ROADMAKING IN EXCESSIVE COLD CONDITIONS

The construction and maintenance of roads in very cold regions where annual temperatures range between +30oC to -35oC has got its own challenges. The phenomena of Freezing and Thawing have got?detrimental effects on the pavement layers.

During freezing the water in Subgrade and the pavement layers expand as it gets converted into ice. The pressure generated by this expansion is estimated to be up to 200 MPa. This high pressure causes heaving on the pavement surface causing damage.

When this ice inside the pours of Subgrade, formed during low temperatures melt during times of higher temperature, the water in the pours generate pour pressure and thus reduces the shear strength of the soil. It is at this time that the traffic movement is maximum. Thus, the combined effect of increased traffic over weaker Subgrade becomes detrimental for the pavement.

The freezing-thawing has got a long-term effect on the Subgrade and pavement layers also. It has been found that the CBR of soil which has gone through at least one cycle of freeze and thaw is lower than the CBR after 4 days of soaking (which is considered to be the worst case). Hence the durability of the pavement also gets affected.

The bitumen used in the Asphalt layer is also important. Normally we use high viscosity low penetration bitumen in order to have better resistance to high temperatures. But those grades of bitumen become brittle at low temperatures. Therefore, bitumen having low viscosity and high penetration value is considered to be more suitable in extremely cold conditions. The IRC:37 recommends VG10 grade for cold conditions (equivalent to 80/100 penetration grade).

Principally three steps are necessary to prevent freezing and thawing

1.???????Prevent water from entering into pavement layers

2.??????Drain the water out of the pavement layers

3.??????Insulate the surface to prevent the effect of excessive freezing

The practical measures to be taken in design and Construction is:

·??????Keep the Sub-grade drained

·??????Clean sands and gravels (small amounts of fine particles) are?non-frost susceptible?(NFS) and ideal for subgrade construction in frost affected area

·??????The layer below the asphalt shall be coarse and well-draining

·??????Re-roll the earthwork left before the winters before dumping the next layer

·??????Maintain camber at all times

·??????Drain-tile piping may be provided

·??????The Sub-grade should be insulated

Measures to be taken during operations

·??????Crack sealing is to be frequently done.

·????The sides?of the pavement should be kept clear for internal drainage

Studies have shown that the insulation material over the Sub-grade reduces the frost penetration into the ground by 50% to 80% depending upon the insulation material, its thickness and type of soil. This reduces the pressure generated by heaving and thus mitigates the damage.

Various insulation materials tried in different researches are

·??????Geofoam (XPS)

·??????Tyre chips

·??????Foamed concrete

·??????Foamed glass aggregates (FGA)

·????A combined?layer of XPS board & modified soil (silty clay modified with fly ash & crumb rubber)

Although the combined layer has been found to be the most effective insulation method but as an individual material, the XPS has been found to be the most effective insulation material for Sub-grade.

The Extruded Poly Styrene (XPS) Boards is a rigid foam sheet having a closed-cell structure and is produced in a fully automated continuous extrusion process. It is manufactured via an?extrusion?process where plastic resin and other additives are combined and?extruded?through a die. The?extruded?foam then cools and expands into its final shape.?

High Compressive strength, low thermal conductivity, resistance to Water vapour diffusion and water absorption, long term durability under buried conditions, lightweight and easy to handle and non-toxic to the environment makes it an ideal insulating material for pavements

The surface on which the geofoam panels or blocks are placed should be free of construction debris, reasonably dry, smooth (levelled to ±10 mm over a 3m distance) and without large (gravel-size or larger) soil or rock particles on the surface.

?Construction vehicles should never traffic directly on the surface of the geofoam. A layer of soil from 150 to 450 mm thick (depending on the size of the compaction equipment to be used) or the next pavement layer should be pushed over the geofoam and then compacted. The remainder of the pavement system can then be constructed in the usual way. If heavy construction vehicles are to traffic over the geofoam, e.g. as a temporary haul road, then it is generally desirable to construct the entire pavement system except for the asphaltic concrete surface layer and place a temporary crushed-stone surface layer before permitting heavy vehicle traffic on the road. Once construction hauling is completed, the temporary crushed-stone surface can be removed or levelled and the asphaltic concrete surface layer placed.

Note - This has been written as a popular article base on a literature review and drawing practical tips from the study.