Click for next page ( 8

The National Academies of Sciences, Engineering, and Medicine
500 Fifth St. N.W. | Washington, D.C. 20001

Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement

Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 7
I. Special Mixture Design Considerations and Methods for Warm Mix Asphalt (WMA) 7 mixtures designed using the same binder and aggregates. A reasonable estimate of the volume of binder absorbed in WMA mixtures is 45% of the volume of water absorbed by the aggregates used in the mixture. This estimate is given in Equation 1 and is used in the software program HMA Tools (available for download at to estimate the binder content by volume for WMA mixtures. VMA Gsb Pwa Vb = VMA - VA + 1 - (1) 100 2.2 where Vb = total asphalt content by volume % VMA = target voids in the mineral aggregate, vol. % VA = design air voids, vol. % Gsb = aggregate bulk specific gravity Pwa = water absorption of the aggregate, weight % As with HMA mixture design, the binder content by volume computed at this point is an esti- mate that will be refined during Step 10, Evaluate and Refine Trial Mixtures, of the design process. For batching, the binder content by volume must be converted to binder content by weight using the specific gravity of the binder and the aggregates in the mixture. These calculations were pre- sented in Chapter 8 and are performed by HMA Tools. Step 7. Calculate Aggregate Content by Volume The total aggregate content by volume is calculated in the same manner as described in Chapter 8 for HMA: 100% minus target VMA. Determination of the total aggregate content by weight will depend on the aggregate specific gravity values, and the specific blend of aggregates used in each mixture. These calculations were presented in Chapter 8 and are performed by HMA Tools. Step 8. Proportion Aggregates for Trial Mixtures Proportioning aggregates for trial WMA mixtures is the same as described in Chapter 8 for design of HMA mixtures. The mix design procedure presented in this manual sets the binder content at a value that will provide the proper VMA once the design air void content is met. Therefore, proportioning aggregates can be thought of as determining the blend of aggregates that will provide the proper air void content for the mixture. Note that the control points given in Chapter 8 are considered guidelines and not specification requirements. Chapter 8 presented a graphical procedure for aggregate blending that uses the continuous maximum density (CMD) plot. This plot quantifies how much various aggregate gradations deviate from the maximum density gradation and is effective at identifying those changes that potentially affect the VMA of the mixture. This procedure should be followed when designing new WMA mixtures. Most WMA mixture design work will be able to adapt a specific WMA process to an existing HMA design. For this type of design, there is no need to change the aggregate proportions from those used in the HMA design; unless the binder absorption is very high, the volumetric prop- erties of the WMA and HMA mixtures will be very similar. When performing a WMA design of an existing HMA mixture in HMA Tools, enter the aggregate and RAP data (if used) in worksheets "Aggregates," and "RAP_Aggregates." Then, the aggregate blend for the existing mix is entered

OCR for page 7
8 Special Mixture Design Considerations and Methods for Warm Mix Asphalt Table 7. Coarse aggregate fractured faces (CAFF) requirements. Percentage of Particles with at Least One/Two Fractured Faces, for Depth Design ESALs (million) of Pavement LayerA, mm 0 to 100 Below 100 < 0.30 55 / --- --- / --- 0.3 to < 3 75 / --- 50 / --- 3 to < 10 85 / 80 60 / --- 10 to < 30 95 / 90 80 / 75 30 or more 98 / 98B 98 / 98B A Depth of pavement layer is measured from pavement surface to surface of pavement layer. B The CAFF requirement for design traffic levels of 30 million ESALs or more may be reduced to 95/95 if experience with local conditions and materials indicate that this would provide HMA mixtures with adequate rut resistance under very heavy traffic. in worksheet "Trial_Blends." The design then proceeds as described in Chapter 8 for HMA, deter- mining the air void content and VMA for trial batches, and then making further refinements in the aggregate gradation as needed, until the desired mix properties are met. The aggregates used in WMA should meet the aggregate specification properties given in Chap- ter 8 for HMA. There are four aggregate specification properties: (1) coarse aggregate fractured faces (CAFF); (2) flat and elongated particles in the coarse aggregate; (3) fine aggregate angularity (FAA); and (4) clay content of the fine aggregate (sand equivalent). These requirements are presented in Tables 7 to 10. Note that the aggregate specification requirements apply to the blended aggregates and should be measured on the final design blend of aggregates. For trial batches, they can be Table 8. Criteria for flat and elongated particles. Maximum Percentage of Flat and Design ESALs (million) Elongated Particles at 5:1 < 0.30 --- 0.3 to < 3 10 3 to < 10 10 10 to < 30 10 30 or more 10 Criteria are presented as percent flat and elongated particles by mass. Table 9. Fine aggregate angularity (FAAAQ15) requirements. Depth of Pavement Layer from Design ESALs (million) Surface, mm 0 to 100a Below 100a < 0.30 ---b --- 0.3 to < 3 40 --- 3 to < 10 45c 40 10 to < 30 45c 45c 30 or more 45c 45c Criteria are presented as percent air voids in loosely compacted fine aggregate. a If Less than 25 percent of a construction lift is within 100 mm of the surface, the lift may be considered to be below 100 mm for mixture design purposes. b Although there is no FAA requirement for design traffic levels below 0.30 million ESALS, consideration should be given to requiring a minimum uncompacted void content of 40 percent for 4.75 mm nominal maximum aggregate size mixes. c The FAA requirement of 45 may be reduced to 43 if experience with local conditions and materials indicate that this would produce HMA mixtures with adequate rut resistance under the given design traffic level.