Accounting for the influence of air void content in the calculation of critical crack length in asphalt concrete

Introduction. The strength and deformation parameters of asphalt concrete are significantly dependent on its temperature. Over a wide temperature range, asphalt concrete shows elastic, viscous and plastic properties. This leads to the fact that at the macrolevel, with the rise of temperature, asphalt concrete strength declines and its deformation resistance decreases. Under temperature conditions below zero, asphalt concrete behaves as a brittle material, while at temperatures above zero, it should be considered a quasi-brittle material. Consequently, it is necessary to implement microlevel material constants (surface energy, fracture energy, critical stress intensity factors or crack toughness, fracture viscosity) into calculation practice for flexible pavements and content design of asphalt concrete mixes. The analysis of pavement design methods currently used in road construction practice has been performed. The objective of the work has been formulated.

Materials and methods. Information on the concepts of brittle and quasi-brittle fracture by A. Griffith and J. Irwin has been provided, and the criterion of crack growth in the form of the Cherepanov-Rice J-integral has been described. It is concluded that one approach for calculating asphalt concrete layers in pavements at zero and subzero temperatures is the application of A. Griffith’s theory of brittle fracture. The application of brittle fracture mechanics allows to determine the critical stress for the given defect size in the asphalt concrete structure, and conversely, the critical crack length for the given stress. The next stage should involve calculations based on stress intensity factors or fracture energy, applied within the framework of linear elastic fracture mechanics, but taking into the account the formation of a plastic zone with small irreversible deformations at the crack tip. Classical Griffith formulas contain material constants, including the elastic modulus, the magnitude of which depends on the air void content. At the microlevel, air voids act as stress concentrators. Therefore, attention to the air void content in determining the elastic modulus of asphalt concrete used in pavement design is a relevant task with a practical significance. A review of scientific works on determining the energy constants of hot mix asphalt concrete in accordance with the variation of different factors has been conducted.

Results. Critical crack length calculation results for hot mix asphalt based on BND bitumen grades, corresponding to permissible air void content standards, are presented. Analysis of the calculation results shows that the increase in air void content leads to the decrease in the elastic modulus of asphalt concrete and to the reduction in the critical crack length. Calculations have been performed for three values of specific surface energy.

Conclusion. The obtained results allow to make more detailed calculation of the road pavement design.

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