Cracking of asphalt pavement is one of the common diseases on pavement. There are four main types of cracks: ① transverse cracks: ② longitudinal cracks; ⑧ Block crack: ④ Cracking. No matter what kind of crack, it is a kind of structural damage to the pavement, and it has no great influence on the driving function of the pavement in the early stage of its development. However, under the action of driving load, temperature and rain, its damage degree will further expand, eventually leading to the destruction of the pavement structure, leading to the reduction of driving function and affecting driving safety.

1 transverse crack

Transverse cracks refer to cracks almost perpendicular to the driving route, occasionally accompanied by a small number of branch joints. Transverse cracks are the most common cracks in asphalt pavement. Usually, the transverse cracks of asphalt pavement can appear through the surface layer, but sometimes the transverse cracks of semi-rigid itself do not affect the surface layer. In this case, although the transverse cracks at the grass-roots level already exist, it is impossible to know whether the pavement has been dug up or whether it has been nondestructive tested.

1. 1 reflection crack

Asphalt pavement is characterized by high strength and stiffness, poor deformability and easy cracking when temperature or humidity changes. When the base is cracked, if the surface layer is well bonded to the base, the surface layer will give the base a constraint, resulting in tensile stress and strain of the asphalt surface layer. Coupled with the stress superposition caused by traffic load, when the tensile stress or strain in the surface layer exceeds the allowable tensile stress or strain of asphalt, the asphalt layer will crack at the bottom corresponding to the cracks in the base. If the pavement is thin, cracks will develop to the surface of the pavement from bottom to top, forming "pavement reflection cracks".

1.2 temperature shrinkage crack

Temperature shrinkage crack is an important manifestation of transverse crack, which mainly includes low temperature shrinkage crack and temperature fatigue crack. Low temperature shrinkage cracks are caused by the rapid gradient change of asphalt surface temperature caused by low temperature or sudden drop of temperature. When the temperature stress exceeds the tensile strength of pavement materials, cracks will occur and lead to cracking. Low temperature shrinkage first occurs in local areas with low temperature and defective asphalt pavement quality. Starting from the surface and extending in the depth direction, the weakest subgrade section.

2 longitudinal cracks

Longitudinal cracks are cracks roughly parallel to the road centerline, sometimes accompanied by a small number of branch cracks. In half-filled, half-dug or high-filled sections, longitudinal cracks often appear due to poor compaction and uneven settlement of subgrade; Longitudinal cracks may occur if the joints between the old and new subgrade are not handled properly when the road is widened. The length of these two kinds of longitudinal cracks is generally long, and the depth can penetrate into the subgrade from the pavement, which is very harmful.

Three large cracks

Generally speaking, cracks with a crack spacing of 0.3-3m and an area of 0. 1- 10 square are considered as massive cracks. Block crack is also a common disease of pavement, which is common in expressways that have been in operation for many years. The main reasons can be divided into two categories: one is the low strength of the base, and the other is the lack of surface strength.

3. 1 Block cracks caused by low base strength

Under the load, the base is broken, and the load deformation of the broken base is large, reflecting that the surface layer leads to large-scale cracks and subsidence diseases. Generally speaking, there are many massive cracks caused by this reason.

3.2 Cylinder cracks caused by low surface strength

Sometimes the integrity of the base is intact, but the asphalt surface itself is not strong enough for various reasons, which will also lead to large-scale cracks. Of course, problems occur at the same time in the surface and base, which is more likely to cause large-scale cracks. In either case, the immersion of rain has played a catalytic role in the formation and development of mass cracks.

3.3 cracking

Cracking is a turtle-shaped crack in local areas of pavement: cracking is often accompanied by sinking and pumping mud. It is generally believed that cracking is the fatigue failure of pavement structure under repeated loads and the embodiment of insufficient structural strength.

(1) The strength of base course is too high, which leads to cracking.

Excessive base strength will not only lead to more dry shrinkage and temperature shrinkage cracks, but also make the reflection cracks more serious and produce cracks. This kind of cracking occurs because of the high strength of the base. When cracking, it starts from the base and then develops upwards.

(2) Cracking caused by poor adhesion between layers

One kind of crack is caused by poor adhesion between layers. That is, due to the poor adhesion between the asphalt layer and the asphalt layer below it, the upper and lower layers are separated, resulting in the asphalt mixture on the surface layer bearing the load and temperature alone, thus causing slight cracking.

(3) Cracking caused by fatigue

But there is another kind of crack on the upper surface, which is not caused by poor adhesion between layers. However, after several years of pavement operation, the pavement is subjected to the repeated action of traffic load and temperature rise and fall, which leads to the aging of asphalt mixture, the increase of stiffness modulus of asphalt mixture and the decline of fatigue performance.

4 preventive measures of asphalt pavement cracks

4. 1 Use reasonable materials with good crack resistance.

High-quality asphalt and dense asphalt concrete pavement are used, because the smaller void ratio makes the asphalt concrete pavement have small water permeability, good water stability, high durability, great resistance to natural factors and can delay the expansion of cracks. Semi-rigid materials with small dry shrinkage coefficient and temperature shrinkage coefficient and high tensile strength are selected for the base. Using rubber asphalt or polymer modified asphalt as the seal coat of asphalt concrete surface layer can improve the temperature crack resistance of surface layer.

4.2 Asphalt pavement should have sufficient thickness.

Using asphalt with low consistency and low temperature sensitivity can reduce or delay pavement cracking. The lower the temperature in the area where the pavement is located, the more serious the cracking is. The aging of asphalt materials is more sensitive to low temperature, which increases the possibility of pavement cracking. Increasing the thickness of asphalt pavement can reduce or delay the cracking of pavement, but it can't completely prevent the recurrence of cracking.

4.3 Improve the strength and stability of subgrade

Subgrade is the foundation of pavement structure. The subgrade has sufficient strength and overall stability, which provides an important guarantee for the pavement structure to bear the automobile load for a long time. The strength and stability of subgrade greatly affect the performance of pavement. Therefore, effective measures must be taken to deal with the key links that affect the stability and strength of subgrade, otherwise uneven settlement will occur, leading to pavement cracking.

4.4 Strict construction procedures and careful organization of construction.

During subgrade construction, the filling technology of subgrade should be strictly controlled, and the degree of compaction should be strictly checked during construction to ensure the strength of subgrade. During base construction, transverse joints and longitudinal joints should be fully compacted, and the water content should be strictly controlled during construction rolling. The water content of the mixture should not exceed the optimum water content required for compaction or the range allowed by the specification. After the rolling is completed, keep in good health in time. After rolling is completed, or at the latest, emulsified asphalt should be used as prime coat or seal coat immediately. After the completion of prime coat or seal coat, asphalt surface layer should be paved as soon as possible to reduce shrinkage cracks.

5 conclusion

In the construction of asphalt concrete pavement, as long as effective measures are taken, the construction is strictly regulated, the maintenance is scientific and reasonable, and the prevention and control are combined, the cracking problem of asphalt pavement can be effectively controlled, the quality of asphalt pavement can be improved and the service life can be prolonged.

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