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Although certain crack widths are allowed in reinforced concrete structures, without having immediate effects on the structural stability, they may impair the durability and service life of the structure in the long term. Cracks wider than 10 μm will result, for instance, in a faster penetration of chlorides into the crack and from there onwards into the concrete matrix. Fortunately, the autogenous healing ability of concrete may close cracks of up to 100 μm completely. The further hydration of binder particles, will be supplemented by the deposition of calcium carbonate crystals in case of wet/dry cycles. In case of marine infrastructures in tidal zones, the presence of magnesium sulfates may enhance the crack sealing by means of brucite precipitation. These processes will result in reduced chloride penetration rates. If the cracks are larger than 100 μm or the conditions are not favourable for autogenous healing, autonomous healing mechanisms can be incorporated. In this case, healing is obtained through encapsulated polymeric healing agents, superabsorbent polymers, microbial agents, expansive additives, etc. With encapsulated polyurethane based healing agents, a reduction of the chloride concentration by 75% or more was obtained in a zone with a 300 μm wide crack after chloride diffusion tests, relative to the case in which cracks were not healed. As a result, the service life of reinforced concrete elements in marine environments could be increased with a factor of about 10. Neutron radiography images obtained during a capillary sorption test indicated that release of encapsulated polyurethane in wet conditions was favourable for the polyurethane reaction. As an alternative to the autonomous healing with encapsulated polyurethane, also the incorporation of encapsulated water repellent agents and corrosion inhibitors, has proven to effectively delay reinforcement corrosion during electrochemical measurement campaigns. Accelerated corrosion tests on cracked, manually treated mortar samples, allowed to rapidly screen different agents for their efficiency.

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.

Cracks serve as preferential pathways for aggressive substances (H2O, O2, Cl-, CO2, etc.) that reduce the durability and service life of steel reinforced cementitious materials. The implementation of an autonomous crack healing mechanism counts as a solution to this problem. A very promising way to achieve that goal consists of incorporating capsules filled with polyurethane (PU) precursor in the mortar/concrete. Upon crack occurrence the capsules break, the PU precursor flows into the crack, reacts with surrounding moisture and seals the crack again once hardened. Now, depending on the specific exposure conditions of the structure (very humid versus dry), the reaction kinetics of the PU can vary. In this research it has been investigated by means of neutron radiography whether the crack healing efficiency is the same regardless of the curing period, i.e., when having direct exposure to water upon release of the healing agent (after 15 min to 3 h) versus when the PU is first given enough time (at least 24 hours) to harden completely. Preferably, the healing agent works under all circumstances. However, it was found that the first scenario resulted in the highest resistance to capillary water absorption for singular cracks, 300 μm in width. Neutron radiography images and water profiles extracted from those images during a two-hour water uptake experiment clearly show that this was the case for both a high viscosity PU precursor that was developed in-house and a commercially available low viscosity PU-based healing agent.

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.

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).

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%.

The deicing salt scaling resistance has been investigated for two types of concrete, i.e., air entrained high-volume fly ash (HVFA) concrete with a 50% cement replacement and non-air entrained concrete containing superabsorbent polymers (SAPs). A full characterization of their air void systems from the moment of casting until the freeze/thaw test was also done. Due to the presence of the highly AEA adsorptive fly ash an increased AEA dosage (7.0 ml/kg binder) was needed to achieve an adequate air void system in terms of air content and spacing factor to keep salt scaling within acceptable limits. For the novel non-air entrained concrete type with SAPs, which are able to absorb up to 500 times their weight in fluids, the salt scaling resistance is surprisingly high. The microstructural analysis revealed the formation of macro-pores due to these SAPs, creating an air void system as can be found in air-entrained concrete. Another advantage is that the strength of concrete with SAPs is much higher than for a conventional air-entrained concrete. This substantiates the further use of these SAPs as admixture in precast concrete road elements.

To achieve higher sustainability of steel reinforced concrete structures, their service life should be extended. When subject to chloride induced steel corrosion, time dependent repair works are most probably inevitable. Evidently, this results in extra concrete manufacturing and thus more environmental impact. Cracks offering direct pathways for the corrosion inducing substances play a very detrimental role in this. This paper presents the potential of using self-healing concrete to cope with this problem. By incorporating a polyurethane (PU)-based healing agent that is adequately released upon crack occurrence, chloride ingress is hindered substantially and onset of active corrosion is postponed. The required number of repair actions within 100 years could then drop to zero. Nevertheless, the implementation of a self-healing mechanism comes along with a higher initial cost and additional environmental impacts. Therefore, the necessary cost and life cycle assessment calculations have been performed as well. It was found that the cost of the PU-based healing agent is very reasonable while the extra costs of the capsules are for the moment still unacceptable. Environmental burdens associated with the PU precursor filled capsules are negligible (0.1-4.8%) in comparison with the impacts related to regular concrete repair to meet the design service life of 100 years.

When studying concrete's susceptibility to carbonation-/chloride-induced corrosion, the focus is usually on the corrosion initiation period. Service life prediction performed as such only quantifies the time to depassivation of the reinforcing steel and does not involve any damage. As a consequence, the actual service life is underestimated. In this paper, it has been evaluated whether the time between steel depassivation and unacceptable corrosion-induced cracking is significant or not. This information is important because it affects the expected number of rehabilitation actions with time and therefore also the sustainability of a concrete composition. Service life calculations done in compliance with the well-known DuraCrete models imply that the propagation period cannot be neglected in a carbonation exposed environment (exposure class XC3), especially for concrete with supplementary cementitious materials such as fly ash and silica fume (propagation periode > 100 years). On the other hand, the propagation period was observed to be very short (max. 8.7 years) in a chloride exposed environment (exposure class XS2). Logically, the differences in global warming potential for not considering the propagation period were mainly important in presence of carbonation. In exposure class XS2, the sole consideration of the initiation period can be acceptable.

Until now, it remains unclear how "green" concrete compositions with high volumes of supplementary cementitious materials really are, especially when subject to carbonation-induced steel corrosion. This paper results from accelerated carbonation tests for high-volume fly ash (HVFA) and fly ash + silica fume (FA+SF) concrete. They served as input for a probabilistic service life prediction on fib Bulletin 34, and a subsequent life cycle assessment. the inverse effective carbonation resistance of the two concrete types was compared with the one of traditional concrete that is normally used in outdoor, sheltered environments. An attempt was also made to determine the model input parameter that accounts for concrete's curing behaviour using literature data. Other input parameters related to meteorological conditions were accounted for with weather station information. In the end, it was found that the estimated time to carbonation-induced steel depassivation for the less carbonation resistant HVFA and FA+SF concrete still exceeds a predefined life span of 100 years by far. As a consequence, global warming potentials (GWPs) calculated for the required concrete volume per unit of strength and service life indicate that an important reduction in greenhouse gas emissions is possible for both concrete types (GWP -29 to -44 %).

Nowadays, a lot of research is being conducted on high-volume fly ash (HVFA) concrete. However, a precise quantification of the environmental benefit is almost never provided. To do this correctly, we adopted a life cycle (LCA) approach. By considering a simple structure and an environment for the material, differences between traditional and HVFA concrete regarding durability and strength were taken into account. This paper presents the LCA results for a column supported isostatic beam made of reinforced HVFA concrete located in a dry environment exposed to carbonation induced corrosion. With a binder content of 425 kg/m3 and a water-to-binder ratio of 0.375, the estimated carbonation depth after 50 years for a 50 % fly ash mixture does not exceed the nominal concrete cover of 20 mm. As a consequence, no additional concrete manufacturing for structure repair needs to be included in the study. Moreover, structure dimensions can be reduced significantly due to a higher strength compared to the reference concrete used in the same environment. In total, about 32 % of cement can be saved this way. The reduction in environmental impact equals 25.8 %, while this is only 11.4 % if the higher material strength is not considered.