First, the characteristics and causes of asphalt highway diseases
Highway asphalt pavement is a common pavement structure in China at present, which has the advantages of smooth surface, firmness, seamless, comfortable driving, wear resistance, low noise, short construction period, simple maintenance, good water absorption, suitable for phased construction and so on, and is widely used. However, due to the difference of asphalt concrete materials and the influence of design and construction level, asphalt pavement often appears eight common diseases, such as cracks, rutting, looseness, frost heaving and boiling, subsidence, crowding, flooding and cracking and boiling. The occurrence of these diseases has seriously affected the driving speed and safety, increased the wear and tear of automobiles and shortened the service life of asphalt pavement.
One is the crack. At the initial stage of asphalt pavement construction, various forms of cracks will inevitably appear. Early cracks will not affect the performance of asphalt pavement, but with the attack of rain and the action of external force, it will gradually cause structural damage to asphalt pavement. Cracks can be divided into three types: transverse cracks, longitudinal cracks and reticular cracks. The reasons are as follows: 1, transverse cracks, often due to the role of temperature stress, fatigue cracks appear on the pavement. This kind of temperature crack often starts from the surface with the largest temperature change rate, and quickly spreads downward, and causes asphalt aging with time. The crack resistance of asphalt pavement decreases year by year, and the temperature cracks also increase. 2. Longitudinal cracks are mainly caused by uneven foundation and fill, especially in the widening section of old subgrade. Due to the nonstandard treatment of soil steps, the thickness and compactness of layered filling are not strictly controlled. 3. Mesh cracks are mostly caused by seepage, scouring and pumping of surface water by early pavement cracks, which leads to settlement and deformation centered on cracks, and at the same time, new cracks or even fragmentation are produced.
The second is rutting. It is mainly the supplementary compaction of pavement structural layer and soil foundation under repeated driving loads, which causes the lateral displacement and cumulative permanent deformation of structural layer materials. There are four main reasons: 1, and the asphalt-aggregate ratio of asphalt mixture is too large; 2. Excessive surface wear; 3. Rainwater intrudes into asphalt concrete; 4. The base contains unstable interlayer, which leads to the formation of wavy ruts when the pavement is pushed sideways. 、
The third is looseness. Looseness is a common problem of asphalt pavement, which directly affects driving safety. Generally speaking, the aggregate particles on the surface layer fall off, the coarse and fine aggregates lose sand, the road surface is worn, the road surface is rough and hemp, and the surface layer is peeled off. If not handled in time, pits and grooves will be formed from the road surface down. The whole road surface may be loose. It may also appear in local areas, generally more serious in the wheel track area. There are seven main reasons for its formation: 1, less asphalt in asphalt mixture, low asphalt-aggregate ratio and poor adhesion between asphalt and aggregate; 2, low temperature construction, low degree of compaction, too large internal gap on asphalt surface; 3. The silt content of aggregate exceeds the standard, the aggregate particles are wrapped by dust, and the asphalt film cannot be bonded to the aggregate particles; 4. The temperature is too high when mixing, which leads to asphalt aging; 5. Loose surface cracks caused by soft foundation; 6. Wrong choice of acid aggregate, poor adhesion to asphalt; 7. Moisture gradually penetrates into the interface between asphalt and aggregate, which reduces the adhesion and cohesive force of asphalt.
The fourth is frost heaving and boiling. Mainly in the freeze-thaw period, due to the invasion of water, the water stability of subgrade soil is poor, and the water on the upper layer of subgrade freezes, which leads to the cracking of pavement. If it is not handled in time, when the frost heaving pavement melts completely, the pavement will boil under the weight of vehicles, which is a great hidden danger to driving safety.
The fifth is subsidence. Subsidence is the most common pavement deformation, which has the characteristics of large area and deep structure, and mainly appears at the junction of excavation section and filling and excavation. There are four reasons: 1, poor drainage of soil cutting, uneven settlement of subgrade under subgrade bed and local settlement of pavement; 2. The pavement strength can't adapt to the increasing traffic volume, and it is prone to fatigue damage; 3. The strength of subgrade or basement is insufficient or the strength of filling and excavation subgrade is inconsistent, and the subgrade or basement structure is damaged and causes subsidence under the action of vehicle load; 4. Uneven settlement of the bridge deck causes settlement and dislocation with the bridge deck.
The sixth is to take a bag. Refers to the local uplift of asphalt pavement. The reasons are: 1, the construction quality causes poor adhesion between the upper and lower layers of pavement; Asphalt mixture paving is uneven, and local fine materials are concentrated; 2. The base or the lower layer is not compacted, with insufficient strength, deformation and displacement; 3. On steep slopes or sections with poor flatness, asphalt pavement mixture is easy to accumulate to the lower part to form a package under the action of driving.
The seventh is flooding. The asphalt in asphalt mixture migrates upward to the surface layer in hot weather, but it can't penetrate to the lower layer in cold weather, so the phenomenon that asphalt accumulates on the pavement surface and forms a shiny asphalt film is rampant. There are five reasons: 1, improper mixture composition design, too much asphalt in the mixture or too small void ratio, and too much asphalt overflowing from the lower part to the pavement. 2. The mixing control of the mixture is not strict, the fine material content is too small, and the specific surface area of the mixture is small, so the asphalt consumption is relatively large. 3. Improper amount of adhesive layer oil, excessive spraying or uneven distribution. 4. The construction quality is poor, the mixture is separated when paving, and the local fine materials are too concentrated. 5, rainwater infiltration makes the lower asphalt and stone peel off, and the asphalt film peels off under the action of water, resulting in surface flooding.
Eight is cracking and pumping. There are two main aspects: internal cause and external cause. Internal factors come from the pavement itself, the quality of base construction is poor, the void ratio of asphalt pavement is too large, and the immersed rainwater stays on the surface, resulting in local segregation and cracks on the pavement surface; The external cause is heavy traffic flow. Under the action of overload and high frequency dynamic load, the pavement surface deflection and structural layer stress increase significantly, leading to pavement cracking.
Second, the maintenance essentials of asphalt highway pavement diseases
The maintenance of asphalt pavement should focus on prevention. In the early stage of the disease, it is necessary to find it in time, start early, take preventive maintenance, minimize the hidden dangers of the disease, and prevent "minor illness from delaying serious illness". The point is "five o'clock". First, the sand escape point, the area of which is less than 10㎡, has a honeycomb surface due to asphalt stripping. The second is the local settlement point, where the settlement exceeds 2㎝ in the vertical and horizontal direction 1m, and the traffic jumps obviously. The third is the pumping point, the pavement cracks, water permeates the base, and the base washes the pump slurry. Fourth, there are ups and downs, and the upper layer of the road surface moves locally longitudinally to form a washboard, and the lateral movement appears rutting. Fifth, there are cracks. Cracks such as transverse joints, longitudinal joints, flexible joints and rigid joints (such as bridge end transition slab joints) are staggered, and the staggered height difference is greater than 5㎜.
There are four main methods of pavement maintenance. One is to glue water to prevent leakage. It is mainly aimed at the network cracks on the pavement, but there is no obvious deformation or pumping. A thin layer of repair glue can be used to prevent water seepage. Second, caulking is waterproof. When cracks appear on the pavement, but there is no obvious dislocation or gnawing edge, hot asphalt can be used for waterproof treatment. The third is hot baking and patching. For cracks, honeycombs, ruts and other points where pavement deformation is not serious, hot baking can be used, new materials can be added appropriately, and the mixture can be mixed evenly by hand and compacted and strengthened. The fourth is digging and filling. If the pavement diseases have spread to the middle and lower surface layers, or even to the grass roots, then all the diseased areas must be dug out and filled in layers. When repairing asphalt pavement, we must pay attention to the following three aspects.
First, hot asphalt caulking repair should pay attention to three points. 1. Before caulking, clean up the sundries and dust in the gap with a blower and an iron hook; 2. When hot asphalt is heated, the temperature should be above 130℃ to maintain its good fluidity; 3. After pouring is completed, when the asphalt is still plastic after cooling to about 60℃, remove the excess asphalt on the surface with a shovel to avoid polluting the pavement.
Second, pay attention to three points when repairing the surface layer. 1. When the pavement with diseases is hot-dried, the temperature must be above 100℃ before the operation can be started; 2. Use an iron rake to loosen the surface of the diseased pavement, remove the large aggregate on the surface, and ensure the repair effect; 3. After adding new materials, comb them evenly and compact them.
Three, pay attention to four points when digging and filling. 1. After chiseling the diseased layer and removing the garbage, blow the repair interface thoroughly with a high-pressure blower to ensure that the repair bed surface is clean and free of sundries and floating ash; 2. The oil spilled on the adhesive layer should be all over the interface, and there should be no flow and no dead angle. 3. The thickness of layered filling should not be greater than 6㎝, and the surrounding joint sealant is slightly higher, and it should be compacted by layers, and the degree of compaction should be above 95%; 4, after the completion of the filling, to use cold glue around the seam, in order to waterproof infiltration.
Three, the prevention and control measures of asphalt highway diseases
The diseases of asphalt concrete pavement are caused by many factors, including design reasons and construction reasons. According to the characteristics of early diseases of asphalt concrete pavement at present, the following three suggestions are put forward.
One is to reduce the thickness of asphalt pavement appropriately. Mainly for the following four considerations. 1, the bearing capacity of semi-rigid base asphalt pavement structure can be borne by semi-rigid material layers (base and subbase), and it is not necessary to use thickened surface layer to improve the bearing capacity. 2. In order to improve the performance of asphalt pavement, high-quality asphalt is used instead of thick asphalt pavement. 3. Cracks in asphalt pavement are not only reflection cracks, but also temperature shrinkage cracks in asphalt pavement itself under normal construction conditions. Generally speaking, thick asphalt pavement is easy to cause rutting. At the same time, we should strengthen the waterproof design of asphalt pavement and choose reasonable base and subbase structure.
The second is to strictly control the quality of asphalt mixture. Asphalt should be high-quality domestic or imported asphalt with good high and low temperature performance, aging resistance, low wax content and high viscosity. Aggregate should be selected with rough surface, hard stone, strong wear resistance, good embedding effect and good adhesion with asphalt. At the same time, the gradation of asphalt mixture should be scientifically designed according to the local climate and traffic conditions, and the road performance such as high temperature stability, fatigue performance, low temperature crack resistance and durability should be taken into account as much as possible.
Third, strictly control the construction quality. According to the requirements of total quality management, asphalt pavement construction must establish and improve an effective quality assurance system, implement target management and process management, clarify responsibilities, strictly check, control and evaluate the quality of each working procedure in the whole construction process, strictly control the mixing quality of asphalt mixture, ensure the smoothness of the top surface of the base, reasonably spread the prime coat and the adhesive layer, improve the paving quality of the surface layer, and ensure that it meets the quality standards.
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