ASPHALT MIXTURE DESIGN CRITERIA

 

Overview of the Asphalt Mixture Design Criteria Chart (Table 1)

The Asphalt Mixture Criteria chart identifies the aggregate, mixture volumetric, and laboratory density requirements for mixtures designed under the gyratory mix design system. The chart is formatted to correspond with the bid item designations. The bid item designations classify each mixture by the maximum 20-year traffic load (ST-Standard Traffic, HT-High Traffic, VT-Very High Traffic), the intended pavement layer (surface, intermediate, base), the mixture size (based on nominal maximum aggregate size), and the surface layer friction requirement. A designation of HMA HT Surface L-3 describes the HMA mixture for high traffic, surface layer, -inch mixture size, with level 3 friction aggregate. Frictional aggregate requirements can be found in Standard Specification 2303.

 

The column to the right of the mixture designation define the required level of compaction (N value) and the target density (expressed as percent of Gmm) associated with each level of compaction. Note that the required density of a given level of compaction varies for different traffic levels and pavement layers. For example, the ST surface/intermediate, Ndes=50, mixture requires 96 percent of Gmm (4.0% air voids). The Ndes=50 base mixture for ST requires 97.0 percent of Gmm (3.0% air voids).

 

The middle column identifies the film thickness requirement.

 

The aggregate properties are defined in the right columns. The quality of the aggregate (Type A or B) is further specified in Standard Specification 4127. The crush value specifies the minimum amount of crushed aggregate required. The Fine Aggregate Angularity and Sand Equivalent values are consensus properties of the fine aggregate portion of the mix. Table Note 1 defines the allowable quantity of flat and elongated aggregate for all mixtures.

 

For any specified asphalt mixture, the mix design criteria are found by reading across the table. The asphalt mixtures are grouped by traffic levels.

 

Gradation Requirements

The individual aggregate gradation requirements for HMA mix designers are contained on Form 955.

 

The combined aggregate shall meet the gradation requirements on Table 2.

 

Table 1

Mix Designation

Gyratory Density

Film Thickness

Aggregate (1)

Ndes

Design (Target) % Gmm

Quality Type

Crush (min)

FAA

Sand Equivalent

ST

Surface

 

50

 

96.0

8.0-15.0

A

60

40

40

Intermediate

B

45

Base

97.0

--

HT

Surface

 

75

96.0

8.0-15.0

A

75

43

45

Intermediate

60

Base

96.5

B

40

VT

Surface

 

95

96.0

8.0-15.0

A

85

45

45

Intermediate

Base

96.5

B

75

40

 

HMA Interlayer (2)

50

98.0

>8.0

A

45

40

50

 

HMA Thin Lift (3)

50

97.0

>8.0

A

45

40

50

 

(1) Flat & Elongated 10% maximum at a 5:1 ratio.

 

(2) See Table 3 for additional requirements.

 

(3) See Table 4 for additional requirements.

 

 

 

Table 2

Aggregate Gradation Control Points

Sieve Size

Mix Size Control Points (% Passing)

1 inch

3/4 inch

1/2 inch

3/8 inch

HMA Interlayer

HMA Thin Lift

min.

max.

min.

max.

min.

max.

min.

max.

min.

max.

min.

max.

1 1/2 inch

100

 

 

 

 

 

 

 

 

1 inch

90

100

100

 

 

 

 

 

 

 

3/4 inch

90

90

100

100

 

 

 

 

 

 

1/2 inch

 

90

90

100

100

 

 

 

 

 

3/8 inch

 

 

90

90

100

100

 

91

100

No. 4

 

 

 

90

80

100

 

90

No. 8

19

45

23

49

28

58

32

67

60

85

27

63

No. 16 (1)

 

28

32

 

40

70

 

 

No. 30 (2)

 

24

25

 

25

55

 

 

No. 50

 

 

 

 

15

35

 

 

No. 100

 

 

 

 

8

20

 

 

No. 200

1

7

2

8

2

10

2

10

6

14

2

10

(1)   Only applies to surface and intermediate mixtures for HMA 30M and above VT designs.

(2)   Only applies to surface and intermediate mixtures for HMA 10M HT designs.

 

 

 

Table 3

Performance Requirements for HMA Interlayer (2)

Test

Requirement

Notes

AASHTO T-321

Minimum 100,000 cycles to failure

1

(1)   Failure criterion at 2,000 microstrain shall be 50% of the initial flexural stress measured at the 200th load cycle.

(2)   Use a PG 58-34E. (Hint: Past experience indicates at least 80%-90% recovery is needed for successful test results) Testing may be verified by the Engineer on field produced mix. Do not open to traffic until mat has cooled to below 200F.

 

 

 

Table 4

Performance Requirements for High Performance Thin Lift (1)

Test

Requirement

Notes

AASHTO T-324

Minimum passes to 4 mm rut depth > 8,000

1

(1)   Use a PG 58-34E binder with a minimum 90% MSCR recovery. Do not open to traffic until mat has cooled to below 200F.