treatments, cost of treatments and discount rate. Regardless of prior pavement condition, all the data was combined and one performance model was developed for each treatment type, namely chip seal, microsurfacing—general system and microsurfacing—priority system. Because of a wide variation in pre-existing pavement conditions, the resulting models showed poor correlation. However, one use of these models was to estimate life of treatments on an average for use in life-cycle analysis.
Ideally, all performance indicators should point to the same prior PCR range. However, the results show differences:
Service life from historical data, for example, is a derivative of the actual practice. In other words, it does not include analysis of variation in pavement condition that existed among projects prior to the installation of each treatment. Pavement conditions in such cases do not relate to a standard frame of reference, threshold PCR for example. However, it reflects the current practices and provides data about the nature and extent of modifications needed to the existing program.
Average performance gain utilizes annual PCR data for individual projects. Although it can provide a rational procedure to judge the effectiveness of treatments, it does not include cost. However, the results can be used to understand the optimal timing of the treatment for maximum effectiveness.
Service-life predictions from models are obtained through the development of performance-prediction models. Service life is measured for a given threshold, and the procedure provides a rational basis to compare various scenarios. This procedure evaluates the effectiveness with respect to “do-nothing” treatment and as such inhibits cost calculations. The advantage of the method is in generating service-life extensions for various pre-existing conditions of pavements.
Cost-effectiveness is somewhat an extension of the life-extension method. Here the performance characteristics of chip-seal and microsurfacing treatments are compared with another treatment whose effectiveness is known beforehand. This method allows the computation of cost-effectiveness of maintenance treatments.
Life-cycle cost analysis combines the attributes of methods 3 and 4. However, the drawback of this method is that the use of multiple treatments during the analysis period is rarely verified and validated in the field. The method also assumes certain conditions of pavements at the end of each treatment cycle, a fact that is highly uncertain in reality.
The foregoing discussion is to suggest that direct comparison of the results should not be made among the five performance indicators derived in this study.
In summary, this study resulted in a critical review and comprehensive understanding of the chip-seal and microsurfacing program in Ohio and provided the basic data needed to determine when and where such preventive-maintenance treatments are appropriate from the standpoint of both economics and performance. The results of this study, in association with similar studies to evaluate preventive-maintenance activities, will enable ODOT staff to better determine what role chip-seal and microsurfacing treatments should play in the overall preventive-maintenance program.
Using the results from this study, field reviews were made by ODOT staff experts, including the project evaluation team. The intent of these field reviews was to inspect pavement sections within the PCR bands studied by the research to determine the best implementation strategy for ODOT’s pavement rehabilitation logic for the PMS. Once this review was made, ODOT consensus was to implement the optimal PCR range of 65 to 80 for both chip sealing and microsurfacing.
It is recommended that ODOT continue with its chip-sealing program on the general system. Care should be exercised to select appropriate candidate pavements for the treatment to ensure maximum performance and benefit. This study did not address materials and mix-design issues. More research is recommended in these areas for improved product and placement and thereby performance. ODOT also may conduct in-house research to verify and validate the performance of successive chip-seal treatments.
Microsurfacing also is a viable preventive-maintenance option. It can provide the same general benefits that chip seals offer, but at a relatively higher cost. The added benefits of using microsurfacing, e.g., rut filling, may offset the additional cost. Because of the marginal benefits observed in the study, it is recommended that ODOT further review the microsurfacing program to enable the department to determine what role such treatments should play in the overall maintenance program. AT