Evaluation of the causal effect of the surface performance-graded (SPG) specification for chip seal binders on performance

Abstract The Texas Department of Transportation (TxDOT) has implemented a Surface Performance-Graded (SPG) specification for chip seal binders as a special provision available for use in their annual statewide chip seal program. The SPG specification was established to improve chip seal field performance by limiting aggregate loss and bleeding during the critical first year after construction. The objective of this study is to evaluate the effect of the SPG specification on chip seal field performance by reviewing selected highway sections (HSs) from previous TxDOT annual statewide chip seal program built from 2011 to 2014. The review of chip seal binder laboratory characterization and field monitoring results is based on: (a) the finalized SPG specification with validated high and low temperature limits, (b) an updated SPG climate-based map with 6 °C increments for practical consideration, and (c) an improved Surface Condition Index calculation method to eliminate subjectivity from field inspection personnel. The review results are used to compare the SPG criteria with field performance. A causal effect analysis is performed to assess the average treatment effect on HSs from implementing SPG specification for chip seal binders. HSs are categorized into treatment and control groups based on SPG results, and 19 well-matched pairs were selected by utilizing a nearest-neighbor matching algorithm to minimize selection bias. Given the random selection of chip seal HSs, an improvement of field performance on HSs that satisfied the SPG specification is validated by a statistical hypothesis test at 95% confidence level.