The main material for the construction of roads is asphalt. Three years later, they need repairs, and then annual restoration of operational properties. The concrete road is significantly superior to asphalt in many ways, but its use is limited.
The reasons for this are as follows:
- insufficient construction budget;
- low productivity;
- climate;
- transport loads;
- lack of necessary cement grades;
- terrain.
In the USA, concrete roads are a national treasure (photo below).
Back in the 50s in America and in the West, they realized their advantage and construction was in full swing.
Previously, concrete roads in Russia were laid out from slabs, driving a car was like the movement of a train at the joints of rails. Now it is poured in place and the coating is smooth.
What are concrete roads for?
Concrete road has the following advantages:
- ease of preparation of concrete mix ;
- high laying speed;
- high strength and durability;
- smooth surface with good grip on tires;
- better visibility compared to asphalt due to good reflectivity.
Concrete roads have operational advantages in transferring loads from a rigid concrete slab to a large part of the foundation. This is especially evident in the spring. Deflection of asphalt often occurs irreversibly, as can be seen by the appearance of ruts and waves. Concrete coating reduces this deflection, while achieving fuel savings of up to 20%.
Environmental benefits are associated with the absence of soil contamination with oil products that are released from asphalt. Reduced fuel consumption reduces emissions. A harder coating is thought to produce more noise, but such an increase is negligible.
The effect of the terrain on road construction
Concrete road is being built using various technologies. Each project is different. In mountainous areas, the road repeats the terrain.
When building highways, they try to straighten it: the hollows fall asleep, the hills are cut, tunnels are drawn through the mountains, flyovers and bridges are built. To ensure a normal speed regime, builders try to avoid steep climbs, descents and turns.
For different road structures, the main classification is made according to the materials of the top coating, which may be asphalt or concrete. Asphalt must be repaired after 3-4 years. Some long roads begin to rebuild when construction is not yet completed. Concrete road costs 80% more, but it does not require repair the first 10 years of operation. Its durability reduces operating costs. If the concrete road is laid with high quality, it serves several decades without major repairs.
Roadway structure
The construction of concrete roads is made of the following layers:
- additional,
- underlying;
- concrete coating.
Soil preparation
Pouring with concrete is preceded by soil preparation and the manufacture of a leveling layer. The soil must be dense. This is easy to verify by inserting a steel rod up to 12 mm thick into it. It should enter a depth of not more than 60 cm. If the seal is insufficient, the cushion will subsequently sag and the concrete will collapse.
Soil is pre-rolled. They pay particular attention to this when adding it. In this case, rolling is performed in layers. Soil compaction is done at optimum humidity. The number of passes and the type of skating rink should be selected experimentally, making trial rolling. If the humidity is below the permissible value, the soil should be moistened. If it is excessively raw, it is dried by loosening, adding sand, slag, or in other ways.
Water disposal
When concrete roads are built, the technology provides for the removal of melt, storm and flood waters from them even before the construction of the subgrade. Work in this regard is carried out both within the city and beyond.
The construction of a concrete road with precipitation removal is necessary in order to increase the service life and improve driving conditions. On the surface of the road, water is dangerous for transport during its movement. Adhesion to the web is deteriorating, splashes from under the wheels interfere with visibility, and ice forms when freezing. To remove it, the pavement must be inclined in the transverse and longitudinal directions, drainage layers are also made. The base under the road is leveled and a slope is created in it, which can be single-pitch or gable. Places where water can accumulate are leveled and covered with non-draining soil.
Outside the city, water from the roadway is removed to the road ditches. Their width is 1-2.5 m. Water is collected in them and discharged into water inlets: a gutter, a natural or artificial pond, and a river bed. To do this, a slope of 1-4% is created in the cuvette, reinforced with lawn grass, cobblestones, debris or concrete stone.
In settlements, water is collected through trays in urban sewers. The devices for its discharge are constantly cleaned to provide the required throughput.
Water that seeps into the ground is dangerous for the road. It is diverted through seepage layers such as gravel. It contains drainage pipes with slots or holes. They can be made of plastic, concrete or ceramic.
Work on the diversion of melt, storm and flood waters from the road strip is done before the construction of the subgrade.
Laying the bed
A sand cushion with a thickness of 20-40 cm is made on the ground. You can do without it, but it significantly prevents the upward flow of moisture from the soil and improves drainage. It is necessary to prevent blurring and frost heaving, leading to the appearance of cavities and the formation of cracks in a concrete monolith. The greatest problems are created by clay, peat, and any other soil that can accumulate water. It is partially cut off and stones of large fractions are poured to the bottom, and then gravel. The height of the layers after rolling is about 30 cm. The costs and how long concrete roads will be built depend on their preparation. The minimum thickness of the base depends on the type of soil and climatic zone. It is determined by the tables. Between all layers of different materials laid geotextile.
Leveling of each layer is carried out in compliance with the design longitudinal and transverse slopes.
Stone base materials are usually reinforced with substances that have astringent properties. This may be cement or industrial waste: granular metallurgical slag with the addition of quicklime, ash from thermal power plants, and ground slag. The layers should be monolithic, which is achieved through the use of cementitious compounds and thorough rolling.
To pass construction machines often requires an increase in the strength of the additional lower layer. To do this, it is strengthened with astringent compounds.
Formwork
The formwork is made of lumber according to the casting height, which is 100-150 mm. Choosing it in height, it should be borne in mind that edges are made at the edges of the concrete slab to increase its strength. The thickness of the boards should be at least 50 mm. They are coated with a composition that facilitates disconnection from the frozen plate. The timber formwork is subject to strength requirements from the spread of fresh concrete and the forces arising from the operation of the tamper.
If heavy road compacting and concrete finishing machines are used, a powerful steel formwork is installed. It does not warp and lasts much longer. At its base there is a sole that increases stability.
Formwork sections are installed in line and securely fastened. This is especially important if concrete vibrations are made heavy machines. In places of lowering the level of the base, layers of lean concrete are poured under the formwork for greater stability.
Concrete roads: slab manufacturing technology
Before laying concrete, temperature joints are installed, allowing the slabs to move vertically and horizontally as they expand or contract.
1. Temperature joints
Filling sections are completely separated. To fill the joints, a material that absorbs energy is used: insulating cardboard, soft wood, cork with bitumen. To protect from debris and stones of the upper section of the seam to a depth of 40-50 mm, it is necessary to waterproof the sealant. If it is not done on time, when expanding the slabs, concrete may break off due to stones.
The distance between the joints in a temperate climate reaches 20-30 m. Reliability of the coating for long plates is 50%, and for short ones - 85%. It is characterized by crack resistance between overhauls. Monolithic coating is supported by steel rods placed in the seams.
They are installed through the side faces or using a special device on the paver. When the gaps between adjacent slabs are at a distance of more than 6 m, intermediate joints are installed on top of the coating to a depth of 1/3 of the thickness of the concrete. Such false seams are made along the axis of the road.
The width of the two lanes of the road is 6–9 m. A temperature-shrinkable seam is also made between them so that longitudinal cracks do not form.
2. Concrete laying
The bedding is covered with waterproof paper, waterproofed or moistened. Concrete is laid in thickness at a time. If reinforcement is used, a layer 30–40 mm thick is first poured, a mesh is laid on it, and then the formwork is completely filled.
The concrete mix must be applied quickly because it is viable for a short time. It cannot be diluted with water, since this will lead to a deterioration in the mechanical properties of the plate.
Due to the large volumes, ready-made factory concrete is delivered to the place of pouring. After unloading from the car, it is leveled by a special machine equipped with driven blades. The mixture is unloaded at 1 m 3 and evenly placed, otherwise a large pile will have a higher density at the base than in other places. In the process of alignment, this unevenness persists, which can lead to increased shrinkage of low-density areas. In this case, depressions form on the surface of the plate. The best option is to evenly lay concrete in 2-3 layers.
3. Concrete compaction
Equipment for concrete compaction is a bar made of wood or metal, which is impacted by pneumatic hammers mounted on it. He plunges into the concrete mix and moves inside it. When the treatment of the site ends, the beam is lifted and transferred to another site.
When using reinforcement, the vibration bar should be 5-7 cm higher from it.
The vibrating machine contains, in addition to the vibrating machine, a leveling beam, which is located in front.
The plasticity of the concrete mixture has special requirements. It should be sufficiently mobile, but not too liquid, otherwise it will swim away, flow through the formwork - strength will deteriorate.
After a little hardening, the concrete slab is watered with water so that cracks from drying do not form. It is covered with sand, straw mats, burlap or tarpaulin. A modern method of protection against evaporation is to spray hardening sealant on the surface of the plate. The film-forming substance is applied to the entire surface and side faces. Before this, it is required to remove excess moisture from the coating.
The low quality of the concrete mix leads to the need for frequent repairs of the road.
Conclusion
Concrete road construction technology is constantly being improved, thanks to the following:
- Further training and experience of specialists.
- The presence of multifunctional and high-performance equipment.
- Improving technical equipment to reduce manual labor.
- The use of new modern materials.
Despite the high cost, the cost of repairing concrete roads is much less than the repair of asphalt.