2/5/2019 Mixture Design Part 2: Maintaining the Balance Shane Buchanan CRH Americas Materials Learning Objectives 1. Review the fundamentals of asphalt mixtures and know what drives performance and economics. Understand the importance of mixture materials consistency. 2. 3. Emphasize how to manage and characterize recycled products to ensure optimum addition levels and benefit. 4. Understand what can go wrong during production relative to the mix design and what steps to take to help ensure quality production. 5. Learn how asphalt performance testing can be used for mixture optimization within or outside a balance mixture design approach. Session Highlights This session will present fundamental principles that will assist in the design and production of a higher quality, more consistent, and more cost effective mixture . Discussion items will include volumetric property fundamentals, materials (binder, aggregate, and recycle) characterization and consistency, stockpile moisture considerations, and mixture performance testing and analysis . 1
2/5/2019 Mix Design Fundamentals The Mix Design A good mix design can help limit the number and magnitude of production issues and make it easier to control or maintain the balance achieved during design. Critical information/relationships must be known during the design process. Information You Must Know! Aggregate / Recycle / Binder Other items Aggregate gravities and absorption Additives (Dosage, Calibration, Impact) Recycled aggregate gravities Liquid anti-strip Current particle size gradings Hydrated lime Relationship between gravities Rejuventors Asphalt binder continuous grading Warm Mix Recycled binder continuous grading Asphalt rubber Volumetrics Air voids, voids in mineral aggregate, voids filled with asphalt, etc. Relationship of each property to one another and to binder content (e.g., how sensitive is the mix to binder changes) Should be done for each unique mix! Phase diagram fundamentals 2
2/5/2019 Aggregates Aggregates Main component of asphalt mixes ~ 95% by mass and ~85% by volume in a typical mix A primary driver of mix consistency and performance Gs Aggregate Gravities Archimedes 3
2/5/2019 Specific Gravity and Absorption: Basic Building Blocks • Specific gravity (G s ) is the ratio of aggregate weight to the weight of an equal volume of water – Dimensionless number (no units attached) Aggregate and Asphalt Mixture Moisture Conditions • Materials can be in varying moisture conditions. Oven dry and saturated surface dry (SSD) are key conditions. • Saturated surface dry is achieved when the surface of an aggregate particle or mix are " dry“ (no free moisture) , but the surface voids are fully saturated with water. • Internal voids are not typically considered for aggregates, but make up the air voids in a compacted asphalt mixture. Internal Voids Surface Voids Oven Dry Partially Saturated Saturated Surface Dry Free Moisture Note: Schematics illustrate an aggregate particle, but the principles hold true for a compacted asphalt mixture (lab or core). Volumetric Naming Convention • Asphalt phase diagram components follow the standard naming convention shown below. 4
2/5/2019 Aggregate Apparent Specific Gravity, G sa Aggregate Specific Gravity Relationships Water Absorption Permeable Surface Voids Filled with Water SSD weight - Oven dry weight Abs, % = X 100 Oven dry weight Aggregate Particle 5
2/5/2019 Aggregate Effective Specific Gravity G se is an aggregate property For a given mix design, the relationship between G se to G sa and G sb should not change (within test variability) Relationship may change based on mix temperature (binder absorption) If mix temperature is constant and a change is seen, something has changed in the aggregate blend (e.g., “heavy vs. light” aggregate). Aggregate Control Chart Develop control chart showing the G sa and G sb of the aggregate blend and the G se determined values. Identify trends before problems show up. Factors Influencing Aggregate Specific Gravity Factor Effect on Gs Aggregate Mineralogy Aggregate mineralogy has inherent variability Fine Aggregate Dust Higher dust contents can lead to falsely low Gsb and high Content absorption Aggregate Shape / Texture Possible. Could impact saturated surface dry determination Aggregate Grading Possible. Perhaps more with fine aggregate Sampling Error Absolutely Questions to ask… 1. Is the specific gravity of a given pit/quarry expected to remain constant? 2. How variable is the specific gravity of your aggregate? 3. How often do you check (test) the specific gravity? If you don’t know the aggregate gravities, you don’t know your mix! Impact of Aggregate Specific Gravity Understand the impact of changing specific gravity Variable gravities will impact volumetrics Changing aggregate gravities can impact binder volume in the mix Heavy aggregate relative to design = higher binder volume (and vice versa), highlight importance to design for desired asphalt volume. 0.1 Gsb ∆ = 0.2% AC ∆ Do your gravities change around your quarry/pit? Probably! How much? 0.01 Gsb ∆ = 0.3% VMA ∆ 6
2/5/2019 Recycled Products 2016 World of Asphalt Mix Optimization Fundamentals Manage and Utilize Recycle Products for Maximum Return • Conduct inventory analysis of recycled products • How much do you have and how much will you use? • How much will you be obtaining? • Can you use this as it’s obtained? (i.e., can you add to stockpiles?) • Evaluate the quality and consistency of the recycled products • What is the RAP source? DOT work, general RAP, plant waste, etc. • What is the RAS source? Manufactured waste, post consumer tear offs • Characterize • Evaluate impact of recycle use on asphalt binder selection and tankage capacity • Treat recycled products during production the same as virgin aggregates. • At higher recycled percentages (~35 to 40%+), this is a MUST! • Cover/paved stockpiles • Multiple recycle bins 2016 World of Asphalt Mix Optimization Fundamentals What Type of RAP Do You Have? Understand that millings and general RAP are different Both can provide a good quality product, but understand they are different 2016 World of Asphalt Mix Optimization Fundamentals 7
2/5/2019 Take Time to Analysis RAP (Especially Grading) Many times, graphs can provide a more complete understanding Suggestion: plot standard deviation along with coefficient of variation (COV) to get complete picture of variation (BTW, do the same for virgin aggregates) 2016 World of Asphalt Mix Optimization Fundamentals RAP Testing Thoughts 2016 World of Asphalt Mix Optimization Fundamentals RAP Testing Thoughts 2016 World of Asphalt Mix Optimization Fundamentals 8
2/5/2019 So What are Acceptable RAP Property Tolerances? NCAT recommendations on RAP AC and grading. Can you make RAP with higher variability work? Yes, but it becomes more difficult! 2016 World of Asphalt Mix Optimization Fundamentals RAP and Virgin Aggregate: Consistency NCAT study evaluated 74 RAP stockpiles in 14 states, and 60 virgin aggregate stockpiles in 6 states RAP would found to have lower grading variability Is this surprising? Probably not, RAP has been sized and processed more than virgin aggregate Is it always the case? No, but likely What is your case? 2016 World of Asphalt Mix Optimization Fundamentals Fractionated RAP is Not Always More Consistent NCAT RAP Study Data (fractionated vs unfractionated RAP) 2016 World of Asphalt Mix Optimization Fundamentals 9
2/5/2019 Utilize RAP Fractionation When Beneficial 2016 World of Asphalt Mix Optimization Fundamentals RAP Bulk Specific Gravity (Gsb) • Substantial VMA errors can occur if the incorrect RAP Gsb is utilized Gsb = 2.700 • Especially critical with high RAP mixes Gsb = 2.660 • VMA errors typically result in lower than calculated effective binder contents. (G mb * P s ) VMA = 100 - G sb Utah Asphalt Conference | February 2015 Binder 10
2/5/2019 Asphalt Binder Grading Up or down in PG 64-22 6 C increments Performance Average 7-day Min pavement Grade max pavement design temp - 22 o C design temp 64 o C PG Binder Classification Useful Temperature Interval (UTI) • Maximum to Minimum temperature range where the binder is expected 64 to perform properly. • PG 64-22 has a UTI of 86° • (64° - (-22°)) = 86° • PG 76-22 has a UTI of 98° • (76° - (-22°)) = 98° Rule of 92 • If UTI < 92°, asphalt binder is probably not modified • Some crude sources are exceptions – California crudes • If UTI = 92°, most asphalt binders are modified • Some crude sources are exceptions – Venezuelan crudes • If UTI > 92°, asphalt binder will be modified • Modification will increase the UTI of asphalt binder -22 11
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