|FHWA > Pavements > Pavement Recycling Guidelines for State and Local Governments > Chapter 7|
Pavement Recycling Guidelines for State and Local Governments
|State||Sample Frequency||Sample Size|
|Arizona||3 cores/1.6 lane-km||150 mm diameter for the full depth of structure|
|Florida||1 set of 3 cores/1.6 lane-km. Minimum 2 sets of 3 cores per lane.||150 mm diameter for the full depth of structure|
|Kansas||3 Cores/1.6 lane-km Minimum 30 cores.||100 mm diameter for the full depth of structure|
|Nevada||1 core/750 lane-m||100 mm diameter for the full depth of structure|
|Texas||10 cores/project||150 mm diameter for the full depth of structure|
|Wisconsin||1 core/800 m||Surface area minimum of 230 cm2|
|Wyoming||2 cores/km||150 mm diameter for the full depth of structure|
Figure 7-1. Flow chart of sampling plans.(modified from 2)
The RAP material needs to be evaluated before the actual mix design. This is because with aging and oxidation certain significant changes occur in the HMA. For the binder, this includes loss of the lighter fractions and a corresponding increase in the proportions of the asphaltenes, hardening (increase in viscosity), and loss of ductility. The gradation of the aggregate may change due to degradation caused by traffic loads and the environment. Hence the composition of the RAP must be determined at the beginning. Most agencies determine aggregate gradation, asphalt content, and asphalt viscosity at 60°C for the reclaimed asphalt pavement. The aged asphalt binder must be extracted from a representative sample of the RAP to determine these properties. The following guidelines are suggested for aggregate and binder evaluation.(4)
Aggregate Evaluation: AASHTO T30, Mechanical Analysis of Extracted Aggregate, or AASHTO T27, Sieve Analysis of Fine and Coarse Aggregates, should be used to perform a sieve analysis of the aggregate extracted from the RAP. Any deficiency can be corrected by blending appropriate sieve fractions of virgin and/or reclaimed aggregate with reclaimed asphalt pavement aggregate. It is also recommended to examine the angularity of both coarse and fine aggregates. Extraction: AASHTO T 164, Quantitative Extraction of Bitumen from Bituminous Paving Mixtures should be used to quantitatively extract the asphalt binder from RAP if the extracted binder is to be recovered and tested for further evaluation. The National Center for Asphalt Technology's (NCAT's) ignition test can be used to determine the RAP composition (asphalt content and aggregate gradation) if the asphalt binder is not intended to be recovered.
Asphalt Binder Evaluation: The extracted asphalt is recovered from the solution by AASHTO T 170. Recovery of Asphalt from Solution by Abson Method. SHRP has developed an improved method of recovering asphalt binder from solution: SHRP Designation B-006 Extraction and Recovery of Asphalt Cement for Rheological Testing. This method is preferable to AASHTO T170. AASHTO T 202 should then be used to check the consistency of the recovered asphalt binder by measuring its viscosity at 60°C, to estimate the amount and grade of virgin asphalt binder required in the recycled mix. Some agencies also test the penetration at 25°C of the recovered asphalt binder.
If no more than 15-20 percent RAP is used in the recycled mix, testing of the extracted asphalt is not required by many highway agencies and the grade of the virgin asphalt binder is kept the same as that of the conventional mix.
There are four basic purposes for using recycling agents.(7) These are:
Recycling agents have been defined as organic materials with chemical and physical characteristics selected to restore aged asphalt to desired specifications.(4) In selecting the recycling agent, the viscosity characteristics of the combined aged asphalt binder and the recycling agent are the determining factors. These agents are also known as softening agents, reclaiming agents, modifiers, fluxing oils, extender oils, and aromatic oils.(7) The Pacific Coast User-Producer Group has defined recycling agent as a hydrocarbon product with physical characteristics selected to restore aged asphalt binder to requirements of current asphalt binder specifications. Under this definition, softer asphalt and specialty products can be classified as recycling agents.(7) Asphalt cements can be used when an increase in total binder content of the recycled mix is required and the specific grade can be blended with the aged asphalt binder in the RAP to yield an asphalt binder meeting the desired specifications. Generally, AC-10, AC-5 or AC 2.5 (85-100, 120-150 or 200-300 pen; AR-4000, AR-2000 or AR-1000) asphalt cements are used for this purpose.(4) Use of such soft grades of asphalt binder in hot mix recycling is more prevalent in the U.S. compared to the use of commercial recycling agents. If the aged asphalt binder has a very high viscosity (or low penetration) or the percentage of RAP in the recycled HMA is much greater than 50 percent, a relatively small amount of a commercial recycling agent can be used to modify the aged asphalt binder without altering the desired binder content. Then, if additional binder is required, the normal grade of asphalt for a virgin mix can be added, though this necessitates that the plant be capable of adding two binder materials.(8) Recycling agents in emulsion form have the potential advantages of improved fluxing, mixing and temperature control to prevent localized overheating in drum mixers. Furthermore, the formulation of the emulsion can be adjusted to provide the design end result viscosity of the binder in the recycled HMA mix. A disadvantage is that additional heat is required to remove the 30 to 35 percent water contained in the emulsion.(9) To ensure the proper function of the modifiers, the following properties are suggested for specification purposes:(7)
Several tests have been investigated by various agencies for evaluation of recycling agents. Table 7-2 shows the physical properties of hot mix recycling agents contained in ASTM Standard Practice D4552 Classifying Hot-Mix Recycling Agents.(10) The important properties mentioned are as follows:
|TEST||ASTM Test Method||RA 1||RA 5||RA 25||RA 75||RA 250||RA 500|
|Viscosity at 140°F, cSt||D 2170 or D 2171||50||175||176||900||901||4500||4501||12500||12501||37500||37501||60000|
|Flash point, COC, °F||D92||425||...||425||...||425||...||425||...||425||...||425||...|
|Saturates, wt%||D 2007||...||30||...||30||...||30||...||30||...||30||...||30|
|Tests on residue from RTFO or TFO oven 325 °F||D 2872 or D 1754|
|Wt change ± %||-||...||4||...||4||...||3||...||3||...||3||...||3|
|Specific gravity||D 70 or D 1298||Report||Report||Report||Report||Report||Report|
The choice of Recycling Agent (RA) grade will depend on the amount and hardness of the asphalt in the aged pavement. In general, the lower viscosity RA types can be used to restore aged asphalts of high viscosity and vice versa.
Additionally, grades RA 1, RA 5, RA 25 and RA 75(10) will generally be most appropriate for hot mix recycling of salvaged asphalt concrete when no more than 30 percent virgin aggregate is added, while grades RA 250 and RA 500 will generally be most appropriate when more than 30 percent virgin aggregate is incorporated into the mix.(10) Specifications for emulsified recycling agents are normally based on the specifications for recycling agents with additional tests that are identical or similar to those for asphalt emulsions (table 7-3). ASTM D5505 Standard Practice for Classifying Emulsified Recycling Agents can be used to select an appropriate emulsified recycling agent. The choice of emulsified recycling agent will be determined by the consistency of the binder in the aged pavement, the methods of recycling planned, the amount, if any, of new aggregates, and other design needs.(10)
|Viscosity, 50°C, SSF||D 244||100||20||450||20||450|
|Sieve, %||D 244||0.1||0.1||0.1|
|Storage stability, 24 h, %||D 244||1.5||1.5||1.5|
|Residue, by distillation, %||D 244||65||65||65|
|Specific gravity||D 70||report||report||report|
|On residue from distillation|
|Viscosity, 60°C, cSt||D 2170||50||200||30||30|
|Saturates, %||D 2007||30|
|Solubility in trichloroethylene||D 2042||97.5||97.5||97.5|
|On residue from distillation after RTFOC|
|Penetration, 4°C, 50 g 5 s||D 5||75||200||5||75|
|RTFO, weight change, %||D 2872||4||4||4|
Figure 7-2 shows the flow chart recommended for the different steps in mix design of recycled mixes.(4) Conventional recycled mix design will be presented first followed by the Superpave recycled mix design.
Figure 7-2. Flow chart for mix design procedure.
|For Asphalt Content|
|by weight of total mix||by weight of aggregate|
|% New Asphalt, Pnb||
|% RAP, Psm||
|% New Aggregate, Pns||r|
|TOTAL||100||100 + Pb|
|% New Asphalt to|
Total Asphalt Content, R
|Psm||=||Percent salvaged mix (RAP) in recycled mix|
|Pb||=||Asphalt content of recycled mix, %|
|Psb||=||Asphalt content of salvaged mix (RAP), %|
|Pnb||=||Additional asphalt and/or recycled agent in recycled mix, %|
|Pns||=||Percent additional aggregate (new aggregate material)|
|r||=||Percent new aggregate material to total aggregate in recycled mix|
|R||=||Percent new asphalt and/or recycling agent to total asphalt in recycled mix|
75 percent new aggregate
100 percent total
The amount of new aggregate is 75 percent. Hence, r = 75. Table 7-4(4) contains formulas for proportioning materials for recycled HMA mixes where the blend of aggregates in the mix is kept constant.
The approximate asphalt demand of the combined aggregates may be determined by the Centrifuge Kerosene Equivalent (CKE) test included in the Asphalt Institute Hveem Method of Mix Design, or calculated by the following empirical formula:
P = 0.035a + 0.045b + Kc + F
|P||=||approximate total asphalt demand of recycled mix, percent by weight of mix|
|a||=||percent of mineral aggregate retained on 2.36 mm sieve, expressed as a whole number|
|b||=||percent of mineral aggregate passing the 2.36 mm sieve and retained on the 75 µm sieve, expressed as a whole number|
|c||=||percent of mineral aggregate passing the 75µm sieve|
|K||=||0.15 for 11-15 percent passing 75 µm sieve, 0.18 for 6-10 percent passing 75µm sieve, and 0.20 for 5 percent or less passing 75 µm sieve|
|F||=||0 to 2.0 percent. Based on absorption of light or heavy aggregate. In the absence of other data, a value of 0.7 is suggested|
With an approximate asphalt demand established, this will provide a basis for a series of trial mixes for a mix design. Trial mixes will vary in asphalt contents in 0.5 increments on either side of the calculated approximate asphalt demand.
For example, suppose that the approximate asphalt demand was calculated to be 6.2 percent. A series of trial mixes then range from 5.0 to 7.0 percent or from 5.5 to 7.5 percent.
|Pnb =||( 1002 - r Psb ) Pb||-||( 100 - r ) Psb|
|100 ( 100 - Psb )||100 - Psb|
|Pnb||=||Percent of new asphalt binder in recycled mix (plus recycling agent, if used), expressed as whole number|
|r||=||new aggregate expressed as a percent of the total aggregate in the recycled mix expressed as a whole number|
|Pb||=||percent, estimated asphalt content of recycled mix (assumed to be the same as that of 100 percent virgin HMA mix or determined as an approximate asphalt demand of combined aggregates in the preceding step)|
|Psb||=||percent, asphalt content of reclaimed asphalt pavement (RAP) (plus recycling agent, if used)|
For example, suppose the asphalt content, Psb of the RAP is 4.7 percent and r = 75 percent, then
|Pnb =||( 1002 - 75 X 4.7 ) Pb||-||( 100 - 75 ) 4.7||= 1.01 Pb - 1.23|
|100 ( 100 - 4.7 )||100 - 4.7|
The percentages of new asphalt binder for any asphalt content may now be readily determined. The formula above is for asphalt content expressed as percent by weight of total mix. If asphalt contents are expressed as percent by weight of aggregate the formula for calculating quantity of new asphalt binder is:
|Pnb = Pb -||( 100 - r ) Psb|
Figure 7-3. Asphalt viscosity blending chart.(4)
The percent of the new asphalt, Pnb, to the total asphalt content, Pb, is expressed by the following formula:
|R =||100 Pnb|
For example, suppose the mix described in Step (3) is to have an estimated total asphalt content of 6.2 percent. The amount of new asphalt to be added (from Step 3) is:
Pnb = 1.01 X 6.2 - 1.23 = 5.0 percent
|R =||100 ( 5.0 )||= 81|
The grade of new asphalt binder (and/or recycling agent) is determined using a log-log viscosity versus percent new asphalt binder blending chart such as figure 7-3.(4) A target viscosity for the blend of recovered asphalt and the new asphalt (and/or recycling agent) is selected. As mentioned earlier, the target viscosity is usually the viscosity of the mid range of the grade of asphalt binder normally used depending on type of construction, climatic conditions, amount and nature of traffic.
Plot the viscosity of the aged asphalt in the RAP on the left hand vertical scale, Point A, as illustrated in figure 7-3. Draw a vertical line representing the percentage of new asphalt binder, R, calculated above and determine its intersection with the horizontal line representing the target viscosity (2,000 poises in this example), Point B. Then draw a straight line from Point A, through Point B and extend it to intersect the right hand scale, Point C. Point C is the viscosity at 60°C (140°F) of the new asphalt binder (and/or recycling agent) required to blend with the asphalt binder in the reclaimed asphalt pavement (RAP) to obtain the target viscosity in the blend. Select the grade of new asphalt binder that has a viscosity range that includes or is closest to the viscosity at Point C. To plot a point using the vertical scale, consider expressing the viscosity using 10 raised to some power. For example, 75,000 poises would be 7.5 X 104. To plot the point on the vertical scale, 7.5 would be interpolated on the scale between 104 and 105. It is suggested that when selecting a grade of asphalt cement for recycling that the following guide be used:
|Up to 15 percent RAP||=||No change in asphalt binder grade (some highway agencies use 20 percent in lieu of 15 percent)|
|16 percent RAP or more||=||Use asphalt binder one grade softer than that normally specified for 100 percent virgin HMA mix. For example, use AC-10 in lieu of AC-20. Do not change more than one viscosity grade unless the recycled HMA mix is checked for resistance to rutting.|
Design Example 1: The reclaimed asphalt pavement has an asphalt content of 5.4 percent by weight of total mix. The viscosity of the asphalt binder recovered from the reclaimed asphalt pavement (RAP) is 46,000 poises at 60°C (140°F). The grade of asphalt cement normally used is AC-20, and the target viscosity at a temperature of 60°C (140°F) is 2,000 poises. Gradation of RAP and new aggregate is:
|Sieve Size||Percent Passing|
|RAP Agg.||New Agg.|
|25.0 mm (1 in)||100||100|
|19.0 mm (3/4 in)||98||93|
|9.5 mm (3/8 in)||85||53|
|4.75 mm (No. 4)||65||30|
|2.36 mm (No. 8)||52||16|
|300 µm (No. 50)||22||5|
|75 µm (No. 200)||8||1|
Approximately 30 percent of RAP was selected because
|Sieve Size||Percent Passing|
|30% RAP Agg.||70% New Agg.||Combination Agg.|
|25.0 mm (1 in)||[100 × 0.3 = 30.0]||[100 X 0.7 = 70.0]||100.0|
|19.0 mm (3/4 in)||[98 × 0.3 = 29.4]||[93 X 0.7 = 65.1]||94.5|
|9.5 mm (3/8 in)||[85 × 0.3 = 25.5]||[53 X 0.7 = 37.1]||62.6|
|4.75 mm (No. 4)||[65 × 0.3 = 19.5]||[30 X 0.7 = 21.0]||40.5|
|2.36 mm (No. 8)||[52 × 0.3 = 15.6]||[16 X 0.7 = 11.2]||26.8|
|300 µm (No. 50)||[22 × 0.3 = 6.6]||[5 X 0.7 = 3.5]||10.1|
|75 µm (No. 200)||[8 × 0.3 = 2.4]||[1 X 0.7 = 0.7]||3.1|
Then: r = 70
The job specification for aggregate gradation (for 19 mm nominal size) is:
|Sieve Size||Percent Passing|
|Max. Size % Pass||Combined Agg. % Pass|
|25.0 mm (1 in)||100||100.0|
|19.0 mm (3/4 in)||90-100||94.5|
|9.5 mm (3/8 in)||56-80||62.6|
|4.75 mm (No. 4)||35-65||40.5|
|2.36 mm (No. 8)||23-49||26.8|
|300 µm (No. 50)||5-19||10.1|
|75 µm (No. 200)||2-8||3.1|
Step 2 - Approximate asphalt demand of combined aggregates
|P||=||0.035a + 0.045b + Kc + F|
|=||0.035 × 73.2 + 0.045 × 23.7 + 0.20 x 3.1 + 1.0|
Step 3 - Estimated percent of new asphalt binder in mix
|=||1.02 Pb - 1.71|
For an approximate asphalt binder demand of 5.2 percent:
Pnb = 1.02 ( 5.2 ) - 1.71 = 3.6 percent
The percent of new asphalt binder, Pnb, to total asphalt, Pb, will then be
|R =||100 ( 3.6 )||= 69 percent|
Step 4 - Select grade of new asphalt binder
On figure 7-4, Point A is the viscosity of the aged asphalt binder at 46,000 poises (4.6 × 104). Point B is located from a target viscosity of 2,000 poises (2.0 × 103) and R = 69. The projected line from Point A through Point B to Point C indicated that the viscosity of the new asphalt binder is 7.0 × 102 (700).
Figure 7-4. Asphalt viscosity blending chart (design example 1).
Since AC-20 is the normal grade of asphalt cement used in the area of construction, climate and traffic, an AC-10 will be chosen for this project. The AC-10 when blended with the aged asphalt binder in the RAP should result in an AC-20 within acceptable tolerances.
Step 5 - Trial mix design
Using an aggregate blend of 70 percent new aggregate and 30 percent RAP aggregate, trial mixes of different asphalt contents (varying in 0.5 percent increments on either side of the estimated asphalt demand) are prepared according to standard Marshall or Hveem mix design procedures.
The formulas in table 7-4 may be used to calculate the percentages of each ingredient in the trial mixes. Since the formula for Pnb was calculated in Step 3, the formulas for proportioning Psm and Pns are:
|=||31.91 - 0.32 Pb|
|Asphalt Content, Pb||4.5||5.0||5.5||6.0||6.5|
|Pnb = 1.02 Pb - 1.71||2.9||3.0||3.9||4.4||4.9|
|Psm = 31.71 - 0.32 Pb||30.3||30.1||29.9||29.8||29.6|
|Pns = 70 - 0.70 Pb||66.8||66.5||66.2||65.8||65.5|
When preparing trial mixes in the laboratory, it is suggested that the RAP be heated to mixing temperature and maintained at that temperature. The new aggregates are normally heated to 10°C (50°F) above the mixing temperature. When the aggregate and RAP have been weighed out, dry mixing should begin to thoroughly blend the materials before adding new asphalt. Keeping the RAP at elevated temperatures should be held to a minimum (not more than one hour). Otherwise, normal mix design procedures are followed.
Step 6 - Select job mix formula
The optimum new asphalt content and the mix design are determined according to established standard Marshall or Hveem mix design criteria (as is used for virgin materials).
Design Example 2: Reclaimed asphalt pavement has an asphalt content of 6.0 percent with a viscosity of 100,000 poises. Gradation of RAP and new aggregate are the same as for Example 1.
Steps 1 and 2 - Same as Example 1
Step 3 - Estimate percent of new asphalt in mix
|=||1.02 Pb - 1.91|
For an approximate asphalt demand of 5.2 percent:
Pnb = 1.02 ( 5.2 ) - 1.91 = 3.4 percent
Step 4 - Select grade of new asphalt binder
On figure 7-5, Point A is the viscosity of the aged asphalt binder at 100,000 poises (1.0 × 105). Point B is located using values of 2,000 poises (2.0 X 103) for target viscosity and R = 57, (100Pnb/Pb = 100 × 3.4/6.0) of new binder asphalt. A line is projected through these two points and intersects the right axis at 1.8 × 102 (180 poises), Point C.
Figure 7-5. Asphalt viscosity blending chart (design example 2).
This is a heavily-traveled roadway where the design engineer is concerned with rutting and normally uses an AC-20 in mix design. Figure 7-6(4) can be used to determine how much of the recycling agent to blend with AC-20 to give an apparent viscosity of 180 poises.
Figure 7-6. Comparison of penetration grades and viscosity grades of asphalt cement (based on RTFOT residue for AR-grades and penetration grades; TFOT residue for AC- grades).
Let the AC-20 be the new asphalt binder and plot 2,000 poises (2.0 × 103) on the left-hand scale, Point D (figure 7-5). The viscosity of the recycling agent is 1 poise. Plot this as Point E on the right-hand scale. Connect Points D and E with a straight line. Now determine what percentage, R, of recycling agent will be required to result in a viscosity of 180 poises for the blend. This is plotted as Point F on the line from D to E. The percentage R on the horizontal scale indicates 22 percent. This means that a tank of AC-20 containing 22 percent of the recycling agent should have a viscosity of approximately 180 poises. When this blend is added to the mix for a total asphalt content of about 5.2 percent, the viscosity of the total asphalt binder in the recycled mix should be 2,000 poises - within acceptable limits.
Step 5- Trial mix design
Using an aggregate blend of 70 percent new aggregate and 30 percent RAP aggregate, trial mixes of different asphalt contents (varying in 0.5 percent increments on either side of the estimated asphalt demand) are prepared according to standard Marshall or Hveem mix design procedures. The formulas in table 7-4 may be used to calculate the percentages of each ingredient in the trial mixes. Since the formula for Pnb was calculated in Step 3, the formulas for proportioning Psm and Pns are:
|=||31.91 - 0.32 Pb|
|Asphalt Content, Pb||4.0||4.5||5.0||5.5||6.0|
|Pnb = 1.02 Pb - 1.91||2.2||2.7||3.2||3.7||4.2|
|Psm = 31.91 - 0.32 Pb||30.6||30.5||30.3||30.1||30.0|
|Pns = 70 - 0.70 Pb||67.2||66.8||66.5||66.2||65.8|
When preparing trial mixes in the laboratory, it is suggested that the RAP be heated to and maintained at the mixing temperature. The aggregate is normally heated to mixing temperature plus 10°C (50°F). When the aggregate and RAP have been weighed out, dry mixing should begin to thoroughly blend the materials before adding new asphalt binder. Keeping the RAP at elevated temperatures should be held to a minimum (not more than one hour). Otherwise, normal mix design procedures are followed.
Step 6 - Select job-mix formula
The optimum new asphalt content and the mix design are determined according to established standard Marshall or Hveem mix-design criteria (as is used for virgin materials). If the Marshall mix design is used, select the optimum asphalt content which gives 4.0 percent air void content.
Superpave technology, a part of the Strategic Highway Research Program (SHRP), developed a performance-based specification for asphalt binders.(11,12,13) The performance grade (PG) of asphalt binder is designed to improve the performance of HMA pavements at three service temperatures. The PG binder specifications consist of
PG grading system contains two numbers which represent high and low service temperatures prevailing at the project site. For example, a PG 64-28 binder is designed to minimize rutting at the high pavement temperature of 64°C (147°F) and to minimize low temperature cracking down to -28°C (-18°F) pavement temperature.
The Superpave technology also consists of a volumetric mix design system in which a Superpave gyratory compactor (SGC) is used.(14) The following six steps, as shown in figure 7-7 are also used when preparing a hot recycled mix design using Superpave technology.(15)
Figure 7-7. Steps in preparing hot recycled mix using Superpave technology.
This step is exactly the same as used for conventional hot recycled mix design given earlier.
This step is also the same as conventional hot recycled mix design given earlier. An alternate method is given in the Superpave volumetric mix design method(16) which calculates estimated asphalt content based on the combined aggregate gradation. However, the procedure is very tedious. It is recommended to assume the estimated asphalt content of the recycled HMA mixture equal to that of 100 percent virgin HMA mixture.
This step is also the same as conventional hot recycled mix design given earlier.
Based on the research conducted at the National Center for Asphalt Technology,(15) the following three tier procedure is recommended to select the Superpave PG grade of the new asphalt binder:
Figure 7-8. Recommended specific grade blending chart with 1.0 and 2.0 kPa stiffness lines.
Figure 7-9. Graphical method to determine minimum and maximum amount of virgin asphalt binder in the recycled asphalt binder.(15)
Suppose PG 64-28 was specified for a paving project. The G*/sinδ measured at 64°C of the aged and virgin asphalt binder (PG 64-28) were 100 kPa and 1.13 kPa, respectively. These values were plotted as Point A and Point B as shown in figure 7-9. The line AB intersected the 2.0 kPa stiffness line at 85 percent. Therefore, the amount of the virgin asphalt binder PG 64-28 that can be added in the recycled mix was 85 to 100 percent (or about 0 to 15 percent RAP). Suppose an asphalt binder PG 58-34 was selected as the virgin asphalt binder. The G*/sinδ as measured at 64°C for the PG 58-34 was 0.65 kPa and plotted as point C in figure 7-9. The line AC intersected the 1.0 kPa and 2.0 kPa stiffness lines at 72 percent and 89 percent, respectively. Therefore, the amount of the virgin asphalt binder PG 58-34 that can be used in the recycled mix is 72 to 89 percent (or about 11 to 28 percent RAP).
It should be noted that the low temperature grade of the selected virgin asphalt binder should always be at least one grade below the specified PG grade. For example, if the specified low temperature grade for 100 percent virgin mixtures is -28°C, then the low temperature grade of the virgin binder should be -34°C.
Note: Research is continuing at the present time (1996) to develop Superpave blending charts based on G*sinδ (fatigue factor) and low temperature properties of asphalt binder in addition to G*sinδ (rutting factor) used here.
Trial mix designs are made using the Superpave gyratory compactor (SGC) following the Superpave volumetric mix design procedures.(15) It may be necessary to evaluate different aggregate gradations to obtain an acceptable aggregate structure meeting the Superpave criteria. The preparation of recycled mixtures such as heating of RAP, new aggregate, and asphalt binder and mixing procedures are similar to those used in conventional recycled mix design given earlier.
The two main steps in design of hot mix recycling are material evaluation and mix design. The materials evaluated include the RAP and the recycling agent. A random sampling plan should be devised to obtain representative samples of the RAP. The sampling plan and frequency should be decided based on historical, construction, and material data, and if needed, the pavement may be divided into different sub sections before sampling. To evaluate the RAP material, which undergoes changes with time and traffic, its gradation, asphalt content, and rheological properties of aged asphalt binder must be determined. The recycling agent should conform to the applicable AASHTO or ASTM standards. Hot recycled mix design involves the determination of the combined gradation of the aggregates and the required amount of new aggregate to meet the target gradation. Next, the amount of new (virgin) asphalt binder required in the recycled mix is estimated. Blending charts (based on viscosity or Superpave rutting factor G*/sinδ) are then used to select the grade of virgin asphalt binder. A series of trial mixes are then made with different asphalt contents. The optimum asphalt content for the recycled mix is selected based on Marshall, Hveem, or Superpave volumetric mix design procedures.
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