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February, 2002 What could a road-builder dream of? Some process that may allow him to save bitumen, lay strong and durable roads at a lesser cost, re-use the aggregate scrapped off the worn out road, open the road built by him to traffic immediately on its laying and continue laying of a road even if a sudden downpour begins! This certainly seems to be a figment of imagination, a far-fetched idea but has become possible with the advent of a new technique: foamed bitumen.
Many countries around the world, including Australia, Brazil, Canada, Finland, Mexico, Norway, South Africa, UK and the USA, are trying the technique in an enthusiastic manner and the results are encouraging. Foamed bitumen has been successfully used in new road construction, in strengthening of existing roads, as a base course material and to correct the subsidence caused in roads due to heavy traffic. Its area of application may further widen as it is showing full stability even under extreme climatic conditions.
Foamed bitumen it is produced when cold water is added to the hot bitumen thus raising a lot of foam. As the bitumen is hot, water coming in contact with it evaporates causing foaming of bitumen along with. The bitumen expands up to 15 to 20 times its original volume, its extent depending upon the quantity of water added and the temperature of hot bitumen. The water is added by the method of injection by use of a specially designed spray-bar and under controlled conditions. The foamed is then mixed with the cold and moist aggregate to produce the ready-mix for laying on roads.
The aggregate used for mixing with foamed bitumen may be the worn out material milled off the already laid road. In the conventional road building process, this aggregate is removed and disposed off and new aggregate is used. But while using foamed bitumen, this aggregate can be put to re-use as there is no reduction in strength or durability of roads on this account. Thus a lot of saving on account of cost of aggregate can be made.
Another highly interesting aspect of foamed bitumen road construction is that the mix of aggregate and foamed bitumen need not be heated in a hot mix plant but can be laid in the cold condition itself. Not only this, if the produced mix does not get laid by the closure of working hours in a day, it may continue to lie on the road side and can be laid in position next day. The mixed material can even be stockpiled on the roadside and remains soft till the time it is laid and compacted. After compaction, the road can immediately be opened to traffic. Thus the road-closing hours due to road work in progress get minimised when foamed bitumen is used. And what may come as a big relief to a road builder, the laying of foamed bitumen mix can continue even if it starts raining. Moreover, the cold process also helps in lesser consumption of fuel thus bringing savings.
As the volume of foamed bitumen increases to many times the original volume of bitumen, its viscosity decreases. This enables a much less quantity of foamed bitumen to coat the aggregate. The foamed bitumen coats only the finer particles of the aggregate and not the large particles thus producing a mastic to bind the material. As a result, the consumption of bitumen decreases thus bringing savings in cost on this front also. However, there is no loss on strength front as a foamed bitumen road gives stiffness values equal or greater than those of a hot mix bitumen road.
With so many advantages in view, including savings in bitumen, fuel and aggregate; recycling of aggregate; cold processing; working in extreme climatic conditions and quicker opening of roads, engineers are working out complete design process of foamed-bitumen-pavements by studying the test results and analysing the behaviour of such laid roads. Germany has developed advanced equipment that mills out old and worn out roads, adds milled aggregate to the mixer drum, adds micro-processor controlled bitumen to the drum, injects micro-processor controlled water to the drum for the foaming of bitumen, mixes all the ingredients and then lays the mixed material along the road — all in one go. The quantities of various ingredients can be controlled with respect to the thickness of layer to be laid and the speed of construction of a road.
There has been an unprecedented growth in road traffic in India. Cargo transport has grown by 60 times during the last 50 years. Passenger traffic has risen by 65 times and the number of vehicles by 80 times. The vehicular and passenger traffic is bound to rise further. Under the circumstances, making use of cost effective techniques such as foamed bitumen construction may prove quite useful for the country.
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