Comparing the Volumetric and Mechanical Properties of Laboratory and Field Specimens of Asphalt Concrete (2016)

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71 This research was intended to quantify the magnitude and cause of the differences of commonly measured parameters of asphalt mixtures among specimen types. This was accom­ plished by evaluating the volumetric and mechanical pro­ perties of three specimen types [design (LL), production (PL), and construction (PF)] from 10 mixtures from various states throughout the country. Variations in the production process were identified and varied throughout the mixtures. Spe­ cifically, variations in the return of baghouse fines, delay in specimen fabrication, aggregate absorption, aggregate hard­ ness, and stockpile moisture content were evaluated for their effects on volumetric properties (AV, VMA, VFA, bulk spe­ cific gravity of the aggregate blend, mixture maximum spe­ cific gravity, AC, and gradation) and mechanical properties (LWT rut depth, axial dynamic modulus, and IDT dynamic modulus) of the three specimen types. Measured differences in volumetric and mechanical prop­ erties were used to develop proposed tolerance values and conversion factors among properties for the three specimen types. In addition, the effects of specimen types on predicted pavement performance were evaluated. Conclusions of this study are discussed in the following sections. 8.1 Effect of Process-Based Factors The research results showed that the effects of the process­ based factors were not as pronounced as originally hypothesized and are only significant between laboratory­mixed specimens (design) and plant­produced specimens (production and con­ struction). The latter finding was expected, because both the production and construction specimens were prepared from plant­produced mixtures, which were subjected to the same process conditions (i.e., plant mixing, baghouse return, and stockpile moisture). Finally, a contractor survey showed that contractors are actively making adjustments based on their experience with the processes in their region. Findings indicated that there were no significant effects of process­based factors on the differences among specimen types for VMA, VFA, Gmm, and Gsb of the mixtures evaluated in this study. Additionally, the process­based factors did not have a significant effect on the differences of mechanical properties among the three specimen types. The lack of the observed effects of process­based factors may result from the variations in the mechanical properties being strongly con­ trolled by compaction effort. Many of the individual mixture comparisons showed that field­compacted specimens (PF) were significantly different from laboratory­compacted spec­ imens (LL and PL). This finding was attributed to differences in compaction effort and confinement conditions between the two compaction processes (laboratory and field). Process­based factors were found to influence the differences among the three specimen types in the following instances: • Stockpile moisture had a significant effect on the difference in air voids between design and production specimens. This may be attributed to aggregates not having sufficient time to dry during production or to improper quantification of stockpile moisture content. • Return of the baghouse fine dust had a significant effect on observed differences in asphalt binder content among design, production, and construction specimens. This finding may warrant the use of baghouse fines during the design of mix­ tures in regions where return of baghouse fines is required. Additionally, aggregate absorption had a significant influence on the difference in asphalt content measured between design and production specimens. • Return of the baghouse fine dust was a significant influ­ ence in the measured difference between the aggregate passing the #200 sieve among the design, production, and construction specimens. This finding may warrant the use of baghouse fines during the design of mixtures in regions where return of baghouse fines is required. In addition, aggregate hardness had a significant effect on the differences in the aggregate passing the #200 sieve among the design, production, and construction specimens. C H A P T E R 8 Summary and Conclusions

72 8.2 Proposed Tolerances Proposed tolerances were developed based on the average difference between specimen comparisons for the 10 mixtures evaluated. Specifying agencies may use these proposed values to evaluate and adjust their current tolerances, as discussed in Chapters 6 and 7. Because these proposed values are based on mixtures from around the United States, agencies may want to use similar procedures to develop regional values. 8.3 Mechanical Conversion Among Specimen Types Conversion factors were developed to enable estimation of the volumetric and mechanical properties of a particular speci­ men type without having to collect additional specimens. In particular, a conversion factor will allow the designer to esti­ mate the mechanical value of the as­built material (i.e., field core) during the laboratory design of the mixture. This may be particularly useful with the implementation of performance­ related specifications. Conversions for LWT, axial dynamic modulus, and IDT dynamic modulus are provided. 8.4 Effect of Specimen Type on Pavement Performance Prediction Results indicate that pavement performance predictions obtained from dynamic moduli measured for different speci­ men types would not be equal without the use of proper cor­ relation factors to account for differences in production and compaction among specimen types.