In recent years, with the continuous and rapid development of China's transportation industry, asphalt roads are increasingly used in urban roads in China, and premature cracking, rutting and squatting are common phenomena in today's asphalt roads. Although the cause of this phenomenon is related to the age of the road, traffic flow, driving load, and design of the pavement structure, it is more related to the lack of solidity of the asphalt surface.
1. The effect of insufficient asphalt compaction on asphalt roads
The lack of compaction of the asphalt pavement makes it easy for the asphalt pavement to crack, rut, and wrap. When the above phenomenon occurs, it not only affects the flatness and appearance of the asphalt pavement, but also the rainwater erosion will damage the adhesion between the asphalt mixture, causing the surface of the asphalt pavement to fall off, causing damage to the road structural layer, thereby reducing the road's Service life. Therefore, controlling the degree of compaction of asphalt pavement is the key to the control of asphalt road construction quality. There are many influencing factors on the solidity of urban asphalt road surface. There are three main aspects: 1 the influence of asphalt mixture performance; 2 the influence of construction equipment; 3 the influence of construction temperature.
2. Control of asphalt mixture performance
The materials of asphalt pavement mainly include asphalt, aggregate, mineral powder and cellulose. The variety of asphalt, the thickness of the aggregate, and the mixing ratio of the mixture determine the compaction effect of the mixture. Therefore, the raw materials, mix ratio and mixing quality of the asphalt mixture should be controlled first to ensure the performance of the mixture meets the design. Claim.
2.1 Quality control of raw materials
Qualified raw materials are the basis for ensuring the performance of the asphalt mixture. The mixing plant shall, in accordance with the provisions of the “Technical Specification for Construction of Highway Asphalt Pavement” (JTG-F40-2014), be tested in batches or sources, and store and store in accordance with the requirements, so that the quality of the asphalt material can be stable and meet the requirements. For the main raw materials such as asphalt, stone and mineral powder, it is necessary to strengthen the evaluation and update of the material supplier. The raw materials entering the site shall be inspected and recorded in batches, and the problems shall be discovered in time. The unqualified materials shall be resolutely withdrawn.
2.2 Control of mix ratio of asphalt mixture
The mix design of urban road asphalt mixture is still designed by volume index. However, the source of raw materials of urban construction enterprise mixing plant is more complicated, and the raw material indexes such as aggregate and mineral material are unstable, resulting in the production of mixed materials and design. The mixing ratio is large, which is not conducive to the stability of the mixture performance. During the construction process, the inspection and control of the raw materials indicators and parameters should be strengthened, so that the raw materials can be matched with the design mix ratio requirements; or the mix ratio of the different batches of raw materials can be used to match the construction mix ratio in real time. The mix ratio deviation is kept within the allowable range. Through the above two aspects of control, the asphalt mixture performance indicators are uniform and stable, in order to facilitate the control of compaction.
3. Influence and control of equipment on compaction
Construction equipment is the premise to ensure construction technology and quality. Asphalt pavement construction tools mainly include paving equipment and compacting equipment. Asphalt pavement construction should be based on the width and thickness of the layer and the type of asphalt mixture, and the paving and compacting equipment should be used reasonably.
3.1 Influence and control of paving equipment on compaction
The effect of the paver on the compaction is that the simultaneous use of the boring hammer and vibration of the paver can greatly increase the primary compactness of the paving surface layer, effectively reduce the temperature reduction speed and reduce paving when the roller is compacted. The degree of material change is time for the roller to be efficiently crushed at a higher temperature to ensure better compaction effect. The paving degree paved by the paver can basically reach 80-90%, but due to the frequent construction transition of the urban road asphalt surface layer, it needs to be assembled every time. Some operators often save time when assembling the screed. If the vibration coupling is not installed as required, or the vibration is turned off artificially, only the tamper is working when paving, resulting in a lower primary compactness of the paving layer. The parameters of the hammer and vibration are generally short strokes when the thickness of the thin layer and the mineral material is small; on the contrary, when the layer thickness and the particle size of the mineral material are large, the long stroke should be selected. The thickness of each layer of the urban road asphalt surface layer is 4-8 cm, the general hammer stroke is 5, the slamming frequency is 15-25 Hz, and the vibration frequency is 4-7. In the construction of urban road asphalt surface layer, the parameters of tamping and vibration must be reasonably selected to ensure the compaction effect of the two in construction and improve the paving density. In addition, changes in the speed of the paver will also affect the compaction of the unit area of the facing. When the paving speed is slow, the vibration and slamming times per unit area are many, and the compactness is good. On the contrary, the number of vibrations and slamming per unit area is small, and the compactness is poor. Therefore, the paver should ensure a constant and stable paving speed during construction. If the speed changes, the frequency of the tamper and vibration should be adjusted in time.
3.2 Influence and control of compaction equipment on compaction
1 Rolling speed is an important factor to ensure the compaction of asphalt pavement. According to the theory of speed and compaction, the compaction speed can reach 98% with only 6 times of compaction at 1.5 m. If the rolling speed is greater than 6 mm, it can only reach 95% even if it is 10 times. During the rolling process of the asphalt surface layer, the compacting machine should always maintain “close and slow pressure” and the speed is uniform.
2 The number of rolling passes is determined according to the thickness of the asphalt surface layer, the temperature, the type of the mixture, etc., and is determined by field experiments. The initial compaction, the recompression, the final pressure, and the three rolling stages make the compaction meet the design requirements. . In general, the initial pressure should be a steel cylinder roller, static pressure 1 to 2 times; the pressure should be followed closely, the vibration roller should be preferred, the vibration pressure 3-4 times; the final final pressure should be more than 12 tons double wheel The cylinder is pressed, and the static pressure is not less than 2 times until the surface is smooth and there is no wheel track.
3 In order to obtain the best rolling effect, choose the appropriate amplitude and frequency when rolling. The vibration frequency mainly affects the surface compaction quality of the asphalt surface layer, so that the impact spacing is smaller than the thickness of the compacted layer to avoid short ripples on the surface. The amplitude mainly affects the compaction depth of the asphalt surface layer. When the thickness is large, the height is selected, and the thickness is selected to be low. The thickness of urban asphalt pavement is generally 4-8 cm, often using high frequency and low amplitude, but in some thick rolling parts, it should be switched to high amplitude low frequency in time.
4. Temperature influence and control
The higher the temperature of the asphalt mixture, the more plastic it is, and it is easier to be compacted. However, if the temperature is too high, it is easy to push the mixture during the rolling process to make the layer thickness uneven; on the contrary, the lower the temperature, the more the mixture Early hardening, not easy to compact, if forced rolling, the structure of the mixture is destroyed, seriously affecting the quality of the asphalt surface layer. Therefore, in the construction of asphalt surface layer, the control of the construction temperature of the asphalt mixture is the most important. The construction temperature is mainly reflected in the paving temperature, the initial pressure temperature and the temperature after the end of the final pressure.
The data shows that the final temperature of the crushing is more than 100 °C, and the compaction degree is above 95%. When the temperature is less than 100 °C, the compaction degree is below 95%, which does not meet the design requirements. Therefore, the final temperature of the crushing should be around 100 °C, the modified asphalt mixture, the initial pressure temperature is not less than 150 ° C, and the final temperature should be greater than or equal to 90 ° C.
The effective compaction time refers to the time required for the asphalt mixture to be reduced from the initial rolling temperature after paving to the minimum allowable rolling temperature. The reduction of effective compaction time reduces the compaction efficiency. The effective compaction time is related to factors such as temperature, wind speed, surface temperature of the underlying layer, thickness of the paving layer, and paving density.
Improve the effective compaction time of the asphalt pavement by increasing the temperature of the mixture before paving or reducing the temperature drop of the mixture during rolling. The influencing factors of the former are: the transportation time is affected by the urban traffic environment, the transportation time is difficult to guarantee, the preparation time of the paving work surface before construction and the waiting time before the paving, the urban road widening section and the construction needs of the intersection The pavers are frequently moved, and the amount of artificial paving is also large. The auxiliary time of the above paving is difficult to control, so the temperature loss of the mixture is large, and the effective compaction time becomes short, which causes the rolling temperature of the asphalt surface layer to be biased. low. In particular, paving is not continuous, and the degree of compaction is low overall, resulting in more early damage to these sections. The latter factors are: temperature, wind speed, thickness of the surface layer, initial compaction of the mixture after paving, initial rolling temperature and so on. The lower the temperature, the higher the wind speed, and the thinner the surface layer, the faster the temperature of the mixture drops. The higher the initial compaction of the paving, the less the voids in the mixture and the slower the temperature drop. Timely initial rolling can also further stabilize the mixture, increase the density and reduce the temperature loss.
To ensure the effective compaction time of the asphalt pavement, firstly, the ex-factory temperature of the asphalt mixture can be appropriately increased, and the insulation measures of the double-layered sponge tarpaulin are adopted for the transportation vehicle, and the transportation route is planned in advance to reduce the temperature loss during transportation. Paving the work surface should be prepared in advance to reduce the waiting time before paving. Secondly, to improve the initial compactness of the paving, the road roller should be kept close to the paver, reduce the temperature loss before the initial pressure, and keep the interval between the paving and the initial pressure not more than 80 meters, so that the entire construction process can be “quickly unloaded”. , fast, fast pressure, early molding", with efficient construction to ensure that the rolling temperature is always within the allowable range.
5. Other measures to control compaction
When conditions permit, the construction of asphalt pavement surface layer should be properly selected, and the thickness of the paving, compacting equipment, compaction method and rolling pass number should be determined through trial and error. According to the type of asphalt, the type of mixture, the construction environment and the layer thickness, the production temperature and paving temperature of the asphalt are determined. When rolling, the roller drive wheel faces the paver and is slowly rolled at a constant speed. After the initial pressure, the asphalt concrete surface layer shall not cause displacement, wave or cracking; after re-pressing, the surface of the asphalt concrete surface layer shall have no obvious wheel tracks; after the final pressure, the surface shall be smooth, smooth and uniform in color. The edge that cannot be reached by the roller should be compacted with a vibrating compactor. During the construction process, when obvious segregation, waves and cracks appear on the asphalt mixture surface layer, the cause should be analyzed and eliminated in time.
6, the conclusion
In summary, in order to improve the quality of urban road asphalt pavement, prevent premature damage, and ensure the compactness of asphalt pavement is one of the main countermeasures. By controlling the raw materials and mix ratio of the asphalt mixture, rationally applying the construction equipment, and strictly controlling the construction temperature of the asphalt mixture, the compactness of the asphalt surface layer can be effectively improved, and the service life of the asphalt road can be guaranteed.
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