Development and Implementation of a Performance-Related Specification for I-65 Tennessee: Final Report
Chapter 6. Summary and Recommendations
This trial implementation of a PRS on I-65 in Nashville, Tennessee, was sponsored by the FHWA in full cooperation and assistance by TDOT. The trial implementation has provided TDOT and the contracting industry with an understanding of the PRS development and implementation process and results achieved. It also has provided useful information for developing future PRS projects by TDOT and other agencies.
The researchers, the FHWA research contract manager, and TDOT staff made significant efforts in advance to develop a practical and effective PRS. Valuable input from the contractor staff was also received. Three AQCs were selected for consideration in the PRS: PCC strength, slab thickness, and smoothness (or PI). Acceptance levels that were selected for these characteristics are shown in table 2. Inputs listed in chapter 3 were used to develop pay factor curves using the PaveSpec 3.0 software available from the FHWA. These pay factor curves were based on economic justification, not opinion as to the impact of changes in AQCs on a project. A detailed but practical plan for field sampling and testing was prepared. The PRS is included in appendix A.
The I-65 PCC paving used to test the PRS was completed between May and October in 2004. Time limitations required that the PRS be applied first to the northbound lanes as a shadow specification, requiring field sampling according to both standard TDOT and PRS formats. The PRS was to be applied formally to the southbound lane paving, but factors unrelated to the PRS precluded this opportunity. The southbound lanes were then constructed according to TDOT specifications, but the strength, thickness, and smoothness data were taken such that they could be readily converted to the PRS sublots and sampling methods. The results of 14 lots were obtained from the northbound and southbound paved lanes, and these data were analyzed using the PRS procedure. Pay factors were determined for all lots and summarized in tables and graphs.
The average pay factor was 106 percent for the project, which indicates that the contractor exceeded the target quality significantly. To determine the impact of this on performance independently of the PaveSpec models, the new Mechanistic–Empirical Design Guide program was used as developed under NCHRP 1-37A.(10) This procedure predicts joint faulting, slab cracking, and IRI over time for a given set of inputs for JPCP. All inputs associated with the target JPCP (target strength, thickness, and smoothness) were used to predict the life of the pavement. Then all inputs associated with the as-built (average as-built strength, thickness, and smoothness) were input and used to predict the life of the pavement. The as-built JPCP showed a 14 percent longer life (due to the higher quality AQCs) than the target JPCP. Thus, for the additional 6 percent invested in incentives at construction, the pavement life is expected to increase approximately 14 percent, which is a significant benefit.
At a meeting held after construction of the north- and southbound lanes, the results from the PRS were presented and discussed. Many interesting comments were received from the contractors, the QC representatives, and the TDOT staff involved. Comments (provided in chapter 5) indicated that all three groups were very supportive of the PRS approach. A few representative comments from each group are provided below:
- PRS “rewards contractor for exceeding 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.”
- Tennessee DOT:
- “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 any more complicated than current specifications.”
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.
The trial PRS worked very well on this major I-65 project and all parties appear to be supportive of constructing future projects fully under PRS. Some key recommendations are provided as follows:
- Carefully define lots and sublots (extremely important).
- Must be very carefully defined to meet the technical requirements of the PRS. This includes clear definition of the sublots and the sampling of all AQCs from each sublot, which are then used to compute the means and standard deviations for the lots and finally the cost pay factor.
- Must also allow for flexibility of unusual situations in the field, such as partial sublots and lots.
- The definitions of lots and sublots developed for I-65 appeared to meet both technical requirements and be practical in the field.
- Carefully select target means and standard deviations of AQCs.
- Carefully consider these selections so that the level of quality for the project is as desired by the owning agency at the 100 percent pay level.
- Determine if the agency wishes to increase the quality level, decrease the quality level, or specify a quality level similar to previous contracts that performed well. Given the typical incentive level provided by the economic analysis, the level of quality will likely increase over that of previous projects.
- Carefully consider impacts of pay factor curves derived using PaveSpec on the highway agency and the contractor.
- The incentives and disincentives must be sufficient to cause the contractor to take actions to consider appropriate AQC targets, but not too large to cause management and political concerns. (Comments indicated that 10 percent was the maximum needed, and if this is exceeded, a change in the specified requirements should be considered.)
- Limits must be placed on each AQC above which no further incentive is paid (MQL) and below which the lot acceptance is decided through other means than pay reduction (RQL). These limits are absolutely essential to avoid problems.
- A small percentage of user cost is needed in the PRS. The level used to develop the PRS pay factor curves was 2 percent. If this is not included, the smoothness curve can be very flat when a conservative JPCP design is used.
- Some practical adjustment may be needed in some of the theoretical, economic-based, pay factor curves to meet the desires of the highway agency.
- Consider tightening subgrade and subbase grade requirements, and encourage contractors to better control and monitor these elevations and profiles. The southbound lanes, paved with stringline, achieved much smoother pavement than the northbound paving, which used the existing pavement as a guide. (5.65 in./mi (88.45 mm/km) versus 3.6 in./mi (56.80 mm/km), or 36 percent reduction using a stringline).
- Provide a methodology to measure PRS pay factor results quickly. More rapid and nondestructive testing for slab thickness would be one solution. Note that Tennessee had done some significant research into using the impact echo procedure for slab thickness, and this procedure was further tested on this project. (However, it was not used in the official measurements of thickness due to problems in identifying the boundary with the permeable asphalt base.) Another technique is used successfully in Wisconsin: a thin metal circular plate is placed on the top of the base, and a probe is inserted into the plastic concrete to determine thickness.
- Adjust smoothness sampling lengths or modify smoothness data analysis method to easily report PI for short lengths.
- Consider methods for increasing the sampling rate and reducing the amount of destructive testing such as the coring for slab thickness measurement.
BENEFITS OF PERFORMANCE-RELATED SPECIFICATION
The clear and rational approach of PRS, with well-defined quality levels that are understandable to the contractor, are expected to lead to significantly improved highway construction quality, improved pavement performance, and a reduction in LCC. The full possibility of PRS may also offer the opportunity to optimize the design and construction process to provide acceptable performance for lower LCCs. Key benefits of PRS are listed below, some of which were demonstrated on this I-65 project:
- Better linkage between design and construction. The very conservative design of I-65 was evident in a relatively flat pay factor curve for thickness.
- Higher quality pavements (through incentives). The overall pay factor was 106 percent, which indicates a significantly higher quality level of construction. The true effect of lower variability (all AQCs had lower standard deviations than the target) may also have benefits that are not known at this time.
- Testing that focuses on key quality characteristics that relate to the pavement's long-term performance. Any factor that is measured and paid by incentive will receive a lot of attention and focus on the project. Other AQCs such as dowel alignment, tie bar alignment, and consolidation around dowels, would add to the comprehensiveness of a PRS project and avoid a disastrous situation where something (such as tie bar location) is not measured until well into the project only to discover that it is out of specifications.
- Incentives and disincentives that are justified through reduction or increase in future LCC. The PaveSpec program provided reasonable pay factors for I-65. An independent estimate of increased life of approximately 14 percent represents a very significant benefit to highway users.
- Specifications that give the contractors more responsibility and flexibility yet increased accountability may benefit both the contractor and owner. Additional full PRS projects are needed to prove this possibility.
- Allow contractors to be more innovative and more competitive.
- Both the contractor and State staff felt that PRS may lead to the elimination of less quality oriented contractors.
PRS may provide a lower “fear factor” for contractors and less administrative complexity and work over the long term for the agency than warrantee specifications.