Constitutive model of early age concrete and its structural performance

The more extensive construction of massive concrete structures strengthens the requirement for the control of early age cracking. Early age cracking not only affects the visual appearance but also reduces the stiffness of structures and leads to increased deformation, harming the usability of underground engineering and hydraulic structures. Moreover it can significantly accelerate the corrosion of steel bars and negatively influence the general durability of concrete structures. On the other hand, a more intensive application of high strength concrete can further increase the cracking risk due to the higher temperature rise and more marked volumetric deformation. Numerical analysis has been widely used to analyse the structural performance. A precise constitutive model is needed for the early age concrete in this respect. Although significant developments concerning constitutive models have already been made for a long time, the early age behaviour of concrete needs further attention. The first objective of this study is to build a hydration model and a basic creep model as a function of the degree of hydration by experimental investigation, including isothermal hydration test, basic creep tests under constant and varying stress level. The second objective of the thesis is to develop a constitutive model for early age concrete that takes into account the creep behaviour under varying stress level. The restrained effect of concrete structures is also studied. The resulting models are of fundamental importance both for a clear understanding of early age concrete and for further improvement of the performance of massive concrete structures. Extended hydration model A hydration model for concrete based on a ternary blend consisting of ordinary Portland cement with strength grade 42.5N/mm2, fly ash and slag is developed. The application of this kind of ternary blend is common practice for massive concrete elements in China. Heat generated by hydration of cement is the main cause of early age thermal cracking. The hydration behavior of early age concrete is accompanied by strongly time and temperature dependent thermal and mechanical material properties. Time is a quite important concept for the performance of early age concrete. Moreover, temperature is as significant as time in the analysis of the performance of early age concrete, which can influence the hydration of cement and then affect the development of shrinkage, creep and so on. Based on a previously developed general hydration model for Portland cement and blast furnace slag cement, a hydration model for concrete based on a ternary blend consisting of ordinary Portland cement with strength grade 42.5N/mm2, fly ash and slag is developed. According to the isothermal hydration tests, the hydration of blended cement can be divided into two parts: a Portland reaction (P- reaction) and a slag reaction (S- reaction). The simulation has been done for both the P- reaction and S- reaction. The influence of fly ash on cement hydration and the start time of slag are discussed in this thesis as well.Basic creep model The creep behaviour of early age concrete can quickly become non-linear. However, creep is a very important phenomenon for early age concrete. Due to relatively high creep deformations and considerable relaxation properties, early age thermal stresses are influenced in a significant way. In literature the calculation of early age creep is focusing on the influence of loading age of concrete and the duration of load. Most creep tests are controlled at constant stress level. This approach however, remains inadequate and falls short to explain or predict the creep behaviour. Moreover, stress levels in reality do not remain constant, especially in the early age stage. An experimental program has been set up in order to examine the basic creep behavior of high performance concrete at early age. Based on the experimental results, a fundamental model based on the degree of hydration for the basic creep under constant stress level and varying stresses are developed, which can be used to simulate the creep behavior of early age concrete. The fictitious degree of hydration method and principle of superposition have been applied to take into account both the loading and unloading of stress during hydration. Constitutive model An incremental non-linear visco-elastic constitutive model for concrete behaviour at early age is developed. The total constitutive formulation, i.e. total strains and stresses have been widely used for the development of constitutive models. The disadvantage of this method is that the total history has to be stored during the calculations. An incremental formulation is applied in the constitutive model developed in this thesis. The time dependent properties, such as shrinkage and creep are described in an incremental formulation. Constitutive models for concrete during early age hydration, and in particular those concerning creep behaviour, are discussed. Finally, the constitutive model of early age concrete is implemented in the general FEM program ABAQUS, which can be used for the analysis of early age concrete and newly cast massive concrete structures that suffer from thermal cracking and shrinkage cracking. Restraint model Restraint is the result of a mechanical interaction between solid bodies. The contact or connection between different structures or structural elements will cause external restraint. Due to restraint actions, stresses will also be induced when a structure is exposed to settlements and/or thermal effects. This phenomenon is quite common during underground construction. Nevertheless, no clear specification exists to estimate the effects of structural restraint during design stage. In order to implement this procedure, any possible restraint on structural members is categorized into three types in this thesis. The parameters defining each restraint type are deduced based on relevant suppositions. Finally, the influence of the boundary restraint on the performance of early age concrete structures is illustrated through examples and a case study