Study On Creep Model Of Cement-based Materials Considering Interface Transition Zone

Cement-based material is a heterogeneous multi-phase composite material,the interface transition zone(ITZ)is the weakest link in each phase,and its mechanical properties have an important impact on the macroscopic properties of cement-based materials.The creep is an inherent time-varying characteristic of cement-based materials,which directly affects the mechanical properties and long-term deformation of structures.The meso-mechanical model of cement-based materials has important guiding significance for engineering practice.The existing meso-model of cement-based materials considering ITZ are mostly used to analyze the elastic properties of materials.There are few studies on the creep model of cement-based materials that directly involving the consideration of the ITZ.Considering the great correlation between the creep problem of cement-based materials and time,these elastic models are slightly lacking in applicability.Based on this,starting from the meso-level in this paper,the cement-based materials were regarded as multiphase composites composed of cement matrix,ITZ and reinforcement.The creep prediction model of cement-based materials considering the ITZ was established by using the meso-mechanical theory,Laplace transform principle and shear lag theory,and the mechanism of ITZ in creep was revealed.The viscoelastic model was used to fit and analyze the test data,determined that the Burgers model was more suitable for characterizing the creep properties of cement-based materials.On this basis,and the concrete creep prediction model considering the viscoelasticity of the ITZ was established by applying the Laplace transformation principle and mesomechanical theory,which was verified by the test data of the research group.The results showed that the proposed creep model of concrete considering the ITZ was consistent with the experimental results.The model could be used to analyze the effect of ITZ on the creep properties of concrete.Compared with the creep model value without considering the ITZ,the specific creep of concrete specimens with different fly ash contents(i.e.,0,30% and 60%)and slag contents(i.e.,30 and 60%)was increased by45.7,25.1,66.7 and 31.3,42.1%,respectively,when the loading age is 365 d due to the ITZ.According to the model,the contribution of the ITZ to concrete creep was discussed.The analytical formula for quantitative calculation of the volume fraction of the ITZ was deduced by using the aggregate grading formula and nearest surface distribution function.The effects of the ITZ thickness,aggregate volume fraction and the fineness of aggregate on the volume fraction and overlapping degree of the ITZ were further discussed.It is determined that the ITZ thickness is the key factor affecting the volume fraction and overlap degree of the ITZ.The ITZ was regarded as a "alone phase",and the volume fraction of the ITZ was calculated by the derived analytical formula.The creep prediction model of cement-based particle reinforced composites with "alone phase" of ITZ was established,which was verified by the test data.According to the model,the effects of the ITZ thickness,the ITZ viscoelastic parameter,the aggregate volume fraction and the fineness of aggregate on the creep properties of concrete and mortar were discussed.It is determined that the ITZ viscoelastic parameter was the key factors affecting the creep of concrete and mortar.Based on the three-phase model and shear lag theory,a creep prediction model of steel fiber reinforced concrete(SFRC)considering the ITZ was proposed,which was verified by the creep data of the research group and revealing the ITZ’s role in it.The results showed that the specific creep values of SFRC at the loading age of 365 d with 1,2,and 3% steel fiber contents exceeded those assessed without the ITZ effect account by 9.8,6.3,and 54.0%,respectively.The effects of the ITZ elastic modulus,ITZ thickness and the steel fiber length-to-diameter ratio on the SFRC creep,were analyzed via the proposed model.It is determined that the ITZ elastic modulus was the key factor affecting the SFRC creep performance.This paper establishes a cement-based material creep model considering the ITZ,reveals the action mechanism of the ITZ on the creep of cement-based materials,and determines the effect of cement-based materials creep performance.It can provide model basis and theoretical reference for active control of the creep of cement-based materials.