Healing efficiency of cementitious materials for durable and intelligent transport infrastructures

Executive Summary In concrete structures, the prevention of crack occurrence is particularly desirable; however, because of the heterogeneity of the concrete material, which affects the mechanical behaviour, concrete is susceptible to cracking. The cracking occurs as a result of various factors, such as the impact of dynamic and static loadings combined with environmental influences, including temperature and humidity variations. As a consequence of the continuous fluctuation of external factors, micro-cracks can form and grow, opening a pathway for aggressive gasses and liquids that may enter into the cementitious matrix and degrade the embedded reinforcement. Irreversibly, this affects the performance and durability of the concrete structure. The cost of crack repair and maintenance is relatively high with limitations on crack detection and repair inside the cementitious matrix. To obtain recovery of the structural performance and mitigate the loss of durability, crack healing has been the subject of intensive research attention over the last two decades. In cementitious materials, self-healing is achievable - just like healing a fractured bone. This study begins by classifying the different categories of self-healing established by various researchers globally. Autogenous self-healing, the intrinsic process of chemical reactions without an additive for self-healing, is achievable over a long period. However, global scientists have made great efforts to speed up the healing process by using additives with crack-filling or healing capabilities, including the biotechnological and polymeric materials, as well as chemical compounds. Equally, scientists have used different methods to transport healing agents into the cementitious matrix using direct, vascular or encapsulation methods and self-healing is triggered based on artificial cracks obtained from conventional mechanical testing methods. In addition, microstructural parameters have been identified to evaluate crack healing in its geometrical and size aspects. An innovative method of inducing the artificial cracks homogenously throughout the entire volume of the concrete specimen at controlled levels was developed for further study of the self-healing phenomena. A theoretical study was undertaken in which a cylindrical concrete specimen was placed into a specifically designed steel mould; at various loading rates, internal stresses in the matrix tend to distribute uniformly. The finite element method (FEM) using ABAQUS software and the experimental program was conducted to evaluate the existing phenomena. The proposed FEM analysis and experimental results showed that the cracking occurrence was approximately identical to higher damage levels and only a slight change was observed at the maximum damage level. Autogenous self-healing, which is a process that requires the availability of water, procuring the dissolution of the undissolved particles and the formation of a newly formed chemical product, results in cracking closure within the cementitious matrix. In this study, autogenous self-healing was investigated using superabsorbent polymers, not as a healing agent but rather to absorb the available water content from the surrounding environment in the concrete material when in a hardened state. The effectiveness of transporting the superabsorbent polymers into the cementitious matrix and various superplasticiser contents without any additional water during the mixing process was also investigated. The in-house fabricated steel mould presented the induced cracks at minor and major controlled damage levels. During the self-healing progress and the formation of the healing product, inter-bonding between the crack walls, mechanical properties tests and advanced microstructural parameters applied on X-ray images demonstrated that crack healing was achievable, typically for minor damage levels and in higher superplasticiser contents. Researchers worldwide have utilised different technologies for the purpose of crack healing, known as autonomous self-healing, depending on the size of the crack in the cementitious matrix. In this thesis, autonomous self-healing was investigated using sodium silicate as the healing agent. Microcapsules containing sodium silicate as the core material and polyurethane as the shell materials were fabricated, with an average diametrical size of 100 to 150 micrometres. Cylindrical concrete specimens containing microcapsules at two different ratios to the weight of cement were cast; subsequently, a cracking pattern was developed homogenously throughout the entire concrete specimens by using the in-house fabricated steel mould at major controlled damage level. Investigations into concrete self-healing were conducted at various times and crack-healing efficiency was evaluated using mechanical and non-destructive testing methods. The investigation produced promising results for the recovery of mechanical properties and crack healing. Concurrently, the proposed system for self-healing was used in cementitious mortar mixes to investigate its effectiveness. Mortar mixes with identical microcapsule contents and preliminary ingredients were obtained for the concrete mixes, using a systematic method that was also applied to the concrete specimens to generate internal cracks; crack development in mortar specimens was also obtained. Mechanical and non-contact test methods were used to evaluate the self-healing efficiency. The results indicated that self-healing is achievable in mortar at approximately the same time as in concrete, as the results of the newly formed calcium-silica hydrate conjoining the crack walls reveal. In a consolidation of the outcomes of the experimental programs conducted in this thesis, autogenous self-healing is achievable when using superabsorbent polymer and a relatively high superplasticiser content. In contrast, the progress of self-healing is substantially accelerated when micro drops of sodium silicate, entrapped in polyurethane material, are mixed in the cementitious matrix.