Experimentally-informed numerical investigation of self-healing cement-based materials

Abstract

Self-healing concrete technology has been widely investigated in the past decade as a solution against degradation and/or loss of functionality in cracked concrete elements. Many self-healing mechanisms have been proposed and proven to work experimentally in the literature. Among these systems, the use of encapsulated mineral-producing healing agents seems to be the best working combination of components to successfully heal the cracks. The involved triggering-healing mechanisms usually consist of complex physical phenomena. Hence proper optimization of the proposed self-healing material is not possible through often resource- and time-consuming experimental campaigns. Instead, numerical modelling could be a useful tool for timely and exhaustive investigation of the self-healing mechanisms, as well as the design of appropriate experimental setups. Yet, while research on self-healing concrete seems to keep growing, comparatively the amount of modelling work devoted to the optimized design of the material has not advanced.
This thesis aims to provide a modelling framework for the study of the main aspects of a capsule-based self-healing cement-based system, namely the mechanical triggering of the self-healing system, the healing process itself and the assessment of the recovered property.
For the self-healing mechanism to work, the triggering of enough capsules along the crack is desired. Notwithstanding, this crack steering optimization comes at the expense of proper mechanical behaviour of the composite. Whereas the earlier aspect has been studied in the past, in this thesis a numerical optimization of the triggering of capsules is carried out taking into account also the achievement of acceptable mechanical performance of the material. To illustrate this, the case of self-healing cement paste with bacteria-embedded polylactic acid (PLA) capsules was selected. A 3D mesoscale lattice model was implemented herein to simulate a uniaxial tensile test on the system composed of cement paste, PLA capsule and their interface. Previous studies on the mechanical behaviour of cement paste with inclusions (i.e. capsules) have shown that the interface transition zone around the inclusion presents microstructural and mechanical properties that are totally different from those of the matrix. Therefore, a meticulous study was first conducted to obtain the mechanical properties of the interface of various types of PLA capsules with respect to bulk cement paste. Nanoindentation was performed to obtain maps of hardness and elastic modulus in the interfaces. 2D microscale lattice modelling of uniaxial tensile test on the mapped locations was performed then to obtain the overall tensile strength and stiffness of the interface. Moreover, hydrates assemblage and chemical composition around the PLA particles were studied through Backscattering Electron images and Energy Dispersive X-ray Spectroscopy. The ratios between resulting tensile strength and elastic modulus of the interface with respect to bulk paste were obtained for each PLA type which were then used as input for the mesoscale model. Cement paste samples with PLA capsules were imaged through X-ray micro Computed Tomography before and after fracture to obtain the capsules distribution to input in the mesoscale model and the fracture surface for validation, respectively. The experimental and simulated stress-strain curves showed excellent correspondence, especially on the elastic phase, hence validating the proposed model. An exhaustive numerical investigation of the material was performed then to analyse the influence of dosage, size and shape of the PLA capsules, as well as of the interface properties on the mechanical behaviour of the composite and the triggering of the PLA capsules. The results show that interface properties close to but lower than the cement matrix do not entail substantial losses of tensile strength and elastic modulus, whereas the amount of triggered capsules is maximized. Optimum dosage, shape and size of the PLA capsules were also obtained.
To illustrate the healing process and the recovery of the functional property within the proposed modelling framework, the case of crack self-sealing in cement mortar with superabsorbent polymers (SAP) was investigated. These healing admixtures steer the crack propagation and become exposed along the fracture surface. Upon contact with ingress water they immediately absorb water and swell, thus providing a water-blocking effect and preventing harming species to further penetrate into the mortar matrix from the crack surfaces. In order to design such self-sealing systems in an efficient way, a three-dimensional mesoscale lattice model is proposed to simulate capillary absorption of water in sound and cracked cement-based materials containing SAP. The numerical results yield the moisture content distribution in cracked and sound domain, as well as the absorption and swelling of SAP embedded in the matrix and in the crack. In a first instance, the model was validated for mortar without SAP, by means of time-resolved X-ray micro Computed Tomography. Additionally, the water absorption and swelling of SAP embedded within the mortar were imaged and quantified over time to better model their role during capillary water absorption in such composite materials. The performance of the model with the presence of SAP was validated by using experimental data from the literature, as well as experimentally-informed input parameters. The validated model was then used to investigate the role of SAP properties and dosage in cementitious mixtures, on the water penetration into the material from cracks. Furthermore different crack widths were considered in the simulations. The model shows good agreement with experimental results. The obtained results show that increasing the SAP water absorption capacity, while reducing their cement solution absorption capacity improves the crack self-sealing effect more efficiently than increasing their dosage. Other guidelines for the selection of appropriate SAP are given for different crack widths. Moreover, it is suggested that capillary water absorption test in cracked concrete is sensitive enough to detect small localized changes in crack width due to the healing of the cracks.