Block 32 SP 13

HMA Characterization Moisture Sensitivity 1

Moisture Sensitivity

Senior/GraduateHMA Course

2HMA Characterization Moisture Sensitivity

Moisture Sensitivity


Reasons for Damage

• Loss of cohesion in asphalt binder film

• Failure of adhesive bond

• Degradation of aggregate

• Freeze/thaw


Loss of Cohesion(Spontaneous Emulsification)

• Inverted emulsion• Aggravated by presence

of emulsifiers• e.g. clays, additives

• Loss of stiffness and strength in asphalt binder


Loss of AdhesionMoist Aggregates

• Internal moisture disrupts asphalt binder film

• Most states have max. moisture content requirement on fresh HMA


Loss of AdhesionHydraulic Scour

• Traffic-induced movement of water “scrubs” asphalt binder off of aggregate

Stress as tire passes


Loss of AdhesionPore Water Pressure

• Usually traffic-related

• Voids decrease and water is trapped

• Moisture gets “pressed” to aggregate surface through breaks in film


Theories of Adhesion

• Mechanical adhesion

• Chemical reaction

• Surface energy

• Molecular orientation


Mechanical Adhesion

• Mechanical interlock

• Surface texture

• Porosity

• Surface coatings

• Surface area

• Particle size


Chemical Reaction

• Better adhesion with basic rather than acidic aggregates

• Basic

• pH > 7

• Positive charge

• Siliceous aggregates tend to strip

• Mixed results and findings in literature


Silica Content, %

Alkaline or Alkaline Earth Oxide, %

0 10 20 30 40 50 60 70 80 90 100

100 90 80 70 60 50 40 30 20 10 0

Silicious Limestone

Basalts Porphyries

LimestoneContent of C3O Diorites

Ophites

Granites

Sandstone

Positive Mixed Negative


Examples of Stripping Potential for Various Minerals and Aggregates

Slight Moderate Severe

Hornblende Quartz Quartz

Basalt Basalt Granite

Siliceous river gravel QuartziteGranite Gneiss

Limestone Limestone ChertSandstone


Weathering of Aggregate

• Over time, aggregate is exposed to numerous cycles of varying temp. and humidity

• Outermost adsorbed water molecules are partially replaced or covered by organic contaminates

• Absorbs oils from air

• Helps improve wetting by asphalt binder

• Stockpiled agg more resistant to stripping than freshly cleaved rock


Surface Energy Theory

• Wetting ability of asphalt binder

• Related to viscosity

• Water better than asphalt binder

• Higher surface tension = better adhesion

• Water will displace asphalt binder due to higher tension


Antistripping Additives

• Liquid antistrips

• Lime


Liquid Antistripping Additives

• Surface-active agents

• Reduce the surface tension between agg and asphalt binder

• Give surface charge opposite of aggregate

• Amines most commonly used

• Form positive ion (R-NH3+) when combined with

water or acid


Lime as an Additive

• Lime

• Hydrated lime Ca (OH)2

• Quick lime CaO• Hydrated lime reacts with most silicate agg.

• Crust of calcium hydroxy silicate• Strong bond with agg• Sufficient porosity to allow pen. of AC• Carboxylic acids and 2-quinlenes of AC

absorbed• Forms insoluble calcium salt


Methods of Adding Lime

• Dry

• Loss of lime in plant

• Results not consistent

• Hydrated lime slurry

• Additional water needed

• Reduces production rates

• Increases fuel costs


Methods of Adding Lime (Continued)

• Dry hydrated lime added to damp agg

• 3-5% moisture

• Mixing in pug mill or tumble mixer

• Quicklime slurry

• Yield of hydrated lime approx. 25% greater for similar cost

• Exothermic (very)


HMA Factors Which Influence Moisture

• Aggregates

• Surface texture (Rough)

• Porosity

• Mineralogy

• Coatings (Clean)

• Surface moisture (Low)

• Surface chemical composition

( ) = Desirable, no info = no consensus



(Continued)

• Asphalt cement

• Viscosity (High)

• Surface chemistry

• Composition

• HMA

• Voids (Low)

• Gradation (Dense)




(Continued)

• Weather Conditions

• Temperature during construction (Warm)

• Rainfall during construction (None)

• Rainfall following construction (Min.)

• Freeze/thaw following construction (Min.)



Boiling Water Test

• Subjects HMA loose mix to boiling water for 10 minutes

• Evaluation of stripping based on subjective evaluation of loss of asphalt binder coating

• Potential for quick field use or initial pass-fail test

• < 95% retained = problem

• Mixed results - no precision statement


Texas Freeze / Thaw Pedestal Test

• Subjects 41 mm diameter by 9.5 mm thick sample to repeated freeze and thaw cycles

• Uses fine portion only

• One Cycle: 10oF for 15 hr, 75oF for 45 min., 120oF oven for 9 hr.

Repeat until sample cracks < 10 cycles = moisture sensitive> 20-25 cycles = resistant to moist.


HMA Voids

• Relationship between strength and air voids

Retained Mix Strength, %

Air Voids, %

0 5 10 15 20

100

0

Impermeable

PessimumVoids

Free Draining


Definitions (Terrel and Swailmi, 1994)

• Impermeable or low void mixtures

• High asphalt binder or mastics

• Offset instability with crushing, large stone, and modified AC

• Pessimum void range

• Conventional dense-graded HMA in US

• Free draining or open graded

• Modified asphalt binder and higher % for thicker films

• Remain open under traffic


QUESTIONS ?

Technology

Block 32 SP 13