Development and Implementation of a Performance-Related Specification for I-65 Tennessee: Final Report

Executive Summary

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).

Figure ES-1. Basic concepts of life-cycle-cost–based performance-related specification pay adjustment for a lot.

Click on the image link for a description of the image.

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:

Concrete compressive strength at 28 days.
Slab thickness.
Initial smoothness (or profile index [PI]).

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:

Testing methods were selected for slab thickness, concrete strength, and PI.
Lots and sublots were defined, and a sampling plan established.
Pay adjustment curves were computed for thickness, compressive strength, and PI.

RESULTS ACHIEVED

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-2. Performance-related smoothness specification results for 14 lots.

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.

Figure ES-3. Summary of performance-related specification pay factor results.

FINDINGS

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:

Contractor:

PRS “rewards contractor for exceeding quality of product requested.” “Incentive promotes quality control.”
More accurate quality measurements can be achieved because PRS “relates actual product back to anticipated [design] product.”
PRS “promotes quality end product. Promotes payment for actual product received.”
“Need faster answers on test results.”
“Need more tests per sublot.”

TDOT:

“I think it [PRS] leads to elimination of less quality-oriented contractors.”
“I like the direction it [PRS] takes us.”
“I see the contractor giving us a more concentrated effort to increase the quality of the product he produces.”
PRS “allows greater pay for better materials and quality of construction.”
“Ultimately it [quality] is up to the contractor and how well they build the road. I think it [PRS] gives the contractor a reason to work harder and do better.”

QC representative:

PRS “would most likely reduce variability, thus increasing quality.”
PRS can provide more accurate quality measurements because “with reduced variability, actual test results are more realistic of actual pavement.”
“From testing and inspection viewpoint, don’t think a PRS is any more complicated than current specifications.”

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:

Better linkage between design and construction.
Higher quality pavements (through incentives) and longer pavement life.
Testing that focuses on key quality characteristics that relate to the pavement long-term performance.
Incentives and disincentives that are justified through reduction or increase in future LCC.
Specifications that give the contractors more responsibility and flexibility yet increased accountability, for the potential benefit of both the contractor and owner.
An environment that allows contractors to be more innovative and more competitive and leads to the success of more quality-oriented contractors.

RECOMMENDATIONS

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:

Develop practical definitions of lots and sublots (extremely important).
Select target means and standard deviations of AQCs to reflect reasonable quality.
Consider the impacts of pay factor curves derived using PaveSpec on the highway agency and the contractor.
Consider tightening subgrade and subbase grade requirements and encouraging contractors to better control and monitor these elevations and profiles.
Provide a methodology to measure PRS pay factor results quickly.
Consider methods for increasing the sampling rate and reducing the amount of destructive testing such as coring for slab thickness measurement.

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Updated: 04/07/2011