Impermeable vs Permeable Voids: Specific Gravity

Impermeable vs. Permeable Voids and Specific Gravity

• Permeable voids allow water to be absorbed by the aggregate, influencing bulk specific gravity because the absorbed water adds weight without increasing solid volume.
• Impermeable voids do not permit water infiltration, affecting material volume without impacting absorption
• Apparent specific gravity only considers volume excluding permeable voids, while bulk specific gravity includes them.
• Specific gravity is vital in asphalt and concrete mix design as it affects density and void content calculations.
• Aggregates featuring high permeable voids can cause excessive water retention, potentially leading to premature deterioration.

Air Voids, Density, and Asphalt Needs

• Air voids (Va) are pockets of air trapped in compacted asphalt
• Density is the mass per unit volume of the compacted asphalt.
• The amount of asphalt needed is determined by voids in mineral aggregate (VMA) and voids filled with asphalt (VFA) to ensure adequate coating of aggregates and to achieve durability.
• Too many air voids result in weak pavement, allows water infiltration and oxidation, leading to premature failure.
• Too few air voids cause risk of rutting and bleeding because asphalt cannot accommodate thermal expansion or loading stresses
• Asphalt content must be optimized for balance between durability (prevent cracking) and stability (prevent rutting).

Criteria for Testing Rut and Crack Resistance (Temperature Ranges)

• Rutting resistance is tested using Dynamic Shear Rheometer (DSR) at high temperatures (~58°C to 82°C) to measure asphalt stiffness under repeated loading.
• G* (complex modulus) and phase angle are measured to evaluate material resistance to deformation.
• Crack resistance is tested using BBR (Bending Beam Rheometer) for low-temperature cracking (~ -6°C to -40°C)
• Stiffness and creep properties are measured to assess asphalt resistance to thermal contraction cracks.
• Direct Tension (DT) Test evaluates fracture properties at cold temperatures.
• Rutting happens because of excessive deformation at high temperatures, requiring a stiffer asphalt binder.
• Cracking occurs when asphalt becomes too brittle in cold weather, requiring a softer, more flexible binder.
• Performance Grade (PG) specifications select binders suited to specific climate conditions.

Modular Ratio: Soft vs. Stiff Materials

• The modular ratio (n) is the ratio of the modulus of elasticity of one material to another.
• If one material is much softer (lower modulus), the stiffer material will carry most of the load while the softer one deforms more.
• In composite materials like asphalt concrete, a stiff binder or aggregate skeleton improves load-bearing capacity.
• A softer binder (low E) increases flexibility but may reduce strength, while a stiffer binder (high E) improves strength but can crack under stress.
• The right balance ensures durability against both rutting and cracking.

Spring, Dashpot, and Slider & Material Behavior

• Spring (Elastic Element): Stores recoverable strain energy; deformation is instantaneous and reversible.
• Dashpot (Viscous Element): Represents time-dependent behavior; deformation depends on applied stress and time, and isn't instantly recoverable
• Slider (Plastic Element): Represents permanent deformation; once moved, it does not return.
• Maxwell Model (Spring + Dashpot in series) represents Linear viscoelasticity, creep, and stress relaxation.
• Kelvin-Voigt Model (Spring + Dashpot in parallel) represents Instantaneous elastic response + delayed deformation.
• These models assist in predicting creep, relaxation, and recovery of materials under load.

Emulsions and Cutbacks

• Emulsions: Asphalt is dispersed in water with emulsifying agents (like soap) to keep it in liquid form.
• Emulsions are used in cold applications such as tack coats and surface treatments.
• Types of emulsions include Anionic (negative charge) and Cationic (positive charge) based on aggregate compatibility.
• Cutbacks: Asphalt is dissolved in solvent (kerosene, naphtha) to reduce viscosity
• Solvent evaporates from cutbacks after application, leaving asphalt behind.
• Types of cutbacks include Rapid-Curing (RC), Medium-Curing (MC), Slow-Curing (SC) based on solvent evaporation speed.
• Emulsions are more environmentally friendly because they contain no solvents.
• Cutbacks cure faster but release volatile organic compounds (VOCs), making them less eco-friendly.
• The choice between cutbacks and emulsions depends on temperature, application type, and setting time needs.

Fineness Modulus (FM) and Coarseness

• Fineness modulus is a numerical index representing the coarseness or fineness of aggregate based on sieve analysis.
• Higher FM = coarser aggregate with more retained on larger sieves
• Lower FM = finer aggregate which allows more to passes through smaller sieves
• High FM (coarse aggregate) improves strength but may reduce workability.
• Low FM (fine aggregate) increases surface area, requiring more binder for coating.
• The right balance ensures durability, workability, and performance in asphalt and concrete.

Archimedes' Principle

• Archimedes’ Principle states that a body submerged in a fluid experiences a buoyant force equal to the weight of the displaced fluid.
• In construction materials, it is used in specific gravity determination of aggregates and asphalt binders.
• Specific gravity is found using water displacement methods.
• Helps determine bulk, apparent, and effective specific gravity, which are critical for mix design.
• Used in density testing which affects mix compaction and stability.
• Determines void content which affects permeability and asphalt binder demand.

Compaction: Temperature, Cement Type, and Asphalt Content (AC%)

• Compaction reduces air voids and increasing density in an asphalt mix.
• Temperature: Asphalt needs to be hot enough for adequate workability; optimal compaction occurs between 275-300°F (binder-dependent).
• Type of Cement: Stiffer binders (higher PG grade) require higher temperatures for proper compaction.
• Higher AC% increases workability but can lead to rutting.
• Lower AC% reduces flexibility and durability, leading to cracking.
• Poor compaction leads to premature rutting, cracking, and moisture damage.
• Optimal AC% ensures a balance between stability and durability.
• Proper temperature ensures effective coating and bonding of aggregates.

Aging: Air Voids and Water

• Oxidation: Asphalt binder reacts with oxygen, becoming stiffer and more brittle over time.
• Moisture Damage: Water infiltrates the mix, stripping asphalt from the aggregate, which leads to raveling and weakening.
• Air Voids: Higher air voids increase the rate of oxidation and aging.
• Aging reduces flexibility, leading to thermal and fatigue cracking.
• Increased air voids accelerate aging, while excessive moisture leads to stripping and loss of bond strength.
• Proper compaction minimizes air voids, slowing aging and improving long-term performance.

Factors Affecting Compaction and Aging

• Compaction Factors:
  • Temperature: Hotter mixes are easier to compact.
  • Asphalt Content: More binder improves compaction but can cause rutting.
  • Aggregate Gradation: Well-graded aggregates compact better than open-graded ones.
  • Roller Type & Passes: Vibratory rollers improve compaction more effectively.
• Aging Factors:
  • Air Voids: More air voids increase oxidation.
  • Binder Type: Harder binders age faster.
  • Moisture Exposure: Water intrusion leads to stripping and binder degradation.
  • UV Exposure & Temperature Cycles: Sunlight and extreme temperature shifts accelerate oxidation.

Asphalt Cement (AC) Content (4-6%)

• Increased AC%:
  • Reduces air voids (Va).
  • Improves coatability of aggregates.
  • Increases flexibility but reduces stability (higher rutting risk).
• Decreased AC%:
  • Increases air voids (Va), leading to oxidation and cracking.
  • Reduces durability due to poor binder-aggregate coating.
• Maintaining AC% within the 4-6% range is critical for balancing stiffness and flexibility.
• Too much asphalt leads to rutting, while too little leads to cracking.

Kelvin, Maxwell, Burger, and Hersch Models

• Maxwell Model (Spring + Dashpot in Series)
  • Represents creep & stress relaxation in asphalt.
  • Free end behavior: exhibits permanent deformation under sustained load.
• Kelvin-Voigt Model (Spring + Dashpot in Parallel)
  • Viscoelastic model representing delayed elastic response.
  • Free end behavior: Recovers completely after removal of load.
• Burger Model (Maxwell + Kelvin in Series)
  • Combines viscoelastic and viscous behavior.
  • Models both immediate and delayed deformation.
  • Free end behavior: Exhibits both permanent deformation and some recovery.
• Hersch Model
  • Advanced model representing non-linear behavior under complex loading conditions.
  • Represents a more accurate and elaborate material behavior under varied stress states.
  • Free end behavior: Accurate depiction of complex viscoplastic responses under irregular load applications.