Mix Design for HMA Recycling · 2013-01-02 · RAP Specific Gravity 1. Measure RAP agg specific...
Transcript of Mix Design for HMA Recycling · 2013-01-02 · RAP Specific Gravity 1. Measure RAP agg specific...
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Mix Design for HMA Recycling
Rebecca McDaniel North Central Superpave Center ISAP Short Course on Recycling
Fortaleza, Brazil September 30, 2012
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What is RAP? Reclaimed Asphalt Pavement (RAP) Existing pavements removed and reused Produced by
Milling -- Upper pavement layers removed and replaced with new pavement
Full-Depth Removal -- Pavement completely removed and reprocessed
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RAP from Milling
Removes old/distressed pavement
Improves smoothness Eliminates costly shoulder
work Maintains drainage
features, curbs, clearance Valuable rehabilitation
option
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What to do with the RAP?
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Composite Sources
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• Usually chunks and slabs from full depth pavement removal • Plant cleanout • Reject material or excess returned from jobs • Excavation • Other sources
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Composite RAP is reprocessed (i.e. crushed, screened, stockpiled and QC tested)
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In Composite Pile
After Processing
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The reprocessed products are very consistent components
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RAP is just another component of HMA
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History of RAP Use First used in 1915 (!) Major emphasis started in 1970’s
Oil embargo and increased oil prices Improved milling machines Counterflow drum plants
Became routine in many states Necessary to be competitive (lower bid prices) RAP contents of 15-20% common
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Today
Strong incentives to increase RAP use Increased material and energy costs Material supply issues Growing environmental concerns
Growing demand to Use RAP in more mixes (i.e. surfaces) Use higher RAP quantities (30% or more)
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Why?
Valuable resource – reduced material costs Cost savings – avoid disposal costs Competitive edge – lower bid prices It is the right thing to do It works!
Produce a recycled mix that performs as well as, or better than, the original mix.
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Typical Asphalt Mix 95% aggregate (approx. 30% of cost) 5% asphalt binder (approx. 70% of cost)
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RAP can be treated as another component of the mix.
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Uses of RAP RAP can be used for
Base Fill Shoulders Alleys
But, the highest and best use of RAP is back into new asphalt mixtures. Most beneficial Most cost effective
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Where Can RAP be Used? Virtually any mix – following best practices
Base and intermediate Potential to use higher amounts
Surface mixes Tendency to allow lower amounts Friction and cracking are potential concerns Increased resistance to rutting
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Note
Cold and hot in-place recycling can reuse essentially 100% of the material
Focus here is on hot mix asphalt for high type pavements
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Another Note
Examples and properties tested in this presentation are based on Superpave mix design practices.
Similar steps and procedures are appropriate for Marshall mix designs as well. Similar aggregate requirements. Use viscosity instead of Performance Graded
binder properties for blending chart.
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Possible Effects of RAP
At low RAP contents, little aged binder and RAP aggregate to affect properties of blend
At higher RAP contents, the hardened RAP binder may stiffen the mix
Good for rutting, not so good for cracking
The aggregate in the RAP may affect structure and stability of the mix
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How can we ensure performance? Account for the RAP aggregate and binder in
the mix design Adjust design as necessary Tiered system
Control variability Treat the RAP like another stockpile Practice good stockpile management Process the RAP, if needed
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Evaluation of RAP
Asphalt Content Aggregate Gradation Aggregate Properties Binder Properties – for high RAP contents
RAP binder stiffness influences how much RAP can be used with minimal testing
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RAP Mix Design Basics
Aggregate Considerations Include RAP aggregate in determinations of:
Specific gravity Gradation Fine aggregate angularity Coarse aggregate angularity Flat and elongated content Other tests required by agency
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RAP Aggregate Evaluation
Extract and test Gradation Coarse aggregate angularity Fine aggregate angularity Flat and elongated particles Aggregate specific gravity (one option)
Include in evaluation of consensus properties of trial blends Evaluate sand equivalent on virgin agg only
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RAP Specific Gravity
1. Measure RAP agg specific gravity after solvent extraction
2. Use RAP agg effective specific gravity, or 3. Backcalculate bulk s.g. from Rice density
and assumed or known absorption.
Agency discretion.
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Recommenda)ons for the Characteriza)on of RAP Aggregate Proper)es Using Tradi)onal
Tes)ng & Mixture Volumetric
Elie Y. Hajj, Peter E. Sebaaly, Randy West, Nathan Morian, Luis Loria
87th AAPT Annual Mee)ng April 1 – 4, 2012 Aus)n, Texas
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Research Approach Extract lab-produced RAP aggregates using:
Centrifuge (TCE) Reflux (TCE) Ignition oven
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Research Approach Tested extracted RAP aggregates & compared results to virgin aggregate properties. SG of RAP & virgin aggregates is required for volumetric calculation.
BSG of each aggregate stockpile, including RAP aggregate needs to be determined for the calculation of BSG of combined aggregates.
Assessed the impact on VMA calculation for various RAP contents
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Conclusions/Recommendations Based on Evaluated Aggregates RAP Asphalt Content Ignition method appears to give the most
accurate results for asphalt content of RAP.
Solvent extraction methods do not appear to remove all of the aged binder from RAP
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Conclusions/Recommendations Based on Evaluated Aggregates RAP Aggregate SG For high RAP content mix designs:
Best method is to use a solvent extraction method to recover the aggregate & then test the coarse & fine parts of recovered aggregate.
Ignition furnace may also be used to recover RAP aggregate except for some aggregate types which undergo significant changes in SG when subjected to the extreme temperatures used in ignition method.
As RAP contents approach 50%, the net effect may be an error in the VMA determination of +/- 0.4%.
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Conclusions/Recommendations Based on Evaluated Aggregates
RAP Aggregate SG (cont’d) (NCHRP Report 452) – Method B
Correct the Gse to an estimated Gsb using an assumed value for RAP asphalt absorption.
Correction is only reliable when asphalt absorption can be assumed with confidence.
Correction is very sensitive to the assumed asphalt absorption value & can lead to errors in VMA that are 0.5% or more.
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Conclusions/Recommendations Based on Evaluated Aggregates
RAP Aggregate SG (cont’d) (NCHRP Report 452) – Method C
Although some agencies use the Gse for the RAP aggregate in the calculation of VMA, the authors strongly advise against this practice.
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RAP Mix Design Basics
Binder Considerations Reduce added binder to account for RAP
binder
For higher RAP contents, use softer virgin binder grade to blend with and “soften” the hardened RAP binder Cost and construction (compaction)
implications
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Conventional Wisdom RAP contains old, hardened binder that
will stiffen the mix
This will help reduce rutting
May increase cracking tendencies
There is research and experience to support conventional wisdom And some that doesn’t.
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Possible Effects of RAP Binder
RAP aggregate with oxidized binder film
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Possible Effects of RAP Binder
RAP aggregate with oxidized binder film plus virgin binder film
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Possible Effects of RAP Binder
If RAP and virgin binders do not blend, effective binder properties will be those of the virgin binder only.
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Possible Effects of RAP Binder
If RAP and virgin binders blend or merge, effective binder properties will be determined by the amount of blending that occurs.
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Does blending happen? With many materials and plants, significant
(nearly complete) blending does occur. Evaluation of blending by looking at mixture and
binder stiffness But, we may be able to go to higher RAP binder
contents before changing grades In other cases -- especially with very hard
binders (shingles), high RAP contents or ???-- complete blending may not occur. Temperature, Time, Compatibility, Plant
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Current US Tiers Based on weight of RAP in mix
Up to 15% RAP, no change in binder grade.
16-25% RAP, lower binder grade by one increment.
More than 25%, create blending charts. Extract, recover and test RAP binder
Some states have established higher limits based on knowing their materials and research.
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RAP Binder Replacement
(AxB) + (CxD) E
x 100%
where A = binder content in RAP, % B = RAP content in mixture, % C = binder content in shingles, % D = shingle content in mixture, % E = total binder content in mixture, %
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RAP Binder Evaluation
Only necessary if using high percentages of RAP
Extract and recover binder Determine high, intermediate and low critical
temperatures for recovered RAP binder. Or use viscosities or penetration at
specification temperature.
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Constructing a Blending Chart
Use critical temperatures Determine Appropriate Grade of New
Binder (Method A), or Determine Maximum and Minimum
Amounts of RAP (Method B)
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Data Needed for Blending Chart Target PG Grade Critical high, intermediate and low temperatures of
recovered RAP binder And either
Critical high, intermediate and low temperatures of new (virgin) binder, or
Desired RAP content
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Example of Method A - Blending at Known RAP Content Desired Final Binder Grade = PG64-22 or
better Desired RAP Content = 30% Recovered RAP Properties Measured
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High Temperature Blending Chart, Method A
52586470768288
0% 20% 40% 60% 80% 100%
Percentage of RAP
Tcr
itic
al, C
54.3
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Intermediate Temperature Blending Chart, Method A
19
22
25
28
31
34
0% 20% 40% 60% 80% 100%
Percentage of RAP
Tcr
itic
al, C
22.6
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Low Temperature Blending Chart, Method A
-24
-18
-12
-6
0
0% 20% 40% 60% 80% 100%
Percentage of RAP
Tcr
itic
al, C
-16.4
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Use PG58-28 for the virgin binder
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Example of Method B - Blending with Known Virgin Binder Desired Final Binder Grade = PG64-22 or
better Virgin binder grade is PG58-28 Recovered RAP is a PG82-10 Critical temperatures of virgin and RAP
binders are determined.
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High Temperature Blending Chart, Method B
52586470768288
0% 20% 40% 60% 80% 100%
Percentage of RAP
Tcr
itic
al, C
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Intermediate Temperature Blending Chart, Method B
1316192225283134
0% 20% 40% 60% 80% 100%
Percentage of RAP
Tcr
itic
al, C
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Low Temperature Blending Chart, Method B
-24
-18
-12
-6
0
0% 20% 40% 60% 80% 100%
Percentage of RAP
Tcr
itic
al, C
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RAP Content
To achieve PG64-22, use between 14 and 36% RAP.
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Rejuvenators
Many on the market, especially for pavement preservation
Some can be used with RAP mixes Add maltenes to bring the oxidized asphalt
“back to life” Most US states just use softer binder grades If using a rejuvenator, use mixture tests to
ensure it is working as intended.
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Handling RAP in the Lab
Heat it gently just enough to heat it through
Recommend 110°C for no more than 2 hours
Heat virgin aggregate about 10°C above the mixing temperature
Then mix, condition and compact as usual.
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Summary Include RAP aggregate in gradation and
determination of consensus properties of trial and final blends.
Evaluate RAP binder properties if RAP is very hard or high percentages are used.
Adjust virgin binder grade by decreasing grade or constructing blending charts, depending on RAP stiffness and content.
Reheat RAP gently and slightly overheat virgin aggregate, then proceed as usual.
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More info:
Rebecca S. McDaniel Technical Director North Central Superpave Center Purdue University West Lafayette, Indiana USA 765/463-2317 ext. 226 [email protected] https://engineering.purdue.edu/NCSC/