Multi-scale modeling of structural concrete performance is presented as a systematic knowledge base of coupled cementitious composites and structural mechanics. An integrated computational scheme is proposed for life-span simulation of reinforced concrete. Conservation of moisture, carbon dioxide, oxygen, chloride, calcium and momentum is solved with hydration, carbonation, corrosion, ion dissolution, damage evolution and their thermodynamic/mechanical equilibrium. The holistic system is verified by the reality.
This paper presents a new method for numerically calculating the concentration profiles of both solid calcium and total chloride ions (Cl−) in concrete in contact with 3% (0.5 mol/l) sodium chloride (NaCl) solution. Since the diffusion of ions present in the pore solution is a primary controlling factor, the application of mutual diffusion coefficients of corresponding ions that are influenced by the concentration of other coexisting ions is proposed. The method of calculation is based on the generalized form of Fick's First Law suggested by Onsager, which is composed of the Onsager phenomenological coefficient and the thermodynamic force between ions, which occurs according to the gradient of electrochemical potential in a multicomponent concentrated solution for the pore solution. In addition, the chemical equilibrium for Ca(OH)2 dissolution and C-S-H decalcification are also modeled and coupled with diffusion. Increased porosity due to dissolved Ca2+ and a chloride binding isotherm are taken into consideration. The concentration profiles of solid calcium and the presence of Friedel's salt in mortar specimens are experimentally identified by the X-ray diffraction method (XRD) and the thermal analysis (TG/DTA) as well as the total chloride profile using an acid extraction method after three years of exposure to 0.5 mol/l NaCl solution. This experimental result verifies the calculation result.
Reinforced concrete structures in a marine environment suffer deterioration caused by the corrosion of reinforcement. Various research projects have investigated this problem, yet few evaluation techniques have been developed until now. In this study, a simulation model for deterioration of concrete structures due to chloride attack was constructed. This model combines chloride and oxygen penetration models and a reinforcement corrosion model, taking into consideration variations in concrete quality and the existence of defects such as cracks. By using the proposed model, it was possible to estimate the progress of reinforcement corrosion and the corrosion crack generation timing in concrete.
A comprehensive numerical system is proposed for the attempt to solve the problem of the deterioration of reinforced concrete structures subjected to chloride-induced corrosion. This numerical system was created by combining the physicochemical models related to the migration of corrosion-related substances in concrete, such as chloride ions, oxygen and vapor, and the electrochemical models dealing with the formation of half-cell potential from the viewpoints of macrocell corrosion and corrosion current of RC members under chloride-induced corrosion. Chloride ions concentration, oxygen concentration, water content, current density and the loss of cross-sectional area at any position of the rebar can be calculated by this system.
It is well known that degradation of properties such as increase in porosity of the surface layer and reduction in strength occur because of material transfer due to dissolution and diffusion of hydration products when concrete comes in contact with water. However, methods for quantitatively evaluating the changes in properties have not yet been established. The authors studied the applicability of immersion tests of cement paste using cation-exchange resin and studied actual concrete structures of ages ranging from 34 to 104 years. The authors also constructed a model of degradation for physical properties such as porosity and strength due to leaching, based on the results of these studies.
Control of thermal cracking in young concrete is of great importance to ensure a desired service lifetime and function of a structure. Young concrete is here defined as the period up to approximately 100 days after casting. Making reliable predictions about thermal stresses, and thereby cracking risks, the creep behaviour forms an important part of the material modelling. Up until now few studies have been made to investigate how different creep modelling influences calculated thermal stresses. Existing creep models for young concrete are often pure mathematical expressions with no direct relation to the material behaviour and thereby complicated to understand and use in a more practical context. In this paper a new basic creep model primarily aimed for early age purposes is outlined. The formulation with its model parameters, which have an easy to understand meaning in the material behaviour, is based on piece-wise linear curves in logarithm of time and therefore denoted the Linear Logarithmic Model (LLM). Comparison with experimental creep data and other more commonly used creep formulations for young concrete is made to achieve an opinion about the accuracy of the new model. The new model is a flexible and robust formulation that can model the behaviour of both young and mature concrete. The robustness enables it to make reliable creep modelling with very few test data. Another advantage with the LLM formulation is that the appearance of negative relaxation in linear viscoelastic modelling is very small and negligible with respect to thermal stresses. This means that the original formulation may be used directly in a thermal stress analysis for young concrete without any adjustment for negative relaxation. The LLM formulation shows very good agreement directly with experimental creep data and indirectly with measured thermal stresses, whereby the formulation has been used to model the viscoelastic behaviour of the concrete. The formulation also has the best correlation with experimental data compared to other commonly used creep models that have been analysed in this paper.
A reliable modelling of the young concrete creep behaviour is of great importance for consistent thermal crack risk estimations that shall contribute to assure a desired service lifetime and function of a structure. All-embracing creep tests aimed for thermal stress analyses are often very time consuming and thereby also costly to perform. Therefore thermal stress calculations in everyday engineering practice are often performed with standard sets of creep data involving no or very limited laboratory testing, which increases the error of the crack risk predictions and consequently also affect the design safety margins. The need for formulations that based on limited test data can make reliable predictions about the creep behaviour of hardening concrete is thus quite evident. This paper is a direct continuation of a previous study by Larson and Jonasson (2003) where a new concrete creep formulation called the Linear Logarithmic Model (LLM) was formulated. Here creep prediction formulas based on the LLM formulation are established and evaluated. It is shown that general model parameters can be established whereby the long-term creep behaviour is clearly dependent on the modulus of elasticity with larger creep deformations for lower E-modulus. An average error related to creep of 15 percent is what can be expected from most thermal stress analyses that are performed with standard sets of creep data today. By use of the prediction formulas based on the proposed LLM formulation for creep compliance it is possible to reduce the error by almost two thirds (2/3) only by adding the results from a test of the modulus of elasticity at the age of 28 days. For more advanced applications, where even better accuracy is required, it is recommended that at least a creep compliance test is performed at two loading ages, of which one at the age of 28 days.
A finite element numerical model for the analysis of composite construction structures has been implemented using the hybrid type formulation for planar frames structures. The hybrid type finite element is regarded as a theoretically exact approach for force and curvature distributions. This approach allows the use of long elements, consequently improving computational efficiency and enhancing the unbonded tendon strain calculation. The computer programme considers bonded and unbonded prestressing, cyclic loading, time-effects and geometrical non-linearity. For composite construction analysis, new layers may be added to the cross-sections at any time, elements may be added to the structure during analysis, reinforcing bars may be included in the elements and bonded or unbonded tendons may be stressed during analysis. Time is a parameter used for the description of loading and construction histories, even in time-independent materials problems. Such approach for composite construction allows the modeling of complex histories in a simple manner. Results are presented comparing numerical and experimental behaviours, including unbonded simply supported prestressed beams, cast-in-place continuity of hollow-core slabs and prestressed cast-in-place continuity of pre-cast beams. Numerical and experimental curves show a good agreement in all examples, demonstrating an adequate performance of the model.
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