Unappreciated materials (2): Asphalt Concrete

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asphaltSome materials catch headlines, are held in awe, but not all.  Some get little respect, despite having changed the world.  They have become commonplace, anonymous, ignored, and (particularly if they are cheap) cast aside when no longer wanted.  If they had feelings, they would be hurt. This brief series of blogs is to draw attention to their plight.

“A hideous combination of …architectural conceit and bad taste.  A cathedral of asphalt”. That was Robert Moses, the master builder of mid-20th century New York City, speaking.   Asphalt (strictly, “asphalt concrete”) does not have a happy reputation.  Yet it is a material on which we depend and use on a larger scale that almost any other[1].

Asphalt concrete is bitumen (asphalt, often loosely called tar) mixed as a bonding agent with gravelly aggregate in the approximate ratio 1:18.  Bitumen, a black, oily hydrocarbon, occurs naturally where oil seeps to the earth’s surface and loses its volatile components by evaporation, leaving a “tar lake”.  Tar has been used for millennia to seal boats, reservoirs, and aqueducts, as a mortar, as an adhesive and even—despite its smell—as a cosmetic. At one time tar-lakes were the only source of bitumen but most now comes as a by-product of petroleum-refining.  It is produced in huge quantities—enough to make 1.6 billion (1.6 x 109) tonnes of asphalt concrete per year, about half of which is recycled when roads are resurfaced.  But despite its many uses, its image remains an unpleasant one: to “tar a reputation” is to blacken and disgrace it.

Like so many great discoveries, asphalt concrete may be the result of an accident.  One story[2] goes like this.  The date is 1901.  Queen Victoria reigns.  Britain is wealthy.  Messrs. Rolls and Royce are building cars.  The need is for roads.  Outside an iron-works in Denby, Derbyshire, a horse stumbles, causing a barrel of tar to fall and burst.  An employee of the iron works is sent out to cover the sticky pool with slag from the iron works.  Passing at the time is the county surveyor, one Edgar Hooley.  He perceives the mix and perceives also its potential.  He patents it.  Tarmac is born.  By June 1903 the Tarmac Group, today the UK’s leading supplier of building materials, was up and running.

It is a nice story, but it is one that is a little cavalier with history, perhaps colored by the Victorian conviction that, if an invention was useful, it was certain to be British.  The first documented use of asphalt in road-building is not in 1901 CE in Britain but in 625 BCE in Babylon[3],[4].  The word “asphalt” itself comes  from the Latin “asphaltus”.  Baths, reservoirs, and aqueducts across the Roman empire were lined with it.  The Champs Elysées in Paris was paved with asphalt concrete as early as 1824, as were other French highways shortly thereafter.  By 1896, New York had adopted asphalt concrete as the standard road surface for the city, and from there it spread across the rest of the US.

Asphalt concrete is a messy, complex material, up to 90% aggregate by weight with about 5% bitumen and 3 to 5% of air voids.  Its density lies in the range 2,250 to 2,550 kg/m3.  Most of the tests used to characterize it are industry-specific.  The properties they measure are useful for acceptance and classification purposes but do not relate in any simple way to the more usual properties of engineering.  In recent years attempts have been made to apply methods of Materials Science[5] to the study of asphalt concrete, but this is not easy.  The material is non-linear visco-elastic or visco-plastic in its response to stress[6], meaning that its mechanical properties depend on temperature and on the time the load is applied.  When it is cold and loaded quickly it is elastic, with a modulus in the range 5—20 GPa.  When it is warm and loaded more slowly it creeps or, at high loads, it deforms rapidly and fractures[7]. The viscous (creep) behavior comes from the bitumen and the elastic, or at high stresses, the plastic behavior is dominated by the skeleton of aggregate.  Tire loading on roads is, of course, cyclic and ultimately leads to fatigue failure.  Thermally[8], asphalt concrete is a poor insulator: the thermal conductivity is 1.4 to 1.8 W/m.K, the thermal diffusivity 0.44 to 0.64 x 10-6 m2/s, and the specific heat is 1500 to 1700 J/kg.K.  It is an electrical insulator too, with a resistivity of 1018 to 1019 μΩ.cm, a dielectric constant in the range 2.5 to 3, and an electrical breakdown potential  of order 20—30 MV/m. Attempts have been made to model all this, but in practice design with asphalt concrete remains largely empirical, based on nomograms or graphs that summarize experience.

Today, roughly 90% of the world’s 5 million miles of roads are surfaced with asphalt concrete.  These roads carry some 4 million tons of freight each day; they are the channels along which the commerce of almost every country flows.  It deserves a better press than it gets.

[1] European Asphalt Pavement Association (EAPA)   http://www.eapa.org/asphalt.php  (Comprehensive data for world production of asphalt concrete.)

[2] Tarmac (2010)  http://www.tarmac.co.uk/about_us/about_tarmac/our_history.aspx  (A rather British history of asphalt concrete.)

[3] National Asphalt Pavement Association (NAPA) https://www.asphaltpavement.org/index.php?option=com_content&view=article&id=21&Itemid=41 (Brief, interesting history of asphalt concrete.)

[4] Krishnan JM, and Rajagopal KR. 2003. Review of the uses and modeling of bitumen from ancient to modern times. American Society of Mechanical Engineers, Applied Mechanics Reviews 56(2):149-214.  (An extensively documented review of the history, properties and use of bitumen.)

[5] Cebon, D. (2000) “Handbook of Vehicle-Road Interaction” Taylor and Francis, London.  ISBN-13: 978-90265-1554-5

[6] Read, J. and Whiteoak, D. (2003) “The Shell Bitumen Handbook”, 5th edition, Thomas Telford Publishing, London, UK.  ISBN 0-7277-3220-X (The bible of Bitumen.).

[7] Cheung, CY and Cebon, D.  ‘Deformation mechanisms of pure bitumen’  ASCE Journal of Materials in Civil Engineering, August 1997, Vol 9, No 3, pp 117 – 129.

[8] Luca, J. and Mrawira, D. (2005). ”New Measurement of Thermal Properties of Superpave Asphalt Concrete.” J. Mater. Civ. Eng., 17(1), 72–79.

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