Experimental Study And Numerical Feedback On Creep Poisson Effect Of Multi-Age Dam Concrete Under Loading/Unloading

Creep is an important factor that cannot be ignored in the simulation calculation of the temperature stress field of dam concrete and the feedback analysis of the measured stress.The current elastic creep theory and method widely used in concrete dam projects implicitly assumes that the elastic Poisson’s ratio,creep Poisson’s ratio,and effective creep Poisson’s ratio are equal.However,the differences in creep test conditions and strain measurement methods,etc.,have led to controversy over this assumption.In order to accurately grasp the true stress state of dam concrete,this paper designs and conducts uniaxial compression and conventional triaxial loading and unloading creep tests on dam concrete under uniform test conditions to explore the intrinsic relationship between the elastic Poisson’s ratio,creep Poisson’s ratio and effective creep Poisson’s ratio,and then conducts numerical feedback on the creep calculations of meso-scale secondary-matched dam concrete based on the measured creep data,and explores the variation of the elastic modulus,elastic Poisson’s ratio,specific creep,creep Poisson’s ratio of the mortar in concrete.The main research contents are as follows:（1）Experimental study of uniaxial and conventional triaxial creep tests of dam concrete under loading and unloading.Combined with dam concrete with water binder ratio of 0.5 and fly ash content of 35%,uniaxial compression creep test with different loading ages（3d/14d/28d/60d）and conventional triaxial creep test with different loading ages（14d/28d/60d）under unified test conditions were designed and carried out,and relatively complete loading and unloading creep test curves were obtained.Then the Savitzky-Golay filter is introduced to denoise the creep test data,and based on the denoised creep test data,the strain variation law of uniaxial and conventional triaxial creep tests in each stage is studied.The results show that:①Under uniaxial stress state,the ξ_r-V and ξ_r-H at the same loading age is approximately equal,and increases with the increase of loading age,and the range is generally between 0.7 and 0.9.Under conventional triaxial stress state,theξ_r-V and ξ_r-H of the same loading age is basically the same,and its value has no obvious relationship with the loading age,generally around 0.8.②The creep test data under the unified test conditions have basically the same noise,and the S-G filter can obtain the optimal de-noising effect under the filter parameter combination of W=9 and N=6.③Under uniaxial stress state,the ε_c-V and ε_c-H decreases with the increase of loading age,while the ε_d-V and ε_d-H increases with the increase of loading age.Under conventional triaxial stress state,the ε_c-V of different loading ages is basically the same,the ε_c-H decreases slightly,and the corresponding ε_d-V and ε_d-H has little difference.（2）Study on Poisson’s ratio relationship of dam concrete in different ages and stress states.Based on the whole process strain of creep test after denoising,the elastic Poisson’s ratio of uniaxial compression creep test and conventional triaxial creep test in loading and unloading stage,creep Poisson’s ratio and effective creep Poisson’s ratio in load holding stage,and creep recovery Poisson’s ratio after unloading are calculated and analyzed.The results show that:①Under uniaxial and conventional triaxial stress states,the calculation of creep Poisson’s ratio and creep recovery Poisson’s ratio is more ideal when considering creep（recovery）strain plus elastic strain.At the same time,for the calculation of Poisson’s ratio under conventional triaxial stress state,Jordaan-Illston formula is better.②Under uniaxial stress state,there is a relationship between the Poisson’s ratio of loading and unloading in the whole process of the same loading age,that is,v_d+rec^U≥(v_el^U≈v_rec^U)>v_c+el^U.The v_d+rec^U is the largest,the v_el^U and v_rec^U is not equal to the v_c+el^U.The correlation between Poisson’s ratio and loading age is not strong,and the range is generally between 0.10 and 0.20.③Under the conventional triaxial stress state,the Poisson’s ratio of the same loading age has the same size relationship,viz,(v_el^T≈v_rec^T)≥v_d+rec^T>v_c+el^T.The v_el^T and v_rec^T is also not uqual to the v_c+el^T.The Poisson ratio of the whole loading and unloading process is between 0.25 and 0.30 under different loading age and is larger than that of the corresponding Poisson ratio under uniaxial stress state.④The difference between the v_cp,i and the v_d,i in each direction is large,which indicates that the creep（recovery）strain in each direction is not completely independent under multiaxial stress state.At the same time,the v_cp,i is also different from the v_c+el^T,v_el^T and v_rec^T under the same loading age.（3）Numerical simulation of creep of dam concrete at meso-scale.The concrete is regarded as a three-phase composite material composed of aggregate,mortar and interface transition zone between them,and the creep finite element model of concrete at meso-scale is established.Firstly,based on the measured strain in the loading stage of uniaxial compression creep test,the elastic modulus and Poisson’s ratio of aggregate and the elastic modulus expression and elastic Poisson’s ratio of mortar are optimized.Then,based on the measured creep of concrete in the load holding stage,the specific creep expression and the creep Poisson’s ratio of mortar are fed back.The results show that:①Mortar has similar mechanical properties to concrete.It is reasonable and effective to introduce the combined exponential formula and eight parameter creep model to describe the elastic modulus and specific creep of mortar respectively.②The elastic Poisson’s ratio of aggregate and mortar obtained based on the inversion of measured strains ranged from 0.13 to 0.20,and the mortar creep Poisson’s ratio was about 0.125.Moreover,the specific creep of mortar in concrete was generally 2.5 times higher than the measured specific creep of concrete.③ The inversion results of aggregate and mortar parameters show that it is necessary to invert the mechanical parameters of each component of concrete based on the measured data when the meso-scale numerical calculation of concrete is carried out.For the creep parameters of mortar,the combination of optimization inversion and optimization identification method can be more efficient.