Experiments-based multi-scale modeling of the alkali-silica reaction in concrete

In this study, we investigate the mechanical behavior of large concrete structures affected by the alkali-silica reaction. Particularly, concrete dams are explored. Their permanent contact with water, and thus increased humidity, promotes the ASR. Seasonal variation of the environmental temperature causes different levels of concrete damage in “cold” and “warm” zones. In order to study the behavior of such structures, a thermo-mechanical multi-scale numerical model is developed. It is based on the algorithm proposed by CubaRamos. Simulations are performed at the macro- and meso-scales, both of which are handled by the finiteelement method. An elastic thermo-mechanical simulation, performed at the macro-scale, results in a temperature and displacement field. This information at every integration point of the macro-scale is communicated to a corresponding representative volume element (RVE) at the meso-scale. The temperature value is used to determine the gel expansion rate for the following time step. Advancement of the ASR at the level of RVEs leads to gel expansion, deformation of the matrix, and growth of cracks. Given the solution of an RVE problem, homogenized material properties are extracted and communicated to the corresponding integration point of the macro-structure. In the next loading step, the latter behaves according to local states of material throughout a structure.Concrete damaging process is locally driven by gel volume increase, which in its turn depends on therate of alkali-silica reaction. An equation used to relate the ambient temperature and the amount of available reactants to the gel strain rate is inspired by the Arrhenius law. Parameters of this equation are calibrated based on the experiments by Gautam & Panesar. Cracks in concrete are handled by a continuum damage model. Convergence of the static solution in the presence of multiple cracks is achieved by modification of the sequential linear analysis proposed by Rots and Invernizzi. The cement paste at the meso-scale behaves viscoelastically, which causes relaxation of stresses and reduction in the level of damage. For this purpose, an algorithm coupling the SLA with the general viscoelastic model is developed.

Thermo mechanical effects in Ultra-High Performance Fibre Reinforced Concretes (UHPFRC) at early age

Eugen Brühwiler, Emmanuel Denarié, Aicha Kamen, Hamid Sadouki

The extremely low permeability of Ultra-High Performance Fibre Reinforced Concretes (UHPFRC) associated to their outstanding mechanical properties make them especially suitable to locally "harden" reinforced concrete structures in critical zones subjected to an aggressive environment and to significant mechanical stresses. Composite UHPFRC-concrete structures promise a long-term durability which helps avoid multiple interventions on structures during their service life. Temperature Stress Testing Machines (TSTM) are very well suited to "experimentally simulate" the conditions encountered in composite structures in a new layer, at early age, subjected to restrained shrinkage under complex environmental conditions. They can also be used to determine intrinsic properties such as free shrinkage and creep response under well defined conditions of restraint and temperature. A new TSTM testing setup has been used to characterize the response of strain hardening UHPFRC, with different binders, under various thermo mechanical conditions. Isotherm tests as well as tests under realistic temperature conditions have been performed. Current numerical models have been applied to simulate the tests and the range of their applicability has been determined for temperatures between 10 and 30 °C. The test results showed a significant creep potential due to the high volume of paste. This beneficial effect was reflected by the linearly increasing-relationship between tensile creep and shrinkage. A Maxwell chain model was applied to predict the early age UHPFRC tensile creep at low stress levels. As expected, UHPFRC tensile creep behaviour was also sensitive to the stress level. Above 35 % of the tensile strength at the loading age of 3 days, the material exhibited a non linear behaviour.

2010

Influence of visco-elasticity on the stress development induced by alkali-silica reaction

Cyrille Dunant, Alain Benjamin Giorla, Karen Scrivener

The alkali-silica reaction causes long-term degradation in the microstructure of affected concrete as well as macroscopic expansion. In this paper, a micro-mechanical model based upon an explicit representation of the microstructure has been used to simulate the role of creep on the expansion and damage induced by the reaction. The model accounts for the coupling between damage propagation and stress relaxation in the cement paste. This study indicates that the influence of the visco-elastic nature of the material on the overall expansion is within the experimental scatter. However, it is shown that creep can explain the comparatively low amount of damage in the cement paste as observed experimentally. Creep also increases the amount of damage in the aggregates when the rate of reaction increases. Overall, the results presented in this paper indicate that accelerated experiments may not be representative of the degradation in the field at equivalent degrees of reaction. (C) 2014 Elsevier Ltd. All rights reserved.

Pergamon-Elsevier Science Ltd2015

Multi-scale modelling of concrete structures affected by alkali-silica reaction: Coupling the mesoscopic damage evolution and the macroscopic concrete deterioration

Mauro Corrado, Aurelia Isabel Cuba Ramos, Emil Gallyamov, Jean-François Molinari, Roozbeh Rezakhani

A finite-element approach based on the first-order FE2 homogenisation technique is formulated to analyse the alkali-silica reaction-induced damage in concrete structures, by linking the concrete degradation at the macro-scale to the reaction extent at the meso-scale. At the meso-scale level, concrete is considered as a heterogeneous material consisting of aggregates embedded in a mortar matrix. The mechanical effects of the Alkali-Silica Reaction (ASR) are modelled through the application of temperature-dependent eigenstrains in several localised spots inside the aggregates, and the mechanical degradation of concrete is modelled using continuous damage model, which is capable of reproducing the complex ASR crack networks. Then, the effective stiffness tensor and the effective stress tensor for each macroscopic finite element are computed by homogenising the mechanical response of the corresponding representative volume element (RVE). Convergence between macro- and meso-scales is achieved via an iterative procedure. A 2D model of an ASR laboratory specimen is analysed as a proof of concept. The model is able to account for the loading applied at the macro-scale and the ASR-product expansion at the meso-scale. The results demonstrate that the macroscopic stress state influences the orientation of damage inside the underlying RVEs. The effective stiffness becomes anisotropic in cases where damage is aligned inside the RVE. (C) 2020 The Authors. Published by Elsevier Ltd.

PERGAMON-ELSEVIER SCIENCE LTD2020