Methods for calculating the composition of asphalt concrete mixtures. Methods for selecting the composition of an asphalt concrete mixture Selection of the grain composition of an asphalt concrete mixture

The calculation consists of selecting a rational ratio between the materials that make up the asphalt concrete mixture.

Wide use received a method of calculation using curves of dense mixtures. Greatest strength asphalt concrete is achieved at the maximum density of the mineral matrix, optimal quantity bitumen and mineral powder.

There is a direct relationship between the grain composition of the mineral material and density. Formulations containing grains will be optimal. various sizes, the diameters of which are halved.

Where d 1 - largest grain diameter, set depending on the type of mixture;

d 2 - the smallest grain diameter corresponding to the dust fraction and mineral powder (0.004...0.005 mm).

Grain sizes, according to previous level

(6.6.2)

The number of sizes is determined by the formula

(6.6.3)

Number of factions P per unit less number sizes T

(6.6.4)

Ratio of adjacent fractions by mass

(6.6.5)

Where TO- escape coefficient.

The value showing how many times the amount of the subsequent fraction is less than the previous one is called the escape coefficient. The most dense mixture is obtained with a runoff coefficient of 0.8, but such a mixture is difficult to select, therefore, according to the suggestion of N.N. Ivanova, escape coefficient TO accepted from 0.7 to 0.9.

Knowing the size of the fractions, their number and the accepted runoff coefficient (for example, 0.7), equations of the following form are drawn up:

The sum of all fractions (by weight) is equal to 100%, that is:

at 1 + at 1 To + at 1 To 2 + at 1 To 3 +...+ at 1 To n -1 = 100 (6.6.6)

at 1 (1 + To + To 2 + To 3 +... + To n -1) = 100 (6.6.7)

The amount is indicated in brackets geometric progression and, therefore, the amount of the first fraction in the mixture

(6.6.8)

Similarly, we determine the percentage of the first fraction at 1, for the runoff coefficient To= 0.9. Knowing the amount of the first fraction at 1, easy to identify at 2 , at 3 and so on.

Based on the data obtained, limit curves are constructed that correspond to the accepted runoff coefficients. Compositions calculated using a runoff coefficient of 0.9 contain an increased amount of mineral powder, and when To < 0,7 - уменьшенное количество минерального порошка.

The grain composition curve of the mixture being calculated should be located between the limit curves (Fig. 6.6.1).

Rice. 6.6.1. Grain compositions:
A - fine-grained asphalt concrete mixture with continuous granulometry of types A, B, C; B - mineral part of sand mixtures of types D and D

High performance indicators are achieved by mixtures with a high content of crushed stone and a reduced content of mineral powder. Preference should be given to mixtures with a runoff coefficient of 0.70...0.80.

If it is impossible to calculate a dense mineral mixture using limit curves (the absence of coarse-grained sands and the impossibility of replacing them with seed), the required density can be selected according to the principle of intermittent granulometry. Mixtures with discontinuous granulometry are more resistant to shear due to their rigid frame.

To determine the bitumen consumption, test samples are formed from a mixture with a known low bitumen content, then the volume of voids in the mineral core is determined

(6.6.9)

Where g- volumetric mass of the asphalt concrete sample;

B pr- bitumen content in the test mixture, %;

r m- average density of mineral material:

(6.6.10)

Where y,y n , at mp- content of crushed stone, sand, mineral powder in % by weight;

r,r p , r mp- density of crushed stone, sand, mineral powder.

The calculation formula for determining the optimal bitumen content will be as follows:

(6.6.11)

Where r b- bitumen density;

j- coefficient of filling voids of the mineral mixture with bitumen, depending on the specified residual porosity

Where By- porosity of the mineral core of asphalt concrete, % volume;

P- specified residual porosity of asphalt concrete at 20°C, % volume.

Cold asphalt concrete

The composition of cold asphalt concrete can be calculated using standard compositions or using the methodology used for calculating hot mixes, with mandatory testing of physical and mechanical properties in the laboratory. The amount of liquid bitumen is reduced by 10...15% against the optimum in order to reduce caking.

Characteristic feature What distinguishes cold asphalt concrete from hot asphalt is its ability to remain in a loose state for a long time after preparation. This ability of cold asphalt concrete mixtures is explained by the presence of a thin bitumen film on mineral grains, as a result of which the microstructural bonds in the mixture are so weak that a small force leads to their destruction. Therefore, the prepared mixtures do not cake under the influence of their own weight during storage in stacks and transportation. The mixtures remain in a loose state for a long time (up to 12 months). They can be reloaded relatively easily vehicles and distribute thin layer when constructing road surfaces.

The grain compositions of cold asphalt concrete mixtures differ from the compositions of hot mixtures in the direction of a higher content of mineral powder (up to 20%) - particles smaller than 0.071 mm and a reduced content of crushed stone (up to 50%). An increased amount of mineral powder is caused by the use of liquid bitumen, which requires a larger amount of powder for structure formation, and when the crushed stone content is more than 50%, the conditions for coating formation worsen. Largest size grain size in cold asphalt concrete is 20 mm. Larger crushed stone worsens the conditions for coating formation.

Crushed stone obtained by crushing rocks and metallurgical slag is used as a large component for cold asphalt concrete. These materials must have a compressive strength of at least 80 MPa, and for grade II asphalt concrete - at least 60 MPa.

To prepare cold asphalt concrete, the same mineral powder and sand are used as for hot mixes.

Liquid bitumens must have a viscosity within which corresponds to the brands SG 70/130, MG 70/130. The viscosity and class of bitumen are selected taking into account the expected shelf life of the mixture in warehouses, air temperature during storage and use, as well as the quality of mineral materials. Cold asphalt concrete mixtures are used to construct road surfaces with traffic intensity of up to 2000 cars per day.

Cast asphalt concrete

Cast asphalt concrete is a specially engineered mixture of crushed stone, sand, mineral powder and viscous bitumen, prepared and placed while hot without additional compaction. Cast asphalt concrete differs from hot asphalt concrete in its higher content of mineral powder and bitumen, preparation technology and laying method. Cast asphalt concrete is used as road surface on highways, on the roadway of bridges, as well as for installing floors in industrial buildings. Repair work using cast mixtures can be performed at air temperatures down to -10°C. A special feature of the work is the need for continuous mixing of the cast mixture during its transportation to the installation site.

To prepare cast asphalt concrete, crushed stone (up to 40 mm in size), natural or crushed sand is used. Crushed stone, seedings and sand must be high-grade, as for conventional hot-melt asphalt concrete. BND 40/60 bitumen is used as a binder. In accordance with TU 400-24-158-89, cast mixtures are divided into five types (Table 6.6.11).

Table 6.6.11

Classification of cast asphalt concrete mixtures

TO positive properties Cast asphalt concrete features include durability, low compaction costs, and water resistance. When reconstructing a road, the existing cast asphalt pavement can be reused in its entirety and with almost no need for the addition of new materials.

Tar concrete

Depending on the viscosity of the tar and the temperature of the mixtures during laying, tar concrete is divided into hot and cold. In terms of physical and mechanical properties, tar concrete is inferior to asphalt concrete, since it has lower strength and heat resistance.

Depending on the type of stone material, tar concrete is divided into crushed stone, gravel and sand. To prepare tar concrete, the same mineral materials are used as for asphalt concrete, the requirements for them are similar. Road coal tar is used as a binder: for hot tar concrete - D-6, for cold tar - D-4 and D-5. Tars are used both industrially produced and prepared directly at an asphalt concrete plant by oxidizing or mixing sand with a thinner (anthracene oil, coal tar, etc.).

Calculation of the composition of tar concrete can be done in the same way as asphalt concrete, while the main attention should be paid to careful selection of the amount of tar, since a slight deviation in its content in the mixture significantly affects the properties of tar concrete.

To prepare hot tar concrete, tars with a viscosity significantly lower than the viscosity of bitumen for the corresponding type of asphalt concrete are used. The reduced viscosity of tar causes a weakening of internal structural bonds, which can be compensated by an increase in internal friction of the mineral part. To do this, it is necessary to use stone materials with angular grains and a rough surface, and also to replace part or all of the natural sand with rounded grains with seedings. To prepare tar and concrete mixtures, you can use crushed stone from more acidic rocks (quartz sandstones, quartz-rich granites, etc.).

Dense tar concrete is used for making surfaces on roads of II...IV categories. According to sanitary and hygienic conditions, the device upper layers tar concrete coatings are allowed only outside populated areas. When preparing tar and concrete mixtures, special safety regulations must be observed.

The tar and concrete mixture is prepared in asphalt concrete plants with forced mixers. Due to the reduced viscosity of tar, its envelopment of grains of mineral material proceeds better than when using bitumen, resulting in a shorter time for mixing materials. For the same reason, it is easier to compact mixtures when constructing coatings. The compaction coefficient, which is the ratio of the thickness of the laid mixture layer before compaction to the thickness of the compacted coating, can be equal to 1.3...1.4.

When producing a tar concrete mixture, it is necessary to strictly adhere to the established temperature regime, since tar is more sensitive to temperature changes than bitumen (Table 6.6.12).

Table 6.6.12

Temperature when preparing and laying tar concrete

In terms of physical and mechanical properties, tar concrete is inferior to asphalt concrete: it has less strength and heat resistance. But at the same time it is characterized by increased wear resistance. Tar concrete coating has increased roughness, a higher coefficient of wheel-road adhesion, and increased traffic safety. This is due to the lower viscosity of tars, weaker cohesive forces of intermolecular interaction, and the presence of volatile components. Volatile substances in the composition of tar accelerate the formation of the structure of tar concrete in the coating, and also contribute to a more intensive change in its properties. Tar concrete is less plastic in comparison with asphalt concrete, which is also due to the composition and structure of tars, which consist mainly of aromatic hydrocarbons, which form more rigid structural bonds in binder materials and are poorly deformed at low temperatures, as a result of which cracks form in coatings.

Control over the production of tar concrete mixture at the plant and during installation of tar concrete pavement, as well as testing methods for tar concrete are the same as for asphalt concrete.

The composition of the asphalt concrete mixture is selected according to the instructions drawn up on the basis of the project highway. The assignment specifies the type, type and grade of the asphalt concrete mixture, as well as the structural layer of the road pavement for which it is intended. The selection of the composition of an asphalt concrete mixture includes testing and, based on its results, the selection of component materials, and then the establishment of a rational relationship between them, ensuring the production of asphalt concrete with properties that meet the requirements of the standard. Mineral materials and bitumen are tested in accordance with current standards, and after carrying out the entire set of tests, the suitability of the materials for an asphalt concrete mixture of a given type and grade is established, guided by the provisions of GOST. The choice of a rational relationship between the constituent materials begins with the calculation of the grain composition. The mineral part of coarse and fine-grained asphalt concrete mixtures in the presence of coarse or medium sand, as well as crushing screenings, is recommended to be selected according to continuous grain compositions; in the presence of fine natural sand - according to intermittent compositions, where the frame of crushed stone or gravel is filled with a mixture that practically does not contain grains of size 5-0.63 mm.

The mineral part of hot and warm sand and all types of cold asphalt concrete mixtures is selected only according to continuous grain compositions. For the convenience of calculations, it is advisable to use the curves of the maximum values of grain compositions, constructed in accordance with the requirements of GOST (Fig. 1). A mixture of crushed stone (gravel), sand and mineral powder is selected in such a way that the grain composition curve is located in the area limited by the limit curves and is as smooth as possible. When selecting the grain composition of mixtures based on crushed sand and crushed gravel, as well as on materials from rock crushing screenings, which are characterized by high content fine grains (smaller than 0.071 mm), it is necessary to take into account the amount of the latter in the total content of the mineral powder. When using materials from screenings of crushing igneous rocks complete replacement mineral powder, their finely dispersed part, is allowed in mixtures for dense hot asphalt concrete grades III, as well as in mixtures for porous and highly porous asphalt concrete grades I and II. In mixtures for hot, warm and cold asphalt concrete grades I and II, only partial replacement mineral powder; at the same time, the mass of grains finer than 0.071 mm included in the mixture must contain at least 50% limestone mineral powder that meets the requirements of GOST

When using materials from crushing screenings of carbonate rocks in the composition of hot and warm mixtures for dense asphalt concrete grades II and III, as well as cold mixtures of grades I and II and mixtures for porous and highly porous asphalt concrete grades I and II, mineral powder can be omitted if the content grains finer than 0.071 mm in screenings ensure compliance of grain compositions with GOST requirements, and the properties of grains finer than 0.315 mm in screenings meet GOST requirements for mineral powder. Rice. Continuous grain compositions of the mineral part of hot and warm fine-grained (a) and sand (b) mixtures for dense asphalt concrete used in upper layers coatings

When using polymineral gravel crushing products in asphalt concrete in road-climatic zones IV-V, it is also allowed not to introduce mineral powder into asphalt concrete mixtures of grade II if the mass of grains finer than 0.071 mm contains at least 40% calcium and magnesium carbonates (CaCO3 + MgCO3). As a result of selecting the grain composition, the percentage ratio by weight between the mineral components of asphalt concrete is established: crushed stone (gravel), sand and mineral powder. The bitumen content in the mixture is pre-selected in accordance with the recommendations of Appendix 1 of GOST and taking into account the standard requirements for the residual porosity of asphalt concrete for a specific climatic region. Thus, in road climatic zones IV-V, the use of asphalt concrete with higher residual porosity than in I-II is allowed, therefore the bitumen content in asphalt concrete for these zones is prescribed closer to the lower recommended limits, and in I-II - to the upper.

In the laboratory, three samples are prepared from an asphalt concrete mixture with a pre-selected amount of bitumen and the following are determined: the average density of asphalt concrete, the average and true density of the mineral part, the porosity of the mineral part and the residual porosity of asphalt concrete according to GOST. If the residual porosity does not correspond to the selected one, then the required content is calculated from the obtained characteristics bitumen B (%) according to the formula: B where V°por is the porosity of the mineral part, % volume; Vpor - selected residual porosity, % volume, is accepted in accordance with GOST for a given road-climatic zone; gb - true density of bitumen, g/cm 3; gb = 1 g/cm 3; r°m - average density of the mineral part, g/cm3.

Having calculated the required amount of bitumen, the mixture is prepared again, three samples are formed from it and the residual porosity of the asphalt concrete is determined. If the residual porosity coincides with the selected one, then the calculated amount of bitumen is accepted. An asphalt concrete mixture of selected composition is prepared in the laboratory: coarse-grained kg, fine-grained kg and sand mixture kg. Samples are made from the mixture and their compliance is determined physical and mechanical GOST properties If asphalt concrete of the selected composition does not meet the standard requirements for some indicators, for example, strength at 50 ° C, then it is recommended to increase (within acceptable limits) the content of mineral powder or use more viscous bitumen; if strength values at 0°C are unsatisfactory, the content of mineral powder should be reduced, the viscosity of bitumen should be reduced, or a polymer additive should be added.

If the water resistance of asphalt concrete is insufficient, it is advisable to increase the content of either mineral powder or bitumen; however, the residual porosity and the porosity of the mineral matrix must remain within the limits provided for by the above-mentioned standard. To increase water resistance, surfactants and activated mineral powders are most effective. When assigning bitumen content to cold asphalt concrete mixtures, additional measures should be taken to ensure that the mixture does not caking during storage. To do this, after determining the required amount of bitumen, samples are prepared for caking testing. If the caking indicator exceeds the GOST requirements, then the bitumen content is reduced by 0.5% and the test is repeated. The amount of bitumen should be reduced until satisfactory caking results are obtained, however, it is necessary to ensure that the residual porosity of cold asphalt concrete does not exceed the requirements of GOST. After adjusting the composition of the asphalt concrete mixture, the selected mixture should be tested again. The selection of the composition of the asphalt concrete mixture can be considered complete if all indicators of the properties of asphalt concrete samples meet the requirements of the above-mentioned GOST.

An example of selecting the composition of an asphalt concrete mixture. It is necessary to select the composition of a fine-grained hot asphalt concrete mixture of type B, grade II, for dense asphalt concrete intended for the installation of the top layer of pavement in road III. climatic zone. Available following materials: - granite crushed stone fraction 5-20 mm; - crushed limestone fraction 5-20 mm; - river sand; - material from granite crushing screenings; - material from limestone crushing screenings; - non-activated mineral powder; - oil grade bitumen BND 90/130 (according to the passport). The characteristics of the tested materials are given below. Crushed granite: grade for strength when crushed in a cylinder, grade for wear - I-I, grade for frost resistance - Mrz 25, true density - 2.70 g/cm 3; crushed limestone: grade for strength when crushed in a cylinder - 400, grade for wear - I-IV, grade for frost resistance - Mrz 15, true density - 2.76 g/cm 3; river sand: content of dust and clay particles - 1.8%, clay - 0.2% of mass, true density - 2.68 g/cm 3; material from granite crushing screenings grade 1000:

The content of dust and clay particles is 5%, clay is 0.4% of the mass, the true density is 2.70 g/cm 3; material from screenings of crushing limestone grade 400: content of dust and clay particles - 12%, clay - 0.5% of mass, true density - 2.76 g/cm 3; non-activated mineral powder: porosity - 33% of the volume, swelling of samples from a mixture of powder with bitumen - 2% of the volume, true density - 2.74 g/cm 3, bitumen capacity - 59 g, humidity - 0.3% of the mass; bitumen: needle penetration depth at 25°C - 94×0.1 mm, at 0°C - 31×0.1 mm, softening temperature - 45°C, elongation at 25°C - 80 cm, at 0°C - 6 cm, Fraas brittleness temperature - minus 18°C, flash point - 240°C, withstands adhesion to the mineral part of the asphalt concrete mixture, penetration index - minus 1. According to test results, crushed granite stone can be considered suitable for preparing mixtures of type B, grade II, river sand, material from granite crushing screenings, mineral powder and bitumen grade BND 90/130.

Crushed limestone and material from limestone crushing screenings do not meet the requirements of Table. 10 and 11 GOST for strength indicators. The grain compositions of the selected mineral materials are given in Table. Calculation of the composition of the mineral part of the asphalt concrete mixture begins with determining such a ratio of the masses of crushed stone, sand and mineral powder at which the grain composition of the mixture of these materials satisfies the requirements of Table. 6 GOST Table

Calculation of the amount of crushed stone In accordance with GOST and Fig. 2, and the content of crushed stone particles larger than 5 mm in type B asphalt concrete mixture is 35-50%. For this case We accept crushed stone content Sh = 48%. Since crushed stone contains 95% of grains larger than 5 mm, crushed stone will be required = The resulting value is entered in the table. 7 and calculate the content of each fraction in the crushed stone mixture (take 50% of the amount of each crushed stone fraction). Calculation of the amount of mineral powder In accordance with GOST and Fig. 2, and the content of particles finer than 0.071 mm in the mineral part of the asphalt concrete mixture of type B should be in the range of 6-12%. For calculation, we take the particle content, for example, closer to the lower limit of the requirements, i.e. 7%. If the number of these particles in the mineral powder is 74%, then the content of the mineral powder in the MP mixture =

However, for our conditions, 8% mineral powder should be taken, since sand and material from granite crushing screenings already contain a small amount of particles smaller than 0.071 mm. The data obtained are entered into Table 7 and the content of mineral powder of each fraction is calculated (take 8%). Calculation of the amount of sand The amount of sand P in the mixture will be: P = 100 - (Sh + MP) = (50 + 8) = 42% Since in in this example two types of sand were used (river and materials from granite crushing screenings), it is necessary to determine the amount of each of them separately. The relationship between river sand Pr and material from granite crushing screenings can be established by the content of grains finer than 1.25 mm, which, according to GOST and Fig. 2, and in an asphalt concrete mixture of type B it should be 28-39%. We accept 34%; of which 8%, as calculated above, is the share of mineral powder. Then the share of sand remains 34-8 = 26% of grains finer than 1.25 mm. Considering that the mass fraction of such grains in river sand is 73%, and in the material from granite crushing screenings - 49%, we draw up a proportion to determine the mass fraction of river sand in the mineral part of the asphalt concrete mixture:

For calculation we take Pr = 22%; then the amount of material from granite crushing screenings will be = 20%. Having calculated, similarly to crushed stone and mineral powder, the amount of each fraction in sand and material from granite crushing screenings, we record the obtained data in table. 7. By summing up the number of particles smaller than a given size in each vertical column, we obtain the overall grain composition of the mixture of mineral materials. Comparison of the resulting composition with the requirements of GOST shows that it satisfies them. Similarly, we calculate the mineral part of an asphalt concrete mixture of discontinuous grain composition. Determination of bitumen content Crushed stone, sand, material from granite crushing screenings and mineral powder are mixed with 6% bitumen. This amount of bitumen is the average value recommended in adj. 1. GOST for all road climate zones. Three samples with a diameter and height of 71.4 mm are prepared from the resulting mixture.

Since the asphalt concrete mixture contains 50% crushed stone, the mixture is compacted combined method: vibrating on a vibrating platform for 3 minutes under a load of 0.03 MPa (0.3 kgf/cm 2) and additional compaction on a press for 3 minutes under a load of 20 MPa (200 kgf/cm 2). After an hour, the average density (volumetric mass) of asphalt concrete (samples) and the true density of the mineral part of asphalt concrete r° are determined and, based on these data, the average density and porosity of the mineral part of the samples are calculated. Knowing the true density of all materials and choosing the residual porosity of asphalt concrete Vpor = 4% according to GOST, the approximate amount of bitumen is calculated. The average density of test asphalt concrete samples with a bitumen content of 6.0% (over 100% of the mineral part) is 2.35 g/cm3. In this case

G/cm 3 ; Three samples are made from a control mixture with 6.2% bitumen and the residual porosity is determined. If it is within 4.0 ± 0.5% (as was customary for fine-grained asphalt concrete from type B mixtures), then a new mixture is prepared with the same amount of bitumen, 15 samples are formed and tested in accordance with GOST requirements (three sample for each type of test). If the properties of samples prepared from the selected mixture deviate from GOST requirements, then it is necessary to adjust the composition of the mixture and test it again.

The grain compositions of the mineral part of mixtures and asphalt concretes must correspond to those indicated in the table. Indicators of the physical and mechanical properties of asphalt concrete used in specific road and climatic zones must correspond to those indicated in the table.

Components, formulation and properties The suitability of a powder for use in cast asphalt concrete can be objectively assessed only by testing the asphalt concrete samples produced with it. Taking this important circumstance into account makes it possible to use in some types of cast asphalt concrete even powders that are of little use at first glance, such as loess powder, ground marl, gypsum stone or gypsum, filter-press waste from the sugar industry, waste from soda factories, ferrochrome slag, etc. Sand plays an important technological and economic role in the production of cast asphalt concrete mixtures. When choosing sand, preference is given to natural sand. The denser and larger the grain, the more mobile and dense the mineral mixture and the less bitumen it requires. Unlike mineral powder, most natural sea, river and lake quartz sands does not enter into a chemical reaction with bitumen. For most cast mixtures, we can recommend sands that meet the requirements of the standard and table.

Components, formulation and properties For mixtures of types I and II, the use of crushing screenings containing an increased amount of dust particles is not recommended in order to avoid deterioration in the mobility of the mixtures and an increase in bitumen consumption. It is advisable to use crushed sand only as an additive to natural rounded sand in the preparation of mixtures of types I and II. V pure form they can only be used in mixtures of types III, IV and V. Almost all properties of cast asphalt concrete are significantly improved when a 3-5 mm fraction of hard-to-polish rocks is added to the mixture. The ratio of the 3-5 mm fraction and the 5-10 fraction in the mixture should be taken as 2:1 or 1.5:1. Crushed stone (gravel) for crushed stone (gravel) cast mixtures must meet the requirements and table. 3. It is not recommended to use crushed stone obtained by crushing weak (breakability grade below 600) and porous rocks. Porous crushed stone quickly absorbs bitumen, and to ensure the necessary mobility of the mixture, the bitumen content must be increased.

Components, formulation and properties In mixtures for the top layer, it is necessary to use crushed stone from dense and difficult to polish rocks, cubic in shape with a maximum size of up to 15 (20) mm. Moreover, for mixtures of type I crushed stone, fractions of 3-15 with a grain ratio of 3-5, 5-10 and mm in size as 2.5: 1.5: 1.0 are recommended. For type V mixtures, the maximum grain size can reach 20 mm, and for III - 40 mm. In the latter case, the strength of the original rock may be reduced by %.

Components, formulation and properties Without much damage to asphalt concrete from mixtures of types II, III and V, but with great benefits for production, the requirement for crushability of crushed stone grains can be reduced. Crushing of grains in these asphalt concrete mixtures is unlikely, since the formation of the structure into a monolith occurs under the influence of gravity or vibration and without the participation of heavy rollers. In cast mixtures of type II, III and V, gravel can be successfully used. Due to the rounded shape and ultra-acidic nature of the surface of the grains, the mixture has increased mobility with less bitumen consumption. Bitumen determines the phase composition of the asphalt binder in asphalt concrete, is subject to the greatest changes compared to other components of the mixture and affects the heat resistance of the coating. Therefore, they focus mainly on viscous grades having the properties indicated in table. 4.

Components, formulation and properties If bitumen does not have the complex of specified properties, it is improved by adding natural bitumen, bituminous rocks, elastomers, etc. Very effective additives include natural bitumen, which are well compatible with petroleum bitumen and are easy to use. Natural bitumens were formed from oil in the upper layers earth's crust as a result of the loss of light and medium fractions - natural deasphalting of oil, as well as the processes of interaction of its components with oxygen or sulfur. On the territory of our country, natural bitumen is found in various bituminous rocks and is rarely found in its pure form. Components, formulation and properties Bitumen deposits occur in the form of layers, lenses, veins and on the surface. Largest quantity bitumen is found in reservoir and lens deposits. Vein deposits are rare in our country. A significant amount of natural bitumen is found in surface deposits. In my own way chemical composition these bitumens are similar to petroleum ones. Natural bitumens are hard, viscous and liquid. Hard bitumen (asphaltite). Density of asphaltites kg/m3, softening temperature °C. On average, asphaltite contains 25% oils, 20% resins and 55% asphaltenes. Asphaltites have increased adhesive properties due to great content they contain natural surfactants - asphaltogenic acids and their anhydrides. Asphaltites are resistant to aging when exposed to solar radiation and air oxygen.

Components, formulation and properties Positive results were obtained by introducing crushed polyethylene into the cast mixture, as well as finely ground rubber powder (TIRP) in an amount of 1.5% by weight of mineral materials. As an additive that increases the heat resistance of cast asphalt concrete, it is recommended to use degassed sulfur in lump, granular (granule size up to 6 mm) or liquid form. Sulfur is introduced into the mixer on hot mineral materials, i.e. before feeding bitumen. The amount of sulfur is prescribed within 0.25-0.65 of the bitumen content. In this case, the amount of bitumen with sulfur is 0.4-0.6 of the content of mineral powder.

Components, formulation and properties To summarize the above, you need to keep in mind that most of the listed “know-how” require overcoming serious technical and technological problems, as well as additional financial costs, which not all organizations can solve. By increasing production costs, they do not always contribute to improving the technological properties of mixtures and performance characteristics coatings, as well as human health and the environment. It is recommended to select the mixture recipe according to special technique. The calculation of the component content begins after determining the grain (granulometric) composition of all mineral materials and constructing a sieving curve. The curve must fit within the recommended limits for a particular type of mixture 53 Components, formulation and properties If the sieving curve does not fit within the recommended limits, adjust the content of individual grains by changing their quantity in the mineral mixture. When calculating the amount of mineral powder, it is necessary to make an adjustment for the content of dust from sand and crushed stone in the mineral mixture. Further, guided numerical values phase composition of the asphalt binder (B/MP) and its quantity (B+MP) for the corresponding type of cast mixture, a dose of bitumen (polymer bitumen or other bitumen binder) is introduced and the property indicators are determined. The main indicators of the properties of cast mixture and asphalt concrete samples, for the given values of which the composition is selected, are for types: I and V - mobility, depth of stamp indentation and water saturation; II - mobility, compressive strength at +50 °C and depth of indentation of the stamp; III - mobility and water saturation; IV - water saturation and compressive strength at +50 °C.

Components, formulation and properties The tensile strength in bending and the elastic modulus at 0 °C are optionally determined, as well as the crack resistance coefficient as the ratio of the values of these indicators. If the properties of the mixture and asphalt concrete fully comply with the required ones (table), the selection is considered successful. Table - Physical and mechanical properties of cast asphalt concrete

The asphalt concrete mixture is construction material, obtained artificially. According to the production technology, a rational selection of the main components is carried out, and then the material is compacted with vibrators. Performance Requirements asphalt concrete composition included in GOST 9128.

What ingredients are used in the mixture?

The asphalt concrete solution contains the following ingredients:

components of mineral origin, such as natural or crushed sand, crushed stone (gravel), fine powder admixtures (if necessary);
binders of organic origin, such as bitumen.

Initially, tar was used instead of bitumen. However, it was abandoned due to its harmful effects on human health and the environment. To mix the components, the asphalt concrete mixture is heated. The purpose of asphalt concrete is laying roads for airfields and highways, arranging industrial floors. According to the principle of laying, asphalt concrete is:

compacted;
cast, characterized by high fluidity and high content binder material, therefore allowing masonry to be carried out without compaction.

The composition of asphalt concrete is:

crushed stone;
gravel;
sandy.

The viscosity of bitumen and the maximum temperature of the masonry determine the following types of mixtures:

hot, laid at 120 °C with binders in the form of viscous-liquid road bitumen;
cold, laid down to 5 °C, where liquids act as a binder bituminous materials petroleum origin;
warm for masonry up to 70 °C based on viscous-liquid bitumen.

However, the last type, like separate species, has not been seen since 1999. Types of hot asphalt concrete based on residual percentage porosity:

high-density - 1-2.5%;
highly porous - 10-18%;
dense - 2.5-5%;
porous - 5-10%.

In cold solutions this value is 6-10%. According to the maximum particle size of the mineral component used, the asphalt concrete sheet can be:

coarse-grained with particle size up to 4 cm;
fine-grained with particles up to 2 cm;
sandy with a size of up to 5 cm.

type A, in which the composition of the mineral stone is 50-60%;
type B with stone content 40-50%;
type B, including 30-40% filler.

What algorithms exist for designing the component composition of asphalt concrete?

To select the composition of the asphalt concrete solution, a rational ratio of components is selected. The resulting compositions have a given density and technical properties. There are four design algorithms:

Method of Professor P.V. Sakharov
Modulo saturation method provided by Professor Durieu M.
Design algorithm for the required operating conditions of the coating, obtained through the research of Professor I. A. Rybyev.
Selection based on density curves, developed by Professor N.I. Ivanov with the assistance of SoyuzDorNII.

An example of the optimal selection of asphalt concrete mixture ingredients

As an example of asphalt concrete components, it is proposed to consider the problem: a fine-grained hot mixture of type B of the second grade is needed to create a dense top ball of the road in the third climatic zone. The following ingredients are available:

granite and limestone crushed stone with a grain size of 0.5-2 cm;
river sand;
sifting after grinding granite chips;
screenings after crushing limestone;
non-activated mineral powder;
bitumen material BND 90/130.

The first stage involves testing and comparing the characteristics of the ingredients presented above. Based on the results of testing samples with different ratios of components, it was concluded that river sand, granite dust, mineral powder, and bitumen material are suitable for producing type B and second grade asphalt concrete mixtures.

Limestone and dust from the crushed limestone component did not meet GOST standards for strength parameters. At the second stage, crushed stone is calculated. Its content with a particle size greater than 0.5 cm is 35-50%. The optimal content in mixtures is 48%. The material contains 95% of particles of the specified size, so the formula looks like:

In this way, the amount of crushed stone in the mixture for the fractional composition is calculated.

At the third stage, the composition of the mineral powder is determined. Calculations begin with deriving the mass proportions of crushed stone, sand and mineral powder with a fractional composition, according to GOST. Therefore, the content of grains smaller than 0.0071 cm in the asphalt concrete mineral material should be in the range of 6-12%. For calculations, 7% is taken. When the content of elements with a particle size of 0.0071 cm is 74% in a powder mineral, the calculation formula looks like this:

Due to the presence of particles less than 0.0071 cm from granite screenings in the mixture, the minpowder fraction is taken equal to 8%. At the fourth stage, the amount of sand is calculated. Its general content is:

Sand = 100 - (Crushed stone minpowder) = 100 - (50 8) = 42%.

The example uses river and granite sand screening. Therefore, the proportions of each are determined separately. The percentage of the river component and granite screenings is established by their fraction with a particle size of less than 0.125 cm. For an asphalt concrete mixture, the grains should be in an amount of 28-39%. The average 34% is taken, 8% of which is calculated as the proportion of minpowder. Therefore, sand needs 34-8 = 26% for particles with a particle size of less than 0.125 cm. Since the mass fraction of these grains in river sand material is 73%, granite dust is 49%, the proportion for type B asphalt concrete mixtures is:

We round the resulting value to 22%, therefore, the content of screenings from granite chips is 42 - 22 = 20%. A similar calculation is carried out for each fraction of sand and screenings. The data is summarized in a table and values with dimensions less than those specified for each individual ingredient are summed up, then compared with the requirements of GOST.

At the fifth stage, the content of the bitumen component is calculated. According to the conditions, crushed stone, sand, screenings of crushed granite, mineral powder are mixed with 6% of the binding ingredient, which corresponds to the average value required in the regulatory document. Three samples of the mixture are prepared with a height of 7.14 cm and an appropriate diameter. Next, compaction is performed using a combined method:

three minutes on a vibration platform at a pressure of 0.03 MPa;
three-minute compaction on a vibropress at a pressure of 20 MPa.

After two days, the average density is determined, that is, the mass in terms of the volume of asphalt concrete, the real density of the mineral component of the mixture r°. Based on the data obtained, in addition to density, the porosity of the mineral component of the tested samples is calculated.

The approximate amount of bitumen binder is determined by the actual density of all ingredients, taking into account the residual porosity of asphalt concrete V pores = 4%. At the same time, the average density of asphalt concrete samples with a bitumen content of 6% per 100% minerals is 2.35 g/cm3. Therefore, the calculation formulas look like:

Next, three more asphalt concrete samples with a bitumen content of 6.2% are prepared to determine the residual porosity. If its value is 4.0 ± 0.5%, additional 15 samples of such a mixture are prepared and tested in accordance with GOST 9128-84.

If a discrepancy with the requirements of the regulatory document is detected, the mixture is adjusted and subsequently tested, as indicated above.

3.8. It is necessary to select the composition of a fine-grained hot asphalt concrete mixture of type B, grade II, for dense asphalt concrete intended for the installation of the top layer of pavement in road-climatic zone III.

The following materials are available:

crushed granite stone fraction 5-20 mm;

crushed limestone fraction 5-20 mm;

river sand;

material from granite crushing screenings;

material from limestone crushing screenings;

non-activated mineral powder;

oil grade bitumen BND 90/130 (according to the passport).

The characteristics of the tested materials are given below.

Crushed granite: grade for crushing strength in a cylinder - 1000, grade for wear - I-I, grade for frost resistance - Mrz25, true density - 2.70 g/cm 3 ;

crushed limestone: grade for crushing strength in a cylinder - 400, grade for wear - I-IV, grade for frost resistance - Mrz15, true density - 2.76 g/cm 3 ;

river sand: content of dust and clay particles - 1.8%, clay - 0.2% of mass, true density - 2.68 g/cm 3 ;

material from granite crushing screenings grade 1000:

material from crushing screenings of grade 400 limestone: content of dust and clay particles - 12%, clay - 0.5% of mass, true density - 2.76 g/cm 3 ;

non-activated mineral powder: porosity - 33% of the volume, swelling of samples from a mixture of powder with bitumen - 2% of the volume, true density - 2.74 g/cm 3, bitumen capacity - 59 g, humidity - 0.3% of the mass;

bitumen: needle penetration depth at 25°C - 94×0.1 mm, at 0°C - 31×0.1 mm, softening temperature - 45°C, elongation at 25°C - 80 cm, at 0°C - 6 cm, Fraas brittleness point - minus 18°C, flash point - 240°C, adhesion to the mineral part of the asphalt concrete mixture is maintained, penetration index - minus 1.

According to the test results, crushed granite stone, river sand, material from granite crushing screenings, mineral powder and bitumen grade BND 90/130 can be considered suitable for preparing mixtures of type B grade II.

Table 7

Mineral material	Mass fraction, %, of grains smaller than a given size, mm
Mineral material
Initial data
Crushed granite
River sand
Materials from granite crushing screenings
Mineral powder
Calculated data
Crushed granite (50%)
River sand (22%)
Materials from granite crushing screenings (20%)
Mineral powder (8%)

Requirements GOST 9128-84 for mixtures type B

Crushed limestone and material from limestone crushing screenings do not meet the requirements of Table. 10 and 11 GOST 9128-84 in terms of strength.

The grain compositions of selected mineral materials are given in table 7.

Calculation of the composition of the mineral part of the asphalt concrete mixture begins with determining such a ratio of the masses of crushed stone, sand and mineral powder at which the grain composition of the mixture of these materials satisfies the requirements of Table. 6 GOST 9128-84.