Researchers at the Indian Institute of Technology (IIT) Hyderabad have developed a novel modelling technique which they say could help in the construction of long-lasting roads in the country.

The technique called reliability-based design optimization (RBDO) can predict the performance of road surfaces or pavements and compare these predictions with conventional road designs, researchers said.

“RBDO is a modelling technique that combines optimisation approaches with reliability assessment of structures,” said Professor Sireesh Saride, from Department of Civil Engineering, IIT Hyderabad.

The researchers noted that India has the second largest road network in the world, after the US, and has about 4.37 kilometres of roads per 1,000 people.

The past two decades have seen a drive to pave the Indian roadway network, and as of 2016, 62.5 per cent of Indian roads has been paved, they said.

Construction of the road surface or pavement is a complex process because they must be able to provide for comfortable riding quality, good skid resistance, favourable light reflecting characteristics, and low noise pollution.

The design of pavement is the first and essential step towards building roads that can meet the needs of the traffic demands as well as to balance the demand on natural materials used in paving them, according to the researchers.

“The pavements are complex layered structures influenced by many factors such as material properties, environmental and climatic conditions, traffic volume, subgrade soil profile, construction practices, and pavement ageing process,” Saride said in a statement.

“Hence, transportation agencies require innovative techniques to address the variabilities associated with the influencing factors,” he said.

A road surface or pavement typically consists of superimposed layers of various materials above the natural soil and help in the distribution of the load of the traversing vehicles for a smooth ride.

There are two types of pavements -- rigid and flexible.

While rigid pavements are made of high strength concrete to resist the vehicle load directly, flexible pavements transmit the load downwards from the surface through successive layers of materials.

“The advantages of flexible pavements are that they are adaptable to stage-wise construction, can be made of low-cost materials and can be easily opened and patched,” said Saride.

In modern pavement construction, accurate prediction of pavement performance has become important in order to develop robust design procedures.

The team, including B Munwar Basha, Associate Professor at IIT Hyderabad, and PRT Pranav, Research Scholar, IIT Hyderabad, used RBDO to predict safety of multi-layered flexible pavement structures against fatigue and rutting criteria while simultaneously considering the variability arising from individual design parameters.

The flexible pavement was modelled as containing four layers -- subgrade, granular subbase, base, and bitumenous layers.

Modelling studies showed that the bituminous layer’s thickness and resilient modulus of the base layer are the most influential parameters for the fatigue failure.

The results of the simulation studies were compared with data from American Association of State Highway and Transportation Officials (AASHTO) guide for the design of pavements.

“AASHTO overestimated reliability by 10-40 per cent compared to RBDO because the former did not consider the variability associated with geometrical and material properties,” said Basha.