EVALUATION OF NEW PCC MATURITY TECHNOLOGY

The objective of the project was to evaluate the ability of the new maturity technology to predict compressive strength development of Tennessee portland cement concrete (PCC) pavements. Two field investigations (at I-65 Nashville and I-75 Chattanooga) were conducted to determine the accuracy of the maturity compressive strength predictions. At each location, two maturity-compressive strength correlations were generated using actual field mixtures, and twelve verifications were conducted using a maturity logger embedded in the pavement. For each verification, several types of PCC strength were measured at different times and compared to maturity predictions. The average maturity predicted compressive strengths for 4x8 cores and 6x12 field-cured cylinders (the best measures of in-place PCC pavement strength) were in the range of 91.75% to 100.48% of the average measured values for I-65 Nashville and in the range of 89.54% to 112.10% of the average measured values for I-75 Chattanooga. Maturity prediction accuracy was found to be sensitive to PCC batch-to-batch variability. However, the Modified AASHTO T 276-97 Method is an effective protection from opening PCC pavement to traffic with sub-standard compressive strength. The new maturity method is capable of providing more relevant information on PCC curing progress than lab-cured PCC cylinders. The information can be provided more frequently and more conveniently compared to lab-cured or field-cured PCC cylinders. Lab-cured cylinders represent potential compressive strength at standard curing temperature, not the compressive strength of the PCC pavement which has experienced different curing conditions. Further, the new maturity technology appears to be robust and reliable enough for field use by the Tennessee Department of Transportation (TDOT). A laboratory experiment, using 120 (4 groups of 30 each) 6x12 cylinders cured at different temperatures, was also used to validate the maturity concept. The difference between compressive strengths of 6x12 cylinders lab-cured at the same maturity index for curing temperatures between 7 and 32 deg C (45 and 90 deg F) is in the range of 3.8% to 12.5% for maturity indices of 2400 deg C-hours or more. At lower maturity indices the compressive strength difference in percent was much greater. The research team recommends that TDOT consider using the new maturity technology experimentally on large projects (projects requiring more than 30 batches of the same PCC mixture design). Modified versions of AASHTO Test Methods TP 52-95 and T 276-97 (for use with the new maturity technology) are included in the appendices for guidance. Finally, in situations where the new maturity technology is not appropriate, the research team recommends that 6x12 field-cured cylinders be used to determine when to open a new PCC pavement to traffic.