Study On The Changes Of Mechanical Properties And Impermeability Of Early-age Concrete After The High Temperature

In recent years,with the improvement of social infrastructure,there are numerous cases of fire in buildings or structures under construction or completed conditions,and the direct loss resulting from this has been rising year by year.In addition,the mechanical properties and impermeability of concrete materials will change after the fire accident,and different cooling modes will affect the degree of change of the material properties.Therefore,it is urgent and practical to study the performance indexes of concrete materials after high temperature cooling.However,the study found that most of the trials in the relevant literatures were based on the?28d of the concrete standard-raising age,and rarely related to the attenuation of the high temperature properties of early-age??28d?concrete.However,when a fire occurs in an early-age concrete structure,because the concrete age is earlier,the strength is low,the strength develops fast,the free water content of the hydration part of the cement is high,the evaporation is serious after the high temperature,these coupling factors will have a great damage to the concrete material,thus affecting the long-term performance of the concrete structure,Need to arouse the attention of everyone.In this paper,2 kinds of cooling methods are tested respectively?Including:natural cooling,water spray cooling?,7early age stages?including:3d,7d,14d,21d,28d?and 7 temperature gradients?Including:300?,400?,500?,600?,700??The role of the concrete specimen macroscopic characteristics,mechanical properties and impermeability of the orthogonal test(Orthogonal test includes two parts:"Before Maintenance"means the cooling of the early-age concrete specimens directly test;"After Maintenance"refers to the cooling of the early-age concrete specimens into the standard room continued to be maintained to 150d?Curing time before and after a total of 150d?age and then test again.The main results are as follows:?1?The variation regularity of color change,crack development and mass reduction coefficientK₁?K₂?K₃ after high temperature action of early-age concrete are discussed.?2?The test of rebound value,ultrasonic wave velocity,compressive strength,modulus of elasticity and electric flux of the early-age concrete before and after the high temperature action is completed.And the variation law of each performance index under the influence of the coupling factors such as age,temperature and cooling mode was analyzed,and the empirical formula was obtained by curve fitting of the data.The relationship between compressive strength and electrical flux of concrete is established.?3?After comparing the performance indexes of early age concrete before and after curing,the recovery ability of concrete mechanical properties and impermeability were obtained.?4?Taking a 1-64m tied arch bridge in Qinghai Province as an example,the transient thermal analysis was carried out by ANSYS finite element software on the basis of conventional testing.Considering the material strength reduction,the effective section conversion is carried out according to the test,literature and standard.The finite element model is established by using Midas Civil software,and the carrying capacity checking analysis is carried out.