University of Garyounis Faculty of Engineering Civil Engineering Department
HIGHWAY & T RANSPORTATION LAB CE335 REPORT:
Superpave Mix Design
NAME : J EBREEL OMAR ALSHUKRI. ST
NO
: 13020
DATE : 20-12-2009
Superpave Overview "Superpave" is an overarching term for the results of the asphalt research portion of the 1987 - 1993 Strategic Highway Research Program (SHRP). Superpave consists of (1) an asphalt binder specification, (2) an HMA mix design method and (3) HMA tests and performance prediction models. Each one of these these components is referred to by the term "Superpave". This section provides a brief overview and background of Superpave. Superpave.
WSDOT Superpave Mix Design Process The WSDOT process is quite similar to the Hveem method described in the Asphalt Institute's Superpave Mix Design (SP-02), 3rd edition, however, there are some differences. Some major points to note about about the WSDOT method are: •
WSDOT is responsible for determining the optimum asphalt content and antistrip percentage needed for the contractor-submitted mix design. The contractor produces several trial blends then sends in his/her proposed aggregate blends (three blends total - the proposed blend plus a coarse and a fine blend on either side of the proposal) along with the proposed asphalt binder type and asphalt content. WSDOT then runs the proposal through the mix design process. In many states the individual contractors are responsible for their own mix design.
•
When using the AASHTO tables for the various Superpave requirements (Tables 5.5, 5.6, 5.7, 5.8, 5.10, 5.11), WSDOT uses a 15-year traffic loading instead of the listed 20-year period because WSDOT typically designs overlays for a 15year design life. life. This difference does result in in fewer design design ESALs (because WSDOT uses 5 fewer years) but usually does not result in a selected category being different than if a 20-year traffic loading were used.
•
An approved aggregate stockpile need not be tested for aggregate properties on subsequent mix designs.
•
WSDOT tests asphalt binder contents at the contractor's desired level and typically at ± 0.5 percent.
•
WSDOT makes separate Hveem samples and tests them for stability stability.. This test is not required in Superpave mix design and is only done for informational purposes.
•
WSDOT uses the modifie modified d Lottma Lottman n test to determine the optimum amount of antistrippin antistripping g asphalt binder modifier .
The following is a brief description of the WSDOT Superpave mix design process.
1. Aggregate Selection The contractor who will be doing the paving sends WSDOT three trial aggregate blends
(typically a coarse, fine, and middle ground gradation) along with laboratory data for each of these blends. The contractor indicates which of the gradations gradations he/she would like to use use and designates a design design asphalt content. The aggregate requirements for Superpave are checked by the contractor during his/her trial blend process, then again by WSDOT during the confirmation of the contractor's proposal. The coarse aggregate angularity requirements are determined by the number of ESALs to which the roadway will be subjected. Additionally: •
Unless the aggregate comes from a previously WSDOT-approved stockpile, testing is done to confirm the aggregates meet WSDOT specifications. specifications. As of 2002 aggregate sources are approved for 5 years, although some sources have not been switched over from the previous 10-year approval interval.
•
Each of these trial blends must be within the Superpave gradation zone(although requirements and preferably not pass through the restricted zone(although WSDOT does accept a mix design that goes through the restricted zone if it meets all other requirements).
•
The bulk specifi s pecific c gravity gr avity (Gsb) of the coarse and fine aggregate is determined for each stockpile. In this case, material retained on the the 4.75 mm (No. 4) is considered "coarse", while the rest is considered "fine".
Figure 1: Aggregate Samples
Figure 2: Preparing the Graded Sample
2. Binder Selection The contractor who will be doing the paving sends WSDOT the brand and type of binder and antistrip modifier to be used. Actual asphalt binder samples are sent from the asphalt producers whenever needed throughout the year. Producers typically send anywhere anywhere from 10 to 40 cans (see Figure 3) at a time (depending on the binder type - typically they send fewer of the modified binders). The asphalt binder shall conform to AASHTO MP 1 requirements (Superpave PG binder system). system). WSDOT only allows one asphalt binder type submission for Superpave jobs. WSDOT determines the asphalt asphalt binder's specific gravity for use in the mix design process.
3. Sample Preparation Typically, six initial samples are made: two at the design asphalt content, two at 0.5 percent below the design asphalt c ontent and two at 0.5 percent above the design asphalt content. These six samples are then cured and conditioned conditioned according to AASHTO PP 2 and compacted in the Superpave gyratory compactor in accordance with AASHTO TP 4. Additionally, three samples (one at each of the above asphalt contents) are made and compacted in the California kneading compactor for use in stability tests. •
AASHTO PP 2: Mixture Conditioning of Hot Mix Asphalt
•
AASHTO TP 4: Method for Preparing and Determining the Density of Hot Mix Asphalt (HMA) Specimens by Means of the Superpave Gyratory Compactor
4. Stabilometer The three Hveem-compacted samples are tested for stability. Because Superpave mix designs do not have to pass stabili ty requirements, these tests are done for informational purposes only.
5. Density and Voids Analysis First, bulk specific s pecific gravit gravity y ( Gmb) is determined for each sample and the two results for each asphalt content are averaged. Second, one sample sample from each asphalt asphalt content is broken down for density and volumetric determinations to include theoretical maximum density (abbreviated TMD or called "Rice" density after its originator, and often designated
Gmm), air voids, VMA and VFA. At this time the Gmm at Ninitial , Ndesign and Nmaximum are checked, as well as the dust to asphalt ratio. The effective asphalt content (Pbe) and percent absorbed asphalt content (Pba) are also checked.
6. Selection of Optimum Asphalt Binder Binder Content Content Using the data from the three asphalt contents, the optimum binder content is selected as that which corresponds to 4.0 percent air voids (4.5 percent air voids for Superpave designs that will be paid for based on volumetric properties). Usually, this asphalt content must be be interpolated between two of the sample asphalt contents. For example, a 5.0 percent asphalt sample may have 4.8 percent air voids and a 5.5 percent asphalt sample may have 3.8 percent air voids. In this case the design asphalt asphalt content would be interpolated as 5.4 5.4 percent. This selected asphalt content content must also meet meet VMA, VFA, density and dust-toasphalt requirements.
7. Determine the the Amount of Antistripping Antistripping Modifier Modifier Six samples are mixed per binder type at the determined asphalt content (fr om step 6 above). Note that these samples are 100 mm (4 inch) diameter cylinders instead of the usual Superpave 150 mm (6 inch) diameter diameter cylinders. Two samples are kept kept as controls and the other four samples contain varied amounts of an antistrippin antistripping g modifier. These samples are then cured and compacted the same way as in the Hveem mix design except one of the two that does not contain any any process . Next, all of the samples except antistripping modifier are tested for bulk specific gravity (G mb). The one that is not tested is kept as an unconditioned sample. The remaining 5 samples are then subjected to the modi modifie fied d Lot Lottman tman tes testt to determine moisture susceptibility. The minimum TSR is 0.80. 0.80.
8. Ignition Furnace Calibration Samples are then mixed at the des ign asphalt content and antistripping modifier amount for use in determining an ignition ignition furnace calibration factor. The ignition furnace is used to determine field sample asphalt content during manufacturing/construction.
9. Mix Design Report Finally, the recommended mix design is reported on a standard form that includes the manufacturer's recommended mix and compaction temperatures. temperatures. These reports are quite valuable because they include the contractor's proposed JMF JMF,, the laboratory analysis information from WSDOT and the recommendations for asphalt content and antistrip amount for the particular JMF and aggregate source submitted.
Strategic Highway Research Program (SHRP) In 1987 the U.S. Congress established a 5-year, $150 million applied research program aimed at improving the performance, durability, safety, and efficiency of the Nation’s highway system. Called the Strategic Highway Research Program (SHRP), this program was officially authorized by the Surface Transportation and Uniform Relocation Act of 1987 and consisted of research concentrated in four key areas (FHWA, 1998): •
Asphalt . This area consists of research to develop a completely new approach to HMA mix design.
•
Concrete and structures. structures . This area consists of research in the areas of mix design and assessing, protecting and rehabilitating concrete pavements and structures.
•
Highway operations. operations. This area consists of pavement preservation, work zone safety and snow and ice control research.
•
Pavement performance. performance. This area consists of the Long Term Pavement Performance Program (LTPP), (LTPP), a 20-year study of over 2,000 test sections of inservice U.S. and Canadian pavements to improve guidelines for building and maintaining pavements.
SHRP research activities were completed in 1992 and and SHRP was closed down in 1993. To date, SHRP has produced more than 100 new devices, tests and specifications and, perhaps more importantly, has spawned a full-sc ale on-going implementation drive by such organizations as the FHWA, AASHTO and TRB. Now that this first SHRP effort has reached the implementation stage, Congress has requested that the Transportation Research Board initiate a new process of setting priorities and designing a program for another focused research and development effort. This new study was initiated in 1999 and was completed in 2001 (TRB, 2001).
Superpave The SHRP asphalt research program, the largest SHRP program at $53 million (FHWA, 1998), had three primary objectives (NECEPT, 2001): •
Investigate why some pavements perform well, while others do not.
•
Develop tests and specifications for materials that will out-perform and outlast the pavements being constructed today.
•
Work with highway agencies and industry to have the new specifications put to use.
The final product of this research program is a new system referred to as "Superpave", which stands for SUperior PERforming Asphalt PAVEments. Superpave, in its final form consists of three basic components: specification. specification . This is the PG asphalt binder specification. 1. An asphalt binder specification.
2. A design and analysis system based on the volumetric properties of the asphalt mix . This is the Superpave mix design method . models . This area is not not yet yet 3. Mix analysis tests and performance prediction models. complete. Test development and evaluation is on-going as of 2001. Each one of these components required new specifications and performance standards as well as new testing methods and devices. As of late 2001, most states (48) have adopted or will adopt the Superpave PG asphalt binder specification and 39 states either have adopted or will adopt the Superpave mix design method (NHI, 2000). One of the principal results fr om the Strategic Highway Research Program (SHRP) was the Superpave mix design method. The Superpave mix design method was designed to replace the Hveem and Marshall methods. The volumetric volumetric analysis common to the Hveem and Marshall methods provides the basis for the Superpave mix design method. The Superpave system ties asphalt binder and aggregate selection into the mix design process, and considers traffic and and climate as well. The compaction devices from the Hveem and Marshall procedures have been replaced by agyr a gyrato atory ry comp compact actor or and the compaction effort in mix design is tied to expected traffic. This section consists of a brief history of the Superpave mix design method followed by a general outline of the actual method. method. This outline emphasizes general concepts and rationale over specific procedures. Typical procedures are available in the following documents: •
Roberts, F.L.; Kandhal, P.S.; Brown, Br own, E.R.; Lee, D.Y. and Kennedy, T.W. (1996). Hot Mix Asphalt Materials, Mixture Design, and Construction. Construction. National Asphalt Pavement Association Education Foundation. Lanham, MD.
•
Asphalt Institute. (2001). Superpave Mix Design. Design. Superpave Series No. 2 (SP02). Asphalt Institute. Lexington, KY.
•
American Association of State Highway and Transportation Officials (AASHTO). (2000 and 2001). AASHTO Provisional Standards . American Association of State Highway and Transportation Officials. Washington, D.C.
1 History Under the Strategic Highway Research Program (SHRP), (SHRP), an initiative was undertaken to improve materials selection and mixture design by developing: 1. A new mix design design method method that that accounts accounts for traffic traffic loading loading and environm environmental ental conditions. 2. A new metho method d of asphal asphaltt binder binder evalua evaluatio tion. n. 3. New method methods s of mixtur mixture e analys analysis. is.
When SHRP was completed in 1993 it introduced these three developments and called them the Superior Performing Asphalt Pavement System (Superpave). Although the new new methods of mixture performance testing have not yet been established, the mix design method is well-established.
2 Procedure The Superpave mix design method consists of 7 basic steps: selection. 1. Aggregate selection. selection. 2. Asphalt binder selection. compaction). 3. Sample preparation (including compaction). Tests. 4. Performance Tests. calculations . 5. Density and voids calculations.
6. Optimum asphalt binder content selection. 7. Moisture susceptibility evaluation.
2.1 Aggregate Selection Superpave specifies aggregate in two ways. First, it places restrictions on aggregate gradation by means of broad control points. Second, it places "consensus "consensus requirements" on coarse and fine aggregate angularity, flat and elongated particles, and clay content. Other aggregate criteria, which the Asphalt Institute (2001) calls "source properties" (because they are considered to be source specific) such as L.A. abrasion,, soundnessand water absorption are used in Superpave but since they were abrasion not modified by Superpave they are not discussed here.
WSDOT Superpave Aggregate Source Requirements
As of 2002, once aggregate source properties are tested and prove satisfactory, aggregate sources are approved for 5 years. Property
Los Angeles Abrasion (500 revolutions)
Value
30% maximum
Degradation Factor Wearing Course
30 minimum
Non-Wearing Course
20 minimum
2.1.1 Gradation and Size Aggregate gradation influences such key HMA parameters as stiffness, stiffness, stability, stability, durability, durability, permeability, permeability, workability, workability, fatigue resistance, resistance, frictional resistance and resistance to moisture damage (Roberts et al., 1996). Additionally, the maximum aggregate size can be influential in compaction andlift andlift thickn thi ckness ess determination. determination.
Gradation Specifications Superpave mix design specifies aggregate gradation control points, through which aggregate gradations must pass. These control points are very general and are a starting point for a job a job mix formula. formula .
Superpave Gradation Requirements These tables (data taken from AASHTO MP 2, Standard Specification for Superpave Volumetric Mix Design) Design) show typical Superpave aggregate specifications for 37.5 mm (1.5 inch) down to 9.5 mm mm (0.375 inch) nominal aggregate sizes. sizes. Significant figures are the same as those in AASHTO MP 2. Table 1: 37.5 mm (1.5 inch) Nominal Size Sieve Size
Control Points
Restricted Zone
(mm)
(U.S.)
Lower
Upper
Lower
Upper
50
2 inch
100
-
-
-
37.5
1.5 inch
90
100
-
-
25
1 inch
-
90
-
-
19
3/4 inch
-
-
-
-
12.5
1/2 inch
-
-
-
-
9.5
3/8 inch
-
-
-
-
4.75
No. 4
-
-
34.7
34.7
2.36
No. 8
15
41
23.3
27.3
1.18
No. 16
-
-
15.5
21.5
0.60
No. 30
-
-
11.7
15.7
0.30
No. 50
-
-
10.0
10.0
0.15
No. 100
-
-
-
-
0.075
No. 200
0
6
-
-
Table 2: 25 mm (1 inch) Nominal Size Sieve Size
Control Points
Restricted Zone
(mm)
(U.S.)
Lower
Upper
Lower
Upper
37.5
1.5 inch
100
-
-
-
25
1 inch
90
100
-
-
19
3/4 inch
-
90
-
-
12.5
1/2 inch
-
-
-
-
9.5
3/8 inch
-
-
-
-
4.75
No. 4
-
-
39.5
39.5
2.36
No. 8
19
45
26.8
30.8
1.18
No. 16
-
-
18.1
24.1
0.60
No. 30
-
-
13.6
17.6
0.30
No. 50
-
-
11.4
11.4
0.15
No. 100
-
-
-
-
0.075
No. 200
1
7
-
-
Table 3: 19 mm (3/4 inch) Nominal Size Sieve Size
Control Points
Restricted Zone
(mm)
(U.S.)
Lower
Upper
Lower
Upper
25
1 inch
100
-
-
-
19
3/4 inch
90
100
-
-
12.5
1/2 inch
-
90
-
-
9.5
3/8 inch
-
-
-
-
4.75
No. 4
-
-
-
-
2.36
No. 8
23
49
34.6
34.6
1.18
No. 16
-
-
22.3
28.3
0.60
No. 30
-
-
16.7
20.7
0.30
No. 50
-
-
13.7
13.7
0.15
No. 100
-
-
-
-
0.075
No. 200
2
8
-
-
Table 4: 12.5 mm (1/2 inch) Nominal Size Sieve Size
Control Points
Restricted Zone
(mm)
(U.S.)
Lower
Upper
Lower
Upper
19
3/4 inch
100
-
-
-
12.5
1/2 inch
90
100
-
-
9.5
3/8 inch
-
90
-
-
4.75
No. 4
-
-
-
-
2.36
No. 8
28
58
39.1
39.1
1.18
No. 16
-
-
25.6
31.6
0.60
No. 30
-
-
19.1
23.1
0.30
No. 50
-
-
15.5
15.5
0.15
No. 100
-
-
-
-
0.075
No. 200
2
10
-
-
Table 5: 9.5 mm (3/8 inch) Nominal Size Sieve Size
Control Points
(mm)
(U.S.)
Lower
12.5
1/2 inch
100
9.5
3/8 inch
90
4.75
No. 4
2.36
Restricted Zone
Upper
Lower
Upper
-
-
100
-
-
-
90
-
-
No. 8
32
67
47.2
47.2
1.18
No. 16
-
-
31.6
37.6
0.60
No. 30
-
-
23.5
27.5
0.30
No. 50
-
-
18.7
18.7
0.15
No. 100
-
-
-
-
0.075
No. 200
2
10
-
-
WSDOT Superpave Gradation Requirements
WSDOT uses 9.5 mm (0.375 inch), 12.5 mm (0.5 inch), 19.0 mm (0.75 inch) and 25.0 mm (1 inch) Superpave mixes. WSDOT gradation requirements requirement s are the same as the AASHTO requirements except that the upper and lower control points on the 0.075 mm (No. 200) sieve for the 9.5 mm (0.375 inch), 12.5 mm (0.5 inch) and 19.0 mm (0.75 inch) Superpave mixes are 2.0 and 7.0 percent respectively. respectively. The WSDOT upper and lower control points on the 0.075 mm (No. 200) sieve for the 25.0 mm (1 inch) mix are 1.0 and 7.0 respectively. respectively.
Aggregate Blending It is rare to obtain a desired aggregate gradation from a single aggregate stockpile. Therefore, Superpave mix designs usually draw upon several different aggregate stockpiles and blend them together in a ratio that will produce an acceptable final blended gradation. It is quite common to find a Superpave mix design that uses 3 or 4 different aggregate stockpiles (see Figure 5.11).
Figure 5.11: Screen Shot from HMA View Showing a Typical Aggregate Blend from 4 Stockpiles Typically, several aggregate blends are evaluated prior to performing a complete mix design. Evaluations are done by preparing an HMA sample of each blend at the estimated optimum asphalt binder content then compacting it. Results from this evaluation can show whether or not a particular blend will meet minimum VMA requirements and Ninitial or Nmax requirements. requirements.
HMA Weight-Volume Terms and Relationships Basic HMA weight-volume relationships are important to understand for both mix design and construction purposes. Fundamentally, mix design is meant to to determine the volume of asphalt binder and aggregates necessary to produce a mixture with the desired properties (Roberts et al., 1996). 1996). However, since weight measurements measurements are typically much much easier, they are typically taken then then converted to volume volume by using using specific gravities. The following is a brief discussion of the more important volume properties of HMA.
In general, weight and volume terms are abbreviated as, G xy, where:
x: b
= bind binder er
s = ston stonee (i.e (i.e., .,
aggregate) m = mixtu ixture re
y: b
= bulk ulk
e = effe effect ctiv ivee a = appa pparent rent m = maxi aximum mum
For example, Gmm = gravity, mixture, maximum = the maximum gravity of the mixture. Other common abbreviations are: VT
Total volume of the compacted specimen
WT
=
Total Total weig weight ht of of the the compa compacte cted d speci specimen men
Volume of air voids
WD
=
Dry weight
Volume of asphalt binder
WSSD
=
Satu Saturat rated ed surfa surface ce dry dry (SSD) (SSD) wei weigh ghtt
Volume of effective asphalt binder
Wsub
=
Weig We ight ht subm submer erge ged d in in wat water er
=
Volume of absorbed asphalt binder
W b
=
Weig We ight ht of the the asp aspha halt lt binde binder r
Vagg
=
Volume of aggregate
W be
=
Weight Weight of effecti effective ve asphal asphaltt bind binder er
Veff
=
Effec Effecti tive ve vol volum umee of agg aggre rega gate te = (V (VT - VAC)
W ba
=
Weig We ight ht of abs absorb orbed ed asph asphal altt bind binder er
Wagg
=
Weig We ight ht of aggr aggreg egat atee
Va
V b V be
V ba
=
= = =
Gsa
=
Appare Apparent nt speci specific fic gravi gravity ty of of the aggreg aggregate ate
G b
=
Asphalt binder specific gravity
P b
=
Asphal Asphaltt conte content nt by weight weight of of mix mix (percen (percent) t)
Gsb
=
Bulk specific gravity of the aggregate
Ps
=
Aggrega Aggregate te cont content ent by by weigh weightt of mix mix (per (percent cent))
Gse
=
Effecti Effective ve speci specific fic grav gravity ity of the the aggreg aggregate ate
=
Perc Percen entt air air voi voids ds
Gmb
=
Bulk Bulk specif specific ic gravi gravity ty of the the compa compacte cted d mixtur mixturee
Gmm
=
Maximum theoretical specific gravity of the mixture
=
Unit Unit weig weight ht of wate water r
Specific Gravities
Pa
γW
the mass mass Bulk Specific Gravity of the Compacted Asphalt Mixture (Gmb). The ratio of the in air of a unit volume of a permeable material (including both permeable and impermeable voids normal to the material) at a stated temperature to the mass in air (of equal density) of an equal volume volume of gas-free distilled water at a stated temperature. This value is used to determine weight per unit volume volume of the compacted mixture. mixture. It is very important to measure Gmb as accurately as possible. Since it is used to convert weight measurements to volumes, any small errors in Gmb will be reflected in significant volume errors, which may go undetected.
The standard bulk specific gravity test is: AASHTO T 166: Bulk Specific Gravity of Compacted Bituminous Mixtures Using Saturated Surface-Dry Specimens
Theoretical Maximum Specific Gravity of Bituminous B ituminous Paving Mixtures (Gmm). The ratio of the mass of a given volume of voidless (Va = 0) HMA at a stated temperature (usually 25 °C) to a mass of an equal volume of gas-free distilled water at the same temperature. It is also called Rice Specific Gravity (after James James Rice who developed developed the test procedure). Multiplying Gmm by the unit weight of water gives Theoretical Maximum Density (TMD). The standard TMD test is: AASHTO T 209 and ASTM D 2041: Theoretical Maximum Specific Gravity and Density of Bituminous Paving Mixtures
Voids (expressed as percentages) p ercentages) Air Voids (Va). The total volume of the the small pockets of air between the the coated aggregate particles throughout a compacted paving mixture, expressed as a percent of the bulk volume of the compacted compacted paving mixture. The amount of air voids voids in a mixture is extremely important and closely related to stability and durability. For typical typical dense-graded mixes with 12.5 mm (0.5 inch) nominal maximum aggregate sizes air voids below about 3 percent result in an unstable mixture while air voids above about 8 percent result in a water-permeable mixture.
volume of intergranular intergranular void space between Voids in the Mineral Aggregate (VMA). (VMA) . The volume the aggregate particles of a compacted paving mixture that includes the air voids and the effective asphalt content, expressed as a percent of the total volume of the specimen. When VMA is too low, there is not enough room in the mixture to add sufficient asphalt binder to adequately coat the individual aggregate particles. Also, mixes with a low VMA are more sensitive to small changes in asphalt binder content. Excessive VMA will cause an unacceptably low mixture stability (Roberts et al., 1996). Generally, a minimum minimum VMA is specified and a maximum VMA may or may not be specified.
Voids Filled with Asphalt (VFA). (VFA) . The portion of the voids in the mineral aggregate that contain asphalt binder. This represents the the volume of the effective asphalt content. It can also be described as the percent of the volume of the VMA that is filled with asphalt cement. VFA is inversely related to air voids: as air voids decrease, the VFA increases.
Other Definitions Effective Asphalt Content (Pbe). The total asphalt binder content of the HMA less the portion of asphalt binder that is lost by absorption into the aggregate.
Volume of Absorbed Asphalt (V ba). The volume of asphalt binder in the HMA that has been absorbed into the pore pore structure of the aggregate. It is the volume of the asphalt binder in the HMA that is not accounted for by the effective asphalt content.
Dust- to-Binder Ratio In order to ensure the proper amount a mount of material passing the 0.075 mm (No. 200) sieve (called "silt-clay "silt-clay"" by AASHTO definition and "dust" by Superpave) in the mix, Superpave specifies a range of dust-to-binder ratio by mass. The equation is:
where:
P0.075
=
mass of parti particles cles passing passing the the 0.075 0.075 mm (No. (No. 200) sieve
P be
=
effective effective binder binder conten contentt = the total asphalt asphalt binder binder conten contentt of a paving paving mixture less the portion of asphalt binder that is lost by absorption into the aggregate particles.
Dust-to-binder ratio specifications are normally 0.6 - 1.2, but a ratio of up to 1.6 may be used at an agency's discretion (AASHTO, 2001). WSDOT Superpave Dust-to-Binder Requirements
The WSDOT Superpave dust-to-binder dust-to-binder ratio must fall between 0.6 and 1.6.
2.1.2 Consensus Requirements "Consensus requirements" came about because SHRP did not specifically address aggregate properties and it was thought that there needed to be some guidance associated with the Superpave mix design method. Therefore, an expert group was convened convened and they arrived at a consensus on several aggregate property requirements - the "consensus requirements". This group recommended recommended minimum angularity, angularity, flat or elongated elongated particle and clay content requirements based on: •
The anticipated traffic loading. loading. Desired aggregate properties are different depending upon the amount of traffic loading. Traffic loading numbers are based on the anticipated traffic level on the design lane over a 20-year period regardless of actual roadwaydes roadway desig ign n life li fe (AASHTO, 2000b).
•
Depth below the surface. surface . Desired aggregate properties vary depending upon their intended use as it relates to depth below the pavement surface.
These requirements are imposed on the final aggregate blend and not the individual aggregate sources.
WSDOT Superpave Aggregate Consensus Requirements
WSDOT uses a 15-year traffic loading instead of the 20-year period listed in the consensus requirement tables because WSDOT typically designs overlays for a 15-year design 15-year design life. life. Property
Value
Coarse Aggregate Angularity < 10 million ESALs
90/-*
≥ 10 million ESALs
-/90*
Fine Aggregate Angularity
45 minimum
Flat and Elongated Particles (5:1 ratio or greater)
10% maximum**
Clay Content (Sand (Sand Equivalent) Equivalent)
37% minimum
*The first number is a minimum requirement for one or more fractured faces and the second number is a minimum requirement for two or more fractured faces. **For > 0.3 million ESALs
Coarse Aggregate Angularity Coarse aggregate angularity is important to mix design because smooth, rounded aggregate particles do not interlock with one another nearly as well as angular particles. This lack of interlock makes makes the resultant HMA more susceptible to rutting. Coarse aggregate angularity can be determined by any number of tes t procedures that are designed to determine the percentage of fractured faces. faces. Table 5.5 lists Superpave requirements. Table 5.5: Coarse Aggregate Angularity Requirements (from AASHTO, 2000b) Depth from Surface
20-yr Traffic Loading (in millions of ESALs)
100 mm (4 inches)
> 100 mm (4 inches)
< 0.3
55/-
-/-
0.3 to < 3
75/-
50/-
3 to < 10
85/80
60/-
10 to < 30
95/90
80/75
100/100
100/100
≥
30
Note: The first number is a minimum requirement for one or more fractured faces and the second number is a minimum requirement for two or more fractured faces.
Fine Aggregate Angularity Fine aggregate angularity is important to mix design for the same reasons as coarse aggregate angularity angularity - rut prevention. Fine aggregate angularity is quantified by an indirect method often called the National Aggregate Association (NAA) flow test. This test consists of pouring the fine aggregate into the top end of a cylinder and determining the the amount of voids. The more voids, the more more angular the aggregate. Voids are determined by the following equation: equation:
where:
V
=
volume of cylinder (mL)
W
=
weig weight ht of loos loosee fin finee agg aggre rega gate te to fill fill the the cyl cylin inde derr (g) (g)
Gsb
=
bulk bulk specif specific ic gravi gravity ty of of the the fine fine aggr aggrega egate te
Table 5.6 shows the Superpave recommended fine aggregate angularity.
Table 5.6: Fine Aggregate Angularity Requirements (from AASHTO, 2000b) Depth from Surface
20-yr Traffic Loading (in millions of ESALs)
100 mm (4 inches)
< 0.3
-
0.3 to < 3
40
3 to < 10 10 to < 30
≥
> 100 mm (4 inches)
40
45
30
45
Numbers shown represent the minimum uncompacted void content as a percentage of the total sample volume.
The standard test for fine aggregate angularity is: •
AASHTO T 304: Uncompacted Void Content of Fine Aggregate
Flat or Elongated Particles An excessive amount of flat of flat or elongated aggregate particles can be detrimental to HMA. Flat/elongated particles tend to breakdown during compaction (giving a different gradation than determined in mix design), decrease workability, and lie flat after compaction (resulting in a mixture with low VMA) (Roberts et al., 1996). Flat or elongated particles are typically identified using ASTM D 4791, Flat or Elongated Particles in Coarse Aggregate. Table 5.7 shows the Superpave recommended flat or elongated particle requirements. Figure 5.7: Flat or Elongated Particle Requirements (from AASHTO, 2000b) Maximum Percentage of
20-yr Traffic Loading
Particles with
(in millions of ESALs)
Length/Thickness > 5
< 0.3
-
0.3 to < 3 3 to < 10
10
10 to < 30
≥
30
Clay Content The sand equivalent test measures the amount of clay content in an aggregate sample. sample. If clay content is too high, clay could preferentially adhere to the aggregate over the asphalt binder. This leads to a poor aggregate-asphalt binder bonding bonding and possible stripping stripping.. To prevent excessive clay content, Superpave uses the sand equivalent test requirements of Table 5.8. Table 5.8: Sand Equivalent Requirements (from AASHTO, 2000b) 20-yr Traffic Loading
Minimum Sand Equivalent
(in millions of ESALs)
(%)
< 0.3 0.3 to < 3 3 to < 10 10 to < 30
≥
30
40 45 50
2.2 Asphalt Binder Evaluation Superpave uses its own asphalt binder selection process, which is, of course, tied to the Superpave asphalt binder performance grading (PG) systemand systemand its associated specifications.. Superpave PG asphalt binders specifications binders are selected based on the expected pavement temperature extremes in the area of of their intended use. Superpave software (or a standalone program such as LTPPBind ) is used to calculate these extremes and select the appropriate PG asphalt binder using one of the following three alternate methods (Roberts et al., 1996):
inputs the design pavement temperatures temperature. The designer inputs 1. Pavement temperature. directly. temperature . The designer inputs the local air temperatures, then the 2. Air temperature. software converts them to pavement temperatures. area. The designer simply inputs the project location (i.e. state, 3. Geographic area. county and city). city). From this, the software retrieves climate conditions conditions from a weather database and then converts air temperatures into pavement temperatures. Once the design pavement temperatures are deter mined they can be matched to an appropriate PG asphalt binder.
WSDOT Asphalt Binder Specifications
WSDOT uses the Superpave asphalt binder performance grading system and specifications. specifications. Therefore, asphalt binder must meet the requirements of AASHTO MP 1. 1 . WSDOT uses three baseline asphalt binder performance grades based on geography. These baseline grades are typically typically used and then adjusted as necessary.
Previously, WSDOT WSDOT had used the aged residue (AR) viscosity grading. grading . The commonly used grade in this old system was AR-4000W.
2.2.1 Design Pavement Temperature The Superpave mix design method determines both a high and a low design pavement temperature. These temperatures are determined as follows: •
High pavement temperature temperature - based on the 7-day average high air temperature of the surrounding area.
•
Low pavement temperature - based on the 1- day low air temperature of the surrounding area.
Using these temperatures as a starting point, Superpave then applies a reliability concept to determine the appropriate PG asphalt binder. PG asphalt binders are specified in 6°C increments.
Reliability Concept in PG Asphalt Binder Selection Reliability is defined as the percent probability in a single year that the actual temperature (seven-day high or one-day low) will not exceed the corresponding design temperatures. The animation below describes the basic process for selecting the pavement temperature
extremes for a PG asphalt binder. Note that pavement pavement temperatures are more extreme than air temperatures.
2.2.2 Design Pavement Temperature Adjustments Design pavement temperature calculations are based on HMA pavements subjected to fast moving traffic (Roberts et al., 1996). Specifically, theDyna theDynamic mic Shear She ar Rheome Rhe ometer ter (DSR) (DS R) test is conducted at a rate of 10 radians per second, which corresponds to a traffic speed of about 90 km/hr km/hr (55 mph) (Roberts (Roberts et al., 1996). Pavements subject to significantly slower (or stopped) traffic such as intersections, toll booth lines and bus stops should contain a stiffer asphalt binder than that which would be used for fast-moving traffic. Superpave allows the high temperature grade to be increased by one grade for slow transient loads and by two grades for stationary loads. Additionally, the high temperature grade should be increased by one grade for anticipated anticipated 20-year loading in excess of 30 million ESALs. For pavements with multiple conditions that require grade increases only the largest grade increase should be used. Therefore, for a pavement intended intended to experience slow loads (a potential one grade increase) and greater than 30 million ESALs (a potential one grade increase), the asphalt binder high tempera ture grade should be increased by only one grade. Table 5.9 shows two two examples of design high temperature temperature adjustments - often called "binder bumping". Table 5.9: Examples of Design Pavement Pa vement Temperature Adjustments for Slow and Stationary Loads Original Grade
Grade for Slow Grade for Transient Loads Stationary Loads (increase 1 grade) (increase 2 grades)
20-yr ESALs > 30 million (increase 1 grade)
PG 58-22
PG 64-22
PG 70-22
PG 64-22
PG 70-22*
PG 76-22
PG 82-22
PG 76-22
*the highest possible pavement temperature in North America is about 70 °C but two more high temperature grades were necessary to accommodate transient and stationary loads.
WSDOT Design Pavement Temperature Adjustments ("Binder Bumping")
WSDOT uses the following guidance when considering adjustments to the design high temperature of a PG asphalt binder (sometimes referred to as "binder bumping"):
Situation
Adjustment to High Temperature Grade
15-year design ESALs of 10 - 30 million
Consider Increasing 1 Grade
15-year design ESALs ≥ 30 million
Increase 1 Grade
Slow Traffic (10 - 45 mph)
Increase 1 Grade
Standing Traffic (0 - 10 mph)
Increase 2 Grades
Additionally, all mountain passes should use a base grade of PG 58-34.
2.3 Sample Preparation The Superpave method, like other mix design methods, creates several trial aggregateasphalt binder blends, each with a different different asphalt binder content. content. Then, by evaluating each trial blend's performance, performance, an optimum asphalt binder content can be selected. In order for this concept to work, the trial blends must contain a range of asphalt contents both above and below the optimum asphalt content. Therefore, the first step in sample preparation is to estimate an optimum asphalt asphalt content. Trial blend asphalt contents contents are then determined from this estimate. The Superpave gyratory compactor (Figure 5.12) was developed to improve mix design's ability to simulate actual field compaction particle orientation with laboratory equipment (Roberts, 1996). Each sample is heated to the anticipated mixing temperature, aged for a short time (up to 4 hours) and compacted with the gyratory compactor, a device that applies pressure to a sample through a hydraulically or mechanically operated operated load. Mixing and compaction temperatures are chosen according to asphalt binder properties so that compaction occurs at the same viscosity level for for different mixes. Key parameters of the gyratory compactor compactor are: •
Sample size = 150 mm (6-inch) diameter cylinder approximately 115 mm (4.5 inches) in height (corrections can be made for different sample heights). Nnote that this sample size is larger than those used for the Hveem and Marshall methods (see Figure 5.13).
•
Load = Flat and circular with a diameter of 149.5 mm (5.89 inches) corresponding to an area of 175.5 cm 2 (27.24 in2)
•
Compaction pressure = Typically 600 kPa (87 psi)
•
Number of blows = varies
•
Simulation method = The load is a pplied to the sample top and covers almost the entire sample top area. The sample is inclined at 1.25°and rotates at 30 revolutions per minute minute as the load load is continuously continuously applied. This helps achieve a
sample particle orientation that is somewhat like that achieved in the field after roller compaction.
Figure 4 (left): Gyratory Compactor Figure 5 (below): Superpave Gyratory Compactor Sample (left) vs. Hveem/Marshall Compactor Sample (right)
The Superpave gyratory compactor establishes three different gyration numbers:
1. N initial initial . The number of gyrations used as a measure of mixture compactability during construction. Mixes that compact too too quickly (air voids voids at Ninitial are too low) may be tender during construction and unstable when subjected to traffic. Often, this is a good indication of aggregate quality - HMA with excess natural sand will frequently fail the Ninitial requirement. A mixture mixture designed for greater than or equal to 3 million ESALs with 4 percent air voids at N design should have at least 11 percent air voids at Ninitial.
2. N design design. This is the design number of gyrations required to produce a sample with the same density as that expected in the field after the indicated amount of traffic. A mix with 4 percent air voids at Ndesign is desired in mix design. density that 3. N max max . The number of gyrations required to produce a laboratory density should never be exceeded in the field. If the air voids at Nmax are too low, then the field mixture may compact too much under tra ffic resulting in excessively low air voids and potential rutting. The air void content content at Nmax should never be below 2 percent air voids. Typically, samples are compacted to N design to establish the optimum asphalt binder content and then additional samples are compacted to N max as a check. Previously, samples samples were compacted to Nmax and then Ninitial and Ndesign were back calculated. Table 5.10 lists the
specified number of gyrations for N initial, Ndesign and Nmax while Table 5.11 shows the required densities as a percentage of theoretical maximum density (TMD) for N initial, Ndesign and Nmax. Note that traffic loading numbers are based on the anticipated traffic level on the design lane over a 20-year period regardless of actual roadway design life (AASHTO, 2001). Table 5.10: Number of Gyrations for Ninitial, Ndesign and Nmax (from AASHTO, 2001) 20-yr Traffic Loading
Number of Gyrations
(in millions of
Ninitial
Ndesign
Nmax
< 0 .3
6
50
75
0.3 to < 3
7
75
115
3 to < 10*
8 (7)
100 (75)
160 (115)
10 to < 30
8
100
160
30
9
125
205
ESALs)
≥
* When the estimated 20-year design traffic loading is between 3 and < 10 million ESALs, the agency may, at its discretion, specify Ninitial = 7, Ndesign = 75 and Nmax = 115.
WSDOT Superpave Gyration Requirements
WSDOT gyration gyration requirements are the same as those shown in Table 5.10. WSDOT does not use the discretionary values between < 3 and 10 million ESALs.
Table 5.11: Required Densities for Ninitial, Ndesign and Nmax (from AASHTO, 2001) 20-yr Traffic Loading
Required Density (as a percentage of TMD of TMD))
(in millions of
Ninitial
ESALs)
< 0.3
≤
91.5
0.3 to < 3
≤
90.5
≤
89.0
3 to < 10 10 to < 30
≥
Ndesign
96.0
Nmax
≤
30 WSDOT Superpave Density Requirements
WSDOT Superpave Superpave density requirements are the same as those shown in Table 5.11 except that WSDOT uses a 15-year Traffic Loading instead of a 20-year traffic loading.
98.0
The standard gyratory compactor sample preparation procedure is: •
AASHTO TP4: Preparing and Determining the Density of Hot-Mix Asphalt (HMA) Specimens by Means of the Superpave Gyratory Compactor
2.4 Performance Tests The original intent of the Superpave mix design method was to subject the various tr ial mix designs to a battery of performance of performance tests akin to what the Hveem method does with the stabilometer and cohesiometer, or the Mars hall method does with the stability and flow test. Currently, these performance tests, which constitute the the mixture analysis analysis portion of Superpave, are still under development and review and have not yet been implemented. The most likely performance test, called the Simple Performance Test (SPT) is a Confined Dynamic Modulus Test. Test .
2.5 Density and Voids Analysis All mix design methods use density and voids to determine basic HMA physical characteristics. Two different measures of densities are typically typically taken:
1. Bulk specific gravity (Gmb) - often called "bulk density" 2. Theoretical maximum density (TMD, G mm) These densities are then then used to calculate the volumetric parameters of the HMA. Measured void expressions are usually: •
Air voids (Va), sometimes called voids in the total mix (VTM)
•
Voids in the mineral aggregate (VMA)
•
Voids filled with asphalt (VFA)
Generally, these values must meet local or State criteria. VMA and VFA must meet the values specified in Table 5.12. Note that traffic loading numbers are based on the anticipated traffic level on the design lane over a 20year period regardless of actual roadway design life (AASHTO, 2000b).
Table 5.12: Minimum VMA Requirements and VFA Range Requirements (from AASHTO, 2001) 20-yr Traffic Loading (in millions of ESALs)
< 0.3 0.3 to < 3
Minimum VMA (percent) 9.5 mm
12.5 mm
19.0 mm
25.0 mm
37.5 mm
(0.375 inch)
(0.5 inch)
(0.75 inch)
(1 inch)
(1.5 inch)
15.0
14.0
13.0
12.0
11.0
VFA Range (percent)
70 - 80 65 - 78
3 to < 10 10 to < 30
≥
65 - 75
30
WSDOT Minimum VMA Requirements and VFA Range Requirements
Item
19 mm 12.5 mm 9.5 mm (0.75 (0.5 inch) (0.375 inch) inch) Superpav Superpave Superp e ave Min. Max. Min.
25 mm (1.0 inch) Superpave
Ma Mi M Mi x. n. ax. n.
-
12. 0 %
-
15.0%
-
14.0 %
< 0.3
70
80
70
80
70 70
80 67
80
0.3 to < 3
65
78
65
78
65
78 7 8 65
78
≥3
73
76
65
75
65
75 65
75
VMA
13. 0%
Max.
-
VFA (based on 20-yr traffic loading in millions of ESALs)
2.6 Selection of Optimum Asphalt Binder Content The optimum asphalt binder content is selected as that asphalt binder content that results in 4 percent air voids at Ndesign. This asphalt content then must meet several other other requirements:
1. Air voids at Ninitial > 11 percent (for design ESALs
≥
3 million). million). See Table 5.11 for
specifics. 5.11 for specifics. 2. Air voids at Nmax > 2 percent. See Table 5.11
3. VMA above the minimum listed in Table 5.8. 4. VFA within the range listed in Table 5.8. If requirements 1,2 or 3 are not met the mixture mixture needs to be redesigned. If requirement 4 is not met but close, then asphalt binder content can be slightly adjusted such that the air void content remains near 4 percent but VFA is within limits. This is because VFA is a somewhat redundant term since it is a function of air voids and VMA (Roberts et al., 1996). The process is illustrated in Figure 5.14 (numbers are chosen based on 20-year traffic loading of ≥ 3 million ESALs).
WSDOT Asphalt Binder Content Selection
In general, WSDOT selects the asphalt binder content that corresponds to 4 percent air voids and meets minimum stability criteria.
2.7 Moisture Susceptibility Evaluation Moisture susceptibility testing is the only performance testing incorporated in the Superpave mix design procedure as of early 2002. The modifie modified d Lottman test is used for this purpose. The typical moisture susceptibility test is: •
AASHTO T 283: Resistance of Compacted Bituminous Mixture to MoistureInduced Damage.
3 Summary The Superpave mix design method was developed to address specific mix design issues with the Hveem and Marshall methods. Superpave mix design is a rational method method that accounts for traffic loading and environmental environmental conditions. Although not yet fully fully complete (the performance tests have not been implemented), Superpave mix design produces quality HMA mixtures. As of 2000, 39 states have have adopted, or are planning to to adopt, Superpave as their mix design system (NHI, 2000). The biggest differentiating aspects of the Superpave method are: 1. The use of formal formal aggregate aggregate evaluation evaluation procedures procedures (consens (consensus us requirement requirements). s). 2. The use of the PG asphalt asphalt binder binder grading grading system system and its associate associated d asphalt asphalt binder selection system. 3. The use of the the gyratory gyratory compactor compactor to to simulate simulate field field compaction compaction.. 4. Traffic loading loading and and environ environmenta mentall considerat considerations. ions. 5. Its volu volumet metric ric appro approach ach to to mix desig design. n.
Even given its many differences when compared to the Hveem or Marshall methods, Superpave still uses the same basic mix design steps and still strives for an optimum asphalt binder content that results in 4 percent design air voids. Thus, the method is quite different but the ultimate goals remain fair ly consistent.
