Use of Extruded Polystyrene Sheets (XPS) in the construction of the Zojila Approach Road has been especially effective as XPS is the most suitable, convenient, and cheaper solution for mitigating frost action on pavements, increasing the life of pavements, and safeguarding their structural integrity.
Extruded polystyrene sheet is a thermoplastic polymer which is formed from general purpose polystyrene by the process of extrusion. XPS has a closed cell structure and virtually no air voids and is often stronger, with a higher mechanical performance. Polystyrene is generally a synthetic polymer made from the monomer styrene, which is derived from benzene and ethylene, both of which are petroleum products. Polystyrene foams are 95-98% air. XPS has been widely used for thermal insulation since the last 40 years; it is commonly used as building insulation material, such as in insulating concrete forms and structural insulated panel building systems. Finished boards of XPS can be used for insulation in the temperature range of -150˚C to 70˚C.
Properties of XPS
In general, thermal insulation is primarily based on very low thermal conductivity of gases. Gases possess poor thermal conductivity compared to liquid and solid, and thus makes a good insulation material if they can be trapped (for example in foam-like structures). Air and other gases are good insulators, and the main reason is the absence of convection. Therefore, many insulating materials like XPS function simply by having a large number of gas-filled pockets which prevent large scale convection. Alternation of these gas pockets and solid material causes the heat to be transferred through the many interfaces, causing rapid decrease in the heat transfer coefficient.
|Table 1: Physical and mechanical properties of XPS|
|Color||Light Bluish Green|
|Thickness (mm)||25, 30, 40, 50, & 75|
|Thermal conductivity (W/mK)||0.028|
|Compressive strength (10% deformation)||Min 250 kPa (30 mm thick)|
|Flexural Strength||Min 350 kPa (30 mm thick)|
XPS technology, developed for insulation and pavement frost damage mitigation, is used as an attractive alternative to natural materials for several road engineering projects due to its engineering properties such as durability, strength, thermal efficiency, and moisture resistance. These characteristics make XPS sheets suitable for construction of road embankments, above all, in areas with extreme climatic conditions, representing a very interesting solution for feasibility of building and construction projects that would otherwise be destined (due to time and implementation costs) to be alienated or postponed.
Effect of cold temperature on pavement
Cold weather not only inhibits rehabilitation and construction of transportation infrastructure, but it also accelerates weathering and deterioration of infrastructure system. Cold weather takes a toll on pavements. Most pavements do not “fail” because of design factors such as thickness, but usually because of material problems or environmental distresses such as oxidation, free-thaw damage, scaling in concrete pavements, joint deterioration (spalling), and thermal cracking.
The primary concern for subgrades under concrete pavements subject to freezing is frost action. Frost action in soil divides into two phases: freezing the soil water and thawing the soil water. For pavements, frost action becomes critical when either the freezing phase is accompanied by heaving of the road surface, or the thawing phase is accompanied by a noticeable softening of the roadbed. Heaving of road from frost action is termed as “frost heave”. Frost heave induces uneven support of a pavement. When a heavy load passes over the area of uneven support, cracks might form in the pavement surface layer (as shown in Figure 2). Cracks will typically begin and end at the pavement edges and have a noticeable change in elevation across the crack and the road surface.
The reason behind formation of heaves on the pavement is the formation of “ice lenses” in the subgrade below the pavement (as shown in Figure 3). Water expands 9% by volume when frozen. The size of the ice lens depends upon the quality of free water available within the soil and from the water table and time. When the soil freezes, the free water freezes and expands and once started, ice lenses continue to grow as long as a source of free water is available. Free water migrates through the soil to form ice lens by capillary action and this migration of water can be as far as 6m for certain frost-susceptible soils. When the soil thaws, the ice lenses melt, and consolidation of the soil occurs.
Frost action affects all types of pavements. It is most detrimental during formation of the ice lenses, which result in expansion of the soil. Pavement distress typically involves longitudinal cracking and differential vertical movement of the slabs, which might reduce the durability and functionality of roads.
Zojila project and the climatic conditions of the region
Zojila project is located at NH-1 (from km 82.00 to km 118.00) - part of Srinagar to Leh Highway in the state of Jammu & Kashmir. The climatic conditions in the region are extreme, ranging from about -10 ˚C during winter to about 20˚C. The region is prone to frequent snowfalls and avalanches during winter.
Usage of XPS in Zojila approach road
Flexible pavement is proposed throughout the approach road to the tunnel in accordance with IRC: 37-2018. As the climatic conditions in the region are extremely harsh, the designer suggested to use Extruded Polystyrene Sheets (XPS) in the construction of the Zojila Approach Road. The XPS sheets have been procured from an Indian manufacturer. The XPS sheets are used to prevent the freeze-thaw action on the pavement due to unusual heaving in the subgrade soil by providing insulation to the subgrade. The design of the pavement comprises Sub-grade (500 mm), XPS sheet (50 mm), Cement treated sub-base (CTSB) (235 mm), Cement treated base (CTB) (115 mm), Granular crack relief layer (100 mm), and Bitumen course (50 mm).
Steps for laying XPS Sheets
- Line of the sheets shall be maintained well. At the start point of the work, a line exactly perpendicular to the centre line shall be drawn with lime powder and the first course of the sheets shall be perfectly aligned to this line.
- The next course of the sheets shall then be laid in such a manner that slots provided at the edges of the sheet fit in well with each other.
- It should be checked and ensured that the edge line in longitudinal direction of any two sheets, when adjacent, remains parallel to the centre line.
- The edges of the sheets shall lie in a straight line without making an offset with each other.
- The dumping shall start from one end, from where the laying of sheet has been started.
- First 2-3 tipper trucks shall dump the material over the laid sheets along width of the bed but without letting the tyre to mount over the sheet.
- This first set of dumped heaves shall be broken and spread with the help of a Backhoe Loader (using its hoe) or Dozer or Tractor dozer or Grader fitted with dozer blades in front. But none of the machines shall be allowed to mount over the laid sheets until and unless a cushion is formed to protect direct touch of the machines.
- After spreading to a sufficient length, the next set of dumping shall be done by allowing the dumpers to mount over the laid and spread CTSB, and the process continues.
- After dumping is over a sufficient length, the normal grading and rolling operations shall be commenced.
- The further laying of XPS sheet shall continue in parallel ahead of dumping stretch.
- It is best to lay the XPS sheets over the prepared sub-grade along with dumping of sub-base (CTSB) layer.
- Any construction vehicle should not mount the XPS sheet without a cushion of CTSB in between the tyre of the machine and the sheet.
- Since rate of laying of XPS is slower that rate of dumping and spreading of CTSB, therefore sufficient length of laying shall be completed before starting dumping. A length of minimum 50 m shall be good enough to start with.
From experience, it is known that the extent of damage to roads caused by the effect of freezing the base and sub-base of the roads can be disastrous. The frost action can sometimes damage the whole pavement, deteriorating the quality of the roads and resulting in waste of resources. The main role of an insulating layer (such as XPS) is to interrupt the capillary contact in the base of the highway. XPS have a reducing and smoothing effect on frost heaving, and during the spring thaw, they increase the bearing capacity. This layer is especially effective during winter. Hence, XPS is the most suitable, convenient and cheaper solution for mitigating frost action on pavements, increasing the life of pavements; and safeguarding the structural integrity of pavements.