Research On Permeability Of Concrete After Freezing-thawing Cycles At Early Age

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.