Frost protection of roads and railways

Damages on Norwegian roads caused by frost heave and spring thaw weakening is a severe problem leading to discomfort and longer travel time for road users and extra cost for maintenance of the roads. The problem is most sever on old roads that are not build according to current requirements. However, also for new roads with high amount of traffic severe frost heave has been a problem in recent winters. When we started this project, most of the knowledge was based on the research project called - Frost i Jord (1970-1976). This was a very thorough and extensive project, but there have been some changes in the way we build roads and railways since then. Among other changes we now use almost 100 % crushed rock compared to more use of natural gravels. The project has two main goals: Investigate how fast the cold will penetrate different materials used for road construction and to investigate how much frost heave different types of materials will give if the frost will reach them. To research this we developed/improved several laboratory tests, instrumented some test sections close to Røros and developed numerical models for simulation. A major concern about the use of crushed rocks with large voids that has become common in Norway is development of natural convection (circulation of air due to differences in density between cold and warm air). Convection in road construction material will have a severe effect the heat loss during the winter will increase significantly compared to what is assumed in the current design guidelines. Testing show that several of the materials that could be used can have convection if the temperature gradients become large. To avoid this the material requirements for frost protection materials should be changed depending on the climatic conditions. When we consider a material for frost susceptibility (if it can move water to a freezing front through capillary suction) we have so far, only looked at the amount or fines present. By using a special frost heave test, it is now possible to test directly the risk of frost heave expressed as the segregation potential. This has given us the possibility to investigate the influence of other parameters like mineralogy, grain shape etc. The equipment is also useful for direct testing of materials that could be used. To secure frost from reaching the frost susceptible materials it is possible to use thick layers of crushed rock as insulation. However, this might lead to impractically thick structures in areas with cold climate. In the project, we have also investigated the effect of using two different insulation materials: foam glass (Glasopor) and lightweight aggregates (Leca). These materials will stop the frost penetration more efficiently compared to rock materials. Through the laboratory and field tests, we have documented some of the properties of these materials and concluded that todays design rules are too conservative. Based on field and laboratory data we have developed/improved numerical methods that could lead to an improved practical tool to help road engineers to make good compromises between frost heave and construction costs. As a follow up of the project desing tools for practical use are being developed based on knowledge generated by this project and calibrated against the field and laboratory data. The project was done by NTNU in cooperation with SINTEF and Laval University in Quebec. Bane Nor (railway), Norwegian Public Roads Administration, Leca and Glasopor was also partners in the project and contributes with financial support and scientific advice.

The main outcomes from the project are: Open graded materials have very low frost insulation capability and for materials commonly used there is a high risk for natural convection to occur during part of the winter making them even less suited for frost protection. More densely graded material insulates significantly better than the open graded materials. Insulation materials like Leca and foamglass has good insulation properties and could be used to protect against using relative thin layers. The knowledge from the project will be a good basis for improving pavement design to get better quality for road users and increased lifetime for roads. It will be easier to avoid costly mistakes that has been seen in the recent past. Better understanding of the insulation materials (Foam glass and Leca) will make it possible to reduce required layer thicknesses and by this save of materials and cost making this a more viable option.

In cold winters we experience frost heave problems on both newly constructed and existing roads and railways. This accelerates deterioration and reduces the bearing capacity of the structures during spring time, and consequently increases maintenance costs. Since the large research program "Frost i Jord" ended about 40 years ago, very little research has been done in Norway on this problem. Since then, used materials have changed from the well graded natural gravel to more coarse and openly graded crushed/blasted rock. There is no good understanding of the equivalent thermal conductivity of these materials including internal convection and radiation. Existing frost susceptibility criteria for soils and coarse material are mainly based on grain size distribution and allowable fines content. However, the mineralogy of the fines is also an important factor to consider. In this project we plan to investigate materials in the laboratory and field conditions, and build a numerical model to simulate different climate conditions, thickness of structures and material combinations. We will educate two PhD-students in this project. One student will focus on frost susceptibility of subgrade soils and pavement materials. The second one will make an assessment of the frost protection layer. We plan to have good interaction between the two students and the rest of the team. The PhD-students will be co-supervised from University of Laval and NTNU and will spend some time at both universities to maximize the learning and use of experimental resources. The project is supported from the Norwegian road and railway administrations. We plan to have a steering group with representatives from both organizations as well as one representative from the Canadian Ministry of Transportation. University of Laval, NTNU and SINTEF is well experienced in conducting research on materials for road and rail construction. The project will have access to advanced laboratories.