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5Ученый XXI века • 2022 • № 1 (82)
METHODS FOR DETERMINING THE WATERPROOFNESS AND FROST RESISTANCE OF ASPHALT CONCRETE
B.Kh. Kodirov1
doi: 10.15350/24103586.2022.1.3
Abstract
Asphalt concrete is the most common material for the construction of road surfaces. However, under the influence of increasing transport loads and environmental factors, the service life of asphalt concrete coatings is not high enough. In this regard, the main purpose of designing asphalt concrete compositions is to create an optimal structure with predetermined properties that would ensure the required characteristics and durability of the road surface being arranged. Due to the insufficient deformability and frost resistance of asphalt concrete at low temperatures, the actual service life of coatings is often lower than the standard ones. To increase the service life of such coatings, it is necessary to develop comprehensive technical methods.
Key words: asphalt concrete, water resistance, fracture, crack resistance, aging coefficient, binder, deformability.
The influence of the composition of asphalt concrete on its waterproofness
Asphalt concrete coatings with prolonged moisture, due to the weakening of structural bonds, can be destroyed due to the staining of mineral grains, which leads to increased wear of coatings and the formation of potholes. The waterproofness of concrete depends on its density and the stability of adhesive bonds. Water, as a polar liquid, well moistens the hydrophilic surface of mineral grains and is able to partially or completely displace sorbed bitumen. The nature of the displacement of the bitumen film depends on the activity and viscosity of the bitumen, the state of the microstructure of the asphalt binder and the surface properties of mineral grains. In addition, water can wash out soluble compounds from the binder, and in the presence of some salts, form emulsions that are easily washed out with water.
When using crushed stone from limestones, dolomites and metallurgical slags and, in particular, with a microporous texture, the water resistance increases. The use of insufficiently dried mineral materials reduces the water resistance of concrete.
Waterproofness is determined by the amount of swelling and the coefficient of waterproofness Kv (the ratio of the strength of water-saturated samples to the strength of dry samples).
waterproofness can be improved by choosing materials of proper quality, drying mineral materials, the use of surfactants and activators and good compaction of mixtures. The waterproofness is slightly increased by treating the surface of the compacted mixture with mineral powder before the last passes of the roller.
The effect of low temperatures on the destruction of asphalt concrete
As a result of operation, asphalt concrete coatings are exposed to very serious external influences: the force of loads from the wheels of cars, precipitation in the form of rain and snow, as well as temperature changes occurring over time, freezing and thawing, etc.
Therefore, great attention is paid to the construction and repair of asphalt concrete pavements all over the world.
The main reasons causing surface destruction of asphalt concrete pavement are:
-insufficient compaction (compaction coefficient is below the norm according to SNiP 3.06.03-85);
-the use of low-quality raw materials;
-substandard preparation of asphalt concrete mixture (poor mixing in the mixer) followed by the appearance of bitumen stains protruding on the surface of the coating;
1Kodirov Bekzod Khomidjonovich - Assistant, Fergana Polytechnic Institute, Uzbekistan.
YHeHbiH XXI BeKa • 2022 • № 1 (82)
-overdose of bitumen, causing increased fat content of asphalt concrete mixture.
In accordance with the Bailey criterion, to assess the crack resistance of asphalt concrete, it is necessary to know the totality of the tensile stresses in the coating and the time of their action. It is accepted that tensile stresses in asphalt concrete pavement arise as a result of the action of transport loads, from a decrease in the temperature of asphalt concrete, from the temperature reduction of adjacent plates of a more rigid block base, from warping of the base plates due to a temperature gradient along their thickness and from uneven subsidence and swelling of the base.
Also known is the approach of B. I. Ladygin, who received an expression for determining the durability (service life) of asphalt concrete by crack resistance T in years, taking into account the fact that at the time of crack formation, the viscosity of asphalt concrete reaches the peak of the permissible viscosity under crack resistance conditions:
= \g^] - 1,1 • \g^H nc
where [q] - is the limit value of the viscosity of crack-resistant asphalt concrete at the calculated low temperature;
qH - initial viscosity of asphalt concrete at the same temperature (viscosity at the time of laying);
nc - is the coefficient of aging of asphalt concrete, expresses the annual intensity of growth of the value of the logarithm of viscosity;
lg - is the coefficient of reduction of the initial viscosity to the conditionally initial one, corresponding to a linear change in the logarithm of viscosity over time.
Analysis of a large number of scientific publications and regulatory literature allows us to conclude that there are many approaches to assessing the crack resistance of road surfaces. However, the influence of transport load, temperature fluctuations, and the design features of the pavement are taken into account to one degree or another in the crack resistance index, or the influence of several factors is taken into account simultaneously in the complex.
According to Haas, only for the calculation of temperature shrinkage cracks in the USA, the critical temperature of crack formation in asphalt concrete pavement is predicted based on the calculated and measured values of asphalt concrete stiffness and simulated cooling conditions and resistance of asphalt concrete to linear shrinkage under field conditions. The probability of crack formation in this method is estimated by comparing the calculated temperature of asphalt concrete cracking with the estimated minimum coating temperature in a given region.
Ways to increase frost resistance
The durability of asphalt concrete coatings significantly depends on the quality of the components used for the preparation of asphalt concrete mixture. The effect of negative temperatures on the water-saturated asphalt concrete pavement, simultaneously with dynamic mechanical loads of variable intensity, leads to the occurrence and accumulation of residual deformations in road surfaces and their premature destruction. It is known that the viscosity of the bitumen used, the genesis of mineral materials, the degree of pre-humidification and the magnitude of the negative temperature, as well as the rate of temperature change, have a significant impact on the frost resistance of asphalt concrete coatings.
Granite and limestone powders were used to study the influence of the nature of mineral powders on the frost resistance of asphalt binder as a microstructural component of asphalt concrete. The results of the frost resistance study show (Table. 1) that after 42 cycles of alternating freezing and thawing, the frost resistance coefficient of asphalt binder made of granite mineral powder is 39% lower compared to the value of this indicator for asphalt binder based on limestone powder. Given the same dispersion of powder grains, it can be noted that the asphalt binder based on limestone powder is less sensitive to the aggressive effects of water and temperature, compared with the asphalt binder on granite powder.
The results obtained show that at the stage of choosing mineral powders for the production of asphalt concrete mixtures, preference should be given to limestone powders. This will ensure increased frost resistance of asphalt concrete coatings, especially in comparison with asphalt concrete coatings containing the mineral part exclusively from granite stone materials.
Ученый XXI века • 2022 • № 1 (82) Table 1
Values of frost resistance coefficients of asphalt binders
Type of powder Number of freeze-thaw cycles
5 14 28 42
Granite 0,82 0,60 0,49 0,39
Limestone 0,92 0,81 0,72 0,63
The bitumen currently produced does not adequately meet the requirements that ensure the long-term operation of asphalt concrete pavement.
One of the ways to increase the stability of asphalt concrete to the effects of heavy transport loads and extreme temperatures is the modification of bitumen by introducing polymer additives.
In cooperation with German specialists, after analyzing the properties of the bitumen supplied to us, a "polymer" corresponding in chemical composition was selected, which, when introduced into the asphalt concrete mixture, allowed to improve its properties.
Work was carried out on the comparative analysis of tests of physical and mechanical properties of a conventional asphalt concrete mixture with a "polymer".
The tests were carried out in several independent laboratories. As a result of the analysis of these tests, it can be concluded that the average density of the mixture with the "polymer" slightly increased;
-strength characteristics increased at positive temperatures and decreased accordingly at (0 ° C);
-water resistance indicators increased, and what is especially important with prolonged water saturation;
-there is an improvement in shear resistance and crack resistance.
The improvement of these indicators makes it possible to predict an increase in the service life of asphalt concrete pavement with a "polymer".
Conclusions:
Modern trends in the field of asphalt concrete design are based on increasing the accuracy of forecasting the durability of materials in road surfaces, depending on specific operating conditions.
An integrated approach to the assessment of operational indicators, including shear resistance and crack resistance of coatings, taking into account the influence of temperature-time loading conditions on the stress-strain state of asphalt concrete will allow designing the most optimal road surfaces and asphalt concrete compositions.
References
1. V.A. Zolotarev, A.V. Kosmin Tests of road-building materials, Kharkiv KHNADU 2011
2. K.B. Khomidjonovich - «Composite materials». International Journal of Discoveries and Innovations in Applied Sciences (Vol.1 No. 5 (2021): IJDIAS, 04.10.2021 p.p 9697).
3. Otakulov, B. A., Kodirov, B. Kh., & Solijonov, H.S. 0.G.L.(2021). Assessment of the quality of source materials for asphalt concrete. Scientific progress, 2(8), 396-402.
4. Otakulov, B. A., Kodirov, B. Kh., & Solijonov, H.S.O.G.L. (2021). Calculating the composition of the mineral part. Scientific progress, 2(8), 403-408.
5. Otakulov, B. A., Kodirov, B. Kh., & Solijonov, H.S. O.G.L.(2021). Determination of asphalt concrete composition. Scientific progress, 2(8), 409-414.
6. Otakulov, B. A., Kodirov, B. Kh., & Solijonov, H. S. O. G. L. (2021). Selecting the optimal bitumen content. Scientific progress, 2(8), 415-420.
7. Otakulov, B. A., Kodirov, B. Kh., & Solijonov, H.S. O. G. L. (2021). Asphalt concrete preparation technology. Scientific progress, 2(8), 421-425.
© B.Kh. Kodirov, 2022.
