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OCR for page 161
Reclaimed Asphalt Pavement 161 in RAP is likely much harder than new asphalt binders used in HMA mix designs and, when significant amounts of RAP are added to a mix, the binder from the RAP will blend with the new asphalt binder added to the mix to produce a blended binder that can be substantially harder than the new binder added to the HMA mixture. For this reason, the amount of RAP that can be added to a mixture can be limited not only by variability, but also by the blended binder grade. The issue of determining blended binder grades in HMA mix designs containing RAP is discussed later in this chapter. RAP Aggregate Properties In addition to gradation, bulk specific gravity, angularity, and particle shape are properties of the RAP aggregate that are needed to effectively design HMA with RAP. These properties are measured on a representative sample of the RAP stockpile that is formed by combining samples from each of the stockpile locations. Some tests must be performed on bare aggregate after removing the binder from the RAP. When experience with local aggregates indicates that they are not substantially altered by exposure to the high temperature in the ignition oven, AASHTO T 308 is the preferred method for removing the binder from the RAP aggregate for testing. Otherwise solvent extraction, AASHTO T 164, must be used. If the ignition oven is used, the aggregates must be cooled to room temperature before further handling and testing. If solvent extraction is used, the aggregates must be thoroughly dried in an oven and cooled to room temperature before further handling and testing. When the ignition oven is used, some aggregates may exhibit significant breakdown as a result of the high temperatures used in this procedure; this breakdown can alter the resulting gradation. The representative sample should be large enough to provide sufficient material for all of the tests being performed; Table 9-6 lists the required sample sizes for laboratory tests normally performed on RAP aggregate. If the nominal maximum aggregate size in the RAP is 12.5 mm or less, a 4 kg sample will be sufficient. If the nominal maximum aggregate size in the RAP is 19.0 mm or more, a 10 kg sample will be required. RAP Aggregate Bulk Specific Gravity The bulk specific gravity of each aggregate stockpile used in an HMA mixture is needed for the computation of the voids in the mineral aggregate (VMA). Two methods can be used to determine the bulk specific gravity of the RAP aggregate. The first is to estimate the bulk specific gravity of the RAP aggregate from the RAP binder content, the maximum specific gravity of the RAP, and estimates of the binder absorption in the RAP and the specific gravity of the RAP binder. The second is to measure the bulk specific gravity of the coarse and fine fraction of the Table 9-6. Sample size for RAP aggregate tests. Sample Size, kg 12.5-mm 19.0-mm Property Method Fraction NMAS NMAS Specific Gravity of Coarse AASHTO T 85 +2.36 mm 2 3 Aggregate Specific Gravity of Fine Aggregate AASHTO T 84 -2.36 mm 1 1 Coarse Aggregate Fractured Faces ASTM D 5821 +4.75 mm 0.5 1.5 Fine Aggregate Angularity AASHTO T -2.36 mm 0.5 0.5 304 Method A Flat and Elongated Particles ASTM D 4791 +4.75 mm 2 5

OCR for page 161
162 A Manual for Design of Hot Mix Asphalt with Commentary RAP aggregate after removing the binder with the ignition oven or solvent extraction. Details of these approaches are discussed below. Estimating RAP Aggregate Bulk Specific Gravity In this approach, the maximum specific gravity of the RAP is measured in accordance with AASHTO T 209. The maximum specific gravity is measured on a sample split from the repre- sentative sample formed for the RAP aggregate and binder analysis. The measured maximum specific gravity, the average RAP binder content from the variability analysis, and an estimate of the RAP binder specific gravity are then used to calculate the effective specific gravity of the RAP aggregate using Equation 9-1. (100 - Pb ) Gse = (9-1) 100 Pb - Gmm Gb where Gse = effective specific gravity of the RAP aggregate Gmm = maximum specific gravity of the RAP measured by AASHTO T 209 Pb = RAP binder content, wt % Gb = estimated specific gravity of the RAP binder The bulk specific gravity of the RAP aggregate can then be estimated from Equation 9-2, which is a rearranged version of the equation used in volumetric analysis to compute asphalt absorption. Gse Gsb = (9-2) PbaGse +1 100 Gb where Gsb = estimated bulk specific gravity of the RAP aggregate Gse = effective specific gravity of the RAP aggregate from Equation 9-1 Pba = estimated binder absorption for the RAP, wt % of aggregate Gb = estimated specific gravity of the RAP binder The overall error associated with this analysis is difficult to quantify. It depends on (1) the precision of the maximum specific gravity measurement and (2) the accuracy of the RAP binder content measurement and the estimated RAP binder absorption and specific gravity. As shown in the analysis below, the accuracy of the RAP binder content in turn depends on the accuracy of the correction factor used to analyze the ignition oven data. The single-operator precision of the maximum specific gravity test, AASHTO T 209, is 0.011 when the dry-back procedure is not required and 0.018 when it is. These are somewhat better than the single-operator precision of the aggregate bulk specific gravity tests which are 0.032 for fine aggregate (AASHTO T 84) and 0.025 for coarse aggregate (AASHTO T 85). The potential error associated with estimating the specific gravity of the RAP binder is small. For a typical mixture it is only 0.002 for a 0.010 error in the bulk specific gravity of the binder. Potential errors associated with errors in the RAP binder content or the RAP binder absorption are signifi- cantly larger. These errors are shown in Figure 9-7 for RAP having a maximum specific gravity of 2.500, a total binder content of 4%, and binder absorption of 0.5%. In this case, underestimating the absorbed binder by 0.3% results in an overestimation of the bulk specific gravity of the RAP

OCR for page 161
Reclaimed Asphalt Pavement 163 0.040 RAP Binder Content RAP Binder Absorption ERROR IN BULK SPECIFIC GRAVITY OF RAP 0.030 0.020 0.010 AGGREGATE 0.000 -0.010 -0.020 -0.030 -0.040 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 ERROR IN RAP BINDER CONTENT OR ABSORPTION, % Figure 9-7. Potential errors in bulk specific gravity of the RAP aggregate for errors in RAP binder content and binder absorption. aggregate of 0.020. Underestimating the total binder content of the RAP by 0.5% results in an underestimation of the bulk specific gravity of the RAP aggregate of 0.021. Thus the accuracy of estimating the RAP aggregate specific gravity from the maximum specific gravity and binder content of the RAP depends mostly on the accuracy of the estimated correction factor used to determine binder content with the ignition oven and the accuracy of the assumed binder absorption. The correction factor for the ignition oven should not be in error by more than 0.3% and the assumed binder content should not be in error by more than 0.2% to obtain estimated RAP aggregate specific gravity values with an accuracy similar to those measured in AASHTO T 84 and T 85. As discussed earlier, correction factors for the ignition oven can be established by performing both the ignition oven and solvent extraction analyses on split samples from at least three locations in the RAP stockpile. Measuring RAP Aggregate Specific Gravity If a reasonable estimate of the binder absorption for the RAP is not available, the specific gravity of the RAP aggregate can be measured after removing the RAP binder using an ignition oven or solvent extraction. The specific gravities of the coarse and fine fractions of the RAP aggregate are measured in accordance with AASHTO T 85 and AASHTO T 84, respectively. HMA Tools and RAP Aggregate Specific Gravity HMA Tools is designed so that either approach discussed above can be used to estimate RAP aggregate specific gravity values. If the specific gravity values are to be estimated from maximum theoretical specific gravity, binder content, and related information, the data are entered in cells C6:F9 in the worksheet "RAP_Aggregates." If actual measured values for RAP aggregate specific gravity are used, these are entered in cells C11:F14. The calculated values for bulk and apparent specific gravity for each of up to four RAP stockpiles then appear in cells C16:F17. If data for both