Use of encapsulated healing agents to limit water uptake through crackis in mortar

Crack formation is a common issue that leads to durability problems in concrete structures. Fluids containing aggressive substances can rapidly penetrate in the concrete matrix through the cracks and cause steel corrosion or deterioration of the concrete. In order to avoid major damage to the concrete structures, repair of cracks is in most cases necessary. Creating a self-healing cementitious material is a possible solution to avoid major repair works of concrete structures. In this research, autonomous healing of cracks by encapsulated healing agents was investigated to reduce water ingress through cracks in mortar. The two polymeric healing agents that were used in this study were able to reduce the water ingress through cracks. The low viscosity polyurethane created a complete and consistent crack healing, reducing the water absorption to values even lower than uncracked cementitious material. For the high viscosity polyurethane the results showed more scatter due to uncomplete crack healing for some specimens. The reduction of water ingress due to the incorporation of the self-healing mechanism has a positive effect on the durability of cementitious materials and hence can prolong the service life of concrete structures

Neutron radiography based visualization and profiling of water uptake in (un)cracked and autonomously healed cementitious materials

Given their low tensile strength, cement-based materials are very susceptible to cracking. These cracks serve as preferential pathways for corrosion inducing substances. For large concrete infrastructure works, currently available time-consuming manual repair techniques are not always an option. Often, one simply cannot reach the damaged areas and when making those areas accessible anyway (e.g., by redirecting traffic), the economic impacts involved would be enormous. Under those circumstances, it might be useful to have concrete with an embedded autonomous healing mechanism. In this paper, the effectiveness of incorporating encapsulated high and low viscosity polyurethane-based healing agents to ensure (multiple) crack healing has been investigated by means of capillary absorption tests on mortar while monitoring the time-dependent water ingress with neutron radiography. Overall visual interpretation and water front/sample cross-section area ratios as well as water profiles representing the area around the crack and their integrals do not show a preference for the high or low viscosity healing agent. Another observation is that in presence of two cracks, only one is properly healed, especially when using the latter healing agent. Exposure to water immediately after release of the healing agent stimulates the foaming reaction of the polyurethane and ensures a better crack closure

Sustainability effects of including concrete cracking and healing in service life prediction for marine environments

With today’s focus on sustainable design, it is necessary to adequately predict and prolong service life of concrete in marine environments. By introducing self-healing properties, service life extension can be achieved. However, in prediction models, the required concrete mix specific input is usually not available. Moreover, little attention goes to the unavoidable presence of cracks. Finally, autonomous crack healing has almost never been taken into account. In this paper, the relevant model input was estimated from experimental chloride profiles. It enabled an adequate prediction of the chloride-induced steel depassivation period for cracked and uncracked 15% fly ash concrete (8–104 years, respectively). Comparison with self-healing by means of encapsulated polyurethane indicated a 48–76% self-healing efficiency. It could extend the corrosion initiation period to 36–68 years. Being much less subject to time-dependent repair, PU based self-healing concrete has a 77–88% lower environmental impact than traditional (cracked) concrete

Resistance to chloride penetration of self-healing concrete with encapsulated polyuretyhane

Reinforcement corrosion induced by diffusion of chlorides is one of the most important damage mechanisms that leads to the deterioration of reinforced concrete structures. Cracking of reinforced concrete structures during their service life is almost inevitable. Cracks form preferential pathways for the ingress of chlorides and will accelerate the onset of corrosion and its propagation. In this paper, autonomous self-healing of cracks by encapsulated polyurethane is investigated as a possible method to heal cracks and reduce chloride ingress through cracks without human intervention. Cracks in concrete specimens were created in two ways: by means of thin metal plates to create standardized artificial cracks and by means of splitting tests to create realistic cracks. A crack width of 0.3 mm was chosen since most design codes limit the crack width to that value. The resistance to chloride penetration of autonomously healed concrete was evaluated by the diffusion test as described in NT Build 443. Uncracked, cracked and healed specimens were subjected to a 165 g/l NaCl solution for 7 weeks. After that period chloride profiles in the crack region and in an area further away from the crack were obtained by potentiometric titrations. From the resulting chloride profiles it was concluded that the polyurethane was very well able to seal both artificial and realistic cracks and reduce the chloride content in the crack zone significantly. At depths below the surface larger than 14 mm, healing was able to reduce the total chloride content in the crack zone by more than 70%

Effect of the service life assessment approach of the environmental benefit of using self-healing concrete in marine environments

To reduce concrete’s susceptibility to cracking, full autonomous healing mechanisms are being studied today. A promising technique consists of incorporating encapsulated polyurethane-based healing agents. Upon crack occurrence, small capsules in close vicinity of the damaged area break. A polyurethane (PU) pre-polymer flows into the crack and after reaction with the surrounding moisture, the hardened PU prevents further accelerated ingress of aggressive substances at the original crack location. So far, promising results have mainly been obtained in proof-of-concept experiments. However, these tests do not allow for a proper estimation of the service life extension and environmental benefit that is possible when using concrete with selfhealing properties. In this paper, such calculations have been performed for marine fly ash concrete with an in-house developed encapsulated high viscosity PU precursor. Since the service life calculation outcome very much depends on the probabilistic prediction model used and the underlying experimental input, two commonly used strategies were considered: one based on natural diffusion tests with chloride profiling at various exposure times and another based on accelerated chloride migration experiments at different ages. The first approach allows for a simultaneous fitting of the chloride diffusion coëfficiënt, surface concentration and ageing exponent, while with the second one the chloride resistance after many years (time infinity) can be taken into account. Both approaches indicate a substantial prolongation of the service life when cracks, 300 pm in width, are healed autonomously, even if only partially. Nevertheless, the outcome of the two prediction methods is not the same (service life: 61-97 years versus 12- 69 years, respectively). Depending on the applied approach, the required number of rehabilitation actions in time for the cracked (reference) concrete varies. As a consequence, service life related life cycle assessment performed in the SimaPro software clearly proves that the environmental benefits of the self-healing concrete will also differ for the ten baseline impact categories of the CML-IA impact method (56-74% versus 59-88%, respectively)

Self-healing of concrete cracks by the release of embedded water repellent agents and corrosion inhibitors to reduce the risk for reinforcement corrosion

From the worldwide steel production, approximately 50 per cent is required to replace corroded steel [1]. In the case of reinforced concrete structures, corrosion of the reinforcement steel causes crack formation and spalling which leads to serviceability problems. Especially when small cracks are already present in the cementitious matrix in combination with aggressive ions present within the environment, a high risk for corrosion exists. Therefore, regular inspection, maintenance and crack repair are insurmountable for concrete structures. However, costs related to repair works mount up as not only the direct costs of the repair but also the indirect costs resulting from traffic jams and possible loss in productivity need to be taken into account. Self-repair of concrete cracks will have a high economic benefit as the indirect costs as well as a part of the direct costs can be avoided. In addition, it is assumed that self-repair will lead to more durable concrete structures as the risk for reinforcement corrosion may be decreased. The possibility to implement self-healing properties in concrete has been investigated for several years now. One of the studied self-healing approaches relies on the use of encapsulated healing agents which are embedded in the matrix. When cracks appear, the capsules break and the healing agent is released in the crack, causing crack repair. In previous research [2, 3] it was shown that by using this approach, part of the mechanical properties and the water tightness of cracks was restored. In this study we investigate whether by encapsulation and embedment of a water repellent agent (WRA) and/or a corrosion inhibitor (CI), we can reduce the risk for reinforcement corrosion. A selection of WRA and/or CI were encapsulated and embedded inside reinforced concrete beams which were cracked to trigger the self-healing mechanism. By electrochemical measurements it was shown that the risk for reinforcement corrosion was reduced in comparison to untreated cracks when the cracked beams, containing encapsulated WRA and/or CI, were exposed to a chloride solution

Internal curing of cement pastes by superabsorbent polymers studied by means of neutron radiography

Autogenous shrinkage is a problem in cementitious materials with a low water-to-binder ratio. When the internal relative humidity decreases due to the ongoing hydration reaction and selfdesiccation, autogenous shrinkage takes place if no external or internal water source is present. This may lead to cracking and eventually cause durability problems in constructions. Ideally, the internal relative humidity should be maintained during hydration of the cement paste. Superabsorbent polymers (SAPs) may be used to mitigate autogenous shrinkage. When self-desiccation occurs, these polymers will release their absorbed additional mixing water due to increasing capillary forces to stimulate internal curing. This release of water towards the cementitious matrix and the effect on the cementitious matrix itself can be studied by means of neutron radiography. In this study, thin samples of cement paste were casted between glass plates and the evolution of the internal water amount was studied as a function of time. In specimens without SAPs and a water-to-binder ratio of 0.30, shrinkage was seen. Furthermore, autogenous shrinkage was reduced in cement pastes when using SAPs and an additional entrained water-to-binder ratio of 0.054. The release of water from smaller SAPs (100 μm dry size) seemed to be more promising compared to bigger SAPs (500 μm) with the same absorption properties. The technique of neutron radiography supports the findings of shrinkage tests where SAPs were already proven to be useful. This opens additional insights towards the application of SAPs in the construction area

Perpendicular-to-crack chloride ingress in cracked and autonomously healed concrete

Cracks in reinforced concrete structures exposed to a marine environment or de-icing salts can cause major durability issues due do accelerated ingress of chloride ions. In this study, the influence of autonomous crack healing by means of encapsulated polyurethane on the chloride ingress perpendicular to cracks was evaluated. This was done quantitatively by determining perpendicular-to-crack chloride profiles by means of profile grinding followed by potentiometric titration and qualitatively through visualization of the chloride penetration front by means of the AgNO3 spray method. The resulting chloride profiles showed that the healing mechanism was able to reduce the chloride concentrations in the direct vicinity of the crack to a large extent and to reduce the perpendicular-to-crack chloride penetration, especially further away from the exposed surface. Visualization of the chloride penetration front showed some variation in crack healing. For some healed samples almost no additional chloride ingress was found compared to uncracked samples, others showed a slightly enhanced ingress at the crack location but less perpendicular-to-crack chloride penetration compared to untreated cracked samples. Generally, the reduced amount of chlorides present in the concrete matrix due to crack healing will enhance the durability and service life of concrete structures