Possible Problems With Asphalt Pavements

1. Rutting on asphalt pavement

Rutting is the cumulative permanent deformation caused by the lateral displacement of the structural layer material due to the supplementary compaction of the pavement structural layer and soil foundation under the action of repeated driving loads. Rutting is a kind of deformation, which refers to the longitudinal strip-shaped grooves on the road surface along the tracks of running wheels, with a depth of more than 1.5cm. Rutting is a belt-shaped groove formed by the accumulation of permanent deformation on the road surface under repeated driving loads. Ruts reduce the smoothness of the road surface. When the ruts reach a certain depth, due to the accumulation of water in the ruts, it is very easy for cars to slip and cause traffic accidents. Rutting is mainly caused by unreasonable design and severe overloading of vehicles. The main factors affecting the rutting depth of asphalt pavement are the internal factors of asphalt pavement structure and asphalt concrete itself, as well as the external factors such as climate, traffic volume and traffic composition.

The main causes of rutting are:

(1) The bituminous mixture ratio is too large

(2) Excessive surface wear

(3) Rainwater intrudes into the interior of asphalt concrete

(4) Due to the unstable interlayer in the base layer, the pavement is pushed laterally to form a wave rut.


2. Move the package

It is mainly caused by insufficient shear strength of asphalt mixture pavement under horizontal load. The internal reasons leading to the insufficient shear strength of this kind of asphalt mixture mainly include: too much oil used in the mixture, too much fine aggregate or filler, improper selection of asphalt grade, poor surface smoothness before the asphalt mixture is paved, The smooth contact between the upper and lower layers, no interlayer cohesion, etc., the actual reason is the combined effect of one or several reasons. The external reasons may be the long time of high temperature in summer, heavy traffic volume, and slow speed, especially on road sections with more brakes, which are prone to shifting and congestion.


3. Oily

Oil bleed is the result of excess asphalt inside the asphalt mixture migrating to the surface of the asphalt pavement under the action of vehicle loads. The main reason for oil flooding is that the amount of asphalt is too large or the residual void ratio of the compacted asphalt mixture is too small.

4. Cracks

The main reasons for asphalt pavement cracking can be divided into two categories: one is the structural damage cracks caused by the driving load, which are generally called load-type cracks. The other is mainly due to temperature changes in the asphalt pavement temperature cracks, including low-temperature shrinkage cracks and fatigue cracks, generally known as non-load cracks.

1) Non-load cracks

a. Transverse shrinkage cracks

The asphalt mixture structure layer on the pavement surface is directly affected by temperature changes. When the temperature stress accumulates to exceed the ultimate tensile strength of the asphalt mixture, cracks will appear on the pavement to release the stress. Additionally, asphalt near the surface is more susceptible to aging than interior asphalt. The ultimate tensile strain of the asphalt mixture is small, and the stress relaxation becomes poor, which is also an important factor that is prone to cracks. Asphalt materials have good stress relaxation properties under relatively high-temperature conditions, and the deformation caused by temperature rise and fall will not cause excessive temperature stress. However, when the temperature drops sharply, asphalt materials gradually harden and begin to shrink. Due to the limited width of the asphalt pavement, the shrinkage is limited by the mutual constraints of the pavement structure, so the low-temperature cracks are mainly transverse.

b. Temperature fatigue cracks

It is the asphalt concrete layer that produces low-temperature cracks, and such cracks mainly occur in areas with large daily temperature differences. Due to the temperature stress fatigue of the asphalt pavement due to repeated temperature fluctuations, the ultimate tensile strain (or stiffness modulus) of the asphalt mixture becomes smaller, and the aging of the asphalt increases the stiffness of the asphalt and reduces the stress relaxation performance, finally reaching Ultimate tensile strength cracks the pavement.

c. Reflective cracks

Asphalt materials have good stress relaxation performance under relatively high-temperature conditions, and the deformation caused by temperature rise and fall will not produce excessive temperature stress. However, when the temperature drops sharply in winter, the soil foundation and pavement base will expand due to temperature changes and freezing in winter, causing cracks in the road base and base and reflecting to the asphalt surface layer. The stress relaxation of the asphalt mixture cannot keep up with the increase in temperature stress. When the stiffness increases sharply, cracking will occur if the ultimate strength or ultimate tensile strain of the mixture is exceeded. That is due to the shrinkage of the semi-rigid base stabilized by cement, lime, and fly ash or the combined effect of the stress concentration in the crack of the cracked semi-rigid base and the low-temperature shrinkage and load of the asphalt surface. Temperature cracks are more frequently generated. These cracks are actually composite cracks of temperature shrinkage cracks and reflective cracks of semi-rigid base shrinkage cracks.

(1) Reflective cracks and corresponding cracks on the semi-rigid pavement

In winter or in cold regions, under the well-bonded asphalt surface, the horizontal displacement of the cracked semi-rigid base causes a large tensile stress or strain in the surface directly above the crack. The layer is usually hard, and it can only bear small tensile stress or tensile strain, so it is easy to be cracked, and the crack propagation path is from bottom to top. The thinner the asphalt pavement, the earlier and more reflective cracks will be formed.

(2) Reflective cracks or corresponding cracks caused by shrinkage cracking of the semi-rigid base

For the newly laid semi-rigid base, as the moisture in the mixture decreases, shrinkage and shrinkage stress will occur; the faster the moisture is reduced, the greater the shrinkage stress and shrinkage strain will be. The asphalt surface layer is paved on the semi-rigid base layer that has already produced shrinkage cracks. In the case of a thin asphalt surface layer, the cracks in the semi-rigid base layer will cause the bottom of the surface layer to crack first due to temperature stress, and reflective cracks will be formed quickly. .

d. Freeze shrinkage cracks

Frost shrinkage cracks are mainly cracks caused by subgrade frost heaving and shrinkage. These cracks are most common where the pavement meets the shoulder.
Factors affecting the low-temperature cracking resistance of asphalt mixture The main factors affecting the low-temperature cracking resistance of asphalt mixture are: material properties such as asphalt temperature sensitivity, time sensitivity, aging performance, etc., pavement structure geometric dimensions such as surface layer thickness, etc., temperature and other environmental factors such as temperature difference.

2) Load-type cracks

(1) The structural failure cracks of asphalt pavement are mainly caused by the traffic load. Under the action of the wheel load, when the tensile strength of the semi-rigid base material is greater than that, the bottom of the semi-rigid base will crack quickly. Under the repeated action of traffic load, the cracks at the bottom will gradually expand to the upper part, and the asphalt pavement will also be cracked and damaged. The main factors affecting the tensile stress are the thickness of the surface layer, the thickness of the base layer itself, the elastic modulus of the base layer, and the elastic modulus of the underlying layer. Different thicknesses of asphalt pavement and semi-rigid base were selected, and the relationship curve between the tensile stress at the bottom of the semi-rigid base and the elastic modulus of the semi-rigid material was obtained through experiments.

(2) The use of semi-rigid materials as the sub-base under the semi-rigid base can significantly reduce the tensile stress on the bottom surface of the base due to the driving load, even less than the tensile stress generated by the bottom of the semi-rigid sub-base, which is a good bearing for the semi-rigid base. The repeated action of traffic load is very beneficial.

Cracks are caused by:

(1), properties of asphalt and asphalt binder

This is the most important reason for the temperature cracking of asphalt pavement. The variety and grade of asphalt are also important factors affecting the cracking of asphalt pavement. In long-term practical experience, asphalt with high viscosity and low consistency is selected, which has low-temperature sensitivity and can delay the occurrence of temperature cracks.

(2), the nature of the base material

The smaller the shrinkage of the base material, the fewer cracks in the surface layer. It is beneficial to have penetrating oil on the base layer to strengthen the bond with the surface layer and resist cracking. The type of base material has a significant impact on the crack rate of the asphalt surface layer.

(3), climate conditions

The asphalt has not reached the quality standard suitable for the climatic conditions and use requirements of the region, and its low-temperature deformation resistance is poor, resulting in shrinkage and cracking of the asphalt surface layer at low temperatures. Improper foundation treatment and uneven rolling of the roadbed lead to uneven settlement of the roadbed.
(4), traffic volume and vehicle type [23] With the rapid development of transportation. The strength of the original pavement is increasingly insufficient, and the pavement cannot meet the needs of the rapid growth of traffic volume and the obvious increase of vehicle load. The fatigue failure of the asphalt pavement occurs prematurely, and the asphalt pavement cracks quickly.

(5) The original structural design was unreasonable, various unfavorable factors were not fully considered, the construction quality was not good, the thickness of the asphalt pavement surface layer was insufficient, the quality of the raw materials of the asphalt pavement did not meet the requirements of the design specifications, and the strength of the pavement obviously could not meet the driving requirements. Under the action of driving, especially the frequent rolling of oversized tonnage vehicles, the asphalt pavement cracks quickly.

(6), construction factors

It mainly refers to the rolling water content of the semi-rigid base material, the exposure time after the semi-rigid base is completed, and other factors.


5. Loose asphalt pavement

Looseness is a road surface disease that directly affects driving safety, and looseness may appear on the entire pavement surface. It may also appear in local areas, but due to the driving effect, it is generally more serious in the wheel tracks. The main reasons for it are:

(1) Pavement damage caused by uneven settlement of local subgrade and base;

(2) The gravel contains weathered particles, which will cause asphalt stripping after water intrusion;

(3) As the use time increases, the bonding performance of the asphalt binder itself decreases, which promotes the wear of asphalt in the contact part between the surface layer and the tire, resulting in a decrease in asphalt content and loss of fine aggregate;

(4) Mechanical damage or oil pollution.


Loose asphalt pavement


6. Water damage to asphalt pavement

In the presence of moisture, the asphalt pavement is subject to the repeated action of traffic load and temperature expansion and contraction. On the one hand, the moisture gradually invades the interface between the asphalt and the aggregate, and at the same time, due to the effect of hydrodynamic force. The asphalt film gradually peels off from the surface of the aggregate, which leads to the loss of cohesion between the aggregates and pavement failure. The causes of water damage to asphalt pavement mainly include materials, design, construction, soil foundation and base, and overloaded vehicles. Peeling (loose) asphalt pavement peeling refers to the lamellar peeling off of the pavement surface, with an area of more than 0.1 square meters. The main cause of peeling asphalt pavements is water damage.


7. Frost heaving and slurrying of asphalt pavement

Frost heave and muddying of asphalt pavement mainly occur during the freeze-thaw period, because of water intrusion and poor water stability of roadbed soil. Due to the effect of freezing, the moisture accumulated in the upper layer of the roadbed freezes and causes the road surface to swell and crack. Road grouting is the result of the combined effects of five main factors: water, soil quality, temperature, road surface, and traffic load. Among them, water, soil, and temperature constitute the three natural factors of mud-turning, and it is impossible to form mud-turning without any one of these factors.


8. Subsidence of asphalt pavement

Subsidence is the most common type of pavement deformation. It is characterized by a large area and involves deep structural layers. It mainly occurs at the excavation section and the junction of filling and excavation. The main reasons for its generation are:

(1) The drainage of the soil cutting is not smooth, and the subgrade under the roadbed is too wet to produce an uneven settlement, causing local subsidence of the road surface;

(2) The strength of the pavement cannot adapt to the increasing traffic volume, and fatigue damage is prone to occur:


(3) The strength of the subgrade or the base is insufficient or the strength of the filled and excavated subgrade is inconsistent. Under the action of the vehicle load, the subgrade or the base structure is damaged and causes subsidence;


(4) The settlement of the road surface at the bridgehead is not uniform, which causes subsidence and misalignment with the bridge deck.


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