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**MODULE 4 ASPHALT MIX DESIGN***

What’s it all about? (*) Information taken from Asphalt Institute’s MS-2 manual, 6th Ed.

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**SCOPE What is a mix design?**

Would you recognize one if it walked up to you and asked for spare change? Basically, its just a recipe for making hot mix. What are the ingredients?

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**Well first you need a binder: That would be the asphalt cement.**

SCOPE Well first you need a binder: That would be the asphalt cement. What’s being bound together? That would be the aggregate. The objective of the design process is to “determine the proportions of asphalt cement and aggregate that will give long lasting performance as part of the pavement structure” (Asphalt Institute, MS-2)

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SCOPE the percent asphalt cement is in terms of the total mix mass in this course in order to get the right balance between aggregate and binder we need to know some properties of these materials the basic property of the aggregate is its grain size distribution or gradation this, more than any other property will affect the performance of the mix

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OBJECTIVES Objective 3.01 “to determine a cost-effective blend and gradation of aggregates and asphalt (cement) that yields a mix having: sufficient asphalt (cement) to ensure a durable pavement sufficient mix stability to carry traffic without distortion or displacement sufficient voids in the total compacted mix to allow for a slight amount of asphalt expansion due to temperature increases without flushing, bleeding and loss of stability

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OBJECTIVES Objective 3.01 a maximum void content to limit the permeability of harmful air and moisture into the mix sufficient workability to permit efficient placement of the mix without segregation and without sacrificing stability and performance for surface mixes, proper aggregate texture and hardness to provide sufficient skid resistance in unfavourable weather conditions” (Asphalt Institute, MS-2)

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MIX TYPES Objective 3.02 The Asphalt Institute (MS-2) identifies 4 different types of asphalt paving mixtures based on their functions within the pavement: Surface Course Mixtures (fine MPS*) Binder Course Mixtures (medium MPS) Base Course Mixtures (coarsest MPS) Sand-Asphalt Mixes (finest MPS) (*) MPS = Maximum Particle Size

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**Surface Course Mixtures**

for wearing surfaces, so fine aggregate needed to give a smoother texture must still have sufficient stability and durability for traffic loads Maximum Particle Sizes range between mm and 19 mm (⅜” to ¾”) open-graded friction course (OGFC), a.k.a. porous friction course or popcorn mix used to reduce hydroplaning and increase skid resistance Objective 3.02

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**Binder Course Mixtures**

intermediate layer between surface and base larger Maximum Particle Sizes 19 mm - 38 mm (¾” – 1 ½”) can be used as a base course or as a surface course where heavy wheel loads and tight radius power steering turns are involved such as is port facilities, logging yards and industrial loading docks Objective 3.02

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Base Course Mixtures can be placed directly on compacted subgrade or over a granular base Maximum Particle Sizes range up to 75 mm (3”) larger particle sizes result in higher stability in dense-graded mixes and facilitate drainage in open-graded mixes Objective 3.02

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Sand-Asphalt Mixes Objective 3.02 produce tightest surface texture and with proper aggregate type (hardness and shape) can be highly skid resistant minimum lift thickness is 15 mm (0.6”) which is useful as overlay padding (scratch course, leveling course) Maximum Particle Sizes range between mm and 1.18 mm (No. 4 to No. 16) a.k.a. plant mix seal or sheet asphalt

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MIX TYPES Objective 3.02 The percent asphalt cement required for mixes with the required properties varies among these mix types according to the maximum particle size: Sand-Asphalt Mixes (highest %AC) Surface Course Mixtures (lower % AC) Binder Course Mixtures (lower % AC) Base Course Mixtures (lowest % AC)

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MIX DESIGN PROCESS The Marshall mix design method involves preparing and testing mix specimen at trial % AC contents. The steps that we will be covering are: Sieve analyses of stock aggregates (stone, sand, mineral filler) Proportion stock aggregates to produce a blend that meets specs Measure specific gravities of stock aggregates and asphalt cement Laboratory compaction of trial mix specimens (6 trial % AC’s) Stability (or strength) and volumetric testing of each specimen Analysis of results (determination of mix properties and optimum %AC) Objective 3.02

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**Tests on Asphalt Cement**

Objective 3.02 Test/Property ASTM Designation AASHTO Designation Penetration D 5 T 49 Absolute Viscosity D 2171 T 202 Kinematic Viscosity D 2170 T 201 Flash Point D 92 T 48 Thin Film Oven Test D 1754 T 179 Rolling Thin Film Oven Test D 2872 T 240 Ductility D 113 T 51 Solubility D 2042 T 44 Specific Gravity D 70 T 228

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**Tests on Mineral Aggregates**

Objective 3.02 Test/Property ASTM Designation AASHTO Designation Los Angeles Abrasion C 131 or C 535 T 96 Unit Weight C 29 T 19 Sieve Analysis (Aggregates) C 136 T 27 Sieve Analysis (Filler) D 546 T 37 Specific Gravity (Coarse) C 127 T 85 Specific Gravity (Fine) C 128 T 84 Specific Gravity (Filler) D 854 or C188 T 100 or T 133 Sulphate Soundness C 88 T 104 Sand Equivalent D 2419 T 176 Particle Shape D 4791 -

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**Tests on Paving Mixture**

Objective 3.02 Test/Property ASTM Designation AASHTO Designation Asphalt Content (Extraction) D 2172 T 164 Asphalt Content (Nuclear) D 4125 T 287 Recovery of Asphalt D 1856 T 170 Density and Voids Analysis See next slide Maximum Specific Gravity D 2041 T 209 Bulk Specific Gravity D 1188 or D 2726 T 166

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**Steps for Voids Analysis**

(T is AASHTO Test and D or C is ASTM Test) Measure bulk specific gravities of coarse aggregate (T 85 or C 127) and fine aggregate (T 84 or C 128). Measure specific gravity of asphalt cement (T 228 or D 70) and of mineral filler (T 100 or D 854). Calculate the bulk specific gravity of the aggregate combination in the paving mixture. Measure maximum specific gravity of loose paving mixture (D 2041). Measure bulk specific gravity of compacted paving mixture (D 1188 or D 2726). Calculate the effective specific gravity of the aggregate. Calculate the asphalt absorption of the aggregate.

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