Research On Creep Properties And Damage Rupture Mechanism Of Concrete Under Freeze-Thaw Effect

With the implementation of strategies in China such as the western development, the revitalization of northeast old industrial zone, "the Belt and Road", many large concrete structures are being built rapidly. These works play an important role on people’s life and economic development. But harsh environmental conditions, such as the freeze-thaw environment, make their life reduce greatly and bring major economic loss to the state. Creep characteristics and the mechanism of injury rupture of concrete under freeze-thaw environment are the basis for solving such problems of engineering structural durability. In this paper, concrete specimens mixed with fly ash were selected for the study. And general mechanical properties,creep characteristics, macroscopic fracture characteristics, mesoscopic pore structure and fracture characteristics, rheological model theory and the mechanism of damage creep rupture were studied systematically by using KDS-60 rapid freeze-thawing test machine, computer controlled servo creep testing system and other modern testing technology, along with theoretical analysis. The main research results obtained are as follows:(1) Experimental study on creep properties of concrete without fly ash under the effect of freeze-thaw environment was done by using the rapid freeze-thaw testing machine and computer controlled servo creep experiment system. The variation of creep-time curves,peak stress,peak strain, modulus of elasticity, creep deformation modulus, instantaneous strain, creep rate, macroscopic fracture characteristics with freeze-thaw cycles was obtained.(2) The creep properties and that under the effect of freeze-thaw of concrete mixed with fly ash were measured systemly. The variation of creep deformation modulus and creep rate with fly ash content was obtained. The variation of creep-time curves,peak stress,peak strain, modulus of elasticity, creep deformation modulus, instantaneous strain, creep rate, macroscopic fracture characteristics of concrete mixed fly ash with freeze-thaw cycles was obtained.(3) By means of a scanning electron microscope system, the mesoscopic structural characteristics of concrete damaged under the effect of freeze-thaw and creep were measured. The variation of pore structure and fracture morphology with the number of freeze-thaw and content of fly ash was obtained. We found four kinds of pore morphology such as haptophore, a cluster of pores unconnected, a cluster of pores connected and pores formed by fly ash, five kinds of brittle fracture such as river-pattern, step-pattern, laminate-pattern, hemisphere surface-pattern and irregular shape-pattern, two kinds of ductile fracture such as dimple and peak-forest-pattern.(4) Based on rheological mechanics and damage mechanics theory, SunJun-NVPB and MSSB-NVPB rheological models were established which can describe the rheological property of concrete mixed with fly ash under the effect of freeze-thaw or not respectively. And the constitutive equation, creep equation, release equation and relaxation equation were derived respectively. Combined with the experimental data of concrete creep test under the effect of freeze-thaw, model parameters were solved, and the variation of model parameters with the content of fly ash and the number of freeze-thaw was obtained. The model theoretical and experimental values are in good agreement.(5) The damage evolution equation considering the influence of strain, content of fly ash m and the number of freeze-thaw n was established. MSSB-NVPB and SunJun-NVPB nonlinear viscous-elastic-plastic creep damage model were proposed, and the constitutive equations and creep equation were obtained considering the effect of damage.