air-blown asphalt

Air-blown asphalt is a bituminous material created by bubbling compressed air through hot, liquid asphalt cement at temperatures typically between 200 and 260 degrees Celsius. The oxidation that occurs during this process stiffens the binder, raising its softening point from around 50 degrees Celsius for straight-run asphalt to between 65 and 90 degrees Celsius depending on blowing time. This controlled hardening makes the material suitable for applications where thermal resistance and reduced rutting are priorities.

The blowing process itself is relatively straightforward: compressed air, often preheated, is passed through the molten asphalt via a pipe or sparger. As the air bubbles contact the liquid, oxygen reacts with the hydrocarbon chains, forming heavier molecular structures. The duration of blowing determines the final grade, with longer exposure producing stiffer, more oxidized products. Some manufacturers also introduce air as a fine mist to accelerate oxidation and control the reaction more precisely.

Properties and industrial use

Air-blown asphalts were historically the standard for roofing membranes and pavements exposed to extreme temperature fluctuations. They resist shoving and permanent deformation better than straight-run binders, particularly in hot climates or under sustained loading. The material also exhibits lower temperature susceptibility, meaning its stiffness changes less dramatically between seasons. Modern specifications often refer to these as blown bitumens and classify them by their penetration at 25 degrees Celsius or by softening point alone.

In road construction, air-blown asphalt saw wide adoption for wearing courses and binder courses where durability under traffic was essential. Built-up roofing, which dominated commercial flat roof work from the 1950s through early 2000s, relied heavily on these oxidized binders because their higher softening point prevented flows and slippage in exposed membranes. The material bonds well to aggregate and to itself during hot layering, which made application straightforward with conventional equipment.

The downside of oxidized binders emerges in cold climates: their reduced flexibility and higher brittle point make them prone to thermal cracking in freeze-thaw cycles. This limitation, combined with the development of polymer-modified and synthetic binders offering better overall performance, has shifted air-blown asphalt toward specialty roles in waterproofing compounds, adhesives, and some industrial roofing applications. Quality control during blowing is critical; under-blowing leaves the material too soft, while over-blowing produces a hard, brittle product that loses adhesive strength.