Study On Frost Resistance Of Magnesite High Strength Concrete Under Coupling Stress And Freezing-Thawing

The normal working environment of concrete structure is very complex,and it will be deteriorated under the coupling effect of multiple factors.Freeze-thaw damage is the second major factor for the damage of concrete members,which is only second to the corrosion damage of reinforcement.The damage speed under the coupling effect is much larger than the simple superposition of various factors.The reserves of magnesite in China ranks first in the world,with Liaoning as the largest one.However,low-grade magnesite is mostly accumulated and not fully utilized.The rational use of low-grade magnesite will bring coordinated economic and environmental development to Liaoning.Prestressed specimens were made of C60 and C70 concrete with low-grade magnesite gravel as coarse aggregate.Four kinds of prestressed specimens and non-prestressed specimens were designed,including axial compressive stress,eccentric compressive stress,axial tensile stress and bending stress,to study the frost resistance of high-strength magnesite concrete under the coupling effect of stress and freeze-thaw.The main research contents and results are as follows :(1)The C60 and C70 magnesite concrete specimens with different prestress and stress levels were designed,and the freeze-thaw test was carried out after loading prestress.The stress levels of axial compressive stress specimens are 0.25,0.37,0.50;the stress level of eccentric compressive stress specimens is 0.50;the stress levels of bending stress specimens are 0.15,0.17,0.19;the stress levels of C60 and C70 concrete specimens under axial tensile stress are 0.06,0.09,0.12 and 0.09,0.12 and 0.15,respectively.The test results show that the frost resistance of high strength concrete is good,and the higher the strength grade is,the better the frost resistance is.The loading prestress will accelerate the damage rate of concrete,and the higher the prestress level is,the faster the damage rate is.The axial compressive stress at low stress level can inhibit the damage rate of concrete at the early stage of freezing and thawing.(2)The pre-stress loss test is carried out before loading compressive stress,the purpose is that the pre-stress loss of steel strands can basically reach the load design value after 48 hours when the specimen is loaded by super tension.The reasonable over-tension coefficient η is1.119,the 48 h prestress loss ratio is 10.70 %,and the relative deviation is about 5 %.During the freeze-thaw period,the prestress loss of compressive stress specimen is slow due to shrinkage and creep,and the higher the stress level,the faster the prestress loss.(3)Using the experimental data of magnesite high strength concrete under the coupling effect of axial stress and freeze-thaw,the axial stress correction coefficient obtained by regression analysis is introduced to modify the freeze-thaw life prediction model of Li Jinyu to predict the service life of concrete.