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 ½ L-3” describes the HMA mixture for high traffic, surface layer, ½-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 G_mm) 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, N_des=50, mixture requires 96 percent of G_mm (4.0% air voids). The N_des=50 base mixture for ST requires 97.0 percent of G_mm (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⁾
		N_des	Design (Target) % G_mm		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	98.0	>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 VT designs.
(2) Only applies to surface and intermediate mixtures for HMA 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 200^th 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 200°F.

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 200°F.