Research On Anti-cracking Performance Of Semi-rigid Base Course In High-grad Highway

On the base of analyzing the forming mechanism of semi-rigid base course, this thesis presents a measure to advance the anti-cracking performance of semi-rigid base course by improving the properties of base materials, which is mixing polypropylene fiber into cement stabilized macadam. According to a great deal of tests, each mechanics property and anti-cracking performance of polypropylene fiber cement stabilized macadam were investigated systemically. The main content includes:(1) The formulae of shrinkage stresses of semi-rigid base course were deduced. This thesis analyzed each influencing factor of the maximum value of shrinkage stress and the cracking rule of semi-rigid base course. At the same time, the corresponding prevention measures of the cracks were presented.(2) According to the compaction test of polypropylene fiber cement stabilized macadam, the maximum dry density and optimum moisture content of each mix proportion were obtained. Based on the shaping principle of standard specimen in the criterion, a suite of steel beam models and the corresponding liftout method of the specimen were designed. The tests indicate that the designed model, shaping and liftout method can ensure the maximum dry density of the specimens.(3) On the base of large amount of tests, this thesis analyzes the impacts of testing period, fiber dosage and cement dosage upon the compressive strength, splitting tensile strength, compressive resilient modulus, flexural strength and flexural elastic modulus of polypropylene fiber cement stabilized macadam, and the corresponding impact rule was found. The results show that the addition of polypropylene fiber has little impact on each strength of cement stabilized macadam, but it can reduce the compressive resilient modulus and flexural elastic modulus evidently.(4) By adopting the method of millesimal gauges, the dry shrinkage property was measured, and by adopting the method of resistance strain gauge, the thermal shrinkage property was measured. The impacts of testing period, fiber dosage and cement dosage upon the average dry shrinkage coefficient and average thermal shrinkage coefficient of polypropylene fiber cement stabilized macadam. The results show that the addition of polypropylene fiber can reduce the dry shrinkage coefficient and thermal shrinkage coefficient of cement stabilized macadam effectively. (5) Adopting three-point-bending method, a large amount of experimental researches was carried on the fractural properties of cement stabilized macadam and polypropylene fiber cement stabilized macadam. During the course of the testing, the maximum load, mid-span deflection, crack mouth opening displacement, crack tip opening displacement was measured. Therefore, the fracture toughness and fracture energy of the material were calculated. The testing results indicate that the addition of polypropylene fiber can increase the fracture toughness and fracture energy of cement stabilized macadam. The fiber can increase the critical crack mouth opening displacement and critical crack tip opening displacement appreciably, buy it can increase the ultimate crack mouth opening displacement and ultimate crack tip opening displacement of cement stabilized macadam evidently. Polypropylene fiber can improve the property after the cracking of cement stabilized macadam and it also can extensively improve the strain filed ahead of the crack tip. The addition of polypropylene fiber improves the fracture parameters of cement stabilized macadam.(6) This thesis evaluates some kinds of common evaluation indices of anti-cracking performance applying grey relational analysis. The result shows that the optimal evaluation indices of anti-cracking performance of semi-rigid base course are dry shrinkage energy anti-cracking coefficient and thermal shrinkage energy anti-cracking coefficient. By the test, a conclusion is drawn that the addition of polypropylene fiber can improve dry shrinkage energy anti-cracking coefficient and thermal shrinkage energy anti-cracking coefficient of cement stabilized macadam, furthermore the anti-cracking performance is improved.(7) Applying fiber distance theory, disorder discontinuous fiber composite theory and the corresponding knowledge of fracture mechanics, the strengthening and anti-cracking mechanism of polypropylene fiber was analyzed. There are two main influencing factors of anti-cracking effect of polypropylene fiber on cement stabilized macadam, which are the bond strength between cement stabilized macadam and the fiber, and the fiber distance.(8) A length of test road was constructed in Ye-Xin expressway and the following observation was conducted. The observation and investigation of the core samples indicate that polypropylene fiber can increase the compressive strength to some extent. And it can decrease the number of cracks of base courses, enlarge the average distance between two cracks and decrease the crack ratio. The addition of polypropylene fiber can obviously improve the resistance to the forming and expanding of shrinkage cracks, and thus the cracks of asphalt pavements were restricted effectively.