Chemical and Mechanical Mechanisms of Moisture Damage in Hot
Transcript of Chemical and Mechanical Mechanisms of Moisture Damage in Hot
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Chemical and Mechanical Mechanisms of Moisture
Damage in Hot Mix Asphalt Pavements
Dallas N. Little – Texas A&M University
David R. Jones – Owens Corning
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Moisture Damage –Loss of strengthand durability dueto effects ofMoisture
loss of bondweakening of mastic US 89
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Air Voids
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The literature suggests a number of mechanisms
• Detachment• Displacement• Spontaneous emulsification• Pore pressure• Hydraulic scour• pH instability• Environmental effects
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Adhesion Theories• Chemical reaction
– Overall polarity of organic molecules promotes attraction to polar aggregate
– Charges are non-uniform (no net charge)– Aggregate charge distribution affected by
environment– Some polar components of asphalt adhere
more tenaciously and with more durability than others
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Adhesion Theories, cont’d• Surface energy and molecular orientation
– Associated with wetability– Synergistic effects– Surface energy defines bond strength
• Mechanical – Function of: surface texture, porosity, surface
coatings, surface area, particle size– Seek to maximize area and provide acceptable
texture
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Cohesion
• Influenced by mastic rheology– Function of asphalt binder and filler– Terrel and Al-Swailmi – describe how
water can weaken by saturation and void swelling
– Illustrated by classic Schmidt and Graf (1972) experiment
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Reversible Effect of Moisture on Mr
Schmidt and Graf, 1972
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Sand-asphalt Sample Installed
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Impact of Mineral Filler on ResistanceOf Mastic to Damage
1.E+03
1.E+04
1.E+05
1.E+06
1.E-01 1.E+00
Strain (%)
Cum
ulat
ive
DPS
E to
Fa
ilure
(*10
^10)
AAD-1AAM-1AAD+LSAAD+HLAAM+HL
0.28% Strain
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Universal SorptionDevice
Surface Energy Measurements: WilhelmyPlate and USD
Bitumen-aggregateBond is calculated From surface free energy
Wilhelmy Plate
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Start with Schapery’s First Principles Fracture Theory
• Load-induced energy that causes fracture
• Balanced by energy stored on newly formed cracks
Energy supplied byloads
Energystored oncracksurfaces
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Cohesive Fracture Adhesive/Dry
Adhesive/wet
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Asphalt Asphalt FilmFilm
Aluminum Aluminum FoilFoil
Adsorbed VaporAdsorbed Adsorbed VaporVapor
Measure of adsorbed and absorbedSolutes
Absorbed Absorbed VaporVapor
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Diffusivity and Absorbed WaterDiffusivity and Absorbed Water
0.00E+00
5.00E-04
1.00E-03
1.50E-03
2.00E-03
2.50E-03
3.00E-03
0 0.2 0.4 0.6 0.8 1 1.20
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.016
0.018
Water Content, g/g
Water Vapor Pressure
Diffusivity, m2/s
AAD
AAM
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85% Saturation
Laboratory DamageEvaluation
DamageSoftening
Load hardeningmix
Permanent Strain
Load Cycles
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Comparison of Free Energy of Adhesion (ergs/cm2) and Triaxial Testing (Saturated)
Mix Cycles to accel. damage
Free energy of adhesion (dry)
Free energy of adhesion (wet)
DLS 425 141 (614) -67 MLS 550 205 (889) -31 DGG 350 153 (158) -48 MGG 455 199 (206) -30
values in ( ) represent ergs/gm x 103
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Adhesive v. cohesive bond failure: a function of film thickness
0
100
200
300
400
500
600
700
0 0.01 0.01 0.01 0.01 0.02 0.02 0.02
AdhesiveCohesive
Asphalt Film Thickness, in.
Tensile Strength, psi
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Effect of Asphalt Composition
• No net asphalt charge but non-uniform charge distribution
• Aggregate surface charges also vary widely
• Charge arises from polar-polar attraction • Strength and tenacity of bitumen-
aggregate bond a function of asphalt polar component
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Tenacity and Durability Asphalt Component to Bond with Aggregate
• Robertson (2000)– Nitrogen compounds held tenaciously– Monovalent salts of acids (RCOOH) are water
susceptible– Divalent salts of acids are moisture resistant
• Curtis (1992)– Sulphoxide > carboxylic acid > non-basic
nitrogen / ketone > basic nitrogen > phenol– But sulphoxide and carboxlyic acid
susceptible to moisture
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Effect of Aggregate Properties
• Pore volume and area– Yoon and Tarrer (1988) – bond defined
by surface area, pore volume, pore size– Asphalt penetration synergistically
dependent on pore size and asphalt viscosity
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Effect of Aggregate Properties, cont’d
• pH of contacting water– Hughes et al. (1960) and Scott (1982) –
bond in presence of water weakens as pH increased from 7 to 9
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Effect of Aggregate Properties, cont’d
• pH of contacting water, cont’d– Tarrer (1988) – pH of aggregate fines in
water will become asymptotic to relatively high levels
• Approx. 10 for limestone• Approx. 8.8 for granite• Approx. 8 for quartz sand and gravel• Approx. 7 for chert gravel
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• pH of contacting water, cont’d– Yoon and Tarrer (1988) – aggregate
selectively adsorbs some asphalt components and H-bonds or salt links are formed
– Yoon and Tarrer (1988) – Presence of ketones and phenols are important to reduce stripping resistance; carboxylic acids, anhydrides, and 2-quinolones increase moisture sensitivity
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•pH of contacting water, cont’d–Thomas (2002) – interaction of moisture sensitive components from asphalt with hydrated lime may also allow bonds with nitrogen compounds to proliferate–Thomas (2002) also points out that lime may react with asphalt components that can further oxide to increase viscosity
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•pH of contacting water, cont’d–Yoon and Tarrer (1988) – metal ions affect stripping–Alkaline earths for salts not easily dissociated in high pH–Hydrated lime can tie up carboxylic acids and 2-quinolones and prevent them from interacting with H-bond forming functionalities
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Interaction of alkaline aggregates and asphalt with acidic components
CaC03 + 2RC00H (RC00-)2 Ca + CO2 + H20
CaO + 2RC00H (RC00-)2 Ca + H20
Ca(OH)2 + 2RC00H (RC00-)2 Ca + 2H20
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In a similar manner to reaction betweenacidic compounds and alkaline aggregatesor hydrated lime, amine compoundspresent in asphalt or added in the form ofantistripping agents can will react withacidic surfaces as in the case of siliceousaggregates to form a surface compound.
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Interaction of acidic aggregates and asphalt with alkaline amine components
--SiOH + RNH2 --SiO- RN+H3
H
HN
Polar End Non-Polar Hydrocarbon ChainGroup
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Asphalt aggregate interaction in the presence of suitable compounds in asphalt
Aggregate
AsphaltChemicalBonding
OH+H2 NSiAnti-StrippingAgentSilicious
Aggregate
Si O H3 N- +
Coating with chemical bonding
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Surface Potential• R-COOH in asphalt forms R-COO-
and H+ causing asphalt surface to have negative polarity at the interface
• Aggregate in presence of water is also negative – repulsion
• As pH of the water increases –negative charge becomes greater
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Yoon and Tarrer (1988) Used Zeta Potential to Measure Aggregate
Surface Charge
0
20
40
60
80
100
0 -10 -20 -30 -40
Limestone
Dolomite
Quartz gravel
Chert gravel
Granite
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SHRP Adhesion Model• Stripping controlled by cohesive failure within
aggregate rather than at bitumen-aggregate interface
• Surface rich in alkaline earth metals promote formation of water-insoluble salts and are more moisture resistant
• Stripping of siliceous aggregates may occur due to dissociation of silica generated by solubilization of alkaline earth cations and soaps formed between acid anions (bitumen) and alkali metal cations on aggregate surface
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Methods of Improvement• Interaction of acidic aggregates with
alkaline amine compounds– Amine ionized to R-NH3 with positive charge– Length of hydrocarbon chain ( R) and number
of amine groups influence adhesion – Fatty amines enable asphalt to wet aggregate
surface– Hydrophobic, hydrocarbon chain of the fatty
amine is anchored in bitumen (bridge)
H
HN
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Methods of Improvement, cont’d
• Hydrated lime– In siliceous aggregates, H-bond between H2O
and and aggregate SiOH group is preferred over that between SiOH and COOH
– Ca(OH)2 provided Ca++ to react with COOH to allow stronger bond with nitrogen groups (Petersen et al., 1987)
– Ca++ also migrates to aggregate surface to replace H+, Na+, K+, and other cations (Schmidt and Graf, 1972)
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• Hydrated lime, cont’d– Mg and Ca salts are relatively
hydrophobic, not very soluble– RCOOH in bitumen bonds very strongly
with aggregate surface, but high sensitive to moisture disruption
– Conversion of RCOOH to Ca++ based salts before mixing with aggregate could prevent adsorption of water-sensitive free acids to begin with
– Could hydrated lime introduced in the refinery or at the plant achieve this? (WRI, 1997)
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Impact of Freeze/Thaw Cycles on TSR
0
0.2
0.4
0.6
0.8
1
1.2
0 2 4 6 8 10 12 14
Number of Freeze-Thaw Cycles
TSR, 25oC
AAB-1
AAB-1 + HLDirectly to binder
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Dusty and Dirty Aggregates
• Dusty – coated with materials smaller than 75 µm – interruption of bitumen-aggregate bond
• Dirty – coated with clay minerals– high surface area– negative charge– affinity for water
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KaoliniteSSA = 20 m2/gCEC = 20 me/g
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SmectiteSSA = 800 m2/gCEC = 100 me/g
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Double Diffused Water Layer (DDL)
-
---
--
-
--
-
+
+
+
+
+
+
++
+
+
+
++
++
++
++
+
+
+
+
++
+
+
++
+
+
++
+
+
+
+
+
------
------
Water Dipole
Cation
ClaySurface
ClaySurface
H2O Diffusion
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Effect of Cation Adsorption on Attracted Water Layer
Na+ SaturationCa++ Saturation
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Ca++
+ =Soil Silica
Ca++
Ca++
CAHCAHSoil Alumina
CSHCSHH2O ( pH )
Ca++
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Conclusions• Moisture damage a synergistic
process• Non-uniform distributions in asphalt
and aggregate produce bond• Certain asphalt polars produce a
more tenacious and durable bond • Bond is affected by aggregate
mineralogy, texture, and porosity
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Conclusions, cont’d• In addition to strong bond, asphalt
must be able to wet and penetrate surface voids
• Bond is dynamic and is affected by pH shifts
• Moisture resistance is derived not only from bond strength but also mastic strength