Northwestern,Northeastern and North China are located in cold region,negative temperature has brought much difficult and inconvenience about winter construction and curing systems of concrete,among them damaging effects from freezing-thawing cycles are particularly serious to early age concrete structure development and hardened concrete durability. Finding problems will provide projects to researchers;solving problems will make contribution to science and technology.Many researchers from home and abroad have studied related projects,but most of them are about that freezing-thawing cycles affect hardened concrete durability, freeze-thaw ways have been adopted include slow-freezing method,quick-freezing method,varied minus-temperature, freezing in water and thawing in air,freezing in water and thawing in water;characterization on frozen injury have been adopted include compressive strength loss,dynamic elastic modulus loss,mass loss,anti-permeability loss;description method about pore structure have been adopted include mercury intrusion method,nitrogen adsorption method,scanning electron microscopy method.By NEL method,the paper has studied variation tendency of compressive strength and anti-permeability before and after freezing-thawing cycles,after recovery of standard curing on normal concrete and the concrete mixed with fly ash and admixture,then discussed the evolution of pore structure when concrete experienced freezing-thawing cycles at early age . The paper simulates temperature variation of winter day and night , takes advantage of chloride-ion-diffusion-coefficient variation,responds permeability and pore structure of concrete truly and integrally .Experiment shows that compared with concrete before -5/-20℃freezing-thawing cycles,the concrete after -5/-20℃freezing-thawing cycles have become more and more close-grained;compared with concrete which have always standard curing,the concrete after-5/-20℃freezing-thawing cycles have produced different degrees of damage.The concrete with three different water-cement ratios have produced different degrees of damage when experience-5℃freezing-thawing cycles,the degree of damage have become more and more bigger since the water-cement ratio grew bigger;the recovery of standard curing can cure the structure of concrete,the amplitude of curing has become more and more bigger since the water-cement ratio grew bigger. After-20℃freezing-thawing cycles,the concrete with small water-cement ratio emerge boundary structure when be standard cured 3d at early age,the concrete with big water-cement ratio emerge boundary structure when be standard cured 4d at early age. The concrete with big water-cement ratio produced frost damage after -5℃freezing-thawing cycles;the concrete with small water-cement ratio produced migration and connection damage after freezing-thawing cycles,lead to nonuniform distribution of concrete structure. The concrete processes migration and connection pore and other types when experience -20℃freezing-thawing cycles . In the recovery of standard curing,migration and connection pore is difficult to be cured,so it is serious hidden danger to concrete durability.Compared with blank concrete,the anti-permeability of concrete mixed fly ash after freezing-thawing cycles is bad,but it is closed or better than blank after the recovery of standard curing. Freezing temperature affect seriously anti-permeability of concrete mixed with fly ash and admixture in the recovery of standard curing. |