[Introduction] This paper analyzes the formation, harm, types and causes of cracks in highway asphalt pavement, and puts forward the treatment measures for cracks. Yang Zhen (Guangdong Jingtong Highway Engineering Construction Group Co., Ltd.)
This paper analyzes the formation, harm, types and causes of cracks in highway asphalt pavement, and puts forward the treatment measures for cracks.
Keywords: asphalt pavement low temperature reflection crack treatment
Introduction to 0
Because asphalt pavement has the advantages of smooth surface, seamless, comfortable driving, wear resistance, small vibration, low noise, short construction period and simple maintenance, it has been widely used in expressway construction in China. However, the asphalt pavement of many newly-built expressways cracked at the initial stage of construction, which greatly affected the service life of asphalt pavement.
According to the causes of asphalt pavement cracks, cracks can be divided into load cracks and non-load cracks. Load-type cracks are mainly due to poor pavement design or construction reasons, and the strength of the structural layer itself is not enough to adapt to the increasing traffic volume and axial load. At first, it generally showed longitudinal cracks, and later it developed into network cracks. Because semi-rigid base asphalt pavement design is widely used in China, this load crack is not the main one. There are mainly two kinds of non-load cracks, one is the reflection crack formed by the cracking of the base course on the pavement, and the other is the low-temperature crack produced by the asphalt pavement itself. The early pavement cracks of newly-built expressways in China are mainly non-load cracks, which should be paid great attention to.
Formation, influencing factors and preventive measures of low temperature cracks on 1 asphalt pavement
The asphalt structure layer located on the pavement surface is directly affected by the temperature change. When the temperature drops, the asphalt surface will shrink and deform. Because there is no shrinkage joint on the asphalt pavement, this deformation will be constrained by the friction of the base layer on the pavement and the shrinkage deformation of the infinite continuous slab on the pavement, which will cause tensile stress in the asphalt pavement. When the tensile stress exceeds the ultimate tensile strength of asphalt concrete due to the sudden drop of air temperature, cracks will appear on the pavement to release the stress.
The main factors affecting the temperature crack of asphalt pavement are:
1. 1 asphalt properties
1. 1. 1 is first affected by asphalt oil source. Relatively speaking, heavy oil asphalt can withstand large tensile strain at low temperature, so its crack resistance is much higher.
Asphalt penetration is 1. 1.2. When the oil source of asphalt is the same, the asphalt with high penetration has lower stiffness modulus and fewer cracks than hard asphalt.
The ductility of 1. 1.3 asphalt, especially the ductility of 15℃, has a certain relationship with cracking. Relevant research shows that the use of 15℃ high ductility asphalt can reduce the occurrence of low temperature cracks.
1. 1.4 aging performance of asphalt. After aging, the penetration and ductility of asphalt will decrease, and asphalt will become hard and brittle with low resistance.
1. 1.5 will greatly reduce the ability of temperature cracking, resulting in serious pavement cracking.
1. 1.6 The asphalt with high wax content has increased brittleness, high temperature sensitivity, small tensile strain and poor low-temperature cracking resistance.
1.2 composition of asphalt mixture
1.2. 1 asphalt content Generally speaking, the stress relaxation performance of the mixture increases with the increase of asphalt content, but the shrinkage strain of the mixture will also increase accordingly.
1.2.2 mineral aggregate composition gradation, the temperature stress growth law of the mixture with different gradation composition is quite different. The mixture with coarse particle size and large porosity has many micro-voids, and the stress relaxation limit temperature decreases, which reduces the temperature stress. As the mineral powder is added to the asphalt mixture, the asphalt will become more viscous after being combined with the mineral powder, and the ability to resist low-temperature cracking will decrease.
1.2.3 The variety of aggregate and the adhesion between aggregate and asphalt have great influence on the occurrence of cracks. If the adhesion is small, the binding force between asphalt and aggregate is low, the tensile strength of asphalt concrete becomes small, the ability to resist temperature shrinkage stress is low, and cracks are easy to form.
1.3 construction cracks, all kinds of vertical and horizontal construction, due to poor joint treatment, the ability to resist temperature shrinkage stress in this part is low, resulting in the release of temperature shrinkage stress concentrated here, resulting in cracks. The temperature of rolling and paving is too low, and the tensile stress of mechanical equipment on asphalt pavement causes micro-cracks on the surface of new pavement, which may crack in these places in the future. The classified query of periodical articles is in the periodical database.
In view of various factors causing low-temperature cracks, the following measures should be taken to improve the crack resistance of asphalt concrete and reduce the occurrence of cracks.
1.3. 1 Select high-quality heavy oil asphalt with good oil source, which should have low wax content, good high and low temperature performance and anti-aging during construction.
1.3.2 Alkaline macadam with high adhesion to asphalt shall be crushed by impact. At the same time, 1%~2% cement is added to the mixture to improve the adhesion between asphalt and aggregate, thus improving the crack resistance of asphalt concrete.
1.3.3 Strictly control the mix design, optimize it as reasonably as possible, and strictly control and optimize the mineral aggregate gradation composition in the mix design; The porosity and stability in the mixing ratio should be adjusted; Strive to improve the strength of asphalt concrete. Strictly control the dosage of mineral powder, and the powder-binder ratio should be controlled at about 0.8~ 1.2.
The asphalt dosage 1.3.4 has little effect on cracks when Marshall's optimal dosage is 0.5%, so the asphalt dosage should be guaranteed in the mixing building during construction, because a large amount of asphalt is beneficial to low-temperature crack resistance, but it should not exceed this range, otherwise the high-temperature stability will be reduced.
1.3.5 Adding asphalt modifier to improve the aging resistance of asphalt.
1.3.6 Strictly control the temperature of paving and rolling during construction, strictly organize the construction, and stop paving and rolling when it is windy and rainy; Full-width multi-machine full-width paving should be adopted to reduce longitudinal joints. Before the construction of transverse joints, the screed of paver should be used to coat the adhesive layer oil and preheat it to ensure the close connection between the two ends of the joints. When paving, the upper and lower longitudinal joints should be staggered over 15cm (hot joint) or 30~40cm (cold joint), and the transverse joints should be staggered over1m.
2 asphalt pavement reflection crack formation and prevention measures
At present, semi-rigid materials such as cement stabilized macadam are mostly used to build the pavement base in the asphalt pavement of expressways built in China. However, semi-rigid materials have the disadvantage of low deformation resistance and are prone to cracks when the temperature and humidity change. The stress concentration phenomenon at the crack makes the tensile stress generated by traffic load in the lower part of the surface layer greater than that in the crack-free part. When the concentrated tensile stress is greater than the ultimate strength of asphalt concrete, the asphalt pavement will also crack here. Once cracks appear in asphalt pavement, it will easily lead to water infiltration. Under the action of external load, it will scour the interior of structural layer and accelerate the development of cracks. However, the poor water stability of semi-rigid base is easy to lead to water damage and loose damage of base, which eventually leads to structural damage of pavement and affects the use function of pavement.
To reduce the occurrence of reflection cracks, we should control them from the following aspects.
2. 1 Semi-rigid materials with good scour resistance, dry shrinkage, low low-temperature shrinkage coefficient and high tensile capacity are selected for the base course, and aggregates with low-temperature expansion coefficient are preferred; For semi-rigid base with cement as inorganic binder, the amount of cement should be reduced as much as possible while ensuring the strength; Thereby reducing the probability of shrinkage cracks in the base itself.
2.2 Set an intermediate layer of geotextile and geogrid between the base course and the surface course, or spread a layer of slurry seal on the base course as a stress absorption layer, so that the stress at the cracks at the base course is concentrated and diffused through the absorption of the intermediate layer, and then uniformly transmitted to the asphalt surface course.
2.3 Optimize the mixture ratio of semi-rigid base, reduce the mixture ratio of fine aggregate as much as possible, thus reducing the occurrence of shrinkage cracks.
2.4 When rolling, the water content of semi-rigid base is controlled at 0.9 times of the optimal water content, and the degree of compaction meets the specification requirements. After the completion of rolling, it should be kept moist and maintained in time to prevent the base from drying. After curing, asphalt emulsion should be sprayed immediately to make it a prime coat or a sticky coat, and then the asphalt surface layer should be paved as soon as possible. For the layered paving base, the joint position of the upper and lower layers should be staggered by 3~4m.
2.5 At present, in China's asphalt pavement expressway, the base is generally laid in three layers. Therefore, when laying the upper base, the cracks in the lower bearing layer must be treated. Cracks should be cut into grooves with a width of 1cm and a depth of 2cm, filled with modified emulsified asphalt, and then covered with geotextile or geogrid; The location of cracks should be accurately recorded during construction, and geotextile or geogrid must be laid at this location for reinforcement when laying a higher structural layer in the future.
3 Conclusion
There are many reasons for asphalt pavement cracks, but only by strengthening design and construction management, strictly controlling the quality of raw materials, optimizing mix design and improving construction quality can asphalt pavement cracks be effectively controlled.
References:
[1] Shen Jinan. Road performance of asphalt and asphalt mixture. People's Communications Press.2001.1.4.
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