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Asphalt Pavement Recycling: US Experience
PSWNA Meeting Warsaw, Poland October 28, 2011
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Rebecca S. McDaniel, PE, PhD
Technical Director
North Central Superpave Center
Purdue University
West Lafayette, IN
765/463-2317 ext 226
https://engineering.purdue.edu/NCSC
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Reclaimed Asphalt Pavement (RAP)
In the USA recycling began over 40 years ago because of: • Oil embargo – shortages and high prices • Environmental concerns • Development of milling machines
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Milling
Removes old/distressed pavement Improves smoothness Eliminates costly shoulder work Maintains drainage features, curbs,
overhead clearance Valuable rehabilitation option
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Typical Asphalt Mix
95% aggregate 5% asphalt binder
Reusing: Reduces asphalt demand Reduces need to quarry more aggregate
• Difficult to open a new pit or quarry Reduces energy/costs to produce,
process, transport aggregate
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Today in the USA
Asphalt pavement is the most widely recycled material 100 million tons reclaimed annually 95% is reused or recycled $1.8 billion in savings Reduces demand for new aggregates and
binder and the energy to produce them Strong incentives to use more RAP in more
mixes – economic and environmental
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Current US Guidelines
Adjust grade of binder added to account for the hard, oxidized binder in the RAP • 0 to 15% RAP, no binder grade change • 16-25% RAP, decrease virgin binder grade • Over 25% RAP, test RAP binder to determine
appropriate virgin grade (or allowable RAP content)
Percentage by weight of RAP in the mixture. Based on non-fractionated mixes with about
5% binder in RAP and new mix.
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Surface Mixes
Typically specifications allow lower RAP contents because of: • Friction of unknown aggregate types • Potential for cracking of stiffened mixes Thinner lifts have less overall aggregate
demand than intermediate and base mixes But, they are more frequently replaced Potential big impact by using more RAP in
surfaces
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50 US States
Every state is different! National specifications are guidelines, not
requirements States have adapted the guidelines to suit
their methods, perceptions and experience.
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0% 10% 15% 20% 25% ≥30% No answer
Maximum RAP in Base Mixes – Specified 2010
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0% 1-10% 10-20% 20-30% ≥30% No answer
Base Mixes – Average Use 2010
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Changes Occurring in US Practice
States are moving to higher RAP contents in more mixtures. More contractors are splitting the RAP into
different size fractions, called fractionating. More states are changing to expressing
RAP content in terms of percent of RAP binder that is replacing new binder. More interest in recycling asphalt shingles.
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RAS = Recycled Asphalt Shingles
Very high binder contents, as high as 30% • Greatly reduces demand for new binder • Hard, angular fine aggregate and fibers
But, shingle binder is very stiff (oxidized) so there is concern about cracking
So, allowable shingle content is about 20-25% as high as allowable RAP content
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Recycled Shingles
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RAP and RAS 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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What We Have Learned
High RAP contents can work – can perform well – if properly designed, produced and constructed.
Start with good mix design that accounts for the RAP.
But, need attention to detail during construction.
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Some Keys to Success Processing
the RAP Stockpiling
the RAP Control
during production
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Composite Stockpiles
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•Chunks and slabs from full depth pavement removal •Plant cleanout •Rejected material or excess returned from jobs •Other sources Variable Material
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Processing RAP
Mixed RAP can be variable
• Crush/Screen to break up clumps
• Improve uniformity
• Uniformity is essential to meet specifications
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In Composite Pile
After Processing
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Crushing and Screening
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The reprocessed products are very consistent components
Before
After
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Fractionated RAP = Crushed and screened into different sizes Improves uniformity (remixes) Allows use of different sizes to meet mix design Better control of gradation (and binder content)
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FRAP
Fractionated RAP
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Stockpiling Practices
Avoid segregation
Avoid contamination
Reduce stockpile moisture
Test the RAP stockpiles regularly – know what is in your stockpiles!
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Segregation
Coarse Area
Fine Area
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Contamination-Not Good
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Reduce Stockpile Moisture Lose ~12% production capacity for every
percent stockpile moisture above 2% Reduce fuel consumption and drying
costs by keeping your materials dry Lower moisture leads to increased
production capacity Lower maintenance and fuel costs for
loaders Lower paving costs How can we reduce moisture?
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Paved and Sloped Stockpiles Paving prevents mud at the bottom of pile. Slope grade 3 to 4° Moisture drains to bottom of pile and runs
along the slope. Pick off high side of pile Face slope towards sun for more drying Can reduce moisture 2% overall
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Covered Stockpiles Still rare but useful,
especially in high moisture areas
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Mix Plant Operations
RAP Collar
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100% Recycle Plant 33
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Plant Control for RAP Mixes
Control plant inputs (cold feeds) Control material variability Follow-up Quality Control test results Watch drum flighting – maintain
protective veil inside drum Avoid overheating mix Normal production care and attention
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Summary of RAP Best Practices Mill layers separately when you can Process RAP and stockpile properly Fractionate RAP Avoid contamination Keep the RAP and aggregates dry –
paved and sloped area, covered stockpile Test the RAP stockpiles regularly Watch plant production
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Performance: RAP vs. Virgin
Using data from Long Term Pavement Performance (LTPP) 20-year study Five performance measurements
• Rutting, mm • Roughness (IRI), m/km • Fatigue cracking, m2
• Transverse cracking, # per section • Longitudinal cracking, m 18 sites with ≥ 30% RAP and control
section with no RAP
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Performance of RAP Mixes Pavements with ≥ 30% RAP perform equal to
or better than virgin in most cases Somewhat more transverse and fatigue
cracking with RAP compared all virgin materials Differences in cracking may have been due to
lower asphalt contents and/or higher dust contents (poor mix design)
West, et al., NCAT
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Cost/Benefits of RAP
Milling or Pavement Salvage Costs • Mill, haul & stockpile: ≈$6.50/ ton • Excavate, haul & stockpile: ≈ $7.00/ton • Reprocessing: ≈ $5.00/ton
Virgin Material Costs • Coarse Aggregate ≈ $12.00/ton • Fine Aggregate ≈ $8.00/ton • PG Binder ≈ $450.00/ton (when analysis was done) • Ton of Virgin Mix = $50/ton for Intermediate,
$60/ton for surface course
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Cost Savings using RAP for a typical 19.0 mm Intermediate
% RAP Per ton Savings $ (materials only)
Notes
0 0 15 $3.40 25 $5.50 Does not include
premium PG58-28 40 $6.80 Includes premium
PG58-28
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25% RAP in an intermediate 19.0mm HMA will save approximately 11% per ton. Assuming a 3” lift; for every $1 million resurfacing project this will equate to approximately 2.2 lane miles of additional paving.
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Another Way to Look at Savings One medium sized US paving contractor uses
900,000 tons RAP per year That is equivalent to 1,460,000 gallons of gas Which is enough to fuel 1,650 big Ford
Expeditions
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Conclusions
RAP has long history of successful use. Asphalt recycling is sustainable. Asphalt recycling is economical. Asphalt recycling works!
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Rebecca S. McDaniel, PE, PhD Technical Director
North Central Superpave Center Purdue University
West Lafayette, IN 765/463-2317 ext 226 [email protected]
https://engineering.purdue.edu/NCSC
Or through [email protected]
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