Asphalt dissolved on the highway because of the record heat? The news has been turning everywhere since yesterday, but the reality is that nothing has been melted. The high temperatures of these days have created Discomfort and kilometer queues in the A4 motorwaywhere at the height of Verona In the Brennero A22 junction in the direction of Milan, for a few hours the authorities had to close the first lane and the entrances of Verona Sud and Verona Est for safety reasons. The reason? The asphalt of the highway has deformed visibly, forming the classic “depressions” in the roadway along the trajectories of the tires that in technical jargon are called footprint. The media speak of “melted asphalt”, or “liquefied asphalt”, but from a chemical-physical point of view it is wrong and totally misleading. The asphalt does not deform because it passes from the solid state to the liquid statebut because as the temperature grows it undergoes a process called regret in which he loses consistency and softens, becoming deformable under sufficient stresses.
From a technical point of view, the asphiclate is a Bituminous conglomeratethat is, a mixture composed of bitumen (a derivative of oil) that acts as a binder and conglomerates such as Ghiaia, Pietrisco and sand. Now, a feature of bitumen is that it does not have a well -defined melting point. If they ask you at what temperature the ice melts, you would immediately answer “at 0 ° C”: the ice melsted (it is wrong to say “melts”) at a very precise temperature. Here, the bitumen no. What happens is that, with the increase in temperature, it becomes increasingly “soft”, that is, less resistant to deformations. It is therefore not a “discontinuous” but gradual phenomenon.
Technically, therefore, the high temperatures do not “melt” the asphalt, it regret. And the higher, the more the bitumen contained in the asphalt is deformed if subjected to a weight. Ok, but so when he regrets exactly the asphalt? Being a gradual process, there is no precise point where bitumen becomes “soft”. In the science of materials we speak of rimollimento point To indicate the temperature (but it would be more correct to speak of temperature interval) in which the material reaches a certain deformation threshold in response to a standard stress established a priori.
In the case of bitumen, a classic recaling test consists in warming gradually and uniformly the material and calculating the average of the temperatures in which two small steel spheres of 3.5 grams have 2.54 centimeters in the bitumen.
For common bitumes, typical regrets can reach 65 ° C. They are not impossible temperatures to be reached in the summer, on the contrary: the asphalt warms up more quickly than the air, therefore if exposed to the sun it can reach the regret with air temperatures of 35 ° C: They are conditions of intense heat, but common in Italy especially during a heat bubble like the one on our country these days.
Be careful, however: this does not mean that with 35 ° C automatically the asphalt will deform because of the regret. In fact, in addition to sufficient weight, also a sufficient pressure time.
