# Accelerated Creep of Asphalt Concrete at Medium Temperatures
## 1 Introduction  
Asphalt concrete is the most widely used road material in the world. It is mainly used for the construction of the upper durable layers of road pavements [1; 2; 3]. Despite the fact that the cost of bitumen is constantly increasing, asphalt concrete will apparently remain the main road material in the coming decades, as it is today. It can be said that, since the 1930s, road asphalt concretes have been used for practical applications. Since then, work has been carried out to improve their quality, test methods have been developed and their mechanical behaviors have been studied. Despite all this, in our opinion, the physical and mechanical properties of road asphalt concretes, particularly their mechanical behavior under conditions close to real ones, have not been studied sufficiently. For example, in Kazakhstan [4], and in many countries around the world, the strength of road asphalt concrete is determined by testing cylindrical samples on a mechanical press until complete failure. The duration of such laboratory tests is only a few seconds, while, in real operating conditions, the service life of asphalt concrete pavements can be 20 years or more [5; 6]. It is known that the service life of asphalt concrete pavements and road surfaces depends not only on mechanical loads but also on climatic factors, including freezing and thawing of road pavement structures and subgrade. The results of our experimental studies show that the mechanical behavior of asphalt concrete under loads at different characteristic moments of time is different. Thus, in work [7], it is shown that an asphalt concrete creep curve consists of three characteristic sections: sections of unsteady, steady and accelerated creep. The features of an asphalt concrete deformation in regard to unsteady creep, as well as its mathematical description and mechanical interpretation, are contained in papers [7]. A detailed study of the steady creep of asphalt concrete is contained in works [8; 9]. The results of the experimental studies presented in this paper show that, at temperatures of 22-24 \({}^{\circ}\)C, the duration of the accelerated creep of asphalt concrete, regardlessof the stress, is 35% of its long-term strength (the time from the moment the load is applied to complete failure). In other words, the failure of asphalt concrete does not occur \"instantaneously\" (not quickly) but after a long period of time. In this case, damage first accumulates in the material, and only then do main cracks appear [10]. In the last two decades, two of the most popular methods for assessing the crack resistance of road asphalt concretes have been the so-called \"single-edge notched beam test\" and \"semi-secular bending test\", which are now carried out according to the AASHTO TP 124 standard [11]. According to these methods, asphalt concrete samples in the form of a rectangular beam [12; 13] and in the form of a semi-circular disk [14; 15] are preliminarily subjected to an artificial initial crack, and then these samples with cracks are tested. Currently, in order to assess the crack resistance of road asphalt concrete, methods of testing samples in the form of rectangular beams (without pre-applied artificial cracks) for direct tension according to European standards EN 12697-46 [16] are also widely used. The results of the works [8; 9; 17] show that a visible to the eye (main) crack in the test sample of an asphalt concrete appears only at the end of the third characteristic section of deformation (at the stage of accelerated creep). Before the main crack appears, the test sample of an asphalt concrete must go through the stages of unsteady-state and steady-state creep, as well as through the first substage of accelerated creep, in which intensive accumulation of damage occurs. As is known, creep is a fundamental property of viscoelastic materials [18; 19; 20; 21; 22; 23]. By testing materials for creep, one can obtain a lot of important information about their mechanical behavior.