Transportation agencies and industry are continually looking for ways to innovate to improve performance. The quality of highway construction translates directly to performance. The better the quality, the better the performance and greater the life of the infrastructure product. Performance related specifications (PRS) provide a mechanism for agencies to incentivize good quality and recover loss of service life by dis-incentivizing marginal quality in a rational and defensible manner, while also allowing flexibility and innovation. PRS provide a bridge between design, construction quality, and long-term performance and thus, a direct linkage between design expectations and construction quality.
A statistically sound quality assurance (QA) approach requires understanding of the risks to the buyer and seller. This approach ensures owner agencies are accounting for the interplay and risks associated with specification limits, acceptable/rejectable quality levels, sample/lot sizes, and payment provisions to incentivize and reward best practices and performance. It further discourages poor quality materials, design, production, and construction practices. Historically, efforts have focused on QA and risks in the context of percent within limits (PWL) specifications with less emphasis on the ultimate long-term performance of the accepted and purchased pavement.
PRS are improved QA specifications, where the quality characteristics used for acceptance are related to fundamental engineering properties and to the performance of the in-place product through performance prediction modeling. Their use allows performance, quality, and costs to be uniquely connected in a structured way in order to analyze tradeoffs and how they impact pavement life. PRS represents a progression toward increased use of properties that are more indicative of how the finished infrastructure product will perform over time. In the long run, this will result in flexibility to optimize construction practices to meet performance expectations, increased flexibility for innovation, provide improvements to pavement life, and thus reduce long-term costs to the agency and traveling public.
The Federal Highway Administration (FHWA) has developed mechanistically based models to predict the cracking and rutting performance of asphalt mixtures. These models form the basis of the asphalt PRS development effort and are being implemented in the PASSFlexTM software program. The predictive cracking and rutting tests used to model performance can also be used as pass/fail or index-based tests in specifications. These fatigue and rutting index parameters are based on cyclic fatigue (AASHTO TP 107-18, PP 99-19, and TP 133-19) and stress sweep rutting (SSR) (AASHTO R 83-17 and TP 134-19) standard methods, respectively. A key component of the PRS effort is the linking of mixture design, construction, acceptance, and payment. PRS provides a total QA system that emphasizes pavement performance with associated risks of accepting poor performing pavements or rejecting well performing pavements. Efforts associated with determining payment provisions have historically been left up to the specifier based on empirical relationships to performance or to controlling variability with much less emphasis on understanding performance and the risks inherent with all elements of a comprehensive QA system and approach.
PRS concepts further highlight and emphasize the need for performance tests (PT), while PT demand continues to rise with the introduction of new materials and increased use of reclaimed materials. PT selection ideally should be based on the tests’ ability to integrate mixture design with pavement performance prediction, construction, and acceptance. PT that are more directly related to performance, even when used as a pass/fail screening test, can also provide added value and use for performance modeling to compare what was designed to what was constructed. It is important to understand PT impacts across design, construction, and acceptance in the quest for longer pavement life. Research is needed to integrate these concepts within PRS efforts; performance prediction capabilities are fundamental to robust and defensible specifications that will ensure long-term pavement performance and reduced life cycle costs.
The objective of this research is to develop guidance to integrate the performance predictive capabilities of the PASSFlexTM software and its suite of tools (FlexPAVETM version 2.0, FlexMATTM, and FlexMIXTM) within a statistically sound QA system in a PRS framework.
The research shall address:
(a) the use of the cyclic fatigue Sapp and SSR allowable traffic for rutting (ATR) index test parameters, index thresholds, and acceptance limits in support of performance engineered mixture design (PEMD) approaches and to facilitate further implementation of the tests and performance predictions,
(b) material selection and mixture design changes that can impact the test results (cyclic fatigue, SSR, and their index parameters) and trends associated with owner agency specified performance thresholds, and
(c) the major elements of a QA system (per TRB E-Circular 235, Glossary of Transportation Construction Quality Assurance Terms ( https://onlinepubs.trb.org/onlinepubs/circulars/ec235.pdf ) and associated buyer/seller and payment risks.
Research in progress.