Evaluation of High L.A. Abrasion Aggregate in Stone Matrix Asphalt

Tommy M. James, P.E.

Objectives• Develop multiple Stone Matrix Asphalt

(SMA) mix designs• Compare laboratory performance

properties• Evaluate results when compared to

specifications and historical data• Provide results and recommendations

for implementation

Literature Review• SMAs rely on stone skeleton for strength and

durability• Asphalt Binder and Mineral Filler create

mastic.• Breakdown of stone during compaction has

an effect strength• Laboratory compaction method and effort has

significant role in breakdown• L.A. Abrasion and Flat & Elongated properties

impact volumetrics

Literature Review• Hard aggregates used due to initial

design need of resistance to studded tire damage

• Hardness of aggregate linked to reflective cracking resistance

• FAA study showed design gyrations should be lowered for higher L.A. Abrasion stone

Purpose• Range of aggregate hardness available in

South Carolina• Using lower design gyration level to try and

prevent breakdown during compaction• Less breakdown allows for use of local

materials• Use of local materials provides cost savings

while taking advantage of SMA’s cracking resistance

Scope• SCDOT’s Draft SMA specification to develop

similar designs with different L.A. Abrasion values

• Develop multiple 12.5 mm NMAS SMA mix designs

• Select aggregate sources with L.A. Abrasion values of approximately 45, 52 and ≥55 (Grading C)

• Compare performance testing on designs to determine if there is a difference

Methodology• Acquire aggregate from selected sources

and obtain samples of RAP from local contractor using the same material.

• All virgin material from the same aggregate source

• Use of Fly Ash as mineral filler and a binder grade of PG 76-22

• Use VCA and VMA of trial blends to select optimum design.

Methodology• Using an L.A. Abrasion value of 45

(Jefferson) and 52 (Pacolet), perform the following performance testing:– Draindown (SC-T-90)– Aggregate Breakdown During Laboratory

Compaction– Rutting Susceptibility (AASHTO T340)– Hamburg Wheel-Tracking (AASHTO T324)– Indirect Tensile Asphalt (IDEAL) Cracking Test

Mix Design GradationsSieve Size, mm Design

Requirements Pacolet Jefferson Mix 1

Jefferson Mix 2

19.0 100.0 100 100 10012.5 85.0 – 90.0 85 86 859.5 60.0 – 80.0 61 64 63

4.75 25.0 – 32.0 27 32 302.36 18.0 – 24.0 19 21 191.18 17 17 160.6 12.0 – 20.0 15 14 140.3 12 12 12

0.15 9.0 – 15.0 10 10 100.075 8.0 – 12.0 8.3 8.0 8.0

Fiber Content, % 0.2 – 0.4 0.3 0.3 0.3

Mix Design PropertiesDesign

Requirements Pacolet Jefferson Mix 1

Jefferson Mix 2

Asphalt Content, % 5.60 – 7.00 6.3 6.3 6.5RAP Content, % NA 12.0 21.0 21.0

Binder Contribution from RAP, % 0.6 0.9 0.9

% Binder Contribution ≤ 15.0 9.5 14.3 14.3

Design Voids, % 3.5 ± 0.5 3.5 3.5 3.5VMA, % 16.5 min. 17.7 17.6 18.1VFA, % 65.0 – 85.0 80.2 80.1 80.6

VCA Ratio ≤ 1.0 0.98 1.03 0.98L.A. Abrasion Loss

of Blend, % 44.8 - 36.4

Draindown• SC –T-90 uses 350 ± 5 °F• AASHTO T-305 uses compaction

temperature plus 27 °F

Mix Fiber?320 °F 347 °F

Rep. 1 Rep. 2 Average Rep. 1 Rep. 2 Average

Jefferson Mix 1 Yes 0.11 0.06 0.08 0.11 0.18 0.15

Jefferson Mix 2 Yes 0.11 0.06 0.08 0.11 0.18 0.15

Pacolet Yes 0.09 0.01 0.05 0.07 0.12 0.09

Aggregate Breakdown• Using NCAT Ignition Oven to evaluate

breakdown under compactive effortSieve Size, mm

Jefferson Mix 2 PacoletJMF Mixed

ExtractedCompacted Extracted

JMF Mixed Extracted

Compacted Extracted

19 100 100 100 100 100 9912.5 85 88 87 85 85 869.5 63 71 69 61 60 64

4.75 30 38 40 27 26 332.36 19 22 26 19 19 241.18 16 18 20 17 17 200.6 14 15 16 15 15 170.3 12 12 13 12 12 14

0.15 10 10 11 10 10 110.075 8.0 8.2 8.4 8.3 7.9 8.5

Asphalt Pavement Analyzer• Using 76°C test temperature, Load of 100 lbs,

Pressure of 100 psi, and Air Void target of 4.0%• Maximum Allowable Rut Depth of Surface Type A is

3.0 mmJefferson Mix 1 Jefferson Mix 2 Pacolet

AC, % 6.3 6.5 6.3Average Voids, % 4.2 4.1 4.4

Average Rut Depth, mm 3.0 1.4 2.8

STD Rut Depth, mm 0.20 0.23 0.68

Comparisons versus PacoletP (F ≤ f) one-tail 0.038 0.052P(T ≤ t) two-tail 0.575 0.008

Significant? No Yes

Hamburg Wheel-Tracking

Hamburg Wheel-Tracking• Using TxDOT criteria values are passing• FAA SMA study showed values ranging from

5.94 -14.99 mm

Jefferson Mix 1 Jefferson Mix 2 Pacolet

AC, % 6.3 6.5 6.3Average Voids, % 7.5 6.9 7.2

Average Rut Depth, mm 4.66 6.68 7.06

STD Rut Depth, mm 1.40 0.33 0.26

Comparisons versus PacoletP (F ≤ f) one-tail 0.117 0.425P(T ≤ t) two-tail 0.139 0.326

Significant? No No

IDEAL-CT • Intermediate temperature cracking test

performed at 25 °C• Uses fracture energy and post peak load

similar to IFIT• Developed at Texas A&M • Simple sample fabrication for ease of

implementation as QC test

IDEAL-CT Jefferson

Mix 1Jefferson

Mix 2 Pacolet

AC, % 6.3 6.5 6.3Average Voids, % 6.9 7.0 7.3

Avg. Tensile Strength, kPa 666.8 738.0 547.0

COV1 Tensile Strength, % 12.4 9.2 11.2Avg. CT Index 417.0 302.1 434.1COV CT Index 25.7 14.9 8.1

Comparisons versus PacoletTS P (F ≤ f) one-tail 0.117 0.425TS P(T ≤ t) two-tail 0.139 0.326

Significant? No NoCT-Index P (F ≤ f) one-tail 0.026 0.316

CT-Index P(T ≤ t) two-tail 0.748 0.002Significant? No Yes

Preliminary Conclusions• With 0.3% fiber draindown is not an issue• All APA data meets SCDOT’s Surface Type A

mix requirements.• Hamburg data showed no stripping and were

consistent with previous research study• CTindex showed statistical difference with

higher LA Abrasion loss indicating better performance. Values were significantly higher than good performing dense graded designs

Current and Future Work

• Develop and evaluate an SMA design with an aggregate source with L.A. Abrasion of 55 or greater

• Possibly design and evaluate source with L.A. Abrasion of 35

• Alter designs and evaluate when fiber is removed and WMA is used

Questions?