ReviewAutonomous healing in concrete by bio-based healing agents – A review
Introduction
Cracking is inevitable in concrete due to its relatively lower tensile strength and action of different load and non-load factors. Sources of cracking may be varied including plastic shrinkage, drying shrinkage, thermal stresses, external loading and rebar corrosion or coupled effect of multiple factors. For example, micro-cracks may form due to shrinkage but may propagate at a lower stress level when external loading is applied causing network of cracks. Such network of cracks gives easy access to moisture and chemicals to seep into the structure or degrade concrete chemically. Such problems are worse in the tropics due to higher moisture content in the air and high precipitation. Cracks can be manually repaired but there are several problems associated with manual maintenance and repair operations such as impact on environment, accessibility and cost. Different chemical and cement based repair materials are used presently. Cement production is associated with about 7% of global anthropogenic CO2 emission [1] while chemical healing agents present threats including material incompatibility, health and environmental hazards [2]. Micro-cracks may originate in concrete just after construction or at advanced stage which is often unnoticed until it leads to major durability or structural issues. It is also a financial burden to carry out routine repair operations on facilities. Therefore, there is a need to find a sustainable way of healing cracks which involve less cost and eliminate the need of manual intervention.
Self-healing is an emerging concept of delivering high quality materials combined with the capability to heal damages and it has received much attention in past decade for application in building structures. Therefore, an effective self-healing mechanism may be able to reduce repair and maintenance works substantially and concomitant environmental and economic impacts. Recently, a sustainable mechanism of self-healing using microbial induced precipitation of calcium carbonate has been intensively studied to seal and heal cracks. Self-healing by microbes involve precipitation of calcium carbonate in cracks by direct action of bacteria species including Bacillus Subtilis on calcium compound such as calcium lactate [3] or by decomposition of urea by ureolytic bacteria such as Bacillus Sphaericus [4], [5]. Calcium carbonate precipitation by microbes is compatible with concrete and the process of formation is environmentally friendly [6]. Bacillus Sphaericus is known to be harmless to human [7]. Additionally, during the process oxygen is consumed and therefore it also reduces the chance of reinforcement corrosion. The genre Bacillus has qualities including tolerance to high alkaline environment and moisture and capacity to form spores which make it suitable to use as self-healing agent in concrete. Therefore, Bacillus has been most commonly used in research studies as the bio-agent for calcite precipitation [3], [4], [5], [8], [9], [10].
The review aims at evaluation of self-healing in concrete by biological action when the bacteria are directly added to concrete or added after encapsulation in mineral or chemical compounds. The discussion is conducted in the light of several key criteria including capsule material and encapsulation of bio-agents, survival of capsules during concrete mixing, viability of bacteria, effect of addition of bio-agents or capsules on concrete properties, and sealing ability and recovery of mechanical and durability properties.
Section snippets
Bio-based self-healing-mechanisms and approaches
Mentioned by Hammes and Verstraete [11], precipitation of calcium carbonate in natural environment is ideally influenced by concentration of calcium ions, pH of the solution, concentration of dissolved inorganic carbon and availability of nucleation sites. While the first three conditions relate to concrete matrix, the fourth one is provided by the bacterial cell itself. Bacterial precipitation may be achieved through different pathways like conversion of calcium compound such as Ca-lactate or
Evaluation of bio-based self-healing system
In general, ensuring efficient self-healing action would require optimization and evaluation of different aspects like effect of bio-agents, capsule and overall system design [21]. Key aspects covered in this review are
- a)
Capsule material and encapsulation of bio-agents.
- b)
Survival of capsules during concrete mixing.
- c)
Effect of addition of bio-agents and capsules on concrete properties.
- d)
Sealing ability and recovery of mechanical and durability properties.
Conclusion and scope of future work
It is a fact that the initial cost of incorporating bio-agents may be high at present. There may be two possible ways to make it commercially viable – by reducing the cost of production of bio-agents or by designing the self-healing action for longer life so that it works well under multiple cycles of loading and adverse environment condition to which structures are often subjected to. Presently, high cost is incurred due to a number of repair activities to handle several deterioration events
References (49)
- et al.
Microbial carbonate precipitation in construction materials: a review
Ecol. Eng.
(2010) - et al.
Self-healing concrete by use of microencapsulated bacterial spores
Cem. Concr. Res.
(2014) - et al.
Use of silica gel or polyurethane immobilized bacteria for self-healing concrete
Constr. Build. Mater.
(2012) - et al.
Use of bacterial cell walls to improve the mechanical performance of concrete
Cem. Concr. Compos.
(2013) - et al.
Application of bacteria as self-healing agent for the development of sustainable concrete
Ecol. Eng.
(2010) - et al.
Application of hydrogel encapsulated carbonate precipitating bacteria for approaching a realistic self-healing in concrete
Constr. Build. Mater.
(2014) - et al.
Factors affecting crack repairing capacity of bacteria-based self-healing concrete
Constr. Build. Mater.
(2015) - et al.
Multiscale mechanical quantification of self-healing concrete incorporating non-ureolytic bacteria-based healing agent
Cem. Concr. Res.
(2014) - et al.
Quantification of crack-healing in novel bacteria-based self-healing concrete
Cem. Concr. Compos.
(2011) - et al.
Crack healing in concrete using various bio influenced self-healing techniques
Constr. Build. Mater.
(2016)