Prof. Prithvi Singh Kandhal
There is a fundamental engineering requirement with respect to the bituminous mix used in road construction. The mix should be dense so that water cannot penetrate it. In fact, water can destroy both rural roads as well as national highways as it can penetrate both the semi-dense and open graded bituminous mix used in road construction. Despite this knowledge and technical research that dissuades the use of outdated technologies, such mixes are permitted and are being used widely across India.
But why do we permit such bituminous mixes?
Though, it is important to consider the quality of materials and methods used for road construction, many bituminous roads constructed in India fail prematurely and develop potholes, especially during monsoons. Today, even the “aam aadmi” knows that water is Enemy Number 1 of bitumen but many of our highway engineers ignore it and use the so-called “semi dense” or “open” graded bituminous mixes in road construction, which gives an open invitation to water.
This, despite technical research that dissuades use of outdated technologies, many new, innovative materials and solutions being introduced by specialized companies.
Road specifications issued by the Ministry of Road Transport and Highways (MoRTH) contain 10 types of bituminous mixes. These can be categorized as dense, semi-dense, and open graded as follows:
- Dense graded bituminous mixes are DBM Grading 2, BC Grading 1 and BC Grading 2. DBM Grading 1 is not listed as due to large nominal aggregate size of 37.5 mm, it is permeable by water and fails prematurely.
- Semi dense graded bituminous mixes are SDBC Grading 1, SDBC Grading 2 and Mixed Seal Surfacing (MSS).
- Open graded bituminous mixes are BM Grading 1 and 2, and Premix carpet (PMC) with seal coat.
Initiatives taken to support the contention
To support these observations, three technical papers (authored by Prof. Kandhal and Prof. Veeraragavan) have been published by the Indian Roads Congress (IRC) since 2008, and were also presented as formal papers in IRC annual sessions. The third paper was presented in November 2017 at IRC’s Bangalore session. A recent research work at IIT Guwahati has shown similar results.
MORTH and IRC engineers generally believe in status quo
Unfortunately, engineers in MoRTH and IRC do not pay any attention to such published technical papers, and are continuing with outdated technologies. They are aware that many open bituminous mixes such as BM and PMC were developed a long time ago, and now we have hot mix plants that are producing dense bituminous mixes.
To justify use of the seven semi-dense and open bituminous mixes, the engineers say that these mixes have served the road construction sector well, and India cannot afford dense mixes (even though the relatively cheap bituminous mixes may last for only 1-2 years), and they are also good in convincing the government officials to sanction their usage.
What our engineering experts opine?
IITs and NITs, should conduct a thorough research on the bituminous mixes, and the Central Road Research Institute (CRRI) should take suo motu cognizance of this subject after reading the three IRC technical papers, and give their opinion to MoRTH and IRC in the interest of the nation’s infra development. But unfortunately, nobody wants to create any waves.
Most highway engineers in the public and private sector are also silent on the subject, even though these mixes are not used in developed countries. The general public is of the opinion that highway engineers intentionally construct bad roads so that they can get a ‘fat’ road budget year after year.
Engineering experts opine that only three dense graded bituminous mixes should be used for all types of roads. These mixes, which are listed in the orange book, are as follows:
- Base Course: DBM Grading 2 (nominal aggregate size 25 mm)
- Binder Course: BC Grading 1 (nominal aggregate size 19 mm)
- Wearing Course BC: Grading 2 (nominal aggregate size 12.5 mm)
The open graded premix carpet (PMC), which is being used extensively on our rural roads, should be replaced by mechanized surface dressing with pre-coated aggregate. This will effectively waterproof the road, and thus minimize potholes. Surface dressing costs only 40% of what PMC does, and lasts much longer. This process would revolutionize the Pradhan Mantri Grameen Sadak Yojna (PMGSY) program by saving thousands of crores of rupees every year and increasing the life of rural roads. It should be noted that NO country in the world would like to use “water-trapping” open graded PMC, and they prefer surface dressing on low to medium traffic roads.
Bad roads are a national problem. Many people die due to potholes every day. Representations made to the government/MoRTH/IRC stating the drawbacks of bituminous mixes based on technical facts, have gone unanswered. One can only hope that the younger generation of upcoming, progressive engineers, who truly believe in strong fundamentals of highway engineering, will bring about a radical change in road construction. To quote John F. Kennedy, “American roads are good not because America is rich, but America is rich because American roads are good.”
About the Writer
Prof. Kandhal has been a practicing highway engineer in India for over 20 years and in the US for 30 years. Recently he has drafted many standards for the Indian Roads Congress including specifications for dense graded bituminous mixes, stone matrix asphalt and readymade pothole patching mix.