Waste not... when making new roads

The Science of Surfaces is a four-part weekly science series that looks at the surfaces we walk on so often but know so little about. In this third part, we look at the material used to make our roads anti-skid. Next week, we look at sand.
Maria Almenoar, Straits Times 23 Jan 10;

WASTE from steel factories is what keeps a tight grip on car tyres in wet weather conditions.

Road pavers have found that steel slag, or a by-product from making steel, provides a better anti-skid property than conventional materials such as granite.

Increasingly, the Land Transport Authority (LTA), which oversees road paving, is adding recycled or waste materials into Singapore's roads.

The aim is to increase the composition of these recycled materials in roads to 70 per cent, up from the current 25 per cent.

LTA's acting manager for road infrastructure management, Mr Yoong Chin Chong, said: 'In the old days, all four layers of the road were made from granite stone but nowadays, we are looking at using recycled material which can do the same or even a better job.'

Recycled materials minimise Singapore's dependence on imports from neighbours Indonesia and Malaysia, reduce the rate at which landfills pile up and save the authorities millions of dollars.

Steel slag is one waste material, for example, that is being used in the top layer of roads, called the asphalt wearing course.

Different sizes of granite stones and steel slag are heated at about 170 deg C to remove any moisture before being mixed with a dark sticky liquid called bitumen, or what is commonly known to motorists as tar.

The steel slag with its grooves gives the road surface a honeycomb texture and makes melding with the bitumen easier and, in turn, the surface stronger.

The use of steel slag also helps to reduce the price of the top layer, which with the quality of the granite aggregate being used is now about the same as that for the bottom layers - about $11 per sq m - despite being about one-sixth the thickness.

On the top two layers, the LTA is testing the use of some of the 'old roads' or roads dug up during resurfacing.

'Old road' surfaces are processed into reclaimed asphalt, which is then crushed and screened into various sizes.

Said pavement specialist Kelvin Lee from Samwoh Corporation, one of the four asphalt production factories in Singapore: 'The field test results of using asphalt with the reclaimed asphalt pavement have been encouraging and we envisage that it will be approved for usage in the near future.'

For the bottom two layers, which need to be between 200mm and 300mm thick, the LTA is testing using processed incinerated waste from landfills.

This is made up of mostly discarded household items and contains mainly silica, glass, ceramic and metal.

The waste is exposed to weather for three months to dry before it is screened to remove unwanted materials.

It is then sieved to the required sizes before being treated by chemicals.

Already, for the bottom two layers, the LTA uses discarded blocks of concrete from construction sites when, for example, buildings are demolished.

The steel reinforcement rods are removed and the concrete is treated before it is used as road-building material.