A numerical and experimental investigation into multi-ionic reactive transport behaviour in cementitious materials

This thesis describes a FE approach to the simulation of reactive transport problems and a simple experimental procedure for the determination of transport parameters in cementitious materials. A comprehensive fully coupled reactive-thermo-hygro-chemical model was developed based on the governing equations of mass and enthalpy balance. The model takes into consideration advective-dispersive transport of solutes, heat flow, advective-diffusive moisture flow, and chemical reactions. The FEM, Euler backward difference scheme and Newton-Raphson iteration procedure were employed to solve the system of nonlinear equations. To address the numerical challenge associated with such coupled simulations, three problem reduction schemes were proposed, each of which uses a reduced set of species, termed ‘indicators’, for full computation. The response of the remaining species is computed at each time step from the transport of the indicators. The difference between the schemes lies in the number of indicator species used and in the method employed for calculating the transport of the remaining species. Firstly the development of the experimental procedure is presented including the design of a porous concrete mix, a discussion of the problems encountered and the results of an advective-diffusive case. Following this, the model is validated and verified against a number of problems, beginning with a moisture transport problem and ending with a multi-ionic reactive transport problem. It was found that the model was able to accurately capture the transport behaviour. The range of applicability of each of the reduction schemes is then investigated through an example problem concerning the reactive transport of 16 chemical species, before verifying each of the schemes against the full model through the consideration of three example problems. The reduction schemes were found to perform well in accurately capturing the transport behaviour whilst greatly reducing the number of coupled equations to be solved, and the computational cost of the simulatio

A multi-point constraint unfitted finite element method

In this work a multi-point constraint unfitted finite element method for the solution of the Poisson equation is presented. Key features of the approach are the strong enforcement of essential boundary, and interface conditions. This, along with the stability of the method, is achieved through the use of multi-point constraints that are applied to the so-called ghost nodes that lie outside of the physical domain. Another key benefit of the approach lies in the fact that, as the degrees of freedom associated with ghost nodes are constrained, they can be removed from the system of equations. This enables the method to capture both strong and weak discontinuities with no additional degrees of freedom. In addition, the method does not require penalty parameters and can capture discontinuities using only the standard finite element basis functions. Finally, numerical results show that the method converges optimally with mesh refinement and remains well conditioned

A 3D coupled finite element model for simulating mechanical regain in self-healing cementitious materials

This study presents a new 3D coupled model for simulating self-healing cementitious materials. The mechanical behaviour is described using a damage-healing cohesive zone model that is implemented using a new embedded strong discontinuity hexahedral element. The transport component of the model considers the flow of healing agent through discrete cracks, governed by the mass balance equation with Darcy’s law being employed for the healing agent flux. The dependency of the mechanical response on the healing agent transport is accounted for through a local crack filling function that represents the amount of healing agent available to undergo healing. The healing itself is described by a generalised healing front model that simulates the accumulation of healed material within the crack, emanating from the crack faces. The performance of the model is demonstrated through the consideration of a healing front study and experimental tests on self-healing cementitious specimens. The examples consider a vascular self-healing cementitious specimen that uses a sodium silicate solution as the healing agent and the autogenous healing of a cementitious specimen with and without crystalline admixtures. The results of the validations show that the model is able to reproduce the experimentally observed behaviour with good accuracy

The application of a curing front model to simulate healing in a cementitious microbial system

This study investigates the ability of a coupled finite element model to simulate Microbially Induced Calcium Carbonate Precipitation (MICP) and associated healing behaviour in cementitious samples. This recent coupled 3D model was first developed for simulating the behaviour of autonomic healing systems in cementitious structural elements. It employs a cohesive zone constitutive model for simulating the damage-healing behaviour of an embedded interface within 3D continuum elements. Fluid flow is simulated using a mass balance equation and Darcy’s law. Healing is computed via a generalised curing front model that simulates the accumulation of healed material within a crack. The research reported in this article demonstrates that the curing front model can be calibrated to predict healing from MICP in cementitious specimens with good accuracy

A crack-opening-dependent numerical model for self-healing cementitious materials

A new damage-healing model for self-healing cementitious materials is described. The model is formulated using results from a discrete ligament model and guided by the findings of a linked experimental study. Healing is simulated using the interaction of curing fronts propagating from opposing crack faces within a body of healing-agent. This approach accounts for the dependency of the healing response on the crack opening displacement (COD) and its rate. The new damage-healing cohesive-zone model is applied to an element with an embedded strong-discontinuity within a coupled finite-element code, which simulates healing-agent transport and mechanical behaviour. The model is validated using data from tests with different CODs and COD rates. The validations show that the coupled model represents the mechanical and flow behaviour of an autonomic self-healing system with good accuracy for a range of cracking configurations and load paths. Previous ar

A specialised finite element for simulating self-healing quasi-brittle materials

A new specialised finite element for simulating the cracking and healing behaviour of quasi-brittle materials is presented. The element employs a strong discontinuity approach to represent displacement jumps associated with cracks. A particular feature of the work is the introduction of healing into the element formulation. The healing variables are introduced at the element level, which ensures consistency with the internal degrees freedom that represent the crack; namely, the crack opening, crack sliding and rotation. In the present work, the element is combined with a new cohesive zone model to simulate damage-healing behaviour and implemented with a crack tracking algorithm. To demonstrate the performance of the new element and constitutive models, a convergence test and two validation examples are presented that consider the response of a vascular self-healing cementitious material system for three different specimens. The examples show that the model is able to accurately capture the cracking and healing behaviour of this type of self-healing material system with good accuracy

Mechanical response of a vascular self-healing cementitious material system under varying loading conditions

The paper presents results from two groups of experimental tests on a pressurised vascular self-healing cementitious material system, in which low viscosity cyanoacrylate was employed as the healing-agent. The first group comprised three series of tests on plain concrete notched prismatic beams. These tests examined the effects on the mechanical response of varying the healing period, the rate of loading and the healing-agent pressure. The second group involved two series of direct tension tests on doubly notched prismatic specimens, each of which had a different crack opening displacement during the healing period. In this second group of tests, healing was allowed to take place in cracks that were held stationary for a period of time, with the degree of mechanical healing being measured for different healing periods. The paper also presents a simplified damage-healing model that is used to interpret the test results and to bring clarity to the indices used to evaluate the degree of healing. The tests were designed to provide new data on simultaneous damage-healing behaviour as well as on the effects of varying pressure, static healing periods and cracking configurations on the mechanical response of this self-healing cementitious material (SHCM) system. These data have been used to guide the development of a new numerical model for SHCMs (reported elsewhere) and should be useful to others who are developing design procedures and/or computational models for similar material systems

An indicator-based problem reduction scheme for coupled reactive transport models

A number of effective models have been developed for simulating chemical transport in porous media; however, when a reactive chemical problem comprises multiple species within a substantial domain for a long period of time, the computational cost can become prohibitively expensive. This issue is addressed here by proposing a new numerical procedure to reduce the number of transport equations to be solved. This new problem reduction scheme (PRS) uses a predictor-corrector approach, which ‘predicts’ the transport of a set of non-indicator species using results from a set of indicator species before ‘correcting’ the non-indicator concentrations using a mass balance error measure. The full chemical transport model is described along with an experimental validation. The PRS scheme is then presented together with an investigation, based on a 16 species reactive advective-diffusion problem, which determines the range of applicability of different orders of PRS. The results of a further study are presented in which a set of PRS simulations are compared with those from full model predictions. The application of the scheme to the intermediate-sized problems considered in the present study showed reductions of up to 82 % in CPU time with good levels of accuracy maintained

Mechanical response and predictive modelling of vascular self-healing cementitious materials using novel healing agents

Self-healing systems represent an effective means of increasing the resilience of cementitious structures, extending service life and reducing cement production. This is achieved through the mitigation of cracking related durability problems. The success of a self-healing system is critically dependent on the selection of an appropriate healing agent, which depends upon the specific application, as well as a number of criteria including crack filling ability and the degree of mechanical healing required. In the present study, we develop modified formulations of a cyanoacrylate-based adhesive, suitable for use in a vascular self-healing cementitious material. The aim is to develop an ‘ideal’ healing agent for the self-healing system that has an extended shelf life and maximises load recovery. To this end, modified cyanoacrylates are tailored using a combination of predictive modelling and physical testing. The physical tests investigate both the mechanical, flow and chemical properties of the different healing agent formulations, including tensile strength, viscosity and curing. The predictive modelling employs a coupled chemo-mechanical model that is used to guide the physical testing programme through the prediction of the performance of different formulations. The results of the investigation show that a tailored formulation of a cyanoacrylate based healing agent increases the load recovery by 48% relative to the best performing original formulation. In addition, it is shown that the numerical model is able to predict the load response of new formulations with good accuracy