The present work is a comprehensive collection of recently published research articles on Structural Health Monitoring (SHM) campaigns performed by means of Distributed Optical Fiber Sensors (DOFS). The latter are cutting-edge strain, temperature and vibration monitoring tools with a large potential pool, namely their minimal intrusiveness, accuracy, ease of deployment and more. Its most state-of-the-art feature, though, is the ability to perform measurements with very small spatial resolutions . This review article intends to introduce, inform and advise the readers on various DOFS deployment methodologies for the assessment of the residual ability of a structure to continue serving its intended purpose. By collecting in a single place these recent efforts, advancements and findings, the authors intend to contribute to the goal of collective growth towards an efficient SHM. The current work is structured in a manner that allows for the single consultation of any specific DOFS application field, i.e., laboratory experimentation, the built environment , geotechnical constructions, tunnels, pipelines and wind turbines. Beforehand, a brief section was constructed around the recent progress on the study of the strain transfer mechanisms occurring in the multi-layered sensing system inherent to any DOFS deployment . Finally, a section is also dedicated to ideas and concepts for those novel DOFS applications which may very well represent the future of SHM. The present work reports numerous Distributed Optical Fiber Sensors (DOFS) monitoring campaigns spread over multiple Structural and Civil Engineering fields. Whilst a sequential reading of the article’s diverse sections may provide a global point of view on the evolution of DOFS for the Structural Health Monitoring (SHM) panorama, the authors are intent on also providing the readers with the possibility of skipping directly to the section of interest.. Introduction provides a very general overview on the topic of SHM and the importance of the implementation of DOFS. Before presenting the most recently published DOFS-SHM integrations, Section 3. DOFS Coatings and Bonding Adhesives: The Strain Transfer Process and the Optimal Bonding Technique tackles an issue of great relevance which intrinsically affects any DOFS monitoring. The latter, as foreshadowed by the section’s title, is the strain transfer process occurring in the multi-layered sensing system that characterizes a bonded DOFS. In more detail, due to the stiffness diversity of the adhesives, cladding and coating layers that separate a DOFS’ sensing core from a structural host surface, a certain distortion of the sampled data occurs which, in return, decreases the reliability of the DOFS measured strain. The authors deemed mandatory reporting some recently published findings on this crucial and intrinsic limitation before reporting any in-situ DOFS-monitoring campaign.Section 2Finally, the reported DOFS-SHM applications are separated into the following sections: introduces the reader to those DOFS-SHM novelties that may very well become key DOFS applications in the future.Finally, Acknowledging the broadness of the above-listed application fields and the readers’ engineering backgrounds differences, for the proper understanding of all the DOFS applications, the authors started each section and subsection with a small introduction aimed at providing a certain degree of contextualization. At the end of each section, instead, a brief summary will be presented encapsulating the key findings and DOFS deployments tips.Furthermore, the authors would like to remark that the hereafter reported figures will all be taken from the referred articles and are, therefore, uniquely the product of their respective writers’ efforts. In line with the concept of research and its goal of collective growth towards the solution of society’s biggest problems, the figures were selected with the aim of clarifying the reported DOFS deployment techniques and their outputs in addition to sharing any particular ingenious technique that other monitoring campaigns might draw inspiration from.Finally, the present work situates itself as a spiritual successor to the work developed by Barrias et al.  which stAs clearly stated by Brownjohn , civil iYet, as stated by Cawley , despiteModern monitoring technologies are aimed at tackling the above-mentioned limitations thus boosting the sensors’ precision, automation and data management speed. These include amongst others Optical Fiber Sensors (OFS), Global Positioning Systems (GPS), radars, Micro Electro Mechanical Systems (MEMS) and Image Processing Techniques such as Digital Image Correlation (DIC). As a matter of fact, as properly put by Seo , Civil EFocusing in particular on OFS, these are dielectric devices used to confine and guide light. They consist of several layers: fiber core, cladding and, occasionally, an external jacket aimed at providing mechanical resistance to the fiber (an example is provided in 2) glass cores and claddings. The refractive index of the latter is lower than that of the core in order to satisfy the condition of Snell’s law for total internal reflection and thus confine the propagation of the light along the fiber core only [The majority of OFS used in sensing applications have silica . When no strain or temperature is imposed on the system, light propagates and gets reflected throughout the imperfections with a given signature. Instead, when strain or temperature values vary, the frequency of the back scattered light is shifted. The measured strain values are then related to the frequency shift.As a matter of fact, three different types of light scattering and backscattering may occur in an OFS, namely Raman, Brillouin and Rayleigh. All hold particular optical features that make one more suitable than the others relative to the research objectives. Brillouin scattering, thanks to its extended measurement range capability , is the most studied and used OFS system in Civil and Geotechnical Engineering. The Raman back-scattering, instead, is characterized by a high dependence on temperature but can also be used to extract physical and chemical information of a material  such as Whilst multiple sections of the present article could be dedicated to the explanation of the different photonic working principles of all the commercially available kinds of OFS, this topic has been discussed innumerable times in almost every OFS application article both new and old. Therefore, the authors believe its insertion would result in redundancy. In its stead, Finally, for additional reading on the subject, the authors refer the readers to the comprehensive state-of-the-art reviews on FOS, namely Udd , Kersey OFS sensors have recently improved their performance, drastically increasing their potential pool. The latter was immediately picked up and tapped into by the worldwide scientific community as evident from the increasingly larger number of relative published research articles. Additionally, according to a market survey report conducted by the Photonic Sensor Consortium and published by Information Gatekeepers , the incIt is worthy of note that, according to They allow for completely distributed monitoring (in the case of DOFS) with monitoring points spaced less than 1 mm;Their small diameter and minimal stiffness allow for a very high degree of deployment configuration complexity, no matter if these imply circumferential surfaces, sharp corners, surface irregularities and more. It is even possible to embed them inside structural elements with a minimal level of intrusiveness;Their ease of deployment as can be achieved by simply applying an adhesive over it.Their monitoring length is very flexible and can vary from halves of millimeter to tens of kilometers;Intrinsic immunity to electro-magnetic interference;They are designed with a long life cycle in mind. Indeed, its main component, silica, is highly resistant to corrosion and can withstand high tensile loading;Silica core OFS are highly resistant to temperature and can measure temperatures from −200 °C to 800 °C.Now, if to focus strictly on OFS’s application to the Civil and Structural Engineering field, it can easily be asserted that it has slowly but steadily increased in the past years ,37. As cThe great majority of photonic sensing technology applied to SHM is constituted by discrete sensors such as Fiber Bragg Gratings (FBG)  as theseThe lack of sufficient monitoring points along a fiber deployment  is an issue entirely surpassed by DOFS  left. AsThe distributed nature of DOFS enables the mapping of temperature, strain and vibration distributions in two or even three dimensions (achievable with a DOFS mesh deployment) and their identification at any point along a fiber. Combined, these aspects allow for the painting of a clear picture of the global behavior of a structure rather than reporting the tensile state of a limited number of points. Despite such, a discrete variety of applications have been performed including bridges, dams, tunnels, pipelines and slopes. The goal of the present article is to shine a spotlight on the most recent of such DOFS SHM applications and help the collective growth towards an efficient SHM by collecting in a single place these recent efforts, advancements and findings. In particular, as mentioned in Ideally, the strains measured by a DOFS silica core should be equivalent to the actual strains present in the host structure on which the former is bonded. If this requirement was fully met then both DOFS structural monitoring and laboratory structural investigation would simply boil down to analyzing the OBR’s output data, no matter how minute the object of investigation is or how accurate the measurement requirements are. Unfortunately, this is not always the case. Indeed, the packaging and the installation of the sensors involves intermediate layers  whose presence leads to the failure of the complete strain transfer from the host material to the sensing fiber . InsteadThe reader should consider that Alj et al. continued reporting that several analytical models had been proposed in the literature among which the one of Feng et al.  who deveWhenever an experimental investigation requires the maximum precision possible, the shear lag induced by any host-surface/DOFS-core intermediate layers should be minimal, thus the preferable fiber is a simply cladded non-coated DOFS. In this case, though, the protective function against external damages falls entirely on the adhesive layers. Numerous experimental campaigns have been performed in the latest year to assess which is the optimal adhesive for any specific function, i.e., bonded to a structure’s surface or embedded inside the structure itself bonded to rebar, for instance.As can be evinced, the optimal DOFS claddings/coatings and bonding techniques are still elusive concepts and, therefore, still under heavy investigation. The authors believe in the importance of reporting the recent discoveries and discussions on the present topic first as it inherently influences every DOFS monitoring campaign henceforth.Wang et al.  recentlyBassil et al.  studied Different types of optical cables are available on the market;Some cables are conceived to be embedded inside the structure during construction, whilst others are more suitable for surface installation;The cables, constituted of different materials and shapes, lead to a different strain transfer response due to the shear lag effect in the intermediate layers;Little information is available on the mechanical properties of the cable and its constituent elements;Little information is also available on the long-term behavior and performance of these cables.Furthermore, the authors summarized the modern-day expertise on DOFS cables:−1 whilst the others between 20 m−1 and 60 m−1. This meant that the Polyimide fiber reported crack-induced strain distributions 5 to 15 times higher than the other tested cables, which allowed for the accurate detection of tiny crack propagations.In response to some of the above lackings, the authors used in their test multiple kinds of OFDR DOFS cables see c. The exZhang et al.  proposedAlj  developeThe above project spawned a research article on the topic of the durability of DOFS . Here, tThe results of the previous test are a demonstration of the large experimental effort still required by the scientific community to fully dominate the DOFS-powered SHM monitoring science.Moving on to the second topic of the present section, specifically the research of the optimal adhesive and bonding technique for the RC structures instrumentation with non-coated OFDR DOFS. Bado et al.  assertedThese incisions further had varying steepness degrees in order to simulate different transmission lengths. Finally, different loading speeds were applied (rapid/impact and slow monotonic one). The results showed the bonding adhesives having an influence on the DOFS performance but not on the rise of anomalies. Expectedly, considering the difference between the modulus of elasticity of silicone and cyanoacrylate, the former seemed to delay the appearance of SRAs to the expense of the strain measurement precision in addition to redistributing the strains along the fiber. Oppositely, the rigid but fragile nature of the cyanoacrylate reportedly provided the results with superior precision but at the expense of the adhesive resistance and endurance. The combination of the two provided a hybrid behavior. Finally, the reasons triggering the SRAs were narrowed down to a strain threshold and to the friction with concrete present whenever the DOFS is embedded inside an RC structure.DOFS positioned inside a groove and bonded with cyanoacrylate;DOFS glued to the rebar’s surface with cyanoacrylate with a protective layer of silicone on top;DOFS positioned inside a groove, bonded with cyanoacrylate and further covered with silicone.This last conclusion was later demonstrated to be the main reason for the rise in SRAs in Bado et al. . The artThe authors also indicated as a crucial requirement the absence of any influence of the bonding technique on the DOFS sampled strains, thus, the guarantee that what is actually reported by the fiber are the actual strains occurring in the monitored surface rather than a strain present solely in the fiber. The first phase of the test (performed on a bare rebar) assessed that the presence of a layer of silicone and/or the one of a groove had no influence on the extrapolated DOFS when compared to readings performed with a DOFS simply bonded to the rebar with cyanoacrylate. This was proved to be valid both in tension and in bending. The second phase of the test saw the embedment of the instrumented rebars inside a prism of concrete to form multiple RC tensile elements . These RC specimens were of two kinds, cracking and non-cracking ones designed expressly to study these two different scenarios. Interestingly, during this second test phase, the authors were able to monitor the steel rebars’ strains well beyond the yield limit as testified by the peaks of Additionally, as also visible in The authors continued reporting an unexpected phenomenon, possibly connected to the under-performance of the bonding techniques, which caused an alteration of the DOFS strain readings whenever the fiber was in the presence of extra bending-induced tension (smoothing out strain profiles) and/or monitoring strains in cracked cross-sections (magnifying their amount). Additional work on the involved transfer mechanisms could be the key to such phenomena. Finally, it was asserted that, whilst none of the studied bonding techniques could completely guarantee the removal of all SRAs, the combination of a longitudinally incised groove, cyanoacrylate and silicone was the bonding technique that led to the smallest amount of SRA thus preserving the readability and reliability of the strain data see .Barrias et al.  reportedThe last observation was very probably due to the aforesaid  siliconeAlj et al.  investigLater, mechanical tests on cylindrical concrete specimens instrumented with a polyimide coated  OFDR DOFS bonded/sealed using several adhesives proved that strain measurements provided by DOFS were consistent with those from conventional sensors. Furthermore, they confirmed that bonding DOFS to the concrete structure using soft adhesives allowed to mitigate the amplitude of local strain peaks induced by crack openings, which may prevent the sensor from early breakage. Unfortunately, this particular result, also corroborated by the previous articles, cuts both ways as the strain alleviation reduces the sensitivity of bonded DOFS and may alter specifically sought after strain readings. This was evident in the strain profiles provided by the DOFS see  which waThe DOFS bonded with Bi-component epoxy revealed the development of a strain peak in the central part of the profile from the actuator displacement of 0.2 mm. DOFS bonded with the two silicone adhesives, instead, did not show clear peaks development but rather small triangular shaped profiles visible only from the actuator displacement of 0.3 mm. Indeed, as already observed in ,50, the In Barrias et al. , the autFeng et al.  and HenaBassil et al.  proposedAlj et al.  indicateArticles ,52 preseWhenever an experimental investigation requires the maximum precision possible, the strain shear lag induced by any host–surface/DOFS-core intermediate layers should be minimal, thus the preferable fibers are simply cladded non-coated DOFS;It was experimentally assessed that simple Polyimide-cladded cables had a strain lag parameter inferior to other coating-including ones, thus allowing for accurate detection of tiny crack propagations;On the durability of DOFS cables embedded inside RC structures, an experimental campaign with accelerated aging of the RC saw a larger DOFS/concrete interface degradation for cables with an external coating made of a polyethylene than the ones made out of soft elastomeric material (olefin-type elastomer).Conclusively, the present section introduced several recently published results and observations on the matters of strain transfer in the multi-layered DOFS bonding systems and of the ideal bonding technology for laboratory-targeted coating-less DOFS. The main takeaways on the former were:Numerous researchers reported the appearance of unrealistic strain readings, defined as Strain Reading Anomalies (SRAs), which prevented the extraction of reliable strain data;Whenever DOFS are embedded inside an RC structure, SRAs are mainly caused by the friction with concrete or  by steep strain gradients like the ones occurring in the proximity of surface non-linearities, such as cracks;Whilst it could not completely guarantee the absence of SRAs, the combination of a longitudinally incised groove, CYN and SI was reportedly the bonding technique that led to the smallest amount of SRAs;It was observed on DOFS instrumented RC ties that load increments increased the magnitude of the influence of the ribs on the strain profile of steel rebars;Several DOFS-powered experimental campaigns detected a behavioral feature of silicone whenever it was used for the protection of DOFS. That is a certain redistribution of the strains present in a specific point/section of the DOFS along a larger segment, consequently sampling lower strains than the ones originally present;This is in line with the results of a performed FEM analysis stating that the softer the adhesive is (in this case silicone can be characterized as a “soft” adhesive), the lower the strain transferred to the DOFS glass core due to redistribution over a broader length of the sensor;Following a test on DOFS-instrumented RC beams subjected to fatigue testing (2 million load cycles) the most resilient bonding adhesives were reportedly epoxy and cyanoacrylate.On the best bonding/protection techniques of thin coating-less fibers on RC structures:DOFS cladding, coating and reinforcementsAdhesives or technologies used to achieve the DOFS/structure bonding and protectionThe layout of the deploymentPositioning of the DOFS: bonded to a structure’s surface versus embedded inside itThe application of DOFS for structural monitoring is still rather young, especially when compared to classic tools such as SG, LVDT and inclinometers. It can be stated that an “instrumental maturity” is still lacking when it comes to optical fiber sensing, a shortcoming that numerous research groups and projects are attempting to overcome through laboratory experimentation. Indeed, most research articles published in the latest year still motivate their experimental efforts with the study of the applicability, suitability and performance of DOFS deployments on any particular structure, under any particular testing conditions on the lookout for any particular mechanical response. Furthermore, it is common to see a comparison between DOFS’ outputs and parallelly sampled ones by means of common and well-established monitoring tools. The most common variables under investigation are the following:Considering all the above variables, the solution for the best DOFS deployment is branching and permuting in nature. Consequently, over the years, multiple techniques have been proposed which will eventually “converge” in a single one in the close future, in accordance with the objective at hand. Only under such conditions, it would be possible to develop standards and guidelines for DOFS monitoring.The present section is aimed at presenting and putting side to side the laboratory experimental efforts performed in the latest years in an attempt to help to hasten the above-mentioned “instrumental converge”.It is worth mentioning, as will be noticeable further on, the large number of laboratory experimental campaigns that integrate DOFS monitoring with a novel technology under the name of Digital Image Correlation (DIC). Briefly, DIC is an innovative, cheap, non-contact optical technique for measuring strains and displacements on any primed surface. It works by comparing digital photographs of a component or test piece at different deformation stages and, by tracking blocks of pixels of a complex stochastic pattern painted on the monitored surface, it measures its surface displacement, building 2D and 3D deformation vector fields and strain maps . As it iThe former explored the combined use of DIC and OFDR DOFS for the strain measurements of two 600 × 600 × 125 cm reinforced panels subjected to diagonal tension. The OFDR DOFS were glued with epoxy inside 1 × 1 mm grooves carved along both legs of every stirrup, summing up to 24 measuring lines across the whole specimen. As visible in The same conclusions were also reached in Bado et al.  who moniTherefore, with this frame of mind, the following sub-sections will explore recent research articles on laboratory experimental campaigns deploying DOFS to RC structures and more. They will be grouped in different sections exploring DOFS deployments on RC beams, RC ties, RC slabs, corrosion tests and finally on miscellaneous experimental campaigns such as fire resistance, concrete–textile hybrids and more.Finally, additional reading on laboratory applications of DOFS can be found in Huo et al. , Bassil The present section collects the large research effort performed throughout the last years on DOFS distributed strain sensing for RC beams experimental campaigns.Berrocal et al. , for exaThis range is understandable considering the different depths at which the transversal cracks were measured and their probable lack of perpendicularity compared to the beams’ axis. An additional takeaway was the need for a certain amount of strain noise-removing data post-processing in order to determine the crack position. The crack width and evolution were further calculated by integration of the strain profile over a certain length adjacent to the crack and through the removal of the contribution of the tension stiffening between cracks. A good agreement (±20 μm) was generally observed between DOFS and DIC crack widths monitoring, as can be seen in Berrocal et al.  also invThe results revealed that a certain strain attenuation occurred in the embedded robust DOFS compared to the commonly used thin polyamide coated DOFS. However, the presence of the protective sheath prevented the appearance of SRAs. Performance wise, the robust DOFS were able to provide a good estimate of the beam deflections with errors of between 12.3 and 6.5%. Similarly, crack widths computed on the base of the DOFS strain measurements differed by as little as ±20 µm with the DIC results.Brault and Hoult  monitoreSimilar RC beams were also used in Brault and Hoult  with theAn additional DOFS-DIC integration was reported in Poldon et al. . Here, aShear failure was also analyzed in Rodriguez et al. ,65 who iApart from the definition of the crack pattern, an algorithm was also proposed to calculate the crack widths based on the strain profiles in both horizontal and vertical directions. On the extraction of the above shear crack widths, the proposed DOFS method was compared with the more traditional one based on the deployment of displacement rosettes in . The resShear failure was also studied in Fischer et al.  who perfThe authors suggested that in structural concrete construction, the robustness and durability of the monitoring DOFS (in addition to their basic suitability and sensitivity) should reflect their application . Thus, the authors investigated both conventional polyimide and nylon coated DOFS but also more robust measuring cables  which include various plastic and metal layers (thus comprising comparatively thicker cross-sections). The formers were bonded to the surface of the test specimen with a cyanoacrylate-based adhesive whilst the latter were embedded inside it. During the test, all the observed flexural cracks formed peaks in the strain curve of the measuring cable. Similarly so, the formation and opening of shear cracks close to the supports could be identified by the upwardly guided sensors V9, V1, and N3 in those specific points where the DOFS and shear cracks cross. The robust measuring cables V9 and V1 were reportedly more adapt to reporting the steel strains beyond the cracking stage.Mehdi Mirzazadeh and Green  studied Sawicki et al.  proceedeThe authors further observed that polyimide-coated fibers allowed for the early and accurate detection and localization of micro-cracks. Conclusively, Sawicki et al. highlighted the importance of a deep understanding of the sensor properties (like crack and temperature sensitivity), durability and aging , lonBroth and Hoult  investigFinally, the authors highlighted the fact that DOFS was able to withstand cyclic loadings all the way to failure.However, at the present day, still few tests are available regarding the issues of DOFS-monitored fatigue and deterioration.Barrias et al.  carried Bassil et al.  proceedeThe beam was also instrumented with SG and Linear Variable Displacement Transformers (LVDT). The DOFS was glued to the beam’s front and back surfaces, to the front bottom rebar, mid top rebar, mid bottom rebar and back bottom rebar. The results showed that the implementation of Coda Wave Interferometry and DOFS allowed for the detection of early subtle changes in RC structures until crack formation. On DOFS, the authors concluded that, beyond the latter stage, their ability to quantify the crack openings permitted effective monitoring of RC structures.Wu et al.  tested aChen et al.  brought Malek et al.  utilizedSounthararajah et al.  presenteRC ties are often used to illustrate cracking, deformation and bond behavior of RC structures thanks to their simplicity and a reasonably good representation of the distribution of internal forces and strains in the tensile zone of RC structures . As suchBado et al. , for exaAccording to the authors, before the advent of DOFS, due to the lack of tools able to assess the rebar strains in an accurate, completely-distributed and un-intrusive fashion, most structural analyses relying on such knowledge (such as tension stiffening) resorted to theoretical, empirical or numerical solutions. They continued suggesting that the introduction of DOFS allowed to finally acquire that insight. In the reported experiment, the RC ties’ crack positions detected by DOFS were compared against the ones detected by a simultaneous DIC monitoring. Briefly, thanks to DOFS, the authors detected some specimen’s hybrid bond/slip behavior falling between a crack formation stage’s and stabilized cracking stage’s whenever they were characterized by a length dangerously close to surpassing the maximum theoretical crack spacing length. The latter was translated into different bond-slip profiles. The authors concluded by noting that for future DOFS applications a combination of gluing adhesive (cyanoacrylate for example) with the addition of a protective layer (silicone) was the preferable DOFS/rebar bonding technique as it removed undesired SRA-related uncertainties.Cantone et al.  also proAmongst the experimental campaign conclusions, the followings are particularly relevant. The observed strain gradients and high strain values sampled during the pullout test confirmed that this test methodology could prove to be unsuitable to characterize the response of embedded reinforcement. During the multi-cycle double pullout tests, by means of DOFS, the authors were successfully able to track the increasing bond degradation see  providinZhang et al.  investigSimilarly to , the autSienko et al.  presenteDavis et al.  instrumeA proper shrinkage strain measurement is imperative for a proper assessment of the tension stiffening as it may lead to an apparent underestimation . To remoThe present sub-section starts acknowledging 2013’s pioneering work by Villalba and Casas  which reMore recently, in Nurmi et al. , a proofThe DOFS sampled strain data  enabled Sienko et al.  glued OFLiao et al.  instrumeAccording to the authors, DOFS demonstrated that the top surface of the slab had a fast reaction rate to the heating/cooling process whilst the bottom one suffered a time-lag resulting in a top-vs-bottom temperature difference. In conclusion, the authors stated that DOFS can provide a comprehensive picture of the longitudinal temperature distributions, the heat flux transfer process and the consequent strain distributions thus returning an immediate idea of the deformed shapes of the pavement slab.Teguedy et al.  sustaineThe above illustrated distributed sensing system allowed for the detection of the changes of the composite action between timber and concrete  in addition to highlighting the considerable structural changes induced by mechanisms such as concrete creep and environmental thermos-hygrometric variations.The corrosion of steel reinforcement bars decreases the load-carrying capacity of RC structures by reducing the reinforcement area and decreasing the latter’s bond with the surrounding concrete. With regards to this deterioration phenomenon, DOFS has the potential of becoming a crucial SHM tool thanks to their immunity to corrosion and the potential of providing an insight into the internal corrosion levels, areas of pitting corrosion, crack locations and bond deterioration .Davis et al. , buildinThe beams were subjected to varying levels of corrosion by placing them in a bath of water with 3.5% of the weight NaCl and imposing a current to accelerate the oxidation process of the reinforcement. The beams were finally tested under three-point bending. The authors’ conclusions were preceded by a warning on the possible exacerbation of the corrosion effects induced by the test setup when compared to in-situ corrosion. Overall, the FOS monitoring system was reportedly able to detect the strain distribution changes along the longitudinal reinforcement due to the corrosion-induced loss of bond between the concrete and reinforcement. Furthermore, it was found that 1.1% and 4.0% corrosion levels of the bottom longitudinal reinforcement with short watertight protective sleeves at its ends could cause a decrease in ultimate load of 13% and 39%, respectively. For the beams with long protective sleeves, instead, 1.4% and 3.8% corrosion lead to a decrease in ultimate capacity by 38% and 37%, respectively.Lv et al.  attempteFan et al.  investigIn the first place, the authors studied the potential influence of the sensing cable on the steel–concrete bond strength through several pull-out tests. It was noticed that whilst no bond alterations were observed between a fiber-less reference specimen and longitudinally bonded fiber specimen, the spirally bonded one led to a certain bond reduction . Following such, electrochemical testing was carried out to assess the influence of the fiber installation method on the corrosion resistance of the RC specimen. In particular, the measured strain distributions were used to quantify the corrosion layer thickness and to estimate the corroded volume.The same team published another research article  on the PAdditional similar work from Fan et al.  was aimeAccording to the authors, the DOFS results showed that the steel bar’s corrosion process occurred in three stages at each of which the strains increased approximately linearly but at increasing rates. The thickness of the concrete cover reportedly influenced the duration of each of the three stages. Meanwhile, the increase of the water-to-cement ratio led to a shortening of the first stage and an increased volume of rust filling the pores near the interface between steel and the concrete.The present subsection is intended to collect experimental campaigns that, despite carrying significant weight in each relative field, are characterized by more diverse traits when compared to the above reported DOFS-powered monitoring campaigns. Therefore, the following assortment presents a high level of heterogeneity.Bao et al.  discusseThe resulting DOFS measurements, combined with the measured temperatures and the building code recommended material parameters, yielded an enhanced thermomechanical analysis of simply supported steel beams subjected to the combined action of thermal and mechanical loading. The simulated strains and deflections were then validated using measurements from a second distributed DOFS strain sensor and two linear potentiometers resulting in accurate predictions of the temperature-dependent material properties. Of the four investigated strain predicting building codes, the European building code provided the best predictions.In Bao et al. , insteadSaidi and Gabor  presenteJaafari et al.  stated tLiu et al.  performeWoods et al.  tested aThe fibers were bonded horizontally see  to the sThe present section was focused on the DOFS instrumentation of various structural elements subjected to laboratory experimentation. The reported efforts were categorized as RC beams, RC ties, RC slabs, corrosion testing and miscellaneous applications. Many of these, integrated DOFS monitoring with a novel technology named Digital Image Correlation (DIC) for the observation of superficial strains and for the cracking patterns of RC structures.The authors of most articles were successful in the identification of the flexural strains inside RC beams and a Cemented Pavement Material (CPM) beam, in the detection of the cracking patterns and in the monitoring of both the widening and the propagation of cracks;Both thin coating-less DOFS and high-resistance coated DOFS cables were successfully used to achieve the previous point;Polyimide-cladded fibers allowed for the early and accurate detection and localization of micro-cracks;For the bonding of thin coating-less DOFS on the surface of steel bars, the fibers were commonly positioned at the concavity created by the longitudinal ridge  before gluing them with cyanoacrylate adhesive with the addition of a protective layer of a one-component water-proof oxygen-free silicone rubber;The strain of steel rebars embedded inside RC beams measured by means of DOFS and SG were found to be in good agreement both at crack locations and between the latter;In Berrocal et al. , the DOFFor the monitoring of shear failure in RC beams different but wholly valid solutions were proposed. The first saw the bonding of DOFS not only on the longitudinal rebars but also on 16 stirrups. The second saw the external deployment of an OFDR DOFS in a 2D grid on the shear span of a beam in order to obtain its shear cracking pattern. The third, deployed multiple fibers both internally and externally to the beam including an upward guided one running perpendicularly to the oblique shear cracks. All three deployment kinds successfully reported the steel strains, crack patterns and crack widths;OFDR DOFS bonded to longitudinal RC beams’ rebars were able to monitor their strains whilst withstanding cyclic loadings all the way to failure;OFDR DOFS were successfully employed for the study of the steel/concrete bond degradation due to reinforcement yielding inside RC beams subjected to lateral loading, thus demonstrating their monitoring potential even for high magnitude strains.In the former category, the main observations were:Zhang et al.  reportedSienko et al.  raised tThe DOFS monitoring of RC ties was mainly used to illustrate the cracking, deformation and bond behavior of RC structures thanks to the latter simplicity and reasonably good representation of the distribution of internal forces and strains in the tensile zone of RC structures. Indeed, most authors reported being able to extract plausible concrete–steel bond-stress values by means of RC ties double pull-out testing. Some key observations follow here:DOFS integration to RC and Timber–Concrete Composite (TCC) slabs was commended by several authors who successfully detected the internal strains up until the onset of reinforcement yielding in addition to obtaining a comprehensive picture of longitudinal temperature distributions, the heat flux transfer process and the consequent strain distributions.DOFS has the potential of becoming a crucial SHM tool thanks to their immunity to corrosion;DOFS monitoring was reportedly able to detect the strain distribution changes along the longitudinal reinforcement due to the corrosion-induced loss of bond between the concrete and reinforcement;Lv et al.  introducThe influence of a winded DOFS on the steel–concrete bond strength was studied by means of several pull-out tests. It was noticed that whilst no bond alterations were observed between a fiber-less reference specimen and longitudinally bonded fiber specimen, the spirally bonded one led to a certain bond reduction;The spiraling DOFS around a steel rebar deployment technique was used for the assessment of the influence of various parameters  on the amount of corrosion suffered by the rebar.On DOFS-integrated corrosion testing:Finally, particularly noteworthy miscellaneous experimental DOFS applications included steel and RC structures fire testing, the testing of Textile Reinforced Cementitious Matrix Composites and the shear testing of a Reinforced Polymer (FRP) RC wall.The present section focuses on the built environment branch of Civil Engineering of which its most conspicuous examples are buildings, bridges and roads but which also includes power plants, dams, railroads and airports amongst others. These structures cover key roles in the proper functioning, security and comfort of any society, thus their time-induced deterioration and serviceability-jeopardizing issues occurring throughout their service lifetime  should be treated with the equivalent criticality. Revealing data, averagely representative of the situation of most countries worldwide, is provided by the American Society of Civil Engineers (ASCE)’s 2017 Infrastructure Report Card . AccordiThe applicability and performance of DOFS to the built environment SHM has been tested and proved possible in pioneering publications such as Regier and Hoult  Glišić eWebb et al.  picked uThe bridge, a three-span, pre-tensioned, prestressed concrete beam-and-slab bridge was monitored with the goal of observing the long-term behavior of the structure for damage detection purposes and in order to compare the in-situ measurements to the various empirical creep and shrinkage models used in the design phase. Six beams were instrumented with fiber-optic cables as in  after thIn particular, two types of fiber-optic cables were installed in the six beams; one for the sensing of mechanical strains  and one for the temperature strains (defined as temperature fibers). Interestingly, the latter cable was filled with the gel inside which the sensing fibers were suspended . With such dual fiber deployment, the temperature effects on the Brillouin frequency shift could be removed, such that the true strain in the fiber  could be calculated. The authors concluded by remarking that the measured creep and shrinkage-induced strains were found to be in reasonably good agreement with both Eurocode 2  and CollSimilar investigation procedures were undertaken by Cong et al.  whilst aThe measured prestress losses were compared with the predicted ones calculated according to both the European and American standards. The outcome was an apparent underestimation from the code’s part (possibly due to the inaccurate estimations of various input parameters) which was more pronounced in the early-age versus compared to the two to three years’ mark.An additional bridge OFDR DOFS monitoring is reported in Barrias et al. . Here, SThe authors reported the strain evolution during the construction phase and calculated that, despite the strain variation (biggest one being −304 με), excessive stresses were not transmitted to the concrete during the construction works (maximum 11.42 MPa).A critical scenario during Civil Engineering works might ensue after the pouring of mass concrete structures . Indeed, after the concrete is poured, the cement hydration reaction produces a large internal temperature upsurge. Due to the poor thermal conductivity of concrete, a huge gradient forms soon after between the extremely hot core of the pouring and its quickly cooling outer surface. Considering the poor tensile strength of early age concrete, the rising thermal tensile stresses at the surface might cause thermal cracking. This could potentially lead to a structure prematurely failing to reach its serviceability requirements or even suffering a structural failure. Some solutions for the prevention of this phenomenon are aggregates precooling, pipe cooling, superficial thermal insulation and alternative bay construction . NeverthOn such a topic, Li et al.  reportedOuyang et al.  deployedMoving on from the bridge’s SHM, interesting results are reported by De Battista et al. ,114 regaThe goal, assessing the combined structural shortening caused by the superimposed load, creep and shrinkage during the construction phase. Indeed, the authors highlighted the importance of establishing the floor-to-floor axial shortening of vertical load-bearing elements and their differential shortening (whenever they are characterized by different stiffness) for the installation of finishes and partitions on the lower floors. Thus the importance of integrating the designers’ empirical shortening calculations (adjusting them if necessary) with in-situ continuous distributed measurements. DOFS was then the ideal candidate for such goal, allowing for the calculation of the axial deformation at any time during the construction and along the whole height of the completed elements. Consequently, each monitored element was instrumented with two DOFS (measuring strains and temperature) fixed to the reinforcement as in b before This also occurred, with a smaller magnitude, between two columns with different cross-sections for example at the end of July 2017, the maximum shortening recorded at level 31 on column C8 was 36.6 mm whilst that on column C9 was 32.6 mm, approximately 11% less.Qi et al.  also insBroth and Hoult  assessedBrault and Hoult  investigProceeding to the third and last topic of the present section, Wang et al.  used fivRabaiotti et al.  acknowleIn the present project, BOTDR and Rayleigh-based Swept Wavelength Interferometry DOFS were embedded in two pavement layers in 150 m long loops. The deployment steps were performed as follows. Initially, the fibers were laid out on the substrate layer, followed by their pre-straining and fixing in place c,d. AfteThe authors observed that the stiffer V3 cable seemed less capable of following the pavement’s high strain gradients versus its V9 counterpart. Additionally, the strains measured in the lowest layer (13.5 cm depth) were expectedly smaller. In conclusion, this DOFS-based testing methodology was commended for the detection of problematic zones and quality issues in the realized pavement in addition to validating the design assumptions.Finally, an additional transportation-focused SHM application is also reported in Wheeler et al.  where OFVehicular loading under low vibration conditions (hi-rail vehicle) and high vibration conditions (in-service passenger train) was applied on the rail. The authors reported that it was feasible to study the variability in track support along a section of rail on the grounds of DOFS rail curvature measurements. Yet, they also acknowledged, that despite dynamic distributed rail strains producing low vibration could be measured, the same thing could not be said in the presence of high vibrations. Indeed, the quality factor of the strains resulting from the transit of the passenger train was below the DOFS manufacturer’s minimum recommended threshold.DOFS can be embedded inside prestressed concrete beams, preferably combining a strain sensing fiber with a temperature sensing one in order to remove any temperature effect on the Brillouin frequency shift, such that the true strain in the DOFS can be calculated. Alternatively, for box girder beams, DOFS can be simply glued inside their hollow cross-section.A reported application described the temperature sensing cable to be filled with gel inside which the DOFS was suspended in order to prevent the shear transfer between the concrete and the sensing core;DOFS was successfully implemented inside bridge RC beams for the purpose of monitoring damages and both immediate and time-dependent effects (prestress losses for instance).Conclusively, the present sub-section started describing various DOFS applications for the SHM assessment of bridges. The following key observations were reported:Furthermore, the temperature monitoring during the pouring of mass concrete structures  was successfully performed by means of both BOTDA DOFS and Raman backscattering-based DOFS with an accuracy of 0.3 °C.The shrinkage and creep-induced shortenings  of the columns and structural walls of high-rise buildings can be monitored with a single BOTDA DOFS stretching across all the building’s floors. The sampled differential shortenings in such kind of structures can be very relevant such as was the case in the reported assessment of the shortenings of the Principal Tower C8 column (60.3 mm) and W2 wall (10.2 mm);Following both static and dynamic testing of multiple buildings’ RC beams, the authors of the relative publications state that, through the use of the OFDR DOFS system, both static and dynamic concrete strains could be successfully measured;The above mentioned DOFS-powered tests further reported an effective capturing of structural inflection points, moment transfer at the supports, crack locations and crack openings;In all cases, DOFS were bonded to the surface of RC elements after sanding the latter along the fiber pathway, cleaning it with water and 99% isopropyl alcohol.On the monitoring of buildings by means of DOFS:The strains transferred to a multilayered asphalt pavement can be monitored by means of embedded DOFS. This fiber deployment’s main criticality is during the asphalt compaction process which further involves high temperatures. To counteract any such adversity specific high-resistance DOFS cables were employed in addition to manually covering the DOFS with finer asphalt, before pouring the overlying layer;OFDR DOFS (OBR Luna ODiSI-B model) was employed to successfully assess the distributed dynamic strains of a steel rail during the passage of low-speed and low-vibration rail traffic but were unsuccessful for high-speed high-vibration ones;OFDR DOFS reported very similar reading qualities between a DOFS cable bonded to a steel rail with minimal and optimal surface preparation (grinding and cleaning);A suggested DOFS deployment methodology was bonding DOFS to the rail foot and to the railhead which, despite being prone to be affected by localized strains under heavy train wheels, allows for more accurate rail curvature calculations thanks to the greater distance between the two DOFS.Finally, on the monitoring of roads by means of DOFS:For the readers interested in further reading on DOFS-based SHM of rails, the authors refer to two recent comprehensive literature review articles, namely Du et al.  and SasiGeotechnical Engineering covers a wide range of applications amongst which pile foundations, soil movements (landslides and soil subsidence), stabilization anchors, mining and more. Each of these has the potential for a DOFS-powered SHM integration and their relative literature review will receive independent scrutiny in the present section.Starting from pile foundations, these are commonly used whenever vertical loads are too high to be carried by shallow foundations (like in the case of high-rise buildings) and/or when the soil’s bearing capacity is not suitable to carry the designed loads. Now, considering the rapid urbanization process occurring nowadays all over the world, skyscrapers and mega infrastructure are becoming commonplace solutions to the novel requirements of our modern-day society. As such, pile foundations have seen increased use in the latest years, bringing along a stronger focus on the methodologies for testing and monitoring their performances. In particular, large research efforts have been dedicated to surpassing the limitations intrinsic to the traditional monitoring tools (such as SG) commonly employed for performing the required tests. Indeed, traditional sensors are easily destroyed in complex geotechnical environments and are prone to rust damage and electromagnetic interference . PredictThe two kinds of pile whose testing methodologies have received a significant performance boost from the integration of DOFS are the Continuous Flight Auger (CFA) or Auger Cast Piling and the Rotary Bored Piling. Research articles on their testing procedures are herein reported.Starting from the CFA, when constructing piles under groundwater level or soft ground conditions, in order to prevent the collapse of the soil after drilling the borehole, continuous down–up concreting is performed as the auger drill is extracted (the concrete is pumped through a hollow shaft in the auger). The concrete hinders the penetration of groundwater and soil in the borehole. A reinforcement steel cage is then inserted in the filled borehole as soon as the concrete injection is terminated. Yet, as stated by Seo , the useSeo  instrumeA similar testing methodology was presented in Kania et al.  who moniThe authors studied and reported the steel pile’s strains induced by its driving in the ground a, the CFSienko et al.  also perMoving on to Rotary Bored Piling, differently than CFA, after drilling these require additional ground support such as casing or drilling fluids but can reach much deeper depths ~60 m (versus its CFA counterpart ~25 m) thus being able to accommodate higher loads, bypass underground impediments and infiltrate the ground which could otherwise be too hard for CFA drilling.Cheng et al.  reportedThe authors reported witnessing a consistent DOFS strain increment as the load increased along with unexpected spikes (between 16 m to 43 m depth) on all four independent fibers likely due to the pile’s stiffness variation (similarly to ). DespitXu et al.  and Pei The maximum lateral wall deflection was assessed to be 0.05–0.1% and the measurements obtained from the BOTDA DOFS were found to be in good agreement with the one coming from parallelly installed inclinometers.Guo and Zhao  performeFinally, Gao et al.  proceedeGround deformations are key study points for Geotechnical Engineering  as serious geological disasters may occur on their back. Their monitoring and the development of related early warning systems are crucial for the prevention and control of such disasters. Conventional technologies such as tiltmeters, inclinometers and multipoint extensometers have been adopted for this purpose for decades but the high costs and logistical issues associated with the creation of automated systems make them impractical for long term and massive deployments . In respThe first scrutinized geotechnical issue will be land subsidence, explicitly the gentle down warping and/or sudden sinking of discrete segments of the ground surface . One of Liu et al.  performeWu et al.  applied The same authors published in Zhang et al.  the resuAfter the 200 m deep borehole was excavated, the DOFS cable was attached to a cone guide and lowered into the former. The hole was then backfilled with a mixture of sand, clay and gravel. Then, by means of direct integration, the authors converted the DOFS strains sampled between December 2012 and November 2014 into a numerical estimation of the compaction or rebound that occurred in that time. The results highlighted the rise of positive strains between 60.3 m and 67.9 m depth indicative of an overall rebound of the pumped aquifer.Liu et al.  performeAccording to Cheng et al. , an addiSoil shear deformations are a supplementary geotechnical parameter to closely monitor for stability assessments and for early warnings of potential geo-hazards . The folWang et al.  reportedZheng et al.  attempteOn the same topic, Wu et al.  raised tThe test results showed that for small soil shear displacement (9–15 mm), good soil displacement estimations could be extracted even with no additional anchorage (V3). Beyond such displacements, though, the non-anchored cables slipped from the surrounding soil  whilst the block and tube anchored ones still maintained a well-coupled state. The authors concluded by suggesting the use of block anchors for shallowly-embedded field applications and tube anchors for high overburden pressure scenarios.Shear deformations were also studied in Schenato et al.  in the fZhang et al. , Chen etAfter discussing soil deformations and landslide DOFS monitoring, the logical next step is the monitoring of stabilization anchors. Starting from Cola et al. , the autOnce the instrumented anchors were installed the system permitted a millimetric evaluation of the axial force distribution in the anchor, of the soil–anchor interface actions and of the stabilizing capability associated with the specific hydrogeological conditions of the site. According to the authors, the extracted strain profiles had similar profiles. All had their respective maximum strains sampled just behind the external plate and slowly decreasing strain values when moving inside the ground. The strains decreased to almost zero at about 16 to 17 m from the slope due to the activation of distributed shear stress along the soil–bar interface.Lienhart et al.  proceedeThe load tests included a stepwise load increase. Initially, the authors reported almost uniform strains along the free length of the anchor and decreasing strains in the tendons with increasing depth (due to the load transfer from the anchor to the soil). As the load increased, the load transfer area widened suggesting a progressive failure of the tendon/grout interface. The same article also reported a DOFS monitoring of the strain variation along the top and bottom of soil nails placed into a slope in a road construction site as in .On the grounds of the distance between the top and bottom fibers, curvature changes were derived from their strain differences and a 7 mm vertical displacement at the front end of the nail was observed after roughly one month.The last geotechnical sub-topic under scrutiny is DOFS monitoring for mining applications. In Zhang et al.  a similaA similar test setup was used in Chai et al.  in orderZhang and Sun  suggesteFinally, Lanciano and Salvini  presenteThe most commonly employed deployment technique was bonding DOFS to a pile’s steel cage before embedding the latter inside concrete. DOFS deployment techniques were also suggested for steel pipes (bonding the fiber on its surface with the addition of steel rebars welded to its surface as a guide and protection for the DOFS) and on pre-cast piles ;Oppositely mounted strain sensors (or a single U-looping one) were used to measure both compressive and tensile strains in those situations where flexural strains are expected ;The installation of two fibers at each location was recommended for both redundancy purposes and, in case of damage or unexpected results, in order to have access to independent verification data;In more than one article, DOFS reportedly provided crucial data on the pile stiffness reductions possibly due to the decrease of its cross-sectional area and/or loose gravel and sand mixing with the concrete.The present sub-section presented various DOFS SHM applications for the assessment of the performance of foundation piles, for the monitoring of soil movements , for the monitoring of soil anchors and mining-induced deformations. On the assessment of the performance of foundation piles:For the monitoring of soil subsidence, multiple publications positioned DOFS inside previously excavated boreholes before backfilling them (preferably in accordance with the surrounding stratum type) and assessing through the extracted profiles the occurrence of soil rebound and compaction;Superficial soil deformations, induced for example by soil subsidence, by a landslide or by a rockfall, were successfully monitored positioning a high-strength DOFS inside a superficially excavated trench.On the DOFS soil-embedment, low performance was detected for acrylate coated DOFS due to their fragility and the poor interfacial coupling with the soil.On the use of DOFS for the monitoring of large shear displacements, the suggested procedure was the bonding of DOFS to the soil/rock with heat-shrinking tubes or aluminum block anchors.On the monitoring of soil movements:On the monitoring of soil stabilization anchors and nails, the most commonly used method was the coupling of one or more DOFS to their tendons. On one occasion, two fiber loops were additionally deployed in the grout material surrounding the anchors. Overall the reported articles testified to a good DOFS strain monitoring performance and the extraction of crucial data on the anchor load-carrying capacity.Finally, PPP-BOTDA and BOTDR DOFS were successful in the monitoring of movements in the overlying strata and of the advanced abutment pressure during coal seam mining. DOFS was also successfully employed for the monitoring of the rock mass stability conditions in a marble quarry.For the readers interested in further reading, the authors refer them to the following comprehensive literature review articles on geotechnical , geophysThere is no doubt that Geotechnical Engineering and Tunnel Engineering go hand in hand. This statement can be even stretched as far as asserting that they are one and the same especially considering that proper geotechnical monitoring systems are crucial for safe underground construction and operations. Nevertheless, keeping in mind the difference in boundary conditions and vast application diversity, for simplicity’s sake the present article distinguishes their DOFS monitoring in two different sections, the previous and the present one.The ever-growing urbanization throughout the world has been the catalyst for congestion, air pollution and surface scarceness issues. The accompanying awareness that this situation could be alleviated by means of an underground placement of some key facilities  has led to an increased tunneling activity. The amount of tunnels is not the only growth factor, though. Their design complexity is rising too, encompassing longer and wider tunnels, excavations through increasingly difficult ground conditions, under densely populated zones (thus requiring very high safety standards) and in highly seismic areas .The need for a performant tunnel SHM is particularly self-explanatory, for example, in the aftermath of a seismic event. especially when attempting to assess its residual capacity to survive consecutive seismic aftershocks. This withstanding, SHM is equally crucial during a tunnel’s construction and service-life. As a matter of fact, both new and existing tunnels suffer an inherent structural risk connected to the surrounding geotechnical and hydrological conditions (new ground level constructions and excessive rain periods amongst others) as well as to unknowns related to design assumptions and construction materials . These rWhilst the common practice for the detection of the above-mentioned tunnel structural issues (or of their precursor signs) are regular visual inspections, these have very logical performance limitations and often require to stop the tunnel traffic, thus strongly limiting their frequency. Alternatively, geodetic target points can be mounted to a tunnel linings’ surface in order to measure their displacements by means of total stations. However, these measurements also miss out on the internal strain distribution data in addition to being taken averagely only once per day . FinallyDOFS’ potential application to tunneling was proved feasible and satisfactory in a few pioneering publications such as 2010’s Shi et al.   which is executed through excavation sequences that allow for the surrounding ground/rock to support itself . The metMonsberg et al.  reportedThe several weeks-long measurements began from the moment the shotcrete started curing and continued over the advancement of the tunnel drive construction. In order to prove the suitability of the DOFS-based monitoring system, the results were compared with the ones coming from Vibrating Wire Sensors VWS in  and geodThen, 48 h after the construction of the bench/invert, the deformations were reported to be almost constant along the entire supporting ring suggesting that it formed a load-bearing system capable of coping with the remaining loads. The authors concluded by remarking that DOFS allowed to precisely localize the maximum strain within the cross-section instantly  along with its deformation history. Yet, they also noted that the position of the sensor versus the direction of the tunnel drive significantly influenced the results.Whilst the presence of temporary shorings ensure the stabilization of large cross-section tunnels excavated according to the NATM, their consequent dismantling during the secondary lining’s construction process is amongst the most dangerous steps of the whole construction development due to alteration of the tunnel lining’s original structural equilibrium. As such, Li et al.  performeBOTDR was also employed by Fajkus et al.  in orderThe authors reported the load changes (referred to in the article as changes of the Brillouin frequency) occurring over a period of time of 5 months and hypothesize their origin to be a combination of the temperature variation (of the concrete and the surrounding rocks) and concrete shrinkage.2 could be found right above the tunnel). In particular, a 50 m long fiber was glued to the tunnel’s cross-section expected to suffer the largest deformations , to a segment beneath the tracks and to a segment of the linen parallel to the tunnel . The orientation of the sensors compared to the direction of the tunnel drive was seen to have a large influence. For instance, in one of the reported campaigns, due to the DOFS longitudinal deployment, the use of an inversion model of the vault settlement curve based on a numerical simulation was necessary to establish the tunnel’s horizontal displacements occurring during the removal of temporary shoring. Furthermore, the use of expressly designed resistant DOFS was necessary for a successful sensor embedment inside concrete. This precaution was not necessary for externally bonded DOFS, as was the case for most monitoring of already existent tunnels where the fibers were simply glued to the linens’ intrados. For the latter, the main takeaway was the successful monitoring of minute but relevant deformations induced, for example, by the construction above the tunnel of a residential building or by water accumulating behind a tunnel’s concrete linings or even by the continuous effect of an earthflow occurring above the tunnel.Pipelines, whether underground, above ground or subsea, are one of the most important ways to transport across extensive distances key resources such as water, oil, natural gases and telecommunication cables. Furthermore, pipeline transportation is substantially more efficient  compared to other ground transportation alternatives such as trucks and trains. Yet, the economic importance of laying and maintaining pipelines is absolutely non-neglectable and should be pondered seriously. Maintenance, in particular, is crucial in those common situations where the pipelines are located in complex and hazardous environments  which might lead to issues such as pitting and patch corrosion, cracking and to an overall degradation of a pipeline’s structural integrity. The above could even represent a great safety hazard and risk of environmental pollution in case of pipe leaks and bursts. Thus, the constant monitoring of a pipeline’s structural health is crucial to guaranteeing its safe functioning and to averting the substantial economic, environmental and social consequences of its failure.Generally, the SHM for pipelines demands the observation of their deformation, curvature, eventual leaks and the movements of their surrounding ground. Yet, considering their kilometric lengths and beforehand-unknown failure location, accurate structural monitoring of their entire extension is challenging for discrete sensors. This is not the case for DOFS which completely bypasses the need to install thousands of discrete sensors (with their complex cabling) in order to cover all the potential failure locations. This by itself makes DOFS the ideal pick for a pipeline’s SHM.This was clearly established by pioneering publications such as Glisic and Yao  . The latter, also referred to as Distributed Vibration Sensing (DVS), belongs to the latest developments of DOFS technologies and are part of the Rayleigh-based DOFS family. DAS is an ideal fit for pipeline SHM as their base principle, heterodyne coherent optical time-domain reflectometry (Φ-OTDR), grants them the ability to detect highly-dynamic vibration signals up to the kHz range. Here, the elastic waves phase change produced by perturbing objects in the fiber’s proximity is detected by an Acoustic Optic Modulator. Thus, the phase-sensitive OTDR can be used for real-time vibration monitoring of extended objects, e.g., bridges, dams, highways, railway tracks, pipelines, paved paths, and many others.In 2018, Stajanca et al.  performeThe goal, studying DAS’ potential to detect a pipe’s weak pinhole leak-induced vibrations for various combinations of leak sizes and pipeline internal pressures. The results revealed that the DAS deployment was capable of detecting the pipeline’s vibration modes induced by the broadband leak-noise excitation. In particular, according to the authors, leaking through increasingly larger hole sizes led to excitation of higher frequencies vibrational modes.A different DAS deployment methodology is proposed in Zuo et al. . In partAs previously mentioned, along with a pipes’ leak monitoring, its deformation monitoring is also of crucial importance. As such, Feng et al.  reportedThe deformation of pipelines was also studied by Zhang et al.  who instConclusively, multiple DOFS SHM studies were dedicated to the detection of pipeline leaks. This was performed both by means of standard DOFS and DAS. When a standard strain sensing DOFS was bonded locally, in the proximity of a corrosion-damaged patch, the results showed the need of the fiber to be close no more than 40 mm from the damage to actually be able to detect it. Alternatively, DOFS or DAS could be wrapped along the entire extension of the pipe to detect any potential peaking of hoop strains and vibration, respectively, indicative of corrosion and/or leaking. The authors reported successfully detecting and localizing leaks by means of this technique. Alternatively, DAS could be suspended outside of the pipe in order to detect any leak-induced vibrations . Finally, in order to monitor the occurrence of pipeline deformation, DOFS was deployed on the latter’s surface by means of a double and opposite fiber run or by means of a triple fiber run at a radial distance of 2π/3 from each other. Tests performed with these methodologies both yielded key deformation data.For further reading, the author suggests the following recently published review article, Inaudi .The popularity of wind turbines as an additional/alternative renewable source of power is steadily increasing over the years, contributing to the reduction of fossil fuel reliance. According to a report from the World Wind Energy Association  the windCoscetta et al. , for exaIn Raman et al. , the autThe above-mentioned wind turbine blade’s size surge has the additional side effect of increasing the overturning moment withstood by a pile’s foundation. Zhang et al.  used a DThe monitored concrete-filled/steel composite pile was a test sample mounted on the same offshore platform as the actual study case one. The author’s aim was pin-pointing the pile foundation’s maximum stress location under horizontal loading . The test, subdivided into 15 consecutive load stages with load increments of 60 kN, yielded the maximum strain (1436 με) at the last load stage at y coordinate −19.63 m  and one more on the pile. Throughout the articles, both static and dynamic tests were performed, all successfully yielding the sought-after results. As such, all the authors commended DOFS for the SHM of wind turbines.The present section intends to concisely report novel DOFS applications/studies that may very well pioneer new ways of performing SHM. Whilst not all the following applications can be tagged as strictly Civil Engineering-focused, the potential that they bring forth could definitely benefit the field in the near future.For example, Fernandez-Ruiz et al.  introducBy means of the optical fibers’ ubiquity and ability to perform distributed and highly sensitive DAS vibration monitoring, it would be possible to recognize patterns of potentially hazardous seismic activity. In particular, the authors tested the feasibility of such a plan by monitoring a simulated seismic wave with both a seismometer and DAS sensing fibers. The comparison of the extracted seismometer spectrogram and DAS’s output (combined with a linear 2D bandpass filtering technique and a 1D adaptive LMS filter) showed an encouraging time–frequency dynamics similarity , showed that both the DAS and the FBG-based acceleration sensors could successfully identify the different loads.In Liu et al. , insteadMoving on from macroscale DOFS applications, Li et al.  collecteYou et al.  also broWhen it comes to modern-day OBR interrogators and BOTDA, the use of post-processing algorithms is an important step for the proper study of the extracted strain data. Three examples of these are provided by Bado et al. , Heinze In Heinze and Echtermeyer , a novelFinally, Song et al.  argued tThe present work was intended to introduce, inform and advise the readers on various Distributed Optical Fiber Sensors (DOFS) deployment methodologies for Structural Health Monitoring (SHM) purposes, i.e., the assessment of the residual ability of a structure to continue serving its intended purpose. By collecting in a single article the most recent efforts, advancements and findings on such DOFS SHM integrations the authors intend to contribute to the goal of collective growth towards an efficient SHM.The current work analyzed numerous DOFS applications such as laboratory experimentation, the built environment , geotechnical constructions, tunnels, pipelines and wind turbines. Relevant specific conclusions are reported at the end of each application section. A common conclusion shared by the largest portion of the reported articles is the efficiency and performance of DOFS for the monitoring of the structures under both serviceability and ultimate conditions. To a certain extent, this shows the instrumental maturity already achieved in some Civil Engineering areas. For instance, in the case of concrete structures, it was shown how the technique is fully operative in the field of crack monitoring both in the controlled environment of a laboratory and in-situ. Crack detection, mapping and width quantification was achieved both in bending and shear. The technique’s efficiency was also fully demonstrated for the calculation of deflections by the integration of measured curvatures and for the distribution of temperatures inside the concrete. Furthermore, DOFS deployments were also shown to provide relevant results in the monitoring of corrosion in the reinforcing steel. Overall, DOFS were commendable SHM tools for the monitoring of geotechnical and underground structures too .Multiple deployments techniques were described for each kind of structure and for the varied sought-after results. For example, different deployment techniques were suggested for the monitoring of cast-in-situ RC piles versus prefabricated ones or for the cross-sectional versus longitudinal monitoring of a tunnel linen’s intrados or even for the detection of steel pipe leaks versus axial deformations. Furthermore, the increasingly numerous types of commercially available DOFS cables allowed for the surpassing of the problem of fiber fragility and the need of protecting it during the concrete pouring operations as well as increasing their resistance to the high temperatures that may appear during the concrete hardening process or in the case of their embedding into hot asphalt pavements.The increasingly larger number of applications in each of the reported fields showed a growing interest and recognition of DOFS’ potential, yet, a large number of such publications called for a deeper understanding of the mechanical performance (short-term and long-term) of the multi-layered system with which the fibers are bonded to a structure. Indeed, the strain transfer from a host structure to a DOFS core was seen to depend on the mechanical and geometrical properties of the different intermediate layers, i.e., coating, jacket, reinforcements, adhesives, etc. Therefore, whilst extensive recommendations are now available on optimal bonding/protective materials and on embedding procedures, further work is required for the interpretation of the extracted strain profiles. Additionally, more research is still needed on the DOFS long-term applicability. Indeed, few studies have addressed their performance to fatigue and under cyclic loading, a condition that may affect the behavior of the bonding agents and of the fiber itself. Durability aspects are also under current investigation branching in two main directions: (1) the ability to monitor material degradation (2) the degradation of the fiber and bonding agents themselves when subject to harsh and aggressive environments .Overall, though, with the already existent potential of DOFS, their number of routine industrial applications for SHM is certain to increase.


COVID-19 has triggered various changes in our everyday lives and how we conceptualize the functions of governments. Some areas require stricter forms of regulation while others call for deregulation. The challenge for the regulatory authorities is to manage these potentially conflicting demands in regulation and define coherently their overall regulatory rationale. The precision regulation approach can be a helpful approach. It is defined here as a streamlined approach to regulation to deliver the right methods of regulation for the right group of people at the right time. This problem-solving innovation in regulation triggered by the recent epidemiologic crisis in South Korea demonstrates the emergence of the precision regulation approach. South Korea has implemented streamlined fast-track services for the biotechnology industry to produce test kits swiftly. This article expands the definition of precision regulation from AI regulation literature, and positions the term as a new regulatory rationale, not as a regulatory tool, using the case study from South Korea. Crises drive various changes in our lives. COVID-19 is a global pandemic with 48.5 million cases, and 1,231,017 death confirmed worldwide as of November 6th, 2020  used emergency fast-track procedures to promote COVID-19 test kits . InfectiHence, the precision regulation approach in this case studyhas two critical characteristics: (i) embracing the urgency of the problem in regulations and (ii) involving plural actors to effectively resolve the problem. However, addressing the regulatory problem promptly while upholding quality standards is not an easy task and can be very costly. Indeed, the prerequisites for the precision regulation are likely to be reasonably well-established public health infrastructure, high level of inter-agency trust, and efficient intersectoral communication skills among the policy actors. Those characteristics helped to accelerate the discussions and enable feedback mechanisms to expedite the political processin the South Korean case.Nonetheless, creating opportunities for discussion and negotiating ways forward has not been the traditional task for policy designers . The notThe binary approach to regulation looks at the regulation functioning like a flip switch that turns either on or off . This apBased on the case study, the precision regulation for the pandemic can be used to reinforce meta-governance with the explicit goal of public value delivery. For the case of deregulation, effective regulation should aim to satisfy political expectations and operational feasibility . PreciseThis article identifies the precision regulation approach, using the case study of biotechnology industry regulation in South Korea. The regulatory approach worldwide is primarily divided between flexible arrangements and deregulation, depending on the sectors and the urgency created by the disease. The article points out that such binary understanding of regulation may fall short of adequately addressing the wide spectrum and the interconnected aftermath of the pandemic. It may be too soon to declare South Korea's regulatory approach as precision regulation because the regulatory responses continue to evolve as the battle against COVID-19 continues. Nonetheless, the South Korean COVID-19 regulatory response on the biotechnology industry can guide other nations struggling to balance the binary scale of regulatory flexibilities. The essence of the South Korean case is the focused attention on the specific problem, striving to incorporate multiple aspects of the problem, and an active engagement between private and public sectors, which can be intuitively applied to various countries. Furthermore, future studies may find more examples of the precision regulatory approach in countries with relatively higher quality of public health infrastructure and high inter-agency trust. It may be timely for scholars worldwide to discuss the new rationale for regulation in post-COVID-19 governance.The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found in the article/supplementary material.SL and WK have contributed equally to the conceptualization of the article. SL has drafted the article. WK added his insights. SL and WK revised the article together. Both authors contributed to the article and approved the submitted version.The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.


Liver ischaemia–reperfusion injury (IRI) is an intrinsic part of the transplantation process and damages the parenchymal cells of the liver including hepatocytes, endothelial cells and cholangiocytes. Many biomarkers of IRI have been described over the past two decades that have attempted to quantify the extent of IRI involving different hepatic cellular compartments, with the aim to allow clinicians to predict the suitability of donor livers for transplantation. The advent of machine perfusion has added an additional layer of complexity to this field and has forced researchers to re-evaluate the utility of IRI biomarkers in different machine preservation techniques. In this review, we summarise the current understanding of liver IRI biomarkers and discuss them in the context of machine perfusion. Liver transplantation (LT) remains the only curative treatment for patients with end-stage liver disease or fulminant hepatic failure. There is a worldwide discrepancy between the number of eligible patients in need of LT and the number of available donor livers. This shortage causes ongoing waiting list mortality  and the The liver IRI was first described by Toledo-Pererya in experiments performed in canine models . This isThe traditionally accepted model of liver IRI involves a period of cold IRI whilst the donor liver is being procured, followed by a period of warm IRI during the implantation phase of transplantation . It is iThe traditionally accepted model of IRI is undergoing a further paradigm shift with the advent of MP of donor livers and it is likely that in the coming years, the concept of IRI will continue to evolve . We expeIt is well known that IRI targets and damages all parenchymal cells of the liver, namely hepatocytes, liver sinusoidal endothelial cells (LSEC) and cholangiocytes . During The complex nature of the liver IRI can make the evaluation of potential biomarkers difficult, which in turn impacts upon their utility. The ideal biomarker in liver transplantation would predict the extent of graft injury pre-transplant , and stratify the risk of EAD, primary non-function (PNF) or biliary complications. A single biomarker is unlikely to be able to fulfil these ideal requirements and it is likely that a combination of biomarkers will be required to assess different aspects of donor organ quality. Furthermore, the different types of MP and their individual impacts upon the global IRI might require specific biomarkers for different types of MPs, in the context of LT.Traditionally, the donor livers immerged in preservation fluid are placed on ice for static cold storage (SCS), after procurement. This organ preservation method remains the standard practice worldwide, which allows the cold IRI to occur. MP attempts to provide vascular flow and oxygen to the donor liver at some point in this process. At present, the liver MP field is broadly divided into two separate areas . The norThe hypothermic MP (HMP) involves liver perfusion with cold oxygenated artificial fluids, following a period of SCS, delivered either through the portal vein alone or through the portal vein and hepatic artery combined. The published HMP series results suggest that a relatively short perfusion that last 60–90 min, allows sufficient functional graft recovery prior to implantation, and have demonstrated good outcomes, particularly with respect to biliary complications .Much of the translational and clinical research with NMP and HMP is occurring simultaneously and many groups, such as our own, are working with both techniques. Ultimately, the aim of both perfusion techniques is to prevent or reverse the decline of the donor liver quality caused by SCS and IRI, and to enable a safer utilisation of the extended criteria grafts. Development of the clinical biomarkers of IRI will allow an objective assessment of allograft transplantability and assist in deriving maximal patient benefit.The literature contains a large number of markers and molecules which purport to assess the liver IRI . Many haThe following section discusses potential IRI biomarkers recently detailed within the literature. We caution the reader that many of these biomarkers might be model specific, and have been described in the context of warm ischaemia reperfusion only. A summary of the biomarkers is shown in miRNAs are one class of small noncoding RNAs of 19–24 nucleotides that are important gene regulators at the post-transcriptional level . Many miIn a porcine model of DCD liver transplantation coupled with human liver graft biopsies, Li et al. demonstrated that expression of human miR-146b-5p, which is homologous to porcine miR-146b, successfully correlated with the extent of EAD . It is aWhilst miRNA shows promise as biomarkers for hepatocyte injury, and miR-122 in particular has been shown in a variety of acute liver injury models, to date there is insufficient evidence to support the clinical use of miRNA to assess transplantability during MP evaluation.IL-17 is a pro-inflammatory cytokine, which is secreted by a subset of CD4+ T-cells in response to interleukin 23 and has been linked to the development of autoimmunity. The presence of IL-17 during IRI is likely to be a result of the associated sterile inflammation.A recent study by Li et al.  reportedIL-33 is a cytokine that specifically drives the production of Th2-associated cytokines such as IL-4. In the normal liver, LSEC are the primary source of IL-33, but in liver fibrosis IL-33 is found within activated hepatic stellate cells . TransplFGF21 is a hepatokine secreted by the liver that regulates glucose uptake by adipocytes, with its effects being additive to those of insulin. In a recent study involving patients after liver transplantation, serum levels of FGF21 demonstrated that it might reflect the scale of liver IRI as its levels were found to be elevated 20-fold, relative to healthy subjects . TemporaET-1 is a potent vasoconstrictor that is produced by vascular endothelial cells and is one of three isoforms of endothelin. It has long been known that in patients with portal hypertension and cirrhosis, ET-1 is elevated. Whilst the biomarkers discussed above all pertain to primarily hepatocellular injury, there is an urgent need to develop measures to monitor cholangiocyte damage that might be predictive of biliary complications, which remain a significant cause of transplant-related morbidity, particularly in DCD grafts . Long-teThe fluid, circulating through the liver during MP, provides researchers with a unique opportunity to assess potential biomarkers, specific only to the liver, in a real-time setting. Many of the biomarkers detailed above have not yet been assessed and several others have been identified in the machine perfusion setting instead. There is, however, discordance between the markers discovered by basic science and those discovered during MP experiments.There would be clear clinical benefits in discovering usable biomarkers to assess liver transplantability, but although the experiments are performed on an isolated organ perfusion model, the variability in the quality of the studied livers and the multiple MP types used, adds another layer of complexity. This can be demonstrated by the example of the perfusion fluid, varying from acellular fluids  and oxygWhilst the biomarkers discussed above are not exhaustive, it does demonstrate that for a meaningful clinical use it might be necessary to develop panels of biomarkers that are able to quantify hepatocellular injury, EAD, risk of rejection or biliary complications, post-transplantation. Some of the currently utilised biomarkers assessed during MP have been adopted from clinical observations during liver transplantation , and theIn 2011, Guarrera et al. reported that an increase in ICAM-1, IL-8 and TNF-α correlated with IRI in the HMP setting . TNF-α aRegarding miR-122 as a marker of IRI during MP, correlation was shown with the extent of IRI in preclinical and animal studies . Selten Linares-Cervantes et al. described four biomarkers in an NMP setting relating to biliary and hepatocellular viability in an experimental model with different degrees of ischaemia . The aut+) to assess hepatocellular and cholangiocyte viability [As alluded to above, the development of ischaemic cholangiopathy is a risk in livers from DCD donors, and many teams have investigated biomarkers of biliary function during NMP. In a study by Linares-Cervantes et al., the authors suggested that a bile to perfusate glucose ratio <0.6717 and a bile glucose level <15 mmol/L resulted in 0% incidence of ischaemic cholangiopathy, at long-term follow-up . In addiiability . Furtheriability . Op den iability . The lowiability .More recent data have demonstrated improving sophistication with the development of biomarkers and has built upon the above studies. It is well known that the mitochondria are the essential energy producer for the cell and have a critical role in both the generation of oxidative stress and the recovery from it, following IRI. The precise role of the mitochondria is beyond the remit of this review and was covered in a recent comprehensive review by Hu et al. . The mitThe development of clinically relevant biomarkers of liver IRI, EAD and biliary complications during MP continues to evolve. The perfusion process allows for real-time biomarker assessment that further increases their relevance and translation to clinical use. It is likely that a combination of perfusate and bile biomarkers will allow a superior assessment of global function of donor livers. The challenge for the transplant community is to interpret what the biomarkers mean for particular donor liver utilisation and how it might predict the immediate and long-term post-transplant outcomes of the graft.


In this paper we use spatial econometric specifications to model daily infection rates of COVID-19 across countries. Using recent advances in Bayesian spatial econometric techniques, we particularly focus on the time-dependent importance of alternative spatial linkage structures such as the number of flight connections, relationships in international trade, and common borders. The flexible model setup allows to study the intensity and type of spatial spillover structures over time. Our results show notable spatial spillover mechanisms in the early stages of the virus with international flight linkages as the main transmission channel. In later stages, our model shows a sharp drop in the intensity spatial spillovers due to national travel bans, indicating that travel restrictions led to a reduction of cross-country spillovers. Are spatial econometric methods suitable to model the recent global spread of the coronavirus (COVID-19)? Overall, there is a vast literature on spatial data analysis with a rather heterogeneous treatment of spatial dependence and spillover structures , the virus was often seen as a Chinese and later an Italian problem. However, due to the rapid spread of the virus across the globe, almost all Western countries reacted by employing drastic measures to contain or delay the further spread of the virus. These measures entail considerable restrictions in every day social and economic life. Most notable policy measures comprise closings of borders and general curfews to curb the spread. At the beginning of the outbreak, national linkages thus appear of particular importance to explain the spread across the globe.In this paper we use spatial econometric frameworks to model COVID-19 infections across the globe. Our spatial econometric specification pays particular attention to different types of spatial dependence including information on geographic neighbourhood, travel linkages as well as trade ties. By using daily data on country-specific infections, we moreover allow the strength of spatial dependence to vary over time. Both features appear to be of particular importance for adequately modelling the spread of the virus.N countries over T days. The model can be written as follows:t  dynamic panel model by particularly focussing on time-dependent spatial dependence structures during the spread of the virus, which we aim to model for t . Second, the intensity of bilateral flight connections, measured in terms of the total number of weekly commercial flights between country pairs (Source: openflights.com). Third, trade intensity, using aggregate trade in 2010 USD to construct a k-nearest neighbour type spatial weight matrix, where the seven partners with highest aggregate trade value were considered (Source: WITS trade database).free movement of people, where if a country pair has signed a treaty allowing for free movement of people, they are considered to be neighbours  of Table Turning attention to Fig. First, the daily estimates of spatial dependence confirm that initially spatial spillovers played a key and also statistically significant role in virus transmission: at the end of February only 4 countries within the sample  introduced flight suspension and Second, the key role of flight travel is revealed to be only of importance in the first two months of our sample. Coupled with the suspension of international airline traffic in the beginning of March, the posterior importance of flight connections across all country significantly decreases. This is accompanied by a slight increase in the posterior importance of other measures of neighbourhood, particularly free movement of people within the EU.The results of the Poisson SEM specification in Fig. Overall, we find that spatial dependence notably decreased over time as countries elected to implement regional and global movement restrictions in the form of border closures, flight suspensions and even complete lockdowns. Additionally, air travel connections played a particularly important role in the early stages of the virus, suggesting that the shutdown of flight connections was indeed an important measure in mitigating early transmissions.We examine the virtues of spatial econometric specifications to study the spread of the recent coronavirus pandemic. Our results indicate that cross-country spatial spillover processes, specifically via international flight connections, played a particularly important role in the early stages of the virus spread. When countries began restricting airline traffic, the relative importance of flight connections as well as spatial autocorrelation decreased. Overall, our results imply that the shutdown of international airports and border closures were important policies to prevent further spillovers across countries. Moreover, the estimated spatial dependence structures seem to trace the process of the virus spread very well. Recent spatial econometric methods thus appear useful tools to model the global spread of coronavirus.