Development And Self-healing Mechanism Of Joint Sealant With Shape Memory Effect In Cement Concrete Pavement

Cement concrete pavement has become one of the main paving types of high-grade pavement due to its advantage in strength,stability and service life.Most cement concrete pavements are jointed cement roads.The joint is one of the weakest parts of cement pavement,which is the main factor leading to the damage of cement pavement.The sealing effect and durability of sealant directly affect the waterproofness,tightness and smoothness of cement pavement joints,and further affect the life span of the pavement,the comfort level of driving,and maintenance and repair costs.However,the sealant of the cement pavement at presentcannot perfectly adapt to the environmental conditions of the joint and the cyclic expansion and contraction of the joint width.Furthermore,there is no self-healing function,which cannot meet the use and development demands of the cement pavement.In this study,a self-healing composite material based on shape memory polyurethane?SMPU?matrix as a sealant is developed,and a biaxial programming method is proposed to impart two-dimensional shape memory performance to the sealant,which allows sensing and responding to environmental changes,and actively adapt to the cyclical variation of joint width of cement pavement,in order to realize self-deformation and self-healing its cracks,as well as reveal the two-step self-healing mechanism based on two-dimensional shape memory effect.This improves the durability and sealing effect of sealant,and prolongs the service life of cement pavement,thus enhances the quality of the road.Firstly,SMPU was developed as sealant,and its properties were studied to reveal the shape memory mechanism.The results show that the phase transition temperature of SMPU matches that of road surface in high temperature season of general cement pavement.The natural ambient temperature can be used as the excitation temperature of SMPU to recover to its original state,which lays a foundation for self-healing of sealant.SMPU has obvious microphase structure separation between the soft and the hard segments,and determines the suitable content of soft segment and hard segment.The result improves that the shape memory performance of SMPU is determined.The SMPU after uniaxial pre-deformation has significant molecular orientation,showing obvious anisotropic characteristics and a driving force being provided to return to SMPU's original state.SMPU has good mechanical properties and can be used as a substrate for preparing sealant materials.Then,nano SiO₂ was selected as the SMPU reinforcement phase,and the optimum mixing amount of SiO₂ was determined.The SiO₂/SMPU composite was prepared as a sealant.The shape memory effect of SiO₂/SMPU and theproperties and other changes during the micro-phase structure of shape memory process were studied,which revealed the SiO₂/SMPU's shape memory mechanism.It is found that when the SiO₂ content is 15%?volume ratio?,the shape fixture and recovery rate of SiO₂/SMPU are more than 97%,which has excellent shape memory effect;the nano-SiO₂ and SMPU physically crosslink to form hydrogen bonds,which enhanced the molecular force and reduced thephase transition temperature of SiO₂/SMPU to 70?,with remarkably improvement of the recovery force.The programmed SiO₂/SMPU produces obvious molecular and characteristic functional group orientation,exhibiting anisotropic characteristics and improving mechanical properties.The phase transition temperature of SiO₂/SMPU,shape memory performance,and mechanical properties meet the requirements for the use of cement pavement sealant.Secondly,a biaxial programming method is proposed,which simultaneously performs pre-compression and pre-pull deformation in horizontal and vertical directions,and imparts shape memory effect to SiO₂/SMPU sealant in two-dimensional direction.The results show that the two-axis programmed sealant has good two-dimensional shape memory effect.The sealant is installed in the joint ata low temperature.The horizontal preloading deformation is restored when the ambient temperature is higher than the installation temperature.The restoring force generated by the shape memory effect causes the sealant to actively act on the two side walls in the joint,and the good sealing effect solves the problems of easy falling off,cracking,water seepage and the like at low temperature.The sealant recovers to its initial shape in the vertical direction,and the sealant shrinks actively with the restoring force generated by the shape,which solves the problem of easy extrusion?overflow?at high temperature.The biaxial pre-deformed sealant produces obvious microphase separation,molecular and functional group orientation,and the microstructure is more compacted,which improves self-healing efficiency and mechanical properties.Biaxial programmed process protects the microstructure of SMPU molecular chains and nano-SiO₂ particles in the sealant,laying a foundation for repeated self-healing.In addition,a suitable amount of thermoplastic polycaprolactone?PCL?is added in the sealant.Particles as a repairing agent,simulating the self-healing process after rupture of organisms,propose a two-step self-repair method.It is found that PCL has almost no effect on the phase transition temperature,dynamic thermodynamics,shape memory and mechanics properties of SiO₂/SMPU sealant when PCL is 10%,PCL effectively crosslink with SMPU to make sealant molecules.The chain arrangement is more orderly,which improves the mechanical properties of the sealant.After the biaxial pre-deformation,the sealant can effectively close the macro-fracture free surface driven by the shape memory effect,and then realize the crack healing on the microscopic scale of the molecule through PCL melting.The repair method improves the self-repairing efficiency of the sealant material and has good repeatability.Then,the road performance of self-healing SiO₂/SMPU sealant under the influence of climate,rain,oil,and force were studied.The results show that the surface of the sealant will slightly turn yellow after comprehensive accelerated aging,and the shape fixing rate is more than80%and recovery rate is more than 85%.It still has a good shape memory effect,and the mechanical properties still meet the requirements of cement pavement sealant material.The appearance of sealant soaked in rainwater,diesel,engine oil and gasoline is basically unchanged.The double-axis programmed also plays an important role in improving the water and oil resistance of the sealant,indicating that the self-healingsealant have good anti-aging properties which improves its lifespan.Finally,the self-repairing SiO₂/SMPU sealant with two-dimensional shape memory effect developed in this paper is better adapted to the working environment conditions of cement pavement joints,and cracks or damages on sealant are timely and effectively repaired,improving the durability of sealant materials and reducing the various damages of cement concrete pavement caused by the sealant failure.It also decreases the maintenance and repair costs,improving the service quality of cement concrete pavement,and therefore it promotes the sound and rapid development of cement concrete pavement,and plays an important role in the national road network structure.