1 pavement structure design
1. 1 asphalt pavement structure selection and mixture type selection
The choice of asphalt pavement structure and mixture type is the key to determine pavement quality. According to specifications for design of highway asphalt pavement (JTGD 50-2006), asphalt pavement should not only meet the requirements of vehicle traffic, but also meet the requirements of rainwater impermeability. Therefore, it is advisable to use graded mixture with small particle size and small gap, and try to use asphalt concrete with small particle size to improve the impermeability of asphalt pavement surface. For asphalt pavement with medium-coarse grained asphalt concrete or open graded or semi-open graded asphalt macadam, a lower seal coat must be set under the asphalt surface to prevent rainwater from infiltrating into the base.
The basis of pavement thickness design is the cumulative equivalent axle times in the design period. For the convenience of calculation, the design unit generally divides the traffic volume of the designed highway into the standard traffic volume of one vehicle type and the non-standard traffic volume of another vehicle type, then converts the axle number of the non-standard vehicle of a certain vehicle type into the equivalent axle number of the standard axle load, and finally calculates the design deflection and structural thickness of the pavement within the design life by using the equivalent axle number.
After a lot of observation, it is found that the actual axle load of vehicles is much greater than the design axle load after expressway is completed and put into operation (most freight vehicles are overloaded). According to the calculation formula of equivalent shaft order, the equivalent shaft order is directly proportional to the 4.35th power of shaft load. Therefore, the actual equivalent axle times of the pavement are much larger than the annual cumulative equivalent axle times as the design basis, that is, the newly-built highway pavement at this stage has reached the annual cumulative equivalent axle times in a short time (such as 1 ~ 2 years). This is one of the important reasons for the early damage of pavement.
1.2 asphalt mixture ratio design
According to the specification requirements, the asphalt mixture mix design should go through four stages: target mix design, production mix design, production mix verification and trial mixing and paving, and each stage should achieve the expected purpose. In the construction, some units compress for 2 ~ 3 stages, and some directly construct by experience, which leads to the low quality of asphalt mixture. On the other hand, in many areas, the highway construction period is short, the price is low, the gravel yard is not standardized, the yard is scattered, the equipment is backward, the management is chaotic, and the material homogeneity and quality stability are poor. Although most units sampled and screened before construction, and tested and adjusted the mixture ratio during construction, due to poor accuracy, the asphalt-aggregate ratio and gradation were changing, leading to early diseases of pavement towels.
2 site construction quality control issues
Pavement use process is the key link of pavement quality formation. The construction links that directly affect the quality of the surface layer are mainly the construction of the surface layer itself, the construction of the base layer and the construction of related connecting layers.
2. 1 quality control of surface construction
If the mixture ratio of asphalt mixture is not well controlled, especially the dosage of mineral powder and asphalt is not allowed, the asphalt pavement will be pushed, wrapped in oil, loose, bare and pitted prematurely. Outdated and inappropriate mechanical equipment will affect the metering, mixing uniformity, compactness and flatness. The temperature control of asphalt mixture should be strengthened on site. The specification requires that the mixing temperature of asphalt mixture should be between 120℃ and 165℃, but in fact, it is unstable from time to time. When the asphalt mixture is transported to the ground, it is sometimes close to 180℃ and sometimes less than 1 10℃. Too high temperature may lead to asphalt aging, and viscosity reduction makes asphalt mixture loose; If the temperature is too low, the asphalt mixture will be mixed unevenly, and white aggregate will appear, which will affect the gradation. If the rolling temperature is too high, the mixture can not be compacted, and it will continue to crack, which is difficult to meet the requirements of the specification.
2.2 Road pollution in the construction process
At present, many bidding projects in expressway are too finely divided, and subgrade, bridge and culvert, pavement and traffic are seriously affecting the smoothness of pavement. On the same road section, there are many units and the construction period is tight. Parallel operation, mutual influence. For example, in highway construction, when paving the bottom and middle layers of asphalt mixture, subgrade construction units have to brush slopes and dig ditches, and vehicles in other sections have to pass, which leads to serious road pollution and affects interlayer bonding. Especially when the asphalt pavement is thin, the asphalt pavement will fall off and push under the high-speed driving load of vehicles, resulting in twisted cracks. This is the common reason of bridge deck pavement cracking. In addition, after the pavement is paved, some machinery in other processes (including traffic engineering, median and subgrade filling) stay on the pavement, spilling diesel oil to pollute the pavement, and even causing local loosening and peeling of the pavement.
2.3 grass-roots construction
The base is the main bearing layer that bears the vehicle load transmitted by the surface layer, and its strength and stability directly affect the strength and stability of the surface layer. The main problem of base construction is that the surface of base and subbase is not clean. Before paving the previous structural layer, if the floating soil, floating ash and floating sand on the pavement structural layer and subgrade surface are not cleaned up, under the action of rain, the fine materials of the floating layer will be softened, squeezed by traffic and washed into slurry by high-pressure water flow, and then spread to the asphalt pavement surface. Causes the asphalt layer to crack, loosen and pit. When the maximum dry density is determined, the compactness of the base is closely related to the proportion of coarse and fine aggregate in the mixture, especially the content of coarse aggregate. When the content of coarse particles is large, even if the degree of compaction exceeds 100%, the compactness of the base can not be guaranteed. Therefore, it is necessary to appropriately increase the rolling tonnage. Increase the number of rolling passes to ensure that the base reaches the specified degree of compaction.
3. After putting into operation, the management of overloaded vehicles is lax.
At present, for flexible pavement, the national design code still adopts deflection value control, and the service life adopts cumulative standard load times as the control index. However, the influence of heavy vehicles, especially super-heavy vehicles, on the strength of pavement structure and the bearing capacity of pavement are not considered. Driving beyond the ultimate load will cause serious damage to the pavement structure, leading to pavement cracking, pushing and even local subsidence. Highway management departments should strictly control over-limit transport vehicles in accordance with the requirements of the national highway law and the regulations of the Ministry of Communications on the driving of over-limit transport vehicles, and carry out compulsory unloading of over-limit transport vehicles. At present, super-heavy vehicles and super-heavy vehicles can be seen everywhere on high-grade highways or other highways, which is very harmful to asphalt pavement.
4 Prevention and control measures
The prevention and control of asphalt pavement diseases must first be analyzed by core drilling sampling test, deflection, friction coefficient and flatness, and the right medicine can achieve results.
4. 1 disease treatment
(1) According to the degree of flooding, lay coarse-grained mineral materials.
(2) It is difficult to deal with wave deformation, and slight waves can be forcibly leveled in hot season. Severe waves need to be filled with hot-mixed asphalt mixture.
(3) The asphalt used has low consistency and too much dosage, or the mineral aggregate gradation in the mixture is not good and there are too many fine materials; Or because the surface layer is thin and the adhesion between the surface layer and the base layer is poor, it is best to push the bag with a shovel.
(4) For smaller longitudinal joints and transverse joints, pouring hot asphalt material is generally adopted for sealing. For larger cracks, the method of filling asphalt stone chips mixture is used to deal with them, and for large-area cracks and reticular cracks. Usually, overlay seal or asphalt surface treatment is adopted. If the cracks and net cracks are serious, they should be reinforced or thoroughly overhauled.
(5) The pit and groove treatment method is to dig the pit and groove into a rectangle, and the groove wall should be vertical. After asphalt is brushed around, it is filled and compacted with the same material as the original structure from the base to the surface.
(6) For the loose diseases of asphalt pavement at the initial stage of use, the treatment method is: the base is wet and soft, and the loose asphalt surface should be removed, then compacted again, and the road surface layer should be paved after the base is dry.
(7) The treatment methods of edge-gnawing diseases are setting kerbs, widening the road surface and strengthening the shoulder. If conditions permit, we can try to widen the pavement base to 20 cm ~ 30 cm beyond the width of the surface layer.
4.2 Limit vehicle load
Asphalt pavement is designed according to the theory of multi-layer elastic continuous system under the action of double-circle vertical uniform load. When the design deflection value is taken as the index to check the tensile stress of asphalt layer, the times of axle load at all levels with axle load greater than 25kN and less than 130kN should be converted into the equivalent times of standard axle load BZZ- 100 according to the standard formula. When the single axle is greater than 130kN, the conversion formula cannot be established, which shows that the axle weight of running vehicles is generally less than 130kN, but it is not consistent with the actual situation. At present, the Ministry of Communications has promulgated the Regulations on the Administration of Vehicles with Overloaded Highway. Restricting the transportation of over-limit vehicles will prolong the service life of asphalt pavement.
4.3 Adoption of new technologies
Adopting and exploring advanced materials and construction technology will better improve the performance of asphalt pavement. One method is to improve the gradation of mineral aggregate and adopt stone matrix asphalt; The second is to improve the asphalt binder and adopt modified asphalt.
5 concluding remarks
In a word, the early diseases of asphalt pavement are caused by many factors, both in design and construction. In view of the characteristics of early diseases of asphalt pavement at present, while optimizing the design, it is more important to strengthen construction management, improve the quality of on-site construction, standardize the construction, strive to improve the performance of asphalt pavement, prolong the service life and improve the investment benefit.
The detailed information of "Causes of Early Diseases of Asphalt Pavement and Preventive Measures" can be found in Zhong Da Consulting & Construction Link, and all relevant building construction information you want is available.
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