Effects of Autogenous and Stimulated Self-Healing on Durability and Mechanical Performance of UHPFRC: Validation of Tailored Test Method through Multi-Performance Healing-Induced Recovery Indices

de Souza Oliveira A., da Fonseca Martins Gomes O., Ferrara L., de Moraes Rego Fairbairn E., Toledo Filho R.D.

This work presents an overview of the most relevant studies developed to understand the enhancement promoted by the double effect of the crystalline admixtures (CAs), both as permeability-reducers and as self-healing stimulators in cement-based materials. Thus, an in-depth investigation was carried out of the main mechanisms of CAs disclosed in the literature in order to associate the relationship between healing products and performance improvement in cementitious materials. Further, an examination of the impact of different factors in cementitious systems with CAs was reported, as well as the synergetic effects of CAs with other constituents. Finally, conclusions were drawn highlighting research needs and addressing future works in order to provide a substantial overview of the latest information in the literature for those who are working or intend to work with this type of admixture. • This paper presents a literature meta-analysis about the effect of Crystalline Admixtures (CAs). • The primary mechanisms along with the healing products provided by CAs are addressed. • The effects of CAs on the properties of cement-based materials as permeability-reducing admixtures are discussed. • The enhancement promoted by CAs as healing stimulators is analyzed focusing on the mechanical and durability recoveries. • The influence of different aspects, such as the environmental exposure, repeated preloading, crack width, and synergic effect are also analyzed.

Al-Obaidi S., Bamonte P., Animato F., Lo Monte F., Mazzantini I., Luchini M., Scalari S., Ferrara L.

The structure presented in this paper is intended to be used as a prototype reservoir for collecting water coming from the cooling tower of a geothermal plant, and is primarily designed to compare the performance of different materials (traditional reinforced concrete and Ultra-High-Performance Fiber-Reinforced Concrete (UHPFRC)) as well to assess the performance of different structural solutions (wall with constant thickness versus wall provided with stiffening buttresses). In the absence of specific code provisions, given the novelty of the UHPFRC used, the main properties used for the design were determined through a dedicated experimental campaign (tensile/flexural properties and shrinkage). The main focus of the design was on the Serviceability Limit States, more specifically the requirements regarding water tightness. Given the rather simple structural layout, especially in the compartments where no stiffening buttresses are present, linear elastic analysis was used to determine the internal actions. The nonlinear behavior ensuing from the peculiar tensile constitutive response of the material was taken into account locally, in order to determine the stress level, the depth of the compression zone and the crack width. The performance was finally compared with the reference compartment (made with ordinary reinforced concrete), through on-site observations and measurements.

Gupta S., Al-Obaidi S., Ferrara L.

Concrete and cement-based materials inherently possess an autogenous self-healing capacity. Despite the huge amount of literature on the topic, self-healing concepts still fail to consistently enter design strategies able to effectively quantify their benefits on structural performance. This study aims to develop quantitative relationships through statistical models and artificial neural network (ANN) by establishing a correlation between the mix proportions, exposure type and time, and width of the initial crack against suitably defined self-healing indices (SHI), quantifying the recovery of material performance. Furthermore, it is intended to pave the way towards consistent incorporation of self-healing concepts into durability-based design approaches for reinforced concrete structures, aimed at quantifying, with reliable confidence, the benefits in terms of slower degradation of the structural performance and extension of the service lifespan. It has been observed that the exposure type, crack width and presence of healing stimulators such as crystalline admixtures has the most significant effect on enhancing SHI and hence self-healing efficiency. However, other parameters, such as the amount of fibers and Supplementary Cementitious Materials have less impact on the autogenous self-healing. The study proposes, through suitably built design charts and ANN analysis, a straightforward input–output model to quickly predict and evaluate, and hence “design”, the self-healing efficiency of cement-based materials.

Lo Monte F., Ferrara L.

• The adoption of strain-hardening cement composites significantly promotes self-healing. • Multi-test approach for self-healing assessment appeared to be robust in durability monitoring. • Self-healing is very effective already after 1 month of curing for the UHDC studied. • The mixes showed sizable self-healing with even almost full crack-sealing after 6 months. • Re-cracking has a detrimental effect in self-healing for all the considered parameters. Within the framework of the Research Project ReSHEALience, new advanced Ultra High-Performance Fibre-Reinforced Cement Composites with enhanced durability, hereafter denoted as Ultra High Durability Concretes, are under investigation to characterize their tensile behaviour and durability performance in aggressive conditions, devoting particular attention to the phenomenon of self-healing. Three different mixes are under scrutiny, based on the combination of cement (CEM I or CEM III), slag, small aggregates (sand with a maximum size of 2 mm), and steel or metallic-alloy amorphous fibre. Self-healing capability has been investigated in aggressive environment (namely, under immersion in geothermal water) via 3 different test setups: (1) water permeability test on pre-cracked concrete disks, (2) 4-Point Bending Test – 4PBT on 100 × 100 × 500mm 3 prismatic beam specimens and (3) on 25 × 100 × 500mm 3 thin beams. In the case of beams, self-healing has been assessed via visual inspection of cracks trough digital microscope and via mechanical re-loading, so to investigate both crack-sealing capability and mechanical recovery. The results of this assessment aim at providing the starting point for a data base finalized at defining a design approach explicitly taking into account self-healing in the evaluation of structural durability. In particular, it has been observed as the adoption of strain-hardening cement composites significantly promotes self-healing phenomenon, thanks to smeared cracking in the tensile region and to consequent low values of crack opening. Self-healing proved to be very effective already after 1 month of curing.

Cuenca E., D'Ambrosio L., Lizunov D., Tretjakov A., Volobujeva O., Ferrara L.

The effects of alumina nano-fibres are investigated in this paper on the mechanical performance of Ultra High Performance Fibre Reinforced Cementitious Concrete and their efficacy in enhancing the durability of the cementitious composite when exposed to extremely aggressive conditions, with main reference to the stimulated autogenous crack sealing and self-healing capacity. A tailored characterization of the flexural and tensile behaviour of the composite has been first of all performed, also with the purpose of validating an experimental and analytical approach for the identification of the tensile stress vs. strain/crack opening constitutive relationship, which makes use of a purposely conceived indirect tensile test methodology, called Double Edge Wedge Splitting test. Secondly the crack sealing and self-healing capacity have been investigated, considering the recovery of both mechanical flexural performance and durability properties (water permeability) and cross analysing the results for a thorough validation. Microstructural investigations have complemented the aforementioned experimental programme to confirm the efficacy of alumina nano-fibres in enhancing the durability performance of the investigated composites. Superior performance of the mix with alumina nano-fibres with respect to parent companion ones has been highlighted and explained through both a nano-scale reinforcing effects which helps in controlling the cracking process since its very onset as well as through their hydrophilic nature which is likely to foster cement and binder hydration reactions, which can usefully stimulate crack sealing and performance healing recovery at both the macroscopic and mesoscopic fibre-matrix interface) level.

Xue C., Li W., Luo Z., Wang K., Castel A.

In this study, the effect of chloride environment containing various concentrations of chloride ion (Cl − ) on self-healing performance of pre-cracked cementitious composite containing crystalline admixture (CA) and MgO expansive agent (MEA) was investigated under wet-dry cycles in chloride solutions. The results revealed that the Cl − changed the mineralogy of self-healing products, and consequently, affected the crack closure ratio and mechanical strength recovery. When self-healing occurred in distilled water, a large amount of ettringite (AFt) were detected, whereas in Cl − solution, monosulfate (AFm) was consumed by Cl − to form Friedel's salt (Fs), and then the Fs was decomposed to Al(OH) 3 (AH 3 ) due to carbonation. During the multiphase conversion process, hydroxide (OH − ) was released into crack solution, therefore the dissolved carbon dioxide (CO 2 ) concentration was increased. The carbonation of the crystals formed in cracks was accelerated with the volume expansion, which achieved rapid crack sealing but contributed little to the mechanical performance recovery. • Crack closure under wet/dry cycles in chloride solution is higher than the one in distilled water. • Element analysis shows that calcium is vital for crack healing either in water or chloride solution. • AFm is consumed by chloride ion to form Fs, and Fs were decomposed to AH 3 by carbonation. • Soluble reactive silica reacts with calcium hydrate to form C-S-H gel to heal concrete cracks. • Expansion of magnesium hydroxide facilitates crack self-healing by interconnected network.

Cuenca E., Rigamonti S., Gastaldo Brac E., Ferrara L.

AbstractThe requirements on service life of reinforced concrete structures, as prescribed by design codes, may be difficult to be fulfilled in highly aggressive environments such as marine ones, in...

Lo Monte F., Ferrara L.

Within the framework of the European Programme Horizon 2020, the Research Project ReSHEALience is currently running with the objective of developing a new approach for the design of structures exposed to extremely aggressive environments, based on Durability Assessment based Design and Life Cycle Analysis. To this aim, new advanced Ultra-High Performance Fibre Reinforced Cementitious Composites with improved durability, called Ultra-High Durability Concretes, are under investigation to characterize their tensile response in both ordinary and very aggressive conditions. In this context, the first step is to develop an effective approach for identifying the main parameters describing the overall behaviour in tension. In the present study, indirect tension tests have been performed via two techniques, based on Double Edge Wedge Splitting and 4-Point Bending Tests. Starting from the test results, a combined experimental-numerical identification procedure has been implemented in order to evaluate the effective material behaviour in direct tension in terms of stress–strain law. In the paper, the mechanical characterization for the reference mix is reported so to describe the identification procedure adopted.

Al-Obaidi S., Bamonte P., Luchini M., Mazzantini I., Ferrara L.

This paper provides the formulation and description of the framework and methodology for a Durability Assessment-based Design approach for structures made of the Ultra-High-Durability Concrete materials conceived, produced and investigated in the project ReSHEALience (Rethinking coastal defence and Green-energy Service infrastructures through enHancEd-durAbiLity high-performance cement-based materials) funded by the European Commission within the Horizon 2020 Research and Innovation programme (Call NMBP 2016–2017 topic 06-2017 GA 780624). The project consortium, coordinated by Politecnico di Milano, gathers 13 partners from 7 countries, including 6 academic institutions and 7 industrial partners, covering the whole value chain of the concrete construction industry. The innovative design concept informing the whole approach herein presented has been formulated shifting from a set of prescriptions, mainly referring to material composition and also including, in case, an allowable level of damage defined and quantified in order not to compromise the intended level of “passive” protection of sensitive material and structural parts (deemed-to-satisfy approach; avoidance-of-deterioration approach), to the prediction of the evolution of the serviceability and ultimate limit state performance indicators, as relevant to the application, as a function of scenario-based aging and degradation mechanisms. The new material and design concepts developed in the project are being validated through design, construction and long-term monitoring in six full-scale proofs-of concept, selected as representative of cutting edge economy sectors, such as green energy, Blue Growth and conservation of R/C heritage. As a case study example, in this paper, the approach is applied to a basin for collecting water from a geothermal power plant which has been built using tailored Ultra-High-Durability Concrete (UHDC) mixtures and implementing an innovative precast slab-and-buttress structural concept in order to significantly reduce the thickness of the basin walls. The geothermal water contains a high amount of sulphates and chlorides, hence acting both as static load and chemical aggressive. The main focus of the analysis, and the main novelty of the proposed approach is the prediction of the long-term performance of UHDC structures, combining classical structural design methodologies, including, e.g., cross-section and yield line design approaches, with material degradation laws calibrated through tailored tests. This will allow us to anticipate the evolution of the structural performance, as a function of exposure time to the aggressive environment, which will be validated against continuous monitoring, and pave the way towards a holistic design approach. This moves from the material to the structural durability level, anticipating the evolution of the structural performance and quantifying the remarkable resulting increase in the service life of structures made of UHDC, as compared to companion analogous ones made with ordinary reinforced concrete solutions.

Qiu J., He S., Yang E.

This study examines autogenous healing of interface between micro polymeric fiber and hydraulic cement matrix. Experiments were carried out to evaluate the healing-induced recovery of fiber/matrix interface bond strength and to characterize the morphology and chemical compositions of the healing products. Results show that interface frictional bond strength is significantly recovered after eight cycles of water/air conditioning. The healing products are mainly CaCO3 which has a crystal-like morphology with a particle size of 0.5–2 μm. Fiber/matrix interface healing results in significant recovery of fiber-bridging strength of pre-cracked FRCC. Furthermore, inclusion of fly ash in the mix composition remarkably enhances the autogenous healing of fiber/matrix interface as the healing degree of interface frictional bond strength increases from 63% to 94% and that of the fiber-bridging strength improves from 82% to 127%.

Zhang Y., Zuo L., Yang J., Cai X., Zhao Y., Zeng X.

Li G., Huang X., Lin J., Jiang X., Zhang X.

Activated chemicals play a key role in the waterproof performance of cementitious capillary crystalline waterproofing materials (CCCW). We proposed sodium carbonate, sodium silicate, sodium aluminate, tetrasodium EDTA and glycine as activated chemicals of CCCW. The proportion of activated chemicals of CCCW was optimized by two key parameters evaluation of the second permeability ratio and pre-pressure self-healing rate for their self-healing ability of CCCW. Results show that sodium silicate has the most significant impact on the self-healing ability of CCCW. In the self-healing progress of CCCW, the consumption of Ca(OH)2 (CH) and the production of Calcium Silicate Hydrate (CSH) gel, calcium carbonate and ettringite were observed through XRD and SEM analysis. The white crystals with an average diameter of 0.2 mm are produced on both sides of cracks, and consequently the cracks were healed by the accumulation of crystals. The self-healing mechanism of CCCW was proposed based on the self-healing experimental results.

Jiang J., Zheng X., Wu S., Liu Z., Zheng Q.

Ultra-high performance concrete (UHPC) contains a large number of unhydrated particles which are prone for self-healing in the presence of water. In this work, three-dimensional X-ray microscope was employed to investigate the self-healing degree. To aid in understanding the evolution of self-healing, a hydration model CEMHYD3D was utilized to simulate the distribution of solids and pores phase in a cracked microstructure. Based on the random walk model, the influence of crack self-healing degree on chloride ion transport process was analysed. The results show that the smaller crack width and the longer healing time reflect a lager self-healing degree. According to the rehydration products, it was found that portlandite crystallizes in the middle of the crack while C-S-H gel accumulates only on the crack surface. Moreover, the ions transport depths in cracked UHPC was simulated by finite difference method. As time increases, chloride ions not only transport vertically along the crack, but also propagate in the horizontal direction of the crack. Mixture proportions have little influence on the diffusion coefficient of chloride ion.

Van Belleghem B., Kessler S., Van den Heede P., Van Tittelboom K., De Belie N.

Cracks in reinforced concrete structures accelerate the ingress of chlorides and therefore cause a higher risk for corrosion. In this research, autonomous healing of cracks by encapsulated polyurethane was investigated as a possible method to reduce reinforcement corrosion. Reinforced concrete beams were exposed weekly to a chloride solution and electrochemical parameters were measured to determine the influence of the self-healing mechanism on the corrosion process. The rebars were visually examined afterwards. For the cracked beams an active state of corrosion was detected within an exposure period of 10 weeks and clear pitting corrosion was observed on the rebars. Autonomous crack healing with low viscosity polyurethane could significantly reduce the corrosion in the propagation stage. For these specimens no visual damage to the rebars was detected. In conclusion, the application of self-healing concrete with a low viscosity polyurethane is able to enhance the durability of reinforced concrete structures in marine environments.

Cuenca E., Tejedor A., Ferrara L.

This paper analyzes the autogenous and stimulated self-sealing capacity of steel fiber reinforced concretes, with and without crystalline admixtures, under repeated cracking and healing cycles. To this purpose, the performance under cracking and healing cycles was investigated on 150 × 150 × 50 mm3 specimens, cracked by means of an indirect tensile test called Double Edge Wedge Splitting (DEWS) test. Two concrete mixes (with and without crystalline admixtures) and three healing exposure conditions were investigated: water immersion, open-air exposure and wet/dry cycles. Initially, the specimens were cracked up to a crack opening of 0.25 mm and were then subjected to the different aforementioned exposure conditions for 1, 3 and 6 months. At the end of each period, the specimens were cracked again and were subjected to the different exposure conditions for an additional 1 or 2 months, repeating the cracking and healing procedure up until a total duration of one year. The crack closure was analyzed using image processing methods. The results show that, for the same healing period, the specimens immersed in water reached the largest crack closures. In addition, it was observed that the crystalline admixture may favor long-term self-sealing capacity under repeated cracking and healing events.

Soave F., Ferrara L.

Huang Z., Cuenca E., Ferrara L.

He S., Sayadi S., Patel R.A., Schultheiß A.L., Mihai I.C., Jefferson A., Jonkers H.M., Luković M., Schlangen E., Dehn F.

AbstractSelf‐healing concrete, with its ability to autonomously repair damages, holds promise in enhancing its structural durability and resilience. Research on self‐healing concrete in the past decade has advanced in understanding the mechanisms behind healing, exploring various healing agents, and assessing their effectiveness in concrete structures. However, the full potential of self‐healing concrete remains untapped unless its effects are effectively integrated into the design practices of reinforced concrete structures. Realizing this challenge, this paper synthesizes the current research progress and discusses the possibilities to consider self‐healing into design codes. The focus was placed on two specific benefits of applying self‐healing concrete: one centered on durability and the other on mechanical performance. Specifically, the effect of self‐healing on impeding chloride penetration into cracked reinforced concrete was discussed first. Modifications of parameters in existing predictive models based on different types of healing approaches were recommended. Furthermore, the possible impact of the self‐healing capacity in mitigating the stiffness reduction of concrete was also discussed. Equations that can describe the stiffness regained due to healing action are presented. In each part of the case study, limitations and challenges still hindering standardization and wider application in the construction field are discussed.

Zhou X., Yu F., Ashour A., Yang W., Luo Y., Han B.

Cibelli A., Ferrara L., Di Luzio G.

Cappellesso V., Ferrara L., Gruyaert E., Van Tittelboom K., De Belie N.

Ultra-high performance (fibre-reinforced) concrete (UHPC) is recognised for its mechanical properties, such as tensile strain hardening, and exceptional durability, including autogenous healing capabilities due to, among others, the addition of supplementary cementitious materials and fibres. However, it is unknown how the cracked concrete can deal with freeze-thaw (FT) cycles with de-icing salts and if healing agents added to the mix could improve the durability of this type of concrete. This study investigates the influence of a crystalline admixture (CA) in UHPC under FT conditions with focus on single cracks with a width of around 120 μm, specifically focusing on the ability of the healing products of CA to survive and the ability to re-heal after a healing regime following FT exposure. The results revealed that UHPC incorporating healing agents exhibited remarkable resilience against cyclic FT conditions. These healing agents successfully generated healing products that provided exceptional resistance to FT-induced damage and demonstrated the capability to undergo re-healing after exposure to FT.

Neves A., Almeida J.A., Miranda T., Cunha V.M., Pereira E.

This research investigates the objectivity of current methodologies to assess self-healing ability in Fiber Reinforced Concrete (FRC) when compared to a traditional Reference Concrete (REF). FRC was obtained by partially substituting of natural aggregates with Electric Arc Furnace Slag (EAFS) and Fly Ash (FA), and Recycled Tyre Steel Fibers (RTSF) were incorporated in order to maximize the incorporation of non-biodegradable waste. Self-healing was evaluated by measuring the water permeability and by visual inspection of cracks, with both a digital microscope and the use of a Linear Variable Differential Transducers (LVDTs) during the tensile splitting test. The results of this assessment are intended to provide improvements in self-healing assessment methodologies based on cracking imaging and the water permeability test. The composition of the FRC_EAFS concrete, with a maximum EAFS and FA content of 70% and 10%, respectively, had lower compressive and tensile splitting strength compared to the REF concrete, which was compensated by the improved ductility and energy absorption capacity under compressive loading. Permeability recovery was only accomplished by REF mixture.

Huang Z., Cuenca E., Ferrara L.

Ultra-high-performance concrete (UHPC) stands out as a crucial construction material, boasting outstanding mechanical properties and exceptional durability in its uncracked state. The distinctive strain-hardening tensile behavior of UHPC necessitates a consideration of material and structural durability in the cracked state, prompting a rethinking of structural concepts and design approaches. Consequently, various competing mechanisms, including material deterioration, self-sealing, and self-healing capabilities, require meticulous assessment. The autogenous nature of the self-healing capacity of the material, crafted with compositions tailored to specific mechanical properties, further underscores this evaluation. This study elucidates above concepts by compiling and analyzing an extensive database of crack closure data obtained and processed through image processing techniques. This research specifically delves into appraising the self-sealing capacity of UHPC under structural service conditions, encompassing challenges such as chloride and sulfate attacks. Additionally, it endeavors to distinguish the crack healing kinetics of diverse UHPC mix designs, calibrating them across varying crack widths (0–20, 20–50, 50–100, 100–300 μm) and diverse healing environments. These findings assume significance in establishing the "healable width threshold" and the "self-healing coefficients of the crack healing kinetics law" under "structural service conditions".

Zhong R., Ai X., Pan M., Yao Y., Cheng Z., Peng X., Wang J., Zhou W.

The durability of ultra-high performance concrete (UHPC) with initial micro-cracking subjected to coupled freeze-thaw and chloride salt (FT-CS) attacks was investigated. The initial micro-cracking was introduced under controlled condition in the laboratory by pre-tensioning the dog-bone specimens and unloading them at a target strain of 1500 με. The micro-cracked UHPC specimens were then submerged in a sodium chloride solution with the concentration of 4 wt% and subjected to freeze-thaw (F-T) cycles. The micro-cracked UHPCs exhibited excellent chloride penetration resistance as evidenced by the consistently low chloride ion concentration (generally less than 0.5%) and the limited penetration depth (not exceeding 6 mm) even after undergoing 300 cycles of the coupled FT-CS attacks. The tensile strength and compressive strength of the micro-cracked UHPC were improved by 28.0% and 18.3% at 200 F-T cycles, respectively. The improvement in the mechanical properties can be attributed to the self-healing effect which densified the fiber-matrix interface and the matrix. Secondary hydration and carbonation were confirmed quantitatively by thermogravimetric test and qualitatively by scanning electron microscopy test. However, the detrimental effects stemming from the coupled FT-CS attacks outweighed the beneficial effect of self-healing at 300 F-T cycles, resulting in a decline in mechanical performance.

Ferrara L., Lo Gatto V., Rizzieri G., Snoek D.

Autogenous and stimulated healing capacity of UHPC is well known, though in the entirety of studies cracks induced by means of flexural or direct tension tests have been studied. In the attempt of widening the case study database and promote self-healing cement-based materials into a variety of structural applications and broaden its concept to the upkeep of the material and structural load bearing capacity, this study focuses on the effects of self-healing in UHPC under torsional behaviour. Both autogenous and stimulated, via crystalline admixture, healing capacity have been considered, investigating cylinder specimens submitted to torsional behaviour. The capacity of the material not only to heal the induced skew cracks, under wet/dry exposure conditions, but also to maintain its multiple cracking capacity and to promote, upon successive reloading after healing, the formation of new cracks instead of the simple reopening of closed ones. A validation of the experimental campaign is also proposed via fracture-mechanics based finite element analysis.

Rosario D R., Viado M.J.

Transportation networks must be resilient to withstand the effects of climate change and natural calamities. Concrete infrastructure must endure extreme weather, flooding, and seismic catastrophes better than many other types of construction to guarantee the sustainability of transportation services. Self-healing concrete is unquestionably the material of the future that could address these issues. Researchers have discovered a self-healing process in automatic repairing the concrete cracks up to 1.8 mm width. This is made possible by ureolytic and non-ureolytic microorganisms from Bacillus family that cause bacterial precipitation and production of calcite that seal cracks, which could extend the serviceability of concrete. Concrete structures can be restored, and damage prevented through different self-healing mechanisms, such as microvascular healing, bacterial healing, capsule-based healing, and autogenous repair. Research reveals that concrete's capacity to repair itself is greatly enhanced by a mixture of self-healing mechanisms. Moreover, the encapsulation of immobilized bacteria with expanded clay, calcium alginate beads, or other porous materials that can hold onto nutrients and bacteria for an extended period resulted in a considerable improvement in the healing ratio. The main objective of this study is to enumerate all the potential challenges and limitations of the recent studies in self-healing concrete to draw a viable conclusion which is necessary for establishing rules and testing procedures for up-scale implementation.

Xi B., Huang Z., Al-Obaidi S., Ferrara L.

This study investigates the self-healing capabilities of Ultra-High Performance Concrete (UHPC) under the combined influence of mechanical and environmental factors. Specifically, it delves into the long-term self-healing process in pre-cracked UHPC samples that endure continuous sustained tensile stresses across the cracks and are exposed to aggressive environmental conditions for one year. The results reveal that UHPC with narrow cracks exhibits a higher degree of self-healing, especially when exposed to tap water, where its self-healing capacity is most pronounced and improves with extended exposure. Furthermore, these self-healing mechanisms contribute to the restoration of mechanical properties and prevent chloride ion penetration by sealing the cracks. While a reduced level of self-healing is observed in saltwater and geothermal water exposure, prolonged exposure mitigates the inhibitory effect of aggressive ions on self-healing. SEM and EDS results provide evidence that samples subjected to extended exposure to salt and geothermal water exhibit a substantial presence of self-healing product-CaCO3. This study not only emphasizes that pre-cracked UHPC, when exposed to both mechanical stresses and aggressive environments, can maintain excellent durability and mechanical strength due to its self-healing effect but also lays the foundation for evaluating the self-healing potential of cement-based materials under conditions representative of real-world structural scenarios. This is essential for advancing the integration of self-healing advantages into design concepts and performance-based verification approaches.

Lo Monte F., Repesa L., Snoeck D., Doostkami H., Roig-Flores M., Jackson S.J., Alvarez A.B., Nasner M., Borg R.P., Schröfl C., Giménez M., Alonso M.C., Ros P.S., De Belie N., Ferrara L.

The huge benefits brought by the use of Ultra High-Performance Fibre-Reinforced Cementitious Composites (UHPFRCCs) include their high “intrinsic” durability, which is guaranteed by (1) the compact microstructure and (2) the positive interaction between stable multiple-cracking response and autogenous self-healing capability. Hence, self-healing capability must be properly characterized addressing different performances, thus providing all the tools for completely exploiting such large potential. Within this context, the need is clear for a well-established protocol for self-healing characterization. To this end, in the framework of the Cost Action CA15202 SARCOS, six Round Robin Tests involving 30 partners all around Europe were launched addressing different materials, spanning from ordinary concrete to UHPFRCC, and employing different self-healing technologies. In this paper, the tailored experimental methodology is presented and discussed for the specific case of autogenous and crystalline-admixture stimulated healing of UHPFRCC, starting from the comparison of the results from seven different laboratories. The methodology is based on chloride penetration and water permeability tests in cracked disks together with flexural tests on small beams. The latter ones are specifically aimed at assessing the flexural performance recovery of UHPFRCCs, which stands as their signature design “parameter” according to the most recent internationally recognized design approaches. This multi-fold test approach allows to address both inherent durability properties, such as through-crack chloride penetration and apparent water permeability, and more structural/mechanical aspects, such as flexural strength and stiffness.

Cibelli A., Di Luzio G., Ferrara L.

di Summa D., Parpanesi M., Ferrara L., De Belie N.

AbstractThe development of innovative cementitious materials such as Ultra High Performance Concrete (UHPC) requires tailored approaches to assess both the environmental and economic impact of structural applications employing them. For this purpose, in this paper, Life Cycle Assessment (LCA) and Life Cycle Cost (LCC) methodologies are integrated into a Durability Assessment‐Based Design (DAD) workflow which combines structural design algorithms for UHPC with the assessment of the durability performance, with the aim of predicting the evolution of the structural performance all along the service life (SL) in the intended scenarios. As a case study a water tank made of UHPC has been herein selected and compared to a reference made of ordinary reinforced concrete (ORC). While the ORC solution was designed with cantilever cast in situ walls, two different design concepts were assessed for the UHPC basin: one with cast in situ walls and one with precast slabs supported by ORC columns. Moreover, two different mix designs (mainly differing on the alternative presence of silica fume or slag) have been investigated for the UHPC basin and a SL equal to 50 years has been taken into account for each structure. The optimized design, together with the reduced frequency of the maintenance activities for the UHPC structure, allowed by the UHPC superior material and structural durability, resulted into consistent reductions of environmental impacts, up to 76% as for Human Toxicity and Fresh Water Aquatic Ecotoxicity in comparison to the ORC solution. In addition to this, an assessment of the overall construction and maintenance costs that occur during the lifetime of the structures showed a cost reduction higher than 40% for both UHPC solutions, mainly due to a reduction of up to 6% during the construction phase and 91% for the maintenance activities. This also highlights the importance of the correct metrics in evaluating the sustainability of UHPC structural applications, which has to move forward from the units volume or mass of material and its individual constituents to functional units, representative of the benefits of using advanced cement based materials in structurally and environmentally challenging service scenarios.