Thermophysical properties of road asphalt concrete

. The article presents the results of experimental research of the thermophysical properties of asphalt concrete of various compositions based on oil bitumen and based on oil bituminous rock, which have been carried out on the laboratory unit LKT-1 using the horizontal flat layer method. It is shown that the use of oil bituminous rocks in the composition of road asphalt concrete makes it possible to obtain a material not only with the best physical and mechanical properties, but also with low thermal conductivity, which contributes to ensuring resistance to shear deformations in the road pavement during the entire period of high summer temperatures with a hot climate.


Introduction
Roads with asphalt concrete pavements are an important object of transport infrastructure. Used asphalt concrete itself is one of the most complex building materials. This complexity is mainly due to the peculiarities of its structure, as well as the great dependence of properties on various factors. The properties of asphalt concrete pavement -as a roadbuilding material -are determined by its strength and deformation properties, which depend on temperature changes. For example, as the temperature rises, the viscosity of the bitumen contained in asphalt concrete decreases, the bonds between the mineral particles weaken, which leads to a strength reduction. Therefore, it undergoes many trials and tests, during which its thermophysical properties are determined. Thermophysical properties show how thermal energy from the environment affects the quality of the asphalt concrete pavement. They are especially relevant for regions with a hot or, conversely, cold climate.
Asphalt concrete pavements are known to operate at elevated temperatures in the summer. It is at this time that an intensive process of formation of a road rut takes place. The ability of the composite to resist the load during heating is decisive in assessing its qualities.
The scientists have carried out a comparative test of the strength of non-thermostatted samples of the composition "granite-bitumen" and thermostatted samples of the same composition at a temperature of 140°C for 25-28 hours [1]. The tests have been carried out in the temperature range of +5-40°C. The results of the adhesive strength test have shown that in the case of conventional asphalt concrete, already at a temperature of 40°C (and in summer the road pavements are heated to a higher temperature in the sun), an intensive process of rutting is inevitable. Thermostatting, an almost cost-free change in technology, can significantly reduce this negative phenomenon.
The distinctive features of road pavements include significant changes in their characteristics during the service life due to the influence of climatic factors, fatigue and aging processes on the properties of asphalt concrete. This feature of asphalt concrete pavements requires an in-depth analysis of their deformed behavior in various modes of exposure to operating loads, and an analysis of the patterns of change in their strength characteristics over time. Such deformations are especially frequent in places of fixed vehicle stops (road crossings, public transport stops) and on transit sections of roads associated with changes in vehicle speed. During the movement of modern cars, significant horizontal (shear) forces may appear in road surfaces, reaching the vertical values and amounting to 0.6-0.8 MPa. Horizontal forces occur during braking, starting off and changing the speed. These forces cause shear stresses and accompanying deformations in the asphalt concrete pavement.
The work [2] has revealed the main patterns of changes in the properties of asphalt concrete under the complex cumulative impact of operational factors: traffic loads, weather and climatic factors, deicing agents. The dynamics of destructive processes occurring in asphalt concrete when working in road pavements during operation has been established. The constituent components and compositions of asphalt concretes that are most resistant to operational factors have been determined.
In the road sector, there has been an active interest in issues related to the thermophysical properties of building materials and structures from the initial stage of the formation of road science. At the end of the 30s of the last century, the treatment of soils with bituminous materials began to be widely used for the arrangement of various heaving protection measures. The relevance of the first studies of changes in the thermophysical properties of the bituminous composites has been determined by the need for placing bitumized soils in the lower layer of the pavement or arrangement of the insulating layer to create a favorable water and thermal regime of the earth roadbed, eliminating the possibility of frost heaving in soils.
At present, many scientific works are devoted to the study of thermophysical properties of asphalt concrete.
The article [3] discusses the main stages of planning and conducting an experiment to study the thermophysical properties of road-building materials. The results of experimental research of asphalt concrete mixtures are presented. The methodology of statistical analysis of experimental data is described.
The paper [4] shows the results of the study of thermophysical properties of various road pavement systems by developing a complex methodology. It was established that with an increase in specific heat capacity, thermal conductivity, and temperature conductivity, a decrease in the maximum surface temperature of the road pavement of various mixtures was observed.
Scientists have developed prediction models of road pavement temperature based on the characteristics of thermophysical properties for assessing the urban climate in a built-up environment [5]. The models include predicting the temperature on the surface and at a depth of 40 mm using indicators of thermophysical properties and registered climatological parameters: air temperature, wind speed and relative humidity. It has been established that an increase in heat capacity leads to a rise of the surface temperature of the road pavement, which indicates that more energy is required to increase the temperature of the road pavement. An increase in thermal conductivity has led to a decrease in the temperature of the road pavement, which indicates that the road pavement will accumulate more heat in the system for a longer time and can release this heat at certain time frames, changing the city climate at this moment. An increase in wind speed by about 1 m/s has raised the temperature of the road pavement by 1 °C.
In the paper [6], to study the thermophysical properties of porous asphalt concrete, such indicators as thermal conductivity, specific heat capacity and temperature conductivity were determined. For comparison, porous and dense asphalt concrete were taken. Relationships between thermophysical properties and moisture content of porous asphalt concrete were analyzed. The results of theoretical calculations show that the thermal conductivity of porous asphalt concrete is reduced by 20%, and temperature conductivity by 10%. The research results show that porous asphalt concrete is highly resistant to changes in temperature load.
The article [7] presents the results of studies of the thermophysical properties of the road pavement, by comparing and analyzing the absorbed heat for nine different sections of the road pavement. Temperature sensors were installed in each section to measure the temperature at the top, middle, and bottom parts of the asphalt concrete and at a depth of three inches in the underlying aggregate layer. Temperatures were recorded hourly for one month -from August to September 2009. The results have shown that increasing the density can also increase the temperature of the road pavement and accumulated energy. In addition, changing the components or structure of an asphalt concrete mixture can change its thermophysical properties, as well as the amount of energy that the road pavement is able to accumulate.
Due to the viscoelastic properties of bituminous concrete, temperature drops lead to significant mechanical effects. A method was proposed [8], for evaluating, firstly, the intrinsic thermophysical parameters (thermal conductivity and heat capacity) of a bituminous mixture, and then for characterizing its mechanical properties by conducting complex tests for the module. The determination of the thermal properties of bituminous mixtures is carried out based on reverse analysis using finite element software. The obtained intrinsic thermal parameters are important for the fatigue law, which depends on the dissipation of the viscous energy of bituminous concrete.
An improved laboratory test procedure for measuring the thermal conductivity of an asphalt concrete pavement has been proposed [9]. The results show that thermal conductivity and temperature conductivity depend on the degree of compaction, the degree of saturation with moisture and on the temperature. It is determined that the road pavement is subject to daily temperature cycles, the amplitude of which decreases with depth and changes dramatically on the surface of the road pavement. In addition, with the frequency of temperature changes, from January to March, the effect of thawing and re-freezing of the road pavement gradually increases.
During periods of cold weather, the construction of asphalt concrete roads is often suspended because the hot asphalt concrete cools too quickly for proper compaction. The research results presented in the article [10] describe the prediction method both in the laboratory and in the field. The results of laboratory measurements of thermophysical properties of asphalt concrete are also presented.
A review of the literature shows that in our country and abroad, studies are being carried out on the thermophysical properties of materials used in road construction. It is noted that one of the most common types of defects found on road surfaces are plastic deformations in the form of ruts, waves, and beads, which significantly worsen technical and operational indicators: the speed of traffic decreases, safety conditions deteriorate, etc. Among the many reasons for this situation, an important place is occupied by the imperfection of methods and criteria for assessing and predicting the shear resistance of asphalt concrete both at the stage of designing compositions of asphalt concrete mixtures and at the stage of operation of asphalt concrete pavements.
In this regard, it is relevant to develop a method and criterion for assessing the thermophysical properties of road pavements, to which this article is devoted.
We have carried out a number of experimental research of the thermophysical properties of asphalt concrete of various compositions based on oil bitumen, as well as fine-grained asphalt concrete based on oil bituminous rock from the Iman Kara deposit [11]. Research has been carried out on the action of temperature on the properties of asphalt concrete pavement with different composition of structural layers [12]. Laboratory research has determined the dependence of the change in the thermal conductivity of asphalt concrete on the grain size composition and moisture content [13].
As is commonly known, oil bituminous rock (OBR), the organic part of which is close to oil bitumen in properties, is a valuable material in road construction and is an additional source of hydrocarbon raw materials, alternative to oil bitumen. Due to the OBR in the production of asphalt concrete it is possible to completely or partially eliminate the use of expensive oil bitumen.
The first attempts to use oil bituminous rocks in road construction began in the 1950s of the last century. Many scientists of Kazakhstan have identified and proposed for construction various optimal compositions of asphalt concrete based on OBR that meet the basic requirements for road pavements [14].
Based on the data of a detailed analysis of the chemical and mineralogical compositions, physicochemical and physical-mechanical properties of OBR, argillous raw materials and waste from chemical factories in Western Kazakhstan and the Aral Sea region, waste-free technologies for the extraction, processing and rational use of OBR have been developed as a new universal raw material, alternative oil product, for the production of a wide range of building materials [14,[15][16][17].

Materials and research methods
In this work, samples of road asphalt concrete based on oil bituminous rock from the Iman Kara deposit with a natural organic content of 12-19% have been taken as starting materials for the research. Asphalt concrete mixture, consisting of crushed stone, screenings, mineral filler and oil bituminous rock and having an optimal grain size composition, has been prepared according to the developed technology [11] in a laboratory asphalt mixer. The samples have been made in the form of cylinders with dimensions of d=h=71.4 mm by pressing on a hydraulic press under a pressure of 200 MPa.
For comparison, samples have been taken in the form of drilled cylindrical cores from the section of the structural layer of the road pavement, prepared using oil bitumen.

Results and discussion
Experimental research of the thermophysical properties were carried out on the laboratory unit LKT-1 (Fig. 1) using the horizontal flat layer method. The test sample was placed on the oven thermostat (Fig. 2), and a ballast calorimeter was placed on top (Fig. 1, item 2). In this case, the calorimeter sensor must be connected to the D2 input of the IST-2M measuring system (Fig. 1, item 3). The front panel of the IST-2M measuring system has all the controls, as well as a four-charge digital display. When the toggle switch "on" is turned on, the oven electric heaters are connected to the network. In this position, the pointer of the temperature control knob must be brought to the reading of 700C so that the oven is thermostated at this temperature. As soon as the oven is thermostated, measurements can be started.
To establish the dependence of the temperature of the calorimeter on time, it is measured at intervals of 60 seconds for 15-20 minutes (i.e., 12-20 measurements), also controlling the temperature of the plate. The thermal conductivity of the test sample is determined from the heating rate of the calorimeter: where T 0 is the temperature of the oven; Т is the temperature of the calorimeter; h is the thickness of the test sample; S is the contact area of the calorimeter cylinder with the sample; с is the heat capacity of the aluminum cylinder of the calorimeter, с = 125 J/K. When moving on to the next sample, it is required to cool the calorimeter by placing it on something cold. The results of the experiment and calculation were entered into the table of experimental data, which was filled in for each sample separately in the form shown in Table 1-6.  Table 3. Results of laboratory research of the thermophysical properties of sand asphalt concrete based on oil bitumen (sample No. 1 in the form of a core from the section of the structural layer of the road pavement)   To compare the results obtained, the values of the thermal conductivity coefficients are entered in the summary tables (Tables 7,8). The results presented in Table 6 show that of the tested types of samples, sand asphalt concrete has the lowest thermal conductivity, and coarse-grained asphalt concrete has the highest. This is due to the fact that crushed stone has a higher thermal conductivity than other components of asphalt concrete. However, it is not recommended to solve the problem of increasing the shear resistance of the pavement only by using coarse-grained asphalt concrete as the upper layer.
If we compare the obtained values of the thermal conductivity of fine-grained asphalt concrete based on oil bituminous rock (Table 7), it can be seen that the composition of the sample based on OBR being studied has a lower thermal conductivity than sand asphalt concrete based on oil bitumen. In order to explain the low thermal conductivity of asphalt concrete based on OBR, we have determined the thermal conductivity of oil bitumen and OBR. As it turned out, OBR has a lower thermal conductivity (0.06 W/(m K)) than bitumen (0.093 W/(m K)). This is explained by the fact that, being a complex natural system, oil bituminous rock consists of the mineral part (mainly fine sand of dune type) and natural bitumen. In the system, the organic part in the form of a film enveloping the mineral material is in a structured state, which provides low thermal conductivity, and thus the resulting asphalt concrete.

Conclusions
Thus, the research results show that the use of oil bituminous rocks in the composition of road asphalt concrete makes it possible to obtain a material not only with the best physical and mechanical properties, as has been previously established, but also with low thermal conductivity, which contributes to ensuring resistance to shear deformations in the road pavement during the entire period of high summer temperatures with a hot climate.