Story of the Week  
Economical Road Design

Construction of road (pavement) involves huge outlay – one kilometer of a 6 lane expressway costs Rupees 5 to 10 crores in India, depending on the site conditions. Small reduction in design thickness of pavement structure and a judicious selection of road materials can, therefore, lead to a large saving in the total cost of construction. The current Pavement Engineering research at IIT Kanpur focuses on the issues related to the improvement of the bituminous pavement design system.

The fatigue life of a bituminous layer can be increased by increasing the bitumen content. But, Voids in mineral aggregates, being fixed for a given aggregate gradation, increase in bitumen content will result in less air voids, which is undesirable for a mix. Thus, there is a need to deviate from the specified gradation in order to come up with new gradations ( non-standard ), which possibly can give rise to better fatigue lives, yet without compromising with volumetric and strength requirements. Softer grades of bitumen exhibit relatively higher fatigue life compared to harder grades of bitumen. Similarly, for a given grade of bitumen, the fatigue life increases with the increase in the bitumen content. Higher bitumen content or softer grade of bitumen enhances the fatigue life but the stiffness modulus value may fall. A designer's objective is to achieve both high stiffness and high fatigue life for an economical design. This mutually contradictory situation can be handled by using bituminous layer(s) which is made richer in bitumen content (or softer grade of bitumen) towards the bottom portion. Since fatigue cracks start from bottom of bituminous layer, higher bitumen content contributes better restraint against fatigue cracking. These considerations, taken together, are expected to result in economy in the design of bituminous pavement.


Fatigue testing of bituminous mixes ( Transportation Engineering Laboratory, IIT Kanpur )

The locally available marginal aggregates, which can not otherwise be used directly for road construction, can possible be acceptable in cemented form (by using lime/ cement/ bitumen etc). The cemented material used as base / sub-base layer can impart extra fatigue life to the pavement. The depleting natural resources, and disposal problem of deteriorated pavement material necessitate adoption of pavement recycling technology. Pavement recycling also preserves road geometry and conserves energy. In mix design for recycled mixes, the reclaimed pavement material , fresh aggregates, virgin bitumen, and rejuvenator are added in such a proportion that the resultant mix performs ‘as good as' the fresh mix. A suitable formulation has been developed for hot central-plant recycled mix design, so that all the specification limits are satisfied simultaneously , as well as the overall material cost is minimized.

Research effort is continuing to evolve an integrated approach to the bituminous mix design and structural design of the pavement, where designer can achieve a better economy in overall material cost, without compromising with the reliability of pavement performance.

Dr. Animesh Das
Department of Civil Engineering
Indian Institute of Technology Kanpur

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