Evaluation Of Frozen Damage Of Concrete At Early Age By Electrical Resistivity

The concrete in winter construction is easy to be affected by the cold climate and suffers from the early age frost,which leads to the damage of the material.The surface of concrete directly exposed to negative temperature will freeze fastest,while the mass concrete will be frozen after hours or days due to hydration and heat releasing.The damage results from low temperature will do harm to strength and durability of fresh concrete,which contributes threat to practical engineering.The electrical resistivity is investigated in this study as a nondestructive method when the fresh concrete subject to freeze.The resistivity varies with hydration and the formation of ice,and people can know the variation of the interior conveniently and real time.As for surface frozen and interior frozen of fresh concrete,two curing systems are used in this paper,which are frozen immediately and after pre-curing.Temperature,water cement ratio(w/c),admixture and mineral admixture were regarded as four important factors to the resistivity of early age frozen concrete.Then we will analyze their influence to ice content.Based on all experimental data,a typical development law about early age concrete subject to freezing was proposed in this paper.The resistivity development curve follows by the four parameter Logistic function.Furthermore,the fresh frozen concrete can be devided into three stages according to the increase of resistivity change rate.The three stages are temperature drop and crystal formation stage,crystal rapidly growing stage and ice formation stable stage.Concrete freezes faster at the lower temperature,and pre-curing can reduce the amount of ice then the resistivity will be down?However,if the pre-curing time reaches 8 hours,extending the pre-curing time does little to improve the concrete.For concrete frozen immediately,the increase of w/c contributes larger resistivity,but it is no obviously different in lower w/c concrete when they have pre-curing before subject to negative temperature.The influence of chemical admixture on frozen concrete is determined in detail.Antifreeze reduces the resistivity of immediately frozen concrete significantly.Early strength agent makes no difference to immediately frozen concrete.When the concrete is pre-curing,for the antifreeze and early strength agent,too long pre-curing time makes no influence to concrete resistivity after freezing,while resistivity depresses with the increase of pre-curing time.Fly ash and slag make the resistivity growth,nevertheless large proportion silica fume decreases resistivity when concrete subject to immediately freezing.If the pre-curing time is not enough,the concrete whose cement is partly replaced by mineral admixtures resists freezing badly.A relationships is proposed in this study that the largest electrical resistivity when concrete is frozen and the strength or strength damage are follow a power function.The value of ?? has a great influence on the compressive strength and loss rate of compressive strength when it changes less than a certain range,while it has little influence if it increases to a large magnitudes.