Causes of unevenness in asphalt pavements and remedial measures

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1 Introduction With the rapid development of high-grade highways, there are increasing demands for road surface smoothness. The pass rate of road surface smoothness not only reflects the comfort level of driving but also indicates the competence of the construction teams. Over the past three years, the asphalt pavement projects carried out by our unit on the Meixian section of G312, the Fengmei section of G312, and the Anlin section of S304 have exhibited various degrees of surface irregularities such as potholes, joint steps, wavy surfaces, rolling ridges, uneven connections at bridges and culverts with the pavement, and vehicle bounce. Based on these irregularities, I conduct an analysis to explore the causes of such problems in asphalt pavements as well as possible solutions. 2 Main causes of unevenness in asphalt pavements. There are many factors that influence the construction of asphalt pavements. Just the levelness of the pavement itself is related to the competence of the construction workers, the quality of subgrade construction, the proper handling of areas around bridges and culverts as well as bridge expansion joints, the construction of the pavement’s base and subbase layers, the selection of machinery used for pavement construction, and the quality of the pavement materials. These factors are precisely those that affect the levelness of the pavement. 2.1 Uneven settlement of the subgrade leads to potholes in the paved road surface. The subgrade serves as the foundation for the road surface, and uneven settlement of the subgrade inevitably results in an uneven road surface. The reasons for this can be summarized as follows: (1) Poor control of the subgrade fill material; for example, in the Pingliang urban area section of the Meiyan route, the filling was carried out by ** of Pingliang City, which caused the road surface to become uneven. After maintenance workers dug into the road surface, they found that in some sections the subgrade was filled with construction waste and industrial waste. In the Anlin section, due to soil conditions, high-limit clay was used for filling, resulting in uneven settlement of the subgrade to varying degrees. ⑵Improper handling of the joints in semi-excavated and semi-filled subgrades, as well as insufficient compaction of the subgrades, are issues that arise. The Pinghua Road is part of a project to renovate an existing road, and it features many semi-excavated and semi-filled subgrades. After the road surface was completed, settlement and cracks appeared; this was due to high moisture content in the subgrade fill material, insufficient capacity on the part of the construction team to carry out excavation work in accordance with standard procedures, which led to cracks and settlement at the joints between the fill materials. Additionally, there was a lack of equipment for compacting the subgrade, resulting in low density of the soil and increased permeability, which caused water to accumulate and erode the subgrade, leading to softening of the soil and uneven settlement. ⑶In sections with special foundations, the protection and drainage systems for the subgrade are inadequate. For example, some subgrades in the Fengmei section have settled due to insufficient exploration of the original soil conditions; part of the subgrade was built on soft soil, and because of the high compressibility of such soil, settlement occurs under its own weight. In other sections, inadequate protection and drainage systems for the subgrade lead to uneven settlement of collapsible loess and poor water flow, thereby causing deformation of the subgrade. 2.2 Vehicle bounce at both ends of bridges and culverts, as well as at the expansion joints of bridges, severely affects the overall smoothness of the road surface. Defects in the subgrade at the ends of bridges and culverts are a fairly common phenomenon and one of the most frequent road-related problems. Such issues occur to varying degrees, whether on the secondary roads in the Anlin section or on the better-maintained primary roads in the Fengmei section; the main problems include: (1) Inadequate compaction of the fill soil behind bridges and culverts due to the limited working area of the compaction equipment, which leads to compression and settlement of the subgrade once traffic resumes. ⑵The large difference in stiffness between the backfill and the platform body leads to uneven settlement. ⑶At the junctions of bridges, culverts, and roadbeds, fine cracking often occurs. When rainwater seeps in, it causes problems in the roadbed, leading to subsidence in that area. ⑷Improper consideration and handling during the selection and construction of bridge expansion joints lead to vehicle bouncing phenomena. 2.3 The impact of uneven subbases on pavement smoothness: The subbases in the Meixian section and Anlin section are of the sub-high-grade pavement type, and the construction requirements for them are not strict. During construction, if the subbases are not made evenly, then no matter how carefully the surface layer is laid, unevenness will still occur in the pavement due to differences in the thickness of the material used for compaction ； The Fengmei grade represents a high-level pavement base layer, and strict construction requirements apply. Both the sub-base layer and the base layer are paved using ABG pavers. However, since the allowable deviation for the flatness of the base layer surface is 10 mm, even though the surface of the asphalt concrete is leveled during paving, the excess 10 mm in thickness results in low spots after compaction. This shows that unevenness in the base layer has a significant impact on the flatness of the pavement. 2.4 The pavement paving machinery and techniques have a significant impact on smoothness. The paver is the main equipment used in the construction of asphalt pavement surfaces, and its performance and operation greatly affect the smoothness of the pavement. Unstable structural parameters of the paver, slipping of the traveling mechanism, uneven paving speed of the paver, sudden starts and emergency brakes of the machinery, as well as fluctuations in the speed of the feeding system, can all cause irregularities and waves in the surface layer. ⑴The performance of paving machinery determines the smoothness of the road surface layer. The three road construction projects carried out by our unit in recent years serve as a good example: the Meixian section was paved using a 45m small asphalt paver; there were many joints in the road surface, and during paving, work was almost entirely done manually, so there was hardly any talk of road surface smoothness ； The Anlin section was paved using two 6.0m asphalt pavers. Although this approach requires less effort in road construction compared to the Meixian section and results in better road smoothness, the values remain high, barely meeting the acceptance standards for class II roads ； For the Fengmei section, two 12.0m tan-4233abg large-scale asphalt pavers were used, which led to a significant improvement in the smoothness of the road surface. During the final project inspection, the mean square deviation of surface smoothness was 1.79 mm, far exceeding the standard of 2.5 mm for secondary roads. ⑵ The control of the paver’s baseline also affects road surface smoothness. Most of the pavers in use today are equipped with automatic leveling devices, and the paving process is controlled based on pre-set benchmarks. However, construction companies often do not pay enough attention to this, or due to errors in level adjustment, the benchmarks are not properly controlled; furthermore, the benchmark lines may warp because of insufficient tension forces or too large spacing between supports, resulting in wavy surfaces in the paved layer ； Inaccurate elevation measurement using a string, errors in measuring the line, or movement of the pile positions will all be reflected on the corresponding paving section through the instruments mounted on the steel wire, resulting in uneven surfaces. ⑶Improper operation of the paver is the main cause of wavy or uneven surfaces on the road. Regardless of the model of paver used in construction, if the operator is not skilled, issues such as the paver moving in a curved path, the material carrier hitting the paver while dumping material, the paver not moving smoothly, or fluctuations in the height of the screed during movement, can all lead to uneven surfaces on the road ； The screed of the paver was not properly preheated, resulting in the mixture sticking together and failing to be leveled properly ； Due to poor coordination between the transport vehicle and the paver, during unloading, the mixture that spilled onto the lower layer was not removed in time, which affected the ground level of the tracks and consequently impacted the cross-slope and levelness of the paved layer. 2.5 The quality of the surface paving material has an impact on levelness. The construction quality of asphalt pavements also depends on the quality of the main materials, the design of the asphalt mixture ratio, and the mixing process of the asphalt mixture. ⑴ The mix design of the asphalt mixture is unreasonable; specifically, the ratio of oil to aggregate is too high, which results in bulging and oil bleeding on the paved road surface ； The oil stone is relatively small, causing the road surface to become loose ； The poor quality of the mineral aggregates – with low crushing values and compressive strength, as well as a high content of long, flat particles – reduces the stability of the road surface mixture, making it prone to various road defects. ⑵ The mixing of asphalt mixtures is uneven; this occurs when there are problems with the mixing equipment, when the machine has just started operating or when the material temperature is low and the moisture content is high, resulting in uneven material temperatures ； When problems occur in the screening system, it leads to significant changes in the aggregate gradation ； Sometimes grayish materials also appear, making it difficult to pave the road surface properly ； Excessively high temperatures cause asphalt to age, which prevents ensuring the quality of asphalt concrete paving ； The mixing capacity is too low, resulting in pauses in production while waiting for materials; this leads to a decrease in temperature at the joints, creating temperature differences that give rise to various bumps ； When the transportation equipment is inadequate or the driver’s skills are poor, it can collide with the paver, causing the machine’s rear to move backward and creating steps. 2.6 The impact of rolling on smoothness: Rolling after the laying of the asphalt surface layer has a significant effect on its smoothness. The choice of rolling equipment, rolling temperature, speed, route, and sequence all influence the smoothness of the road surface. This is manifested in the following ways: (1) When it comes to selecting the type of roller, if a roller with low frequency and high amplitude is used, it can cause uneven compaction, thereby damaging the smoothness of the road surface. An excessively high initial rolling pressure from the roller can also cause pushing deformation in the freshly paved road surface. ⑵In terms of controlling the rolling temperature, when the initial pressing temperature is too high, the wheel tracks left by the road roller are obvious, and the asphalt material shifts significantly back and forth, resulting in instability ； Excessively high recompaction temperature can cause the rubber-tyred roller to stick to the asphalt fines, resulting in the splashing of small fragments and affecting the surface gradation ； If the temperature is too low, it is difficult to compact and level it. ⑶Regarding the adjustment of the rolling speed, uneven rolling speeds by the roller, sudden braking and starting, arbitrary stopping and turning, and leaving the vibrating device on while parked on a road that has already been compacted can all cause ripples on the road surface ； Parking on uncooled pavement results in indentation grooves. ⑷When following the rolling route, an improper rolling path and a lack of attention to rolling with alternating wheels, as well as making turns at the same cross-section each time, can cause unevenness in the road surface. ⑸Regarding the determination of the number of passes for rolling: insufficient passes result in inadequate compaction, which leads to the formation of ruts once traffic starts using the road ； Too many passes of rolling are carried out; repeated rolling over a short period of time causes the already formed pavement to shift, resulting in crack cores and wave patterns. ⑹Regarding the front-back arrangement of the drive wheels and steering wheels, if the driven wheels are at the front, since these wheels have no driving force of their own and rely on the rear wheels for propulsion, the mixture is pushed forward; when moving backward, waves are formed in front of the wheels. 2.7 Poor joint treatment: Joints include longitudinal joints and transverse joints (working joints). Common defects that arise when joints are not properly treated are bulges or protrusions at the joint areas, as well as cracks or looseness due to insufficient compaction and weak bonding strength. These issues occur to varying degrees on several roads. 3 Measures to improve the flatness of the subgrade and pavement base 3.1 Treatment of the original ground surface before embankment filling The construction quality of the subgrade is crucial for the entire road project, and it determines whether the subgrade and pavement can withstand the effects of time, vehicle loads, as well as weather conditions during the rainy and winter seasons. To carry out subgrade work properly, it is essential to fill the subgrade thoroughly, with particular attention paid to the treatment of the original ground surface and the slopes: (1) When filling the embankment, the original ground surface should be treated first. When the fill height of the embankment is 1.0 m or more, care should be taken to remove all tree roots and grasses within the subgrade area. If the topsoil of the foundation is humus soil, it must be removed and replaced using an excavator or by manual labor; the thickness required depends on the specific circumstances, but generally it should be no less than 30 cm, with each layer being compacted properly. If peat layers, mouse holes, or cracks are found, soil that meets the requirements should be used for backfilling, and compaction should be carried out in accordance with regulations. When the embankment passes through arable land, it must be leveled and compacted in advance before embankment construction begins. If it contains a high amount of organic matter and other impurities, the excessive elasticity during rolling makes it difficult to compact it, in which case the soil should be replaced. ⑵Treatment of the slope base. When the slope is gentle (with a cross-slope of less than 1:5), it is only necessary to remove the surface layer on the slope, and the treatment method remains the same as above. However, when the slope is steep (with a horizontal slope greater than 1:5), the slope surface should be made into steps to allow the filler to settle fully into the foundation, thereby preventing the embankment from sliding. The dimensions of the steps vary depending on the soil type, terrain, and construction methods. Generally, the width at the base should not be less than 1 meter, and the top surface of the steps should have a slope of 3%-5% leaning inward toward the embankment, with each layer being compacted firmly. Once all the steps are filled, normal soil filling can be carried out. 3.2 Embankment Filling: Embankment filling should generally consist of gravel and soil with plasticity indices and moisture contents that meet the specifications; sediment, marsh soil, frozen soil, organic soil, soil containing grass, household waste, and soil containing humus should not be used. For soils with a liquid limit greater than 50 and a plastic index greater than 26, they are generally not suitable as fill material for roadbeds. However, in the Anlin section, due to cost considerations and the local soil conditions as well as constraints imposed by the site conditions, it is necessary to use such soils; accordingly, the following measures were taken: (1) Controlling the optimal moisture content to ensure that the soil achieves its maximum compaction at that level of moisture. Due to the particularly high moisture content in the local soil, it is dried in the sun to achieve its optimal moisture level. ⑵Improvement through the addition of admixtures. For soils with high water content and high plasticity, or other materials with insufficient strength such as sandy soils containing a large amount of fine sand, stabilizers such as lime, cement industry waste, or other materials are added to improve the properties of the soil so that it meets the requirements for filling. ⑶When filling embankments with different types of soil, the following measures should be taken: ① The number of layers should be minimized; the total thickness of each structural layer must be at least 0.5m. Random mixing during filling should be avoided to prevent the formation of water pockets or sliding surfaces ； ②When soil with poor water permeability is used in the lower layer, its surface is given a certain slope to ensure that water from the more permeable fill material in the upper layer can be drained away promptly ； ③Arrange the different soil layers in a proper manner, using high-quality soils that do not change in volume due to moisture or ice melting for the upper layers, while soils with lower strength should be used for the lower layers ； ④At the joints of embankments filled with different types of soil, slopes should be created, with soil of poor water permeability placed at the lower part of these slopes. 3.3 Compaction of filled subgrades: During subgrade construction, it is necessary to strictly follow the requirements of the current \"Technical Specifications for Highway Subgrade Construction.\" It is also essential to determine, through test sections, the optimal moisture content for different fill materials when compacted by various machines, the appropriate loose thickness, as well as the number of passes required for compaction. Additionally, the best combination of machinery and the most effective construction organization must be identified, and a skilled construction team must be involved to ensure proper execution of the work. 3.4 Special foundation treatment Soft soil foundations can be highly destructive. Although there is no completely unified consensus regarding their classification, in general, whenever external loads may cause harmful excessive deformation or insufficient strength in the soil foundation, it should be regarded as soft soil and treated with due attention, along with appropriate treatment measures. Generally, it can be divided into foundation treatment and embankment treatment based on the area to be treated. The methods used include: (1) In areas where the embankment height is not high and the layers of soft soil or silt are thin, such as the Anlin section, we employ methods such as sand bedding, replacement filling, counterpressure barriers, and stone dumping to displace the silt, thereby strengthening the embankment. ⑵For drained foundations, depending on the actual conditions in areas such as the Anlin section and the Meixian section, methods such as sand cushioning, bagged sand wells, sand piles, plastic sheet drainage, and replacement filling are employed for treatment. ⑶For complex foundation conditions in soft soil foundations or collapsible loess areas, such as the Fengmei section, methods including geotextile pads, gravel piles, reinforced soil piles, and dynamic compaction are employed for treatment. ⑷For the treatment of soft soil embankments, methods such as using geotextiles for cushioning and separation, adding geogrids, and using geogrids are employed. 3.5 Improving drainage facilities: To keep the subgrade in a dry, solid, and stable condition at all times, it is necessary to intercept surface water that could affect its stability and drain it outside the subgrade area, thereby preventing flooding, accumulation, and seepage. At the same time, groundwater that affects the stability of the subgrade should be intercepted, drained, and its water level reduced, with it being directed away from the subgrade area; this ensures that ditches, pipelines, bridges, and culverts along the entire route form a complete drainage system. For drainage systems in loess areas, attention should be paid to erosion control, seepage prevention, and soil and water conservation. ⑴On ordinary road sections, the subgrade drainage ditches include side ditches, catch ditches, and drainage ditches; it is necessary to pay attention to preventing seepage and erosion, and to take measures for reinforcement and leakage prevention ； In loess areas, using mortar-laid stones to reinforce roadside ditches yields good results ； The drainage ditch should be located at a distance of not less than 10 m from the edge of the trench top; its cross-section should not be too large, and the longitudinal slope of the ditch bottom should be between 0.5% and 2.0%. When guiding the water from the side ditch at the junction of filled and excavated areas, care should be taken to reinforce the outlet. ⑵In areas such as passes, deep cuts, high embankments, landslides, and sinkholes, slope runoff can be regulated and controlled through methods such as digging fish-scale ditches, creating horizontal grooves, planting grass, and trees ； Plant trees at the head of gullies to prevent upstream erosion by these gullies, which could damage the roadbed ； The route is laid in the valley, where dams are built to silt up the area and protect the base of the road embankments from erosion by water ； It can also be used for slope protection embankments, waterlogging ponds, water kilns, etc. 3.6 Preventive measures for bridgeheads, both ends of culverts, and expansion joints The problem of vehicle bounce caused by bridgeheads and both ends of culverts is a major issue that needs to be addressed on various road networks. To effectively resolve this problem, I believe the following steps should be taken: (1) Foundation reinforcement – To eliminate differential settlement and deformation in the areas around bridge abutments and the fill soil behind them, it is necessary to reinforce the foundation. Special treatment is required for certain types of subgrades, such as soft soil subgrades, collapsible loess subgrades, and riverine alluvial subgrades. Soft soil is characterized by high compressibility and significant deformation; for such subgrades, methods such as the use of plastic sheets or sand bags to facilitate drainage and reinforcement should be employed first. Additionally, reinforcement measures such as powder jetting piles or compaction piles should be used, based on the pressure exerted by the fill soil ； River alluvium, which has accumulated over the years, consists of a variety of sediment types; it is necessary to thoroughly analyze its composition, carry out proper design, and implement gradual reinforcement of the foundation ； For collapsible loess, proper drainage and waterproofing designs should be implemented, and reinforcement methods such as dynamic compaction should be used. ⑵Designing a transition section at the bridge head, by laying a transitional pavement or installing bridging plates over a certain length, allows the significant settlement that occurs in sections with flexible structures to be transferred to the bridge and structural elements, thereby preventing vehicles from bouncing during travel. ⑶For the selection of backfill material at the area behind the embankment in areas where excavation takes place, and given that the space there is extremely limited, high-quality local fill materials such as stone debris and gravel can be used ； In arch culverts with high fill heights, as well as at the junctions where culverts meet side walls, it is advisable to use filling materials with a large internal friction angle. During construction, care must be taken to maintain balance in the soil pressure exerted by the filling material to prevent any displacement, which could lead to engineering accidents. ⑷Necessary underground drainage facilities should be installed behind the structures. In addition, cushion layers can be placed beneath the pavement at the junctions of bridge abutments and fill areas, as well as in the transition zones, to prevent water from seeping beneath the pavement into the fill material. At the junctions where the fill material is gravel in the middle and soil on both sides, longitudinal and lateral drainage pipes should be installed to drain the water that seeps between the fill material and the reinforced foundation. ⑸Strengthen construction quality management, improve the construction quality of the road embankments at both ends of bridges and culverts, and refine construction techniques and methods as well as enhance management. To accommodate the narrow construction sites for embankments at the ends of bridges and culverts, the irregular shapes of the compaction areas, and the tight deadlines, specialized small-scale compaction machinery should be used. 3.7 Precautions for pavement base construction: ⑴ The construction of the sub-base and base layers must be carried out in strict accordance with the requirements of the \"Technical Specifications for Highway Pavement Base Construction\" (JTJ034-93). For expressways and first-class highways, except that the sub-base in contact with the soil foundation can be constructed using the road-mixing method, all higher layers must be constructed through centralized mixing and paving to ensure that the elevation, cross-slope, strength, and smoothness meet the design requirements. When using a paver for base layer construction, in order to eliminate the phenomenon of a higher center and lower edges, the lateral diagonal rods on both sides of the paver can be adjusted appropriately so that the screed is lower in the center and rises at the ends. ⑵Strengthen surface curing: after the surface construction is completed, use impermeable membranes or wet sand for curing, or apply asphalt emulsion spray for protection. In the absence of the above conditions, watering can be used for maintenance, and traffic must be strictly controlled. If it is not possible to close off the traffic, the passage of heavy vehicles should be restricted, with their speed not exceeding 30 km/h; at the same time, attention must be paid to any damage that other traffic facilities may cause to the underlying surface. If the grooves (pits) become loose, they should be repaired and compacted using the same material; alternatively, they can be filled with lean concrete, compacted, followed by the application of a layer of oilcloth before proceeding with road surface construction. Filling with loose granules is strictly prohibited. ⑶Strict control is exercised over the levelness of the base layer; before laying the surface layer, a 3m straightedge is used to check its levelness. Sections with poor levelness and deviations greater than 8mm must be leveled. Thoroughly clean the surface of the base layer before laying the surface layer to ensure it is clean, free of loose material and impurities. If there is any soil, it should be washed away with pressurized water. If there is loss of the surface coating asphalt or bottom seal coat in certain areas of the base layer, the affected area of the base layer surface should be cleaned thoroughly, after which surface coating asphalt should be reapplied or a new bottom seal coat should be applied. Carefully level and set out the lines to ensure that the elevation of the base layer and the elevation of the reference line are accurate. When the base elevation exceeds the allowable range, the higher areas must be leveled, while the lower areas can be filled in with the underlying layer. If the surface layer is contaminated by other processes prior to its laying, such as cement droplets forming hard residues on the surface, these should be removed promptly to ensure the flatness of the surface layer. 3.8 Mechanized paving process and control for asphalt pavements ⑴ Control of the paver’s reference line: When the paver performs automatic leveling, it requires an accurate reference surface (line). Two methods for establishing such a reference surface (line) are described below; users can choose the appropriate method based on the structure of the pavement and the location of construction. Its basic principle is: when height control is the primary concern, walking on a wire is the appropriate approach ； When controlling thickness is the primary focus, the floating reference beam method is adopted. Generally, the bottom layer uses the wire walking method, while the middle and top layers use the floating reference beam method. Laying the base course – the reference wire rope (wire walking) method involves installing reference wire ropes on both sides of the road surface; it should be noted, however, that the spacing between the rebar posts that support these wire ropes should not be too large, usually ranging from 5 to 10 meters ； Use two precision levels to measure the elevation of the control rebar; the rebar should be 1–2 mm higher than the designed elevation, and it is necessary to ensure that its elevation remains accurate throughout the paving process ； High-strength steel strands with a diameter of φ2mm–φ3mm are generally used; they are tightened using tensioners and mounted on the adjustment rods attached to the pillars. The deflection of the steel cables between every two steel pillars should not exceed 2mm, and the tension applied to these cables is usually around 800N ； The baseline should be as close as possible to the ironing plate in order to reduce the thickness increment ； To ensure continuous operation, each side of the steel wire rope should have three strands of 200–250 meters in length; by the time one section of the steel wire is completed, the next section has already been installed. Paving the intermediate and surface layers – the floating reference beam method is used; this beam helps to maintain a constant height difference between the front and rear parts of the paver, thereby ensuring the proper paving thickness and improving surface smoothness. Steel wires are also used in conjunction with this system on structures, as the thickness of the asphalt layer on those structures differs from that of the surface layer. The floating reference beam consists of 2–4 wheel frames, each 2–3 meters long, and each frame has 3–44 pairs of small wheels that move along the sub-base in front of the paver. At the rear of the floating reference beam is a slide plate measuring approximately 0.5m x 10m (commonly known as a sliding shoe), which slides on the surface of the paved layer. To reduce reference errors and the errors associated with the auto-leveling device, it is necessary to pay attention to the following during its installation and adjustment: the installation error of the slope sensor should be less than +0.1% ； The sliding base surface of the floating reference beam should be parallel to the paving base surface, with the same cross-slope value ； Check the operating pressure of the hydraulic system at any time to ensure it remains within normal limits ； Check at any time whether the paving thickness and cross-slope values meet the design values. ⑵For the control of the paving progress with a paver, it should pave continuously at a constant speed without any pauses; it is strictly prohibited to vary the speed. Changes in the paving speed inevitably lead to changes in the paving thickness. To maintain a constant thickness, it is necessary to adjust the thickness regulator, as well as the excitation forces of the rammer and the screed, along with the stroke of the tamping beam. However, manual adjustment relies on such adjustments, and changes in speed can lead to variations in the pre-compaction density after paving, thereby resulting in differences in the final compacted thickness and affecting the smoothness of the road surface. During the paving process, shutdowns should be avoided as much as possible; the points where shutdowns are necessary on a daily basis to interrupt paving should be located at the end of the structure, at the position designated for contraction joints. In the event of a shutdown midway, the paver’s screed should be locked to prevent it from sinking ； When the temperature is above 10°C, the downtime should not exceed 10 minutes. If the pause time exceeds 30 minutes or the mixture temperature is below 100, the joint should be re-sealed using the method for handling cold joints. ⑶Operation control measures for pavers: hire skilled paver operators and provide pre-job training ； During the paving process, the material transport vehicle should stop at a distance of 10–30 meters from the paver, put it in neutral gear, and move forward slowly under the push of the paver; meanwhile, a dedicated person should direct the unloading vehicle to unload the material ； Ensure the continuous operation of the paver’s feeding system, that is, maintain a stable, uniform, and consistent material level inside the idler wheels (conveyor wheels); it is advisable to keep this material level at 2/3 of the height of the blades above the central axis. If the interruption in paving is short and it is only due to the mixture in the hopper having hardened, the hardened mixture inside the hopper should first be cleared out, after which feeding can resume ； Assign a dedicated person to clean up the spilled granules promptly ； Before paving, the screed must be cleaned thoroughly; after adjusting its height and slope, preheat the screed. The preheating temperature of the screed should be close to that of the asphalt mixture; it can generally be heated to 85–90°C. 3.9 Compaction Quality Control The compaction of the asphalt concrete surface layer is usually carried out in three stages: initial compaction, intermediate compaction, and final compaction. ⑴Initial pressure, the initial pressure in the first stage* is usually referred to as the pressure stabilization stage. Since the asphalt mixture has been preliminarily tamped and compacted in front of the paver’s screed, and the temperature of the freshly laid mixture is high (usually around 140°F), a moderate level of compaction is sufficient to achieve good stability. Typically, a 6-8t two-wheel vibratory roller is used to compact the material 2-3 times at a speed of around 2 km/h. The roller’s drive wheel moves forward at a constant speed under static pressure; when moving backward, it travels along the tracks left by its forward movement to carry out vibratory rolling. Initial compaction can also be carried out using a combined steel-wheel-tire (four wide tires spaced equally apart) roller (with the steel wheels positioned near the paver). When moving forward, it performs uniform rolling under static pressure; when moving backward, it follows the tracks left by the wheels during forward movement and carries out vibrating rolling. ⑵Recompaction; the second stage of recompaction is the main compaction phase. At this stage, the specified degree of compaction must be achieved; therefore, re-compaction should be carried out at a higher temperature and immediately following the initial compaction. During the recompaction phase, the temperature should not be lower than 100–110°C. Typically, a two-wheel vibratory roller is used for vibration compaction, or heavy static two-wheel rollers as well as tire rollers with a weight of over 16 tons are employed in sequence. A combined steel-tire roller can also be used alongside vibratory rollers and tire rollers for compaction. The number of rolling passes is determined based on that obtained from the paving test section; it is usually not less than 8 passes, and the rolling method is the same as that used for initial compaction. ⑶Final pressure: The final pressure in the third stage is the last step to eliminate defects and ensure good flatness of the surface layer. Since the final pressure is required to eliminate the irregularities remaining on the surface during the recompaction process, the asphalt mixture also needs to be at a high temperature. The final compaction is usually carried out using a static double-drum roller and should be done immediately after the secondary compaction. The temperature at the end of final rolling should not be lower than 70, as specified in the specifications for asphalt pavement construction; final rolling should be completed at the highest possible temperature. At the construction site, well-organized rolling should involve rollers for initial rolling, re-rolling, and final rolling each rolling on sequentially connected sections while moving forward in line with the paving speed. Of course, during the actual rolling process, the roller will cross the boundary between the secondary compaction and the initial and final compactions ； To help the road roller operator easily identify the section they should roll, colored flags or other markers can be placed at the boundaries between the initial rolling, the subsequent rolling passes, and the final rolling. These markers can be moved depending on the temperature of the asphalt mixture and the number of rolling passes, thereby guiding the operator to proceed to the next section for rolling in a timely manner. ⑷To ensure that the rolling operations at each stage are carried out while the mixture remains in a stable state, the rolling should be performed according to the following rules: from bottom to top (along the longitudinal and transverse slopes) ； Static compaction first, followed by vibratory rolling ； For the initial and final compaction, double-drum rollers are used; for the initial compaction, combined steel-wheel/tire rollers can also be employed, while vibration rollers and tire rollers are used for the subsequent compactions ； During rolling, the drive wheel is at the front and the driven wheel is at the back ； Drive along the tracks left by the wheels moving forward when moving backward ； The rolling distance of the roller should be balanced with the paving speed of the paver, as the paver moves forward ； The roller turns back on the same cross-section, but in a stepped shape ； On the asphalt concrete layer that has been compacted on that day and has not yet cooled, no construction equipment should be parked there (including compactors left there temporarily), to prevent deformation ； Traffic can only be allowed to proceed once the compacted asphalt surface has fully cooled down. ⑸Rolling of the transverse joints is an important step in the process. Rolling of the transverse joints is a crucial part of this procedure. During rolling, a two-wheel roller should be used first for rolling in the transverse direction (that is, perpendicular to the center line of the road surface); if necessary, timber pads should be placed on the outside of the paving layer for the roller to move on. During rolling, the roller should be positioned mainly on the already compacted mixture layer, with its extension into the newly laid mixture not exceeding 20 cm. Then, with each pass, move about 20 cm toward the newly laid mixture, until the roller has compacted the entire newly laid surface. Then normal longitudinal rolling is carried out. When a cold longitudinal joint must be created where the adjacent paving layers have already been completed, a steel wheel roller can first be used to roll back and forth; the rolling width on the newly laid layer should be 15–20 cm, after which rolling is carried out horizontally along the longitudinal joint. Normal longitudinal rolling is carried out after the lateral rolling is completed. ⑹Compaction of longitudinal joints: The roller first moves over the already compacted road surface, compacting the newly laid mixture by 10–15 cm; then it compacts the newly laid mixture while moving 10–15 cm across the already compacted road surface, thereby compacting the joint tightly. 3.10 Measures for joint treatment ⑴ For longitudinal joints, at the point where two paving strips meet, there must be some overlap to ensure that this area has the same thickness as the surrounding areas. The width of the overlap should be consistent front to back. There are two types of lap construction: cold laps and hot laps. Cold jointing in construction refers to the joining of a newly laid layer with the compacted existing layer. When hot joints cannot be used in half-width construction, retaining plates should be installed or the edges should be trimmed with a cutter. Before laying the other half, the seam edges must be cleaned thoroughly, and a small amount of adhesive primer should be applied. During paving, it should overlap the previously laid layer by 5–10 cm; after paving, the mixture that has been placed on the upper half should be removed manually, followed by rolling. It should be noted that the loose thickness of the newly paved strip must be the same as that of the previous paved strip. Hot joining is generally used in construction when two or more pavers operate in a tandem. At this time, the mixtures in the two adjacent paving strips are still in a hot state before compaction; therefore, the longitudinal joints are easy to handle, and the connection strength is good. During construction, a strip 10–20 cm wide of the already laid mixture should be left unrolled as a elevation reference for the subsequent paving section; once that section is completed, both areas should be rolled together across the joint. Whether using the cold joining method or the hot joining method, the edges of the paving strip must be neat, which requires the machine to maintain its correct position while moving in a straight line or on curves. To this end, a guide line can be laid along one side of the paving strip, and a rod with a chain can be installed on the machinery; the driver simply needs to drive while keeping an eye on the chain, which should be aligned with the guide line. ⑵For transverse joints, the transverse joints of adjacent panels as well as those in the upper and lower layers should be offset by more than 1 meter. There are two types of transverse joints: oblique joints and straight joints. For the lateral joints in the middle and lower layers of expressways and first-class highways, diagonal joints can be used, while vertical straight joints should be employed in the upper layer; diagonal joints can be used for all layers of roads of other grades. When laying the joint, some hot mix can be placed on the compacted portion to preheat and soften it, thereby enhancing the bond between the old and new mixtures. However, the preheating mixture should be removed before starting the rolling. The overlap length of the diagonal joint is related to the layer thickness, generally ranging from 0.4 to 0.8 meters. The joint should be cleaned thoroughly and coated with a bonding agent. When the coarse aggregate particles in the mixture at the joint exceed the thickness of the compacted layer, they should be removed and replaced with fine aggregate. The diagonal joints should be thoroughly compacted and overlapped evenly. Flat joints should be tightly bonded, fully compacted, and have a smooth connection. The following methods can be used for construction: ① At the end of construction, the paver should lift its screed slightly about 1 meter before reaching the end of the area and then leave the site; the mixture at the end should be leveled manually before being rolled down. A 3-meter straightedge can then be used to check the levelness. While the mixture is still not completely cooled, any areas with insufficient thickness at the end should be removed vertically, so that a right-angle connection can be achieved during the next phase of construction. ②First, spread a thin layer of sand at the end of the designated paving section. After pouring the mixture, create a gap in the paved layer at the junction where the sand was spread; insert a wooden board or steel bar with a thickness equal to that of the compacted layer into this gap. Once compacting is complete, remove the sand that was spread there, sweep away all the sand, take out the wooden board or steel bar, and then spread the adhesive asphalt at the end before proceeding with paving. ③At the end of the designated paving section, a layer of burlap or kraft paper is laid first; the surface is then rolled into a slope. During the next construction phase, the portion covered with burlap or kraft paper is removed manually, and an asphalt layer is applied to that area before continuing with paving. ④First, spread a thin layer of sand at the end of the designated paving section, then lay down the mixture. Once the mixture has cooled slightly, use a cutter to trim the area covered with sand neatly and remove it; afterward, use a dry mop to absorb any excess cooled water. After the area is completely dry, apply tack coat asphalt at the end and then proceed with paving. It is not allowed to lay the mixture when there is water or moisture at the joints. For transverse joints, before continuing to lay the mixture at the joint, a 3m straightedge should be used to check the flatness of the end of the paved surface; if it does not meet the requirements, it must be removed. When paving the new mixture, the reserved height should be adjusted properly. After the construction of the joint paving layer is completed, a 3m straightedge should be used to check the levelness; any areas that do not meet the requirements must be addressed immediately while the mixture is still warm, in order to ensure the levelness of the road surface at the horizontal joints. 4 Conclusion: To ensure road surface smoothness and thus passenger comfort, attention must be paid from the initial stage of subgrade construction. All construction companies involved in highway projects bear an undeniable responsibility to strengthen construction management, improve construction techniques and methods, and enhance construction quality. Only in this way can problems be addressed at their root, ensuring both social benefits and quality. References: 1. “Pavement Engineering”, December 1989, People’s Transport Press. 2. “Subgrade Engineering”. 3. “Comprehensive Guide to Practical Techniques for the Prevention, Detection, and Repair of Defects in Highway and Bridge Engineering”, July 1999, Changchun Publishing House