Frost modelling and pavement temperatures: summer pavement temperatures and frost modelling: licentiate thesis

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Åke Hermansson

Temperature and moisture are very essential parameters when describing the condition of a pavement. In most cases, a high moisture content involves a decreased bearing capacity and, consequently, a shorter durability of the pavement. A frozen pavement has a greater bearing capacity than the corresponding construction in spring or late autumn. However, the freezing itself also implies strains to the pavement, as it heaves to different extent and in different directions in connection with the frost heave. The properties of an asphalt concrete pavement vary dramatically according to temperature. A cold asphalt concrete is hard, stiff and brittle, and therefore, cracks easily occur, whereas its bearing capacity decreases at high temperatures as softening progresses.

A numerical model has been developed for calculation of the temperatures in a road pavement during summer condition, especially emphasizing the asphalt concrete. Further, in order to also model temperatures and other conditions, occurring during winter conditions in the pavement, such as frost heave, a frost heave model has been developed. The aim of this is to gain a better insight into the freezing process of a road structure. The model also provides an efficient tool for a better understanding of important factors related to frost depth and frost heave. In the present work, numerical analysis of frost heave and frost front propagation has been performed and compared with some field observations.

Furthermore, equipment for freezing tests in laboratory has also been developed. Experiences from such tests and field measurements have been used when developing the numerical model for the freezing of pavements. At the laboratory freezing tests, a special interest has been devoted to heave rate, water intake rate and cooling rate. The experiences, obtained from both the laboratory tests, as well as the field observations, have been compared to what has been reported in literature.

Temperatures obtained from the numerical model for summer temperatures have turned out to correspond well to measurements of pavement temperatures on three different levels below the road surface on a road west of Stockholm, Sweden. Calculated temperatures were also compared with temperatures calculated by using a model presented by SUPERPAVE (i.e. the asphalt binder specification, developed under the Strategic Highway Research Program (SHRP) USA). This model gives the highest temperature of asphalt concrete during daytime. According to the opinion of the author it is found that SUPERPAVE uses an erroneous assumption that there is equilibrium when the highest temperature is reached in the pavement on a hot summer day. This, of course, leads to an overestimation of the temperature, which is compensated in SUPERPAVE by assuming that the highest temperature is reached at a relatively high wind velocity of 4.5 m/s instead of at feeble winds, which is more realistic according to the author.

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