Self-healing and repair of concrete structures: COST action CA15202 SARCOS and lessons learnt from FP7 project HEALCON

The appearance of small cracks in concrete can result in a loss of performance and functionality in the long term. Smart self-healing materials are developed as preventive solutions to avoid the need for extensive repair works. Although several approaches for promoting the self-healing of concrete structures have been developed during recent decades, they will only be viable when comparative characterization techniques for assessing their performance and efficiency are properly established. Furthermore, modelling the healing mechanisms taking place for the different designs and predicting the associated service life increase will help consolidate the implementation of these preventive repair approaches. The SARCOS Action is focused on the concept of preventive repair, with the objective of sealing small cracks at the earliest stage of damage, both for new and existing structures, and on looking for standardizing methodologies to evaluate the mechanical and durable performance of the treated structures, with continuous feedback from the modelling of self-healing mechanisms. This is reflected in the scopes of the three SARCOS Working Groups. The presentation aims to give a general vision on the advances attained within the SARCOS Action, including the revision of the state-of-the art of the different aspects addressed within the Action: self-healing approaches, techniques for characterizing self-healing performance and self-healing modelling. Furthermore, an overview of the results of the recently finished EU-FP7 project HEALCON is provided. Its aim was to design smart concrete with self-healing properties to create durable and sustainable concrete structures. While superabsorbent polymers and bacterial healing agents were used for healing of early age cracks in structures which require liquid tightness, elastic polymers were proposed for healing of cracks under dynamic load. The efficiency with regard to mechanical behaviour, liquid-tightness and durability was quantified by (non)-destructive monitoring techniques in small and large scale tests. Computer models were developed to simulate fracturing and self-healing

Repeated autogenous healing in strain-hardening cementitious composites by using superabsorbent polymers

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Effect of sample age on the self-healing properties of cementitious mateials with superabsorbent polymers

To obtain sustainable construction materials, occurring cracks in concrete should be repaired. However, as this often manual repair is time-consuming and expensive, self-healing may provide a solution. Autogenous healing is an already-present feature in cementitious materials, but it is an inferior mechanism as it can only heal cracks up to 30 μm in the presence of water. Therefore, a cementitious material with synthetic microfibres and superabsorbent polymers (SAPs) is proposed. Synthetic microfibres cause multiple crack formation with small healable cracks. Furthermore, SAPs are able to extract moisture from the environment and to provide it to the cementitious matrix for autogenous healing. But, if the building blocks are exhausted in time due to ongoing hydration, healing may be less efficient. In this study, the ability of (promoted) autogenous healing in time (7 days, 28 days, 3 months and 1 year) is investigated by comparing the mechanical properties after performing four-point-bending tests in different mixtures. The specimens were first loaded to 1% strain, stored in wet/dry cycles for 28 days and were subsequently reloaded. The results show that, with increasing age, the crack width decreases and a higher water-to-binder ratio tends to increase the mean and maximum crack width. All specimens are able to heal and to regain some of the mechanical properties after being preloaded and pre-cracked under four-point-bending. If SAPs are added, there is even healing in an environment without liquid water (relative humidity of more than 90%). At early age, the healing is governed by further hydration and calcium carbonate crystallization. At later ages (after one month), the main autogenous healing mechanism is the formation of calcium carbonate crystals. The amount of white precipitated healing products was higher in specimens with a higher water-to-cement ratio. Cracks in the specimens with fly ash were better closed in comparison with the samples with pure cement and blast-furnace-slag-blended mortars. Mixtures with SAPs showed more healing. The cementitious composite with microfibres and SAPs thus shows good self-healing in time

Service life estimation of cracked and healed concrete in marine environment

In the aggressive seawater environment, the durability of concrete is strongly influenced by the presence of chlorides and sulfates. Marine structures mostly have an important social function with a high economic impact, which makes durability a key issue. In addition, early-age cracks are a common problem, specifically for massive structural components. Repair of cracks is expensive and often impossible due to inaccessibility. Self-healing concrete is a promising solution to make marine structures more durable. In this study, capsules containing Polyurethane (PU) prepolymers were embedded in the concrete to release their contents when cracks appear. In cracked mortar, the chloride diffusion coefficients in the zone immediately around the crack significantly increased compared to uncracked mortar. The crack width dependency could be introduced into the service life model using a crack effect function. For crack widths in the range of 100 mu m to 300 mu m a service life decrease of around 80% was calculated. Autonomous crack healing had a beneficial influence on the resistance against chloride diffusion. However, for about one third of the cracks the healing mechanism failed, probably due to shifting of the tubes, tubes not rupturing properly, too high capillary forces in the tubes, etc. Nevertheless, on average, the service life of autonomously healed structures by means of encapsulated polyurethane increased with around 100% compared to cracked, unhealed structures. Moreover, in the most beneficial situation of proper healing, a service life increase of 150-550% was obtained, reaching values similar as for sound structures

Self-healing and repair of concrete structures: COST action CA15202 SARCOS and lessons learnt from FP7 project HEALCON

Contiene únicamente el abstract de la ponencia[EN] The appearance of small cracks in concrete can result in a loss of performance and functionality in the long term. Smart self-healing materials are developed as preventive solutions to avoid the need for extensive repair works. Although several approaches for promoting the self-healing of concrete structures have been developed during recent decades, they will only be viable when comparative characterization techniques for assessing their performance and efficiency are properly established. Furthermore, modelling the healing mechanisms taking place for the different designs and predicting the associated service life increase will help consolidate the implementation of these preventive repair approaches. The SARCOS Action is focused on the concept of preventive repair, with the objective of sealing small cracks at the earliest stage of damage, both for new and existing structures, and on looking for standardizing methodologies to evaluate the mechanical and durable performance of the treated structures, with continuous feedback from the modelling of selfhealing mechanisms. This is reflected in the scopes of the three SARCOS Working Groups. The presentation aims to give a general vision on the advances attained within the SARCOS Action, including the revision of the state-of-the art of the different aspects addressed within the Action: self-healing approaches, techniques for characterizing selfhealing performance and self-healing modelling. Furthermore, an overview of the results of the recently finished EU-FP7 project HEALCON is provided. Its aim was to design smart concrete with self-healing properties to create durable and sustainable concrete structures. While superabsorbent polymers and bacterial healing agents were used for healing of early age cracks in structures which require liquid tightness, elastic polymers were proposed for healing of cracks under dynamic load. The efficiency with regard to mechanical behaviour, liquid-tightness and durability was quantified by (non)-destructive monitoring techniques in small and large scale tests. Computer models were developed to simulate fracturing and self-healingDe Belie, N. (2018). Self-healing and repair of concrete structures: COST action CA15202 SARCOS and lessons learnt from FP7 project HEALCON. En HAC 2018. V Congreso Iberoamericano de hormigón autocompactable y hormigones especiales. Editorial Universitat Politècnica de València. 15-15. https://doi.org/10.4995/HAC2018.2018.8262OCS151

Application of bacteria in concrete : a critical evaluation of the current status

Microbially induced carbonate precipitation has been tested over more than a decade as a technique to enhance concrete properties. Mainly bacteria following the pathways of urea decomposition, oxidation of organic acids, or nitrate reduction have been studied for this purpose. For bacteria mixed into fresh concrete, it is difficult to prove that they actively contribute to calcium carbonate precipitation and the effects on concrete strength are variable. Application of bacteria for surface consolidation has been shown to reduce water absorption and increase durability. Microbial self-healing of cracks in concrete shows promising results at the laboratory scale. Especially the use of self-protected mixed cultures opens perspectives for practical application. However, their self-healing efficiency needs to be further proven in larger concrete elements, and under non-ideal conditions. The use of denitrifying cultures for concurrent self-healing and production of corrosion inhibiting nitrites is a promising new strategy