Self-healing concrete research in the European projects SARCOS and SMARTINCS
Abstract: Self-healing concrete and preventive repair of structures will slow down the development of cracks and/or arrest the ingress of aggressive agents. When the cracks are closed or a decrease in crack width is achieved, this will be associated with improved durability of the structure. This paper describes the literature review and inter-laboratory comparison carried out within the COST Action CA15202 (SARCOS), as well as the research planned within the recently started International Training Network SMARTINCS.
Reference of this article: De Belie, N., Van Tittelboom, K., Sánchez Moreno, M., Ferrara, L., Gruyaert, E. (2020). Self-healing concrete research in the European projects SARCOS and SMARTINCS. Plenary keynote lecture. RILEM Spring Convention, University of Minho, Guimarães, Portugal, 9-10 March 2020. (In press).
Keywords: Self-healing Concrete, Repair, Durability, Healing Efficiency
Nele De Belie, Kim Van Tittelboom: Magnel-Vandepitte Laboratory for Structural Engineering and Building Materials, Ghent University, Belgium
Mercedes Sánchez Moreno: Inorganic Chemistry Department, University of Córdoba, Spain
Liberato Ferrara: Department of Civil and Environmental Engineering, Politecnico di Milano, Italy
Elke Gruyaert: Department of Civil Engineering, KU Leuven, Belgium
The search for smart self-healing materials and preventive repair methods is justified by the increasing sustainability and safety requirements of structures. The appearance of small cracks in concrete is unavoidable, not necessarily causing a risk of collapse for the structure, but certainly impairing their serviceability, accelerating its degradation and diminishing the service life and sustainability of constructions. That loss of performance and functionality promotes an increasing investment on maintenance and/or intensive repair/strengthening works. Therefore, the COST Action CA 15202 (SARCOS)  firstly focuses on preventive repair solutions including self-healing approaches and innovative external repair methods for existing concrete elements. Despite the promising potential of the developed healing technologies, they will only find their way to the market when sound characterization techniques for performance verification are developed, being the SARCOS's second focus. The third focus deals with modelling the healing mechanisms taking place for the different designs and with predicting the service life increase achieved by these methods.
The added value of networking in SARCOS mainly lays in the expertise exchange between the most recognized international research groups working in the topic. The discussion at international level between worldwide leading groups covers a wide range of approaches, methodologies and applications, ensuring the objective of creating guidelines and recommendations for real applications.
From the sustainability point of view, self-healing concrete and preventive repair of structures may slow down the development of cracks, even achieving a decrease in crack width with associated benefits for the structures. Filling of the cracks with healing agent will furthermore reduce their negative effects regarding penetration of aggressive substances in the concrete. These approaches are also associated to the optimal application of different repair technologies, involving environmentally friendly technologies, and to the minimization of the use of raw materials and of the worker’s risk associated to traditional repair operations, which will be a benefit in the long-term. The new functionalities included in these advanced cement based materials and the higher performance, associated to the developed methodologies, are expected to delay the ageing of concrete structures built with these materials, thus leading not only to higher service life but also to better performance in more aggressive environments and under more demanding conditions.
A recent review paper by SARCOS members  reveals the key challenge that the self-healing additions up-to-date are produced at lab scale and self-healing efficiency is only shown at paste/mortar level. The few existing demonstrators at the concrete level often show insufficient or not yet proven self-healing efficiency. Further, factors such as effect of variability in the design parameters and longevity of the embedded system may be questioned. One of the reasons for reduced efficiency after upscaling to concrete is the significant dilution of the additives when maintaining the dosage relative to cement weight; however, keeping the same dosage in proportion to the total volume, results mostly in an unacceptable strength decrease and high healing agent cost. Another challenge is that the durability of self-healing concrete elements has only been scarcely investigated. No long-term durability results of self-healing concrete are available, and even results for accelerated durability testing in lab conditions are scarce. Mostly, durability is assessed indirectly through parameters such as gas and water permeability, surface resistivity and capillary absorption. It is suggested that further research should focus on durability of the healed structures e.g. resistance to chloride diffusion and carbonation, corrosion, freeze/thaw, salt crystallization, etc. .
A second SARCOS review paper  relates to the experimental methods and techniques, which have been employed to characterize and quantify the self-sealing and/or self-healing capacity of cement-based materials, together with the methods for the analysis of the chemical composition and intrinsic nature of the self-healing products. This article also addresses the correlation between crack closure and recovery of mechanical properties. Especially the experimental characterization of the self-healing capacity under sustained loads has been highlighted as an important research need to provide a basis for incorporation of self-healing concepts and to allow incorporation of self-healing functionalities in predictive models and durability based design approaches.
The third review paper , which is related to SARCOS’ third objective, discusses research progress on numerical models for self-healing cementitious materials. This article provides a summary of self-healing techniques and discusses mechanical models for self-healing, transport processes in materials with embedded healing systems, fully coupled models and other modelling techniques used to simulate self-healing behaviour. The models discussed include those based on continuum-damage-healing mechanics, micro-mechanics, as well as models that use discrete elements and particle methods. The article also covers transport models and the simulation of carbonation in concrete since this mechanism governs self-healing based on calcite precipitation in cracks. The article highlights the lack of fully-coupled models, although approaches that couple some aspects of transport and mechanical healing behaviour are discussed. One bottleneck pointed out is that many models are presented with only very limited experimental validation. It seems that there has been insufficient interaction between numerical and experimental research teams. Other limitations include that often only one cycle of healing can be simulated, healing takes place under zero-strain conditions, damage and healing are not coinciding and healing takes place instantaneously. The statistical variations have only been considered in a few exploratory investigations.
Currently, the SARCOS partners are conducting a Round Robin Test campaign to verify the different characterization techniques and evaluation criteria for the preventive repair approaches. The outcome of this test campaign will be used as input for recommendations and guidelines. Important in this regard is to standardize a procedure including pre-cracking and multiple healing measurements. The self-healing evaluation includes microscopic crack closure quantification, regain of water tightness (through water absorption and water flow tests), regain of resistance against chloride ingress, and in some cases recovery of mechanical properties. The tested types of self-healing specimens include (1) concrete with mineral additions, (2) concrete with MgO, (3) concrete with crystalline admixtures, (4) high performance fibre reinforced concrete with crystalline admixtures, (5) mortar and concrete with macrocapsules containing a polymeric healing agent and (6) concrete with encapsulated bacteria. For some test series, first results are already available. For instance, the inter-laboratory test on self-healing mortar with macrocapsules showed that the target crack width of 300 µm could be obtained with great accuracy as a result of the applied active crack width control technique. This resulted in similar results for the water permeability test, yet additional specimens will be investigated to eliminate all critical points. It was much more difficult to obtain the target crack width in the concrete specimens used for capillary water absorption testing, since no active crack width control was foreseen in this case and due to the presence of large aggregates. This resulted in a high variability regarding the measured capillary water absorption.
To fill in the need for further research in the area of self-healing concrete and repair materials, a team of SARCOS participants has taken the initiative to establish an International Training Network (ITN) with the name SMARTINCS (Smart, Multi-functional, Advanced Repair Technologies In Cementitious Systems) . The project was launched on 1 December 2019.
SMARTINCS will implement new life-cycle thinking and durability-based approaches to the concept and design of concrete structures, with self-healing concrete, repair mortars and grouts as key enabling technologies. This will create a breakthrough in the current practice of the construction industry, which is characterized by huge economic costs related to inspection, maintenance, repair and eventually demolition activities and additional indirect costs caused by traffic congestions during maintenance and environmental effects.
SMARTINCS will train a new generation of creative and entrepreneurial early-stage researchers in prevention of deterioration of (i) new concrete infrastructure by innovative, multifunctional self-healing strategies and (ii) existing concrete infrastructure by advanced repair technologies. The project brings together the complementary expertise of research institutes pioneering in smart cementitious materials, strengthened by leading companies along the SMARTINCS value chain, as well as certification and pre-standardization agencies. They will intensively train 15 early stage researchers to respond to the clear demand to implement new life-cycle thinking and durability-based approaches to the concept and design of concrete structures, minimizing both the use of resources and production of waste in line with Europe’s Circular Economy strategy. The new generation of researchers will be immediately employable to support the introduction of the novel technologies allowing the expected spectacular growth of the self-healing materials market to take place. By combined experimental research and enhanced coupled multiscale numerical models for prediction of the self-healing behaviour, SMARTINCS strives to move beyond the state-of-the-art.
The scientific objectives are attained by joint PhD research and envisage:
(i) To develop and model innovative self-healing strategies for bulk and local application, including optimization of mix designs and development of multi-functional self-healing agents with attention to cost, applicability and environmental impact.
(ii) To scientifically substantiate and model the durability of self-healed concrete and repaired systems for an accurate service life prediction and to integrate self-healing into innovative service-life based structural design approaches to foster the market penetration through an innovative life-cycle thinking.
(iii) To quantify and prove the eco-efficiency of newly developed smart concrete / mortars by life cycle assessment modelling.
The author(s) would like to acknowledge networking support by the COST Action CA15202 “SARCOS”.
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