| The temperature of the atmosphere at high-latitude coastal areas is relatively cold in winter,and the temperature difference between the coastal surface water and the atmosphere is relatively large.Due to the periodic movement of tides,the reinforced concrete(the following are referred to as RC)piers of offshore bridges have been suffering from the seawater freezethaw cycles for a long time.Combined with the existing research,it was known that the durability and mechanical properties of concrete would be significantly reduced in the high salt environment.The decrease of the durability of concrete would shorten the life of the bridges,and the decrease of the mechanical properties of concrete would affect the seismic performance of the bridges.So far,the research on the mechanical properties of concrete under the action of seawater freeze-thaw cycles was not sufficient,and the research on the seismic performance of RC bridge piers after seawater freeze-thaw cycles was rarely reported.In this dissertation,the uniaxial repeated compression constitutive of concrete,the freeze-thaw depth of RC piers,and the seismic performance of RC piers after seawater freeze-thaw cycles are studied by combining experimental tests and numerical simulation.The main work and conclusions of this dissertation are as follows:(1)Uniaxial repeated compression tests were carried out on concrete specimens after seawater freeze-thaw cycles,and the effects of seawater freeze-thaw cycles on the mechanical properties of concrete under uniaxial repeated compression were compared and analyzed in this dissertation.The results showed that:The deterioration of the concrete mechanical properties has been accelerated after seawater freeze-thaw cycles;In the uniaxial compression test after seawater freeze-thaw cycles,with the increase of freeze-thaw cycles,the initial modulus and peak stress decreased significantly,and the peak strain increased gradually;The degradation trend of the reloading modulus was not affected by the number of freeze-thaw cycles.Based on the results of repeated uniaxial compression tests of concrete specimens,the uniaxial compression damage calculation model for concrete after seawater freeze-thaw cycles was proposed.Combined with the formula of the skeleton curve in the Mander model,the uniaxial repeated compression constitutive relation calculation method of concrete was given based on the seawater freeze-thaw cycles test results.(2)According to the natural environment of offshore bridges in cold areas,a test scheme of water freezing and water thawing for RC piers of the offshore bridges was designed by taking precast segment assembled RC piers as the test object.The cement mortar spall conditions on the surface of pier segments under different seawater freeze-thaw cycles were compared and analyzed through the freeze-thaw cycles tests in a seawater immersion environment.Based on the seawater freeze-thaw cycles tests,bridge pier models after seawater freeze-thaw cycles were designed and made,and the influence of seawater freeze-thaw cycles on the seismic performance of RC piers was studied through the low cyclic loading tests.The results showed that:Under the same design variables,the seawater freeze-thaw cycles could not only reduce the peak load,initial stiffness,and cumulative energy of RC piers but also increase the peak displacement;Under the same number of seawater freeze-thaw cycles,the seismic performance of RC bridge piers after seawater freeze-thaw cycles could be improved by reducing the axial compression ration,increasing the longitudinal reinforcement diameter and decreasing the stirrup spacing of specimens.Based on the experimental research and theoretical derivation.the calculation method of restoring force model on RC piers after seawater freeze-thaw cycles was given,which provides a reference for seismic analysis of offshore bridges in the cold areas.(3)The identification method of concrete freeze-thaw damage was established by using ultrasonic tomography,and the freeze-thaw damage distribution nephogram of RC piers crosssection under different seawater freeze-thaw cycles was drawn in this dissertation.Based on this,the relationship between the number of seawater freeze-thaw cycles and the freeze-thaw depth of RC piers was determined.The results showed that:In the initial stage of the seawater freeze-thaw cycles tests,the RC piers had no obvious freeze-thaw damage;When the cement mortar on the surface of the RC piers has been peeled off in a large area,the damage distribution nephogram showed an obvious freeze-thaw damage boundary;In the later period of seawater freeze-thaw cycles tests,the variation value of RC pier freeze-thaw depth tended to be flat,indicating that the RC pier has the ultimate freeze-thaw depth in seawater freeze-thaw cycles.A numerical simulation method for the ultimate freeze-thaw depth of RC piers was given based on the thermodynamic principle in this dissertation,which provided a reference for the life prediction and numerical simulation analysis of offshore bridges in service.(4)Based on the random aggregate model of concrete,the equivalent meso-element model of concrete after seawater freeze-thaw cycles were established utilizing the meso-element equivalence method,and the two-parameter Weibull distribution of the elastic modulus of the concrete element after seawater freeze-thaw cycles were determined in this dissertation.Based on the research and combined with the results of the freeze-thaw depth of RC piers,a layered simulation method was proposed.Finally,the solid finite element model of RC pier was established which could consider the freeze-thaw damage characteristics of concrete,and the low cyclic loading tests of RC piers after seawater freeze-thaw cycles were numerically simulated.The numerical simulation results were in good agreement with the experimental results,which indicated that the modeling method proposed in this dissertation could provide a reference for seismic response analysis of offshore bridges in cold areas. |
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