3. HMA Mix Design
• Objective:
• Develop an economical blend of
aggregates and asphalt binder that meet
design and functional requirements
• Historical mix design methods
• Marshall
• Hveem
• New
• Superpave gyratory
HMA Superpave Mix Design 3
4. Requirements in Common
• Sufficient asphalt binder to ensure a durable
pavement
• Sufficient stability under traffic loads
• Sufficient air voids
• Upper limit to prevent excessive environmental
damage
• Lower limit to allow room for initial densification
due to traffic
• Sufficient workability
HMA Superpave Mix Design 4
5. Goals of Compaction Method
• Simulate field densification
• traffic
• climate
• Accommodate large aggregates
• Measure compactability
• Conducive to QC
HMA Superpave Mix Design 5
11. General Notes to Revised Table
• Traffic Level is Based Upon 20 Year
Pavement Design Life
• Slow / Standing Traffic : Increase Ndesign
by 1 Level.
HMA Superpave Mix Design 11
12. Superpave Gyratory Compaction
• Select mixing and compaction temperature
based on asphalt binder properties
• Select number of gyrations to use based on
design traffic level
HMA Superpave Mix Design 12
14. Step 1: Materials Selection
• Materials Selection consists of:
• Choosing the correct asphalt binder
• Choosing the aggregates that meet the
quality requirements for the mix
HMA Superpave Mix Design 14
15. Superpave Asphalt Binder Specification
The grading system is based on Climate
PG 64 - 22
Min pavement
Performance temperature
Grade
Average 7-day max
pavement temperature
HMA Superpave Mix Design 15
20. Establish Trial Blends
• Develop three gradations based on
• Stockpile gradation information
• Gradation specification
• Optimize use of materials in the most
economical blends
• Estimate properties of combined stockpiles
HMA Superpave Mix Design 20
28. Short Term Aging Important
• Allows time for aggregate to absorb asphalt
binder
• Helps minimize variability in volumetric
calculations
• Most terms dependent upon volumes which
change with changes in the amount (volume) of
absorbed asphalt binder
HMA Superpave Mix Design 28
29. Determine the sample mass
• Estimate an asphalt binder content
• Mix up a sample & determine Gmm
• Calculate the bulk gravity needed to achieve
4 % air voids (Va)
• Calculate the weight for a pill with a height of
150 mm
HMA Superpave Mix Design 29
30. Sample Mass
Sample mass = (Est. Gmb) (Sample Volume)
π d 2 hx
Sample Volume = Vmx = * 0.001 cm3/mm3
4
Where: Vmx = volume of specimen in mold)
d = diameter of mold (150 mm)
hx = height of specimen in mold
h d
32. Overview of Compaction Procedure
• Initialize Compactor
• verify/set ram pressure at 600 kPa
• verify/set number of gyrations for Ndes
• Fill Gyratory Mold With HMA
• paper disk on bottom
• one lift of HMA
• slightly round top of HMA
• paper disk on top
• Load Mold into Gyratory Compactor
HMA Superpave Mix Design 32
33. Compaction
After aging, take mix and preheated mold
from oven. Place paper in bottom of mold.
HMA Superpave Mix Design 33
34. Compaction
Place mix in mold
HMA Superpave Mix Design 34
35. Compaction
Place another
paper disc on top
of the mix
HMA Superpave Mix Design 35
43. General Guidance
• Compact the trial mixtures in accordance with
AASHTO T 312 which now requires
specimens be compacted to the design
number of gyrations
• When doing a mix design when you compact
a pair of samples to Nmaximum and check them to
see if the Nmaximum value of 98% is exceeded.
HMA Superpave Mix Design 43
45. Superpave Mixture Requirements
• Mixture Volumetrics
• Air Voids (Va)
• Mixture Density Characteristics
• Voids in the Mineral Aggregate (VMA)
• Voids Filled with Asphalt (VFA)
• Dust Proportion
• Moisture Sensitivity
HMA Superpave Mix Design 45
46. Mix VMA Requirements VMA
Voids in the Mineral Aggregate
Table 334-9
% asphalt binder
Mix type
Minimum VMA, %
9.5 mm 15.0
12.5 mm 14.0
19.0 mm 13.0
HMA Superpave Mix Design 46
47. VFA
Mix VFA Requirements
Voids Filled with Asphalt
% asphalt binder
Traffic Level Design VFA, %
A 70 – 80
B 65 – 78
C 65 – 75
D 65 – 75
E 65 - 75
HMA Superpave Mix Design 47
48. Mix Requirement 100
for Dust Proportion 100
92
83
65
% weight of - 0.075 material 48
0.6 < < 1.6 36
% weight of effective
asphalt binder
22
15
9
4
HMA Superpave Mix Design 48
49. Selection of Design Asphalt Binder Content
VMA
VFA
% asphalt binder % asphalt binder
%Gmm
at Nmax
Va
% asphalt binder
% asphalt binder
%Gmm
at Nini
DP
% asphalt binder % asphalt binder
HMA Superpave Mix Design 49
50. Classroom Example
• Using the data on the next sheet,
determine:
• The design asphalt binder content
• The VMA at the design asphalt binder
• The VFA at the design asphalt binder
• The dust to asphalt ratio
HMA Superpave Mix Design 50
51. Classroom Example
% AC Va VMA
4.5 5.5 15.1
5.0 4.5 15.0
5.5 3.3 14.9
6.0 2.4 15.0
P = 0.4 % & the percent of minus 200 is 6%
ba
HMA Superpave Mix Design 51
56. DEFINITION
Stripping is the breaking of the
adhesive bond between the
aggregate surface and the
asphalt binder
HMA Superpave Mix Design 56
57. Stripping potential is controlled by
• Asphalt binder properties
• Aggregate properties
• Mixture characteristics
• Climate
• Traffic
• Construction practices
HMA Superpave Mix Design 57
58. Surface Chemistry
• Hydrophilic - “water loving”
• Those with high silica content
• Granites
• Hydrophobic - “water hating”
• Those with high carbon content
• Limestones
• But - it depends
HMA Superpave Mix Design 58
59. ANTISTRIP ADDITIVES
Surface Active Agents
• Generally they are chemical compounds
containing amines
• Amines are basic compounds derived from
ammonia
• Heat stability can be a problem
• Dosage rate is generally 0.5 % (but it depends)
• Can change the properties of the asphalt cement
- generally soften
HMA Superpave Mix Design 59
60. ANTISTRIP ADDITIVES
Lime
• Hydrated lime - Ca(OH)
• AASHTO Specification -
• The result is a bonding of the calcium with the
silicates in the aggregate
• Or an interaction or modification of the acidic
portions of the asphalt
• Dosage rate is generally 1 to 1.5%
HMA Superpave Mix Design 60
61. T-283 Procedure
• Six specimens are made at optimum asphalt
binder content
• VTM is 7.0 + 0.5 % for all other mixes
• Three specimens are vacuum saturated
• 90 % saturation minimum
• One freeze-thaw cycle
• Determine the indirect tensile strength of for all
six of the specimens
• Determine the percent retained strength
HMA Superpave Mix Design 61
62. Treatment with admixtures
• Liquid antistrip
• Asphalt binder is heated to 325 F
• Add liquid antistrip
• Stir for 2 minutes
• Lime
• Dry mixed to the hot aggregate or damp
aggregate immediately before the asphalt
binder is added and mixed (the process used
should match that being used in the field).
HMA Superpave Mix Design 62
63. Vacuum Saturation
• Place the specimen in vacuum chamber covering
with at least one-inch of water
• Drop the pressure by 26 inches of mercury for 30
minutes
• Tap the chamber to dislodge trapped bubbles
• Release the vacuum and leave in water for 30
minutes.
HMA Superpave Mix Design 63
64. Vacuum saturation
• After 30 minutes determine the percent saturation
% Saturation = {(100) (D-A)}
{(C-B)(E)}
A: Dry wt
B: Wt in water before saturation
C: SSD wt. Before vacuum
D: SSD wt. After vacuum
E: Percent air voids in specimen
HMA Superpave Mix Design 64
The objective of this module is to walk through each of the steps associated with the development of a Superpave mix design.
The mix design process should provide an HMA mix that will resist permanent deformation, should provide fatigue resistance, durability and resistance to moisture damage.
The objective of a mix design is outlined on this slide. Historically the Hveem and Marshall design methods have been used. They have been described in previous modules.
No matter which mix design procedure is used that the requirements for the resultant hot mix asphalt remain the same.
If we could start from scratch, what do we need or want from our laboratory compaction method? 1. We would want a compactor or compaction process that would simulate what happens to the HMA mix during construction and in service. 2. We would want to be able to evaluate larger sized aggregates. In the past (with the 4” Marshall), we may have “allowed the mold to design road” by not designing with large aggregates. 3. By monitoring the compaction throughout the process, we can get an idea of how the mix will behave under the roller. 4. We would want a machine that can be moved from place to place to follow an HMA plant and thus be used for quality control.
This slide shows three gyratories manufactured by different manufacturers. There are now (summer 2001) eight different manufacturers of gyratory compactors. They all produce similar results.
When the SGC was developed existing equipment - the Corps of Engineers, French, Texas, or Australian devices were evaluated and they took parts (vertical pressure, angle, RPM) from many sources to optimize the equipment eventually developed. A big advantage of the SGC is the capability to record height during the process.
This figure shows the various parts of the gyratory compactor.
The reaction frame provides a stiff structure against which the loading ram can push when compacting specimens. The base of the SGC rotates and is affixed to the loading frame. It supports the mold while compaction occurs. The SGC uses a mold with an inside diameter of 150 mm and a nominal height of at least 250 mm. A base plate fits in the bottom of the mold to afford specimen confinement during compaction. Reaction bearings are used to position the mold at a compaction angle of 1.25 degrees, which is the compaction angle of the SGC. An electric motor drives the rotating base at a constant speed of 30 revolutions per minute.
The AASHTO Subcommittee on Materials (SOM) has established levels for compaction with the SGC. Select the numbers of gyrations based on the anticipated traffic level for a 20 year design life.
The performance of HMA mixes is dependent on the rate of loading. Therefore, criteria was established to define the traffic levels described in the table in the previous slide.
There are four steps in the Superpave mix design: Materials selection, gradation selection, identification of optimum binder content, and the determination of the moisture sensitivity of the HMA at optimum asphalt binder content.
The first step in the mix design process is picking the asphalt binder that is going to be used and finding aggregates that meet the quality requirements.
The first step in asphalt binder selection is to choose an asphalt binder based on the climate in the region. The selection process for the asphalt binder was discussed in an earlier module.
Another step is to select that aggregate based on the properties needed to provide a long life HMA pavement. The column headings indicate the different angularity requirements based on the depth of the layer from the surface. For example, < 100 mm means this column is for the aggregate properties for the lift that is less than 100 mm from the surface (that is, for the upper 100 mm of the pavement). The two values in the coarse aggregate angularity column are values for the percent aggregate with one or more crushed faces and two or more crushed faces, respectively. Note that the requirements for crushing increase with increasing traffic levels.
Sand equivalent, to control the amount of clay in the mix, and flat and elongated properties, to control the non-uniformity of the aggregate shape are also specified. Many agencies will also have requirements for sulfate soundness and abrasion included in their specifications.
After you have approval of materials – you need determine what gradation can be used to provide the mix properties that you desire.
Selection of the design aggregate structure is a trial-and-error process. This step consists of blending available aggregate stockpiles at different percentages to arrive at aggregate gradations that meet Superpave requirements. Three trial blends are normally employed for this purpose. A trial blend is considered acceptable if it possesses suitable volumetric properties (based on traffic and environmental conditions) at a predicted design binder content. Once selected, the trial blend becomes the design aggregate structure.
There are two methods that can be used to establish a starting point for the asphalt binder content. There is a procedure the Superpave design manual that uses the specific gravities of the aggregates and the gradation. The most common method is engineering judgment which basically means that someone makes an educated guess based on past experience with a particular aggregate or similar aggregates.
The first three volumetric properties were defined in the HMA volumetrics section. Refer to that section for the necessary equations and definitions.
Use the volumentric information to calculate the initial binder content, P bi .
Pour the heated asphalt binder into a bowl with the heated aggregate. Typically the temperature for this is about 275 to 325 C.
The mixture is typically mixed in a mechanical mixer. The mix should be observed to insure that the aggregate and asphalt binder are thoroughly mixed.
The sample is aged in the plant – therefore we age it in the lab.
The short term aging is used to simulate what is happening in the hot mix plant during the mixing, storage and placement operations.
Follow these steps to determine the sample mass for the gyratory compactor.
Using the measured bulk specific gravity of the final specimen and the recorded change in height during compaction, the change in density (%G mm ) with number of gyrations can be calculated. It is typically plotted on a semi-log scale. A smooth sided cylinder is assumed initially and then later corrected based on the measured value for specific gravity
This slide shows the calculations associated with the volume. This is a standard that when multiplied times the height will give provide the volume of the specimen. The 0.0001 is for the needed unit conversion.
Once the sample mass needed to produce a sample of the required height is determined, the specimen is prepared. This slide presents the first steps associated with the compaction of a test specimen. The following slides show the lab work needed to make a specimen.
The mold is made ready by first placing a paper disk in the bottom.
The charging of the mold should be done in one mass operation. The mix is not to be rodded. A scoop or a gyroloader (as shown in the picture) are acceptable. The process should be done quickly to avoid the loss of heat. Care should be taken to ensure that the fine and coarse fractions are not separated.
After the mix has been placed in the mold and the top slightly rounded, a paper disk is placed on top of the mix.
The filled mold is placed in the compactor, the number of gyrations are set on the computer keypad that controls the instrument. The door is closed and the machine started.
This slide outlines the remaining steps for compacting a specimen.
The machine is started.
After sample is compacted it is extruded from the mold. Care should be taken to not damage the specimen when removing it from the mold.
There are three points of interest on the Superpave gyratory compaction (SGC) curve – N initial which is a measure of mixture compatibility. Mixtures that compact too quickly are believed to be tender during construction and may be unstable when subjected to traffic. N design is the number of gyrations required to produce a density in the mix that is equivalent to the expected density in the field after the indicated amount of traffic. In the mix design process, as asphalt binder content is selected that will provide four (4) percent air voids when the mix is compacted to N design gyrations. N maximum is the number of gyrations required to produce a density which some safety factor to ensure that the mixture does not densify too much resulting in low in-place voids thus leading to rutting. The air voids at N maximum are required to be a least two percent. Mixtures that have less than two percent air voids at N maximum are believed to be more susceptible to rutting than mixtures exceeding two percent air voids. The current practice is too compact the laboratory mixtures to N design during the design process and then check the mix to insure that the criteria for N maximum is not violated.
The HMA properties of each of the three trial gradations are compared. The gradation with the best properties is selected for use in the next step (determining the optimum asphalt binder content). In this example, blend 3 is selected because the VMA is higher than the minimum of 13%. While blend 2 meets the minimum, it will be difficult to consistently produce HMA with VMA values over this minimum.
The third step in the process is to determine the design binder content and to determine if the mix meets the requirements for the mix design. The requirements in this course are based on the overall national requirements for the US as determined by the Federal Highway Administration. The local requirements in each state may be different.
The HMA mix will be designed by compacting the specimens to N design but it is necessary to ensure that the mixture does not densify too much resulting in low in-place voids thus leading to rutting by checking the final mix by compacting two samples to N maximum and checking to see if if the mix has more that 2% air voids.
When the mix is compacted at different asphalt binder contents it will produce curves similar to these. As the asphalt binder increases the the curves will shift vertically up the axis.
This slide presents a listing of the requirements that are included in most HMA specifications.
As the nominal maximum aggregate size increases the VMA requirement will drop because less asphalt binder is needed to coat the aggregates. Larger aggregate has less surface area.
Dust proportion is the ratio of the percent passing the 0.075 mm sieve to the percent of the effective asphalt binder. This is the amount of asphalt binder that is available on the surface of the aggregate for “sticking” the aggregate together.
The next step in the process is to plot up your data, then pick a design asphalt binder and then check to see if your mixture meets the design criteria.
Given the data shown on this slide the desire is to determine the optimum asphalt binder content for an HMA Mix Design. The data is plotted in the forms shown earlier.
This is the plot of G mm at design gyrations versus the asphalt binder content. At 96 % of G mm or 4 % air voids the design asphalt binder content is 5.2 %.
At 5.2 % asphalt the VMA is 14.9 % which if the specification is 14% you are OK if it is 15 % the mix does not meet the specification requirements and must be redesigned.
The Voids Filled with Asphalt can be calculated from the VMA information and the air voids information. It is plotted against the asphalt binder content. In this case the VFA is 74 %.
The third step in the process is to determine the design asphalt binder content and to determine if the mix meets the requirements for the mix design. The requirements in this course are based on the overall national requirements for the US as determined by the Federal Highway Administration. The local requirements in each state may be different.
The purpose of the moisture sensitivity testing is to determine if there is a possibility of the asphalt binder stripping off the aggregate due to the moisture.
There are a number of factors that affect whether or not an HMA mixture will exhibit moisture problems. These factors are listed in this slide. 1. Asphalt binder properties – The stripping (moisture susceptibility) problem is basically a chemical compatibility problem. The surface charges on the asphalt molecules either attract or repeal the surface molecules on the aggregate particles. Therefore, the type and composition of the asphalt cement can affect the moisture susceptibility of the mixture. 2. Aggregate properties – As noted above – stripping is a chemical compatibility problem. It has been learned from experience that two aggregate types – granite and sand & gravel – are more prone to have stripping problems. The problem is that these two materials are very common aggregate types. 3. Mixture characteristics – A open mix with a low asphalt content is more prone to strip that a dense mix with a high asphalt content. This has to do with film coating. 4. Climate – To have a stripping problem you need to have a moisture susceptible mix (aggregate and asphalt incompatibility), moisture and high traffic levels. Therefore, it would seem reasonable to conclude that stripping problems would not occur in the desert Southwestern United States. But, they do – it does not require much moisture to cause a stripping problem. The freeze-thaw actions of the northeast do seem to accelerate the problems. 5. Traffic – You need to have high traffic volumes to have a stripping mix. 6. Construction practices – If density is not achieved during construction – the mix may be more susceptible to stripping.
As discussed earlier – the high silica aggregates (granite and sand & gravel) are more prone to stripping because by nature of their chemical structure they are more “water loving” than limestones. But, that does not mean that all granite mixes will strip or that no limestone mix will strip. It depends on the characteristics of the mix as a whole – air voids, VMA, asphalt binder content (type and amount), etc.
A common solution is to use liquid antistrips or lime.
This standard procedure for evaluating moisture susceptibility is AASHTO Test Procedure T-283. This slide outlines the test procedure.
It is suggested that if admixtures that they be added using the procedure outlined in this slide.
Test procedure continued.
This equation would be used to calculate the saturation level for the specimens.
The samples are vacuum saturated.
Then they are placed in a hot water bath for conditioning.
After being placed in the water bath they are placed in a chamber that controls the temperature for further conditioning.
Both the conditioning and unconditioned specimens are tested to determine their indirect tensile strength.
This equation is used to calculate the strength of the specimen.
The average dry tensile strength is divided into the average conditioned (wet) tensile strength to calculate a Tensile Strength Ratio (TSR). The typical requirement in the United States is a minimum of 80%.
When you have gone through these steps you have produced a mix design. If the mix developed does not meet the design requirements it will be necessary to repeat the process.
