According to many years' construction practice of asphalt concrete pavement, this paper analyzes the smoothness of subgrade pavement, asphalt concrete mixing quality, paver machinery and technology, compaction machinery and rolling technology, and the treatment of longitudinal and transverse construction joints, and puts forward the factors affecting the smoothness of asphalt concrete pavement and the corresponding solutions.

Keywords: factors and methods of smoothness of asphalt concrete pavement, asphalt concrete structural layers are more and more used in expressways and ordinary trunk highways. The smoothness of asphalt pavement is one of the main indexes to evaluate the quality and performance of pavement. The higher the highway grade, the higher the requirement for pavement smoothness. Pavement smoothness is not only directly related to the safety and comfort of driving, but also affects the economic indicators such as vehicle fuel consumption, tire wear and transportation aging. Moreover, the uneven road surface will lead to the impact and vibration of vehicles on the road surface, and then accelerate the damage of the road surface. The Standard for Quality Inspection and Evaluation of Highway Engineering (JTJ07 1-98) requires that the mean square deviation of pavement smoothness measured by continuous flatness meter is δ <1.2 mm. However, there are many factors that affect the smoothness of asphalt concrete pavement, and even a small mistake in each link will reduce the smoothness index. Here, the author briefly analyzes the influencing factors of smoothness from the aspects of pavement machinery configuration and construction technology, and puts forward corresponding solutions. 1, the influence of the smoothness of subgrade, subbase and base on the smoothness of asphalt concrete pavement. The smoothness of asphalt concrete pavement is not completely determined by the last side. If the smoothness of subgrade, subbase, base and sub-level is quite different, and the loose laying thickness of each layer is also different, the surface of each layer will be uneven after rolling. Even if the automatic leveling device can eliminate some errors, the allowable errors of the paver will still lead to the decrease of the final pavement smoothness. (1) starts from the smoothness of subgrade. At present, most of the requirements and standards for improving the smoothness of subgrade are to improve the automatic leveling ability of grader scraper when subgrade is formed. Generally, laser or sonar control is used instead of manual control. The scraper is equipped with a laser receiver or sonar lock tracker, which can keep the roadbed smoothness at a good level. (2) Strictly control the elevation and smoothness of subbase and base, strictly control the elevation, and ensure the surface thickness. Subbase and base are required to be paved with pavers to ensure the smoothness is improved step by step. (3) The transverse slope of each layer should be strictly controlled. Control the positive and negative deviation of embedded kerbs to prevent the change of transverse paving thickness caused by composite transverse slope and affect the flatness. If the asphalt concrete pavement adopts multi-layer structure, as long as the embedded kerb elevation is accurate, the paver with dynamic compaction and automatic leveling function can adjust the pavement elevation, cross slope and smoothness at one time when paving forward according to the kerb top elevation benchmark. The density of asphalt concrete can reach more than 80% after dynamic compaction by paver, which largely eliminates the impact of bumps at the base. The equal thickness paving of the middle surface layer and the upper layer is less affected by the base layer. (4) Influence of subgrade deformation and settlement of bridge and culvert head on pavement smoothness. These influences come from the poor quality control effect of subgrade and bridge construction. Subgrade deformation will cause deformation, cracks and even dislocation of asphalt concrete pavement, which can be compensated by restoring non-pavement flatness. Quality control of filling and compaction must be done well in the construction of subgrade, bridge and culvert, and technical measures such as bridge end transition slab should be taken to overcome it. 2. The influence of asphalt concrete quality on pavement smoothness has the following four situations: (1) the specification and quality of aggregate. The quality such as needle-like content and crushing value in aggregate must meet the design requirements. The primary distribution of specification materials into the hopper should meet the requirements, so as to avoid the concentration of intermediate particle size during automatic grading of the mixer, which makes the compaction coefficient fluctuate and affects the smoothness of the pavement. Especially, after the oversized stones enter the paving, the local loose paving thickness will change due to the filling effect, or the paving surface will be scratched, and convex edges will appear after rolling, resulting in unevenness. (2) Mixing of asphalt concrete. When the machine is just started, the material temperature is low, which will cause uneven compaction, reduce flatness or increase aggregate water content, resulting in uneven material temperature; Low material temperature and short mixing time lead to ash, which affects paving quality; However, too high material temperature will cause asphalt aging, affect the quality of asphalt concrete, and the pavement will appear early loose damage. This can be overcome as long as enough attention is paid to production control. (3) The production capacity of mixing equipment is insufficient. The production capacity of the mixing station does not match the paving equipment, which leads to the discontinuous paving of the paver and the phenomenon of machine and other materials. There will be a temperature difference at the joints, which will affect the flatness. Therefore, in the configuration of construction equipment, mixing, transportation, paving and rolling equipment should be coordinated. (4) Paving of asphalt concrete. Most mixing stations are equipped with hot silos, and asphalt concrete enters the silos through the hoppers and falls into the transport vehicles, resulting in certain segregation. In the process of paving, many pavers are used to turning the wing up at the end of paving. All these behaviors make the isolated asphalt concrete spread on the surface, which affects the smoothness because of the different compaction coefficients of coarse and fine aggregates. The correct way is: when mixing normally, the car moves back and forth when receiving materials; When discharging to the paver, the lifting of the hopper (discharging amount) is coordinated with the paving amount of the paver, and the mixing amount, transportation amount and paving amount are coordinated. Paver operators should not always put up their wings. In addition, except for special occasions, try to avoid extra-wide paving, that is, to avoid asphalt concrete segregation caused by widening screw conveyor. 3. The influence of paver machinery and paving operation on smoothness. The performance and operation level of paver are the direct factors affecting the smoothness of asphalt concrete pavement. Unstable structural parameters of paver, skid of walking device, uneven paving speed, discontinuous paving, frequent stopping and violent starting, uneven dumping of hopper when reversing, or abnormal braking when discharging, abnormal operation of screed, and untimely removal of asphalt concrete spread on the lower layer will all cause uneven pavement and even waves. (1) According to the pavement grade, choose the paver with excellent performance, stable structural parameters, high degree of automation of leveling device and matching the capacity of mixer. (2) The paving speed should be uniform. Generally, it should be controlled at 2~6m per minute, and the paving speed should be kept constant (on the principle of non-stop according to the feeding situation of mixers and transport vehicles). If the mixer is a little small, you can use asphalt concrete produced overtime in the storage bin to ensure that the paver does not stop. The instantaneous speed change of paver leads to uneven surface flatness and uneven vibration interval, which will inevitably lead to the decline of flatness. Therefore, once the paver speed is selected, it should be balanced and not changed frequently.

(3) The truck shall not collide with the paver, and the unloading shall not be too violent, nor shall it brake during the unloading process to increase the driving resistance of the paver, otherwise the speed of the paver will change, resulting in "waves" and "washboard". The contact between the truck and the driving wheel of the paver should be gentle, and the hopper should be lifted evenly when the truck is discharged. In the process of discharging, put neutral, and the paver advances smoothly. (4) The vibration frequency of the screed should be properly adjusted, because the frequency is too small, the pre-compaction degree can not meet the requirements, and the rolling slip is serious; If the vibration frequency is too high, it is easy to cause vibration of screed, especially when the layer is thin, it will cause "whitening" phenomenon. In addition, the screed should be fully preheated before starting work, but the temperature should not be too high to avoid deformation of the screed or coking of asphalt. The start of the paver should be synchronized with the vibration of the screed, so as to ensure that the asphalt concrete will be evenly distributed after starting a small vibration, so that the paving layer can reach a relatively uniform density. If the starting and vibration are not synchronized, the local density of surface materials will be inconsistent and the flatness will be reduced. (5) Loose materials scattered on the ground due to truck unloading or other reasons should be cleaned up in time, otherwise, when the grounding elevation of the tracks on both sides is inconsistent with the cross slope, waves will be generated, which will affect the flatness. 4, the influence of rolling operation on flatness (1) compaction machinery. Roller compaction of asphalt concrete pavement generally includes initial compaction, secondary compaction and final compaction. Double steel wheels should be used for initial pressure; Vibrating roller and tire roller for recompression; The final pressure can be achieved by double steel roller or vibratory roller. The specifications, quantity and running speed of the roller should match the construction width and speed of the paver. If the selection is improper, there will be channels, cracks, wheel tracks, bumps, potholes, washboards, waves and other phenomena, resulting in a decline in flatness. (2) Compaction process. (1) Initial pressure shall be carried out at higher temperature. The initial pressure temperature has always been determined by pressure test according to asphalt grade, roller model, original initial density of paver screed and other factors. According to the author's practical experience, the compaction temperature immediately after the paver is mostly around 120℃~ 140℃, and there will be no displacement and cracking. (2) When rolling, the driving wheel is in front and the driven wheel is behind, which can avoid the hot material from being extruded and uplifted, and drive along the track of forward rolling when retreating. From outside to inside, from low to high, static pressure first and then vibration rolling. Improper methods of starting, reversing and reversing in the rolling process will cause road surface movement, compaction, potholes and wheel tracks. Therefore, the direction and braking should be avoided as far as possible during the rolling process, and the rolling must overlap trapezoid to prevent overpressure or pressure relief. When the vibratory roller is used for re-pressing or final pressing, the vibration should be stopped when reversing, and the vibration should be started before starting when moving forward, so as to avoid crowding or grooves. The roller shall not turn, turn around or wait on the road surface that has not been cooled by rolling. Vibrating roller should turn off vibration when driving on the formed road surface. Construction or other equipment and materials shall not be parked on the asphalt concrete pavement that has been rolled but not cooled that day to prevent deformation. Traffic can only be opened after it is completely cooled. 5. Influence of construction joints (vertical and horizontal) on smoothness In asphalt pavement construction, construction joints are inevitable, and joints often have uneven phenomena such as deformation, cracking, looseness or dislocation due to insufficient bonding strength. (1) longitudinal thermal connection. Set aside a certain width on the side where asphalt concrete is paved first, and then compact it together after the paver at the back is paved. So you can't see the seams. The hot spot rolling shall be carried out at the end of staggered wheel rolling, and it shall be completed by a roller, and the machine shall not be changed. (2) longitudinal cold joint. For longitudinal joints that are not paved at the same time, they should be compacted in time, and the edges should be cut into vertical planes before the next paving. The paved asphalt concrete will overlap the paved part by 5~ 10cm to ensure the loose paving thickness, and then the asphalt concrete on the original pavement will be removed manually. Finally, the longitudinal joints are compacted with a steel wheel roller with a spacing of 15cm. When implementing longitudinal cold joint, the upper and lower longitudinal joints shall not overlap, and the stagger shall not be less than 50cm. (3) transverse joints. During construction, asphalt concrete pavement should be laid continuously as far as possible, and the fewer transverse joints, the better. For transverse joints, before the next paving, check the flatness of the ends with a planer or a 3m ruler, cut off the ends that do not meet the requirements with a cutting machine, coat the cut surface with emulsified asphalt, iron the original compacted parts with a paver or preheat or even soften them with hot materials, ensure the loose paving thickness when paving, manually scrape and sprinkle a layer of fine materials when necessary, and then roll them horizontally with a steel wheel roller, from cold to hot, with a width of 20cm at a time, and move in the direction of new paving until all. When changing to longitudinal rolling, do not roll vertically on the transverse joints, so as not to cause the new and old layers to stagger, and measure the flatness in time, and handle it in time if it does not conform. 6. Conclusion The above analysis and discussion on the factors influencing the smoothness of asphalt concrete pavement show that there are many factors that cause the smoothness of asphalt concrete pavement, but as long as it is treated seriously and scientifically, the materials are excellent, the equipment configuration is advanced and reasonable, the construction technology is advanced, the construction organization is strict, the quality awareness of all staff is enhanced, and all links are not relaxed, the purpose of maintaining the smoothness of asphalt concrete pavement can be achieved.