| The asphalt pavement with cement-treated macadam base has the advantages of high strength, sound flatness and good anti-fatigue performance, and has become the main pavement structure of high-grade highway in China. However, premature failure caused by sever cracking and shearing slip damage has long been the main technical hurdles in asphalt pavement with cement-treated base. Base on the in-depth analysis of the basic reason of pavement cracking and shearing slip, the Continuous Construction Method(CCM) was proposed to reform the traditional construction process, decreasing the reflective cracking caused by the base construction and improving the bonding quality at the interface between base layer and surface layer. The low-cost anti-cracking agent was developed and added into the cement-treated base materials to prolong the hydration process of base materials and improve the shrinkage performance of base materials. By adding the anti-cracking agent, traditional3to7days of curing period of cement-treated macadam base was eliminated by CCM and the asphalt mixture can be paved immediately after the construction of cement-treated macadam base. The surface layer and base layer were compacted and bonded tightly as a continuous whole and higher structural capacities against loads and cracks were obtained.The CCM differs from the traditional construction method both in materials and construction process. On the one hand, the anti-cracking agent can make the cement-treated materials micro-expand after hydration and increase its densification, making the shrinkage caused by the temperature and humidity change decreased significantly and achieving the purpose of reducing road base cracks fundamentally. On the other hand, CCM changes the interface bonding condition between the surface layer and base layer. Aggregates of the asphalt mixture and cement stabilized mixture embedded and squeezed into each other a certain depth, forming a mutually staggered contact surface at the interlayer, better friction resistance and bond property can be acquired then. The combination of anti-cracking agent and CCM makes it possible to solve the cracking and shearing slip damage from the construction technical aspect, the construction period and cost can also be saved at the same time. However, researches on CCM is so poor that lack of systematic study on the developing of the micro-expansion anti-cracking agent, the base-surface bonding strength of CCM and its contact mechanism, quality control measures on compaction and construction, which restricts the application of CCM in engineering practices. Based on the issues discussed above, this paper focus on the following works:(1) The soluble phosphorus and fluorine contains in phosphogypsum have significant effect on retarding the cement hydration, while the CaSO4ยท2H2O contains in phosphogypsum can stimulate the generation of micro-expansile hydration products which can decrease the chemical shrinkage and dry shrinkage of cement. Based on the comprehensive effects of phosphogypsum on properties of cement, the phosphogypsum was used to develop an anti-cracking agent, with the citric acid as an auxiliary set retarder. On the one hand, prolonging the initial setting time of the base materials to8-10hours to guarantee, on the other hand, micro-expansion substances generated from the pozzolanic reaction between cement-treated materials and anti-cracking agent contribute to reducing the porosity of cement-treated base and the contraction space for shrinkage. Strength, setting time and dry shrinkage were used as the evaluation indicators in laboratory tests to determine the suitable dosage of the addition agent. The anti-cracking mechanism was analyzed from the microscopic perspective by the scanning electronic microscope and X-ray diffraction.(2) Based on many laboratory tests, the mechanical properties and shrinkage performance of cement-treated materials with anti-cracking agent were analyzed. The influences of anti-cracking agent dosage, retarded molding time and cure period on the properties of cement-treated materials were studied. The results show that the mechanical properties and shrinkage properties can be improved significantly by adding appropriate dosage of anti-cracking agent, with the compressive strength, splitting strength and flexural strength increased, the modulus of resilience and the shrinkage coefficient decreased.(3) Shear test and pull-out test were conducted with an own designed road surface-base shear-pull tester to contrast the interlayer bonding performance of the traditional construction method and CCM. Test core samples molded by CCM show better friction resistance and bonding properties than samples molded by traditional construction method. Researches on the influences of the initial compaction degree of base layer on the bonding properties indicate that there exists a proper initial compaction degree of base layer which enables the interlayer between the surface layer and base layer to obtain the largest bond strength. Meanwhile, researches on the influences of retarded molding time on the bonding properties show that the bond strength decrease after the cement-treated materials hardened Researches on the squeeze depth of the CCM constructed core samples corroborated that under the action of the vibration rolling compaction on surface layer, aggregates of asphalt mixture and cement stabilized mixture embedded and squeezed into each other a certain depth, the surface layer and base layer can be effectively bonded to bear loads as a whole.(4)To explain the surface-base contact mechanism of the CCM constructed roads, fractal theory and interlayer contact theory was used in this paper. Based on the fractal properties of aggregate particle and aggregate gradation, fractal dimension of the rough curved contact surface in CCM was calculated. Contact model of two rough surfaces consisting of an array of asperities was proposed. The model is appropriate for the study of CCM surface-base contact and provides the statistical distributions of both heights and curvatures. On this basis, the real contact area and critical contact area of CCM constructed road interface were calculated and contrasted with that of the traditional road, the CCM constructed roads has larger real contact area between the surface layer and base layer, contributing to better resistance to plastic deformation at the interface. Finally, correlation analysis between the interface bonding strength and the fractal dimension of aggregate gradation was conducted by test results of the interlayer shear test and pull-out test.(5) The nonlinear finite element analysis software ABAQUS was used to simulate the actual interlayer contact condition by establishing the continuous construction interlayer contact model. The mechanical response results of the contact model and the continuous model under vehicle load were compared. Characteristics of each mechanical index varies with the interfacial contact condition and influences of the interfacial contact condition on cracking and shearing slip were analyzed. The analyses conclude that the bad interfacial contact condition will lead to the increase of the deflection on the pavement surface, the maximum principal stress and maximum shearing stress at the bottom of surface layer and base layer. Combination use of anti-cracking agent and CCM brings the road with more dense base and better interfacial bonding condition to resist the cracking and shearing slip damage.(6) Test roads were constructed and contracted to demonstrated the feasibility of this approach. Construction organization of mixing, transport, spreading and rolling processes of CCM was studies. The following tracking detections of the test roads were conducted and the CCM test roads showed a superior anti-cracking performance.In summary, through the studies on CCM in this paper, key questions about the developing of anti-cracking agent, the interlayer contact mechanism, the cracking resistance mechanism and the quality control measures on compaction and construction have been solved, which provide theoretical basis for the CCM to be popularized and applied in practical engineering. |
PDF Full Text Request