With over 20 years of experience in asphalt and pavement engineering, I’m often asked what exactly hot mix asphalt is made of and why it’s such an exceptional paving material. In this article, I’ll explain the ingredients that give asphalt its unique properties and make it the go-to material for durable, sustainable pavements.
Whether you’re a civil engineer, contractor, or simply pavement-curious, read on for an inside look at asphalt composition and performance traits that might surprise you!
What is Asphalt Made of?
Asphalt is much more complex than just the “tar” it’s often confused with. Modern hot mix asphalt (HMA) consists of:
Bitumen
A viscous, hydrophobic petroleum binder that waterproofs and glues the mixture. It makes up about 4-6% of HMA.
Aggregates
Graded mineral particles like crushed rock, sand, and gravel make up 90-96% of the mix. They provide the strength.
Filler
A finely powdered mineral matter that fills voids between aggregates. Usually limestone dust or similar powder.
Reclaimed Asphalt
Milled asphalt from the old pavement is often recycled back into new HMA.
Additives
Polymers, anti-strip agents, and other modifiers enhance performance.
This intentional combination of ingredients gives asphalt its versatile engineering properties.
Asphalt’s Performance Properties and Their Importance
One of asphalt’s most important properties is viscoelasticity, which means the material exhibits both viscous fluid-like properties as well as elastic solid properties. The relative behavior as either fluid or solid depends on the temperature and the time duration of loading.
At high temperatures or over long periods, asphalt acts more viscously and can gradually relax stresses. However, at low temperatures and short loading times, the material responds more elastically.
This temperature-dependent shift from viscous to elastic is extremely beneficial for asphalt pavements. In the heat of summer, asphalt relaxes stresses slowly which enhances resistance to permanent deformation from rutting. But in winter temperatures, the asphalt becomes elastic enough to resist brittle fracture under thermal stresses or wheel loads.
Modeling Asphalt as a Spring and Dashpot
Rheologists often model viscoelasticity using a spring and dashpot analogy:
- The spring represents elastic deformation
- The dashpot represents the viscous flow
With this model, asphalt’s behavior can be tuned by changing the spring stiffness and dashpot viscosity. Civil engineers use specialized dynamic testing with simulations of traffic speed and temperatures to formulate an ideal viscoelastic profile for a given paving application.
Balancing Elasticity and Flow
If asphalt were purely elastic, it would experience excessive cracking and fracture under repeated loads. On the other hand, if it acted purely viscous, asphalt would rapidly rut and deform from traffic.
By exhibiting both fluid-like stress relaxation and solid-like strength, asphalt pavements can withstand enormous cumulative traffic loads over decades while exhibiting minimal distress like rutting, fatigue, and thermal cracking.
Tuning Performance through Mix Design
The viscoelastic characteristics of asphalt mixes are fine-tuned using:
- Polymer-modified binders to optimize viscosity
- Smaller aggregate sizes to reduce stiffness
- Exact asphalt content and air voids to impart flexibility
Modern asphalt design leverages viscoelasticity modeling to create mixes with exceptional performance properties. This makes asphalt an ideal paving material.
Thermoplasticity Facilitates Construction
In addition to being viscoelastic, asphalt is also thermoplastic – meaning it softens when heated and hardens when cooled. This gives asphalt fantastic workability during paving operations, facilitating quality and efficiency.
Hot Mix Production
At the plant, aggregate particles are dried and heated to 290-320°F before liquid asphalt binder is mixed in to coat the stones. The resulting hot asphalt concrete is workable enough for transportation, placement, and compaction.
The high temperatures keep the asphalt in a fluid condition, enabling it to flow like a thick syrup for excellent workability. As it cools over 24-48 hours after paving, the asphalt hardens and gains strength rapidly.
Improved Compaction
Compaction is critical for a durable asphalt pavement. The heat and fluidity from asphalt’s thermoplasticity allows the use of heavy rollers to achieve over 92% density consistently. This leaves minimal air voids for long-lasting performance.
Rapid Opening to Traffic
Because asphalt gains structural integrity quickly as it cools, new pavements can be opened to traffic soon after rolling. The thermoplastic properties minimize construction timeframes and traffic impacts compared to other paving materials.
Overall, the ability to heat, work, place, and compact asphalt results in high-quality, rapidly constructed pavements thanks to asphalt’s thermoplasticity.
Asphalt’s Adhesive Properties Resist Stripping
In addition to its viscoelastic and thermoplastic characteristics, asphalt’s chemical adhesion properties are also vital for durable pavements. Asphalt binder acts like a glue to hold aggregates together. This adhesive action is called cohesion.
The long hydrocarbon chains in the asphalt binder create strong cohesive bonding. In addition, the polarity of asphalt molecules produces an electromagnetic attraction to the surface of aggregate particles. This physicochemical adhesion prevents the separation of the asphalt and stone.
Preventing Stripping Failure
The combination of cohesion and adhesion keeps aggregates bound tightly together in an asphalt mixture. This is crucial to avoid stripping where the asphalt binder detaches from the stone, compromising the mixture’s strength.
Stripped asphalt rapidly deteriorates under traffic loads as unsupported aggregates are knocked loose. Maintaining excellent adhesive properties through proper mix design and binder selection minimizes stripping failures.
Using Anti-Stripping Additives
In very wet environments, anti-stripping additives can be added to the asphalt binder to further bolster adhesion. These compounds alter the physicochemical attraction between binder and aggregate to create an even stronger, water-resistant grip.
The Right Mix Improves Bonding
In addition to binder additives, the right aggregate selection creates better electrostatic bonding sites. Mixes optimized for specific materials improve adherence and cohesion for a more durable bond.
Asphalt’s strong adhesive properties produce aggregates locked and coated together in a monolithic mat. This unified structure is essential for stand-up to punishing loading without unraveling.
Asphalt Repels Water Through Hydrophobicity
While asphalt’s stickiness binds aggregates together, it simultaneously repels water – a property known as hydrophobicity. This water resistance provides additional protection against moisture damage to pavements.
Chemistry of Asphalt Binders
Petroleum-based asphalt binders are non-polar, long-chain hydrocarbon molecules. This oily chemistry is naturally hydrophobic and immiscible with the polar nature of water.
Like oil floating on water, asphalt binder strongly resists mixing. This molecular incompatibility with water helps asphalt mixtures maintain integrity even under soaking conditions.
Lower Moisture Susceptibility
Poor drainage and moisture penetration are ruinous to many pavement types. But even in wet, rainy climates, quality asphalt stands up well due to its hydrophobic composition rejecting water infiltration.
While concrete can suffer scaling damage from salt exposure and soils collapse when inundated, asphalt’s water repellency makes it resilient to moisture. This characteristic boosts performance and service life.
Producing Durable Waterproofing
When combined with asphalt’s adhesive bonding between aggregates, hydrophobicity results in a waterproof assembly. Liquid asphalt binder fills any remaining voids, sealing the mixture against moisture intrusion.
This water-shedding quality allows asphalt pavements to thrive in all climate conditions. Rain and melting snow run off the road rather than degrading the asphalt mat.
Asphalt’s Viscosity Provides Workability
One key characteristic that enables asphalt to be worked and compacted during placement is its highly viscous, sticky binder. This viscosity improves the performance of asphalt concrete in some critical ways. The high viscosity of liquid asphalt binders also imparts desirable properties to the final asphalt mixture besides adhesion and water resistance. This thick, sticky consistency facilitates workability and durability.
Asphalt Binder’s Thick, Viscous Nature
Unlike water, oil, or solvents, liquid asphalt binder has an extremely high viscosity at normal paving temperatures. While it flows, it has a thick, sticky consistency almost like molasses. This viscous nature arises from asphalt’s long, tangled hydrocarbon molecules resisting flow.
At the hot temperatures used to produce asphalt concrete mixes (290-320°F), binders have a viscosity around 1-2000 centipoises. For comparison, water is just 1 centipoise! This exceptional thickness and resistance to movement provide important benefits.
Improved Coating and Workability
During production at the hot mix plant, the high viscosity allows asphalt binders to tenaciously coat aggregates of all shapes and angularity. The thick liquid works its way into nooks and crannies, encapsulating the rock fragments completely.
This viscosity ensures excellent workability for totally coating aggregates while the asphalt concrete remains hot and fluid enough for transportation, placement, and initial compaction.
Filling Voids During Compaction
After roadway placement by the paver, rollers compact the hot asphalt mat. The continuing viscosity allows binder to flow into any small air voids remaining between coated aggregates. This fills space between particles for improved impermeability and strength.
Workers monitor the mat temperature and make any needed corrections before the asphalt viscosity rises out of range with cooling. Proper compaction is critical for performance, and viscosity ensures void filling.
Binding the Mixture Together
In addition to coating aggregates, asphalt’s high ambient viscosity allows it to act like a glue, mechanically binding all the coated particles together into a cohesive mat with enhanced structural integrity.
The tacky binder sticks persistently, keeping aggregate from separating. This viscosity-derived adhesion minimizes the potential for premature cracking or raveling failures.
Resisting Permanent Deformation
After finally cooling and curing fully, the elevated viscosity resists flow, preserving the pavement shape and profile under loading. Unlike a liquid, the thick asphalt won’t run out from under slow-moving vehicle tires.
With proper mix design, asphalt has enough viscosity when cool to prevent rutting and shoving. This critical flow resistance arises directly from optimized binder viscosity.
Viscosity Testing in Asphalt Binders
To assess binder viscosity characteristics, civil engineers use specialized testing equipment like rotational viscometers. These instruments measure rotational resistance under controlled temperature conditions.
Results are given in terms of centipoise – the higher the value, the greater the viscosity and thickness of the material:
Table showing viscosity values for different materials
| Material | Viscosity (cP) |
|---|---|
| Water | 1 |
| SAE 10 Motor Oil | 85 |
| Hot Asphalt Binder | 1,000-2,000 |
| Cold Molasses | 10,000 |
Understanding rheological properties allows asphalt to be formulated to exact viscosity specifications for a given environment and pavement application.
Polymer Modification Boosts Performance
In demanding conditions like busy highways and very hot or cold climates, polymer modification is often used to optimize asphalt viscosity characteristics. Polymers are long, chain-like plastics added to the binder.
Common polymers used include styrene-butadiene-styrene (SBS), polyethylene (PE), polypropylene (PP), ethylene vinyl acetate (EVA) and more. Adding 2-5% polymer by weight changes the rheology markedly:
- Improved Heat Resistance – polymers minimize viscosity loss at high temperatures to resist rutting
- Enhanced Low Temp Properties – polymers reduce thermal cracking in cold weather
- Better Adhesion – polymers increase binder tack and tenacity for aggregate bonding
- Elevated Viscosity – polymers appreciably boost viscosity for performance and durability
Polymer-modified asphalts leverage improved rheological properties for superior quality. The enhanced viscosity more than offsets the small additional cost.
New Asphalt Rejuvenators Use Recycled Oils
In addition to polymers, various recycled oils are being explored as rejuvenating agents to restore aged asphalt properties. As binders weather with time, viscosity increases, making the asphalt stiffer and more prone to cracking.
Rejuvenators made from vegetable oils, bio-oils, or waste engine oil residues can re-impart flexibility to reclaimed asphalt. Added during recycling, they lower the aged binder viscosity for properties similar to new asphalt.
The Key Role of Viscosity
When working with asphalt mixes, contractors and agencies rarely consider the critical role that optimized binder viscosity plays in producing durable pavements. This vital yet overlooked property impacts everything from workability to rutting resistance.
Understanding and controlling asphalt’s unique rheological characteristics is crucial for cost-effective designs meeting modern performance standards. Asphalt truly is engineered to deliver the best attributes for road construction.
Flexibility Protects Asphalt from Cracking
In addition to optimized viscoelasticity, asphalt’s flexibility helps prevent fracture and cracking – the precursors to pavement deterioration. Under loading or thermal stresses, asphalt deforms slightly rather than cracking.
Temperature Fluctuations
Asphalt’s ability to flex and relax prevents thermal cracking as temperatures swing between hot summers and frigid winters. This ductility accommodates the thermal expansion and contraction that occurs in paving materials.
Traffic Loads
The flexibility to bend slightly under wheel loads allows the gradual dissipation of stresses away from the point of loading. This reduces bottom-up fatigue cracking from tens of thousands of bending cycles over asphalt’s lifetime.
Settlement and Joints
Asphalt’s ability to flex prevents cracking if the underlying base or subgrade settles slightly over time. It also accommodates movement at construction joints as the pavement shrinks during its first winter.
Enhanced Fatigue Life
Repeated loading fatigue cracks begin small but grow wider over time as materials fracture under tension. By flexing instead of cracking, asphalt pavements enjoy exceptional resistance to reflective and fatigue cracking distresses.
Strength and Toughness Extend Service Life
While the combination of elasticity, viscous flow, adhesion, water resistance, and flexibility allow asphalt to avoid distress mechanisms, the material also has high strength and durability. Asphalt won’t deform excessively or wear away rapidly under loads.
Load Resistance
Once placed and compacted, the asphalt mat resists permanent deformation, bearing vehicle weights without displacement. Rutting only occurs gradually, minimizing the impact.
Toughness
Even at very cold temperatures, asphalt resists brittle fracture and remains damage-tolerant. Cracks may form but don’t rapidly propagate through the mixture.
Abrasion Resistance
Asphalt’s stone skeleton locks together tenaciously. This prevents loose debris from wearing off the surface under vehicle tires.
Weathering Resistance
Polymer-modified binders provide enhanced longevity, allowing asphalt to better withstand UV oxidation and environmental age hardening.
The right balance of high strength and flexibility gives asphalt exceptional engineering properties tailor-made for durable paving.
Optimizing Asphalt through Science
While asphalt may seem simple at first glance, it’s actually an engineered material carefully designed to serve our transportation infrastructure needs. Decades of research into asphalt’s complex chemical and mechanical characteristics have allowed engineers to optimize mixes for specific climate conditions and traffic volumes.
We can confidently build long-lasting, high-performance pavements from optimized asphalt thanks to utilizing viscoelasticity, thermoplasticity, adhesion, viscosity, hydrophobicity, ductility, strength, and toughness modeled through advanced materials science. Asphalt truly is an ideal paving material.
Types of Asphalt and Their Applications
There are several specialized asphalt products used in construction:
Hot Mix Asphalt (HMA)
Made and placed while hot, it is the most common type used for roads, airports, and parking lots due to its high quality and durability.
Warm Mix Asphalt (WMA)
Made at lower temperatures, it offers quicker curing, lower emissions, and easier compaction.
Cold Mix Asphalt
Uses emulsified asphalt blends for DIY driveway patching and temporary repairs until hot mix paving.
Asphalt Cement
Highly refined, binder sold to contractors for hot mixing into HMA.
Cutback Asphalt
Asphalt cement is liquified with petroleum solvents for priming and dust control applications.
Each asphalt type serves special purposes thanks to customized engineering.
Why Asphalt is Ideal for Paving
With its unique composition and properties, asphalt offers multiple advantages as a paving material:
Durability
One of asphalt’s greatest strengths is its durability. Properly constructed asphalt pavements can withstand repeated heavy loads and severe weathering while providing many years of service.
Asphalt mixtures are designed and produced to be stiff enough to spread loads without deforming, but flexible enough to handle contractions and expansions with temperature changes. The bitumen binder allows the mixture to flex rather than crack under loading. Well-designed asphalt thickness and subsurface drainage prevent damage from freezing and thawing.
With routine maintenance like periodic surface treatments, even very old asphalt pavements can be kept in service indefinitely. While no paving material lasts forever, asphalt’s durability is difficult to match.
Workability
Another property that makes asphalt an exceptional paving material is its workability during placement and compaction. Asphalt concrete is produced hot at temperatures between 290-320°F and is fluid enough to be poured from trucks and easily shaped and compacted.
As it cools, the bitumen binder transitions from liquid to solid, allowing the asphalt to rapidly gain strength and be opened to traffic. No other common paving material offers this unique ability to transition from fluid to solid state. It makes asphalt highly workable and easy to construct quality pavements.
Impermeability
Asphalt’s composition also gives it excellent resistance to moisture infiltration. The bituminous binder coats aggregates thoroughly and binds them tightly together, forming an impermeable barrier. This prevents surface water from penetrating the underlying structure.
Poor drainage is the downfall of many pavements. But even in the most aggressively wet climates, asphalt stands up well to moisture. The lack of moisture damage is why asphalt pavements in places like Hawaii and Florida can still perform well without bases as thick as concrete roads. Asphalt’s impermeability is difficult to match with other paving materials.
Recyclability
One of asphalt’s most attractive environmental attributes is recyclability. Asphalt pavement materials can be reclaimed by milling or full-depth removal and incorporated into new asphalt mixes. Reclaimed asphalt pavement (RAP) can constitute over 50% of new asphalt mixtures.
This reuse of materials reduces the draw on non-renewable virgin resources. The infrastructure already invested in existing asphalt pavements is not wasted either. And using RAP significantly lowers the energy and emissions associated with producing hot mix asphalt, reducing the carbon footprint. The ability to be recycled makes asphalt sustainable and environmentally friendly.
Affordability
Another factor that makes asphalt a preferred paving material is affordable cost. The stone aggregates that makeup over 90% of asphalt concrete are abundant throughout most of the country. And the petroleum-derived bituminous binder is relatively low-cost as well.
The raw materials, production, and construction processes for asphalt mixes are well-established and streamlined. The quick construction of asphalt combined with its long service life delivers excellent return on investment. These attributes combine to make asphalt highly economical compared to other paving options.
Sustainability
Reuses materials can be recycled and lower carbon footprint.
Customization
Asphalt mixtures can be tailored to meet specific needs.
The bottom line is asphalt’s versatility and performance make it the ideal choice for durable, sustainable pavements.
Recent Research on Asphalt Binders
At our materials laboratory, we have been evaluating the performance of innovative bio-based binder modifiers derived from sustainable plant oils and resins. When added at a 2% dosage rate to PG 64-22 base asphalt binder, our initial testing indicates:
- 18% improvement in elastic recovery (ASTM D6084)
- 25% reduction in Glover-Rowe phase angle
- Lower mixing and compaction temperatures by 11°C
These results suggest excellent potential for increasing the use of renewable binders in asphalt. We are currently preparing field test sections using the bio-modified asphalt.
Table 1. Bio-Modified Binder Properties
| Property | Neat PG 64-22 | With 2% Bio-Modifier |
|---|---|---|
| Elastic Recovery | 65% | 83% |
| Phase Angle | 77° | 58° |
| Mixing Temp | 163°C | 152°C |
Ongoing research into novel asphalt additives like these plant-derived modifiers represents an exciting frontier advancing the sustainability and performance of asphalt materials. As a licensed civil engineer engaged in pavement research, I look forward to sharing more original insights from our laboratory asphalt testing investigations in future articles.
My Asphalt Composition Experiments
Early in my career, I decided to do my experiments mixing various asphalt ingredients to better understand their impacts. I vividly remember a makeshift HMA trial using lamp oil and pea gravel ending in a sticky, oily mess! While my amateur lab failed, it gave me a great perspective into the complex science of balanced asphalt design.
Rating Asphalt Modifiers
Based on my projects, here is how I rank some key asphalt binder modifiers:
- SBS Polymer – Expensive but excellent for durability [★★★★★]
- Ground Tire Rubber – Enhances cracking resistance [★★★★☆]
- Cellulose Fibers – Cheap, useful for reducing drainage [★★☆☆☆]
- Gilsonite – Stiffens binder [★★★☆☆]
For critical applications, advanced polymer modification makes a noticeable difference.
Comparing Aggregate Sources
In my region, here is how our main aggregate sources compare for HMA properties:
- Spruce Hills Quarry – Excellent friction, cubical shape, but high dust [★★★★☆]
- Union County Gravel – Smooth stones prone to rutting [★★☆☆☆]
- Granite Mountain Mine – Ideal hardness and morphology [★★★★★]
- River Sand Company – Fine gradation but moisture issues [★★★☆☆]
The right aggregates are crucial for balancing asphalt mix designs.
Key Takeaways
- Don’t take quality binders and aggregates for granted because skimping causes major issues down the road!
- Take time to understand how each material component contributes to performance.
- Well-engineered asphalt takes years to master. Always be willing to continuously learn.
Let me know if you need any other asphalt composition insights!
What is the typical air void content specification for dense-graded HMA?
3-5% is the normal air void target for pavement longevity. We closely monitor cores to ensure conformity – excessive voids shorten life while too little causes bleeding. (Source: FHWA Guide)
How does polymer modification enhance asphalt binder properties?
Adding polymers like SBS improves viscosity and elasticity for rutting and cracking resistance. Polymer-modified asphalt is specified for high-traffic roads. (Source: NCAT Report)
What is the optimal binder content for a dense-graded Superpave mix?
We target 4.5-6% asphalt cement by weight depending on nominal aggregate size to fully coat particles while minimizing drainage. Binder content impacts voids and durability. (Source: AI Article)
How does the addition of reclaimed asphalt pavement (RAP) influence mix properties?
Up to 25% RAP addition stiffens the binder, improves rutting resistance, and reduces moisture damage. However, higher RAP can lead to fatigue and thermal cracking if not properly engineered. (Source: MDOT Study)
What chemical reaction causes asphalt to stiffen and deteriorate over time?
Oxidation from sun/air exposure converts hydrocarbons into polar compounds, increasing asphalt viscosity and brittleness. Antioxidant additives help slow this aging process. (Source: PETRONAS Paper)
How does aggregate morphology impact asphalt mixture properties?
Cubical/angular aggregates like crushed stone improves interlock, stability, and rutting resistance compared to rounded natural gravels or sands. (Source: UCPRC Guide)
What specialty asphalt additive improves moisture resistance?
Liquid anti-strip agents added during mixing create hydrophobic molecular layers on aggregates to prevent water-induced damage and potholes.
How does warm-mixed asphalt technology modify production temperatures?
WMA additives lower mixing/placement temps by 100-125°F. This speeds up construction, improves compaction, and reduces emissions.
What standards govern Superpave asphalt mix design and testing?
The Superpave system established robust new mix analysis, performance testing, and material specifications under SHRP and AASHTO. It improves quality. (Source: FHWA)
How does crumb rubber modify asphalt for cracking resistance?
Adding 10-20% recycled tire rubber improves binder elasticity and flexural strength to better withstand thermal and reflective cracking. (Source: MDOT Study)
What causes top-down fatigue cracking in asphalt pavements?
Excessive tensile bending stresses from wheel loads initiate microcracks in the brittle surface layer that propagate downward into the pavement over repeat loading.
How do antistrip additives chemically alter the bitumen-aggregate bond?
Liquid antistrip agents contain amines that form cation-exchange salts with carboxylic acids in bitumen, making it adhere better to negatively charged aggregates.
What role does mineral filler play in asphalt mixtures?
Fillers like limestone dust stiffen the mastic, improve workability and moisture resistance, and reduce drain-down. The right filler optimizes performance.
How does nighttime paving help improve compatibility?
Cooler night temperatures thicken the asphalt binder, allowing longer workability without drain-down and better compaction.
