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Engineering, Civil
Developments in low-carbon concrete technology are of peak interest today under the scrutiny of emerging policy pressures. Existing research in self-healing concrete (SHC) has often concerned the scope of material development and evaluation with inconclusive field testing, hindering its progress towards structural feasibility. This paper presents an overview of recent progress in SHC, and possible opportunities and challenges of popular healing systems are discussed. Moreover, trends are observed to investigate SHC’s influence on the engineering properties of concrete, and future projections of SHC are suggested with identification of potential research needs.
- low-carbon concrete
- sustainability
- structural resilience
- self-healing concrete
1. Introduction
As a primary engineering material, concrete contributes greatly to the impact of the construction industry on the global environment emitting approximately 8% of the global carbon dioxide emissions [1,2], a poor trend that may rise with growing populations. However, the urgency for green concrete is globally expanding as legal regulations intervene, placing a new challenge on existing means of concrete manufacturing and application. The United Nations Sustainable Development Goals [3] and independent regulations of individual countries have placed the industry under unprecedented scrutiny to control its carbon footprint.
In addressing some of the main influencing factors contributing to unsustainable practices such as the phenomenon of concrete cracking, the construction industry can expect promising lifecycles of various structures. Cracking creates an open path for the ingress of harmful substances into concrete structures, exposing steel reinforcement to the risk of corrosion and overall degradation of structural integrity. Similarly, the anticipated atmospheric deterioration due to climate change effects may also threaten the lifecycle and resilience of existing concrete structures. Consequently, increased concrete production becomes a requirement with maintenance and demolition of deteriorated structures, hence directly influencing the industry’s carbon footprint. Within this context, costly maintenance is in demand; however, in addition to being uneconomical, some cracks and defects are hard to detect and reach in certain structures and/or aging infrastructure. Current practices geared towards controlling concrete cracking and improving durability mainly constitute the use of supplementary cementitious materials (SCMs) and various admixtures, as well as the traditional approach of steel reinforcement. The SCMs adopted are often low-carbon industrial by-products or landfill waste such as ground granulated blast-furnace slag (GGBS), pulverised fuel ash (PFA), coal bottom ash (CBA), glass, ceramics, etc., where their use in concrete contributes to a circular economy. Most SCMs can enhance the Ordinary Portland Cement (OPC) concrete properties such as reduced porosity, heat generation, and subsequently improved hydration, hence enhancing durability, quality, and overall practicality in versatile environments. A relatively novel approach to improving structural resilience is the enhancement of the concrete itself in preference to reinforcement reliance. This is achieved through enhancing the intrinsically regenerative nature of concrete with complimentary admixtures or through the inclusion of self-healing agents (SHA) and microorganisms capable of producing self-healing concrete (SHC).
3. SHC Structural Engineering Performance
In evaluating the self-healing performance of concrete, mechanical strength recovery, durability improvement and microstructural analysis are of main interest following exposure to deterioration and cracking. In specific, the influence of different healing systems on the engineering properties is substantial to support understanding the role of invoked healing processes in the development of mechanical and durability properties paramount to the structural engineering performance.
2.1. Mechanical Properties
A non-exhaustive summary of existing research is shown, where figures are limited to the highest engineering performance of the respective study selected for conforming to Eurocodes, Canadian, American and Indian Standards. The selected studies are therefore repeatable and allow confirmatory corroboration for further reliability and mitigation of testing disparity in SHC. Generally, a trend is observed where the mechanical properties of concrete are improved in utilising healing systems.
In using crystalline admixtures, the sealing functionality is stimulated at an accelerated rate in comparison to ordinary concrete such that a study has found sealing efficiency of SHC between 1 and 3 months to be comparable to ordinary concrete’s capacity after 3–6 months [37]. An agreeing experiment testing the self-healing functionality of crystalline admixtures in concrete has shown poor compressive strength development in contrast with ordinary concrete [38]; however, the SHC rate of strength recovery was superior. It is recommended that cracking is induced during early ages to attain the advantages of autogenous healing prominent at younger ages. In some cases, it was found that the precracking induced at 7 days has resulted in 10% higher strength recovery and crack closure in contrast to 28 days cracking; however, the former had larger crack widths which accumulated chloride ions. This recommendation may be explained by the premature flexural properties’ development of younger concrete, and therefore minimal control of crack, whereas mature concrete possesses greater control of cracking width and extent.
In using chemical SHC systems, Giannaros et al. [35] have reported superior 28 days compressive strengths of the SHC samples in comparison to the control; however, at 56 days, the control samples had comparatively improved. A possible explanation for this may be that there was capsule rupture causing an accelerated early hydration which enhanced strength development up to 28 days; this is also seen in field case studies [39]. The flexural strength of the smaller capsules exceeded the values of the larger capsules; this may be due to microstructural integrity. However, from a structural view, this may be compared to standard steel reinforcement where reduced reinforcement (reduced capsule size) contributes to the favourable ductile failure, and increased reinforcement (larger capsule size) may compromise flexural properties. Moreover, there was no correlation between the crack widths reported and the mechanical performance such that the smaller capsule SHC had smaller crack widths with low compressive strength values, whereas the larger capsules had larger cracks and showed higher compressive strength values.
The typically autogenous approach of using basalt fibres has proven to be an efficient healing system with enhanced compressive strength and flexural strength when combined with bacteria [30]. Moreover, engineering properties have been reported to have been recovered to a high degree following the application of 60% of load-bearing capacity at 28 days. The combined action of fibres restricting crack width and bacteria filling the cracks may sufficiently address unwarranted healing activation.
In a study of bacterial RC beams incorporating a microbial-induced carbonate precipitation healing system [36], the deflection of the SHC beams was reported to increase progressively with increasing crack widths; however, higher loads were sustained. Moreover, assessed recovery of the flexural strength of the SHC was found to be 73%, whereas OPC concrete has shown a decrease of 41%. Hence, the study has concluded that improved flexural stiffness and load-bearing capacity may be achieved with bacterial RC beams. This may be attributed to the general increase of compressive strength reported in bacterial concrete, which has improved ductility. The latter response characteristic can be useful for the application of SHC in structures exposed to extreme natural and/or man-made hazards, e.g., earthquakes, floods, strong winds, explosions, and impacts. The study recommends for bacterial SHC to include the incorporation of a nutrient source and the introduction of cracking at an early age of 7 days for optimal tensile strength recovery; this is also applicable in using autogenous SHC incorporating crystalline admixtures [33].
2.2. Durability Properties
The durability of SHC is mainly assessed by the improvement of durability following exposure to degradation. Improvements in transport properties (i.e., permeability, sorptivity, and diffusivity), resistance to corrosion and chemical attacks, etc., are of interest when investigating durability properties in SHC.
In investigating the corrosion resistance of bacterial RC beams and cylinders [36], a 90% reduction in corrosion probability is reported; this is possibly linked to the increased watertightness. The decreased water absorption rate in bacterial SHC has been relatively consistent in research compared to OPC concrete [40,41,42,43,44,45], hence lower porosity is often found in microstructural analysis [28]. In using crystalline admixtures, its nature as water-resistant and permeability-reducing [12] hints at an ability to sustain a lower water absorption rate; this has been established in various experiments with different components of the admixture [18,19,33,34,35]. However, in the use of crystalline admixtures, there is a lack of comparability in research [16,17] due to nonlinearity in admixture composition and testing methods [12,16], this hinders understanding its feasibility in relation to comparably well-documented performance of bacterial SHC.
In practical applications, SHC may be an attractive technology for seismically risky locations where the smallest of impact loads present a risk of cracking in concrete; this is especially true due to the shared parametric interests regarding repeatability of self-healing under cyclic loading (i.e., earthquake dynamic loads). In a study testing the dynamic behaviour of SHC utilising microencapsulated epoxy-resin subjected to impact loading [46], the SHC was shown to have an enhanced energy absorption capacity and a dynamic strength increase parallel to the increasing strain rate. The ability of SHC to have an improved self-healing functionality in exposure to water also suggests that it may be appropriate for humid, rainy, seaside, breakwater, and underground foundation applications. This is especially useful in the face of rising sea levels, where the current and upcoming approaches of employing sea defenses or managed realignment strategies may benefit from SHC for its tendency to thrive in such conditions.
Conclusively, two factors are identified to interfere with crack healing in SHC: an increase in hydration that decreases porosity and subsequently restricted the healing system’s transportation, and the likely subsequent increase in crack age, likely due to a decreased number of viable bacteria remaining after consequent pore filling. Hence, it may be feasible to employ mineral admixtures (SCMs) for their contribution to delayed hydration to counteract the high initial hydration interference. The six robustness criteria established to predict self-healing functionality may be explored further to aid in the selection of an appropriate concrete approach for specific structural applications [47]. Preliminary aspects must be addressed to achieve maximum compatibility of SHC with a specific structural application such as width and dynamic of anticipated cracking, probability and extent of water exposure, and most importantly to tailor the mix for the specified application, it may be useful to detail the desired SHC properties and possibly categorise targets by priority according to the subjected environment (i.e., recovery of mechanical properties, liquid tightness, limiting crack width, etc.).
3. SHC Market Feasibility
The experimental progress of self-healing concrete is an emergent domain that lacks in field applications where practical testing illustrates realistic structural feasibility. Government support for the development of SHC was seen in major European projects such as RM4L [48] and HEALCON [49]. However, the lack of industrial collaboration in the theoretical progress of SHC research may reduce SHC prospects and the pace of adoption in real-life applications. Another barrier to the market adoption and upscale of SHC lies in the short duration of experimental testing. The current approach towards commercialisation and long-term performance prediction makes use of computer modeling to carry out lifecycle assessments and material optimisation based on simulated output. This approach conforms to the standard of design by testing and reduces the high costs associated with producing and testing SHC in field applications given its currently limited understanding.
It may be deduced that a shortcoming in SHC progress towards commercialisation lies in the restrictions associated with short- to medium-term experimental testing compensated with accelerated tests. In addition, the persuasion of industry interest may also be hindered due to a lack of consistent testing methods and inconclusive field tests that poorly correlate with laboratory results.
Field studies where SHC was used on a newly built structure have all shown no signs of cracking, which has disallowed the evaluation of self-healing performance on a real-life scale. However, valuable output can be observed regarding the feasibility of mass production of SHC for newly built structural applications. In contrast with self-healing application in new structures, mostly successful applications are reported in using SHC as a repair agent on existing structures and infrastructure with crack width closure and permeability reduction. This presents an opportunity for the use of SHC to non-invasively regenerate historical structures. The application of the SHC roof slab [50] was reported to have imposed a requirement for extending the mixing time on site, which presented the issue of an increased air content, concluding with a suggestion to avoid the direct addition of healing systems into an industrial concrete mixer.
In another application for an underground structure [39], noticeable crack formation was found within 7 days after pouring the SHC, the cracks were healed by continued wetting; however, the healing product formed was seen leaking externally, a possible indication of inadequate control of the extent of warranted healing response. This issue is especially crucial for structural applications such that self-healing system incorporation costs more than traditional concrete and, therefore, wasted product has serious economic and durability implications. Arguably, leaking product may be due to a nonuniform or inconsistent dispersion of capsules within the concrete matrix. This occurrence perhaps reveals the relevance of studying the efficiency of the extent of response of self-healing upon stimulation. Conclusively, the rapid crack formation seen in this project is concerning and may indicate that the fresh properties of SHC require further scrutiny. This is supported by the appearance of fluctuations at the early-age temperature and strain monitoring, which was translated as accelerated hydration, possibly due to potential breakage of microcapsules. Early microcapsule breakage is a common phenomenon; however, researchers have used capsules protected with low-alkali cement. Therefore, it is worth noting that the standard procedures of concrete mixing and pouring may have contributed to the inefficient self-healing functionality.
An irrevocable appeal of SHC is the noted reduction in carbon emissions. This is portrayed by the use of SCMs in most SHC systems where utilised cement and reinforcement are
simultaneously reduced. However, it is challenging to quantify the estimated emission reduction in SHC due to the wide range of composition, production and testing variability. Nonetheless, several lifecycle assessments (LCA) have been conducted on varying SHC systems. It is worth noting that the initial environmental and economic impacts of using SHC systems may be higher due to associated cradle-to-gate processes; therefore, some assessments conclude with urging that the repair costs may be relatively diminished in efficient SHC structures, offsetting initial costs with an overall improved lifecycle. Noticeably, a shortcoming was found in LCA studies of SHC regarding bacterial concrete, where most literature scopes have involved the self-healing ECC systems.
This entry is adapted from the peer-reviewed paper 10.3390/buildings12081196
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