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.@en

The possible beneficial impact of self-healing on chloride transport through cracks has been assessed for two bacteria-based self-healing mortar mixtures in comparison with Ordinary Portland cement mortar. Intact self-healing specimens featured lower chloride transport coefficients thanks to a denser microstructure and to the formation of a layer of calcium carbonate on their surface. However, self-healing of cracks of cracked mortar specimens did not significantly reduce chloride penetration during 28 days of chloride exposure compared to cracked and non-healed specimens. On the other hand, this study demonstrated that self-healing of 150–200 µm wide cracks reduced chloride ingress during shorter term (14 days) chloride exposure in comparison to cracked but non-healed specimens. The results of this study suggest that self-healing of cracks through calcium carbonate formation results in water blockage (sealing) of cracks but that the limited amount of limestone formed creates an imperfect barrier against diffusion of chloride ions through the apparent porous limestone. The short-term positive effect of self-healing with respect to reduced chloride ingress could be beneficial in applications where chloride exposure is limited or non-permanent such as concrete structures irregularly exposed to deicing salts or located in the splash zone in marine environments.

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In this study, the applicability of two bacteria-based healing agents (e.g., poly-lactic acid and polyhydroxyalkanoate) in blast furnace slag cement (BFSC) mortar has been assessed. An experimental campaign on the functional properties, self-healing capacity, freezing–thawing and carbonation resistance has been conducted in comparison with plain mortar (Ctrl). Due to the relatively low alkalinity of the mixture, the addition of poly-lactic acid healing agents (PLA) caused coarsening of the micro-structure, decrease of strength and did not improve the self-healing capacity of the material. Among other consequences, the mass loss due to the freezing–thawing of PLA specimens was about 5% higher than that of the Ctrl specimens. On the contrary, no detrimental effect of the mortar functional properties was measured when polyhydroxyalkanoate healing agents (AKD) were added. The self-healing capacity of AKD specimens was higher than that of the Ctrl specimens, reaching a maximum healed crack width of 559 µm after 168 days of self-healing, while it was 439 µm for the Ctrl specimens and 385 µm for PLA specimens. The air void content of the AKD mixture was 0.9% higher than that of the Ctrl, increasing its resistance against freezing–thawing cycles. This study aims to confirm the potential applicability of AKD particles as self-healing agents in low-alkaline cementitious mixtures. @en

The elastic modulus of corrosion product (E_cp) has been reported with significant variations in the literature. This study aims to investigate the E_cp of naturally-generated chloride-induced corrosion products formed in different concrete mixes. Microstructural characterization was conducted through nano-indentation, electron microscopy and Raman spectroscopy. The corrosion products were mainly composed of a goethite matrix with portions of maghemite, independently of the concrete composition. Microscopic analysis suggest that layers of corrosion products grow at different times and under different physico-chemical conditions. Our measurements showed that E_cp varied between 80−100 GPa, which can be suggested for numerical models of corrosion induced cracking.

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Over the last few years, the United States has experienced a shortage of fly ash and slag that consequently created a need for an alternative material that is locally available, sustainable, and provides desirable concrete properties. Recent studies have shown that Ground Glass Pozzolan (GGP) offers favorable attributes as a supplementary cementitious material (SCM) for concrete. However, there are limited studies demonstrating freeze-thaw (FT) resistance of concrete with GGP, as well as assessing the FT resistance in relation with the air-void system of GGP mixtures. In response, this study aimed to evaluate both macro- and micro-level behavior of GGP on FT resistance, and characterize mixtures with different contents of GGP. Six concrete mixtures were evaluated: three mixtures with 20, 30, and 40% GGP as cement replacements and three other reference mixtures with 30% fly ash and 40% slag and 100% Ordinary Portland cement (OPC). Following ASTM standards, concrete beam samples were tested for accelerated FT resistance and dynamic modulus of elasticity up to 1000 cycles. All concretes showed high FT resistance with a durability factor over 90% and, consequently, minimal deterioration and scaling. Core samples extracted from the FT conditioned beams were scanned with the X-ray micro-tomography (CTscan) to identify air-void parameters. Through image analysis a quantification of air-void parameters was obtained, and their relationship to FT resistance was established. Using CT scan analysis, we demonstrated that concretes with the highest cement replacement with GGP and slag developed the most desirable spacing factor and specific surface for FT resistance.@en

An important input parameter for numerical models that simulate cracking of the concrete cover due to reinforcement corrosion is the Elastic modulus of corrosion product (Ecp). Despite its relevance, Ecp is subject of significant variations according to the values reported in the literature, which vary from less than 100 MPa up to 360 GPa. Furthermore, Ecp values proposed in most of the present literature are representative of the corrosion product generated by anodic accelerated corrosion or extracted from the steel/concrete interface (SCI), which might differ from that formed in real corroding structures. Therefore, this study aims to investigate the Elastic modulus of naturally-generated corrosion product present at the SCI through nano-indentation conducted on six reinforced concrete polished sections. The polished sections were obtained from six 20-year-old reinforced concrete prisms cast with different cement type (CEM I, CEM II/B-V, CEM III/B, CEM V/A), same water/binder ratio (0.55) and which were previously exposed to NaCl solution wet/dry cycles. This study revealed that the range of Ecp did not considerably vary between corrosion products formed in different concrete mixes. However, corrosion product was microscopically found to consist of overlapping bands with different Ecp, varying for up to around 70 GPa between each other. Through Environmental Scanning-Electron Microscopy (ESEM) and Energy Dispersive Xray Spectrometry (EDS) analysis of the indented locations, it was found that Ecp is highly dependent on the presence of interfacial cracks and inversely proportional to the concentration of Si and Ca, representative for corrosion product mixed with the surrounding concrete. Furthermore, higher concentration of Fe leads to higher Ecp. Based on this study, an average range of values for Ecp between 80-100 GPa can be suggested for use in numerical models for corrosion induced cracking, regardless of cement type of the structure under investigation.@en

Coupling of carbonation and chlorides ingress mechanisms is very common in concrete under certain exposure conditions such as coastal environments. The aggravation/ mitigation of corrosion by the existence of carbonation lies on the fact that microstructural changes due to carbonation result in changes on the transport properties of the material. In this study we investigate and quantify evolving transport properties of ordinary Portland cement paste, such as porosity, tortuosity and intrinsic permeability. Dual X-ray micro computed tomography (micro CT) is used for the quantification of porosity. Furthermore Dynamic Vapour Sorption (DVS) measurements are carried out to resolve water retention and relative permeability curves. The authors expect to provide insights into the mechanisms of accelerated carbonation in both types of cement paste, as well as data for input and validation of numerical and analytical models on this degradation phenomenon.@en

Recent studies have shown promising potential for using Glass Pozzolan (GP) as an alternative supplementary cementitious material (SCM) due to the scarcity of fly ash and slag in the United States. However, comprehensive studies on the freeze-thaw (FT) resistance and air void system of mixtures containing GP are lacking. Therefore, this study aimed to evaluate GP’s effect on FT resistance and characterize mixtures with different GP contents, both macro- and microscopically. In this study, six concrete mixes were considered: Three mixes with 20%, 30% and 40% GP as cement replacements and two other comparable mixes with 30% fly ash and 40% slag, as well as a mix with 100% Ordinary Portland cement (OPC) as a reference. Concrete samples were prepared, cured and tested according to the ASTM standards for accelerated FT resistance for 1000 cycles and corresponding dynamic modulus of elasticity (Ed). All the samples showed minimal deterioration and scaling and high F/T resistance with a durability factor of over 90%. The relationships among FT resistance parameters, air-pressured method measurements of fresh concretes and air void analysis parameters of hardened concretes were examined in this study. X-ray micro-tomography (micro-CT scan) was used to evaluate micro-cracks development after 1000 freeze-thaw cycles and to determine spatial parameters of air voids in the concretes. Pore structure properties obtained from mercury intrusion porosimetry (MIP) and N2 adsorption method showed refined pore structure for higher cement replacement with GP, indicating more gel formation (C-S-H) which was verified by thermogravimetric analysis (TGA).

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Polyhydroxyalkanoate (PHA) production is a promising opportunity to recover organic carbon from waste streams. However, widespread application of waste-derived PHA as biodegradable plastic is restricted by expensive purification steps, high quality requirements, and a fierce competition with the conventional plastic market. To overcome these challenges, we propose a new application for waste-derived PHA, using it as bacterial substrate in self-healing concrete. Self-healing concrete is an established technology developed to overcome the inevitable problem of crack formation in concrete structures, by incorporating a so-called bacteria-based healing agent. Currently, this technology is hampered by the cost involved in the preparation of this healing agent. This study provides a proof-of-concept for the use of waste-derived PHA as bacterial substrate in healing agent. The results show that a PHA-based healing agent, produced from PHA unsuitable for thermoplastic applications, can induce crack healing in concrete specimens, thereby reducing the water permeability of the cracks significantly compared to specimens without a healing agent. For the first time these two emerging fields of engineering, waste-derived PHA and self-healing concrete, both driven by the need for environmental sustainability, are successfully linked. We foresee that this new application will facilitate the implementation of waste-derived PHA technology, while simultaneously supplying circular and potentially more affordable raw materials for self-healing concrete.

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In this study, a novel non-toxic, biodegradable bacteria-based healing agent known as alkanoate derivatives (AKD) derived from wastewater was investigated for its self-healing efficiency of the mortar specimens in comparison to the already-developed healing agents made of lactic acid derivatives (PLA). Mortar with different percentages of healing agent inclusions (2.6% and 5% w/w cement) have been evaluated in this study. To assess the self-healing improvements of the mortar incorporated bacteria-based bio-plastic healing agent's self-healing capacity, quantification of self-healing efficiency was performed at two different healing intervals of 28 and 56 days through optical determination of crack closure by stereomicroscope, recovery of water tightness by rapid water permeability test and determination of mass percentage increase of calcium carbonate by thermogravimetric analysis (TGA). The healing products formed in the cracks were analyzed by TGA and X-ray diffraction (XRD). Furthermore, a statistical analysis was performed to understand the variability in the crack width, water flow and the correlation of self-healing ratios between the stereomicroscope and permeability measurements. The results revealed that series containing the healing agents displayed a higher crack closure ratio compared to plain mortar series for initial crack widths greater than 0.4 mm at 56 days of healing. Moreover, the recovery of water tightness for series containing bacteria were greater compared to plain mortar for initial crack widths greater than 0.6 mm at 56 days of healing. However, in healing agent incorporated mortar series, only alkanoate derivatives at 5% dosage reported an increase in mass % of calcium carbonate precipitation at 56 days of healing. From the statistical analysis, it was confirmed that the influence of internal crack geometry plays a significant role in the degree of healing and variation of the water flow for smaller crack widths as the healing period increases.

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Bacteria-based self-healing concrete has the ability to heal cracks due to the bacterial conversion of incorporated organic compounds into calcium carbonate. Precipitates seal the cracks, theoretically increasing the service life of constructions. The aim of this paper is to propose a precursor for bacteria-based self-healing concrete derived from organic waste streams, produced is in line with the circular economy principle and ideally more affordable than other substrates. To verify the applicability of the proposed healing agent, some fundamental requirements of the proposed system are studied, such as its influence on functional properties, crack sealing capacity and evidence of bacterial activity in concrete.

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The Service life evaluation of reinforced concrete structures is usually limited to initiation of corrosion, whilst in practice corrosion in many structures has already reached the propagation stage. To better understand the processes that lead to the cracking and detachment of concrete cover during this phase, knowledge of corrosion products’ development over time is required. This paper investigates corrosion products found in blast furnace slag cement concrete, in which natural carbonation acted upon original chloride-induced corrosion. The sample was cast in 1998, after curing subjected to wet-dry cycles to enhance chloride penetration, and later was exposed to unsheltered outdoor conditions. Corrosion products and textures at the concrete-steel interface and late carbonate veinlets within them have been characterized by a combination of optical microscopy, SEM, Raman spectroscopy and CT scanning.

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Since self-healing of cementitious materials can theoretically improve the service-life of concrete structures, it has gathered significant attention from both researchers and industry during the last two decades. Many researchers have proposed different methods to assess and quantify the self-healing capacity (i.e. the ability of cementitious materials to heal cracks) that is generated in concrete autogenously as well as autonomously. Even though many methodologies can be found in the literature, a way to accurately quantify the healing products produced by any self-healing mechanism has not been yet achieved. In this study, a methodology is proposed to observe and to quantify in-time formation of healing products based on active thin sections. Thin sections of Portland cement paste have been prepared with no epoxy impregnation to facilitate reactions between the cement matrix and the surrounding environment. Artificial cracks (260 μm wide) were induced at 28 days of age and the crystal growth was continuously monitored up to 28 days of self-healing. Through image analysis of the micrographs, it was calculated that the autogenous self-healing capacity of paste (triggered by portlandite carbonation in uncontrolled indoor conditions) was around 55% after 28 days of self-healing. Healing products were further characterised through Environmental Scanning Electron Microscope analysis. Based on the results obtained in this study, the proposed methodology seems to be promising to compare the self-healing mechanisms triggered by different healing agents.

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In this study, the interface between different types of bacteria-embedded self-healing polylactic acid capsules (PLA) and cement paste is investigated. Particularly, the changes in microstructure and mechanical properties of the interface with respect to bulk cement paste were studied. First, nanoindentation was performed to obtain maps of hardness and elastic modulus in the interfaces. Lattice modeling 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. The results suggest that PLA can be tailored to optimize the physico-mechanical properties of the interface and hence, the mechanical behavior and triggering efficiency of the self-healing system.@en

Although reinforcement corrosion is a well-known issue, which are the locations of the steel/concrete interface most sensitive to pitting corrosion is still an unclear issue. In this study, X-ray computed tomography is used to characterize eight 20-years-old reinforced concrete cores naturally deteriorated due to chloride-induced corrosion. The deepest and most frequent corrosion pits were observed at the portion of the reinforcement oriented to the outdoor environment and in proximity to interfacial air voids. Therefore, the presence of interfacial air voids should be considered as a relevant factor when assessing the risk of corrosion of reinforced concrete structures.

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Soft inclusions, such as capsules and other particulate admixtures are increasingly being used in cementitious materials for functional purposes (i.e. self-healing and self-sensing of concrete). Yet, their influence on the fracture behaviour of the material is sometimes overlooked and requires in-depth study for the optimization of mechanical and/or smart properties. An experimental investigation is presented herein on the role of bacteria-based lactate-derived particles on the fracture behaviour of cement paste in tensile configuration. These admixtures are currently used for the purpose of self-healing. Digital Image Correlation was used to obtain strain contours on the surface of the samples during the test. The influence of soft particles addition and age of the samples on the fracture mechanics of the composite were investigated.@en

Although corrosion of reinforcement is a well-known issue for the construction industry, there are still open questions about some fundamentals of corrosion in reinforced concrete. These points include, among others, which are the most sensitive locations of the steel/concrete interface for pitting corrosion to initiate and to propagate. In this study, X-ray computed tomography (CT-scan) is used to characterize eight 20-years-old reinforced concrete cores naturally deteriorated due to chloride-induced corrosion. The volume loss due to corrosion of the reinforcement was quantified through image analysis of CT-scans. The volume loss of the steel was found to be higher for steel rebars embedded in Portland cement specimens rather than in blended cement specimens. Furthermore, CT-scans revealed that the deepest and most frequent corrosion pits, as well as the consequent highest volume loss of steel, were present at the portion of the reinforcement closer to the outdoor environment and in proximity to air voids at the steel/concrete interface. As a consequence, the highest decrease of structural performance of the rebars would be likely localized at those locations. Therefore, the presence of interfacial air voids should be considered as relevant factor when assessing the risk of corrosion of reinforced concrete structures.

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