What do the public paved roads in your nearest city look like? Are they smooth and dark, with a topcoat of asphalt? Maybe they’re pale grey, made from concrete. Regardless of their makeup, they’re unlikely to be pristine. Instead, most roads are covered in defects. Cracks, depressions, bumps, potholes, patches of bitumen from previous repairs… all of these surface scars tell a story of that road’s usage and its internal structure. In engineering parlance, these scars are called ‘pavement failures’, and ‘pavement’ is the formal term for “any ground surface prepared for transport”. By that definition, even a soccer pitch counts as a pavement, but in most cases, we’re talking about a road.
Despite what you might think, roads (or pavements) are rather complex structures, made of multiple layers. At the very bottom is the subgrade – this is the existing soil. On top of that, they lay one or more thick layers of a granular material – often crushed stone or aggregate. This forms the subbase. After that, things deviate, depending on what type of surface is used.
If you’re building a rigid concrete road (widely used in the US), the next step is to lay a reinforced or unreinforced concrete slab on top of the subbase. The concrete’s stiffness acts to transfer the load from the traffic that will eventually run along it into the subbase. In many cases, including Germany’s Autobahn, multiple layers of reinforced concrete are laid on top of one another. There are also joints in the concrete to minimize cracking.
If you’re building a flexible asphalt road (typical of those found in the UK, New Zealand and other parts of the world), the subbase is followed by a layer of finer aggregate which is compacted and acts as the base. In this pavement design, the base’s job is to distribute the traffic-induced stresses. The aggregate in this layer can either be mixed with bitumen, or left unbound. Next up is the binder course. As its name suggests, it is an intermediate layer which sits between the base and the surface layer, binding them together. It is made from aggregate coated in bitumen, and like the base, it plays an important role in strengthening the road, providing structural support and distributing the load from traffic. The surface layer – the part of the road you drive or cycle on – is again made with a mix of small stones and bitumen. If hot bitumen is laid down first, followed by a layer of similarly-sized stones, the resulting surface is called chipseal. If the bitumen is first mixed with stones that fall within a range of sizes, and then that mixture is laid down, the surface is called asphalt.
When you see a pothole or a pavement failure of any type, it’s because something has happened to compromise one or more of the road’s layers. It turns out that the terminology for describing pavement failure is a rich tapestry. Some of my favorite new-to-me terms include rutting (visible wheel grooves in the pavement caused by repetitive heavy traffic loads), shoving (the formation of ripples across a pavement resulting from horizontal stresses), and raveling (detachment of aggregate particles from the surface, leaving a rough and loose texture). The causes of these failures are wide and varied, and include poor soils, improper materials, and substandard design.
However, there are two main contributors to pavement failure, and unfortunately, they’re both set to get worse in the coming years.
Weather and environment.
Moisture and temperature are known to have a significant effect on pavement performance. Potholes, for example, are typically the result of prolonged water infiltration through the surface, which weakens the underlying layers. Water can find its way through even the smallest gaps. Though bitumen is hydrophobic (water-repelling), the stones mixed into it are not, and the interface between those materials is weak. The weight of vehicles causes larger cracks to form too, giving water even more opportunity to seep down through the pavement structure, eroding materials along the way. In cold climates, water can freeze and expand beneath the surface, causing additional stresses on the pavement, causing potholes to form more quickly.
Winter 2022 here in Aotearoa New Zealand was exceptionally wet; the wettest since 1971. The rain, combined with an historic eight-year ‘freeze’ in road maintenance budgets, resulted in a record number of potholes on our road network. Observations and climate change projections for NZ suggest that precipitation patterns will change significantly in the next few decades, with some parts of the country seeing less rain, while others will see more extreme downpours. Globally, the implications of climate change for road infrastructure are stark.
Where rainfall is set to become more frequent and more intense, we’ll see increased flooding and greatly elevated soil moisture. This can reduce the structural integrity of the underlying soil, causing sagging of the road. In some cases, the subgrade can be washed away by flooding, causing the pavement to collapse from within. In others, landslides caused by saturated soils will block roads. Flooding events (as well as extreme snow events) also apply additional stress to pavement – with overloaded drainage systems, floodwaters remain on the road; that extra weight can cause surface cracking, and compromising the base layers.
Roads in coastal areas are particularly vulnerable to flooding from storm surges and sea level rise. According to the U.S. Global Change Research Program, more than 60,000 miles of coastal roads are already at risk, and in the Gulf Coast, “2,400 miles of major roadway could be permanently flooded by sea level rise in the next 50 to 100 years.” The salinity of seawater means that coastal floods don’t just cause structural damage; they can lead to chemical degradation of roading materials, too.
Where more drought-like conditions are predicted, there could be ground shrinkage below road infrastructure, threatening its integrity. Wildfires cause surface bitumen to melt, deform and crack. But even less extreme temperature rises will cause pavement to soften and expand, which can result in rutting. Permafrost, which supports roads, railways, and airport runways in high latitude regions in Alaska, Norway, and China, is already starting to thaw. This causes the subgrade to sag, which distorts the pavement layers, cracks the surface, and causes raveling and the formation of potholes.
The costs involved in responding to these risks are significant. The EU-funded WEATHER project estimated that in 2010, the total costs of extreme weather events to Europe’s road network was €1.8 billion/year. The researchers also concluded that by 2040, climate change would have increased those costs by 20%. A separate study said that weather stresses represent 30 – 50% of Europe’s current road maintenance costs; equivalent to between €8 billion and 13 billion each year.
I could go on, but you get it. Climate change is an expensive, disruptive process. Now to discuss the second major contributor to pavement failure.
Traffic using the road.
In the 1950s, the American Association of State Highway and Transportation Officials (AASHTO) carried out a series of ambitious experiments to explore “the performance of pavement and bridge structures of known characteristics under moving loads of known magnitude and frequency.” Colloquially known as the AASHO Road Test, it involved the construction six two-lane test track loops along a segment of what would later become Interstate 80. The pavement design was varied along the lengths of each track. Vehicles of different weights and number of axles were repeatedly driven along them, in order to test the road’s performance, and monitor its degradation, over time.
Described as the “largest & most substantive pavement research performed in the 20th century,” and costing US$27 million in 1960 (equivalent to almost $279 million today), the AASHO road test established many of the standards for road construction still in use today, particularly in the US. Amongst many other findings, the test showed that a road’s service life and condition depend on the thickness of the pavement, the number of vehicle passes along that pavement (i.e., the busyness of the road), and the axle load of the vehicles using it. The relationship between these factors is the so-called “fourth-power law”. Though more of a general rule of thumb than a true scientific law, it tells us that heavy vehicles, such as multi-axle trucks, have a much greater impact on a road’s performance than lighter vehicles such as cars or bicycles.
To give you an example of that impact, let’s do a quick calculation. Here in New Zealand, the heaviest vehicle allowed on (some of) our roads is the 50MAX truck. It has nine axles and a total weight of 50 tonnes, so the load-per-axle is 5.55 tonnes. The best-selling car in NZ in 2022 was the Mitsubishi Outlander. It weighs 1.76 tonnes, so its load-per axle is 0.88 tonnes. The fourth-power law says that to calculate the relative stress that these two vehicles apply to a road, you take the ratio of their loads-per-axle and raise the result to the fourth power. In this case, (5.55 / 0.88)4 = 1582. In practical terms, it means that a 50MAX truck applies as much stress to a road as 1,582 cars (or quite literally billions of bicycles)
This analysis is not exact. In reality, the exponent isn’t always four (it can range from 3 to 6), and factors including the stiffness of the road, or, as we’ve seen, the weather, contribute to its deterioration. Nonetheless, the “law” is an oft-quoted guideline; one that can inform policies and regulations. It is partly why trucks pay more in terms of road taxes than cars (though, they don’t pay 1582x more…).
As you’d expect, pavement engineers use a much more detailed approach to designing and building a road, choosing materials that can best withstand the loads it will experience during its service life. Because trucks exert considerably larger forces – and cause more wear-and-tear – than anything else on road networks, the number of trucks expected to use a road (and frequency of journeys) is what will define that road’s design. Factors such as the thickness of the base, the hardness and durability of the aggregate, and the surface material can all be tuned in order to produce a road that is fit-for-purpose. Highways and other roads that carry numerous trucks are therefore built to be able to carry them. Roads in a low-traffic areas or residential neighborhoods can be made with thinner and less expensive road-base materials, reflecting the lower loads they experience. If trucks travel on roads not designed for them, those roads will degrade more quickly than planned.
According to NZ’s Ministry of Transport, in 2021 there were 15,357 trucks weighing more than 30 tonnes on the country’s roads. And the annual distance covered by these freight-carrying trucks continues to increase year-on-year. The European Automobile Manufacturers Association recently announced that registrations of new trucks increased by 20% in the first half of 2023, equating to nearly 180,000 additional trucks in use. Astonishingly, more than a fifth of the total distance traveled by road freight vehicles in Europe in 2020 was “deadhead mileage” – the trucks carried no freight whatsoever (most often reflecting the return leg of a journey). Having a growing fleet of trucks – as well as increasingly-heavy trucks – on our roads will undoubtedly have an impact on the road structure. Stresses will increase, failures become more frequent, and maintenance more critical. As well as that, road freight is a high emissions business. It currently generates 15 % of European CO2 emissions, and it’s the cause of 80% of the global increase in diesel consumption since 2000…. but that is an article in itself.
Road transport is one of the key contributors to humanity’s emissions footprint, and as such, is an important driver of climate change. It is also one of the sectors most susceptible to the effects of climate change. Let’s hope a sense of self-preservation kicks in, sooner rather than later.
