Why asphalt roads fail

There is no doubt that Ahmedabad is living up to its Jahangir-assigned name of Gardabad due to the state of our roads. As road making is a simple technology that is around since ages, it is a bit surprising that we are still not able to make our roads properly.

Road making consists of making a good strong base from large stones and laying a layer of smaller stones (aggregates) on top that are cemented together with asphalt.

Looking at the above process, road damages can come in two forms, i.e. failure of the base on which road rests and failure of the top layer that provides a levelled surface to drive.

Failure of base is a structural problem leading to collapse of the road, but what is bothering us in our city more is the failure of the layer cemented by asphalt.

Asphalt is a petro-product that is very viscous and thus acts as a binding glue. To make it workable, it requires to be heated. So, the process of road-making is melting asphalt, adding aggregates, mixing them to form a semi-solid hot mix and laying it to form the top layer of the road.

Our roads are reduced to dirt-tracks as asphalt binding is broken and aggregates have become loose. This is at such a massive scale that traffic is raising dust clouds that can be extremely hazardous for health.

So, what could have caused the asphalt glue to fail?

It requires no Sherlock Holmes to deduce that it is extended exposure to water that has led to weakening of the bond between aggregates and asphalt. As the animosity between water and asphalt is well known, a number of scientific studies are conducted on the subject, and it is likely that our engineers have not learnt from them.

One scientifically established reason for reduction in asphalt’s binding capacity with aggregates when exposed to water is acidity of aggregates. It is noted that acidic aggregates like basalt fare poorly compared to basic aggregates like limestone. Silica usually causes a reduction in bond-strength between asphalt and aggregate; so the limestone aggregate containing less SiO2 than basalt shows a better resistance to water.

The problem faced by our administration is that, we citizens are keen to jump to the conclusion that it is corruption that is root cause of every problem.

While there is no denying that systemic problems like corruption and bad workmanship are real, but there is also an equally strong possibility that the malaise of flaky roads has a scientific reason. And, it is just as important that it is studied and resolved.

Looking at the evidence at hand, i.e. city-wide damage to top layer of roads, there appears to be a universal reason beyond corruption, and it is, most probably hidden in the type of aggregates used.

What is more interesting is the fact that the scale of damage is far greater this year, which hints at a possibility of introduction of a new additive in the mix that had increased the water-induced weakness of asphalt, possibly by increasing acidity.

There is also an outside chance of increased acidity in rain water, as acid rain is a phenomenon getting increasingly common with rising pollution levels.

For the sake of the city, we need to stop crying corruption wolf all the time and allow AMC to engage in scientific study of this problem in consultation with an academic institution.

We need good roads, and if our age old ally asphalt is failing due to an insurmountable technical reason, we need to shift to making RCC roads.

DNA: 6/8/17

I am happy to admit that I have managed surviving till now with minimum effort as all my intellect has be used to avoid doing anything meaningful. As I needed to while all the free time I generated in course of being lazy, science has been my favorite muse that I have enjoyed company of. As an effort to kill time (in a way, to get even with it) one fine day I decided to write a science column, more for my personal amusement than to attract readers. After getting educated about the attention span of modern readers from my editor, it became more like a challenge to tackle esoteric subjects in 600 words that I have managed to remain interested in for more than a year now. I do not want to add my worldly profile here as these are ideas that need to be considered only on the merits they carry and not as an opinion of a certain human being.

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