Tag: bitumen

A cure for the Great British Pothole Plague?

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Anyone who read the manifestos of the mainstream political parties in the UK – there may not be many who did – would have been amused to see that all promised to resolve the plague of potholes in the countries roads, both major and minor. For decades road users have been alarmed when hitting a pothole and in some cases had damage inflicted on their vehicles, and in the case of those on two wheels, on themselves. The RAC (Royal Automobile Club) has estimated that there are, on average, six potholes per mile on Britain’s roads: the greatest density in Europe. The AA (Automobile Association) estimated that almost £0.6 billion was spent in 2024 repairing pothole-damaged vehicles. This is not a new phenomenon. Before the advent of turnpike trusts in the late 18th century, which maintained roads travelled by Britain’s mail coach services, it was not uncommon to encounter potholes up to two metres deep. Legend has it that on one such route through northern Nottinghamshire two coach horses fell into a pothole and drowned. Scottish engineer, John Loudon McAdam invented a solution around 1820: crushed stone laid on the road surface in slightly convex layers, the topmost being bonded with stone dust. This ‘macadam’ surface created cambered highways that drained rainwater to the sides and downwards. Modern roads are still based on that principle, with the addition of tar or bitumen to the top layer to produce a hard, impermeable surface, which also prevents aggregate and dust being sucked from the surface by fast moving vehicles.

A spore of the club moss Lycopodium

So, why the potholes? Several reasons: increased traffic; heavier vehicles; less maintenance; patching rather than resurfacing. Most important: the materials and the weather. Dry, hot weather softens the bitumen and drives out volatile hydrocarbons making the bitumen less plastic. The pounding of tyres in cooler weather fractures the now stiffened bitumen, mainly at microscopic scales. Wetting of the tarmac seeps water into the microfractures. The formation of ice films jacks opens the microfractures and produces more in the cold stiff bitumen, eventually to separate the particles of aggregate in the asphalt. The wearing course begins to crumble so that aggregate grains escape and scatter. Thus weakened, the top layer breaks up into larger fragments and a pit forms to join up with others so that a pothole forms and grows. Wheels of traffic bounce when they cross a pothole, the shock of which causes the centre of degradation to shift and create more cavities. Simply filling the existing potholes merely serves to create new ones: a vicious cycle that can only be broken by complete resurfacing: the traffic cones come out!.

All this has been known for well over a century by civil engineers. Around the start of the 21st century – maybe slightly earlier – it dawned on engineers that the critical problem was degradation of bitumen. A petroleum derivative, occurring naturally as surface seeps in some oilfields, bitumen is chemically complex: a combination of asphaltenes and maltenes (resins and oils). Deterioration of bitumen through evaporation, oxidation and exposure to ultraviolet radiation decreases the maltene content and stiffens the binding agent in asphalt. So the earliest attempts at reducing pothole formation centred on rejuvenation by periodically adding substitutes for maltenes to road surfaces. Diesel (gas-oil) works, but is obviously hazardous. More suitable are vegetable oils such as waste cooking oils or those produced by pyrolysis of cotton, straw, wood waste and even animal manure. The problem is getting the rejuvenators into existing asphalt surfaces: clearly, simply spraying them on the surface seems a recipe for disaster! A solution that dawned on engineers around 2005 was to make bitumen that is ‘self-healing’.

Schematic of the production of microcapsules from club moss spores to contain sunflower oil to be used in self-healing asphalt (Credit: Alpizar-Reyes, E. et al. 2022)

Simply mixing rejuvenators into bitumen during asphalt manufacture will not do the trick, for the result would be a weakened binding agent at the outset. For the last 15 years researchers have sought means of adding rejuvenators in  porous capsules, to release them as microfractures begin to form: on demand, as it were. There have been dozens of publications about experiments that found ‘sticking points’. However, in early 2025 what seems to be a viable breakthrough splashed in the British press. It was made by an interdisciplinary team of scientists from King’s College London and Swansea University, in collaboration with scientists in Chile. They chemically treated spores of Lycopodium club mosses to perforate their cell walls and clear out their contents to be replaced by sunflower oil, an effective bitumen rejuvenator. Experiments showed that such microcapsules released the oil to heal cracks in aged  bitumen samples in around an hour. Mixed into bitumen to be added to asphalt they would remain ‘dormant’ until a microfracture formed in their vicinity released it, thereby making the asphalt binder self healing.

Will such an advance finally resolve the pothole plague? It may take a while …

See: Alpizar-Reyes, E. et al. 2022. Biobased spore microcapsules for asphalt self-healing. ACS Applied Materials & Interfaces, v. 14, p. 31296-31311; DOI: 10.1021/acsami.2c07301

A fully revised edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

Neanderthals and the earliest ‘plastic’ handles

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February 2024 was a notable month for discoveries about ancient technology: first that of an ancient tool probably used in rope making and now signs of the use of a composite ‘plastic’ material in stone-tool hafts. Both are from Neanderthal sites in France, the first dated around 52 to 41 ka and the second in the Le Moustier rock shelters of the Dordogne – the type locality for the Mousterian culture associated with Neanderthals (Schmidt, P. et al. 2024. Ochre-based compound adhesives at the Mousterian type-site document complex cognition and high investment. Science Advances, v. 10, article ead10822; DOI: 10.1126/sciadv.adl0822), dated at around 56 to 40 ka. The second discovery resulted from the first detailed analysis of unstudied artifacts unearthed from Le Moustier in 1907 by Swiss archaeologist Otto Hauser that had been tucked away in a Berlin Museum.

Patrick Schmidt of the University of Tubingen in Germany and colleagues  from Germany, the US and Kazakhstan identified stone artifacts that show traces of red and yellow colorants. At first sight it could be suggested that they are decorations of some kind. However, they coat only parts of the stone flakes and are sharply distinct from the fresh rock surface and the sharpest edges. Another feature discovered during chemical analysis is that the colour is due to iron hydroxides (goethite) but this ochre is mixed with natural bitumen: the coating is a composite of an adhesive and filler not far different from what can be purchased in any hardware store.

LEFT: Stone flake from the Le Moustier site in France, partly coated with a reddish iron-rich colorant. RIGHT: Experimental stone flakes with 55:44 mix of goethite and bitumen (top) and pure bitumen (bottom) being handled. (Credit: Schmidt et al. Figs 1A, 3).

The authors tested the properties of the mixtures against those of bitumen alone – an adhesive known to have been used along with various tree resins to haft blades to spears in earlier times. In particular they examined the results of ‘cooking’ the substances. Whether unheated or ‘cooked’ a mixture of ochre and bitumen is up to three times stronger than pure bitumen. A further advantage is that the mixed ingredients are not sticky when cooked and cooled, whereas bitumen remains sticky, as the illustration clearly shows. Anyone who has handled a stone blade realises how sharp they are, and not just around the cutting edges. So Schmidt and colleagues tried to use the composite material as a protective handle when stone flake tools were gripped for cutting or carving. The composite handles worked well on scrapers and blades, even in the softer, ‘uncooked’ form

Similar composite adhesives are known from older sites in Africa associated with anatomically modern humans, but not for this particular, very practical use. It is perhaps possible that the use of bitumen mixed with ochre was brought into Europe by AMH migrants and adopted by Neanderthals who came into contact with them. Yet the limestones of the Dordogne valley yield both bitumen in liquid and solid forms, and ochers are easily found because of their striking colours. Long exposure of petroleum seeps drives off lighter petroleum compounds to leave solid residues that can be melted easily to tarry consistency. So there is every reason to believe that Neanderthals developed this technology unaided. As Schmidt has commented, “Compound adhesives are considered to be among the first expressions of the modern cognitive processes that are still active today”.

When and why did the North American Pleistocene megafauna collapse?

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The US city of Los Angeles, originally known as El Pueblo de Nuestra Señora la Reina de los Ángeles (The Town of Our Lady the Queen of the Angels), was founded in 1781 by 44 Spanish settlers. It remained a small cattle-centred town after the annexation of California from Mexico by the USA in 1847. Once it was reached by the transcontinental Southern Pacific railroad in 1876 it had the potential for growth. But it took the discovery of oil within its limits in 1892 for its population to increase rapidly. The Los Angeles City Oil Field became the top producer in California with 200 separate oil companies crammed cheek by jowl by 1901. Now only one remains, producing just 3.5 barrels per day. That crude oil was there for the taking was pretty obvious as bitumen seeps had long been exploited by native people and the original Spanish colonists. The oilfield was developed near one such seep: the Rancho La Brea tar pits.

Rancho La Brea tar pit and derricks of the Los Angeles City Oil Field in 1901

By 1901 perfectly preserved bones of a huge variety of animals – 231 vertebrate species – as well as plants and invertebrates began to be collected from the continually roiling pond of bitumen. Thousands of specimens have been collected since then, both predators and prey of all sizes. Famous for mastodons and sabre-toothed cats, La Brea is a repository of almost the entire western Californian fauna through much of the Late Pleistocene: before about 100 ka the area lay beneath the Pacific Ocean. Tar pits are traps for unwary animals of any kind, especially as shallow water often hides the danger. Carnivores seeking easy, abundant food end up trapped too.

Because of the anaerobic nature of bitumen, bacterial decay is suppressed. Many of the bones still contain undegraded collagen: the most abundant protein in mammals, which can be dated using the radiocarbon method. So, despite the lack of stratigraphy in the tar pits, it is possible to track the history of the ecosystem by painstaking dating of individual fossils (OKeefe, F.R and 18 others 2023. Pre–Younger Dryas megafaunal extirpation at Rancho La Brea linked to fire-driven state shift. Science, v. 381, article eabo3594; DOI: 10.1126/science.abo3594). Robin OKeefe and colleagues dated 169 specimens of eight large mammal species most commonly found in the bitumen: sabre-toothed cat (Smilodon fatalis); dire wolf (Aenocyon dirus); coyote (Canis latrans); American lion (Panthera atrox); ancient bison (Bison antiquus); western horse (Equus occidentalis); Harlan’s ground sloth (Paramylodon harlani); and yesterday’s camel (Camelops hesternus).

The authors focussed on precisely dated specimens spanning the 15.6 to 10.0 ka time range. This would allow the disappearance times of individual species to be compared with stages in the rapid change in the Californian climate during post glacial maximum warming, those during the Younger Dryas abrupt cooling (12.9 to 11.7 ka) and the earliest Holocene warming that succeeded it. The first to go extinct were the camels and giant sloths about 13.6 ka ago. At 13.2 ka the other mammals declined very rapidly, the two remaining herbivores vanishing more quickly than the four predators. By 12.9 ka the only surviving species of the chosen eight was the coyote. So seven members of the Pleistocene mammalian megafauna became extinct before the onset of the Younger Dryas cold millennium.

Part of the team examined pollen from a core through sediments deposited in a lake 100 km south of La Brea. They found that flora, and probably climate, had not changed at the time of camel and sloth extinctions around 13.6 ka. However a 300 year period between 13.2 and 12.9 ka witnessed a collapse in deciduous tree species while conifers, grasses and drought-tolerant shrubs increased. A woodland ecosystem had been replaced by semi-arid chaparral. Another feature of the lake-bed sediments was that charcoal fragments increased explosively during that 300-year episode that ended both the woodland ecosystem and the megafauna that exploited it: undoubtedly three centuries of regular wildfires. What remained was the chaparral ecosystem based on drought-tolerant, fire-adapted plants.

Were the megafauna collapse and a change in ecology results of a climatic harbinger for the Younger Dryas cool millennium, or some other cause? Interestingly, tangible evidence for the Clovis hunting culture of North America, which has long been implicated in the faunal ‘extirpation’, does not appear until 12.9 ka, and in California neither does any implicating other human groups. Yet evidence is accumulating for much earlier entry of humans into North America. Occupation sites are very rare on land, but human presence here and there implies such earlier migration, probably along the west coast that avoided the frigid interior further north than California. The question posed by OKeefe ­et al. is, ‘Were the fires ignited by humans over a 300 year period just before the Younger Dryas’? It remains to be confirmed … First human arrivals coinciding with evidence for wildfires in Australia, New Zealand and a few other areas do suggest that it is a possibility. There needs to be a motive, such as producing lush clearings in forest to attract game, or removing cover to make hunting easier. In this case, the fires immediately preceded a global climatic downturn with terrestrial drying, so they may have had natural causes: the potentially incendiary chaparral flora had been increasing steadily beforehand and decreased rapidly after the evidence for wildfires

See also: Price, M. 2023.  Death by fire. Science, v. 381, p. 724-727; DOI: 10.1126/science.adk3291