Study On The Asphalt Pavement Crack And Preventive Treatment In Jinan Area

It is the main structure style to use semi-rigid base asphalt pavement for high-type highway, especially in recent years, almost all the arterial highway adopt it in Ji Nan. As the asphalt pavement has the characters of comfortable traffic, good antiskid property, cleanness, also convenient for construction and maintenance, it is a kind ideal pavement structure. However, because of the asphalt concrete itself characters and various problems in construction, the crack usually is unavoidable. It is certified that by improving the property of materials, adopting reasonable ratio of mixture, and various effective measures, the asphalt pavement cracks can be reduced, prolonged the life. The occurrence time can also be delayed greatly.The pavement structure of national trunk highway 104 in Jinan is complex, all kinds of cracks may occur, so we must investigate carefully before construction, make clear what kind of type the crack is, find the reason, analyze the mechanism, and then take reasonable measures to avoid them guarantying construction quality. Now, we will describe them as below according to the types:1. reflection crackThe discontinuity of subjacent layer cause reflection crack. Asphalt pavement which was paved on surface of the old cement concrete pavement can yield cracks as easy as on the surface of the old asphalt pavement.The basic mechanism of reflection crack includ tensile fatigue, tensile yield and shear yield, on the action of unique or combined action of them, reflection crack will occur.Elimination of the asphalt pavement reflection crack should be consideredfrom the aspects: eliminating reflection resource, strengthening tension strength and setting up stilling base. The methods usually adopted are using geotextile polymer, smashing old cement concrete pavement, setting up stress absorption layer or transition layer, etc.2. semi-rigid base crackCement-bound granular materials base is a kind of semi-rigid base, the main reasons causing crack are: (1) great thermal difference between day and night; (2) long exposure time, causing thermal fatigue crack; (3) no uniformity mixture causing irregular crack; (4) incorrect curing, causing desiccation fissure; (5) incorrect construction technology causing tiny fissure.As semi-rigid base has properties of thermal expansion, becoming dry, fatigue, etc, the crack was caused mainly because of thermal shrinkage, dry shrinkage and fatigue load. Tri-phase of inorganic binder interacts during the process of thermal reduction, causing volume shrinkage. Dry shrinkage is a kind of holistic macroscopically change due to the action of capillary tensile force caused by moisture evaporation, force of absorption water molecule, confined water in mineral crystal or gel, carbonation dehydration. As semi-rigid base materials has far less tension strength than compression strength, on the repeated action of vehicle load, thermal and humidity stress, crack will occur at the bottom of the structure layer, when flexural tensile stress or strain exceed fatigue stress, and then extend to the surface gradually.Measures to control cracks should be taken from raw material, ratio of mixture, construction technology and curing. Maximum grain size should be no less than 31.5mm, on condition of satisfying design strength, cement dosage should be lesser. Cement dosage in base is about 5%, and the max dosage does not exceed 5.5%. Grading curve should be smooth and at the same time it is better to be closer to thelower limit of the gradation bound. During the process of mixing, the mount of cement and water will be controlled strictly, no water sprinkling and slurry absorption, guarantying rolling on condition of optimum water content.3. Temperature affect crackThere are two kinds of crack, one is named thermal shrinkage crack which is caused by lower temperature only. Lower temperature will cause thermal stress . The crack occurs when the stress exceed tensile stress which has the characters of vertical to the traffic direction, equal space. Another kind is thermal fatigue crack. Asphalt pavement comes into fatigue after long time temperature affection, cracks occur even thermal stress is less than tensile stress. Lower temperature crack is related with properties of asphalt, mix composition, thickness and category of pavement structure, environment and construction technology, also with traffic.4. Water damage affectionIf porosity of semi-open-graded bituminous paving mixtures is big or degree of compaction is not enough and has great asymmetry, water on surface can enter into surface layer, separating asphalt and mineral powder, giving birth to pot hole and net-shaped crack.There are two disabled models in asphalt-aggregate system: adhesive capacity destroys and cohesion destroy. As attraction force to water of aggregate is bigger than asphalt, water can penetrate between asphalt and aggregate, cutting down touching angle between them, descending bond force inducing asphalt film peeled off. This is adhesive capacity destroy. As water penetrate, making asphalt soft, descending viscosity. All of this will cut down whole strength of asphalt mixture and it is called cohesion destroy. So, adhesive capacity loss and cohesion loss are the asphalt pavement water damage mechanism.Main measures reducing water damage on asphalt pavement are to strengthenwaterproof and drainage. Utilizing dense-graded asphalt mixtures and seal coat, preventing rain water penetrating into asphalt pavement, this will reduce the action of water to asphalt mixture. Another method is designing reasonable drainage system, discharging water as quickly as possible, decreasing the action time of the asphalt and asphalt mixture. Waterproof and drainage are just temporary solution not permanent cure. There is a very important tache needed, that is improving the ability to endure water. It is the basic method solving water damage, such as improving adhesive capacity between asphalt and aggregate, optimizing gradation, reasonable porosity, setting up appropriate interior drainage system.Asphalt pavement crack is a very complex problem, only if we explore and study at work, can we find the reason, and then set down feasible solutions. However, it is also impossible to solve the entire problem.