Experimental Study On Durability Performance Of Concrete Subjected To Early-age Freeze-thaw Damage

Current studies on durability performance of concrete mostly focus on the concrete specimens cured of 28 days.However,the concrete structures in winter construction or cold environment often suffer some unrecoverable frost or freeze-thaw damage in early-age,which pose a potential threat to the subsequent performance development and service life of concrete.Regarding the research of early-age frost damage concrete,most studies mainly focus on the antifreeze critical strength and later-age mechanical properties,whereas the studies on long-term durability are rare.It is of great significance to further study the mechanical properties and durability of concrete subjected to early-age freeze-thaw damage under different influencing factors for effectively preventing early frost damage of concrete.The main studies of the paper are as follows.(1)The resistivity of fresh concrete was measured for the period from casting to the age of 72 h by a non-contact electrical resistivity measurement,which was used to analyze the hydration process of concrete during early-age stage.Based on the Krstulovic-Dabic hydration model,the parameters of kinetics model were obtained and the hydration kinetics process was characterized.The results show the early-age hydration process is divided into five stages,which are ion dissolution,induction,acceleration,transition and deceleration.The incorporation of fly ash delays the hydration process of the concrete and silica fume has less effect on the early age hydration process of concrete.The parameters of the Krstulovic-Dabic hydration model can be fitted using the resistivity data of concrete,and the fitting effect is good.The compressive strength and electrical resistivity of concrete show a good linear correlation at the early-age period.(2)The influences of pre-curing time,the number and environment of early-age freeze-thaw damage,later-age curing methods and mineral admixture on later-age compressive strength,resistance to chloride penetration and resistance to freeze-thaw of concrete were studied.The recovery percentage of different later-age performance indexes based on the results of control concrete specimens with no experiencing early-age freeze-thaw damage was compared and analyzed.The results show that all three later-age performance indexes improve gradually as the pre-curing time prolongs.The variation rate is large between18 h and 24 h,and the change rate gradually decreases after 24 h.Both the increase of numbers of early-age freeze-thaw cycle and the existence of salt in freeze-thaw environment are adverse effect for the recovery of later-age performances.In all later-age curing methods,the restoration effect of water curing on later-age performance is the best.Compared with the standard curing,the restoration effect of natural air curing on later-age compressive strength and resistance to chloride penetration of concrete is better,but that of resistance to freeze-thaw is worse.When fly ash is incorporated,the later-age performance of concrete decreases obviously,while the incorporation of silica fume has the significant adverse effect on the resistance to freeze-thaw.Generally speaking,the recovery percentage of later-age durability indexes of concrete subjected to early-age freeze-thaw damage,especially the resistance to freeze-thaw,is always lower than that of compressive strength.(3)Based on the experiment results above,the correlation among the later-age performance indexes of concrete subjected to early-age freeze-thaw damage was established,as was the relationship between early-age and later-age performance indexes,and a predicted method of minimum pre-curing time was proposed.The results show that there is a high correlation between the later-age compressive strength and the chloride ion electric flux of concrete subjected to early-age freeze-thaw damage,and the resistance to chloride penetration can be evaluated according to the compressive strength.When the compressive strength and resistance to chloride penetration of some experiment groups meet the requirements,the resistance to freeze-thaw is still poor.Compared with the compressive strength before early-age freeze-thaw damage,the linear correlation between the compressive strength after early-age freeze-thaw damage and the later-age performance indexes of concrete is better.As a nondestructive testing method,the early-age resistivity can predict well the later-age performance of concrete that has been precured for more than 24 hours and subjected to early-age freeze-thaw damage.The minimum pre-curing time predicted based on the principle of 95% recovery rate is determined by the later-age durability indexes of concrete.For concrete subjected to early-age freeze-thaw damage,it is more important to ensure the recovery of later-age durability.