Asphalt pavement cracking is one of the most common quality diseases in municipal roads and highway engineering, and it is also the core inducement to induce pavement seepage, base softening, pit, subsidence and pavement damage aggravation.Pavement cracking not only affects the appearance smoothness and driving comfort of the road, but also greatly reduces the overall service life of the road and increases the maintenance cost in the later period.In order to effectively control asphalt pavement crack disease and reduce crack problem from the source, combining with asphalt pavement construction specification and field construction experience, the causes of crack are systematically analyzed, and targeted prevention and special treatment measures are put forward.

Common crack types of asphalt pavement

Asphalt pavement cracks can be divided into four major categories according to their morphology and causes, namely transverse cracks, longitudinal cracks, network cracks (cracks), and irregular cracks. There are obvious differences in the generation mechanisms and treatment methods of different crack diseases.Transverse cracks are mostly caused by temperature stress contraction, longitudinal cracks are mostly caused by subgrade settlement and improper treatment of construction joints, network cracks are mainly caused by pavement fatigue damage, insufficient compaction and insufficient base strength, irregular cracks are mostly caused by irregular construction technology and impact of external load.

Analysis on the Causes of Pavement Crack Core

temperature factor

Asphalt material has obvious temperature denaturation, asphalt brittleness increases and flexibility decreases under low temperature environment, road surface is affected by temperature difference between day and night and seasonal temperature difference to produce shrinkage stress, when stress exceeds tensile strength of asphalt mixture, road surface will produce temperature shrinkage crack.At the same time, asphalt pavement softening in high temperature season, deformation accumulation under repeated rolling load, long-term reciprocating will cause fatigue cracking, which is the main reason for early cracking of pavement.

Non-standard construction technology

The artificial core factor of pavement cracking is that the process control is not in place in the construction stage.Firstly, asphalt paving and rolling temperature are not up to standard, low temperature rolling leads to poor adhesion of mixture, excessive internal void ratio, insufficient pavement compactness and easy cracking in later stage; secondly, construction joint treatment is rough, vertical and horizontal joint cutting is uneven, adhesive layer oil painting is uneven, joint rolling is not dense, the integrity of new and old pavement connection is poor, and joint cracking is easy to occur after stress; Third, unstable paving speed, insufficient compaction times and pressure leakage blind zones cause uneven overall pavement compactness and local stress concentration to cause cracking.

Subgrade and base quality defects

Subgrade and base course are the stress foundation of asphalt surface course. If the compaction degree of base course is insufficient, the strength is insufficient, the water content exceeds the standard, or the subgrade has problems such as uneven settlement and insufficient modulus of resilience, it will lead to uneven stress of asphalt surface course and structural cracking under the action of traffic load.At the same time, the base maintenance is not in place, the surface layer is loose and peeling, which will directly transmit stress to the asphalt surface layer and induce large-area crack disease.

Traffic Load and Natural Environment Erosion

Long-term heavy vehicle repeated rolling, vehicle sudden braking, sharp turning and other external impact, will cause asphalt pavement fatigue damage, gradually produce fine cracks and continue to expand.In addition, rainwater infiltration, freeze-thaw cycle, ultraviolet exposure and other natural factors will accelerate asphalt aging, adhesion attenuation, resulting in increased brittleness of pavement, continuous development, extension and penetration of cracks.

asphalt pavement cracking source prevention measures

Strictly control the quality of raw materials and mixtures

Priority shall be given to the selection of high-quality asphalt with ductility, softening point and low-temperature toughness index up to standard, and the crushing value, mud content and grading parameters of aggregate shall be strictly controlled to prevent impurities and inferior aggregates from being put into construction.Optimize asphalt mixture mix ratio design, reasonably control asphalt aggregate ratio, porosity and saturation, improve mixture high temperature stability, low temperature crack resistance and fatigue resistance, and improve pavement crack resistance from material level.

Standardize temperature control in the whole process of construction

The asphalt construction temperature standard shall be strictly observed. The temperature of SBS modified asphalt mixture at each stage of appearance, paving and rolling shall be strictly up to standard. Low-temperature construction is strictly prohibited.The mixture shall be covered with heat preservation in the whole transportation process, so as to reduce the temperature drop of transportation, follow the rolling operation after paving, shorten the interval time between paving and rolling, ensure that the mixture is compacted and formed in the optimal temperature range, ensure that the aggregate is tightly bonded, the structural integrity is strong, and effectively avoid temperature cracks.

Optimization of paving and rolling construction technology

The paver shall be used for continuous paving at a constant speed, so as to eliminate pause and uneven speed in the middle, ensure uniform paving thickness and road flatness up to standard.Strictly implement the compaction process of "initial compaction, recompaction and final compaction", standardize the compaction sequence, speed and times, focus on strengthening the compaction of weak areas such as pavement edges, inspection wells and construction joints, eliminate pressure leakage and underpressure, ensure that the compaction degree of the whole pavement is uniform and up to standard, and reduce structural cracks.

Standardized treatment of construction joints

Strictly standardize the construction technology of vertical and horizontal joints. The hot joints shall be paved and rolled synchronously in echelon to ensure the overall compactness of the joints; the cold joints shall be cut flat, the dust and sundries of the joints shall be thoroughly cleaned, and the adhesive layer oil shall be evenly coated to ensure the close adhesion of the new and old pavement.The joint position adopts the cross-joint rolling technology to eliminate the joint height difference and stress defects, and eliminate the joint cracking and misplacement diseases.

Strengthen the construction quality of subgrade and base course

During subgrade and base construction, the degree of compaction, flatness, and water content shall be strictly controlled to eliminate uneven settlement.After the completion of the base construction, it shall be fully maintained to ensure that the base strength meets the standard, the surface layer is dense and stable, and there is no loose, peeling and cracking problem, so as to provide a uniform and stable stress foundation for the asphalt surface layer and fundamentally eliminate the pavement cracking caused by the deformation of the base layer.

Strictly implement the pavement maintenance management system

After asphalt pavement is formed, the closed maintenance system shall be strictly implemented. During the maintenance period, vehicles and pedestrians are prohibited to pass ahead of time. Heavy vehicles are prohibited to roll and brake sharply.In high temperature weather, the road surface shall be protected from sun and moisture, and in low temperature season, anti-freezing and heat preservation measures shall be taken to reduce the damage of temperature difference stress to the road surface, delay asphalt aging and improve the overall durability of the road surface.

Special treatment and repair technology for asphalt pavement cracks 

Fine crack treatment (crack width <3mm) 

Aiming at the fine hairline crack of pavement surface layer, emulsified asphalt is used to seal the crack. Firstly, dust, sundries and floating materials on the crack surface shall be cleaned to keep the crack dry and clean. Special spraying equipment shall be used to spray emulsified asphalt evenly to make asphalt fully penetrate into the crack, seal the crack and bond the surface aggregate to prevent rainwater from penetrating and expanding the disease. Fine aggregate protection shall be spread after completion to ensure the smooth and beautiful pavement. 

Conventional crack treatment (crack width 3~10mm) 

For medium penetrating cracks, special hot asphalt joint pouring adhesive shall be used for pouring repair. First, use high-pressure blower and slotting cleaning equipment to clean up impurities and accumulated water inside cracks, ensure that there is no impurity in cracks, heat joint pouring glue to standard temperature, pour cracks at constant speed, ensure that joint pouring glue is full, dense and free of gaps, and the surface is flat and flush with the road surface. After the completion of joint pouring, it is allowed to stand and cool, and after the maintenance reaches the standard, the traffic is opened, effectively plugging cracks and blocking seepage channels. 

Severe crack and crack treatment (crack width>10mm, network crack) 

In view of the serious diseases such as large area crack, wide crack and loose settlement of road surface, the local milling and resurfacing technology is adopted to treat them. Accurately delimit the disease scope, mill the damaged asphalt surface layer, check the condition of the lower base layer, if the base layer is damaged, softened or insufficient in strength, mill the base layer synchronously and lay and compact it again. After the acceptance of the base course is qualified, the asphalt mixture shall be re-paved, compacted layer by layer, the joints between the old and new pavements shall be treated in a standardized manner, the flatness and compactness of the repaired pavement shall meet the standards, and the cracking disease shall be completely eradicated. 

epilogue 

Asphalt pavement cracking disease has multiple causes of material, construction, structure and environment. The core of treatment lies in "prevention first, prevention combined, early detection and early treatment". Strictly control the core links of materials, temperature, technology, base course and joints in the construction stage to avoid cracking hidden dangers from the source; inspect the pavement condition regularly in the operation and maintenance stage, and timely close and repair the fine cracks to avoid the expansion of diseases. Through systematic prevention and control and standardized repair technology, the cracking probability of asphalt pavement can be effectively reduced, the service life of road can be prolonged, and the traffic safety and traffic quality of road can be ensured.