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

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68 7.1 Effect of Process-Based Factors The following section discusses the process-based factors affecting the differences among specimen types evaluated in this research study. This experiment was designed to evalu- ate the effects of five specific processes (i.e., baghouse fines, reheating, aggregate absorption, aggregate degradation, and aggregate stockpile moisture). Although the results of the study showed that the effects of these processes were not significant for most parameters evaluated, Table 7-1 summa- rizes factors that had a significant effect on those parameters. Federal, state, and local transportation officials may be able to use these findings to determine whether these processes may affect mixtures in their respective regions. • With respect to air voids, the producer should ensure that stockpile moisture content is accounted for. This practice minimizes the magnitude of the difference between pro- duction and design specimens. • Regarding asphalt binder content, if the owner agency requires the return of baghouse fines during production, mixture designs should consider the return of baghouse fines during specimen preparation. • Regarding gradation, the return of baghouse fines, aggre- gate hardness, and stockpile moisture all had a significant effect on the laboratory-produced and plant-produced mixtures. Therefore, design specimens should account for baghouse dust and aggregate breakdown. Process-based factors did not have a significant effect on the VMA, VFA, Gmm, and Gsb of the mixtures evaluated in this study. Process-based factors did not have a significant effect on comparisons between production (PL) specimens and construction (PF) specimens. This is logical because these mixtures were produced through the asphalt plant and, therefore, experienced the same processes (i.e., stockpile moisture, baghouse return, and breakdown from plant mix- ing). Process-based factors did not have a significant effect on differences in mechanical properties among the three speci- men types. 7.2 Volumetric Properties Tolerance Recommendation Table 7-2 presents the tolerance values developed in this study. The proposed tolerances reflect the average difference among specimen comparisons for the ten mixtures. Based on these findings, specifying agencies may be able to evaluate and adjust their current tolerance values. Section 6.1.1 illus- trates how these tolerances may be used to evaluate current specification tolerances. These tolerance values encompass mixtures from around the country. Therefore, development of regional or local values may be appropriate. 7.3 Conversion of Mechanical Properties Among Specimen Types The following section details how agencies can implement the average conversion factors discussed in Section 6.1.3. 7.3.1 Loaded-Wheel Test Conversion Table 7-3 presents proposed LWT conversion factors, which can be used to assess whether an as-built mixture will be expected to meet performance indicators developed with the laboratory design. The conversion factors indicate that the laboratory-compacted specimens typically resulted in 33% less rut depth than field-compacted specimens. There- fore, if the LWT rut depth of a PF specimen is required to be 6 mm at 20,000 passes, the laboratory-compacted mix- ture should have a rut depth of 4.5 mm at 20,000 passes. This relationship will be important as agencies transition toward performance-based specifications. C H A P T E R 7 Implementation Recommendations

69 Property Comparison Significant Process AV Design (LL) - Producon (PL) Stockpile Moisture VMA None VFA None AC Design (LL) - Producon (PL) Baghouse fine return and aggregate absorpon Design (LL) - Construcon (PF) Baghouse fine return Producon (PL) - Construcon (PF) None Gmm Design (LL) - Producon (PL) None Design (LL) - Construcon (PF) None Producon (PL) - Construcon (PF) None Gsb Design (LL) - Producon (PL) None Design (LL) - Construcon (PF) None Producon (PL) - Construcon (PF) None Gradaon Design (LL) - Producon (PL) Baghouse fine return and aggregate hardness Design (LL) - Construcon (PF) Baghouse fine return, aggregate hardness, and stockpile moisture Producon (PL) - Construcon (PF) None Table 7-1. Effects of process-based factors on volumetric properties. dict the rutting in the pavement. The results of the model may vary based on the specimen type used to determine the dynamic modulus. The predictive models are often cal- ibrated with field data using modulus values determined during design or production. For this reason, agencies may find converting the modulus data to suit their calibration needs beneficial. Figure 7-1 presents how an agency can use the conversion factors presented in this report. 7.3.2 Axial Dynamic Modulus Conversion Table 7-4 presents the average conversion factors for axial dynamic modulus comparisons among design and produc- tion specimens. Typically, moduli of the laboratory-mixed specimens were 80% of those of the plant-mixed specimens at higher testing temperatures. Rutting models in pavement distress prediction programs (e.g., Pavement ME Design) use the dynamic modulus of the asphalt mixture to pre- Table 7-2. Volumetric tolerance recommendations. Property Comparison Tolerance Recommendaon AV, % Design (LL) - Producon (PL) ± 0.8 VMA, % + 1.2 VFA, % ± 5.4 AC, % Design (LL) - Producon (PL) ± 0.2 Design (LL) - Construcon (PF) Producon (PL) - Construcon (PF) Gmm Design (LL) - Producon (PL) ± 0.020 Design (LL) - Construcon (PF) ± 0.013 Producon (PL) - Construcon (PF) ± 0.018 Gsb Design (LL) - Producon (PL) ± 0.014 Design (LL) - Construcon (PF) ± 0.019 Producon (PL) - Construcon (PF) ± 0.017 Aggregate Passing 0.075 mm, % Design (LL) - Producon (PL) ± 0.5 Design (LL) - Construcon (PF) ± 0.7 Producon (PL) - Construcon (PF) ± 0.5 Comparison ConversionFactor Design (LL) / Producon (PL) 1.0 Design (LL) / Construcon (PF) 0.75 Producon (PL) / Construcon (PF) 0.75 Table 7-3. LWT conversion recommendations. Comparison Temperature, °C Conversion Factor Design (LL)/ Producon (PL) -10.0 1.0 4.4 1.0 25.0 0.9 37.8 0.8 54.4 0.8 Table 7-4. Axial dynamic modulus conversion recommendations.

70 7.3.3 IDT Dynamic Modulus Conversion Table 7-5 presents the average conversion factors for IDT dynamic modulus determined for the three specimen types evaluated in this study. The conversion factor for design and production specimens is 1.0. Therefore, no conversion was required between design and production specimens. However, the conversions between laboratory-compacted and field- compacted specimens were more pronounced. This was espe- cially noted for intermediate- and high-temperature conver- sions. A designer can use these conversion factors to estimate the dynamic modulus of the field core from mixture collected during production or mixture produced in the laboratory. Figure 7-1. Dynamic modulus conversion decision tree. Temperature, °C Comparison Conversion -10 Design (LL)/Producon (PL) 1.0 Design (LL)/Construcon (PF) 1.0 Producon (PL)/Construcon (PF) 1.1 10 Design (LL)/Producon (PL) 0.9 Design (LL)/Construcon (PF) 1.2 Producon (PL)/Construcon (PF) 1.3 25-35 Design (LL)/Producon (PL) 1.0 Design (LL)/Construcon (PF) 1.4 Producon (PL)/Construcon (PF) 1.5 Table 7-5. IDT dynamic modulus conversion recommendations. IDT vs. Axial Correlation Factor Low-Temperature Comparison, -10°C 0.81 Intermediate-Temperature Comparison, 10°C 0.75 High-Temperature Comparison, 25 - 35°C 0.90 Table 7-6. IDT vs. axial dynamic modulus correlation. 7.3.4 Correlation Between Axial and IDT Dynamic Modulus Table 7-6 compares proposed conversion factors obtained for axial and IDT dynamic modulus. The conversion factors were determined based on the average percent difference of the mixtures evaluated. An outlier analysis was performed before the correlation factors were determined. This resulted in discarding the percent difference data from Mix 10, because it was not within a 95% confidence band with respect to the population. The data show that the conversion factor for intermediate- and low-temperature values should be nearly 0.80. This means that the modulus determined from IDT was generally 80% of the modulus determined from axial test- ing. The difference between axial and IDT determined at high temperature was much more variable, probably because of the increased influence of the loading mode at high tempera- ture; some mixtures exhibited both higher and lower values of modulus when comparing IDT dynamic modulus with axial dynamic modulus.