India road design 2

[mickmac]

I've been asked to confirm a design for a road job in southern India. The design is for 1 million axles for a 15 year period. Specification is for 200mm sub-base, then 250mm hydraulically bound material, topped with 60mm base hma and 40mm wearing course hma. The sub-base cbr is 8. Is this spec over the top or can someone come up with a non asphaltic alternative?

 

[BigH]

Is the 250mm hydraulically bound material identified as WMM or WBM? Will the subbase be crushed stone or river gravel? Lorries in India are notoriously and severly overloaded - your 1 million standard axles may end up effectively to be much much higher.

For a road of which I am aware - a National Highway - and upwards to well over a hundred million axle loads on the life span, 330 crushed stone subbase, 200mm crushed stone base and 210 total bituminous thickness (170 dense base macadam) and 40 wearing course (BC) - on a CBR requirement (light tamping - standard Proctor) of 6 for the underlying 500mm subgrade.

A few things to consider - the mix designs typically now use 4 to 6 percent air voids - used to be 3 to 5% - but the compaction level required by the agency is 98% Marshall - about 2% higher than normal compaction for highways. Contractors always wanted to exceed (and many times more than 100%) because if below 98%, they lost a portion of their bid rate. If below 95% they lost it all in some cases. Secondly, it is very hot and humid - flushing is a big big problem - try to use a polymer modified bitumen - rubber modified can also work - or specify a 30-40 penetration grade. 60-70 has a softening point in the order of 46 to 47 degrees and with April/May temps approaching 46 to 48 with humidity, well, . . . They have not gone to performance grade which would require viscosity decrease for the penetration grades. Top down cracking is also understood to be a problem that should be addressed although many don't yet recognize the mechanism at present.

Just some thoughts - and goooooooooddd luck.