Getting to know the bitumen asphalt factory
The purpose of preparing asphalt in the asphalt factory and spreading it in the upper last layers of pavement is to build and develop roads, freeways, highways, airport runways, piers, landscaping around buildings, etc., to create a smooth, non-compacting surface. And it is durable, so that the tires of vehicles have a proper contact on it and bring movement, control, steering and ultimately comfort and peace of mind for the driver and the passengers of the cars.
Due to the fact that the roads of each country are considered among the national assets and capitals of each country, the importance of the preparation and correct implementation of the paving layers is more evident.
In order to create such a smooth surface, with a long and long service life, after gaining experience and studies and tests and the special conditions of the country in terms of oil resources, it is recommended to use Asphaltic Concrete.
Asphalt or asphalt concrete, produced in an asphalt factory, is a mixture of aggregates with the desired granularity and the relevant adhesive bitumen, which is mixed in a certain process with specific proportions of aggregates and bitumen, asphalt concrete mixture in factories. Asphalt is produced.
Its advantages can be mentioned as being flexible and controllable when spreading, and if properly mixed, as an impenetrable barrier against fluids including water and unsuitable particles to prevent freezing.
In order to create such a layer of a smooth surface without elevation and with a geometric design and correct slopes along the road, especially in the curves, equipment is needed so that the appropriate asphalt can be produced first and then it can be distributed and compacted. did
For this purpose, in this article, explanations will be given first about the asphalt mixture and its components (aggregates and bitumen), and then the introduction of the asphalt production plant and the technical specifications of its components will be mentioned, and in the continuation of the various and complementary processes, in Asphalt production will be explained.
1 - Warm asphalt mixture
Bituminous mixtures obtained by mixing bitumen and stone materials in an asphalt factory are known as asphalt in Iran, and its various types include black base, binder, topka, asphaltic sand, etc. It is used for paving roads and airports. The properties of asphalt depend on the properties of its constituent materials, i.e. bitumen and stone materials, and the technical specifications and proper performance of the asphalt manufacturing plant.
1-1: stone materials
Stone materials are usually found either naturally in nature or artificially and indirectly in the form of slag from some metal smelting factories. Natural stone materials are sometimes obtained in a ready form from river beds or sand mines that are located along the route of roads, or they are prepared from breaking stones in stone mines (mountain mines) for asphalt factories. According to the amount of stone materials needed for each project, if the amount of crushed stone materials needed for the asphalt of the project is significant, fixed stone crusher plants with high production capacity are used, otherwise, mobile stone crusher plants are used.
Stone breaking is usually done in two, three or four stages. In the first stage, the stones extracted from the mountain mines are converted into smaller pieces by the primary crusher, which is usually of the jaw type, and in the next stages, by the secondary crushers, which are of the hammer, cylindrical or conical type, they are crushed into materials with grains. The smaller ones are converted.
They are separated from each other through the screens installed in the stone crusher complex and are stored separately and granulated by means of a conveyor belt at a suitable distance from the screen.
Usually, for the production of asphalt binder 0-52 in the stone crushing plant, materials are produced and stored in four different sizes: 0-6, 6-25, 25-21, and 21-52 mm, and after being transported to the asphalt factory for production. Asphalt is used.
Crushed stone materials have more bearing capacity and endurance compared to round corner (river) materials because the grains of such materials have sharp edges and rough surface and are better locked together. Usually, the crushed materials obtained from stone crushers are more expensive than the natural materials from the river or round the corner, and from this point of view, in cases where both types of materials are available, economic factors should also be considered in their selection.
In cases where the use of river materials is the only solution, stone materials should be broken and then used in order to increase endurance and carrying capacity.
The stone materials that are used to produce all kinds of asphalt must have special technical specifications such as grain size, hardness, durability, cleanliness, shape of grains and surface quality of grains.
A- Granulation
Granulation of stone materials is one of the most important factors that affect the durability and load capacity of asphalt. The grading of stone materials is determined by conducting the grading test and drawing the grading curve in the laboratory.
The proper grading of asphalt stone materials is selected according to several factors such as: the type of pavement, the type and location of the desired layer in the pavement system, the thickness of the asphalt layer and the size of the largest grain of asphalt materials.
Stone materials can be divided into one of three types from the point of view of granularity: full, hollow (open) and one size. Stone materials with full graining are materials whose grain size varies uniformly from fine to coarse and has enough grains of different sizes that this type of graining is very suitable for making hot asphalt concrete.
Stone materials with hollow or open graining do not have a uniform graining curve, and in comparison with stone materials with full graining, they have less amount of fine grains (rejected from sieve 500) and more porosity.
This type of granulation is used to make the asphalt layers below the pavement. Stone materials with grains of the same size have more or less grains of the same size and are used to make the surface asphalt layer, the base and tops of macadam and waterproofing coatings.
Stone materials with solid graining have more specific weight, endurance and load capacity than stone materials with hollow graining or stone materials with grains of the same size.
B- Difficulty
The stone materials used in the construction of asphalts must have sufficient resistance against the weight of heavy vehicles and also the weight of rollers and should not be broken or crushed due to the stresses caused by their weight. By using the Los Angeles abrasion test, an indication of the resistance of stone materials against the pressures applied to them is obtained.
- Durability
Stone materials used for the preparation of asphalts must be resistant to atmospheric factors such as frost and humidity changes, and materials that break and crumble due to frost or humidity changes should not be used in asphalt pavement layers.
Durability test using sodium or magnesium sulfate is used to determine the resistance of stone materials against atmospheric factors.
D- Cleanliness
Asphalt stone materials to be sent to the asphalt plant must be clean and free from any foreign and harmful substances such as organic materials, soft and short-lasting stones and clay. To determine the relative amount of clay and fine-grained materials in stone materials, the sand equivalent test (sand value) is used.
R- Characteristics of the surface and shape of the grains
The endurance and load capacity of the asphalt layers produced in the asphalt factory is proportional to the endurance and load capacity of its stone materials. Therefore, stone materials must have a large internal friction angle so that its grains are well locked in each other and have great endurance due to external forces.
The more the grains of natural stone materials are broken, the more its endurance ratio increases and its load capacity increases.
The fracture rate of stone materials is determined by performing the fracture percentage test.
Hot asphalt concrete stone materials include coarse stone materials, fine stone materials, and stone powder (filler). Coarse stone materials are materials that remain on sieve number 8 or 20. These materials should be made of crushed stone or sand or a mixture of both and should be clean, hard and durable.
Fine stone materials are materials that pass through sieve No. 8 or 20, and filler is said to be materials that pass through sieve No. 500 and must be free of clay and organic materials.
In cases where the filler obtained from breaking stones and gravel is not enough to provide the amount of filler required for asphalt concrete, or the resulting filler is not of good quality, limestone powder, slaked lime, or Portland cement can be used.
The most important role of filler in asphalt concrete is to increase the life of pavement and increase its resistance against the effect of water. In addition to these, the use of filler in asphalt concrete increases the bearing capacity, reduces relative deformation, increases impact resistance, increases shear and compressive strength, increases mental retardation, and reduces brittleness.
At the same time, using more than the permissible limit of filler in asphalt causes a decrease in porosity, an increase in resistance to compaction, and a decrease in endurance (due to a decrease in internal friction) of the material.
Bitumen: 1-2
The bitumen that is used to make hot asphalt concrete in the asphalt factory is pure bitumen with a penetration degree of 20-00, 60-00, 200-82, 220-250, and the appropriate penetration degree should be based on the weather conditions of the region and the amount of rain. And heavy vehicles were chosen.
The amount of bitumen in hot asphalt concrete depends on the characteristics and granulation of selected stone materials and its amount is obtained by performing the Marshall endurance test.
Bitumen, which is used in making asphalt concrete, increases the strength of asphalt concrete due to coating the grains of stone materials and gluing them together. Asphalt concrete endurance increases by increasing the percentage of bitumen consumed, until the endurance reaches a maximum value, and after that, with the increase of bitumen amount, the endurance of asphalt concrete decreases drastically.
In addition to reducing the strength of asphalt concrete, excessive use of bitumen also reduces its shear resistance, which changes the shape of asphalt concrete pavements with low shear resistance due to the thrust of the wheels of heavy vehicles and creates waves on the pavement surface.
It should not be filled with bitumen by using too much bitumen, because the excessive reduction of asphaltic concrete void space has the possibility of bitumen exposure in hot weather.
On the other hand, with the use of bitumen, the amount of void space in asphalt concrete is too much, which reduces its durability and formability, and as a result, reduces its durability. For this reason, based on the experimental results, the void volume of asphalt concrete is limited between 5-6%.
Marshall's test curves are used to determine the appropriate bitumen percentage to be used in asphalt concrete. The percentage of bitumen that causes the highest amount of durability of asphalt concrete may not be the same percentage of bitumen that causes the highest specific weight, or the void space of asphalt concrete may not be the most appropriate.
Therefore, the amount of bitumen that is used to make asphalt concrete in the asphalt factory is the average of the amounts that cause the highest endurance, the highest specific weight, and the most suitable empty space in asphalt concrete. The amount of bitumen consumed should be such that the technical specifications of the obtained asphalt concrete are within the limits of the technical specifications of the regulations.
After preparing the appropriate design of asphalt concrete in the laboratory using the Marshall endurance test, and announcing the optimal bitumen percentage and the mixing percentage of stone materials (granulation) and presenting it to the contractor, the contractor based on the design provided and the asphalt materials produced in the crusher who has transported to the asphalt factory; By using a set of asphalt factories, which are often fully automatic today, it produces the asphalt required by the project.
In the asphalt factory, based on the asphalt plan provided by the laboratory (mixing plan), the factory operator calculates the amount of stone materials, filler and bitumen needed to produce a batch of asphalt based on the weight percentage (which depends on the type and capacity of the factory, the weight amount of each batch) It varies from 2 to 2.5 tons (it enters the factory's batching computer).
After entering the above values into the batching system of the asphalt factory, due to the automaticity of the factory's production system, the desired asphalt is produced and used in the project according to the design and the proportions entered.
2 - A view of the asphalt factory
It is shown in the form of a view of the asphalt factory and its various components.
3 - Process set in asphalt production
Since the production of asphalt is mixing bitumen with broken and dust-free aggregates at high temperature, asphalt factories are producing asphalt in different departments with a specific process design.
The intended processes are as follows:
2-3 processes related to aggregates in the asphalt factory
5-3 process related to filler separation in asphalt factory
3-3 process of exhausting smoke and gases in the asphalt factory
0-3 Bitumen preparation process in asphalt factory
1-3: The process related to the flow of aggregates in the asphalt factory:
Broken material
Compressor or transfer loader
Cold tanks
Transfer conveyor
Dryer
Turbojet heat dust separation
Elevator
Hot aggregate transfer elevator
Top of the tower
Empty shooting
Sarandha
Vibrating nets of the sieve
Hot spots
Adjustable drain valve
tank
weighing
Drain valve
mixer
Drain valve
Asphalt silo
Drain valve
Compress
The crushed materials produced in the crusher are transported to the asphalt plant by a compressor and stored separately (sand, peas, almonds).
Then, using a loader, it is transferred from the depots to the cold tanks of the asphalt factory (usually every asphalt factory has four cold silos next to each other) and it is transferred to the drier of the factory through a variable speed conveyor belt under the cold silos. .
Inside the dryer, by using a diesel or gas burner or with diesel fuel, the materials reach a certain temperature, and from there, the materials are transported to the top of the factory tower that has been installed by means of an elevator.
There, the materials are separated from each other by the use of a sieve and are stored in the granulators located under the granulator (usually every asphalt factory has four granulators with suitable capacity).
Then, according to the asphalt plan, the required amount according to the weight percentage of the materials are transferred to the chamber of the weighing scale, and after weighing, it is poured into the mixer of the factory, inside the mixer, after mixing with bitumen and filler and Asphalt mixing and production is transferred to the asphalt silo of the factory and then transferred to the asphalt transport truck to be transported to the distribution site in the project.
2-3: The process related to the separation of filler in the asphalt factory
dryer
Turbojet heat dust separation
Dust extraction
cyclone
Rotary separation of filler
spiral
Filler transfer lift
top of the tower
Empty shooting
Filler tank
Adjustable drain valve
Filler weighing tank
Drain valve
mixer
Drain valve
Asphalt silo
Drain valve
Compress
The materials that have entered the dryer of the factory, after being heated with the torch of the factory and drying its moisture, the dust and filler, which have a lower specific weight, are sucked out of the dryer by the suction device (exosphen) and placed into the cyclone between the suction device and the dryer. It is being dragged.
Inside the cyclone, by using the rotation mode given to the material, the heavier material which is the filler is separated from the dust and poured into the filler tank which is located under the cyclones, and the dust is also separated through the exosphene. It is directed to the filter.
The filler material deposited in the tank under the cyclones is transferred to the filler elevator by the filler transfer screw, and by the elevator, it is moved to the top of the factory tower where the filler tank is installed, and it is shot into the filler tank.
From there, using a special valve, the filler is transferred to the weighing tank and after weighing to the required amount according to the plan.
Asphalt enters the mixer of the factory and is mixed with materials and bitumen. It is transported to the silo of the asphalt factory along with the produced asphalt, and from there it is poured into the asphalt transport truck.
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