Asphalt cement is made by refining crude oil. Heavy crude petroleum enters a distillation tower, where progressively higher temperatures boil off lighter products like gasoline, kerosene, and diesel. What remains at the bottom of the tower is a thick, dark residue, and that residue is the raw material for asphalt cement. The entire process is industrial in scale, requiring specialized refinery equipment, but the underlying principle is straightforward: remove everything that isn’t asphalt.
Crude Oil Selection
Not all crude oil produces the same amount of asphalt. Heavier crudes with a higher proportion of large, complex hydrocarbon molecules yield more asphalt residue per barrel. Lighter crudes produce more gasoline and jet fuel but leave behind less of the heavy residue needed for paving-grade binder. Refineries that specialize in asphalt production typically source heavy or medium-heavy crudes for this reason. The conversion factor for finished asphalt is roughly 5.5 barrels per short ton, according to the U.S. Energy Information Administration, which gives a sense of the density of the final product compared to the crude it comes from.
Fractional Distillation
The core manufacturing step is fractional distillation. Crude oil is heated inside a tall distillation tower, and as the temperature rises, different components vaporize at different points. The lightest fractions (gases and light fuels) collect near the top of the tower. Heavier fractions like diesel and lubricating oils collect on trays further down. The heaviest residue, rich in the long-chain hydrocarbons that make up asphalt, stays at the very bottom.
Steam or vacuum pressure is used to push temperatures higher without thermally cracking the molecules. This matters because the goal at this stage is physical separation, not chemical transformation. The refinery gradually raises the temperature to strip away every usable lighter product, leaving behind only the asphalt-bearing residue.
Vacuum Distillation and Further Processing
The heavy residue from the first distillation column typically goes through a second round of processing in a vacuum distillation unit. Lowering the pressure inside the column allows heavier molecules to vaporize at temperatures that would otherwise be too low to move them, protecting the material from breaking down. This step removes additional gas oils and waxes, concentrating what remains into asphalt cement of various grades.
For some applications, refineries use what’s called destructive distillation, which involves much more aggressive conditions: temperatures as high as 600°C and pressures above 5,000 kPa. Unlike standard distillation, this process causes chemical changes in the hydrocarbon molecules themselves. It’s used to produce specialty asphalts or to process crudes that don’t yield enough residue through gentler methods. The result is a range of products from slow-curing to rapid-curing asphalts, depending on how far the processing is taken.
Performance Grading
Once asphalt cement leaves the refinery, it has to meet strict performance specifications before it can be used in road construction. The system used across North America is called performance grading (PG), defined by the AASHTO M 320 standard. Each grade is expressed as two numbers representing temperature limits. A PG 64-22 binder, for example, is engineered to perform well at pavement temperatures up to 64°C and down to -22°C.
The high-temperature number reflects resistance to rutting on hot summer days. The low-temperature number reflects resistance to thermal cracking in winter. Engineers select the grade based on the climate where the road will be built. To verify these properties, labs use a device called a dynamic shear rheometer, which applies oscillating stress to a small asphalt sample and measures how much of the response is elastic (springy, recoverable) versus viscous (flowing, permanent). The ratio between those two behaviors determines whether the binder will hold up under traffic loads at a given temperature.
Some grades carry an “ER” designation, meaning the binder must also pass an elastic recovery test with a minimum 50% recovery. This is typically required for polymer-modified asphalts used on high-traffic roads.
Polymer-Modified Asphalt
Standard asphalt cement works well for moderate conditions, but heavy truck traffic, extreme heat, or harsh winters call for something tougher. Refineries and asphalt terminals improve binder performance by blending in synthetic polymers. The most common modifiers are styrene-butadiene-styrene (SBS), styrene-butadiene rubber (SBR), crumb rubber from recycled tires, and polyethylene.
Polymer content typically ranges from 3% to 6% by weight of the base asphalt. A standard modified binder might contain 4% SBS, which forms a stretchy network inside the asphalt that resists both rutting in heat and cracking in cold. Adding a small amount of a chemical catalyst called polyphosphoric acid (0.5% to 1%) can boost the effect, allowing producers to achieve the same performance as a 6% polymer binder while using only 4% polymer. That substitution lowers material costs while maintaining resistance to deformation, temperature sensitivity, and aging.
Bio-Based Alternatives
A growing area of asphalt production involves partially replacing petroleum-based binder with oils derived from biological sources. These bio-oils come from a wide range of feedstocks: waste cooking oil collected from restaurants, waste wood, swine manure, crop straw, municipal waste, and vegetable oils from soybean, rapeseed, castor, and palm. The conversion process typically involves pyrolysis, where biomass is heated in the absence of oxygen to break it down into a liquid oil.
Lab studies have shown that blending 10% bio-oil with an SBS-modified base can produce a binder that matches the road performance of conventional 50-penetration grade asphalt at a lower cost. Field trials using vegetable-based binders have demonstrated comparable surface texture and regularity to standard petroleum asphalt. The quality of bio-oil still lags behind petroleum fuel in some respects, particularly in oxygen content, water content, and corrosiveness, which limits how much can replace conventional binder in a single mix. But agricultural and forestry residues exist in large enough quantities to support commercial-scale production if the technology continues to improve.
Storage and Handling
Asphalt cement is a solid or semi-solid at room temperature, so it must be kept hot and liquid for transport and use. Storage tanks at terminals and paving plants maintain the binder at elevated temperatures, but overheating is a real concern. Excessive heat causes oxidation, which hardens the asphalt and degrades its flexibility. State transportation departments explicitly prohibit modification of asphalt through “air-blowing” or oxidation, and additive storage temperatures are capped at 150°F to prevent degradation.
Proper circulation inside storage tanks prevents localized overheating and keeps the binder homogeneous. The finished product must be free from water and show no flecks of carbonaceous matter, which would indicate it was exposed to destructively high temperatures during storage or transport.
Worker Safety During Production
Heating asphalt cement produces fumes that pose health risks to workers at refineries, terminals, and paving sites. There is no specific OSHA permissible exposure limit for asphalt fumes, though one was proposed at 5 mg/m³ in 1992 and never finalized. OSHA’s own risk assessment found a significant lung cancer risk at exposures as low as 0.2 mg/m³. The American Conference of Governmental Industrial Hygienists recommends a threshold of 0.5 mg/m³ as an 8-hour average, and NIOSH recommends a ceiling of 5 mg/m³ for any 15-minute period, classifying asphalt fumes as a potential carcinogen.
In practice, this means workers involved in asphalt production and paving rely on respiratory protection, proper ventilation, and personal protective equipment. Keeping asphalt at the lowest effective working temperature reduces fume generation significantly, which is one reason temperature control matters beyond just product quality.