Study On Dynamic Mechanical Properties Of Freeze-Thawed Concrete By Hopkinson Pressure Bar

Freeze-thaw cycles of concrete have significant negative impact on bearing capacity and durability of concrete structures.Current studies on mechanical properties of concrete after freeze-thaw cycles mainly concentrate in the area of statics,while few of studies were on its dynamic mechanical properties.In this respect,experimental studies on dynamic mechanical properties of freeze-thawed concrete by using the Split Hopkinson Pressure Bar(SHPB)were conducted.The main work in this thesis are as followed:(1)Rapid freeze-thaw tests of C40 and C60 concrete,with freeze-thaw cycles of 0,25,50,75,100,125 times for C40 concrete and freeze-thaw cycles of 0,75,125,175,225 times for C60 concrete.The influence of freeze-thaw cycles on the damage of concrete surface,as well as the mass loss and relative dynamic modulus of elasticity of concrete are studied;(2)Static compressive tests on C40 and C60concrete cubes(100mm×100mm× 100mm)and concrete cylinders(φ86mmx45mm)were conducted,by which the influence of freeze-thaw cycles on the static compressive strength of concrete was discussed;(3)SHPB tests on cylinder specimens of freeze-thawed concrete were conducted.Impact velocities of 4m/s,6m/s,8m/s,9m/s and 10m/s for C40 concrete,and 6m/s,8m/s,9m/s,10m/s and 11m/s for C60 concrete respectively.The stress-strain curves of freeze-thawed C40 and C60 concrete under various strain rates were obtained by the“Three Waves Method";(4)Dynamic Increase Factors(DIF)of concrete after freeze-thaw cycles were calculated based on SHPB test results,and the predicting formula of DIF with freeze-thaw cycles and strain rate was obtained;Freeze-thaw damage degradation factor ξ was adopted to describe the relationship between relative value of C40 concrete’s dynamic compressive strength and free-thaw cycles,and the dynamic visco-elastic damage constitutive model for C40 concrete after freeze-thaw cycles was built based on Weibull distribution and component combination theory.Based on the experimental studies above,main conclusions obtained by this thesis are shown as following:(1)Concrete strain rate increases with the increase of impact velocity in SHPB tests.Strain rate of C40 concrete is higher than that of C60 concrete while the impact velocity is the same and C40 and C60 concrete’s formula of strain rate and impact velocity is fitted;(2)With the increase of strain rates,dynamic peak stresses of freeze-thawed C40 and C60 concrete increas,while the peak strains increase at first and then decrease;(3)With the increase of freeze-thaw cycles,dynamic peak stresses of C40 concrete decrease while the dynamic peak strains increase,and dynamic peak stresses of C60 concrete increase then decrease because of excellent frost resistance.After 225 times freeze thaw cycles,the dynamic peak stresses of C60 concrete are still higher than that of C60 concrete without freeze-thaw cycles.And the freeze-thaw cycles have little influence on dynamic peak strains of C60 concrete;(4)The relationship of DIF with logε and freeze-thaw cycles n were obtained by fitting test data,and good predictions were obtained;(5)The dynamic visco-elastic damage constitutive model for C40 concrete has great consistency with the stress-strain curves obtained from the experiment.