Study On The Creep Characteristics Of Hydraulic Concrete And Its Engineering Application—Macro-meso Interacting And Thermo-dynamic Driving Mechanism

The material properties of mass concrete play a vital role in the workability and stability during the long service life of concrete structures(e.g.concrete dams)in hydraulic engineering.Creep is one of the typical and important aging deformations through the whole life cycle of concrete structures,to which special attentions have always been paid in the material and structure designs,temperature analysis and control.Therefore,accurate simulation and prediction of creep is of great significance to the durability and safety of mass concrete structures.Concrete,as an artificial composite material,not only its components are complex,but also its mesostructure and components’ properties vary significantly with aging,temperature and other external conditions.However,due to the limitations of laboratory test and the complex characteristics of concrete itself,it is insufficient to invseitigate the macro-meso interacting mechanism of concrete merely by test,and it is difficult to conduct comprehensive and systematic tests of concrete properties for many engineering projects.Fortunately,meso-mechanics research can make up the situation by studying the relationships between the mesostructure,components’ properties and the macroscopic behaviours of concrete based on its multiphase structure characteristics.However,due to a tremendous computation capability demended,we are encountered with challenges in the numerical simulation of mass concrete structures using,for example,the mesoscale finite element method directly.Meanwhile,the analytical solution using meso-mechanics is rather difficult to get blamed for the complex mesostructure of concrete,especially the hydraulic concrete with high volume fractions of aggregate.Therefore,inspite there are many mesoscale studies on concrete,their engineering applications are less common.In addition,the meso-mechanics research of concrete creep is still insufficient at present,for example,few research is focused on the mechanism of temperature effects on creep at the mesoscale level,making the establishment of well-performed creep prediction model as an unsolved problem.To counter the problems above,this dissertation carries on a systemic multi-scale research from mesoscale(e.g.tests of components’ properties and mesoscopic simulations of multiphase concrete)to macroscale(e.g.tests of concrete properties and macroscopic simulations of homogeneous concrete).Based on the macro-meso mechanism and the thermo-dynamic driving mechanism,this dissertation proposes a meso-macro recursive method to predict the concrete properties(e.g.hydration characteristic,elastic modulus and creep)with the influence of temperature,and applies this method to an actual engineering practice.The main achievements of the research are summarized as follows:(1)A state-of-the-art review is given,including the macro-mechanics and mesomechanics research of composite materials and its application,and the mechanism,test process and prediction methods of concrete creep.Some critical scientific issues and shortcomings are presented,and the main contents and roadmap of the research are introduced.(2)Based on the modeling technique of a 3D mesoscale concrete,the elastic modulus and creep are simulated numerically in mesoscopic.After been validated by experimental data,parameter sensitivity analysis is carried out w.r.t the mesoscale factors such as components’ properties and mesostructure.According to the mesoscale numerical simulation and macro-meso interacting mechanism,an existing prediction model of the elastic modulus is modified,and a new creep prediction model is proposed.Both models are able to better describe the influences of components’ properties,interactions and mesostructure on the macroscopic behaviours of concrete.In other words,a meso-macro prediction methodology is established to obtain the macroscale effective properties of concrete based on the mesoscale components’ properties and the mesostructure,which is useful for the engineering practice lacking of test conditions to predict the properties of multi-grades concrete.(3)The thermal properties of concrete are also studied by the meso-macro recursive research method,in which the hydration characteristic is investigated based on the thermo-dynamic driving mechanism.The impact of the inactive component(i.e.sand)on the hydration of cementitious materials is studied by the hydration heat release rate tests of cement paste and mortar specimens.A hydration model with the consideration of temperature effect is established based on the equivalent age and hydration degree,and is validated by experimental data.The effective thermal parameters of concrete are predicted by mesoscale simulation and homogenization calculation,too.(4)Creep tests of concrete with different aggregate volume fractions(0,20%,40%)under different curing-loading temperatures(20℃,30℃,40℃)are conducted.The test results demonstrate the dual effect of temperature on creep: rising temperature will accelerate the concrete aging and reduce creep,but it also can decrease the viscosity of the flowing phases inside concrete and promote creep;and due to the competition of these two effects,the final influence of temperature on creep varies with aging.The comparison between the experimental data and predictive results of MC2010 model and B4 model indicates that,the existing creep prediction models are still inadequate in covering temperature impact comprehensively.(5)A creep model based on the equivalent age and the reduced time parameter is proposed to take into account the dual effect of temperature,afterwards it is employed to simulate the creep test in(4).According to the mesoscale numerical simulation and the thermo-dynamic driving mechanism,the proposed creep predictive model in(2)is further extended by introducing two functions to describe the temperature impacts on maturity and viscosity,respectively.This is equally to declare that,based on the mortar creep at a certain reference temperature,a meso-macro recursive model to predict the concrete creep at any temperature has been established and validated by experimental data under constant temperature and varying temperatures.(6)For a concrete arch dam,a macroscale finite element simulation from pouring to completion and operation is carried out.In such a computation,the effective properties of concrete predicted by the properties of mortar and aggregate and the mix proportion of concrete,are employed.The simulation results are cross-referenced with the monitoring data to validate the applicability of the proposed meso-macro predictive models in practical engineering.This application case provides a frame for solving the mismatch problem between routine laboratory experiment,mesoscale research and engineering exercise demands.In addition,it demonstrates the necessity of considering the temperature effects on concrete properties,particularly in estimating the workability and cracking safety of mass concrete dams.