Study On Self-Healing Behavior Of Joint Sealant In Cement Concrete Pavement Based On Shape Memory Function

With the rapid development of road traffic in China,cement concrete pavement is one of the main pavement types of high-grade pavements,which is widely used in roads,urban roads,airport runways,and port roads.And it is mainly made of jointed cement concrete pavement with various kinds of filling materials,which not only maintains the integrity,smoothness and water tightness of pavement on the structure,but also improves the durability of the cement concrete pavement and driving comfort.However,under the repeated action of traffic load and environmental factors,the cement concrete pavement sealant is prone to crack or damage due to the stress concentration and aging effect.It causes the failure of the cement concrete pavement surface,such as breaking,dislocation and mud-depositing,which seriously affects the service life and quality of the cement concrete pavement.In addition,the durability of joint filling material has become one of the important bottlenecks,restricting the development and application of cement concrete pavement.Therefore,this paper has developed a joint filling material with self-healing function,which can actively adapt to changes in the width of joints and repair the cracks or damage in time.It is of great significance to improve the durability of joint filling materials,to extend the service life of cement concrete pavement,to improve service quality,to promote the development of cement concrete pavement,and to play a role in the national road network structure.Firstly,based on the research of the shape memory polyurethane（SMPU）in the earlier stage of this research group,the epoxy resin（EP）and nano titanium dioxide（TiO₂）were used to modify and enhance the SMPU.The thermal properties,microstructure and mechanical properties of the two materials were studied by differential scanning calorimetry,Fourier infrared spectroscopy,scanning electron microscopy,and universal testing machines,respectively.The optimal dosage of EP and nanoscale TiO₂ is 10%and 3%,respectively.It is found that the shape memory temperature of the two joint sealant conform to the actual working conditions at the joint,and both of them have better shape memory and tensile properties.However,the compressive properties of the EP/SMPU are far worse than the TiO₂/SMPU.TiO₂/SMPU is more suitable to be joint sealant which is under long pressure.Therefore,3%TiO₂/SMPU is chosen as the base material for cement concrete pavement is chosen as the base material for cement concrete pavement joint sealant in this paper.Secondly,in order to reduce the common damage of extrusion,cracking,stretching,extruding,shedding in cement concrete pavement joint sealant,a two-stage uniaxial programming was proposed in this paper to give the pre-deformation to the TiO₂/SMPU joint sealant,which could enhance the shape memory effect.Also,it can make the joint sealant adapt to the change of the width of the cement concrete pavement joint initiatively,so as to prevent joint sealant from extruding at high temperature,and cracking or falling off at low temperature.The changes of properties of the joint sealant before and after programming were also studied in this paper.The results show that the molecular orientation and the conformational entropy of the TiO₂/SMPU joint sealant have changed significantly before and after the programming.It reveals the mechanism of shape memory effect in the two-dimensional direction.The shape fixity rate and the shape recovery rate of the joint sealant in horizontal and vertical directions are all greater than 90%,which indicates the joint sealant has good shape memory performance.After programming,the tensile strength of joint sealant increased by 43.2%,and elongation at break increased by 13.6%.Compressive yield strength is reduced,but it is still higher than 20MPa,which meets the compressive strength requirement of cement concrete pavement joint sealant.Then,in order to enable the joint sealant to self-heal the cracks and damages while adapting to the cyclic variation of the joint width,the PCL/TiO₂/SMPU composite joint sealant was adopted by using thermoplastic PCL particles as the repair agent.According to the test results of shape memory,mechanics and self-healing of joint sealant,it is determined that the optimum content of thermoplastic PCL is 10%.A two-step self-healing method was proposed in this paper,that is,the natural environment temperature is used as an incentive for shape memory to drive the joint sealant to close the two free surfaces of cracks or damages on the structural scale,and then through the mechanism of molecular migration and diffusion between the two free surfaces to achieve self-healing on the molecular scale.The self-healing rate of the joint sealant is 56.18%,and it is 38.64%after three cycles of self-healing,which indicates that the material has good self-healing property.Finally,the tests of artificial weather accelerated aging,rainwater immersion,oil immersion and mechanics were adopted in this paper to investigate the pavement performance of PCL/TiO₂/SMPU joint sealant.The results show that the surface of the joint sealant is slightly yellow,and it has good shape memory performance and mechanical properties under the comprehensive aging effect.After being soaked in rainwater,machine oil and diesel oil,respectively,the appearance of the joint sealant is basically unchanged,and it has good water resistance and oil resistance.The PCL/TiO₂/SMPU joint sealant prepared in this paper can meet the working requirements of the cement concrete pavement joints.It can actively respond to changes in ambient temperature through shape memory effects,and can perform good self-healing when cracking and damage occur.It not only improves the durability of the joint sealant and the service quality of the cement concrete pavement,but also reduces the damage of the cement concrete pavement,which promotes the sound and rapid development of the cement concrete pavement.