THE TEXAS EXPERIENCE · 2017. 11. 16. · •Fatigue cracking is increasingly becoming our biggest...
Transcript of THE TEXAS EXPERIENCE · 2017. 11. 16. · •Fatigue cracking is increasingly becoming our biggest...
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LIME IN HOT MIX ASPHALTTHE TEXAS EXPERIENCE
Darren Hazlett P.E.Retired Director
Texas Department of TransportationConstruction Division
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Population 28 million
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Texas Department of Transportation is responsible for 190,000 lane‐miles
of roadway
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856 miles/1377 km
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896 miles/1442 km
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Austin
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University of Texas
State Capitol
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Population: 1 Million
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Use lime all mixesUse lime some mixesNo lime or unknown
Use of Hydrated Lime in Hot Mix Asphalt - USA -
Hydrated Lime in Hot Mix Asphalt
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What is the Magnitude?Texas Department of Transportation Use of Lime in HMA• Approximately 25% of HMA contains Lime• In the last 12 months TxDOT contracted for 8.5M tons
(7.7Mt) of HMA• This translates to 80,000 tons (72Kt) of lime for HMA• Estimated that private/other-entity work doubles these
numbers
US HMA production is 370M tons with average 10% lime using lime
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Texas DOT Specs for Antistrip Agent
• Determine the dosage needed to achieve the moisture resistance requirements during design if an antistripping agent is required. Use this addition rate in the production mixture.
• Add between 0.5% and 2.0% of hydrated lime or commercial lime slurry solids by weight of the individual aggregate treated when using lime.
• Add liquid antistripping agent, when used, to the binder, in accordance with the manufacturer’s instructions. Do not exceed the manufacturer’s maximum recommended dosage rate.
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Texas DOT Specs for Antistrip AgentLime. Incorporate lime in a manner that thoroughly and uniformly distributes lime onto the aggregate surface or into the mixture. Use metering equipment, as approved, to ensure the required quantity of lime is used. • Hydrated Lime. Add hydrated lime to the aggregate by one of the
following methods, unless otherwise shown on the plans: ▫ Mix in an approved pug mill mixer with damp aggregate containing water at least
2% above saturated surface dry conditions. ▫ Add into the drum-mix plant immediately before asphalt binder addition or in the
pug mill of the weigh-batch plant before asphalt binder addition. Dry mix aggregates and lime before adding asphalt binder when a weigh batch plant is used.
• Commercial Lime Slurry. Add commercial lime slurry to the aggregate by one of the following methods unless otherwise shown on the plans: ▫ Mix in a suitable pug mill mixer with the aggregate. ▫ Mix with aggregate between the plant cold feeds and the dryer or mixing drum
during mixture production.
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Hydrated Lime can Address(I will Talk About These)
• Moisture Sensitivity (Stripping)• Moisture Sensitivity and Rutting• Binder Hardening (Oxidation)
• Where are we now in Texas
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Moisture Sensitivity (Stripping)
•Boil Test•AASHTO T-283•History
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147
Moisture SensitivityAASHTO T-283
Empirical test used to characterize aggregates
Passing threshold – 80% TSR
TSR (%) Alabama California Illinois South Carolina Texas
Un-treated 81 72 82 61 61
Liquid 83 91 85 81 100
Lime 90 95 87 87 98
Ref: Sebaaly, et al
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In the Past• Up until the late 1990’s TxDOT was experiencing
approximately 3 premature failures per year related to stripping and/or rutting
• Conventional tests such as T-283, HveemStability, Boil Test, Static Creep etc. did not show a very good correlation with performance. Mixes would pass all of these tests yet fail on the roadway
• Two extensive field studies showed that AASHTOT-283 (Tex-531-C) did a poor job identifying mixtures susceptible to moisture damage▫ CTR Study: 8 TxDOT Districts, 92 test sections▫ TTI Study: 3 Districts, over 35 pavements
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Moisture Sensitivity & RuttingHamburg Wheel Tracking Device
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Typical Test Results
Number of Passes
Rut
Dep
th
Creep Slope
Stripping Inflection Point
(SIP)
Stripping Slope
Number of Passes @ Failure
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02468
1012141618
0 2 4 6 8 10 12 14 16 18 20
Number Passes (X1000)
Rut
Dep
th, m
m
No AdditiveLiquidHydrated Lime
Moisture Sensitivity/ Rutting (1) Hamburg Wheel - Corpus Christi Gravel
Ref: Texas Department of Transportation
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Moisture Sensitivity/Rutting (2) Hamburg Wheel - Basalt Mixtures
02468
1012141618
0 2 4 6 8 10 12 14 16 18 20
Number Passes (X1000)
Rut Dep
th, mm
No AdditiveLiquidHydrated Lime
Ref: Texas Department of Transportation
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Stripping Susceptible Mix
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Investigations
Different Aggregate, Same Binder
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District: W.Falls Mix Type: Stone Filled(0.5) Binder: 76-22
Aggr.: Limestone Additive: Lime (2%) ID: 540010
Rutting: 12.5+ mm # of Passes: 18,900* Temp: 50C
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District: W.Falls Mix Type: Stone Filled(0.5) Binder: 76-22
Aggr.: Granite+ Additive: Lime(1%) ID: 540027
Rutting: 2.9 mm # of Passes: 20,000 Temp: 50C
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Investigations
Same Aggregate, Different Binder Source
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Rutting: 12.5 mm # of Passes: 10,200 Temp: 50C
District: Abilene Mix Type: Superpave Binder: 76-22 (Source 1)
CSJ: 0068-07-046 Aggr.: Limestone Additive: None
ID: 01500318 Lab Mix Notes:
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Rutting: 2.8 mm # of Passes: 20,000 Temp: 50C
District: Abilene Mix Type: Superpave Binder: 76-22 (Source 2)
CSJ: 0068-07-046 Aggr.: Limestone Additive: None
ID: 01500380 Lab Mix Notes:
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What Does the Hamburg Test Tell Us?
• Hamburg is a torture test that is an indicator of a hot mix paving mixture’s susceptibility to premature failure that may be attributed to factors including: – A weak aggregate structure– Inadequate binder stiffness– Moisture Damage (Lime still proves an
almost universal antistrip agent.)– Inadequate binder to aggregate adhesion– Murphy’s Law, etc, etc, etc.
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BINDER HARDENING(OXIDATION)
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Effect of HL on Asphalt Binder HardeningUtah DOT Field Study
0
1000
2000
3000
4000
5000
6000
0 1 2 3 4 5 6 7 8
Time in Years
Visc
osity
(60C
), St
okes
No Additive Hydrated Lime
Ref. Jones, 1997
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How do you Specify Lime for Oxidation Resistance?
• Each combination of lime and asphalt react differently
• Can’t quantify the effect and put it in a binder specification
• Could we just say to use lime because we know it is beneficial? (Some TxDOT engineers specify lime as a mineral filler because they know they get higher performing mixtures.)
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Where Are We Now in Texas?(Unintended Consequences)
• We have solved some problems.• We have created other problems• It is COMPLICATED
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What have we learned about Hamburg testing
• Hamburg does a better job of identifying mixtures that are susceptible to premature failure (T-283, Hveem, Creep, etc.)
• Higher (stiffer) PG grade binders do better• Adding liquid antistrips or lime usually improves the
Hamburg results• Harder aggregates do better (Igneous -Vs- limestone)• Stone on stone mixes do better than dense mixes• There are no absolutes………...• Do not assume………... Measure!
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The PresentWe have Solved Some Problems
• Stripping and rutting problems have decreaseddramatically (To practically ZERO)
• The Hamburg Wheel has proven to be the best identifier of mixtures susceptible to premature failure
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TxDOT HMA Specifications
Superpave and TxDOT Traditional Dense Graded Mixtures
Permeable Friction Course and Stone‐Matrix Asphalt Mixtures
• Hamburg in Design• Hamburg in Production• Treatment, if needed, is
contractor’s choice (lime or liquid)
• Lime may be specified as a mineral filler
• Hamburg in Design• Hamburg in Production• Lime is required (may
use liquid also)• Use either
o PG 76, fibers, and lime, oro Asphalt‐Rubber Binder and lime.
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TxDOTSpecifications
Grade Passes @ 12.5 mm rut
PG 64 10,000
PG 70 15,000
PG 76 + 20,000
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We have Created Other Problems• Hamburg only tells one side of the story• Rutting resistance - versus- Fatigue cracking
• Fatigue cracking is increasingly becoming our biggest problem! – Relatively thin (and overly stiff) layers of hot
mix placed on flexible base can be a recipe for disaster
– Thick and stiff (OK)– Thin and flexible (OK)– Thin and stiff (no good)
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TxDOT Funding ProblemsPennies to the Pavement
(2010-2014)• Reduce Cost of HMA.
– Discourage requiring lime as a mineral filler– Binder substitution– More RAP– Use RAS– Thin up overlays
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The Complicated Part(How to Increase Hamburg Passes)
• Change Aggregate (Use Harder Aggregates)
• Change Mix Type or Gradation (More Stone on Stone Contact)
• Use Antistrip Agents (Lime/Liquid Agents)– Lime Works Almost Universally. – Some Liquid agents soften the binder
• Change Asphalt (Some Asphalts are Better)
• Reduce Asphalt Content
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Fixing The Problem
• In addition to Volumetrics• In addition to Hamburg• Need another test to guard against mixtures
being brittle and prone to cracking
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Balanced Mix Design
• Semi-Circular Bend Test (SCB)• Beam Fatigue Test• Others• Texas Overlay Test
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Overlay Test
174
Instrotek AMPT
Troxler
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Balanced Mix Design Approach
y = 2E-07x12.635
R2 = 0.9829
y = 698467x-2.4408
R2 = 0.9505
0
5000
10000
15000
20000
25000
4.0 4.5 5.0 5.5 6.0 6.5 7.0Asphalt Content (%)
Num
ber o
f Pas
ses
(12.
5 m
m R
ut D
epth
)
0
500
1000
1500
2000
2500
3000
3500
4000
4500O
T Num
ber of Cycles
HWTOT
5.3
300
5.7
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Conclusions & The Future• Lime has beneficial effects when added to
HMA• TxDOT has found lime to be almost
universal in its ability to prevent stripping in HMA
• Use of the Hamburg Wheel Test by DOTs is growing
• Many DOTs are investigating balance mix design to protect against rutting and cracking
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Who says Concrete does not Rut?
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Thank You