Winter roads – sugared, salted or heated?
At EPFL, several laboratories are studying the impact of extreme weather conditions on road-traffic infrastructures. In particular, they are focusing on how road surfaces are standing up to winter weather, and the de-icing of bridges.
The increase in road traffic is only one problem factor in terms of making the roads safe in winter. “The maintenance services often end up stuck in traffic, and completely incapable of fulfilling their mission”, explains Professor André-Gilles Dumont of the EPFL laboratory off traffic facilities. To address these kinds of problems, some sections of freeway – for example the Lausanne bypass – have been equipped with valve systems: “They work like automatic lawn sprinklers, and enable us to deliver a preventive salting, which is also ecological, since this method only uses 2 grams of salt per square meter, instead of 15.” Professor Dumont’s laboratory has been working on the method of application of this concept for more than ten years. Today, the use of salt remains the preferred method: studies performed in Berne have shown that the use of gravel or sand is not a solution. Their production, retrieval from the drains, and their decontamination, results in a considerable ecological impact, and although sugar is less abrasive, its current price is too high.
However, solutions will have to be found. Minus 10° Celsius on Tuesday; 8° on Thursday – these sudden changes in temperature around zero degrees are particularly interesting for the laboratory: “When the climate approaches an exotic kind of model, with sudden temperature changes and such rapid differences, road surfaces can be badly affected, with ruts appearing, and the alternate freezing and thawing can alter the characteristics of the surface.” A thesis project has therefore been launched, financed by the Federal Roads Office, in order to understand the effects of these cycles, using a model that simulates the temperatures both on the surface and underneath. “We can observe speeds of variation of surface temperature of the order of 5 degrees per hour”, adds Professor Dumont.
And what if the problem of black ice could be resolved by a heated road surface? Professor Lyesse Laloui, responsible for the EPFL laboratory of soil mechanics studying the thermo-hydro-mechanical behaviour of soils, is exploring this question. The laboratory has been working for around 10 years on a new, renewable energy source – energy-generating geo-structures. It consists of using the foundations of a concrete building to collect and store heat : “After having worked on exchanging energy in buildings, we recently began to look at the problem of black ice on bridges. The idea is to capture the heat on the bridge-deck in summer, and then to use abutments to retrieve it and store it in the earth.” By installing a circuit of tubing under the blacktop surface, like underfloor heating, and then circulating water mixed with a heat-retaining fluid thru it, a temperature is obtained that prevents the formation of a fine layer of black ice.
Professor Laloui’s ideas on the subject go even further: to use the heat of the tunnels, where there is a constant temperature of about 25° Celsius! “This energy is there, and readily available. Our idea is to use the anchoring bars of the tunnel as heat-exchangers. For us, it involves a genuine transfer of knowledge; the solutions used for the buildings now enable us to perform digital simulations and determine their feasibility.”
Auto-cleaning road surfaces, heated bridges, energy-producing tunnels – it’s also on our roads that we will find solutions to future climatic challenges. The snow can keep falling . . .