| Concrete plays an important role in China’s civil engineering construction and national economic development,which is widely used in the construction of buildings,highway bridges and culverts,underground tunnels,and so on.Generally,concrete materials have better durability which determines the safety and service life of concrete engineering structures.However,in the cold areas of the north,freeze-thaw damage has become one of the important reasons for the decline of concrete durability,which seriously affects the safety of concrete materials under service conditions,and brings many economic problems.Therefore,it is necessary to study the preparation methods and freeze-thaw resistance of frost resistant concrete.There are many tiny pores inside or on the surface of concrete,and the medium uses water as carrier to penetrate into the concrete through these pores,causing freeze-thaw damage of the concrete,which seriously reduces its durability and affects its service performance and safety.In this paper,the superhydrophobic coatings of concrete were prepared,which used the hydrophobic effect of carbon ash coating and nano-SiO2 coating to prevent or reduce the water penetration from the surface and improve the freeze-thaw resistance of concrete.In addition,the concrete was modified by adding rubber and nano-SiO2,and the mechanical properties,frost resistance and micro mechanism of the modified concrete were discussed.Fourther more,the freeze-thaw damage model was established.The main conclusions of the study are as follows:(1)Carbon ash coatings and silanized nano-SiO2 coatings with a controllable layer were prepared by using interfacial self-assembly method and sol-gel method,respectively,and the contact angle was 151 ° and 153 ° respectively,which indicate that they were superhydrophobic.The effects of carbon ash concentration,silane coupling agent content and the coating amount on the hydrophobic properties,as well as the contact angle,material composition,micro morphology and composition of the two hydrophobic materials were analyzed.The results showed that:with the increase of the number of carbon ash film layers and the silane coupling agent KH-570,the surface contact angle gradually increases to superhydrophobic state,and then decreases,which was reflecting the transition from Wenzel state to Cassie state and then to Wenzel state.(2)Frost resistant concrete was prepared by using the synergistic effect of rubber particles and nano-SiO2.Then the mechanical properties,frost resistance and modification mechanism of the designed concretes with single or composite doping of nano-SiO2,rubber and silanized rubber particles were analyzed.Based on these,the comprehensive anti freezing effect of hydrophobic coating and internal doping was discussed.The results showed that the frost resistance of concrete mixed with rubber particles was greatly improved,especially after silanization.Nano-SiO2 effectively alleviated the loss of mechanical strength of composite antifreeze concrete caused by rubber particles,especially the early strength.When the content of nano-SiO2 and silylated rubber was 3%and 5%respectively,the modified concrete showed the best frost resistance based on maintaining better mechanical properties.In addition,both carbon ash coating and silylated SiO2 coating could further improve the frost resistance of the modified concrete,and the silylated SiO2 coating was better.(3)The traditional method which was evaluating the pore characteristics of concrete only by the air bubble spacing coefficient had been revised.The relationship equation between the microstructure and the macromechanics and frost resistance of the composite modified frost-resistant concrete system were established using the air bubble density,total pore area and total air content as the microscopic quantitative evaluation indexes of the frost-resistant performance of concrete.The decay equation of freeze-thaw cycle conformed to the inverse S-type model,and the deterioration of pore structure directly determined the mechanical and freeze-thaw performance of concrete.(4)Based on the damage mechanics theory,the evolution equation of freeze-thaw resistance of modified concrete with nano-SiO2 and silanized rubber under freeze-thaw environment was established,respectively,and the relative dynamic elastic modulus was taken as the freeze-thaw damage index.In addition,a multi-factor freeze-thaw damage model of concrete frost resistance was established by analyzing the influence of concrete proportioning parameters on its frost resistance,and the rationality of the model was verified by using the experimental data of nano-SiO2 and silanized rubber compound. |
