LIME IN HOT MIX ASPHALTTHE TEXAS EXPERIENCE
Darren Hazlett P.E.Retired Director
Texas Department of TransportationConstruction Division
17
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
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
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.
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.
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
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
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
Typical Test Results
Number of Passes
Rut
Dep
th
Creep Slope
Stripping Inflection Point
(SIP)
Stripping Slope
Number of Passes @ Failure
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
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
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
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
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:
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:
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.
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
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.)
Where Are We Now in Texas?(Unintended Consequences)
• We have solved some problems.• We have created other problems• It is COMPLICATED
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!
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
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.
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)
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
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
Fixing The Problem
• In addition to Volumetrics• In addition to Hamburg• Need another test to guard against mixtures
being brittle and prone to cracking
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
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