Understanding the Impacts of Healing Agents on the Properties of Fresh and Hardened Self-Healing Concrete: A Review

Self-healing concrete has emerged as one of the prospective materials to be used in future constructions, substituting conventional concrete with the view of extending the service life of the structures. As a proof of concept, over the last several years, many studies have been executed on the effectiveness of the addition of self-healing agents on crack sealing and healing in mortar, while studies on the concrete level are still rather limited. In most cases, mix designs were not optimized regarding the properties of the fresh concrete mixture, properties of the hardened concrete and self-healing efficiency, meaning that the healing agent was just added on top of the normal mix (no adaptations of the concrete mix design for the introduction of healing agents). A comprehensive review has been conducted on the concrete mix design and the impact of healing agents (e.g., crystalline admixtures, bacteria, polymers and minerals, of which some are encapsulated in microcapsules or macrocapsules) on the properties of fresh and hardened concrete. Eventually, the remaining research gaps in knowledge are identified.

Effects of Autogenous and Stimulated Self-Healing on Durability and Mechanical Performance of UHPFRC: Validation of Tailored Test Method through Multi-Performance Healing-Induced Recovery Indices

Chloride diffusion and penetration, and consequently chloride-induced corrosion of reinforcement, are among the most common mechanisms of deterioration of concrete structures, and, as such, the most widely and deeply investigated as well. The benefits of using Ultra-High Performance (Fiber-Reinforced) Concrete—UHP(FR)C to extend the service life of concrete structures in “chloride attack” scenarios have been addressed, mainly focusing on higher “intrinsic” durability of the aforementioned category of materials due to their compact microstructure. Scant, if nil, information exists on the chloride diffusion and penetration resistance of UHPC in the cracked state, which would be of the utmost importance, also considering the peculiar (tensile) behavior of the material and its high inborn autogenous healing capacity. On the other hand, studies aimed at quantifying the delay in chloride penetration promoted by self-healing, both autogenous and autonomous, of cracked (ordinary) concrete have started being promoted, further highlighting the need to investigate the multidirectional features of the phenomenon, in the direction both parallel and orthogonal to cracks. In this paper, a tailored experimental methodology is presented and validated to measure, with reference to its multidirectional features, the chloride penetration in cracked UHPC and the effects on it of self-healing, both autogenous and stimulated via crystalline admixtures. The methodology is based on micro-core drilling in different positions and at different depths of UHPC disks cracked in splitting and submitted to different exposure/healing times in a 33g/L NaCl aqueous solution. Its validation is completed through comparison with visual image analysis of crack sealing on the same specimens as well as with the assessment of crack sealing and of mechanical and permeability healing-induced recovery performed, as previously validated by the authors, on companion specimens.

Methodology of Controlled Crack Introduction in Cementitious Materials

Crack formation is a common and generally inevitable phenomenon in the field of concrete structures. On the other hand, the ever-increasing demand for sustainable construction, thus the structures durability, has led researchers to propose and investigate various crack-sealing methods. This study deals with the key aspect of these investigations – the in-vitro creation of cracks. A large number of the conducted studies have been carried out on artificially cracked specimens, and various methodologies of the controlled crack introduction were presented; however, no specific method was clearly preferred. In this paper, several approaches to the crack introduction are applied: cracking through compressive loading, tensile loading, and 3-point bending. Further, different types of specimens are presented: plain concrete, reinforced with short and long steel fibers, and reinforced with steel rod. The achievable crack characteristics, such as widths or its stability over time, are evaluated and compared. This study thus provides valuable overlook of the possible approaches to the controlled crack creation and points out their potential and limitations. Based on the comparisons presented in this paper, the long steel fiber reinforced concrete specimens subjected to 3-point bending are identified as the most appropriate method of crack induction.

Quantitative Assessments of Crack Sealing Benefits by 3D Laser Technology

Crack sealing is one of the most commonly used methods to preserve asphalt pavements. However, quantification of crack sealing benefits (or the long-term delaying effects of crack sealing on crack propagation) remains unavailable because field crack lengths could not be measured accurately and efficiently. In this study, 3D laser technology is proposed to measure and compare the growth of crack lengths between sealed and non-sealed pavement sections and, for the first time, to accurately and efficiently quantify the crack sealing benefits. To validate the proposed method and find adequate treatment timing (or conditions), nine field sites in Georgia, U.S., with different pavement pre-treatment conditions and roadway environmental factors were monitored over 3 years from December 2016 to September 2019. The study results showed that crack sealing can retard crack growth by 40%–128%, and such delaying effects are more significant under better pavement pre-treatment conditions. The findings suggest that transportation agencies can prolong the service life of pavements by applying crack sealing before the pavement condition becomes poor. In addition, this work has been proved to be very valuable for transportation agencies to determine the best timing and treatment criteria for crack sealing.

Ureolytic MICP-Based Self-Healing Mortar under Artificial Seawater Incubation

Ureolytic microbial-induced calcium carbonate precipitation (MICP) is a promising green technique for addressing sustainable building concerns by promoting self-healing mortar development. This paper deals with bacteria-based self-healing mortar under artificial seawater incubation for the sake of fast crack sealing with sufficient calcium resource supply. The ureolytic MICP mechanism was explored by morphology characterization and compositional analysis. With polyvinyl alcohol fiber reinforcement, self-healing mortar beams were produced and bent to generate 0.4 mm width cracks at the bottom. The crack-sealing capacity was evaluated at an age of 7 days, 14 days, and 28 days, suggesting a 1-week and 2-week healing time for 7-day- and 14-day-old samples. However, the 28-day-old ones failed to heal the cracks completely. The precipitation crystals filling the crack gap were identified as mainly vaterite with cell imprints. Moreover, fiber surface was found to be adhered by bacterial precipitates indicating fiber–matrix interfacial bond repair.

Serpentine crack-seal veins: a unique record of fluid conditions during faulting

<p>Serpentine veins are ubiquitous in hydrated and deformed ultramafic rocks, and have previously been used to track fault kinematics and understand the evolution of environmental conditions during vein formation. However, difficulties in unambiguously identifying and mapping serpentine types at sub-micron to mm scales has limited our understanding of vein precipitation kinetics and growth histories. Using recently developed techniques of Raman spectroscopy mapping, combined with scanning- and transmission-electron microscopy, we describe a new type of mineralogically banded serpentine crack-seal vein in six samples from different settings around the world. In all of the studied samples, individual bands comprise a thin layer (~0.4–2 µm) dominated by chrysotile and a much thicker layer (~0.5–30 µm) dominated by polygonal serpentine/lizardite. Existing field and experimental data suggest that disequilibrium conditions immediately following crack opening may favour rapid precipitation of chrysotile along one of the crack margins. Subsequently, diffusional transport of elements favours slower precipitation of polygonal serpentine/lizardite which leads to crack sealing. The similarities in layer thicknesses and mineralogy exhibited by samples collected from extension and shear veins, dilational jogs, foliation surfaces, and the margins of phacoids, suggest that a common set of processes involving crack opening and sealing are active in a range of different structural sites within serpentinite-dominated shear zones, potentially associated with frequent and repetitive stress drops such as those recorded during episodic tremor and slow slip.</p>