Bituminous Materials
Learning Objectives
- Understand the properties and viscoelastic behavior of asphalt cement.
- Differentiate between binder grading systems including Superpave PG.
- Compare cutback asphalts and asphalt emulsions.
- Explain the Marshall Mix design principles and volumetric properties calculation.
- Identify standard laboratory testing methods for bitumen.
- Describe the Superpave mix design process.
Bituminous materials (primarily asphalt and tar) are complex mixtures of hydrocarbons. In civil engineering, asphalt cement is the most critical bituminous material. It is a viscous, dark, highly sticky, and highly waterproof material produced by the fractional distillation of crude oil. It serves as the flexible, thermoplastic binder in Hot Mix Asphalt (HMA) pavements, holding the aggregate structure together.
Asphalt Cement (Binder)
Unlike Portland cement, which cures via a chemical reaction with water, asphalt is a thermoplastic material. Its physical stateβwhether it acts like a solid glass, a viscous putty, or a flowing liquidβdepends entirely on its temperature and the duration of the load applied to it (rheology).
Viscoelasticity
The property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. Viscous materials resist shear flow and strain linearly with time when a stress is applied, while elastic materials strain when stretched and immediately return to their original state once the stress is removed.
Viscoelastic Behavior
Asphalt is viscoelastic. Under rapid, short-duration loads (like a car driving 60 mph), it behaves like an elastic solid, springing back into shape. Under slow, long-duration loads (like a truck parked on a hot day), it behaves like a viscous fluid, permanently deforming and causing 'rutting' in the pavement.
Binder Grading Systems
Because asphalt's properties change with temperature, engineers must specify the correct "grade" for the local climate.
Binder Grading Systems
- Penetration Grading (ASTM D5): An older empirical system that classifies asphalt based on how deep a standard weighted needle penetrates the sample at (). A "60-70 pen" asphalt is harder than a "120-150 pen" asphalt.
- Viscosity Grading (AC Grading): Classifies asphalt based on its scientifically measured viscosity at (), which represents the maximum expected pavement surface temperature in summer. For example, AC-20 has a viscosity of 2000 poises.
- Performance Grading (Superpave PG): The modern, advanced classification system. A PG 64-22 binder is specifically designed to resist rutting at an average 7-day maximum pavement temperature of and resist thermal cracking at a minimum pavement temperature of .
Liquid Asphalts: Cutbacks and Emulsions
Standard asphalt cement must be heated to over () to become fluid enough to mix with aggregate. For maintenance tasks (like pothole patching) or surface treatments (like tack coats) at ambient temperatures, the asphalt's viscosity must be temporarily reduced without heat.
Interactive Simulation
Use the simulation below to explore the relationships and concepts detailed above.
Asphalt Binder Viscosity
Adjust the temperature to observe how the asphalt binder viscosity changes. Viscosity dictates mixing and compaction properties.
Arrhenius viscosity calculation
Cutback Asphalts
Cutback Asphalt
Asphalt cement that has been temporarily liquefied by dissolving it in a petroleum solvent (distillate). Once applied, the solvent evaporates (cures) into the atmosphere, leaving the solid asphalt behind. Due to severe environmental regulations (VOC emissions) and flammability hazards, cutbacks are largely obsolete.
Types of Cutback Asphalts
- Rapid Curing (RC): Dissolved in highly volatile gasoline/naphtha. Cures very quickly.
- Medium Curing (MC): Dissolved in moderately volatile kerosene.
- Slow Curing (SC): Dissolved in low-volatility diesel or heavy oils. Often used for dust control.
Asphalt Emulsions
Asphalt Emulsion
A heterogeneous, two-phase system where microscopic droplets of asphalt cement are mechanically suspended in water with the aid of a chemical emulsifying agent (like soap). When the emulsion is sprayed onto the road, the water evaporates, and the asphalt droplets fuse together (a process called "breaking").
Emulsions are the modern, environmentally safe, non-flammable alternative to cutbacks.
Types of Asphalt Emulsions
- Anionic Emulsions: The asphalt droplets carry a negative electrical charge. Best used with positively charged aggregates like limestone.
- Cationic Emulsions: The asphalt droplets carry a positive electrical charge. They bond rapidly and strongly with negatively charged siliceous aggregates like gravel and granite. (Denoted by a "C", e.g., CRS-2).
Hot Mix Asphalt (HMA) Design
The goal of HMA mix design (traditionally the Marshall Method, now largely replaced by Superpave) is to find the perfect balance: enough asphalt binder to coat the aggregate and ensure durability, but not so much that the aggregate loses interlock and the pavement ruts under traffic.
Interactive Simulation
Use the simulation below to explore asphalt paving dynamics.
Marshall Mix Design Simulator
Adjust the Asphalt Binder Content (%) to see its effect on Stability and Flow.
Current Properties at 5.0% AC
*Optimal AC typically maximizes stability while keeping flow within specified limits (e.g., 8-14).
Marshall Stability and Flow
In the Marshall Method (ASTM D6926), a compacted asphalt cylinder is heated to and crushed laterally. Stability is the maximum load (in Newtons or lbs) the specimen can withstand before failing. Flow is the amount the specimen deformed (squished) at the exact moment of failure, measured in units of 0.25 mm (0.01 in). High stability with proper flow indicates a mix that will resist rutting but remain flexible enough not to crack.
Marshall Mix Design Principles
The traditional, empirical method of designing HMA. Samples are compacted in a cylindrical mold using a standard drop-hammer. They are then heated to 60Β°C and loaded in compression on their side to failure.
Marshall Mix Properties
- Stability: The maximum load the compacted specimen can support before failure. A measure of resistance to rutting and shoving.
- Flow: The total deformation of the specimen at the point of maximum load. A measure of the mix's flexibility and plasticity. Too high flow indicates the mix will deform permanently under traffic; too low indicates it will be brittle and crack.
- Density/Voids Analysis: The most critical aspect of Marshall design. The optimal binder content is selected primarily to achieve exactly 4% Air Voids () in the compacted mix.
Interactive Simulation
Use the interactive simulation below to perform the Marshall Stability and Flow test. Compact the specimen and analyze the load-deformation response to determine volumetric compliance.
Marshall Asphalt Design
Simulate compacted asphalt specimen testing to evaluate optimum binder content for structural road paving.
Volumetric Properties Calculation
Volumetrics (the calculation of air voids and aggregate space) is the most critical part of mix design. Proper air voids (typically targeted at exactly 4.0% for new mixes) ensure the pavement has room for the asphalt to expand in summer without "flushing" to the surface.
Bulk Specific Gravity ()
The ratio of the weight in air of a compacted HMA specimen to the weight of an equal volume of water. It represents the density of the actual pavement block, including the solid aggregate, the solid asphalt binder, and the air voids trapped inside.
Maximum Theoretical Specific Gravity ()
The specific gravity of a loose, uncompacted HMA mixture with zero air voids. It is the absolute maximum density the mix could achieve if every single microscopic air bubble were removed.
Air Voids ()
The total volume of small pockets of air between the coated aggregate particles throughout a compacted paving mixture, expressed as a percent of the bulk volume of the compacted mixture.
Variables
| Symbol | Description | Unit |
|---|---|---|
| Air Voids | % | |
| Maximum Theoretical Specific Gravity | - | |
| Bulk Specific Gravity of the compacted mix | - |
Voids in Mineral Aggregate (VMA)
The total volume of intergranular void space between the aggregate particles in a compacted paving mixture that includes the air voids and the effective asphalt content, expressed as a percent of the total volume of the sample. Adequate VMA is critical to ensure there is enough room for sufficient asphalt binder to coat the aggregate thoroughly.
Variables
| Symbol | Description | Unit |
|---|---|---|
| Voids in Mineral Aggregate | % | |
| Bulk Specific Gravity of the compacted mix | - | |
| Percent of aggregate in the mixture | % | |
| Bulk Specific Gravity of the total aggregate | - |
Voids Filled with Asphalt (VFA)
The percentage of the VMA that is filled with the effective asphalt binder. It is a measure of the relative volume of binder to the volume of air voids.
Variables
| Symbol | Description | Unit |
|---|---|---|
| Voids Filled with Asphalt | % | |
| Voids in Mineral Aggregate | % | |
| Air Voids | % |
Standard Laboratory Testing of Bitumen
Standard testing ensures the safety and performance of the raw asphalt binder before it is mixed with aggregate.
Standard Laboratory Tests
- Ductility Test (ASTM D113): Measures the distance (in cm) a standard briquette of asphalt can stretch before breaking at . High ductility indicates good flexibility and resistance to cracking.
- Flash Point Test (Cleveland Open Cup - ASTM D92): Determines the lowest temperature at which the asphalt vapors will temporarily ignite (flash) when exposed to a flame. This establishes the absolute maximum safe heating temperature for the refinery and the paving contractor.
- Softening Point Test (Ring and Ball - ASTM D36): Asphalt does not have a distinct melting point. This test determines the specific temperature at which the bitumen transitions from a semi-solid to a softer, fluid state (when a steel ball pushes through a ring of the asphalt).
- Rotational Viscometer (Superpave): Replaces capillary tube viscometers to measure the asphalt's flow properties at high manufacturing temperatures (e.g., ) to ensure it can be pumped at the asphalt plant.
Superpave Mix Design Process
The Superpave (Superior Performing Asphalt Pavements) method was developed in the 1990s to replace the empirical Marshall method. It is a comprehensive system designed to tailor HMA specifically to the project's unique climate and traffic loading.
Superpave Mix Design Process
- 1. Material Selection: Select the appropriate Performance Graded (PG) binder based on the highest and lowest expected pavement temperatures. Select aggregates that meet strict consensus requirements (angularity, flat/elongated particles, clay content).
- 2. Design Aggregate Structure: Blend different aggregate stockpiles to create a combined gradation that fits within the Superpave control points, ensuring a dense, stone-on-stone skeleton that resists rutting.
- 3. Design Binder Content: Compact trial mixes with varying asphalt contents using a Superpave Gyratory Compactor (SGC). The SGC simulates the kneading action of construction rollers and traffic. The optimum binder content is the percentage that yields exactly 4.0% air voids at the design number of gyrations ().
- 4. Moisture Susceptibility Evaluation: The final mix design must pass the Tensile Strength Ratio (TSR) test to ensure the asphalt binder will not strip away from the aggregate when exposed to water and traffic (preventing potholes).
- Thermoplastic & Viscoelastic: Asphalt cement changes from a brittle solid to a viscous fluid depending heavily on temperature and the duration of traffic loading.
- Binder Grading: The modern Superpave Performance Grading (PG) system selects binders based precisely on the local climate's extreme high and low temperatures to prevent rutting and thermal cracking.
- Liquid Asphalts: Emulsions (asphalt suspended in water) have replaced Cutbacks (asphalt dissolved in volatile solvents) for cold-weather applications due to environmental and safety concerns.
- Volumetrics: The calculation of Air Voids () comparing bulk specific gravity () to maximum theoretical specific gravity () is the absolute core of HMA mix design.