Properties of Foamed Asphalt for Warm Mix Asphalt Applications (2015)

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89 A P P E N D I X B Draft Commentary on Guidelines Proposed for Revising Appendix to AASHTO R 35 This appendix provides a commentary on the proposed revisions to the appendix to AASHTO R 35 Special Mixture Design Considerations and Methods for WMA based on the results of NCHRP Project 9-53, “Properties of Foamed Asphalt for Warm Mix Asphalt Applications.” The commentary provides marked changes to the sections on additional laboratory equipment (X2.3), technology-specific specimen fabrication procedures (X2.7), WMA mixture evaluations (X2.8), a justification for proposed revisions, and the need for future research. Specifically, the changes address: 1. More detailed requirements for laboratory foaming units. 2. Two alternate methods for adding foamed asphalt to mixtures in the laboratory. 3. A special provision for mixture preparation for coatability and workability measurements. 4. An objective method for a mixture coatability evaluation test. 5. A mixture workability evaluation. 6. A method to identify the optimum amount of foaming water to be used in WMA mixtures.

90 X2. SPECIAL MIXTURE DESIGN CONSIDERATIONS AND PRACTICES FOR WARM MIX ASPHALT (WMA) X2.2. Summary: X2.2.1. This appendix includes separate sections addressing the following aspects of WMA mixture design: Equipment for Designing WMA, WMA Technology Selection, Binder Grade Selection, RAP in WMA, Technology-Specific Specimen Fabrication Procedures, Determining the Optimum Water Content for Plant Foaming, Evaluation of Workability, Evaluation of Coating, Evaluation of Compactability, Evaluation of Moisture Sensitivity, Evaluation of Rutting Resistance, and Adjusting the Mixture to Meet Specification Requirements. X2.3. Additional Laboratory Equipment: X2.3.3. Plant Foaming Processes: Laboratory-Foamed Asphalt Plant—A laboratory-scale foamed asphalt plant (laboratory foaming unit) that is capable of producing consistent foamed asphalt at the water content used in field production. The unit should be capable of producing foamed asphalt for laboratory batches ranging from approximately 10 to 20 kg. The laboratory foaming unit should be capable of heating the asphalt binder to between 320°F (160°C) and 360°F (182°C). The water supply line should have a flow meter capable of regulating the water input to the foam by ±0.1 percent by weight of the unfoamed binder. The water and asphalt should be combined by a suitable means to achieve a uniform dispersion of water in the asphalt prior to discharge from the unit. Note X6—Research has shown that the range of commercially available laboratory foaming units is capable of producing suitable foamed asphalt for mixture evaluation. X2.7.6. Preparation of Foamed Asphalt Mixtures: X2.7.6.1. 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. X2.7.6.2. Prepare the asphalt binder foaming equipment, and load it with binder per the manufacturer’s instructions. X2.7.6.3. If a liquid anti-stripping additive is required, add it to the binder in the foaming equipment according to the manufacturer’s instructions.

91 X2.7.6.4. Heat the mixing tools, aggregate, and RAP in accordance with Section X2.7.2. X2.7.6.5. Prepare the foamed asphalt binder according to the instructions for the foaming equipment. X2.7.6.6. Mixing Method 1: Foam weighed into aggregate. X2.7.6.6.1. Place the hot mixing bowl on a scale, and tare the scale. X2.7.6.6.2. Charge the mixing bowl with the heated aggregates and RAP, and dry-mix thoroughly. X2.7.6.6.3. Form a crater in the blended aggregate, and add the required amount of foamed asphalt into the mixture to achieve the desired batch weight. X2.7.6.6.4 Remove the mixing bowl from the scale, and mix the materials with a mechanical mixer for 90 60 s for workability evaluation or 90 s for volumetric and performance testing. X2.7.6.7. Mixing Method 2: Foam discharged into aggregate. X2.7.6.7.1. Calibrate the rate of discharge of the laboratory foaming unit into a tared, empty container. Three separate discharge times should be used to determine the amount of foamed asphalt binder being dispensed. A graph of weight versus time should be constructed, and a discharge time equal to the desired weight of binder should be found. X2.7.6.7.2. Charge the heated bucket with the pre-weighed and heated aggregate and place under the foamed asphalt outlet on the laboratory foaming unit. X2.7.6.7.3. Discharge the foamed binder into the mixing bowl or the bucket mixer for the desired time interval from paragraph X2.7.6.7.1. X2.7.6.8. Mix for 90 s for volumetric or performance testing. X2.7.6.8.1. Mix for 60 s ± 3 s for workability testing or 90 s for volumetric and performance testing. Note X21—The laboratory foaming equipment uses a timer to control the amount of foamed asphalt produced. Ensure the batch size is large enough that the required amount of foamed asphalt is within the calibrated range of the foaming device. Depending upon the foaming equipment this operation 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 X2X—If any binder additives have been used in the laboratory foaming unit immediately prior to mixing, clear all residue from the unit as thoroughly as possible before producing other foamed binders that do not contain the same additive. It has been found that running approximately 2 gal. (8 L) of straight asphalt through the unit is adequate to accomplish this. Note X22—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:

92 1. Record the oven-dry weight of the aggregates and RAP, wi. 2. Determine the target total weight of the mixture as follows: (X2.5) Where: wt = target total weight, g; wi = oven-dry weight from Step 1, g; and Pbnew = percent by weight of total mix of new binder in the mixture. 3. Add foamed binder to the bowl to reach wt. X2.7.6.9. 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 a compaction temperature of 240°F (116°C) for WMA or 275°F (135°C) for HMA for 2 hours. Stir the mixture once after 1 hour. X2.8. WMA Mixture Evaluations X2.8.1. At the optimum binder content determined in accordance with R 35, prepare WMA mixtures in accordance with the appropriate procedure from Section X2.7 for the following evaluations: Workability Compactability Coating Moisture sensitivity Rutting resistance X2.8.2. Coating X2.8.2.1. Prepare a sufficient amount of mixture at the design binder content to perform the coating evaluation procedure in T 195 using the appropriate WMA fabrication procedure from Section X2.7. Do not short-term condition the mixture. X2.8.2.2. Evaluate the coating in accordance with T 195. X2.8.2.3. The recommended coating criterion is at least 95 percent of the coarse aggregate particles being fully coated. X2.8.2. Workability to Determine Optimum Water Content for Foamed Mixtures per AASHTO TP XX-XX. X2.8.2.1. Prepare a sufficient amount of mixture at the design binder content for two gyratory specimens at 1, 2, and 3 percent water using a laboratory foaming unit from Section X2.7.6 including the short-term conditioning. new1 100 i t b w w = P

93 X2.8.2.2. Compact the specimens at 240°F (116°C) using a gyratory compacter capable of measuring the shear force (stress) generated during compaction to a point just beyond the maximum shear force (stress). X2.8.2.3. Record the maximum shear stress for each specimen following the procedure in TP XX-XX. Average the two results for each foaming water content. X2.8.2.4. The optimum foaming water content is that which produces the lowest average maximum shear stress. X2.8.2.5. In the event that an optimum water content cannot be determined, use a foaming water content of 1 percent. Note X.X. While almost all asphalt binders will readily foam, there are a few that demonstrate insensitivity to the water content. For these it is recommended that the lowest level of water content be used as the additional water will serve no purpose. X2.8.3. Coating: Method 1 for any WMA material. X2.8.3.1. Prepare a sufficient amount of mixture at the design binder content and design foaming water content to perform the coating evaluation procedure in T 195 using the appropriate WMA fabrication procedure from Section X2.7. Do not short-term condition the mixture. X2.8.3.2. Evaluate the coating in accordance with T 195. X2.8.3.3. The recommended coating criterion is at least 95 percent of the coarse aggregate particles being fully coated. X2.8.4. Coating: Method 2 for laboratory-foamed asphalt mixtures. X2.8.4.1 Prepare a sufficient amount of mixture at the design binder content and design foaming water content to perform the coating evaluation procedure described in TP XX-XX using a laboratory foaming unit and a mixing time of 60 s ± 3 s. Short-term age the sample for 2 hours at 240°F (116°C). X2.8.4.2 Evaluate the coating in accordance with TP XX-XX. X2.8.4.3 The recommended coating criterion is a coatability index of at least 70 percent.

94 Introduction The suggested modifications to the appendix of AASHTO R 35 to optimize the foaming water content through the mix design process were presented in Chapter 1. These changes are suggested on the basis of results obtained in NCHRP Project 9-53, “Properties of Foamed Asphalt for Warm Mix Asphalt Applications.” The original warm mix asphalt (WMA) modifications to the mix design process were the result of NCHRP Project 9-43. A summary of the expanded changes for foamed asphalt applications includes: 1. More detailed requirements for laboratory foaming units. 2. Two alternate methods for adding foamed asphalt to mixtures in the laboratory. 3. A special provision for mixture preparation for coatability and workability measurements. 4. A second method for a mixture coatability evaluation test. 5. A mixture workability evaluation. 6. A method to identify the optimum amount of foaming water to be used in WMA mixtures. Figure B-1 presents the flowchart for the mixture design process developed in NHCRP Project 9-53, including foamed binder and mixture characteristics. The recommended procedure begins after the determination of the optimum asphalt content according to R35. Next, foamed asphalt mixtures are produced with 1%, 2%, and 3% foaming water. These mixtures are evaluated for workability by monitoring shear stress during compaction in a Superpave gyratory compactor (SGC). The mix producing the lowest maximum shear stress during compaction is considered to be at the optimum foaming water content. This mixture is then evaluated for coatability using a new method based on the moisture absorption of the coated aggregate relative to the uncoated aggregate. After the evaluation of coatability, the mixture undergoes the desired performance testing protocol. This appendix will present the justifications for these changes and identify future research needs.

95 Figure B-1. Final proposed foamed asphalt mix design method. Laboratory Foamers At the time of preparation of this report there were three commercially available laboratory foamers, and all three were evaluated during the course of the project. The general characteristics of these units are presented in Table B-1. Although the discharge rate varies from very rapid (approximately 100 g/s) to much slower (approximately 14 g/s), and the three units produced very different foamed asphalt properties (Figure B-2 and Figure B-3), all of the laboratory foamers were capable of successfully producing foamed asphalt that could be incorporated into mixtures to identify an optimum water content. It can be seen that two of the three units produced the same workability optimum water content (1.0%), while the third produced an optimum water content of 2.0% (Figure B-3). In each case, the foamed WMA mixture was more workable than the corresponding hot mix asphalt (HMA) mixture. Furthermore, the differences in mix performance properties were not correlated to the foamed binder properties. The laboratory foaming unit should have an adjustable positive water flow control capable of ±0.1% by weight of the unfoamed binder tolerance on water content. Two methods of binder addition to the aggregate are provided for (1) laboratory foamers capable of discharging directly into the aggregate and (2) laboratory foamers requiring discharge into a separate container for gravimetrically adding the foamed binder to the aggregate on a weigh scale.

96 Table B-1. Summary of characteristics for commercially available laboratory foamers. Characteristic Wirtgen WLB 10S InstroTek Accufoamer PTI foamer Air flow pressure Min. 15 psi (100 kPa) Max. 145 psi (1,000 kPa) Min. 75 psi (517 kPa), Max. 150 psi (1,034 kPa) Min. 80 psi (552 kPa) Max. 110 psi (758 kPa) Water flow pressure Max. 145 psi (1,000 kPa) Max. 30 psi (207 kPa) 33 psi (230 kPa) Binder flow pressure Max. 145 psi (1,000 kPa) Max. 60 psi (413 kPa) The binder is dispensed by gravity Reaction chamber Water and compressed air are injected into the hot binder. Pressurized binder and water meet at a single junction. A small amount of air is used to atomize the water to a fine droplet. Binder temperature 284°F–392°F (140°C–200°C) 320°F–390°F (160°C–200°C) Max 350°F (177°C) Discharge time 100 g/s 16–20 g/s 14–20 g/s Mass control Mass flow control Overhead pressure control Scale control Power requirement Adaptable to various international supplies 208–240 VAC, 220- volt, 30-amp circuit 120 VAC, 20 amp Binder chamber size 5.3 gallon (20 L) 0.3–15.0 lb (150 to 6,800 g) 14 lb (6,350 g) Foaming agent dosage (water content) 0%–5% 0%–9% 1%–7% Foaming agent temperature No heat Max. 180°F (82°C) No heat VAC = volts alternating current. (a) (b) Figure B-2. (a) Foamability index and (b) surface area index (SAI) at 1.0% water content for commercially available laboratory foamers (SAI could not be measured on the PTI foamer).

97 (a) (b) (c) Figure B-3. Workability and coatability results for the control HMA and foamed mixture; (a) produced in the Accufoamer, (b) produced in the PTI foamer, and (c) produced in the Wirtgen foamer.

98 Mixing Methods Two methods for producing foamed asphalt mixtures in the laboratory are presented in the proposed appendix to AASHTO R 35 recommended practice. The design of the laboratory foamers included as part of NCHRP Project 9-53 necessitated that these two options be presented. Two of the units were able to dispense foamed asphalt directly into mixing containers at a prescribed amount, but the third unit did not have room for a mixing container below the outlet. Mixing Method 1 involves dispensing foamed asphalt at the desired water content into an empty container and then pouring the foamed binder into a pre-weighed amount of aggregate in the same way that unfoamed asphalt binder is added to aggregate. The foamed binder and aggregate may then be mixed in either a bucket mixer or planetary mixer. Mixing Method 1 is applicable to any laboratory foaming unit. Mixing Method 2 is applicable for laboratory foamers capable of discharging the foamed binder directly into a mixing bucket or bowl. An important step in this method is calibrating the discharge time for the desired amount of binder. A graph of binder weight versus binder discharge time is constructed by weighing the amount of binder dispensed from the foaming unit at three different times and determining the appropriate amount of time for the desired binder content. The bucket mixer or mixing bowl is then placed directly below the foaming nozzle, and the foamed binder is added to the aggregate according to the desired dispensing time. Method 2 is applicable only to those laboratory foamers capable of direct addition of the binder to the aggregate. For the evaluation of asphalt mix workability and coatability, it is necessary to carefully monitor the mixing time of the heated aggregate and foamed asphalt. The time set during this research project was 60 s ± 3 s for both test methods. This provided a basis to compare foamed asphalt mixtures containing different amounts of foaming water, and allowed researchers to identify and discriminate the effects of water content on the workability and coatability of the mix. Based on research accomplished in NCHRP Projects 9-49 and 9-52, it is recommended that the foam-produced WMA be aged at 240°F for 2 hours prior to compaction in order to simulate aging that occurs in a batch or drum mixing plant. The original recommendation from NCHRP Project 9-43 (Advanced Asphalt Technologies 2012) was that aging should be accomplished at the planned field-compaction temperature, but this is difficult to anticipate during mix design. Technology-Specific Specimen Fabrication Procedures Discussions on the changes to the mixing procedure, calibration techniques for dispensing foamed asphalt, methods of adding foamed binder to the aggregate, the mixing procedure for workability and coatability testing, and the short-term oven aging of the foamed WMA were presented in the previous sections. In summary, the two methods of mixing foamed asphalt into aggregate are (1) dispensing the foamed asphalt into a container and then pouring the desired amount of foamed asphalt into the mixing bucket or bowl containing the heated aggregate and (2) discharging the binder at a calibrated rate directly into the mixing bucket or bowl containing the heated aggregate. For volumetric and performance testing specimens, the mixing time remains at 90 s, as originally proposed in NCHRP Project 9-43 (Advanced Asphalt Technologies

99 2012), and for workability and coatability testing, the mixing time is set at 60 s ± 3 s. Short-term oven aging for 2 hours at 240°F (116°C) should precede specimen preparation. WMA Mixture Evaluation The evaluation of WMA mixtures is focused on two important interrelated aspects of mix behavior. The first of these is the production and placement of the mixture, and the second is the long-term performance. This discussion will present the recommendations from NCHRP Project 9-43 and those from NCHRP Project 9-53 with respect to WMA evaluation. During mix production, the asphalt binder must be evenly distributed throughout the mixture so that the aggregate particles are completely covered. In NCHRP Project 9-43, recommendations were made to evaluate coating using AASHTO T 195, which is a visual inspection of the coarse coated aggregate particles. During placement, the mixture must flow sufficiently to allow compaction to a specified density. For workability, NCHRP Project 9-43 recommended a compactability evaluation as the ratio of the number of gyrations to achieve 92% of maximum density at 30°C below the planned field-compaction temperature to the number of gyrations for the same density level at the planned field-compaction temperature. Recommendations from NCHRP Project 9-53 include an alternative method to assess coatability using the relative difference between uncoated aggregate moisture absorption and coated aggregate moisture absorption as well as a workability test in which the shear stress during gyratory compaction is monitored. The coatability and workability test methods suggested in the proposed appendix to AASHTO R 35 from NCHRP Project 9-53 do not have existing AASHTO test method designations, but they do have proposed procedures presented as deliverables for NCHRP Project 9-53. The second aspect of the mixture’s qualities is its potential long-term performance. In NCHRP Project 9-43, recommendations for performance testing included an evaluation of moisture susceptibility using AASHTO T 283 and an evaluation of rutting using AASHTO TP 79, T 320, T 324, or T 340. The suggestion for performance testing is to maintain the NCHRP Project 9-43 protocol with the addition of a stiffness test and potentially add one or more mixture cracking tests at some point based on the results of NCHRP Project 9-57 and other future projects. In NCHRP Project 9-53, the suggestions for performance testing do not differ from those made in NCHRP Project 9-43 as the goal of the asphalt community is to ensure that the performance of WMA is equal to or better than that of HMA. Based on recommendations from NCHRP Project 9-53, the first step after determining the optimum asphalt content from AASHTO R 35 is to prepare mixtures at the optimum asphalt content and foaming water contents of 1.0%, 2.0%, and 3.0% by weight of asphalt binder. These three water contents were found to span the range of optimum water contents for mixtures that were sensitive to water content during the course of NCHRP Project 9-53. The next step is to evaluate the mixtures with the three different water contents for workability using an SGC capable of measuring shear stress generated during compaction. The mixture at the water content with the lowest maximum shear stress (i.e., greatest workability) is considered to have the optimum water content. Workability Workability describes the ease with which the mixture can be placed, worked by hand, and compacted. It is a function of temperature, binder properties (e.g., viscosity, grade, polymer

100 modification), and aggregate properties (e.g., size, angularity), among other factors. Because the binder is being applied to the aggregate at a reduced temperature in WMA applications, there have been concerns as to whether the binder will flow readily enough to distribute itself among the aggregate particles and whether the mixture will flow sufficiently to compact the mixture to the specified density. The maximum shear stress during compaction with the SGC was selected as a workability metric because it is essentially the amount of effort required to shove the mix around in the SGC mold at its anticipated placement and compaction temperature. Although not all SGCs are equipped to measure shear stress during compaction, it is becoming a common feature on a number of available models. It is suggested that a standard method of quantifying shear stress during compaction be developed under a future NCHRP research project. However, since shear stress is commonly defined in gyratory compaction, simply being able to define the minimum peak shear stress among the mixtures being evaluated should suffice for the time being as a means to determine the optimum moisture content. The best workability is ensured by selecting the foaming water content that produces the lowest maximum shear stress during compaction. This is termed the “optimum foaming water content,” and it is selected from the lowest maximum shear stress obtained at 1.0%, 2.0%, or 3.0% water. In the event that the mixture is not sensitive to water content, then the lowest (i.e., 1.0%) water content should be selected as the optimum. For instance, in Figure B-4, 1.0% water would be selected for the N6 and O6 binders as that was the water content producing the lowest shear stress. For the Y6 binder, 1.0% would also be selected, but that is because the mixture is not sensitive to changes in foaming water content and the use of greater than 1.0% moisture is not justified. Figure B-4. Workability test results for foamed WMA versus HMA.

101 Coatability Next, the mixture with the optimum water content is evaluated to ensure that it can provide an adequate coating of binder to the aggregate. Enough of the mixture is prepared to allow the preparation of two specimens. The loose mixtures are short-term oven aged (STOA) for 2 hours at 240°F (116°C) for WMA specimens, as recommended by NCHRP Project 9-52. Coatability is a measure of the ease with which an asphalt binder distributes itself over the surface area of the aggregate. The current method of evaluating coatability presented in the proposed appendix to AASHTO R 35 is AASHTO T 195, which is a visual rating of the uncoated coarse aggregate particles. Because it is a visual inspection, it is subjective in its determination and is somewhat dependent on (1) the skill of the technician in quantifying uncoated particles and (2) the color of the aggregate. If the aggregate is dark, then it may be difficult to determine the amount of uncoated particles. A less subjective means to determine the coatability of foamed asphalt mixtures was developed based on work done at the University of Wisconsin (Geng et al. 2013; Velasquez et al. 2012). The procedure to determine the coatability of the mixture in the proposed changes to the proposed appendix of AASHTO R 35 is based on the relative difference in measurement of water absorption of bare coarse aggregate (adjusted for the change in surface area from the full aggregate gradation to the coarse aggregate only) and the water absorption of the coated coarse aggregate at the optimum asphalt and foaming water contents. In this method, the coarse aggregate and foamed asphalt are mixed for 60 s and subjected to the STOA protocol of 2 hours at 240°F (116°C) for WMA. Both the coated and uncoated samples are weighed to obtain their oven-dry weights. Samples of uncoated coarse aggregate and coated coarse aggregate are soaked in water for a period of 1 hour. The samples are then brought to a saturated surface dry (SSD) condition and weighed. The relative difference in water absorption between the two samples is the coatability index (CI) and is a measure of how well the binder has coated the aggregate. A threshold value of the CI that relates to field measurements of coating needs to be established for this test from future research efforts. However, as shown in Figure B-5, a CI value of 70% is appropriate for mixtures prepared at the optimum moisture content for two of the three binders.

102 Figure B-5. Coatability test results for foamed WMA versus HMA. Conclusions Suggested changes to Appendix X2 of AASHTO R 35 have been made accompanied by the justifications for these changes reflecting the information gathered under NCHRP Project 9-53. The important changes include: 1. More detailed requirements for laboratory foaming units. 2. Two alternate methods for adding foamed asphalt to mixtures in the laboratory. 3. A special provision for mixture preparation for coatability and workability measurements. 4. A method for a mixture coatability evaluation test. 5. A mixture workability evaluation. 6. A method to identify the optimum amount of foaming water to be used in WMA mixtures. Provisions have been made to allow foamed asphalt production from the existing commercially available laboratory foaming units. The evaluation of workability is based on the shear stress generated in an SGC during the compaction process. It was found that this approach allowed for the determination of an optimum foaming moisture content in mixtures that were sensitive to changes in moisture content. A coatability test based on water absorption is suggested to replace AASHTO T 195. The new method is based on objective measurements rather than subjective judgments on the amount of coating of aggregate particles. This approach to determining an optimum foaming moisture content allows contractors and agencies to better use the foaming equipment at asphalt mix plants for greater mixing and placing efficiency. References Advanced Asphalt Technologies, LLC. 2012. NCHRP Report 714: Special Mixture Design Considerations and Methods for Warm Mix Asphalt: A Supplement to NCHRP Report 673: A

103 Manual for Design of Hot Mix Asphalt with Commentary. Transportation Research Board of the National Academies, Washington, D.C. Geng, H., C. S. Clopotel, and H. U. Bahia. 2013. Effects of High Modulus Asphalt Binders on Performance of Typical Asphalt Pavement Structures. TRB 92nd Annual Meeting Compendium of Papers, Transportation Research Board of the National Academies, Washington, D.C. Velasquez, R., G. Cuciniello, D. Swiertz, R. Bonaquist, and H. U. Bahia. 2012. Methods to Evaluate Aggregate Coating for Asphalt Mixtures Produced at WMA Temperature. Proceeding. Canadian Technical Asphalt Association.