How a 'billion-dirham' map ensures UAE roads don't crack when the heat is on

Roads in the Emirates have to cope with some of the toughest conditions on Earth, and climate change means they will become harsher still

Researchers are making road surfacing an exact science to guard against the UAE's scorching temperatures. The National

Asphalt is something that people often do not think about even when, as in the UAE, it has to cope with some of the toughest conditions imaginable.

Day after day there are countless thousands of vehicles pounding over the road surface, all while the sun shines brightly for hours on end.

When air temperatures climb above 40°C, roads may be tens of degrees warmer, placing them at risk of softening and permanent damage. However, research is ensuring that the UAE’s roads are fit not just for current conditions, but for the even hotter temperatures expected in future because of climate change.

A team of scientists in Sharjah are working to ensure the country's roads remain climate-proof for years to come.

“We’re looking to meet our local conditions, which are some of the harshest in the world when it comes to temperature and when it comes to traffic volumes,” Dr Waleed Zeiada, an associate professor at the University of Sharjah, said.

Paving the way for smooth travel

Dr Zeiada and his colleagues have recently released a “billion-dirham map” of the UAE that indicates the ideal grade of asphalt binder, the most critical ingredient of the asphalt mixture, to be used in any particular location.

The map’s informal name is a reference to its likely ability to save the authorities significant amounts of money by ensuring that roads will be able to cope with the heat and traffic in the location where they are built.

Each grade of asphalt binder offers the optimum compromise between protection against the formation of ruts, depressions parallel to the direction of the road that often develop at higher temperatures and cracks, which may form at intermediate and cooler temperatures.

Dr Zeiada and his co-researchers have worked closely with the UAE authorities, including the Ministry of Energy and Infrastructure, to ensure that their research can be used to optimise the country’s asphalt.

“I assured them this map is worth a billion dirhams because this will save maintenance and rehabilitation money in the near future,” Dr Zeiada said.

Central to the performance of roads is the asphalt binder, which is the “glue” that bonds stones or aggregate. Together, these ingredients make up the asphalt mixture.

Hard binders can withstand higher temperatures and heavier traffic and are better able to prevent ruts from forming, but may be at greater risk of cracking at lower temperatures than softer binders.

Traditional grading systems have been based on variables including the softening point, measured by putting a steel ball on the binder and increasing the temperature so that it softens.

“It’s like chocolate or sugar – when heated up it will become soft and fluid until, at a certain temperature, the ball in the binder will go through the material,” said Dr Haopeng Wang, a lecturer in structural engineering materials at the University of Liverpool, in Britain.

“If the binder is very, very hard, you can increase the temperature to 100ºC and it’s still very stiff.”

Another crucial metric is the penetration value, which involves measuring how far a needle will penetrate the binder. Asphalt binders are made from petroleum and are a mixture of hydrocarbons of different molecular weights, with the main substances being saturates, aromatics, resins and asphaltenes.

“You adjust the proportion of different fractions to change whether they are soft or hard,” Dr Wang said. “Usually hard binders have a high proportion of asphaltenes. Asphaltenes have higher molecular weight.”

Climate-busting plan

Climate change and the high temperatures it brings, along with large traffic volumes, mean that traditional binders sometimes “cannot meet the requirements of use”, Dr Wang said.

As a result, in recent decades, engineers and scientists have modified the binders, often through the use of additives, among them “crumb rubber”, made of granules of old tyres and similar products.

“When you mix the rubber powders with your binder at very high temperatures, it can change the physical and the chemical properties of your binder,” Dr Wang said.

Additives may improve the elasticity or flexibility of binders so that the asphalt can cope with high temperatures without deforming and is more resistant to cracking.

In recent years there has been research involving biological materials, such as using agricultural or food waste, to produce oils that can be used to generate binders. Lignin – the rigid, woody material in plant cell walls – has also been a focus of interest.

“Lignin is a very abundant material from the wood industry,” Dr Wang said. “It has a similar microstructure to certain bitumen components, so they have very good compatibility.”

In a paper published in the International Journal of Pavement Engineering, Dr Zeiada and four University of Sharjah colleagues described how they produced their detailed map of UAE asphalt specifications based on temperatures gathered from 20 weather stations across the Emirates. Their research also looked at traffic volumes.

They found that the previous asphalt specification widely used in the UAE is actually suitable for only about one per cent of the country’s area because of the demanding temperatures and traffic volumes.

They have highlighted a dozen specifications suitable for different areas, employing an updated “Superpave” grading system developed in the US.

“This selection process should consider the specific climate conditions in the UAE. By doing so, balanced mix designs and optimal pavement performance against rutting and cracking can be ensured,” the authors wrote in the study, Development of superpave asphalt binder specifications to meet climate conditions in the UAE.

Making the grade

Their new map shows that for most of the UAE, an asphalt binder or bitumen grade designated as 76-10 is suitable for the temperature conditions.

Such an asphalt should be able to cope with a seven-day average surface temperature – which can be much higher than the air temperature – of up to 76°C and a minimum temperature as low as -10°C.

The next grade down, 70-10, which has recommended maximum and minimum temperatures of 70°C and -10°C, is likely to be adequate for some other parts of the country where temperatures tend to be slightly lower.

Traffic volumes also play a role in determining the suitable asphalt binder.

“High traffic volume roads, such as motorways, will experience more and higher traffic loadings, leading to more deformation and damage. Therefore, a higher-grade asphalt binder is needed,” Dr Wang said.

Dr Zeiada said that there were also variations known as standard (S), heavy (H), very heavy (V) and extremely heavy (E), based on the level of traffic.

He said that a motorway such as Sheikh Zayed Road requires the E form because of the large amounts of lorry traffic, while a quieter local road needs only the S form.

Looking ahead, Dr Zeiada and his colleagues have developed maps that consider what asphalt mixtures will be required in the UAE up to 2050 and 2080 based on various carbon emissions scenarios.

Dr Zeiada is particularly interested in recycling plastic by using it in the asphalt binder and mixture, as this disposes of waste that could end up in landfill sites while enhancing the asphalt mixture’s ability to withstand extreme temperatures and heavy traffic.

“We’re able to work with local industry who are collecting those waste plastics in a primary form that is to be recycled,” he said.

The work highlights how complex it can be to ensure that asphalt, something that few people think about when they step into a car or bus, is suitable for the conditions that it faces.

“A lot of people take our roads for granted. They think it’s a very simple construction, huge infrastructure, but it has huge influence on our everyday lives,” Dr Wang said.

“Because this material is not like steel, not like cement, not like concrete, it’s temperature-dependent. This makes it different, unique, fascinating to scientists.”