1. Pavement thickness design problem

The basis of pavement thickness design is the cumulative equivalent axle times in the design period. The author thinks that the actual axle load of the vehicle is much larger than the design axle load, whether calculated according to the standard axle load or the non-standard axle load. Therefore, the actual equivalent axle number of the designed pavement is much larger than the cumulative equivalent axle number of the design year as the design basis. That is to say, one of the crux of the early cracking damage of newly-built pavement at this stage is that the highway has reached the cumulative equivalent axle number in the design life in a short time (such as 1 ~ 2 years).

2. Grass-roots influence

The strength and stability of base course are related to the strength and stability of surface course. The loose paving coefficient (or elevation) of the base is not strictly controlled, which leads to the secondary repair layer. Because the secondary repair layer can not be closely combined with the lower base, it is small in thickness and easy to loosen, thus causing pavement damage. Uneven foundation strength, mixed subgrade fillers or poor compaction will lead to uneven settlement and poor smoothness of base course, which will lead to uneven thickness and large dispersion of concrete surface course, and will concentrate pavement stress under traffic load and temperature warping stress. When the stress exceeds the ultimate strength, cracks will occur at the weak thickness.

3. Influence of pavement manhole and pipeline

(1) pond slag around pavement manhole (mixture of aeolian fossils and soil; Slag is crushed stone after blasting, with large particles, which is a mixture of earth and rock. ) Filling is the weak link in actual construction. When bulldozers push pond slag, construction units often push large pieces of pond slag to the edge of inspection wells, and rollers rarely press them here, resulting in weak stress areas. In actual construction, pavement cracks mostly occur at inspection wells.

(2) The leakage of rainwater and sewage pipes on the pavement will scour the subgrade, especially quicksand.

4. The influence of concrete quality

(1) When different grades and varieties of cement are mixed, the hardening time and shrinkage are different, and cracks will also form.

(2) The influence of aggregate quality.

(3) Influence of mixing quality. If the stirring time is too short and the stirring is uneven, the surface strength is different, the hardening time is different and the shrinkage is different, resulting in cracks; If the mixing time is too long, the aggregate will be easily broken and separated, which will affect the strength of concrete.

(4) The influence of vibrating quality. Insufficient vibration can easily lead to pores and honeycombs in concrete, which will lead to stress concentration and cracks under the action of traffic load and natural factors; If the vibration is excessive, the coarse aggregate will sink and the concrete will separate, thus affecting its strength.

(5) Influence of maintenance. The curing of concrete is very important for its early strength growth and prevention of shrinkage cracks. Therefore, it is necessary to strengthen the early curing of concrete, and cover and spray water on the surface as soon as possible after manual grouting. At the same time, the curing time must be guaranteed. The actual curing days depend on the increase of concrete strength, which should generally be 14 ~ 2 1 day.

5. Influence of transverse shrinkage joint quality

The purpose of setting transverse shrinkage joint is to reduce shrinkage stress and warping stress. Slotting construction is an important link in concrete construction. If it is not strictly controlled, it will easily lead to cracks.

(1) cutting time. When the concrete reaches 25% ~ 30% of the design strength, the joint cutting machine should be used for cutting. If the seam is cut too early, the coarse aggregate will jump out of the mortar; If the joint is cut too late and the tensile stress is greater than the allowable value of concrete, the concrete slab will crack. When the temperature is high, the strength of concrete increases rapidly, and the cutting time should be advanced. The temperature difference is large, and the cutting time should be earlier. The general principle of sewing time is "cutting as soon as possible, sooner rather than later". A sewing machine with small model, fast rotating speed and small vibration should be adopted, and the concrete can be cut within a few hours after pouring.

(2) Cutting depth. The cutting depth should be controlled at 1/4 ~ 1/5 of the plate thickness. If the cutting depth is too deep, it is difficult to ensure the load transfer ability between plates. Cut too shallow, the strength of concrete section is not weakened enough, and irregular cracks will appear on the surface.

(3) Joint materials. Joint material is the main component to ensure the normal use of concrete slab. If it is not handled well, it is easy to have problems.

6. Influence of tie rod and dowel bar

There are two main points in the quality control of dowel bar of expansion joint: one is that one end of dowel bar should be coated with asphalt and sheathed to ensure its expansion and contraction distance; Second, the dowel must be parallel to the road surface to ensure its telescopic direction. If the former is not handled properly, the concrete at the joint end will crack. The dowel bar is not parallel to the road surface. When the concrete slab expands and contracts, the dowel bar produces compressive stress on the concrete slab, which leads to the concrete slab cracking.

(1) transverse construction joint dowel bar. The dowel bar of transverse construction joint is made of round steel, half of which is coated with asphalt, allowing sliding. In practical application, construction units often use asphalt-free rebar for convenience. The concrete pouring time on both sides of the construction joint is different, the contraction produced in the condensation process is not synchronous, and the dowel bar cannot be extended and contracted, forming a weak link. When the concrete slab shrinks under load or natural factors, this weak link will produce cracks.

(2) Longitudinal seam pull rod. There are generally two kinds of longitudinal joints: longitudinal contraction joints and longitudinal construction joints, and tie rods should be set for both longitudinal joints. The tie rod should be made of steel bars, and the length should meet the design requirements, so as to ensure the bond between the tie rod and concrete, and at the same time, the diameter and spacing of the tie rod should be ensured. In this way, the pull rod can not provide enough tension, and when the concrete panel shrinks, the longitudinal joints are pulled open to form cracks.

7. Cracks caused by temperature change

Concrete has the characteristics of thermal expansion and cold contraction. When the external environment or internal temperature changes, concrete will be deformed. If the deformation is limited, stress will occur in the structure, and when the stress exceeds the tensile strength of concrete, temperature cracks will occur. The main difference between temperature crack and other cracks is that it will expand or close with the change of temperature. The main factors causing temperature change are annual temperature difference, sunshine, sudden cooling, hydration heat, steam curing or improper winter construction measures.

8. Opening traffic prematurely

It was opened to traffic too early, and the strength of concrete was still low, which was not enough to bear the traffic load. Under the load, concrete pavement is prone to cracks.

To sum up, the causes of cracks in cement concrete pavement are not only related to the links such as design and construction before pavement formation, but also closely related to the use and maintenance after pavement formation. Therefore, in order to eliminate the common quality problem of cement concrete pavement cracking, prolong the service life of cement concrete pavement and improve the investment benefit, it is necessary for all parties in design, construction and maintenance management to perform their respective functions strictly according to industry norms and standards and combining with engineering practice. I believe this stubborn disease will be cured.