Durable and Economical Technology for Bituminous Surfacing of Rural Roads

Introduction

The Pradhan Mantri Grameen Sadak Yojna (PMGSY) has been one of the most successful flagship programmes launched by former Prime Minister Atal Bihari Vajpayee in 2000 for constructing a wide network of rural roads across India. During the last 17 years, a total of 5,29,975 km of PMGSY roads have been constructed connecting villages of over 500 people, which is a commendable task!

Two technologies (surface dressing and premix carpet) are specified in India’s ‘Rural Roads Manual’ for bituminous surfacing of rural roads such as those under PMGSY. Either of these two technologies are permitted. Both are also standardized by the Indian Roads Congress (IRC).

Surface dressing (also called chip seal) consists of spraying a thick film of bitumen over the compacted stone base layer (called water bound macadam) with the help of a bitumen truck tank distributor at a specified rate. This is followed by spreading stone chips (aggregate) at a specified rate by a mechanized chip spreader (available from many manufacturers in Gujarat and costing less than Rs. 2.5 lakhs). Stone chips are then compacted with a road roller so as to embed/fix them (about 70%) into the sprayed thick bitumen layer. This completes the surface dressing operation as illustrated in Figure 1.

The premix carpet (PMC) simply consists of single size (about 12 mm) stone chips mixed with 3.5% bitumen by weight at a hot mix plant. PMC is highly open graded because the mix does not contain any fine aggregate (sand). This mix is laid in 20 mm thickness manually or with a paver. A sand seal coat is provided at the top.

The objective of this paper is to discuss these two technologies in detail and recommend the one which is better both in terms of durability and economics.

History of Bituminous Surfacing

When the author was serving as a highway engineer in the Rajasthan PWD during the early 1960s, it was very common to use bituminous surface dressing (SD) or chip sealing on most types of roads. Surface dressing was very effective in water-proofing the water-bound macadam (WBM) roads because of heavy bitumen application rate followed by chip application. Surface dressing was scheduled once in 3 or 4 years on all roads. Very few potholes dotted the roads at that time, and traffic volumes were generally less. Road construction was largely manual and hardly mechanized. Bitumen for surface dressing was applied with perforated tin cans. Spreading the surface aggregate (chips) by hand was an art learnt through practice, usually by swirling the basket containing the aggregate.