The construction and maintenance of roads with flexible pavements in very cold regions where annual temperatures range between +30oC to -35oC has got its own challenges. The bitumen in the asphalt layer becomes hard at such low temperatures and thus become brittle. This results into cracks on the surface of wearing course. Moreover, the phenomena of Freezing and Thawing has a detrimental effect on the non-bituminous pavement layers, especially on the subgrade.
Vivek Singh, Founder, BuildStreet
Bitumen in Cold Weather
The bitumen used in asphalt layer is also important. Normally we use high viscosity low penetration bitumen in order to have better resistance to high temperatures. But these grades of bitumen become brittle at low temperatures. Therefore, bitumen having low viscosity and high penetration value is considered to be more suitable in extreme cold conditions. The IRC:37 recommends VG10 grade for cold conditions (equivalent to 80/100 penetration grade).
Freezing and Thawing
One cycle of freezing and thawing completes when the temperature raises above freezing point and then goes below the freezing point and then again raises above the freezing point. In very cold conditions like the Himalayas, the subgrade goes through several such cycles during the winter season and in some places for even a year.
It is a common phenomenon that water enters into the subgrade from top (through cracks), from bottom (by capillary action or high water table) or from the sides (accumulated water). Low porosity and low draining material of subgrade would get saturated by the ingress of water.
During freezing the water in subgrade and the pavement layers expands as it gets converted into ice. The pressure generated by this expansion is estimated to be up to more than 200 MPa. This high pressure causes heaving on the pavement surface, leading to damage.
When the ice inside the pours of subgrade (formed during low temperatures) melts in higher temperature, the water in the pours generates 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 (due to pore pressure) becomes detrimental for the pavement.
Effect of Freezing & Thawing
Freezing and thawing has a long-term effect on the subgrade and the pavement layers also. It has been found that the CBR of a soil which has gone through at least one cycle of freeze and thaw is lower than the CBR after 4 days soaking (which is considered to be the worst case). Hence, the durability of the pavement also gets affected.
How to prevent Freezing & Thawing
- Prevent water from entering the pavement layers
- Drain water out of the pavement layers
- Insulate the surface to prevent the effect of excessive freezing
- Keep the subgrade drained
- Layer below the asphalt should be coarse and well-draining
- Clean sand and gravel are non-frost susceptible (NFS) and ideal for subgrade construction in frost affected areas
- Re-roll the earth work left before the winters before dumping the next layer
- Maintain camber at all times
- Drain-tile piping may be provided
- The subgrade should be insulated
- Crack sealing to be frequently done
- Sides of pavement kept clear for internal drainage
Studies have shown that the insulation material over the subgrade reduces 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 damage.
Various insulation materials tried in different researches:
- tire chips
- foamed concrete
- foamed glass aggregates (FGA)
- combined layer of XPS board & modified soil (silty clay modified with fly ash & crumb rubber)
Extruded Poly Styrene (XPS)
The XPS board 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, 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.
Laying of XPS Board
Construction vehicles should never travel directly on the surface of the geofoam. End-on tipping is the most suitable technique. 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 travel over the geofoam, i.e. if the road is to be used as a temporary haul road before its completion, 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.
In India, the roads made in places with extreme cold conditions are generally of strategic importance, hence, it is important that they remain serviceable in all the conditions. The effect of extremely low temperatures is detrimental for roads. The freezing and thawing effect not only exerts excess stress on the road pavement but also weakens the subgrade. To prevent damage from this effect, ideally, the ingress of water into the subgrade should be stopped and the subgrade be made of well drained and non-frost susceptible material.
But since a road is subjected to various natural conditions which cannot be easily controlled, it is therefore prudent to take suitable measures so that the freezing and thawing action does not occur even if water is present inside the pores of the subgrade material. This is best done by insulating the subgrade with the help of Extruded Polystyrene Sheets (XPS).