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OCR for page 108
108 A Manual for Design of Hot Mix Asphalt with Commentary In HMA Tools, the worksheet "General" should be filled out with information about the mix design. This includes the date, the name of the engineer or technician performing the mix design, the project name, the aggregate size (NMAS), the required binder grade, the target VMA, and the target air void content. Completion of this worksheet is essential, because much of this information is used in other parts of HMA Tools. Step 2. Select Asphalt Binder Details concerning the properties and specification of asphalt binders are presented in Chapter 3 of this manual; what is presented here is a concise description of the grade selection process. Details of the performance grading system are given in AASHTO M 320, Performance-Graded Asphalt Binder. Engineers and technicians unfamiliar with the performance grading system should review Chapter 3 and AASHTO M 320 before attempting to understand HMA mix design as presented in this manual. Selection of the asphalt binder grade in an HMA mix design will generally depend on five factors: 1. The base performance grade dictated by the climate 2. The grade adjustment required for traffic level and speed 3. The performance grade specified by the state highway agency or other authority 4. The effect of any RAP in the mix on the final effective performance grade of the binder 5. The available asphalt binders. In most cases, the performance grade will be specified by the state highway agency or other owner/agency and will not be selected during the mix design process. However, private clients will often leave binder selection up to the contractor or HMA supplier. Engineers and technicians involved in the design of HMA mixes should keep in mind that in many cases the available binder grades in a given geographic area are limited, given that state highway agencies tend to specify a limited number of binder grades, often referred to as a "binder slate." For example, a state might specify only PG 58-28, PG 64-22, and PG 76-22 binder grades, even though some areas could use a PG 70-22 binder grade--in these locations, a PG 76-22 binder grade would be used. This is done to simplify the mix design process, and to limit the number of binders that must be produced by refineries or delivered to local terminals. In those special cases demanding a binder grade outside those normally available, the binder can be ordered, but the cost might be higher compared to binders within the standard slate. In those cases where the binder grade must be determined as part of the mix design process, the required final performance grade will depend on five factors: 1. Local climate, dictating the base performance grade 2. Design traffic level 3. Average traffic speed 4. The amount of RAP added to the mix 5. The performance grade of the RAP binder Details on handling RAP in HMA mix designs are presented in Chapter 9 of this manual; what follows is a brief discussion of the effect of RAP on binder performance grade selection. The essential concept that must be understood is that RAP contains significant amounts of asphalt binder, and this binder has usually undergone significant age hardening. Therefore, adding RAP to an HMA mixture tends to increase the effective grade of the binder in the final mix. For example, an HMA mix made with a PG 64-22 binder and 25% RAP might have an effective binder grade of PG 70-16. If the required grade for the mix is a PG 64-22, a softer binder grade must then be added to this mix--say, a PG 58-28 rather than a PG 64-22, so that the blend of the new binder and RAP binder meets the requirements of a PG 64-22.

OCR for page 108
Design of Dense-Graded HMA Mixtures 109 The effect of RAP on the final effective binder grade can be handled in three different ways. In the simplest case, if the RAP content is 15% or less, no adjustment is made to the performance grade of the new binder added to the HMA. At higher RAP contents, two approaches can be used in making sure the final effective binder grade meets the given requirements. In the first approach, the desired amount of RAP is selected, and the performance grade of the new binder to be added to the mix is determined so that the final effective performance grade meets the established performance grade requirements. The second approach is to select the performance grade of the new binder and then determine the minimum and maximum amounts of RAP that can be added while still meeting the established performance grade requirements for the final effective binder grade. Either approach can be used with HMA Tools, which gives both the final effective binder grade for a given RAP content and the new binder performance grade, and also the minimum and maximum allowable RAP content for a given new binder performance grade and RAP stockpile or blend of RAP stockpiles. A minimum RAP content requirement can occur when the selected new binder performance grade is softer than the required grade, so that the addition of RAP is needed to effectively stiffen the asphalt binder. For example, a given location might require a PG 70-22 binder, but a PG 64-28 binder is selected because it is expected that the addition of RAP to the mixture will stiffen the final effective binder grade. However, in such a situation, the technician must make sure that enough RAP is actually added to the mix so that the effective binder meets the final grade requirement of PG 70-22. An important issue related to the effect of RAP on binder grade is the effect of RAP on the variability of various mix properties. In some cases, the maximum amount of RAP that can be added to an HMA mix will be limited by variability, rather than by the final binder performance grade. This issue is discussed in detail in Chapter 9. The FHWA's software program, LTPPBind, provides information on climate, base performance grade, and performance grade adjustments for traffic and speed. As of September 2008, information on LTPPBind, including a free download of the program, could be found at gov/pavement/ltpp/ltppbind.cfm. One limitation to the current LTPPBind, Version 3.1, is that only fast and slow traffic speeds are addressed, and the specific speeds in kph corresponding to these categories are not given, although it appears that fast traffic corresponds to an average speed of about 70 kph, and slow traffic to a speed of about 35 kph. Performance grade adjustments for very slow traffic are not addressed. Fortunately, recent research published in NCHRP Report 567: Volumetric Requirements for Superpave Mix Design has provided a model relating binder perfor- mance grade, compaction level (Ndesign), traffic level, traffic speed, and mix composition to rutting. This model was used to develop high-temperature performance grade adjustments, which are given in Table 8-1. These grade adjustments differ only slightly from those given in LTPPBind Table 8-1. High temperature binder grade adjustments for traffic level and speed. Grade Adjustment for Average Vehicle Speed in kph (mph): Design Very Slow Slow Fast Traffic < 25 25 to < 70 70 (MESALs) (< 15) (15 to < 45) ( 45) < 0.3 --- --- --- 0.3 to < 3 12 6 --- 3 to < 10 18* 13 6 10 to < 30 22* 16* 10 30 --- 21* 15* * Consider use of polymer-modified binder. If a polymer- modified binder is used, high-temperature grade may be reduced one grade (6 C), provided rut resistance is verified using suitable performance testing.

OCR for page 108
110 A Manual for Design of Hot Mix Asphalt with Commentary for fast traffic. However, the assumed slow traffic speed in Table 8-1 is somewhat slower than that apparently used in LTPPBind and the adjustments are correspondingly larger. Also, as mentioned above, Table 8-1 includes adjustments for very slow traffic--a speed category not included in LTPPBind v. 3.1. Theoretically, high-temperature performance grades could be reduced for faster traffic at the lowest design traffic level (< 0.3 MESALs). However, construction of such pavements will often be poorly controlled and the use of softer binder grades could result in rutting, shoving, and flushing in many cases. No grade adjustments for very slow speed are given for the highest traffic level because such a combination of traffic speed and volume cannot occur-- if the traffic on a road is very slow, the volume cannot be extremely high because the vehicles are not moving at a fast enough speed. This brings up another important point in applying Table 8-1 and similar grade adjustments, such as those given in LTPPBind: the traffic speeds are average speeds for the road, not minimum speeds. Average speeds should be those determined from traffic studies or other objective procedures, not personal judgment. A final difference between Table 8-1 and the traffic and speed adjustments given in LTPPBind is that the adjustments given in Table 8-1 are for binders that are not polymer modified; as explained in the note for the table, the high-temperature performance grade requirement can be reduced by one grade (6C) if a polymer- modified asphalt binder is used, and if the mix successfully meets appropriate performance testing requirements, as discussed later in this chapter. A second important high-temperature grade adjustment must be made for temporary con- struction. This is necessary for two reasons: (1) lack of age hardening and (2) greater potential for extremely hot weather. A pavement carrying 20 million ESALs over 20 years will experience 1 million ESALs per year. During the first 2 to 3 years, such a pavement will undergo significant age hardening, which will greatly reduce the rate of rutting after this initial loading period. A pavement designed to carry 20 million ESALs over 2 years, on the other hand, will carry 10 million ESALs before significant age hardening occurs, and the final 10 million ESALs will be applied after only a modest amount of age hardening. The result will usually be substantial, even catastrophic failure, unless the high-temperature performance grade is adjusted upward. The other factor that must be considered is variability in climate. Occasional extremely hot summers over a 20-year design life are to be expected, but the damage will usually not be severe, given that the amount of traffic traveling over the pavement over a few such summers is limited. Using the same example of a pavement designed for very heavy traffic over 2 years, a single very hot summer would mean that about half of the design traffic would travel over the pavement while in a very soft condition; again, the results could be catastrophic rutting. For this reason, as in the Superpave system, all HMA mixtures should be designed for the projected 20-year traffic level. For example, if a temporary road is to carry 1,500,000 ESALs over 2 years, the mix should be designed not for 1,500,000 ESALs but for 15 million ESALs. Another question that must sometimes be addressed during binder selection is whether or not to select a polymer-modified asphalt binder. A common rule is that when the two numbers in a performance grade add to a number more than 90, the resulting binder must be modified. For example, most PG 64-22 binders are not modified (64 + 22 = 86), whereas most PG 76-22 binder are (76 + 22 = 98). However, refineries and chemical manufacturers are constantly developing new types of binder modification, and the performance of modified binders is not completely reflected in the current performance grading system. For example, the rut resistance and fatigue resistance exhibited by many commercially available polymer-modified binders is significantly better than non-modified binders of the same grade. For this reason, some states require that polymer-modified asphalt binder be used in certain critical situations. Such specifications often include one or two additional tests, such as elastic recovery, meant to ensure that only certain types of modification be used for these critical applications. Applicable state specifications must be reviewed to determine if such requirements apply. As noted in Table 8-1, consideration should be given to using polymer-modified binders in cases where