Recycled Aggregate / Cement Composite Shrinkage Performance Study

Based on the comprehensive utilization of construction waste in order to promote the sustainable development of the construction industry, experimental study, finite element analysis and theoretical research were used in this paper to study the drying shrinkage, basic mechanical properties and microstructure of recycled aggregate concrete. And drying shrinkage deformation mechanism of recycled aggregate concrete was studied from the point of view of the pore structure, materials wettability and moisture migration.This paper firstly studies the mechanical properties of recycled aggregate concrete in order to ensure that the mechanical properties of recycled aggregate concrete can first meet the domestic and international interrelated standards and actual project needs. On the other hand, that can provide the necessary mechanical properties' parameters for researching on shrinkage deformation of recycled aggregate concrete materials. Secondly, targeting at the situation of almost not considering the recycled aggregate and additive effect (impact factor) in concrete shrinkage mathematical model, the mathematical model of drying shrinkage was established, which considering the influence of the effect coefficient of recycled aggregate, engineering common additives (including fly ash, water reducer and expansion agent), component size, ambient humidity, strength grade of concrete and so on. Then, the paper converted the mathematical model type to strain increment type as field variables, and transformed them into thermal strain as intermediate variables through the_user subroutine USDFLD written in FORTRAN language for doing finite element analysis of drying shrinkage of recycled aggregate concrete, and the necessary verification experiments made to verify the accuracy and reasonableness of.the shrinkage model. At the end, Shrinkage deformation mechanism of recycled aggregate concrete materials was analyzed by means of finite element analysis, experimental research and micro-mechanism analysis. The results showed as followed.Firstly, mix design of natural aggregate concrete C30 is used as reference, the experiment of workability and basic mechanical properties considering the influence of recycled aggregate replacement ratio and mixing amount of fly ash, water reducer, and expansive agent are made, and the mechanism is analyzed, the results show that:the addition of recycled aggregate reduced the liquidity of recycled aggregate concrete. In the combined effect of pore structure and old cement mortar, when the recycled aggregate replacement ratio is lower, the addition of recycled aggregate will increase the strength of recycled aggregate concrete, but the excessive addition of recycled aggregate would result in the mechanical properties of recycled aggregate concrete lower than the natural aggregate concrete under the same conditions. As a result of "rolling effect", the addition of fly ash increases the liquidity of recycled aggregate concrete. On the other hand, because of improving pore structure of recycled aggregate concrete, the addition of fly ash improves the mechanical properties of recycled aggregate concrete. In the case of fixed amount of mixing water and cement content, the addition of water reducing agent is equivalent to increase he relative amount of mixing water, so that can improve the liquidity of recycled concrete aggregate. As a result of increasing the porosity, the strength of recycled aggregate concrete decreases with the water reducer dosage increases. The liquidity of recycled concrete aggregate decreases with the addition of expansion agent, but at the same time, the addition of expansion agent has increased the sticky cohesion and retention of mixture. The addition of proper amount of expansive agent is helpful to improve the strength of recycled aggregate concrete, but the excessive addition of expansive agent can lead to the intensity decrease. Through comparative analysis of experimental test value and calculated value of the elastic modulus of recycled aggregate concrete from existing mathematical models, the paper proposals that the type can be used in predicting elasticity modulus value of RC.Secondly, on the basis of the experiment sdudy and regression fitting which method is Levenberg-Marquard method+GM global optimization, the paper put forward a mathematical model predicting drying shrinkage of recycled aggregate concrete, which consider effect coefficient of recycled aggregate, fly ash, water reducer and expansive. And transform model type into strain increment to complete finite the element analysis of drying shrinkage of recycled aggregate concrete. In above process, it is necessary to transform the field variables into thermal strain as intermediate variables, which can achieve through user subroutines USDFLD written in FORTRAN language. The results show that:In early drying age, the Mises stress and displacement (shrinkage) value of recycled aggregate concrete growth is very fast, but with the development of drying age, the process gradually become slower. The maximum displacement (shrinkage) value appears in the top surface of the recycled aggregate concrete specimen; the displacement change first occurs in the surface of the specimen. Drying shrinkage spread to 50mm depth need to spend 30 to 70 days. The error analysis shows that, the simulated shrinkage values of recycled aggregate concrete agree well with the experimental test data. Under the study conditions in this paper, the displacement and stress value of recycled aggregate concrete demonstrated good linear relationship. Under the same conditions, recycled aggregate concrete shrinkage value was slightly higher than the natural aggregate concrete's.Thirdly, based on the pore structural model, the paper establishes the shrinkage theory of recycled aggregate concrete. Through experiment study, simulation research and micro-mechanism analysis, the influence are explored, of which the addition of recycled aggregate, water reducer, expansive agent and fly ash on pore structure, water-wettability and moisture migration of recycled aggregate concrete, and further analysis was made for drying shrinkage deformation mechanism of recycled aggregate concrete. The conclusions are as follows. The mechanism of shrinkage of recycled aggregate concrete is similar to the natural aggregate concrete's. Drying shrinkage deformation of recycled aggregate concrete is the interaction results of the pore structure, water-wettability, and moisture migration. The drying shrinkage value of recycled aggregate concrete can reduce through improving the pore structure, reducing the water-wettability (increasing contact angle) and decreasing water diffusion rates. Using the above theory can well explain the influence of water-cement ratio, recycled aggregate, water reducer and fly ash to the shrinkage of recycled aggregate concrete. While, expansive recycled aggregate concrete shrinkage mechanism can be explained using the theory of "opposite deformation" and "backward direction deformation".Fourthly, using the mass diffusion module of finite element analysis software ABAQUS, this paper simulation calculated the moisture diffusion behavior of recycled aggregate concrete, and the validation experiment was made. Throuth study research, this paper has analyzed the relationship between shrinkage value and water loss of recycled aggregate concrete. The results show that:The evaporation of water starts first from the surface of recycled aggregate concrete specimen, and the water concentration quickly reaches the equilibrium value, and then gradually extended to the internal in extremely slow rate, and ultimately achieving the balance with the ambient humidity. In the diffusion process, the water concentration takes on a gradient distribution. The water evaporation extended to 50mm thickness needs to spend about 33d to 65days' time, the specific time due to the different mix design of recycled aggregate concrete. Under the same conditions, the shrinkage values and water loss of recycled aggregate concrete is not a simple linear relationship. The error analysis shows that, the calculated value of water loss agrees well with that of the experimental test value.The researches in the thesis would establish basis for studying the drying shrinkage deformation of recycled aggregates concrete and provided the necessary technical support and theoretical basis for resource and scale utilization of recycled aggregates.