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Development and Implementation of a Performance-Related Specification for I-65 Tennessee: Final Report
PURPOSE AND SCOPE OF THE PROJECT
The Tennessee Department of Transportation (TDOT) piloted a performance-related specification (PRS) for portland cement concrete pavement. The trial implementation of this PRS on I-65 in Nashville, Tennessee, was sponsored by the Federal Highway Administration (FHWA). The trial has provided TDOT and the contracting industry with an understanding of the PRS development and implementation processes and the results achieved. The main objective of the PRS is to provide the agency with a methodology to assure that design assumptions are fulfilled, to promote high quality construction, and to protect the agency from poor workmanship. At the same time, the PRS will allow the contractor increased freedom and innovation in deciding how to perform the construction and will provide significant incentives to produce a quality project.
DESCRIPTION OF THE PROJECT
The PRS provides for incentive/disincentive pay to the contractor depending on the level of construction quality achieved in the field, as illustrated in figure ES-1. With PRS, the composite pay adjustment factor for a specific lot of pavement is a calculated value based on the difference between the estimated life-cycle cost (LCC) of the as-designed (target) pavement and the estimated LCC of the as-constructed pavement (lot) as computed by the PaveSpec 3.0 software.
This methodology is defined in the report FHWA-RD-98-155, Guide to Developing Performance-Related Specifications. The FHWA Web site provides additional information about PRS and the PaveSpec 3.0 software (www.tfhrc.gov/pavement/pccp/pavespec/pavespec.htm).
The pay adjustment factor is defined as the percentage of the bid price that the contractor is paid for the construction of a concrete pavement lot and is computed based on the difference between the as-constructed and as-designed LCC. Pay adjustment in these specifications was based on the following key acceptance quality characteristics (AQCs) for the I-65 project:
Other quality characteristics (e.g., consolidation around dowel bar, entrained air content) could have been included if desired. All other acceptance quality characteristics (AQCs) and construction requirements were considered according to TDOT’s existing Standard Specifications. Other aspects of the PRS that were established include the following:
A sample of the results obtained from the construction work for smoothness is shown in figure ES-2. The PI for each lot, plus and minus one standard deviation, is shown. The pay factor associated with each lot is also shown. The southbound exhibited much smoother pavement than the northbound due to use of stringlines, which provided better grade control for the subbase.
Figure ES-3 shows a summary of the PRS pay factors for each of the 14 lots used in the analysis. It also includes an overall pay factor, which averages 106.5 percent for the northbound lots and 105.2 percent for the southbound lots.
The result of using the PRS was that the contractor would receive an average of 106 percent incentive pay for higher quality construction. The following question was posed: will a 6 percent increase in construction cost due to higher quality result in a similar or greater increase in pavement life as well as a lower LCC (on which the pay factor curves are based)? This question was addressed using an independent method to predict pavement life. The NCHRP 1-37A mechanistic–empirical pavement design and analysis software was used to predict the performance of the target (or as-designed) and the as-built lots.
Results showed that the expected life of the target pavement turned out to be in excess of 50 years, which was due in part to the conservatism in the design. The expected life of the as-constructed lots was even longer, by 14 percent, due to the better AQCs. Therefore, for an increase in initial cost of 6 percent (from the positive quality incentives), an even greater percentage increase in pavement life was achieved.
After construction, a meeting was held with the contractors, the quality control (QC) representatives, and the TDOT staff. Independent comments indicated that all three groups supported the PRS approach. A few representative comments from each group are provided below:
BENEFITS OF PERFORMANCE-RELATED SPECIFICATION
This project provides strong support for the concept that a PRS that considers those AQCs that relate directly to performance and are under the control of the contractor is practical and can produce a win–win situation for the contractor and the highway agency. Listed below are key benefits of PRS that were demonstrated on this I-65 project:
The trial PRS worked very well on this major I-65 project, and all parties appeared to be supportive of fully implementing a PRS for future projects. Some key recommendations are provided as follows: