Quantification of crack-healing in novel bacteria-based self-healing concrete
Introduction
As it is strong, durable and relatively inexpensive, concrete is the most used construction material worldwide [1]. However, the presence of cracks may reduce the durability of concrete structures. Micro cracks are an almost unavoidable feature of ordinary concrete. If micro cracks form a continuous network they may substantially contribute to the permeability of the concrete, thereby reducing the concrete’s resistance against ingress of aggressive substances [2]. Nevertheless, not all initial micro cracks develop into harmful or unstable cracks. A number of studies reported that under certain circumstances, small cracks in concrete can heal [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. This phenomenon is known as ‘autogenous healing’ or ‘self-healing’ of concrete. The primary causes of autogenic healing are considered to be based on chemical, physical, and mechanical processes [2], [4]. However, precipitation of calcium carbonate has been reported to be the most significant factor influencing the autogenous healing of concrete [4], [7].
Besides autogenous healing, cracks may also be autonomously repaired by incorporating a specific healing agent within the matrix. Various healing agents have been proposed for enhancing the self-healing capacity of concrete. While most healing agents are chemically based [2], [12], [13], [14], more recently the possible application of bacteria as self-healing agent has also been considered [1], [15], [16], [17], [18]. In a number of published studies the potential of calcite precipitating bacteria for concrete or limestone surface remediation or durability improvement was investigated [15], [17], [19], [20], [21], [22], [23]. The mechanism of bacterially mediated calcite precipitation in latter studies was primarily based on the enzymatic hydrolysis of urea. A potential drawback of this reaction mechanism is that for each carbonate ion two ammonium ions are simultaneously produced which may result in excessive environmental nitrogen loading [16]. Moreover, in these studies bacteria or derived ureolytic enzymes were externally applied on cracked concrete structures or test specimens. Thus the remediation mechanism in those studies cannot be defined as self-healing. Recently, Jonkers et al. [1], [16], [18] developed a two-component self-healing system that is composed of bacterial spores, which after germination catalyze the metabolic conversion of organic compounds (the second component) to calcium carbonate. Both components were mixed with the fresh cement paste, thus becoming an integral part of the concrete. They furthermore showed that incorporated bacteria and certain organic calcium salts such as calcium lactate functioning as calcium carbonate precursor did not negatively affect concrete compressive strength. However, the authors also observed that the functionality of bacterial mineral production of directly (unprotected) incorporated two-component healing agent was limited to young (1–7 days old) concrete specimens. It was hypothesized that the majority of incorporated bacterial spores apparently became crushed or inactivated by high alkalinity, resulting not only in loss of viability but also in decreased mineral-forming capacity in aged specimens.
In the present study protection by immobilization in porous expanded clay particles of the two-component bio-chemical healing agent prior to addition to the concrete mixture was tested as an alternative strategy to the direct mixing in order to substantially increase its service life functionality. In this manner, the expanded clay particles not only represent an internal reservoir but also constitute both a structural element of concrete as well as a protective matrix for the self-healing agent. Such a system should increase the viability and thus the time-related functionality of the bio-chemical self-healing agent. The main aim of this study was therefore to quantify the crack-healing ability of aged concrete specimens based on this two-component bio-chemical agent immobilized in expanded clay particles.
Section snippets
Self-healing agent preparation
The bio-chemical two-component self-healing agent consisted of a mixture of calcium lactate and bacterial spores both embedded in expanded clay particles. Spores of a bacterial isolate obtained from alkaline lake soil (Wadi Natrun, Egypt) were used in this study. Sequence analysis of 16S rRNA gene of this bacterium revealed a 98.7% homology to Bacillus alkalinitrilicus an alkali-resistant soil bacterium [24]. Senescent cultures containing high number of spores were washed by repeated
Optical determination of crack-healing capacity
Fig. 1 shows direct stereomicroscopic observation of cracks from control and bacteria-based specimens before and after 100 days of immersion in tap water. Width of completely healed cracks was significantly larger in bacteria-based specimens (0.46 mm) compared to control specimens (0.18 mm).
Element composition analysis using energy dispersive spectroscopy (EDAX) revealed that the massively formed precipitate on crack surfaces of bacterial specimens was essentially an association of calcium, oxygen
Discussion
The self-healing is the partial or total recovery of at least one property of a material. In this study, the damage considered is the formation of cracks as it reduces the concrete’s resistance against the ingress of aggressive substances; therefore the crack filling and/or closure is closely related to the recovery of the material permeability. Nevertheless, the sample geometry and size required to perform permeability test limit the number of cracks per sample, and the maximum crack width
Conclusion
In conclusion, the results presented in this study show that the applied two-component bio-chemical self-healing agent, consisting of a mixture of bacterial spores and calcium lactate, can be successfully applied to promote and enhance the self-healing capacity of concrete as the maximum healable crack width more than doubled. Moreover, oxygen measurements provided evidence that concrete incorporating bacterial spores embedded in expanded clay particles and derived active bacteria remain viable
Acknowledgments
Arjan Thijssen and Ger Nagtegaal are acknowledged for help with ESEM analysis and laboratory support, Gerard Muyzer and Ben Abbas for 16S rRNA gene sequence analysis of the used bacterial isolate and Serge van Meer for help with FT-IR analysis. The authors acknowledge the financial support from Delft Centre for Materials (DCMat) in the form of Project SHM08704, ‘Bio-chemical self-healing agent to prevent reinforcement corrosion in concrete’.
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