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I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) 9 Table 10. Maximum clay content requirements. Design ESALs (million) Minimum Sand Equivalency Value < 0.30 40 0.3 to < 3 40 3 to < 10 45 10 to < 30 45 30 or more 50 Criteria are presented as sand equivalent value. estimated from the specification property values for the individual aggregates using HMA Tools. These estimates should then be verified by measurements on the final design blend. Step 9. Calculate Trial Mix Proportions By Weight and Check Dust-to-Binder Ratio This step in the design of WMA mixtures is identical to that described in Chapter 8 for HMA mixtures. This step involves calculating the following for each trial blend of aggregates: 1. Bulk specific gravity of the aggregate blend, 2. Weight percentage of binder in the mixture, 3. Weight percentage of total aggregate in the mixture, 4. Effective binder content by weight, 5. Weight percentage of each aggregate in the mixture, 6. Weight percentage of mineral dust in the mixture, and 7. Dust to effective binder ratio. These calculations are performed automatically by HMA Tools and when RAP is included in the mixture, binder from the RAP is properly accounted for in the calculations. WMA mixtures should meet the requirements for the ratio of dust to effective binder content given in Chapter 8 for HMA. These requirements are reproduced in Table 11. Step 10. Evaluate and Refine Trial Mixtures This step involves the preparation and evaluation of laboratory specimens of WMA. The pro- cedure follows that described in Chapter 8 for HMA with slight modification. Table 12 summa- rizes the steps for WMA and HMA design. The modifications required for WMA design are 1. For some processes, the WMA additive must be calculated. 2. Viscosity-based mixing temperatures are not used with WMA. Laboratory mixing is done at the planned production temperature. Table 11. Requirements for dust/binder ratio. Allowable Range for Mix Aggregate Dust/Binder Ratio, by NMAS, mm Weight > 4.75 0.8 to 1.6A 4.75 0.9 to 2.0 A The specifying agency may lower the allowable range for dust/binder ratio to 0.6 to 1.2 if warranted by local conditions and materials. The dust/binder ratio should however not be lowered if VMA requirements are increased above the standard values as listed in Table 13-6.

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10 Special Mixture Design Considerations and Methods for Warm Mix Asphalt Table 12. Comparison of trial specimen fabrication procedures for WMA and HMA design. Step Description HMA WMA Comment 1 Calculate batch weights X X Must calculate WMA additive content for some processes 2 Batch aggregates X X Must batch WMA additive for some processes 3 Heat aggregates and X X Use planned production temperature for asphalt binder WMA 4 Mix aggregates and X X Procedure is WMA process specific binder 5 Mixture conditioning X X WMA uses lower temperature procedures 6 Compact laboratory X X WMA uses lower temperature specimens 7 Calculate volumetric X X composition of laboratory specimens 8 Adjust aggregate X X proportions to meet volumetric requirements 9 Evaluate coating and NA X Used in WMA design in place of viscosity- compactability based mixing and compaction temperatures 10 Conduct performance X X Moisture sensitivity for all mixtures, rutting testing resistance for design traffic levels of 3 m ESALs or greater 3. The short-term conditioning temperature for WMA is the planned compaction temperature. 4. Viscosity-based compaction temperatures are not used with WMA. Laboratory compaction is done at the planned compaction temperature. 5. WMA design includes an evaluation of coating and compactability using the planned pro- duction and compaction temperatures. These modifications are discussed in the sections that follow. Calculate Batch Weights Some WMA processes require an additive to be added either to the binder or to the mixture. The amount of additive needed may be specified by the WMA process supplier as percent by weight of binder or total mixture. The "Additive" sheet in HMA Tools allows the user to specify the dosage rate for up to three additives and whether the dosage rate is based on binder or total mixture weight. Batch Aggregates For most WMA processes, aggregate batching is identical to that for HMA. HMA Tools pro- vides a convenient tool for calculating batch weights for various specimens and degrees of aggre- gate processing. In one WMA process, water is added to a portion of the fine aggregate, then this wet, fine aggregate is added cold to the mixture during the mixing process. For this process, treat the wet portion of the fine aggregate as a separate fine aggregate in HMA Tools. Compute the dry aggregate batch weight for this aggregate, then add the required weight of water to the dry aggregate, mix, cover, and let stand 2 hours before using it in the mixing process. Heat Aggregates and Asphalt Binder The most notable differences between the design of WMA and HMA occur in the specimen fabrication process, which begins with this step and continues through the next three steps. Viscosity-based mixing and compaction criteria cannot be used with the wide range of WMA

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I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) 11 processes available. In fact, research in progress suggests that enhanced lubrication, not viscosity reduction, is the primary mechanism governing the success of WMA processes. The design of WMA mixtures is done using the planned field production and compaction temperatures. The aggregates and binder that will be used are heated in an oven to approximately 27F (15C) above the planned production temperature. Aggregates may be heated overnight. The asphalt binder and RAP, if used, should be heated the minimum time necessary to reach this target temperature. Mix Aggregates and Binder For mixture design purposes, the various WMA processes can be grouped into the following generic categories: 1. Additives blended into the binder, 2. Additives added to the mixture, 3. Wet aggregate mixtures, and 4. Foamed asphalt. This section describes laboratory procedures for preparing each of these types of WMA mixtures. Some WMA processes may include elements from two or more of these processes. The laboratory equipment needed to produce the mixtures is generally the same as that required for HMA. A mechanical mixer capable of mixing 10 to 45 lb (5 to 20 kg) batches is needed for all WMA processes. The mixing times presented later are based on a planetary mixer with a wire whip. Bucket mixers are less efficient than planetary mixers; therefore, the mixing times may need to be increased. For WMA processes requiring the additive to be blended in the binder, a low-shear mechanical stirrer with appropriate size impeller is needed to homoge- neously blend the additive in the binder. Finally, for foamed asphalt mixtures, a laboratory-scale foamed asphalt plant capable of producing consistent foamed asphalt at the water content used in field production is needed. An example of such a device is shown in Figure 1. The device should be capable of producing foamed asphalt for laboratory batches ranging in size from approximately 20 to 45 lb (10 to 20 kg). Note that laboratory foaming plants designed for cold mix applications will require a more precise flow controller to allow foamed asphalt production Figure 1. A foaming device for preparing WMA in the laboratory.

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12 Special Mixture Design Considerations and Methods for Warm Mix Asphalt at the lower water contents used in WMA. Also, because these machines are designed to produce large quantities of foamed asphalt, it will be necessary to produce larger batches of WMA and then split the material needed for the various tests required. Additives Blended in the Binder For WMA processes that require the WMA additive to be blended in the binder, the additive must be blended into the binder before the WMA mixture can be produced. The required dosage rate will be provided by the WMA process supplier who usually will also provide instructions for blending the additive in the binder. HMA Tools will compute the mass of additive to add for a given batch size. If instructions for blending the additive are not provided, use the following procedure: 1. Follow the manufacturer's instruction for storage of the additive (e.g., temperature and humidity) particularly after opening the manufacturer's packaging. 2. Weigh the required amount of the additive into a small container. The additive is typically specified as a percent by weight of binder. For mixtures containing RAP, determine the weight of additive based on the total binder content of the mixture. 3. Heat the asphalt binder in a covered container in an oven set at 135C until the binder is sufficiently fluid to pour. During heating, occasionally stir the binder manually to ensure homogeneity. 4. Add the required amount of additive to the binder and stir with a mechanical stirrer until the additive is totally dispersed in the binder. 5. Store the binder with WMA additive at room temperature in a covered container until needed for use in the mixture design. Some binders are being supplied with the WMA additive pre-blended into the binder. For these binders, it is not necessary to blend the additive, and the preparation of the WMA mixture proceeds as outlined below. Once the WMA additive has been added to the binder, the preparation of the WMA mixture proceeds in a similar manner as that for HMA. The following steps summarize the mixture preparation process: 1. Heat the aggregate, RAP, binder, and mixing tools to approximately 27F (15C) above the planned production temperature. Aggregates may be heated overnight. The asphalt binder and RAP should be heated the minimum time necessary to reach this target temperature. 2. If a liquid antistrip is required, add it to the binder per the manufacturer's instructions. 3. Place the hot mixing bowl on a scale and zero the scale. 4. Charge the mixing bowl with the heated aggregates and RAP and dry mix thoroughly. 5. Form a crater in the blended aggregate and weigh the required amount of asphalt binder into the mixture to achieve the desired batch weight. If the aggregates and RAP have been stored for an extended period of time in a humid environment, then it may be necessary to adjust the weight of binder based on the oven dry weight of the aggregates and RAP as follows: a. Record the oven dry weight of the aggregates and RAP, wi b. Determine the target total weight of the mixture wi wt = p 1 - bnew 100 where wt = target total weight wi = oven dry weight from Step a Pbnew = % by weight of total mix of new binder in the mixture c. Add new binder to the bowl to reach wt

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I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) 13 6. Remove the mixing bowl from the scale and mix with a mechanical mixer for 90 sec. 7. Transfer the mixture to a flat shallow pan at an even thickness of 1 to 2 in (25 to 50 mm) for short-term conditioning. Additives Added to the Mixture Some WMA processes specify that the additive be added to the mixture during plant mixing. The additive dosage rate may be given as a percent of the total mixture mass or a percentage of the binder in the mixture. If the mixture contains RAP and the dosage rate is as a percentage of the binder, remember to include the RAP binder contribution when computing the amount of additive. HMA Tools will compute the mass of additive to add for a given batch size. For these processes, the following mixing procedure should be followed: 1. Follow the manufacturer's instruction for storage of the additive (e.g., temperature and humidity) particularly after opening the manufacturer's packaging. 2. Weigh the required amount of the additive into a small container. 3. Heat the aggregate, RAP, binder, and mixing tools to approximately 27F (15C) above the planned production temperature. Aggregates may be heated overnight. The asphalt binder and RAP should be heated the minimum time necessary to reach this target temperature. 4. If a liquid antistrip is required, add it to the binder per the manufacturer's instructions. 5. Place the hot mixing bowl on a scale and zero the scale. 6. Charge the mixing bowl with the heated aggregates and RAP and dry mix thoroughly. 7. Form a crater in the blended aggregate and weigh the required amount of asphalt binder into the mixture to achieve the desired batch weight. If the aggregates and RAP have been stored for an extended period of time in a humid environment, then it may be necessary to adjust the weight of binder based on the oven dry weight of the aggregates and RAP as follows: a. Record the oven dry weight of the aggregates and RAP, wi b. Determine the target total weight of the mixture wi wt = p 1 - bnew 100 where wt = target total weight wi = oven dry weight from Step a Pbnew = % by weight of total mix of new binder in the mixture c. Add new binder to the bowl to reach wt 8. Pour the WMA additive into the pool of new asphalt binder. 9. Remove the mixing bowl from the scale and mix with a mechanical mixer for 90 sec. 10. Transfer the mixture to a flat shallow pan at an even thickness of 1 to 2 in (25 to 50 mm) for short-term conditioning. Figure 2 shows a WMA additive being added to a mixture in the laboratory. Wet Aggregate Mixtures One WMA process uses cold, wet fine aggregate to produce asphalt concrete at significantly lower discharge temperatures. In this process, a portion of the total aggregate is added wet. The coarse aggregate and dry portion of the fine aggregate are mixed with the binder at normal HMA production temperatures. The percentage of the fine aggregate that will be added wet, the mois- ture content of that portion of the fine aggregate, and the initial mixing temperature are recom- mended by the WMA process supplier. An additive is also added to the binder following the steps described above for Additives Blended in the Binder. In HMA Tools, treat the portion of the fine

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14 Special Mixture Design Considerations and Methods for Warm Mix Asphalt Figure 2. Adding a WMA additive to a mixture in the laboratory. aggregate that will be added wet as a separate aggregate. Compute the dry aggregate batch weight for this aggregate, then the weight of water to add (based on the recommended moisture content), and then proceed as follows: 1. Add the required moisture to the wet fraction of the aggregate, mix thoroughly, then cover and let stand for at least 2 hours before mixing with the heated fraction. 2. Heat the aggregate, RAP, binder, and mixing tools to approximately 27F (15C) above the initial mixing temperature. Aggregates may be heated overnight. The asphalt binder and RAP should be heated the minimum time necessary to reach this target temperature. 3. Place the hot mixing bowl on a scale and zero the scale. 4. Charge the mixing bowl with the heated aggregates and RAP and dry mix thoroughly. 5. Form a crater in the blended aggregate and weigh the required amount of asphalt binder into the mixture to achieve the desired batch weight. If the aggregates and RAP have been stored for an extended period of time in a humid environment, then it may be necessary to adjust the weight of binder based on the oven dry weight of the aggregates and RAP as follows: a. Record the oven dry weight of the heated aggregates and RAP, wi b. Determine the target total weight of the mixture: (w i + w dwf ) wt = p 1 - bnew 100 where wt = target total weight wi = oven dry weight from Step a wdwf = oven dry weight of the wet fraction from the batch sheet Pbnew = percent by weight of total mix of new binder in the mixture c. Determine the target weight of the heated mixture: w thm = w t - w dwf

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I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) 15 where wthm = target weight of the heated mixture wt = target total weight wdwf = oven dry weight of the wet fraction from the batch sheet d. Add new binder to the bowl to reach wthm 6. Remove the mixing bowl from the scale and mix with a mechanical mixer for 30 sec. 7. Stop the mixer and immediately add the wet fraction. 8. Restart the mixer and continue to mix for 60 sec. 9. Transfer the mixture to a flat shallow pan at an even thickness of 1 to 2 in (25 to 50 mm) for short-term conditioning. 10. Check the temperature of the mixture in the pan. It should be between 90 and 100C. Foamed Asphalt The preparation of foamed asphalt mixtures requires special asphalt binder foaming equip- ment that can produce foamed asphalt using the amount of moisture that will be used in field production. The procedure for preparing foamed asphalt mixtures is as follows: 1. Prepare the asphalt binder foaming equipment and load it with binder per the manufacturer's instructions. 2. If a liquid antistrip is required, add it to the binder in the foaming equipment per the manu- facturer's instructions. 3. Heat the aggregate, RAP, and mixing tools to approximately 27F (15C) above the planned production temperature. Aggregates may be heated overnight. The asphalt binder and RAP should be heated the minimum time necessary to reach this target temperature. 4. Prepare the foamed asphalt binder per the instructions for the foaming equipment. 5. Place the hot mixing bowl on a scale and zero the scale. 6. Charge the mixing bowl with the heated aggregates and RAP and dry mix thoroughly. 7. Form a crater in the blended aggregate and add the required amount of foamed asphalt into the mixture to achieve the desired batch weight. If the aggregates and RAP have been stored for an extended period of time in a humid environment, then it may be necessary to adjust the weight of foamed binder based on the oven dry weight of the aggregates and RAP as follows: a. Record the oven dry weight of the aggregates and RAP, wi b. Determine the target total weight of the mixture wi wt = p 1 - bnew 100 where wt = target total weight wi = oven dry weight from Step a Pbnew = percent by weight of total mix of new binder in the mixture c. Add foamed binder to the bowl to reach wt The laboratory foaming equipment uses a timer to control the amount of foamed binder provided. Make sure the batch size is large enough that the required amount of foamed binder is within the calibrated range of the foaming device. This may require producing one batch for the two gyratory specimens and the maximum specific gravity specimen at a given asphalt content and then splitting the individual samples. 8. Remove the mixing bowl from the scale and mix with a mechanical mixer for 90 sec.

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16 Special Mixture Design Considerations and Methods for Warm Mix Asphalt 9. Transfer the mixture to a flat shallow pan at an even thickness of 1 to 2 in (25 to 50 mm) for short-term conditioning. Mixture Conditioning Procedures AASHTO R 30 describes three different procedures for mixture conditioning in a forced draft oven: (1) mixture conditioning for volumetric mix design; (2) short-term conditioning for mixture mechanical property testing; and (3) long-term conditioning for mixture mechanical property testing. The last procedure, long-term conditioning for mixture mechanical property testing, is generally not used in the design and testing of WMA mixtures and is not addressed here. WMA mixtures for both volumetric mixture design and mechanical property testing (performance evaluation) should be conditioned for 2 hours at the planned compaction temperature. The conditioning should follow AASHTO R 30, using a preheated forced-draft oven, a mixture thickness of 1 to 2 in (25 to 50 mm), and stirring of the mixture after 1 hour. The conditioning procedure for volumetric mix design is essentially identical for WMA and HMA, the only difference being in the definition of compaction temperature for the two types of mixes. Short-term conditioning for performance testing differs in that only 2 hours is required for WMA mixtures, while 4 hours is required when conditioning HMA. Also, WMA mixtures for performance testing are conditioned at the planned compaction temperature, whereas AASHTO R 30 specifies a conditioning temperature of 135C for HMA mixtures for mechanical testing. Compact Laboratory Specimens WMA specimens are compacted in the same manner as described in Chapter 8 for HMA using a properly calibrated and maintained Superpave gyratory compactor (see Figure 3). Compact duplicate specimens in accordance with AASHTO T 312. Calculate Volumetric Composition of Laboratory Specimens Volumetric analysis of compacted WMA specimens is the same as described in Chapter 8 for HMA. Recall that the procedure used in this manual sets the binder content at a value that Figure 3. Compacting a WMA specimen in a Superpave gyratory compactor.

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I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) 17 will provide the proper VMA once the design air void content is met, and the gradation was selected to provide an acceptable ratio of dust to effective binder content by mass. Thus the air void content is the primary volumetric factor used to determine the acceptability of the trial mixture. Given that the binder content was initially set using an assumed binder absorp- tion, the effective binder content and VMA of the trial mixture should also be analyzed. The following equations (which were derived in Chapter 5) are used to perform the volumetric analysis. Air Void Content G VA = 100 1 - mb (2) Gmm where VA = Air void content, volume % Gmb = Bulk specific gravity of compacted mixture Gmm = Maximum theoretical specific gravity of loose mixture Total Binder Content by Volume PbGmb VB = (3) Gb where VB = Total asphalt binder content, % by total mix volume Pb = Total asphalt binder content, % by mix mass Gmb = Bulk specific gravity of the mixture Gb = Specific gravity of the asphalt binder Absorbed Binder by Volume P P 100 VBA = Gmb b + s - (4) Gb Gsb Gmm where VBA = Absorbed asphalt content, % by total mixture volume Gmb = Bulk specific gravity of the mixture Pb = Total asphalt binder content, % by mix mass Gb = Specific gravity of the asphalt binder Ps = Total aggregate content, % by mix mass, equal to 100 - Pb Gsb = Average bulk specific gravity for the aggregate blend Gmm = Maximum specific gravity of the mixture Effective Binder Content by Volume VBE = VB - VBA (5) where VBE = Effective asphalt content, % by total mixture volume VB = Asphalt binder content, % by mix volume VBA = Absorbed asphalt content, % by total mixture volume

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18 Special Mixture Design Considerations and Methods for Warm Mix Asphalt Effective Binder Content by Mass VBE Pbe = Pb VB (6) where Pbe = Effective asphalt binder content, % by total mass Pb = Asphalt binder content, % by total mass VBE = Effective asphalt binder content, % by total mixture volume VB = Asphalt binder content, % by mix volume Voids in Mineral Aggregate VMA = VA + VBE (7) where VMA = Voids in the mineral aggregate, % by total mixture volume VA = Air void content, % by total mix volume VBE = Effective binder content, % by total mixture volume Dust Proportion P D B = 0.075 (8) Pbe where D/B = dust/binder ratio, calculated using effective binder content P0.075 = mineral dust content, % by total mix weight Pbe = effective binder content, % by total mix weight HMA Tools performs the volumetric analysis using data on the bulk and maximum specific gravity of the trial mixture. The design procedure in this manual provides acceptable ranges for three volumetric factors: (1) air void content, (2) VMA, and (3) dust proportion. The acceptable ranges are summarized in Table 13. These are the same ranges used for the design of HMA. Adjust Aggregate Proportions to Meet Volumetric Requirements Adjusting WMA mixtures to meet volumetric requirements is the same as discussed in Chapter 8 for HMA. The procedure presented in this manual is designed to provide accept- Table 13. Acceptable range for volumetric factors. Aggregate VMAA Air Voids Dust ProportionB NMAS Minimum Maximum Minimum Maximum Minimum Maximum (mm) (%) (%) (%) (%) 4.75 16.0 18.0 3.5 4.5 0.9 2.0 9.5 15.0 17.0 3.5 4.5 0.8 1.6 12.5 14.0 16.0 3.5 4.5 0.8 1.6 19.0 13.0 15.0 3.5 4.5 0.8 1.6 25.0 12.0 14.0 3.5 4.5 0.8 1.6 37.5 11.0 13.0 3.5 4.5 0.8 1.6 A The specifying agency may increase the minimum and maximum values for VMA by up to 1.0% to obtain mixtures with increased asphalt binder content, which can improve field compaction, fatigue resistance, and general durability. Care should be taken to ensure that the resulting HMA mixtures maintain adequate rut resistance for their intended application. B The specifying agency may lower the allowable range for dust/binder ratio to 0.6 to 1.2 for NMAS mixtures of 9.5 mm and greater if warranted by local conditions and materials. The dust/binder ratio should, however, not be lowered if VMA requirements are increased using Note A above.

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I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) 19 able volumetric properties when specimens of the trial mixture meet the design air void content. After preparing trial specimens, it may be necessary to make minor adjustments to the binder content to account for differences between the assumed and actual binder absorption. If the air voids of the trial specimens are more than a few tenths of a percent out- side the design range, then the aggregate gradation should be adjusted to change the VMA of the mixture. The general rule for adjusting aggregate blends to meet VMA requirements is that the closer an aggregate gradation is to a maximum density gradation, the lower will be its VMA. Evaluate Coating and Compactability The viscosity-based mixing and compaction temperatures used in the design of HMA can- not be used with the wide range of WMA processes currently available. For WMA, the design procedure, therefore, includes an evaluation of coating at the planned production temperature and an evaluation of compactability at the planned compaction temperature. The sections that follow describe these evaluations. Coating Coating is evaluated at the planned production temperature by preparing loose mix of the design mixture following the specimen fabrication procedures presented earlier and evaluating the coating of the coarse aggregate particles using AASHTO T 195, Standard Method of Test for Determining Degree of Particle Coating of Bituminous-Aggregate Mixtures. This test method consists of separating out the coarse aggregates of the mixture and determining the percentage of the coarse aggregate particles that are fully coated. The recommended criterion is 95% of the coarse aggregates fully coated. It should be noted that this criterion and the mixing times given earlier were developed using a planetary mixer with a wire whip. Bucket mixers are not as effi- cient as planetary mixers; therefore, laboratory mixing times may need to be increased if a bucket mixer is used. Compactability Compactability is evaluated by compacting two specimens at the planned compaction temperature and two specimens at 30C below the planned compaction temperature. The num- ber of gyrations required to reach 8% air voids is determined for both sets of specimens. It is recommended that the increase in gyrations to 8% air voids between the planned compaction temperature and 30C below the planned compaction temperature should be less than 25% of the number of gyrations at the planned compaction temperature. The procedure is described in detail below: 1. Prepare a sufficient quantity of the design mixture for four gyratory specimens and one maximum specific gravity measurement using the appropriate WMA fabrication procedure. 2. Determine the theoretical maximum specific gravity (Gmm) according to AASHTO T 209. 3. Compact duplicate specimens at the planned compaction temperature to Ndesign gyrations in accordance with AASHTO T 312. Record the specimen height for each gyration. 4. Determine the bulk specific gravity of each specimen in accordance with AASHTO T 166. 5. Allow the mixture to cool to 30C below the compaction temperature. Compact duplicate specimens to Ndesign gyrations in accordance with AASHTO T 312. Record the specimen height for each gyration 6. Determine the bulk specific gravity of each specimen in accordance with AASHTO T 166. 7. For each specimen determine the height at a relative density of 92.0% using Equation 9.

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20 Special Mixture Design Considerations and Methods for Warm Mix Asphalt %Gmmd h92 = hd 92 (9) where h92 = height at a relative density of 92% hd = height at Ndesign, as measured by the gyratory compactor %Gmmd = relative density at Ndesign 8. For each specimen, determine the number of gyrations to reach 92% relative density. This can be done by looking at the output from the gyratory compactor giving specimen height as a function of gyrations--simply find the number of gyrations at a height where the rela- tive density is 92%, h92 as determined in Step 7 above. 9. Determine the gyration ratio using Equation 10. ( N 92 )T -30 Ratio = (10) ( N 92 )T where Ratio = gyration ratio (N92)T-30 = gyrations to 92% relative density at 30C below the planned compaction temperature. (N92)T = gyrations to 92% relative density at the planned compaction temperature 10. The compactability is acceptable if the gyration ratio is less than 1.25. Conduct Performance Testing Like HMA, the final stage of laboratory work in a WMA design is evaluating the performance of the mixture. Performance evaluation for WMA is the same as HMA and includes evaluation of the resistance to moisture damage for all mixtures and evaluation of rutting resistance for mix- tures designed for traffic levels of 3 million ESALs and higher. As discussed above, the primary difference between WMA and HMA is in mixture conditioning--HMA mixtures are condi- tioned for 4 hours at 135C, whereas WMA mixtures for performance testing are conditioned at the planned compaction temperature for 2 hours. Otherwise, the procedure for short-term con- ditioning for mixture mechanical testing as described in AASHTO R 30 should be followed when preparing WMA specimens for performance testing. The resistance to moisture damage is evaluated using AASHTO T 283, Resistance of Com- pacted Asphalt Mixture to Moisture-Induced Damage. Like HMA, WMA mixtures have accept- able resistance to moisture damage if the tensile strength ratio is equal to or greater than 80% and there is no visual evidence of stripping in the conditioned test specimens. When redesign- ing an existing HMA mixture using a WMA process that does not include an antistrip additive, it is not uncommon to find that the WMA mixture is less resistant to moisture damage. For these mixtures, the use of hydrated lime or an antistrip additive will usually provide acceptable results. Many WMA processes include an antistrip additive. For these processes, consult the process sup- plier for recommendations to improve resistance to moisture damage if unacceptable results are obtained at normal WMA additive dosage rates. The rutting resistance of WMA is evaluated for mixtures designed for 3 million ESALs and higher using the flow number test, AASHTO TP 79, Determining the Dynamic Modulus and Flow Number of Hot Mix Asphalt (HMA) Using the Asphalt Mixture Performance Tester (AMPT). The same testing conditions used for HMA are used with WMA.