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66 APPENDIX A Draft Appendix to AASHTO R 35: Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA)

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67 Draft Appendix to AASHTO R 35 Appendix: Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) 1. PURPOSE 1.1. This appendix presents special mixture design considerations and methods for designing warm mix asphalt (WMA) using AASHTO R 35. WMA refers to asphalt concrete mixtures that are produced at temperatures approximately 50 F (28 C) or more cooler than typically used in the production of HMA. The goal with WMA is to produce mixtures with similar strength, durability, and performance characteristics as HMA using substantially reduced production temperatures. 1.2. The methods in this appendix are applicable to a wide range of WMA processes including: WMA additives that are added to the asphalt binder, WMA additives that are added to the mixture during production, Wet aggregate mixtures, and Plant foaming processes. 1.3. The information in this appendix supplements the standard procedures contained in AASHTO R 35. This appendix assumes the user is proficient with the standard procedures contained in AASHTO R 35. 2. SUMMARY 2.1. This appendix includes separate sections addressing the following aspects of WMA mixture design: Equipment for Designing WMA, WMA Process Selection, Binder Grade Selection, RAP in WMA, Process Specific Specimen Fabrication Procedures, Evaluation of Coating Evaluation of Compactability, Evaluation of Moisture Sensitivity, Evaluation of Rutting Resistance, and Adjusting the Mixture to Meet Specification Requirements.

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68 2.2. In each section, reference is made to the applicable section of AASHTO R 35. 3. ADDITIONAL LABORATORY EQUIPMENT 3.1. All WMA Processes: 3.1.1. Mechanical mixer. A planetary mixer with wire whip having a capacity of 20 qt. or a 5 gal. bucket mixer. Note 1 The mixing times in this appendix were developed using a planetary mixer with wire whip, Blakeslee Model B-20 or equivalent. Appropriate mixing times for bucket mixers should be established by evaluating coating of HMA mixtures prepared at the viscosity based mixing temperatures specified in Section 8.2.1 of AASHTO T 312. 3.2. Binder Additive WMA Processes: 3.2.1. Low shear mechanical stirrer. A low shear mechanical stirrer with appropriate impeller to homogeneously blend the additive in the binder. 3.3. Plant Foaming Processes: 3.3.1. Laboratory foamed asphalt plant. A laboratory scale foamed asphalt plant capable of producing consistent foamed asphalt at the water content used in field production. The device should be capable of producing foamed asphalt for laboratory batches ranging from approximately 10 to 20 kg. 4. WMA PROCESS SELECTION 4.1. There are over 20 WMA processes being marketed in the United States. Select the WMA process that will be used in consultation with the specifying agency and technical assistance personnel from the WMA technology providers. Consideration should be given to a number of factors including: (1) available performance data, (2) the cost of the warm mix additives, (3) planned production and compaction temperatures, (4) planned production rates, (5) plant capabilities, and (6) modifications required to successfully use the WMA process with available field and laboratory equipment. 4.2. Determine the planned production and planned field compaction temperatures. 5. BINDER GRADE SELECTION 5.1. Use the same grade of binder normally used with HMA. Select the performance grade of the binder in accordance with Section 5 of AASHTO M 323 considering the environment and traffic at the project site.

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69 Note 2 For WMA processes having production temperatures that are 100 F (56 C) or more lower than HMA production temperatures, it may be necessary to increase the high temperature performance grade of the binder one grade level to meet the rutting resistance requirements included in this appendix. 6. RAP IN WMA 6.1. For WMA mixtures incorporating RAP, the planned field compaction temperature shall be greater than the as-recovered high temperature grade of the RAP binder. Note 3 This requirement is included to ensure that there is mixing of the new and recycled binders. Laboratory studies showed that new and recycled binders do mix at WMA process temperatures provided this requirement is met and the mixture remains at or above the planned compaction temperature for at least 2 hours. Plant mixing should be verified through an evaluation of volumetric or stiffness properties of plant produced mixtures. 6.2. Select RAP materials in accordance with Section 6 of AASHTO M 323. 6.3. For blending chart analyses, the intermediate and low temperature properties of the virgin binder may be improved using Table 1. Note 4 The intermediate and low temperature grade improvements given in Table 1 will allow additional RAP to be used in WMA mixtures when blending chart analyses are used. An approximately 0.6 C improvement in the low temperature properties will allow approximately 10 percent additional RAP binder to be added to the mixture based on blended binder grade requirements.

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70 Table 1. Recommended Improvement in Virgin Binder Low Temperature Continuous Grade for RAP Blending Chart Analysis for WMA Production Temperatures. Virgin Binder PG Grade 58-28 58-22 64-22 64-16 67-22 Average HMA Production Temperature, oF 285 285 292 292 300 Rate of Improvement of Virgin Binder Low Temperature Grade per oC Reduction in Plant 0.035 0.025 0.025 0.012 0.025 Temperature Recommended Improvement in Virgin Binder Low WMA Production Temperature, oF Temperature Continuous Grade for RAP Blending Chart Analysis, oC 300 NA NA NA NA 0.0 295 NA NA NA NA 0.1 290 NA NA 0.0 0.0 0.1 285 0.0 0.0 0.1 0.0 0.2 280 0.1 0.1 0.2 0.1 0.3 275 0.2 0.1 0.2 0.1 0.3 270 0.3 0.2 0.3 0.1 0.4 265 0.4 0.3 0.4 0.2 0.5 260 0.5 0.3 0.4 0.2 0.6 255 0.6 0.4 0.5 0.2 0.6 250 0.7 0.5 0.6 0.3 0.7 245 0.8 0.6 0.7 0.3 0.8 240 0.9 0.6 0.7 0.3 0.8 235 1.0 0.7 0.8 0.4 0.9 230 1.1 0.8 0.9 0.4 1.0 225 1.2 0.8 0.9 0.4 1.0 220 1.3 0.9 1.0 0.5 1.1 215 1.4 1.0 1.1 0.5 1.2 210 1.5 1.0 1.1 0.5 1.3 7. PROCESS SPECIFIC SPECIMEN FABRICATION PROCEDURES 7.1. Batching 7.1.1. Determine the number and size of specimens that are required. Table 2 summarizes approximate specimen sizes for WMA mixture design. Note 5 The mass of mixture required for the various specimens depends on the specific gravity of the aggregate and the air void content of the specimen. Trial specimens may be required to determine appropriate batch weights for the AASHTO T 283 and flow number testing.

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71 Table 2. Specimen Requirements. Specimen Type Gyratory Specimen Approximate Number Required Size Specimen Mass Maximum NA 500 to 6,000 g 2 per trial blend plus Specific depending on 8 to determine design Gravity maximum aggregate binder content plus 1 size at design binder content for compactability evaluation Volumetric 150 mm diameter by 4,700 g 2 per trial blend plus 8 Design 115 mm high to determine design binder content Coating NA 500 to 6,000 g 1 at the design binder depending on content maximum aggregate size Compactability 150 mm diameter by 4,700 g 4 at the design binder 115 mm high content AASHTO T 150 mm diameter by 3,800 g 6 at the design binder 283 95 mm high content Flow Number 150 mm diameter by 7,000 g 4 at the design binder 175 mm high content 7.1.2. Prepare a batch sheet showing the batch weight of each aggregate fraction, RAP, and the asphalt binder. 7.1.3. Weigh into a pan the weight of each aggregate fraction. Note 6 For WMA processes that use wet aggregate, weigh the portion of the aggregate that will be heated into one pan and weigh the portion of the aggregate that will be wetted into a second pan. 7.1.4. Weigh into a separate pan, the weight of RAP. 7.2. Heating 7.2.1. Place the aggregate in an oven set at approximately 15 C higher than the planned production temperature. Note 7 The aggregate will require 2 to 4 hours to reach the temperature of the oven. Aggregates may be placed in the oven overnight.

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72 7.2.2. Heat the RAP in the oven with the aggregates, but limit the heating time for the RAP to 2 hours. 7.2.3. Heat the binder to the planned production temperature. 7.2.4. Heat mixing bowls and other tools to the planned production temperature. 7.2.5. Preheat a forced draft oven and necessary pans to the planned field compaction temperature for use in short-term conditioning the mixture. 7.3. Preparation of WMA Mixtures With WMA Additives Added to the Binder Note 8 If specific mixing and storage instructions are provided by the WMA additive supplier, follow the supplier's instructions. 7.3.1. Adding WMA Additive to Binder 7.3.1.1. Weigh the required amount of the additive into a small container. Note 9 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. 7.3.1.2. Heat the asphalt binder in a covered container in an oven set at 135 C until the binder is sufficiently fluid to pour. During heating occasionally stir the binder manually to ensure homogeneity. 7.3.1.3. Add the required amount of additive to the binder and stir with a mechanical stirrer until the additive is totally dispersed in the binder. 7.3.1.4. Store the binder with WMA additive at room temperature in a covered container until needed for use in the mixture design. 7.3.2. Preparing WMA Specimens 7.3.2.1. Heat the mixing tools, aggregate, RAP, and binder in accordance with Section 7.2. 7.3.2.2. If a liquid anti-strip is required, add it to the binder per the manufacturer's instructions. 7.3.2.3. Place the hot mixing bowl on a scale and zero the scale. 7.3.2.4. Charge the mixing bowl with the heated aggregates and RAP and dry mix thoroughly.

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73 7.3.2.5. Form a crater in the blended aggregate and weigh the required amount of asphalt binder into the mixture to achieve the desired batch weight. Note 10 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: 1. Record the oven dry weight of the aggregates and RAP, wi 2. Determine the target total weight of the mixture wi wt = pb 1 - new 100 where: wt = target total weight wi = oven dry weight from step 1 pb new = percent by weight of total mix of new binder in the mixture 3. Add new binder to the bowl to reach wt 7.3.2.6. Remove the mixing bowl from the scale and mix with a mechanical mixer for 90 sec. 7.3.2.7. Place the mixture in a flat shallow pan at an even thickness of 25 to 50 mm and place the pan in the forced draft oven at the planned field compaction temperature for 2 hours. Stir the mixture once after the first hour. 7.4. Preparation of WMA Mixtures With WMA Additive Added to the Mixture Note 11 If specific mixing and storage instructions are provided by the WMA additive supplier follow the supplier's instructions. 7.4.1. Weigh the required amount of the additive into a small container. Note 12 The quantity of additive may be specified as a percent by weight of binder or a percent by weight of total mixture. 7.4.2. If a liquid anti-strip is required, add it to the binder per the manufacturer's instructions. 7.4.3. Heat the mixing tools, aggregate, RAP, and binder in accordance with Section 7.2. 7.4.4. Place the hot mixing bowl on a scale and zero the scale. 7.4.5. Charge the mixing bowl with the heated aggregates and RAP and dry mix thoroughly.

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74 7.4.6. Form a crater in the blended aggregate and weigh the required amount of asphalt binder into the mixture to achieve the desired batch weight. Note 13 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: 1. Record the oven dry weight of the aggregates and RAP, wi 2. Determine the target total weight of the mixture wi wt = pb 1 - new 100 where: wt = target total weight wi = oven dry weight from step 1 pb new = percent by weight of total mix of new binder in the mixture 3. Add new binder to the bowl to reach wt 7.4.7. Pour the WMA additive into the pool of new asphalt binder. 7.4.8. Remove the mixing bowl from the scale and mix with a mechanical mixer for 90 sec. 7.4.9. Place the mixture in a flat shallow pan at an even thickness of 25 to 50 mm and place the pan in the forced draft oven at the planned field compaction temperature for 2 hours. Stir the mixture once after the first hour. 7.5. Preparation of WMA Mixtures With A Wet Fraction of Aggregate Note 14 Consult the WMA process supplier for appropriate additive dosage rates, mixing temperatures, percentage of wet aggregate and wet aggregate moisture content. 7.5.1. Adding WMA Additive to Binder 7.5.1.1. Weigh the required amount of the additive into a small container. Note 15 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. 7.5.1.2. Heat the asphalt binder in a covered container in an oven set at 135 C until the binder is sufficiently fluid to pour. During heating occasionally stir the binder manually to ensure homogeneity.

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75 7.5.1.3. Add the required amount of additive to the binder and stir with a mechanical stirrer until the additive is totally dispersed in the binder. 7.5.2. Preparing WMA Specimens 7.5.2.1. 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. 7.5.2.2. Heat the mixing tools, dry aggregate portion, and dry RAP portion to the initial mixing temperature in accordance with Section 7.2. 7.5.2.3. Place the hot mixing bowl on a scale and zero the scale. 7.5.2.4. Charge the mixing bowl with the heated aggregates and RAP and dry mix thoroughly. 7.5.2.5. Form a crater in the blended aggregate and weigh the required amount of asphalt binder into the mixture to achieve the desired batch weight. Note 16 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: 1. Record the oven dry weight of the heated aggregates and RAP, wi 2. Determine the target total weight of the mixture: (wi + wdwf ) wt = pbnew 1- 100 where: wt = target total weight wi = oven dry weight from step 1 wdwf = oven dry weight of the wet fraction from the batch sheet pb new = percent by weight of total mix of new binder in the mixture 3. Determine the target weight of the heated mixture: wthm = wt - wdwf where: wthm = target weight of the heated mixture wt = target total weight wdwf = oven dry weight of the wet fraction from the batch sheet

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76 4. Add new binder to the bowl to reach wthm 7.5.2.6. Add the additive to the binder immediately before mixing with the heated fraction of the aggregate per Section 7.5.1. 7.5.2.7. Remove the mixing bowl from the scale and mix with a mechanical mixer for 30 sec. 7.5.2.8. Stop the mixer and immediately add the wet fraction. 7.5.2.9. Restart the mixer and continue to mix for 60 sec. 7.5.2.10. Place the mixture in a flat shallow pan at an even thickness of 25 to 50 mm. 7.5.2.11. Check the temperature of the mixture in the pan. It shall be between 90 and 100 C. 7.5.2.12. Place the pan in the forced draft oven at the planned field compaction temperature for 2 hours. Stir the mixture once after the first hour. 7.6. Preparation of Foamed Asphalt Mixtures 7.6.1. The preparation of foamed asphalt mixtures requires special asphalt binder foaming equipment that can produce foamed asphalt using the amount of moisture that will be used in field production. 7.6.2. Prepare the asphalt binder foaming equipment and load it with binder per the manufacturer's instructions. 7.6.3. If a liquid anti-strip is required, add it to the binder in the foaming equipment per the manufacturer's instructions. 7.6.4. Heat the mixing tools, aggregate, and RAP in accordance with Section 7.2. 7.6.5. Prepare the foamed asphalt binder per the instructions for the foaming equipment. 7.6.6. Place the hot mixing bowl on a scale and zero the scale. 7.6.7. Charge the mixing bowl with the heated aggregates and RAP and dry mix thoroughly. 7.6.8. Form a crater in the blended aggregate and add the required amount of foamed asphalt into the mixture to achieve the desired batch weight.

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77 Note 17 The laboratory foaming equipment uses a timer to control the amount of foamed asphalt provided. Make sure the batch size is large enough that the required amount of foamed asphalt is within the calibrated range of the foaming device. This may require producing one batch for the two gyratory specimens and the two maximum specific gravity specimens at each asphalt content then splitting the larger batch into individual samples. Note 18 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: 1. Record the oven dry weight of the aggregates and RAP, wi 2. Determine the target total weight of the mixture wi wt = pb 1 - new 100 where: wt = target total weight wi = oven dry weight from step 1 pb new = percent by weight of total mix of new binder in the mixture 3. Add foamed binder to the bowl to reach wt 7.6.9. Remove the mixing bowl from the scale and mix with a mechanical mixer for 90 sec. 7.6.10. Place the mixture in a flat shallow pan at an even thickness of 25 to 50 mm and place the pan in the forced draft oven at the planned field compaction temperature for 2 hours. Stir the mixture once after the first hour. 8. WMA MIXTURE EVALUATIONS 8.1. At the optimum binder content determined in accordance with Section 10 of AASHTO R 35, prepare WMA mixtures in accordance with the appropriate procedure from Section 7 of this appendix for the following evaluations: Coating Compactability Moisture sensitivity Rutting resistance

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78 8.2. Coating 8.2.1. Prepare sufficient mixture at the design binder content to perform AASHTO T 195 using the appropriate WMA fabrication procedure from Section 7 of this appendix. Do not short-term condition the mixture. 8.2.2. Evaluate the coating in accordance with AASHTO T 195. 8.2.3. The recommended coating criterion is at least 95 percent of the coarse aggregate particles fully coated. 8.3. Compactability 8.3.1. Prepare sufficient mixture at the design binder content for 4 gyratory specimens and one maximum specific gravity measurement using the appropriate WMA fabrication procedure from Section 7 of this Appendix including short-term conditioning for 2 hours at the planned compaction temperature. 8.3.2. Determine the theoretical maximum specific gravity (Gmm) according to AASHTO T 209. 8.3.3. Compact duplicate specimens at the planned field compaction temperature to Ndesign gyrations in accordance with AASHTO T 312. Record the specimen height for each gyration. 8.3.4. Determine the bulk specific gravity of each specimen in accordance with AASHTO T 166. 8.3.5. Allow the mixture to cool to 30 C below the planned field compaction temperature. Compact duplicate specimens to Ndesign gyrations in accordance with AASHTO T 312. Record the specimen height for each gyration 8.3.6. Determine the bulk specific gravity of each specimen in accordance with AASHTO T 166. 8.3.7. For each specimen determine the corrected specimen relative densities for each gyration using Equation 1. Gmb hd %GmmN = 100 (1) Gmm hN where: %G mmN = relative density at N gyrations; Gmb = bulk specific gravity of specimen compacted to Ndesign gyrations;

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79 hd = height of the specimen after Ndesign gyrations, from the Superpave gyratory compactor, mm; and hN = height of the specimen after N gyrations, from the Superpave gyratory compactor, mm 8.3.8. For each specimen, determine the number of gyrations to reach 92 percent relative density. 8.3.9. Determine the average number of gyrations to reach 92 percent relative density at the planned field compaction temperature. 8.3.10. Determine the average number of gyrations to reach 92 percent relative density at 30 C below the planned field compaction temperature. 8.3.11. Determine the gyration ratio using Equation 2. ( N 92 ) T -30 Ratio = (2) ( N 92 ) T where: Ratio = gyration ratio (N92)T-30 = gyrations to 92 percent relative density at 30 C below the planned field compaction temperature (N92)T = gyrations to 92 percent relative density at the planned field compaction temperature 8.3.12. The recommended compactability criterion is the gyration ratio should be less than or equal to 1.25. Note 18 The compactability criterion limits the temperature sensitivity of WMA to that for a typical HMA mixture. The criterion is based on limited research conducted in NCHRP 9-43. The criterion should be considered tentative and subject to change as additional data on WMA mixtures are collected. 8.4. Evaluating Moisture Sensitivity 8.4.1. Prepare sufficient mixture at the design binder content for 6 gyratory specimens using the appropriate WMA fabrication procedure from Section 7 of this appendix including short-term conditioning. 8.4.2. Compact test specimens to 7.0 0.5 percent air voids in accordance with AASHTO T 312. 8.4.3. Group, condition and test the specimens in accordance with AASHTO T 283. 8.4.4. The recommended moisture sensitivity criteria are the tensile strength ratio should be greater than 0.80 and there should not be any visual evidence of stripping.

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80 8.5. Evaluating Rutting Resistance 8.5.1. Evaluate rutting using the flow number test in AASHTO TP 79. 8.5.2. Prepare sufficient mixture at the design binder content for four flow number test specimens using the appropriate WMA fabrication procedure from Section 7 of this appendix including short-term conditioning. 8.5.3. The test is conducted on 100 mm diameter by 150 mm high test specimens that are sawed and cored from larger gyratory specimens that are 150 mm diameter by at least 175 mm high. Refer to AASHTO PP 60 for detailed procedures for test specimen fabrication procedures. The short-term conditioning for WMA specimens is 2 hours at the compaction temperature. 8.5.4. Prepare the flow number test specimens to 7.0 1.0 percent air voids. 8.5.5. Perform the flow number test at the design temperature at 50 % reliability as determined using LTPP Bind Version 3.1. The temperature is computed at 20 mm for surface courses, and the top of the pavement layer for intermediate and base courses. 8.5.6. Perform the flow number test unconfined using repeated deviatoric stress of 600 kPa with a contact deviatoric stress of 30 kPa. 8.5.7. Determine the flow number for each specimen, then average the results. Compare the average flow number with the criteria given in Table 3. Table 3. Minimum Flow Number Requirements Traffic Level, Minimum Flow Number Million ESALs <3 NA 3 to < 10 30 10 to < 30 105 30 415 9. ADJUSTING THE MIXTURE TO MEET SPECIFICATION PROPERTIES 9.1. This section provides guidance for adjusting the mixture to meet the evaluation criteria contained in Section 8 of this appendix. For WMA mixtures, this section augments Section 12 in AASHTO R 35.

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81 9.2. Improving Coating- Most WMA processes involve complex chemical reactions and/or thermodynamic processes. Consult the WMA additive supplier for methods to improve coating. 9.3. Improving Compactability- Most WMA processes involve complex chemical reactions and/or thermodynamic processes. Consult the WMA additive supplier for methods to improve compactability. 9.4. Improving the Tensile Strength Ratio Some WMA processes include adhesion promoters to improve resistance to moisture damage. Consult the WMA additive supplier for methods to improve the tensile strength ratio. 9.5. Improving Rutting Resistance- The rutting resistance of WMA can be improved through changes in binder grade and volumetric properties. The following rules of thumb can be used to identify mixture adjustments to improve rutting resistance. Increasing the high temperature performance grade one grade level improves rutting resistance by a factor of 2. Adding 25 to 30 percent RAP will increase the high temperature performance grade approximately one grade level. Increasing the fineness modulus (sum of the percent passing the .075, 0.150, and 0.300 mm sieves) by 50 improves rutting resistance by a factor of 2. Decreasing the design VMA by 1 percent will improve rutting resistance by a factor of 1.2. Increasing Ndesign by one level will improve rutting resistance by factor of 1.2. 10. ADDITIONAL REPORTING REQUIREMENTS FOR WMA 10.1. For WMA mixtures, report the following information in addition to that required in Section 13 of AASHTO R 35. 10.1.1. WMA process description. 10.1.2. Planned production temperature. 10.1.3. Planned field compaction temperature. 10.1.4. High temperature grade of the binder in the RAP for mixtures incorporating RAP. 10.1.5. Coating at the design binder content. 10.1.6. Gyrations to 92 percent relative density for the design binder content at the planned field compaction temperature and 30 C below the planned field compaction temperature 10.1.7. Gyration ratio.

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82 10.1.8. Dry tensile strength, tensile strength ratio, and observed stripping at the design binder content. 10.1.9. Flow number test temperature and the flow number at the design binder content.