1, asphalt quality problem
Due to the national urban infrastructure construction in recent years, there are many urban road construction projects and the construction funds are limited. Therefore, the thickness design of road structure layer and the use of materials are based on the principle of economy and applicability, without focusing on the change of traffic volume and service life. Like high-grade asphalt pavement, provinces and cities use imported asphalt in the upper layer, and domestic asphalt in the middle layer and ground layer. As far as domestic asphalt is concerned, there are not many manufacturers that can meet the specification requirements, and the number is very limited, which can not meet the needs of domestic construction scale.
2. Problems existing in design specifications
At present, the national flexible pavement design code still adopts deflection value control, and the standard axle load for pavement design is a single axle with two wheels, and the axle load is 100kN. Three indexes should be considered in asphalt concrete pavement design: allowable rebound deflection, bending tensile stress and shear stress. When paving asphalt on the branch road with small traffic volume, only allowable deflection value can be used for design; Allowable rebound deflection and shear stress are used to design asphalt macadam surface. The cumulative number of standard axle load and the cumulative number converted into standard axle load within the design life are used as control indexes.
In pavement design, on the one hand, the survey data of traffic vehicles serve the traffic capacity, and the problem of overload is not considered, which makes it impossible to obtain accurate axle load in the design, resulting in the situation that the accumulated standard axle load in the design life is inconsistent with the facts. In this way, for some roads, the cumulative number of standard axles is reduced from the beginning, which makes the design deflection value too large and the tensile stress of the base and low base too small, resulting in insufficient overall stiffness of the pavement and premature failure of the pavement.
In addition, driven by economic benefits, 80% of trucks are overloaded, and it is this part of overloaded vehicles that accelerates the damage of the road surface, prompting the road surface to crack, push and even partially sink.
3. The influence of climate
(1) low temperature cracking. Asphalt material has good stress relaxation performance at higher temperature, and the deformation caused by temperature rise and fall will not produce excessive temperature stress, but when the temperature drops sharply, asphalt material gradually hardens and begins to shrink. At this time, tensile stress will be generated at the bottom of semi-rigid base. When the stress relaxation of tensile stress asphalt mixture can't keep up with the increase of temperature stress, the stiffness of the mixture will increase sharply. Because the asphalt pavement is constrained, once the shrinkage tensile stress or tensile strain generated inside the pavement exceeds the tensile strength of asphalt mixture, the asphalt pavement will crack and produce cracks. Due to the limited width of asphalt pavement, the mutual restraint of shrinkage pavement structure is small, and the low temperature cracks are mainly transverse cracks.
(2) Temperature fatigue crack. This kind of crack mainly occurs in areas with large daily temperature difference. Due to the repeated rise and fall of temperature, the temperature stress of asphalt pavement is exhausted, which makes the ultimate tensile strain of asphalt mixture smaller. In addition, the aging of asphalt increases the stiffness of asphalt, reduces the stress relaxation performance, and finally reaches the ultimate tensile strength, resulting in cracks on the pavement. Asphalt pavement has the characteristics of softening at high temperature. Although the asphalt-aggregate ratio is reduced as much as possible in design and construction, and the high-temperature stability is maximized by using aggregate gradation, rutting will still occur under the long-term action of vehicles. Flooding usually occurs in high temperature weather, and the softening point of asphalt is not suitable due to the increase of temperature.
4. Problems existing in asphalt concrete mix design
According to the specification requirements, asphalt concrete mix design should go through four stages, namely, target mix design stage, production mix design stage, verification stage and trial mixing and paving stage. Each stage has clear requirements for the purpose to be achieved. During construction, some units compress two or three stages, and some units simply rely on experience for construction. Therefore, there is a big deviation between theory and practice, which leads to the inherent deficiency of asphalt concrete. On the other hand, due to the current situation, the construction period of the performance project is short, and the low-priced bid is won, and the gravel yard is not standardized. Most of the materials are borne by individual enterprises, with scattered yards and backward equipment, and the homogeneity and stability of materials are quite different. Although most units have done screening analysis before construction and tested and adjusted the mixture ratio during construction, it is impossible to be very accurate because of the great difference, which leads to some common diseases on the pavement.
5, asphalt concrete mixing temperature control
The mixing temperature of petroleum asphalt is required to be 120 ~ 165℃, but in fact, due to the quality of equipment and personnel, the mixing temperature control of some construction units is unstable. Too high temperature may lead to asphalt deterioration, and there is no viscosity to make asphalt concrete loose; If the temperature is too low, the asphalt mixture will be mixed unevenly, which will affect the gradation. These are also the causes of local looseness or other diseases on asphalt pavement. After the asphalt mixture is transported from the mixing station to the paving site, the temperature difference between the air and the mixture is generally greater than 120℃. In addition, the relative wind speed caused by speed is relatively large, which will cause the temperature of the mixture to drop greatly before it reaches the site. The cooling range gradually decreases from the outside to the inside, and the most serious cooling area appears at the contact surface between the pile surface and the manger. The severity of cooling depends on factors such as transportation time, speed, temperature and heat preservation measures.
6, asphalt concrete paving
At present, the section of paving equipment is widened, and asphalt mixture is transported from the middle to both sides by rubber wheels. Because of the long distance, segregation will inevitably occur, thus changing the production mix ratio of asphalt concrete. Secondly, because the cantilever of the screed is long from the center of the machine to both sides, it will be deformed with the increase of paving times, which also has a great influence on the control of the cross slope of the pavement.
In addition, in the process of unloading the mixture from the transport vehicle to the hopper of the paver, the fabric of the contact surface, especially the fabric of the contact surface of the two cars, is finally sent to the spiral distributor by the scraper in each car, that is, the surface "cold" material with the largest cooling range of each car is distributed centrally, and the surface material has a large cooling range, so the compaction degree is difficult to meet the requirements in the normal rolling process, which is the reason for the looseness, potholes and water seepage damage of the road surface.
7. Influence of parallel crossing operation on pavement quality
Many units participate in urban road construction, such as underground pipelines such as tap water, rainwater and sewage, heating, gas, telecommunications, electricity, cable and road lighting. Due to the parallel crossover operation, the construction period is tight, which affects the pavement quality. For example, after the soil foundation is rolled and formed, pipeline units dig trenches and lay pipes, and pipeline units generally do not pay attention to the compaction quality of backfill soil, resulting in local uneven settlement; Another example is that when asphalt concrete paves the bottom layer and middle layer, the road lighting construction unit has to lay light poles and cables, which can not close the traffic, resulting in serious road pollution, thus affecting the bonding between the layers of the pavement, especially when the asphalt surface layer is thin, the asphalt pavement will fall off, push and twist cracks under the action of high-speed driving load of vehicles.
Second, the prevention and control measures of pavement diseases
1, design
Judging from the current design code, the structural layer design is too thin, and the pavement base and subbase can not meet the role of traffic load. Pavement structure design can meet the requirements of asphalt pavement strength and bearing capacity, especially considering the influence of extra-large vehicle load on subgrade and pavement.
(1) When designing semi-rigid pavement, semi-rigid materials with good scour resistance, small dry shrinkage coefficient and wet shrinkage coefficient and high tensile strength should be selected as the base.
(2) High-quality asphalt with good relaxation performance shall be selected as the asphalt surface course. In the absence of high quality asphalt, measures should be taken to improve the performance of asphalt.
(3) On the premise that the stability meets the requirements, the asphalt with high penetration is preferred as the asphalt surface layer.
(4) Dense asphalt concrete is used as the asphalt surface layer.
(5) Appropriate asphalt pavement thickness should be adopted to ensure that semi-rigid base will not produce shrinkage cracks and temperature shrinkage cracks during its service life.
2. Architecture
Excellent design and reasonable construction period are the basis of high quality of buildings, and scientific construction is the guarantee of high quality. The first is the selection of materials, especially the storage yard should be fixed, and the materials supplied by 1 ~ 2 manufacturers should be selected to ensure the construction progress, so that the material gradation is always under control and cannot deviate too far from the gradation center line. In the process of construction, control from the following aspects:
(1) When semi-rigid base is rolled, the water content shall be strictly controlled, and the water content of the mixture shall not exceed the optimum water content required for compaction or be controlled within the allowable range of construction specifications.
(2) After the semi-rigid base is rolled, it shall be maintained in time.
(3) After the semi-rigid base is rolled or solidified at the latest, emulsified asphalt shall be used as prime coat or seal coat immediately.
(4) After the prime coat or seal coat is completed, the asphalt surface layer shall be paved as soon as possible. The choice of asphalt is very important. To choose asphalt that meets the requirements of various specifications, especially the penetration and ductility of asphalt must be strictly controlled. Due to the climate warming in recent years, low-grade asphalt should be selected as the asphalt label within the specified range. In addition, base oil and base oil asphalt should use the same asphalt as asphalt concrete, especially the selection of oil-stone ratio should consider the influence of base oil and base oil return. During asphalt concrete construction, there must be a special person responsible for traffic control, and non-construction vehicles are prohibited from going on the road to prevent oil leakage from road machinery and keep the road clean.
Step 3 maintain
(1) Take measures in advance to prevent possible diseases on asphalt pavement. According to the use and natural conditions of the pavement, some suitable materials can be selected in advance to prevent and treat the pavement. Through early and effective prevention, some diseases on the pavement can be avoided or delayed.
(2) When the asphalt pavement is damaged, it should be repaired in time. The experience of maintenance management tells us that "small holes are not mended, and big holes suffer". If some diseases on the pavement can be treated in time and effectively, it will not only cause great damage to the pavement, but also save maintenance and management funds and ensure traffic safety.
Third, the treatment methods of pavement diseases
1, pavement crack
(1) For longitudinal and transverse cracks caused by temperature shrinkage and drying shrinkage of pavement base, if the crack width is less than 6mm, it is advisable to clean the cracks with an iron brush, blow off the dust with compressed air, and then caulk and seal them with hot asphalt or emulsified asphalt.
(2) When the joint width is greater than 6mm, asphalt sand or fine asphalt mixture shall be filled, sealed with soldering iron after tamping, and then polished and swept evenly. Conditional can also use modified emulsified asphalt mixture for sealing.
(3) For cracks caused by insufficient strength of soil foundation or pavement base, the soil foundation or base should be treated first, and then the pavement should be repaired.
(4) Through technical and economic comparison, emulsified asphalt slurry seal coat, hot asphalt seal coat or geotextile shall be laid first, and then hot asphalt seal coat shall be applied to the cracks with slightly concentrated area and good subgrade strength.
2. The pit is loose
(1) If the pits are loose due to low-temperature construction, loose materials can be collected. When the temperature rises above 20℃, the loose parts will be cleaned and the oil injection seal will be redone. After spraying hot asphalt 0.8kg/m2 ~ 1.0kg/m2, sprinkle 3 mm ~ 5mm thick stone chips or coarse sand (5m3/100m2 ~ 8m3/100m2), and compact with a light roller. If it is in low temperature and rainy season, emulsified asphalt can also be used for sealing treatment.
(2) Loose diseases caused by excessive temperature and binder gasification should be removed and resurfaced.
(3) When the gene strength of the base or soil is insufficient, soft deformation and loose, the diseases of the base or soil should be treated first, and the pavement surface should be redone after reinforcement meets the requirements.
3, asphalt pavement pits
(1) The pavement base is in good condition, and the repair methods are as follows:
① Determine the scope and depth of the damaged part, and draw a contour line (square or rectangle) which is roughly parallel or vertical to the road centerline according to the principle of "round hole square compensation".
(2) According to the outline of the groove, it should be dug to the stable part of the pit bottom, and its depth should not be less than the maximum depth of the original pit, and the groove wall should be vertical.
(3) Remove loose parts, dust and sundries from the tank bottom and tank wall, and coat the clean tank bottom and tank wall with viscous oil.
(4) Fill in asphalt mixture (emulsified asphalt mixture should be used in wet or low temperature season), and fill and level the pit with single layer or double layer depending on the pit depth.
⑤ Compacted with a small compactor, the newly filled part should be slightly higher than the original pavement, and the double-layer filling should be compacted by layers.
⑥ Using hot repair and maintenance vehicle, heating the pit-side pavement with heating plate, repairing the heated and softened pavement, spraying emulsified asphalt, adding new asphalt mixture, stirring and paving, and compacting with roller.
(2) If the pavement base is damaged, the disease at the base should be treated first, and then the surface layer should be repaired.
(3) In the cold season of continuous rain and snow, in order to control the expansion of the potholes, the existing pavement materials can be used to temporarily fill the potholes, and then the potholes can be repaired according to the specification requirements after the weather improves.
4. Road surface subsidence
(1) For cracks and slight subsidence caused only by uneven settlement of the surface layer, if the soil foundation and base are dense and stable, the subsidence part can be brushed, and the subsidence part can be filled flush with the original pavement after spraying asphalt.
(2) For the settlement caused by the destruction of soil foundation or base structure, the soil foundation and base should be repaired first, and then the surface layer should be repaired.
(3) If the pavement is seriously damaged due to subgrade subsidence and the mineral aggregate has loosened and fallen off to form pits and grooves, it should be treated according to the pit and groove repair method.
(4) If uneven settlement occurs at the abutment back of bridge and culvert due to incompact fill, the following treatment methods can be adopted:
(1) Dig out the asphalt surface course, and redo the surface course after paving the base course in the subsidence part;
(2) For the abutment back fill with insufficient density, tamp it with tamping machine;
③ Soft soil foundation should be replaced with soil, and the depth of soil replacement depends on the thickness of soft soil foundation. Filling and replacement materials should be graded materials with high strength and good water permeability, such as gravel, gravel soil and industrial waste residue;
⑤ Grouting reinforcement is adopted.
Four. conclusion
In a word, the causes of urban road asphalt pavement diseases are various, and there are some shortcomings in design and construction. In practical work, a series of preventive and improvement measures should be taken according to the reasons, from pavement design, raw material entry to concrete construction, to fundamentally reduce the occurrence of asphalt pavement diseases and improve the quality of urban road construction to a new level.
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