Studies On Short-term And Long-term Behavior Of Corroded Prestressed Concrete Beams Under Freeze-thaw Cycle

Prestressed concrete is widely used in civil engineering structures,which has obvious advantages in durability,crack resistance,anti-torsion and material saving compared with ordinary reinforced concrete.However,the prestressed concrete structure will also suffer from the erosion of the external environment,such as freezing damage and corrosion,which often leads to the applicability and durability problems such as structural degradation,excessive deflection and reduced bearing capacity when the structure fails to reach the design service life.This not only increases the cost of detection and maintenance of the late structure,but also may lead to premature failure of the structure when it is more serious,resulting in huge losses of life and property and endangering people ’ s life safety.There are many prestressed concrete structures in the northern coastal areas of China,which are affected by freeze-thaw cycles and corrosion.Under the action of freeze-thaw cycles,the mechanical properties of concrete decrease,the compactness of materials decreases,and the permeability increases.The harmful ions in the environment accelerate to penetrate into the internal structure,which aggravates the corrosion of steel bars and steel strands.With the increase of corrosion rate,the corrosion expansion cracking of concrete and the continuous development of cracks accelerate the deterioration of prestressed concrete structure.Therefore,under the combined action of freeze-thaw and corrosion,the damage of concrete and prestressed reinforcement materials continues to accumulate and promote each other,and prestressed concrete structures are more prone to premature failure.In addition,in the process of long-term service of prestressed concrete structures,in addition to the durability degradation from the external environment,the mechanical properties of concrete and prestressed reinforcement materials will also evolve over time,such as material strength,elastic modulus,etc.Among these time-varying effects,the creep-shrinkage effect of concrete and the relaxation effect of prestressed reinforcement are the most significant and critical effects on the application performance of prestressed concrete beams.Under the action of long-term load,the creep and shrinkage effect of concrete and the relaxation effect of prestressed reinforcement will lead to the increase of prestress loss in prestressed concrete structures.The internal force of the structure is redistributed over time,resulting in the secondary internal force of the structure.The most critical problem is that the deflection increases over time,which seriously even leads to the over-limit deflection of the structure in the design life.It can be seen that the long-term performance of prestressed concrete structure is not affected by a single factor,but by the combined action of load,external corrosive medium,concrete creep and shrinkage,and prestressed reinforcement relaxation.The mechanism of action is not a simple superposition of a single factor,but the promotion of different factors.These factors make the long-term behavior of the structure more complex and must be considered comprehensively in the evaluation process.At present,there is a lack of short-term and long-term research on corroded prestressed concrete structures under the action of freeze-thaw cycles.In order to make up for the research vacancy,this paper conducts a numerical analysis of the shortterm and long-term mechanical properties of corroded prestressed concrete beams under the action of freeze-thaw cycles.The research results are of great significance for the durability evaluation and safety evaluation of prestressed concrete beams in the future,and also provide a theoretical basis for the practical engineering application of prestressed concrete structures in the future.The main work of this paper is as follows :(1)The short-term mechanical behavior of corroded prestressed concrete beams under freeze-thaw cycles was studied,and the short-term numerical analysis model of corroded prestressed concrete beams under freeze-thaw cycles was established.The plastic damage model is used to simulate the material properties of concrete,and the temperature reduction method is used to impose prestress on concrete.The material damage and nonlinearity caused by freeze-thaw cycles and corrosion are considered,and the influence of concrete strength,corrosion rate,freeze-thaw cycles and prestress level on the short-term mechanical properties of prestressed concrete beams is clarified.The results show that the final failure mode of prestressed concrete beams will change due to the different severity of freeze-thaw cycles and corrosion.The flexural performance of prestressed concrete beams under the coupling effect of freeze-thaw cycle and corrosion is significantly lower than that under the single factor effect.The effect of freeze-thaw cycles on the bearing capacity of beams is remarkable.The ultimate bearing capacity of corroded prestressed concrete beams with 150 freeze-thaw cycles is reduced by 17.9 %compared with that without freeze-thaw cycles.Higher prestress level can only improve the early stiffness of corroded prestressed concrete beams under freeze-thaw cycles,and has little effect on the later deflection and ultimate load.The established model can be used to predict the short-term mechanical properties of corroded prestressed concrete beams under freeze-thaw cycles.(2)Based on current standard and specification and relevant literature model,the parameter variables considering the corrosion rate and the number of freeze-thaw cycles are introduced,and the calculation formula of cracking load of corroded prestressed concrete beams under freeze-thaw cycles,the calculation formula of bending capacity under compression and tension failure mode,and the short-term stiffness correction formula after freeze-thaw cycles are proposed.The calculation results of the formula are in good agreement with the simulation results,which proves that the formula proposed in this paper has certain applicability to calculate the short-term mechanical properties of corroded prestressed concrete beams under freeze-thaw cycles.(3)The theoretical analysis model of long-term mechanical behavior of corroded prestressed concrete beams under freeze-thaw cycles is developed.The analysis model considers the material damage caused by creep shrinkage,cracking,relaxation effect of prestressed reinforcement,and freeze-thaw cycle and corrosion of concrete.The model adopts the step-by-step calculation method of the integral creep and shrinkage model which meets the superposition principle.The equation proposed by Magura et al.is used to consider the relaxation effect of prestressed reinforcement,and the diffusion crack model is used to consider the cracking of concrete.Finally,based on the formula of uncracked concrete,a numerical analysis model is developed to simulate the crack in a time process.The established numerical analysis model can be applied to the long-term mechanical performance and life prediction of corroded prestressed concrete beams under freeze-thaw cycles.(4)Based on the above analysis model,the effects of concrete creep,shrinkage,relaxation of prestressed reinforcement,concrete strength,corrosion rate,freeze-thaw cycles and prestress level on the long-term time-varying performance are studied.The results show that the long-term deformation of prestressed beams can be significantly reduced by using high-strength concrete,and the long-term deflection of prestressed concrete beams with concrete grade C35 is 32.7 % higher than that of beams with concrete grade C50;the number of freeze-thaw cycles has the greatest influence on the long-term deformation of the prestressed concrete beam.The long-term deflection of the prestressed concrete beam with 1000 days of load and 150 freeze-thaw cycles is about 4 times that of the beam without freeze-thaw cycles.The corrosion of prestressed reinforcement has little effect on the long-term deflection.The higher the prestress level,the more can inhibit the development of deflection of prestressed concrete beam,but the prestress loss of beam also increases.