NEW DELHI: Learning from the “rutting” (permanent depressions) at several portions of the Mumbai Expressway’s Sohna-Dausa section last year, which attracted huge criticism, the NHAI has directed detailed testing of the stone mastic asphalt (SMA) material used in other highway projects.
Deficiency in the quality of SMA used on this section was identified as one the major reasons for the expressway getting damaged less than two years after its inauguration. Several projects, including the Delhi-Amritsar-Katra Expressway, have been constructed or are being constructed using 50 mm SMA — a gap-graded hot mix asphalt that contains a large percentage of coarse aggregate (over 70%) and bitumen filler mastic.
This is used since it is strong, durable, and resistant to deformation. The investigation into the Sohna-Dausa stretch had found that the highway builder had used SMA having 13 mm aggregate, while 19 mm aggregate was preferable.
The probe also found that rutting mostly developed during the summer when the temperature of the pavement was more than 70 degrees Celsius. An expert panel had observed that the grade of bitumen used could not sustain high temperatures and recommended use of higher-grade bitumen. It recommended the removal of the SMA layers in stretches where the average rut depth was more than 10 mm, and re-lay the stretch with 50 mm SMA, having 19 mm aggregate with higher quality bitumen.
Officials said the work has been completed. “Since this incident came to our notice, we are carrying out tests across all other stretches where SMA has been used to detect if there are similar issues,” said an official.
In a circular, the NHAI has said “some incidences” have been reported regarding the “deficiency in quality of SMA” on completed projects”. It added that agencies must ensure that technology (SMA) is implemented in a right manner so that it can survive.
In case, SMA or DBM, which is often used as a base course for roads, fails in compliance or has a tendency of premature distress or rutting, then more samples from other locations will be sent for testing. Subsequently, the rectification will be carried out in consultation with domain experts.
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