From waste to self-healing concrete: the missing PHA-link

Abstract

Self-healing concrete has attracted increasing attention of researchers and industry over the last decades. Given the brittle nature and the relatively low tensile strength of concrete, the occurrence of cracks is an almost unavoidable phenomenon affecting (reinforced) concrete structures. Cracks allow harmful agents present in the environment
to penetrate more easily in structures, accelerating the material degradation and compromising their service life. Repair and maintenance interventions are often needed in practice to maintain the serviceability of structures and to avoid any premature collapse. However, the costs of these interventions are socially and economically impactful. The
aim of both academia and industry is (and it has been) therefore to limit the need of these interventions through different technologies such as, among others, self-healing
concrete. Several technologies (or healing agents) have been proposed and investigated to improve the self-healing capacity of concrete, such as mineral and crystalline admixtures, superabsorbent polymers, micro- or macro-encapsulated polymers, and bio-concrete. Any technology has its own working principle, and the beneficial effects that the healing agents have on concrete properties are continuously under investigation. Despite the relatively high amount of available healing agents, this research focuses on the potential of innovatively using waste-derived polyhydroxyalkanoates (PHAs) as bacterial substrate for self-healing bio-concrete applications. Differently from other healing agents, PHAs are biodegradable and they can be extracted from biomass. The application of PHAs as substrate for bacterial healing agents in concrete does not ideally need high material requirements (e.g., purity) as other applications may do, hence opening the possibility to obtain efficient and relatively low costs healing agents, of which production would significantly contribute to the principles of circular economy. The aim of this research is, therefore, to investigate the applicability of PHAs as bacterial
substrate for self-healing agents in concrete. To do so, a PHAs extraction procedure has been designed, as well as the healing agent particle formulation process, as reported
in Chapter 3. This chapter aimed to formalize the production of PHAs-based healing agents (AKD), and to demonstrate its compatibility with bacterial metabolic activity. In Chapter 4, an extensive experimental campaign on the compatibility between AKD and Ordinary Portland Cement (OPC) mortar has been conducted. The feasibility of this application was demonstrated by the marginal effect that AKD healing agents have on some fundamental functional properties of cementitious materials (e.g., strength) and
by the improved self-healing capacity of the proposed system compared to plain mortar. In Chapter 5, the effect of self-healing on chloride penetration resistance in mortar specimens has been investigated. The results of this chapter firstly showed that the chloride penetration resistance in sound specimens increased thanks to the formation of a
thin calcium carbonate layer at the surface of the specimens and through a slight densification of the matrix. Secondly, self-healing of cracks was beneficial since it delayed the penetration of chlorides. Nevertheless, the initial chloride penetration resistance (e.g., that of sound specimens) could not be completely restored through self-healing.
In Chapter 6, the applicability of AKD in Blast Furnace Slag Cement (BFSC) mortar was investigated. Results of this chapter demonstrated the compatibility between AKD and low-alkaline mixtures, since functional properties did not get negatively affected and the self-healing capacity significantly improved. These results are different from those
observed when applying poly-lactic acid (PLA) healing agents in BFS mixtures, which on the contrary showed incompatibility with the mixture due to its lower alkalinity. In
Chapter 7, analysis and reflections on the potentially positive impact that self-healing could have on the service-life of infrastructures have been conducted based on idealized
scenarios. The environmental impact of AKD healing agents has been also discussed based on the eco-costs related to available processing treatments of PHAs. In Chapter 8, a methodology based on gas chromatography has been proposed to detect and quantify bio-based healing agents in added in cementitious materials. Even though the applicability of this methodology has been demonstrated for PLA healing agent, gas chromatography can be extended to AKD healing agent as well. In Chapter 9 conclusions based on the analysis conducted over the whole research have been drawn, and recommendations for future research have been made. With the present research the author hopes to provide a valuable contribution for those interested in the field of self-healing materials.