Mohammed Alnaggar

Modeling Time-Dependent Behavior of Concrete Affected by Alkali Silica Reaction in Variable Environmental Conditions

Alkali Silica Reaction (ASR) is known to be a serious problem for concrete worldwide, especially in high humidity and high temperature regions. ASR is a slow process that develops over years to decades and it is influenced by changes in environmental and loading conditions of the structure. The problem becomes even more complicated if one recognizes that other phenomena like creep and shrinkage are coupled with ASR. This results in synergistic mechanisms that can not be easily understood without a comprehensive computational model. In this paper, coupling between creep, shrinkage and ASR is modeled within the Lattice Discrete Particle Model (LDPM) framework. In order to achieve this, a multi-physics formulation is used to compute the evolution of temperature, humidity, cement hydration, and ASR in both space and time, which is then used within physics-based formulations of cracking, creep and shrinkage. The overall model is calibrated and validated on the basis of experimental data available in the literature. Results show that even during free expansions (zero macroscopic stress), a significant degree of coupling exists because ASR induced expansions are relaxed by meso-scale creep driven by self-equilibriated stresses at the meso-scale. This explains and highlights the importance of considering ASR and other time dependent aging and deterioration phenomena at an appropriate length scale in coupled modeling approaches.

Lattice Discrete Particle Modeling for Coupled Concrete Creep and Shrinkage Using the Solidification Microprestress Theory

The macroscopic continuum creep simulation always neglect internal creep/relaxation at lower scales due to the internal self-equilibrated stresses. So far, a comprehensive model for concrete creep at the meso-scale level has been lacking. In this paper, such a shortcoming is over come by the explicit implementation of the solidification-microprestress (SM) theory within the Lattice Discrete Particle Model (LDPM). Aging effect is obtained using a global reaction degree of concrete obtained by a multi physics model evolving temperature, humidity and cement degree of reaction in full coupling over time and space leading to an elegant and simple implementation within the LDPM framework through an imposed eigenstrain. This leaves the features of the LDPM constitutive equation simulating material strength and toughness completely unaltered. To show the superiority of the proposed model, extensive calibration and validation of the model is pursued by numerical simulations of experimental data from literature.

Effect of alkali silica reaction on the mechanical properties of aging mortar bars: Experiments and numerical modeling:

Alkali silica reaction and its effect on concrete and mortar have been studied for many years. Several tests and procedures have been formulated to evaluate this reaction, particularly in terms of aggregate reactivity. However, the data given in the literature concerning the mechanical properties of concrete and mortar are scattered and very little information is available for some properties such as fracture energy. In this study, the mechanical behavior of mortar was evaluated and monitored, under normal and accelerated environmental conditions. Fracture energy, compressive strength and tensile strength were measured for mortar specimens, casted with highly reactive Spratt crushed aggregate, at two different storage temperatures (23℃ and 80℃) and at two different alkali concentrations (immersed in water and in 1 N NaOH solution). Moreover, free expansion tests (according to ASTM C1260) and petrographic observations were performed, in order to relate them to the evolution of the mechanical properties of mortar. Results show a decrease of the mechanical properties associated with specimens at 80℃ in alkali solution and that the deterioration due to alkali silica reaction is counter-balanced by the strengthening of mortar resulting from the hydration process. A multi-physics computational framework, based on the Lattice Discrete Particle Model is then proposed. Numerical simulations based on a complete calibration and validation with the obtained experimental data capture the behavior of mortar subjected to the complex coupled effect of strength build-up and alkali silica reaction at different temperatures and alkali contents.

Multiscale Homogenization Modeling of Alkali-Silica-Reaction Damage in Concrete

Alkali silica reaction (ASR) is one of the main reasons that cause deterioration in concrete structures, such as dams and bridges. ASR is a chemical reaction between alkali ions from cement paste and the silica inside each aggregate piece. ASR gel, which is the product of this reaction, imbibes additional water causing swelling and cracking, which leads to degradation of concrete mechanical properties. In this study, to model ASR-induced damage, the Lattice Discrete Particle Model (LDPM) is adopted, which is a meso-scale discrete model. LDPM simulates concrete at the level of coarse aggregate pieces. ASR effects have been already successfully modeled by LDPM in the recent past. This paper employs a recently developed multiscale homogenization approach to derive macroscopic constitutive equations for ASR-damaged concrete. The adopted homogenized model is used to reproduce experimental data on volumetric expansion of unrestrained concrete prisms.

Lattice Discrete Particle Modeling of Shear Failure in Scaled GFRP Reinforced Concrete Beams without Stirrups

This paper discusses the calibration of a concrete lattice discrete particle model (LDPM), and its preliminary validation for the case of shear failure in scaled glass fiber reinforced polymer (GFRP) reinforced concrete (RC) beams without stirrups. First, the model parameters were defined based on: (a) the design of the concrete mixture that was used to fabricate scaled beam specimens; and (b) a literature database of meso-scale concrete parameters. Second, the calibration was refined to reach satisfactory agreement between numerical and experimental compression stress-strain curves as obtained by testing concrete cylinders in accordance with ASTM C469. The calibrated model was then used for the numerical simulation of four-point bending load tests on two slender GFRP RC beams without stirrups, and having an effective depth of 146 and 292 mm, respectively. The beam computational models are discussed vis-a-vis experimental data based with respect to elastic response, post-cracking stiffness degradation and damage progression, ultimate strength, and failure mode. The proposed model accurately approximates the preand post-cracking flexural stiffness, and holds promise to predict the shear strength of scaled GFRP RC slender beams without stirrups, provided that a suitable rebar-concrete bond stress-slip law is implemented.

Lattice discrete particle modeling (LDPM) of flexural size effect in over reinforced concrete beams

At the macroscopic scale, concrete can be approximated as statistically homogeneous. Nevertheless, its macroscopic behavior shows quasi-brittleness, strain softening, and size effects evidencing a strong influence of material heterogeneity. A model naturally accounting for material heterogeneity is the Lattice Discrete Particle Model (LDPM). LDPM replaces the actual concrete mesostructure by an assemblage of discrete particles interacting through nonlinear and fracturing lattice struts. Each particle represents one coarse aggregate piece. Since the initial development, LDPM has shown superior material modeling capabilities. In this research, LDPM is used to simulate the flexural failure of three groups of over reinforced concrete beams. The groups represent 1D, 2D and 3D geometric similarities. Geometry is generated based on concrete mix design. Then calibration was only guided by the experimentally provided compressive strength. In order to reduce the redundancy of the calibration process, the fracture properties of concrete were estimated using relevant literature. Finally, the rebar assembly was connected to the LDPM mesh using penalty type constraints and the rebars were modeled using 1D beam elements. Numerical results show excellent agreement with experimental data and clear capability of capturing size effects.