The studied methods similarly result in more efficient utilization of resources by lowering cement demand by up to 65%, leading to an expected reduction in all other environmental burdens. These findings show that the flexibility within current concrete design approaches can contribute to climate mitigation without requiring heavy capital investment in alternative manufacturing methods or alternative materials.Growing urban populations and deteriorating infrastructure are driving unprecedented demands for concrete, a material for which there is no alternative that can meet its functional capacity. The production of concrete, more particularly the hydraulic cement that glues the material together, is one of the world’s largest sources of greenhouse gas (GHG) emissions. While this is a well-studied source of emissions, the consequences of efficient structural design decisions on mitigating these emissions are not yet well known. Here, we show that a combination of manufacturing and engineering decisions have the potential to reduce over 76% of the GHG emissions from cement and concrete production, equivalent to 3.6 Gt CO This work quantifies the climate benefits of efficiently utilizing concrete through improved material and structural design, and it shows that over 75% of CO2 emissions from global concrete production could be cut with already implementable measures There are several uses of cement in such materials, such as in concrete and mortar . As the world population grows, the development, maintenance, and extension of urban areas will grow; projected estimates show that by 2030, nearly 1 billion (22% increase compared to 2018) more people will live in urban areas3. With such urban growth, the demand for concrete will continue to rise, with rates exceeding those of population growth4.Cement-based materials are essential for urban development, and there is no alternative material that meets their functional capacity2. Concrete consists of fine and coarse aggregates (sand and crushed rocks), water, admixtures, and a hydraulic binder (cement) that reacts with the water to glue these constituents together into an artificial conglomerate. Significant greenhouse gas (GHG) emissions are attributable to cement-based materials production, ~8% of global anthropogenic CO2 emissions5, which is primarily a function of producing clinker (the precursor to cement). Clinker is a calcined and quenched material that requires high temperatures to create the desired mineralogy, leading to emissions associated with fuels for thermal energy, and chemical-CO2 emissions from limestone decarbonation in its production.Concrete is uniquely poised to meet the needs for many civil infrastructure and building systems because of the broad availability of the primary constituents of concrete, and the strength and durability achievable with this material6, and to do so, the “difficult-to-decarbonize” industries, such as cement and concrete7, must find pathways to mitigation. There are several commonly discussed mitigation strategies for these emissions including use of alternative fuels, use of more efficient equipment, carbon capture, utilization and storage (CCUS), or reducing the demand for clinker through use of supplementary cementitious materials (SCMs)9. CCUS technologies are not well established for the industry10, and while alternative cements and aggregates have been proposed13, their efficacy can be hindered by resource availability, by costs, or by a risk-averse industry15. Critically, improving material efficiency, in which less material is used to achieve the same performance, is a key step in mitigating the environmental impacts from materials production18. This step should be used in unison with low-emissions material alternatives to overcome GHG emissions challenges from the built environment.Society must reach net-zero GHG emissions by 2050 to limit warming to 1.5 °C above pre-industrial levels20. In this work, we systematically quantify the potential role of manufacturing changes, in combination with mixture proportioning and engineering design in the efficient utilization of concrete worldwide  and compressive strength25 and the quantity of steel reinforcement used. Noting that the strength of concrete and the magnitude of steel reinforcement used will affect the volume of each of these materials that must be specified. To address this factor, a model linking concrete strength and reinforcement ratio to environmental impacts of a column or slab26, for a unit structural frame, and based on three different design codes was derived. The impact of service life extension to reduce the future demand for cement was based on average in-use times of cement-based buildings and infrastructure in each of the 10 global regions.To show the opportunity for reduction of GHG emissions within current accepted design, we estimated the average global cradle-to-gate emissions from production of cement-based materials (concrete and mortar), based on 2015 baseline production values. A projection of global emissions from production of cement-based materials between 2015–2100 was modeled based on projected per capita saturation levels, projected population, and average in-use service lives of cement for 10 global regions. The model used was initially developed by Cao et al.To understand the efficacy of structural design in contributing to GHG emissions reductions, this work draws comparisons to the more conventional mitigation methods discussed, which typically revolve around manufacturing improvements for cement and concrete. While there are a variety of manufacturing improvements that can be implemented to reduce GHG emissions from cement and concrete production, here we consider common methods of improving the efficiency of cement kilns, replacement of high emitting kiln fuels with natural gas, and using a low-emissions electricity grid throughout the production of primary constituents .To perform such a comparison, we derive GHG emissions associated with the production of concrete using established data reflecting current global practice as a baseline, and then assess the effects of using manufacturing alterations to lower these emissions. We consider process-based  and energy-based emissions . While we address the role of design decisions and use phase in subsequent stages, initial modeling is for cradle-to-gate impacts  for one cubic meter of concrete. Assumptions for energy resources, initial levels of supplementary cementitious material use, and mixture proportions are stipulated in the Methods and the Supplementary Information.27, indicate that the common manufacturing alterations considered could contribute to GHG emissions reductions of 1% with increased kiln efficiency, ~15% with replacement of higher emitting thermal energy sources in kilns with natural gas, ~6% with wind electricity used to meet all electricity demands, thus ~20% for all manufacturing improvements combined ; a similar difference in median emissions remains even when the manufacturing improvements are implemented (130 kg CO2-eq/m3) , there tends to be greater GHG emissions for higher strength concrete mixtures: for example, without any manufacturing improvements, there is a 75% increase in emissions for a 50 MPa mix relative to a 20 MPa mix . Comparing the 25th percentile of GHG emissions to the median for concrete mixtures capable of achieving the same strength (with no manufacturing improvements), the 25th percentile is 8% lower than the median for 20 MPa, 4.5% lower than the median for 35 MPa, and 9% lower than the median for 50 MPa. These results could have substantial impacts on how we consider prescriptive design . However, variability in GHG emissions per 28-day strength does not reflect other changes in performance characteristics obtained by cement replacement to achieve lower clinker content, such as durability and workability, which are important metrics in concrete mixture design.However, just by selecting appropriate mixtures, such as those capable of achieving the desired strength with lower clinker contentBeyond the benefits gained from selecting desired mixture proportions, which can be implemented in many cases without capital investment in improved manufacturing methods, there are substantial variations in GHG emissions between structural members designed for exactly the same performance requirements. Structural designs are often not optimized because of the trade-off with constructability efficiency, e.g., column dimensions and concrete mix designs can only vary so much for a large construction project before construction becomes over-complex, which is not economically desirable. However, these results show the potential to mitigate GHG emissions through more efficient utilization of materials in structural design, which is currently not addressed because environmental impacts are not included in the design codes. Due to the myriad members and performance requirements that exist, we limit this exploration to columns and slabs, which are among the most common reinforced concrete elements for buildings.The effects of changing steel reinforcement ratio and concrete compressive strength to drive down materials consumption and GHG emissions were explored. Specifically, design of columns and slabs were examined as these make up a significant fraction of the built environment. Using three of the most used and adopted design codes, reflective of 105 countries, allowable member design with steel reinforcement was considered. The role of higher or lower reinforcement ratios on both GHG emissions from the amount of steel needed and the commensurate reduction of concrete needed were examined. Simultaneously, the effects of higher concrete compressive strength on parameters such as member cross-sectional area were addressed. Here, we address emissions savings as they could accrue relative to the median reinforcement ratios and median compressive strengths considered. Details of the calculations performed and assumptions made to quantify these benefits are provided in the Methods and Supplementary Information.3 of steel being much larger than per m3 of concrete, and the fact that the required cross-sectional area depends on concrete strength. As a result, by specifying different volumes of each material, while meeting design requirements such as maximum allowable slab deflection, there can be notable shifts in net GHG emissions. Notably, this work shows that low strength concrete or low reinforcement ratios do not always correlate to low GHG emissions when the whole member design is considered, which is in line with findings by Belizario-Silva et al.29. For structural slabs designed at the yielding and ultimate stage, the lowest emissions for our case study loadings (discussed in the Supplementary Information) occur at the highest reinforcement ratio and lowest concrete compressive strength assessed (highest emissions occur for the lowest reinforcement ratio and highest strength). In the case when crack prevention is the controlling design factor for a reinforced concrete slab , the minimum required reinforcement ratio and low compressive strength is preferable per Fig. 2-eq for the column using the ACI-318 code (this is 70% greater emissions than the lowest column emissions using this code); a difference of >63.1 kg CO2-eq (90% between highest and lowest) for the column using Eurocode 2; and a difference of ~51 kg CO2-eq (60% between highest and lowest) for the column using the Indian Standard code. For slabs designed for bending at the ultimate stage, there is a 58–93% difference between the highest and lowest emissions members that meet design code requirements with the same boundary conditions and loading. In slab design (ultimate), there is a larger difference in GHG emissions for low reinforcement ratio than for higher ratio, which suggests that if a low ratio is used, there is increased reliance on high concrete strength or greater cross-sectional area of concrete (slab thickness), which results in higher impact. However, use of excess reinforcement is inefficient due to the significantly higher volumetric impact of the reinforcement. While trends are similar between codes used in different regions, designing slabs per Eurocode 2 and columns per ACI-318 result in the lowest impact. If all countries/regions were to design for the lowest impact per Eurocode 2 and ACI-318 for slabs and columns, respectively, it would result in a reduction of approximately 67 Gt of GHG emissions between 2015–2100 . The authors recognize that this is a simplified model, but nevertheless useful for the argument at hand. Slabs spanning over multiple supports as well as pre- and post-tensioned slabs are common designs that could yield different results than the modeled simply supported slab. Here, it was assumed that 20% of GHG emissions are from concrete used in other applications than columns and slabs, such as in foundations. Further, if we assume a baseline of 30 MPa (the middle of the strength range considered in this work) and median longitudinal reinforcement ratio , then choosing the optimal combination of strength and reinforcement ratio could lower slab emissions by 20–25%, column emissions by 18–22%, and unit emissions by approximately 23% for these three codes. If instead reinforcing steel with a higher environmental impact is used, the resulting reductions are ~20% for slab, ~30% for column and ~21% for a unit . However, the lower environmental impact of reinforcing steel is used in the analysis herein.Our findings show that within any accepted design code, selection of concrete strength and reinforcement ratio can result in large variations in emissions see Fig. . These v3. The use of SCMs by their incorporation in blended cements increases concrete resistance to chloride ingress, and hence durability in coastal regions31. Though carbonation is another common durability concern, specifically for corrosion of steel reinforcement, due to various factors that need to be present for corrosion to be initiated, as motivated in the Supplementary Information, carbonation was not considered in this analysis. Projections of concrete demand and associated GHG emissions were made to 2100, accounting for shifts in population growth, per capita demands of concrete in use, and national affluence (see Methods). Increased concrete durability in coastal regions was used to estimate benefits to concrete longevity  that SCMs usually allow a reduction of concrete GHG emissions in comparison with plain Portland cement34, thereby improving the sustainability of the system.This work considers that rate of population growth in urban areas is projected to be greater than overall population growth, and that urban areas are predominantly in coastal regionsNew cement production could be curtailed through improved utilization and life extension of in-stock resources for concrete systems; this curtailment in turn would influence the amount of GHG emissions associated by reducing cement production. In this work, we apply this concept by examining the material flows associated with cement and concrete production. Namely, we address regional variability in the magnitude of concrete produced, whether it is used in residential buildings, non-residential buildings, or civil infrastructure, and the current estimated longevity of the concrete in each of those applications for each of 10 regions reflecting the world. Using these baseline statistics on production, utilization, and longevity, we quantify the effects of reducing new cement demand if existing cement applications had longer service lives. Namely, by projecting future demand of concrete , we model the effects of increasing the period before a limit state is reached. Here we assume, if a limit state is not reached, then existing concrete can stay in service and would not require new concrete to replace it. As a result, increasing this period to reaching a limit state mitigates future consumption of concrete.Such modeling requires several assumptions regarding factors such as which regions will have access and ability to utilize SCMs for improved durability, which regions will be susceptible to deterioration and failure mechanisms that will benefit from the utilization of SCMs, and how much elongated service life can be anticipated. Here we consider a scenario in which all conventional Portland cement can be replaced with up to 50% SCMs in coastal areas with chloride-rich environments, leading to the service lifetime of new buildings being extended by threefold and other concrete systems being extended by fourfold (see estimates for service extension in Methods and the Supplementary Information). In this case, a reduction of 175.7 Gt 47.1%) GHG emissions could be achieved  as SCMs would lead to 11% (at 30% replacement) to 34% (at 50% replacement) reductions in GHG emissions; optimizing concrete strength and steel reinforcement for building applications would lead to 18.5% reduction in GHG emissions; increasing concrete system longevity by up to fourfold from current regional average service lives, resulting in a world cement demand reduction by 47.1% and 175.7 Gt of CO2 emissions to achieve net zero within the coming decades in accordance with the findings of the Intergovernmental Panel on Climate Change6. A focus on cement production related emissions has been noted in attempts to meet goals set out in the Paris Agreement35 and in regional regulations . However, due to the difficulties in fully eliminating limestone decarbonation emissions from cement production7, material efficiency strategies that limit the demand for these materials will be a critical aspect of meeting emissions goals18. This needed mechanism for reducing emissions has been accepted by industry as well . Here, we show that within already accepted design, there is huge opportunity to reduce emissions. The inclusion of environmental impact assessments to calculate GHG emissions reduction within conventional codes, material specifications, and procurement/design decisions is critical. Because such methods can already be implemented, they should be put into effect immediately.The degree of GHG emissions mitigation possible through the efficient use of cement and concrete, achieved through design improvements within current design codes, could be used to inform engineers, material scientists, policy makers (or code-writers), and other stakeholders. Many regulatory bodies have emphasized the need for all COIn this work, we compile the anticipated global GHG emissions savings from using several key engineering strategies to reduce emissions during concrete production and efficiently use cement-based materials. To do this, we estimate the GHG emissions to produce cement-based materials worldwide . We then adapt these models to reflect the influence that a variety of manufacturing improvements would have on reducing GHG emissions. Finally, we study the influence that design-based improvements  can have on reaching net-zero GHG emissions goals. The methods for this work are presented below, and details of the methodologies and data used are presented in the Supplementary Information.38. The electricity mix for the full production system was based on world average for the same years from the International Energy Agency (IEA)39. Remaining concrete constituents, including aggregates and water, were determined as a function of cement use, namely in either concrete or mortar ; calculation methods for these ratios are presented in the Supplementary Information. Concrete demand was broken down by strength class, and distributions of concrete constituents were determined using a database of concrete mixtures collected from the academic literature and representative of common concrete constituents and strengths used globally (see discussion of data in Supplementary Information). Distributions were fitted to concrete mixtures in this dataset that fell within the European Ready Mixed Concrete Organization (ERMCO)-specified strength groupings; in these distributions, all cementitious materials were grouped together to form a distribution, which was then sub-divided based on the mineral additive content reported by the GNR Initiative37. To estimate mortar constituents, distributions were modeled based on the standard mortar constituents reported by ASTM International40. As noted, we model non-concrete, cement-based materials as having approximate constituents of mortar. We note this simplification does not capture all cement-based materials. However, due to poor data availability for other cement-based products, and the largest fraction of non-concrete products being mortar, this is an accepted approximation in the academic literature41.Emissions from the production of cement-based materials were modeled for six representative years: 1990, 1995, 2000, 2005, 2010, 2015. For each of these years, kiln efficiency, thermal energy mix, electricity demand, and SCM content for the cementitious materials  were determined based on the Getting the Numbers Right (GNR) InitiativeGHG emissions from the production of concrete and mortar in 2015  was considered to be the baseline herein. These emissions were projected forward to 2100 by using the same emissions per kg of cement consumed and the quantity of cement required in future years, i.e., a do-nothing or business-as-usual scenario. The authors note that the data prior to 1990 were limited; as such, the emissions per kg of cement for all years prior to 1990 were assumed to be equivalent to emissions per kg of cement in 1990, and changes in cumulative emissions in those years were reflective of differences in the quantity of cement produced annually.24. In this modeling approach, the use of cement was broken down by category: residential buildings (Res), non-residential buildings (NonRes), and other civil engineering applications (CE). Because the model by Cao et al.24. focused on production after 1950, for this work, historic data for cement inflow is used for the years 1931–1950. Then, the cement inflow between 1951 and 2100 was captured based on a stock-driven approach, following the procedure outlined by Cao et al.24, which uses per capita saturation levels , the period of time cement-based materials stay in-use , and population projection statistics (based on United Nations data) synthesized by Cao et al.24. Projected cement consumption was determined through use of a combined Gompertz model which calculates the growth curve of future per capita material stock based on in-service lifetimes and stock patterns24. The service lifetimes ranged from ~31 to 100 years for Res, ~31 to 76 years for NonRes, and ~30 to 75 years for CE applications. Generally, shorter in-use lifetimes are seen in developing countries/regions for all three applications. The in-use service life was modeled as a non-deterministic value, i.e., all cement is not taken out of service when the average service life for the country/region in particular is reached, and therefore the cement staying in use is modeled as a distribution, again based on by Cao et al.24.To estimate future cement demand, requirements for 10 countries and regions were projected using the model developed by Cao et al.42 were applied using the medium variant. A sensitivity analysis was performed to capture the impact of alternative population growth patterns between 2020–2100, using the low and high population variant, also from the UN World Population Prospects42. Results from the sensitivity analysis are presented in the Supplementary Information. The ten countries/regions modeled were: (1) North America; (2) Latin America; (3) Europe; (4) Commonwealth of Independent States (CIS); (5) China; (6) India; (7) Africa; (8) the Middle East; (9) Developed Asia & Oceania; and (10) Developing Asia. Cement demand was calculated based on inflow for each application  for each of the countries/regions noted.To capture anticipated changes in populations, projected population data from the UN World Population ProspectsTo examine the effects of typical differences in GHG emissions as a function of concrete constituents and manufacturing methods, the same concrete mixture dataset was used. Mixtures within ±3 MPa of 20 MPa, 35 MPa and 50 MPa compressive strength at 28 days were used for comparisons; mixtures had varying water-to-binder ratios, SCM replacement levels, and aggregate contents to achieve the same 28-day strength.2-eq emissions per m3 of concrete (kg CO2-eq / m3). The GHGs considered in this work are CO2, CH4, and N2O, and they were assessed in terms of CO2-eq using the 100a global warming potentials from the Intergovernmental Panel on Climate Change (IPCC)43. Details on how environmental impact assessments were performed and further assumptions made are presented in the Supplementary Information.Calculations of GHG emissions to produce concrete mixtures were performed for cradle-to-gate production. That is, these calculations included sources of GHG emissions from raw material acquisition through constituent mixing, but not including placement or other construction-related emissions. For this work, it is assumed that changes proposed will have limited effect on construction, use phase, or end-of-life GHG emissions of concrete between alternatives; therefore, GHG fluxes that occur in life cycle phases subsequent to concrete production are modeled as equivalent between alternatives and not incorporated into calculations. The emissions for each mixture were calculated as a volumetric impact, i.e., in terms of kg COTo assess changes that could alter GHG emissions during cradle-to-gate production of concrete, this work examines alterations in both cement manufacturing methods and concrete mixture proportions (focusing on increased utilization of SCMs). For manufacturing methods, this work focuses on the beneficial effects of commonly discussed GHG emissions mitigation strategies during cement production, namely: increasing kiln efficiency, switching higher emitting kiln fuels to natural gas, switching higher emitting electricity resources to wind power, and a combination of these strategies.37, to 30% and 50% SCM content44 The authors note that the supply of GGBS and FA may decrease in the future, but we anticipate that similar performance can be achieved by utilization of NP, and though the availability of certain NP is regional, a wide range of pozzolanic materials could be used 46. Due to variations in GHG emissions from production of different SCMs (see Supplementary Information), additional SCM content is modeled here as having equivalent emissions to NP.Further, to address utilization of SCMs, initial assessment included impacts associated with inclusion of Limestone filler (LS), Natural Pozzolans (NP), Shale Ash (SA), Calcined Clay (CC), Silica Fume (SF), Fly Ash (FA) and Blast Furnace Slag (GGBS) in concrete mixtures. Ranges in environmental impacts from the use of increased SCM content were based on an increase from the 2015 average SCM content (20.3%), as reported by the GNR InitiativeThese more commonly discussed strategies can be compared to mitigation possible from structural design to provide context for the significance of design for efficient concrete use as a mitigation method. Further, these alterations were used to assess a cumulative potential mitigation of GHG emissions from global concrete production and use.25 to link GHG emissions from concrete production to its compressive strength and (2) the methods developed by Kourehpaz26 to link concrete compressive strength and reinforcement ratio to environmental impacts. To apply the relationships derived by Fan25, parameters were derived for the effects of binder content as it relates to both GHG emissions and concrete compressive strength (derivation and parameters determined are presented in the Supplementary Information). The method developed by Kourehpaz26 was based on reinforced concrete members meeting the ACI 318 design code from the American Concrete Institute47. In this work, similar relationships were derived to examine design of reinforced columns (3.5 m standard height) and reinforced slabs (spanning 7 m) at cracking, yielding of reinforcing steel, and ultimate stages according to equivalent members based on Eurocode 248 and the Indian Standard code49. The slabs are designed for bending, and while other structural design aspects such as shear can affect the GHG emissions, those are outside the scope of this analysis. The equations were developed to allow the cross-section area of the column to vary for a fixed applied axial load, and the slab depth to vary for a constant uniform load, for a range of compressive strengths and reinforcement ratios. More detail on assumptions made and application are presented in the Supplementary Information. To address the GHG emissions for reinforcing steel, this work assumes the steel used contains 80% recycled content, with an impact of 1.03 kg CO2-eq per kg of steel ; however, the authors note that different production methods for steel  can lead to different GHG emissions. To address variations in manufacturing methods, a sensitivity analysis was performed to examine the impact of using reinforcing steel with higher environmental impact. The higher value used, 2.29 kg CO2-eq per kg of steel, was chosen based on ref. 50. Contribution from reinforcement other than longitudinal steel, such as stirrups, mesh, or shear reinforcement, have not been considered in this study for simplicity.This phase of study examines the influence of the amount of reinforcing steel and concrete compressive strength in design of concrete members in buildings, to ascertain their ability to contribute to mitigation of GHG emissions. To determine the effect of these factors, this work couples two models from the literature: (1) the mixture proportioning relationships developed by Fan24. for estimating the in-use cement stock, and scales this stock to estimate cement-based materials demand based on the models discussed in depth in the Supplementary Information. Again, this model categorizes cement demand in three different applications (Buildings (which include Res and NonRes), and CE). The ratios of use in each of these sectors and the longevity of service were based on data collected by Cao et al.24, where the service-life modeling is based on a statistical analysis of longevity of concrete systems in the herein 10 countries/regions, that cumulatively represent the world.To analyze the effects on GHG emissions of service life extension for concrete structures and mortar used, several factors were assessed concurrently. This work uses the model developed by Cao et al.51. Binder selection is critical in extending concrete service life. Taking chlorides as the illustrative case for durability: the expected corrosion-free service life of a concrete structure with 40 mm steel cover in a typical marine environment, using different binder types  can be estimated, following31. Based on the generally accepted critical chloride threshold of 0.4% chlorides by mass of binder at the steel as the corrosion initiation threshold, different corrosion-free life can be expected for different binder systems: <5 y for OPC, ~25 y for FA, and ~50 y for GGBS. The problem here is simplified, but nevertheless illustrates a possibility of extending service life substantially by judicious choice of concrete materials, while at the same time retaining all necessary mechanical and physical properties of the concrete. While using SCMs may increase the ability of a concrete structure to carbonate52, this does not necessarily translate to greater corrosion-susceptibility, as indicated in the Supplementary Information.The role of mixture selection and structural design on the longevity of concrete systems was analyzed by altering the in-stock time-horizons modeled for each concrete application. A further benefit is that concretes with blended cements generally have much higher resistivity than plain Portland concretes, which allows possible steel corrosion to be better controlled and managed during the active corrosion phase, thus further extending the service life31. It was assumed that this level of increased longevity would be attained with a 30 or 50% SCM content. The GHG emissions from producing these SCMs was modeled here as having equivalent GHG emissions to natural pozzolans. Due to higher levels of SCMs being utilized within recent years in cement and concrete production, namely a ~10% rise in use has been reported within the past 30 years53, prolonged use was considered to begin with structures currently in-stock for one of the idealized scenarios; however, for the baseline considered in the manuscript, only future stock (2015–2100) is considered to have elongated service life. It was further assumed that the increase in service life would contribute to efficient utilization of resources and, thus, decrease the demand for new cement.This work considers that the mean service lifetime for each of the three application categories was increased, as a result of improved durability, by threefold and fourfold (see Supplementary information) for buildings and infrastructure, respectively, with various service-life extension scenarios considered based on cement use. To estimate the potential effect cement replacement and longer service lives could have had retrospectively, an ideal scenario was modeled, where it was assumed that all countries/regions would have access to 50% SCM and as a result, service lives can increase by fourfold. A more realistic scenario was modeled as well, where it was assumed that only certain parts of the world (50%) will have access to SCMs to replace up to 50% of cement in concrete. For this scenario it was also assumed that access to SCMs would have increased since 1931, and hence a fourfold and a threefold service life extension was modeled for historical and projected data. Results from the sensitivity analysis based on these scenarios are summarized in the Supplementary Information. The service life was modeled as a distribution, and hence all cement in-use was not taken out of service at the same time to reflect the effective in-use time of buildings and infrastructure. This life extension was based on empirical findings on the influence of using FA and GGBS on hindering corrosion of steel reinforcement in coastal regions, and the associated increase in service life of the concrete structuresAt this stage of assessment, the mitigation potential from each strategy was considered. In this component of the analysis, the mitigation potential from using improvements in manufacturing , lower levels of clinker per cubic meter of concrete achieved through use of SCMs, desired concrete compressive strength and reinforcement ratio for infrastructure systems, and increasing longevity from SCM use were examined cumulatively. To assess the influence of compressive strength and reinforcement ratio on GHG emissions at a global scale, differences in emissions were calculated based on strengths, SCM content, and manufacturing methods that would result in lower GHG emissions relative to the global average in the year 2015, and reinforcement ratios that would result in lower GHG emissions relative to the mean of the reinforcement ratios.54. For buildings, the member designs were extended to reflect concrete structures based on a method proposed by Schmidt et al.55 namely, the requirements of members in terms of strength and cross-sectional area were related to relative volume of horizontal members  and vertical members  as well as the degree of loading related to the height of the structure. A simplified approach was applied herein where both NonRes and Res buildings were modeled as having an average height of 3.5 m per story. Vertical members were modeled based on the reinforced column design methodology, and horizonal members were modeled based on the slab designed for bending at the ultimate stage methodology. This method allowed for the assessment of mitigation potential from median reinforcement ratios and from the current average strength concrete used around the world. More detail on assumptions made and application are presented in the Supplementary Information. Code to reproduce the figures in the manuscript have been made available by the authors.To scale member designs to global infrastructure systems, two primary approaches were taken. Concrete used in civil infrastructure was not considered herein for improvements to reinforced members as such concrete can vary from reinforced to substantial uses as non-reinforced or nominally reinforced concrete ; therefore, we model these applications as not to be reinforced like buildingsSupplementary InformationPeer Review FileDescription of Additional Supplementary FilesSupplementary Data 1


Intraoperative arterial hypotension (IOH) leads to increased postoperative morbidity. Norepinephrine is often use to treat IOH. The question regarding the mode of administration in either a bolus or continuous infusion remains unanswered. The aim of the present study was to describe and compare the effects on macrocirculation and microcirculation of a bolus and a continuous infusion of norepinephrine to treat IOH.2. In case of IOH, the patient could receive either a bolus of 10 µg or a continuous infusion of 200 µg/h of norepinephrine. Time analysis for comparison between bolus and continuous infusion were at peak of MAP. The primary outcome was MFI by videocapillaroscopy.We conducted a prospective observational study with adult patients who underwent neurosurgery. Patients with invasive arterial blood pressure and cardiac output (CO) monitoring were screened for inclusion. All patients underwent microcirculation monitoring by video-capillaroscopy, laser doppler, near-infrared spectroscopy technology, and tissular COThirty-five patients were included, with 41 boluses and 33 continuous infusion.P=0,07). For macrocirculatory parameters, continuous infusion was associated with a smaller decrease in CO and stroke volume (p<0.05).Bolus and continuous infusion induced an maximal increase in mean arterial pressure of +30[20-45] and +23[12-34] %, respectively , perfusion index , total vessel density , showed significant opposite variations with bolus and continuous infusion, respectively.For microcirculatory parameters, microvascular flow index , 05/03/2018 At the macrocirculatory level continuous infusion resulted in a smaller decrease in CO and SV and a smaller increase in afterload than bolus administration.At the microcirculatory level continuous infusion enhanced blood flow and vessel density while these variables were impaired with a bolus.These results on macro and microcirculation enlighten the potential benefits of a continuous infusion of norepinephrine rather than a bolus to treat anaesthesia-induced hypotension. Intraoperative arterial hypotension (IOH) leads to increased postoperative morbidity –3. DurinVasopressor agents, especially norepinephrine, are commonly administered to correct IOH . In the These constatations could have clinical implications: recently, continuous infusion of norepinephrine has been associated with better control of peroperative ABP and better renal postoperative outcomes than boluses .Microcirculation monitoring has been intensely developed since it became a topic of interest more than 20 years ago. Indeed, microcirculation has proven to be strongly associated with organ dysfunction and outcome in various shock models , 14, 15.2 and videocapillaroscopy are mostly used  L/min and an SV of 74 [49-86] mL, for an HR of 70 [61-77] pulse/min. Variations of pulsed pressure (ΔPP) was 10 [5-12] %. Macrocirculatory parameters before bolus and continuous infusion were not different.Effects of a bolus of norepinephrine: Norepinephrine boluses were associated with a 30 [20-45] % increase in MAP  and a decrease in HR and SV of 6 [1-11] %  and 12 [4-16] % , respectively, leading to a decrease in CO of 15 [16-29] % . GALA increased by +12 [6-15]°  in median . This was coupled with a drop in HR of -10%  but not in SV, in which variations remained nonsignificant +2%). However, a significant decrease in CO was observed, of 7 [0-14] % . GALA increased by +7 [3-12]°   increase in MAP , continuous infusion was associated with a more limited decrease in CO of 0,6 L/min . As changes in HR were similar, this implies that differences in CO were attributed to differences in the decrease in SV. Indeed, we observed a smaller decrease in SV of 4 ml . This was associated with a smaller increase in the GALA of 7° . The results are exposed in Table p<0.05). Microcirculatory parameters before bolus and continuous infusion were not different.Intra- and interindividual variability for the assessment of the MFI score was good, with kappa coefficients of concordance of 0.82 and 0.87, respectively  and TVD from 14,1  to 13,9  (p=0.22). PVD also increased significantly from 10,1  to 11,8  mm2/mm2 . This was associated with a significant decrease in PI of -12  % . Concomitantly, we observed an early and transient increase in TPU at T30s ). No significant variation concerning the remaining parameters was observed.ne Table : NorepinEffect of a continuous infusion  and a significant increase in TVD from 15,1   to 17,4  mm2/mm2 . Concomitantly, PI also increased but not significantly by + 12 (4-20)%. In contrast, we observed a decrease in StO2 at thenar eminence .on Table : NorepinComparison between bolus and continuous infusion at peak of MAP: bolus and continuous infusion showed overall opposite variations in microcirculatory parameters, as MFI  vs. +0,3, p=0,03), PI  vs. +12(4-20)%, p=0,008), TVD  vs. +2,3mm2/mm2, p=0,002), for bolus and continuous infusion, respectively. The results are exposed in Table  is smaller. This could be at the origin of a greater stimulation of α1-adrenoreceptors and precapillary sphincters.Third, CO is a major determinant of tissue perfusion even if mechanisms of autoregulation exist at the cellular scale.These results could be explained by various reasons:At the end of the process, the combined decrease in CO and greater tissue vasoconstriction may explain the decrease in flow and vessel density.StO2 and videocapillaroscopy seem to show contradictory results when looking at continuous infusion effects.One could argue that variations in regional blood flow at the administration of norepinephrine should be considered differently depending on the organ of focus. Consequently, we can logically suppose that norepinephrine leads to cutaneous vasoconstriction, which is responsible for the decrease in microcirculatory parameters reflecting capillary density in that particular territory (StO2 and/or THI). On the other hand, in the central location , the effects could be opposite, with a central redistribution of blood consecutive to peripheral vasoconstriction.This study had some limitationsThis is an observational study with a small sample size of patients, and clinical implications should be carefully considered. The smaller size of continuous infusion sequences analysed in videocapillaroscopy could be responsible for selection bias. This small sample is partially due to difficulties encountered during recording attempts, with restricted access to the mouth. These limitations are also linked to the device  and explain why videocapillaroscopy, even if very intriguing, is not an easily used bedside tool for daily practice, at least in the operating room. Difficulty in recording is accountable to a certain power in our study.Our results cannot be generalized to other types of shock as we studied a population of carefully selected patients who only presented anaesthesia-induced hypotension. As previously published, the effects on macro- and microcirculatory parameters of norepinephrine could be different depending on the haemodynamic state of the patient before infusion , 33.The absence of randomization might be responsible of selection bias. Because the intervention (bolus or continuous infusion) is at the discretion of the anaesthesiologist, boluses and infusions could be compared in different physiologic situations.As mentioned earlier, the increase in MAP appears to be greater, although not significant, with the bolus mode of administration, which could have influenced our results on the macro- and microcirculatory parameters. The differences observed may be slightly overestimated.It could also be argued that the comparison between different moments  is not relevant. More than the comparison of two moments (T60 vs T300), we tried to compare two ways of administrating the same drug when the effect on MAP was at its maximal. Those times were decided before we started the protocol, and are based on a study we are about to publish, focusing precisely on pharmacodynamics/pharmacokinetics effects of norepinephrine.We decided to put in light the results offered by the videocapillaroscopy more than the other monitors : PI seems to have the same variations but it is true that laser Doppler, tissular CO2 and the StO2 are not always going the same way. The main reason we decided to present the results that way is that today videocapillaroscopy is seen as the gold standard monitor of microcirculation since several years, and the most precise device, even if recording might be difficult. The recent publication of the consensus of 2018 is a reflection of the interest showed by the scientific community. We decided to present all the results, even those evolving in a opposite way, because we found it interesting for the readers. Our final experience remains that some of the monitors we used  have not the capacity of detecting such rapid moves as we wanted to study. Their time of reaction doesn’t allow to make a sure and certain conclusion.Last, we do not report the post-operatives outcomes of the patients we studied. The reason is the study design was not powered for this, as it was a preliminary work. Our results pave the way for an randomized trial.These results on macro and microcirculation enlighten the potential benefits of a continuous infusion of norepinephrine rather than a bolus to treat anaesthesia-induced hypotension.Indeed, continuous infusion of norepinephrine appears to preserve microcirculatory flow and capillary density and results in a smaller decrease in CO compared with the bolus mode of administration. Future studies are still warranted to confirm these results in a larger sample of patients.


In low- and middle-income countries, where antimicrobial access may be erratic and neonatal sepsis pathogens are frequently multidrug-resistant, empiric antibiotic prescribing practices may diverge from the World Health Organization (WHO) guidelines. This study examined antibiotic prescribing for neonatal sepsis at a tertiary referral hospital neonatal unit in Gaborone, Botswana, using data from a prospective cohort of 467 neonates. We reviewed antibiotic prescriptions for the first episode of suspected sepsis, categorized as early-onset  or late-onset . The WHO prescribing guidelines were used to determine whether antibiotics were “guideline-synchronous” or “guideline-divergent”. Logistic regression models examined independent associations between the time of neonatal sepsis onset and estimated gestational age (EGA) with guideline-divergent antibiotic use. The majority  were prescribed one or more antibiotics, and 31 (10%) received guideline-divergent antibiotics. Risk factors for guideline-divergent prescribing included neonates with LOS, compared to EOS ). Prematurity was a risk factor for guideline-divergent prescribing. Every 1-week decrease in EGA resulted in 11% increased odds of guideline-divergent antibiotics ). Premature infants with LOS had higher odds of guideline-divergent prescribing. Studies are needed to define the causes of this differential rate of guideline-divergent prescribing to guide future interventions. Neonatal sepsis is a major cause of neonatal mortality and morbidity globally, accounting for 500,000 to 900,000 neonatal deaths annually ,2. TheseAntimicrobial-resistant (AMR) pathogens are becoming an increasingly common cause of fatal neonatal sepsis; an estimated 214,000 neonatal deaths per year are attributable to infections caused by AMR pathogens . The higNeonatal units have high rates of antibiotic usage ,8,9, mucStreptococcus [Empiric antibiotic treatment guidelines are a key tool designed to improve the quality of antibiotic prescribing by preventing “bug-drug mismatches” and reducing the unnecessary prescribing of antibiotics . Howevertococcus ,19. In Ltococcus ,13,20,21tococcus . Synchrotococcus , limitedtococcus , as welltococcus . Prior wtococcus ,25,26 antococcus ,27,28,29This study examined antibiotic use in a neonatal cohort from a tertiary referral hospital neonatal unit in Gaborone, Botswana, to identify patient-level factors associated with guideline-divergent prescribing compared to the existing WHO guidelines . Our worThe study setting was a 530-bed tertiary public referral hospital in Gaborone, Botswana, where over 8000 deliveries occur annually. The 36-bed neonatal unit is divided into five rooms that provide varying levels of acuity and support. Neonatal care in this unit includes supplemental oxygen and mechanical ventilatory support, cardio–respiratory monitoring, enteral and parenteral nutrition, thermoregulation, phototherapy, and fluid and electrolyte supplementation.Data from a prospective cohort of 467 neonates admitted to the referral hospital’s neonatal unit from November 2020 to December 2021 were utilized for this study. Approximately 75% of admitted neonates were born at the study hospital. Neonates aged < 96 h at admission were eligible for the cohort; neonates whose mothers had a maternal age of <18 years or a maternal diagnosis of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) within the past 10 days were excluded. This study used a sub-cohort of 325 neonates prescribed antibacterial antimicrobials. The full cohort was approved by the institutional review boards of the institution, Botswana Ministry of Health Research and Development Committee, University of Botswana, the University of Pennsylvania, the hospital IRB and Children’s Hospital of Philadelphia.Maternal epidemiologic data was assessed by chart review whereas the mother was interviewed to determine elements of her past medical history and past medication use. Neonates were followed until neonatal unit discharge or death. Study personnel reviewed the neonates’ charts to obtain information on birth history and parameters and daily exposures including antibiotic administration. Antibiotic prescriptions for the first episode of suspected sepsis were reviewed. The primary outcome was defined as “guideline-divergent” or “guideline-synchronous” based on the 2013 WHO neonatal sepsis antibiotic guidelines. First-line antibiotics, per the WHO guidelines, are gentamicin with ampicillin or benzylpenicillin . Two maiStatistical analysis was conducted using R . Descriptive analyses were conducted, with categorical variables described using total counts and frequencies and continuous variables described using medians and interquartile ranges (IQR). The normality of each continuous variable was assessed. The amount of data missing for each variable under study was evaluated, with a cutoff of 10% missingness. Bivariate analysis was conducted to detect significant differences at the 0.05 significance level via Fisher’s exact tests for categorical variables and the Wilcoxon ranked sum test for continuous variables since all were abnormal. The other bivariate analysis results were used to assess for potential confounding.Multivariate analysis was then conducted with logistic regression models to detect any significant differences at the 0.05 significance level. Based on bivariate analyses and conceptual framework, EGA was controlled for in the model of the time of neonatal sepsis onset and the guideline synchrony of the antibiotic. For the model of EGA and guideline-synchrony of antibiotic use, it was determined that the variables under consideration were mediators, not potential confounders, and thus, no variables were controlled for in this model.In this prospective cohort of 467 neonates admitted to the neonatal unit between November 2020 and December 2021, we examined the use of antibacterial and antimicrobial agents prescribed for 325 neonates during their first episode of suspected sepsis. In our cohort, the overall median EGA was 34 weeks (IQR 30–38), and 81 (25%) neonates had perinatal exposure to HIV. The in-hospital mortality rate was 11% (35/325) . Five 1. Of the p = 0.0057). The median day of life at the time of sepsis onset was similar for neonates with EOS who were prescribed guideline-divergent antibiotics compared to guideline-synchronous antibiotics. However, among neonates with LOS, the median day of life at the time of neonatal sepsis onset of neonates prescribed guideline-divergent antibiotics was nine (IQR 5–17.75) compared to four (IQR 4–4) in those prescribed guideline-synchronous antibiotics (Guideline-divergent antibiotics were prescribed for 10% (31/325) of neonates treated empirically for sepsis. The use of guideline-divergent antibiotics was more common in neonates treated for LOS than in those with EOS . Neonates with an EGA < 28 weeks had the highest proportion of guideline-divergent antibiotic use. Most neonates treated for sepsis had a blood culture collected . There was no difference in the rate of blood culture collection between those prescribed guideline-synchronous versus guideline-divergent antibiotics  (Neonates prescribed guideline-divergent antibiotics had a lower median EGA (31 weeks) as compared to neonates prescribed guideline-synchronous antibiotics  .In bivariate analysis, the likelihood of guideline-divergent prescribing was associated with the duration of neonatal unit stay, maternal age, and birth weight. There was no difference in the likelihood of guideline-divergent prescribing based on infant sex, HIV exposure status, delivery mode, or discharge status.p = 0.3551); the mortality rate was 16% in neonates prescribed guideline-divergent antibiotics compared to 10% in neonates prescribed guideline-synchronous antibiotics.The use of guideline-divergent antibiotics was not associated with in-hospital death  . When weIn this cohort of hospitalized neonates in a tertiary hospital in Botswana, a middle-income country, we found a relatively low rate (10%) of guideline-divergent antibiotic use based on first-line prescribed antibiotics. However, specific groups of neonates, those with low EGA and LOS, had higher odds of treatment with guideline-divergent antibiotics, which was aligned with our observation that nearly 40% of organisms isolated from blood cultures were resistant to guideline antibiotics.Previous studies evaluating guideline-divergent antibiotic use for neonatal sepsis, were limited to EOS ,25,26. TPrematurity is a major risk factor for sepsis ,27,28,41Guideline-divergent prescribing has been categorized as “inappropriate” in some settings, and certainly, inappropriate prescribing is occurring in LMICs, where antimicrobial stewardship oversight is often inadequate. However, the associations we identified of time of neonatal sepsis onset and EGA with guideline divergence likely reflect a thoughtful risk calculation on the part of the prescribing clinician, one that incorporates available clinical and laboratory data and a rising concern for nosocomial sepsis due to multidrug-resistant pathogens with each hospital day. Given the existing critiques of the WHO guidelines as being under-representative of LMIC settings and growing AMR, these identified associations may be incorporated into future antibiotic-prescribing algorithms tailored to LMIC settings, which consider both local epidemiology and clinical risk factors.The present study had limitations. The small sample size may have limited the detection of significant effects of certain variables, resulting in the exclusion of these variables as controlling factors in regression analysis. A lack of data prevented us from examining the frequency of guideline-divergent dosing or duration of antibiotics. Similarly, we are unable to comment on provider rationale for guideline-divergent antibiotic choice. Local system factors, such as antibiotic shortages, concurrent outbreaks of multidrug-resistant infections, and prescriber details, may have influenced clinicians’ decisions when prescribing antibiotics for neonatal sepsis, but data on these are either not collected or were not available. Additional patient-level factors and perinatal history that could have been accounted for, including signs and symptoms at initial presentation or leading to sepsis evaluation, potentially introduced residual confounding. Likewise, maternal factors and symptoms were not available. This study was also impacted by intermittent enrollment and data collection pauses due to the COVID-19 pandemic, possibly introducing selection bias. Lastly, due to the complex drivers of neonatal mortality, we have insufficient power to examine the independent impact of guideline-divergent prescribing on mortality or other patient outcomes.Antimicrobial resistance significantly impacts neonatal sepsis patients’ outcomes and influences, as well as results from, guideline-divergent antibiotic use. As suggested by other groups ,22,28,43In conclusion, we observed an association between time of neonatal sepsis onset and lower EGA with increased use of guideline-divergent antibiotic regimens for neonatal sepsis. Local factors, such as antibiotic shortages and rates of multidrug resistant infections prevalent in the neonatal unit, may have influenced the decisions made by healthcare providers when prescribing antibiotics. Since the time of this study, local antibiotic guidelines have been published to allow for adapting to the hospital antibiogram , however


IgG4-related diseases are characterized by marked infiltration and IgG4-positive plasma cells and fibrosis, and involve multiple organs. However, IgG4-related mastitis is rare. We report a case of mastitis associated with IgG4-related disease.A 78-year-old woman visited our hospital with a complaint of multiple subcutaneous nodules. A biopsy of a dorsal subcutaneous mass was performed but did not yield a definitive diagnosis. However, blood tests showed a high level of IgG4, thus she was referred to the department of collagen disease for further examination. Computed tomography (CT) showed a thickening of the skin of the right breast, and the patient was referred to our department. On physical examination, a large area of thickened skin was observed in the right breast without inflammatory breast cancer-like redness, and no mass was palpable. A needle biopsy was performed on an indistinct hypoechoic area in the breast, and she was diagnosed with mastitis associated with IgG4-related disease. Systemic steroid therapy was then administered and the symptoms of multiple skin nodules and mastitis improved.We reached the diagnosis based on a biopsy of the mammary gland enabling the patient to begin treatment for IgG4-related disease. This case was characterized by breast skin thickening, which is different from inflammatory breast cancer. IgG4-related diseases are a relatively new disease concept proposed in the 2000s, and are characterized by marked infiltration of lymphocytes and IgG4-positive plasma cells and fibrosis , 2. SingHerein, we report a case with mastitis as the main feature associated with IgG4-related disease which showed skin thickening.A 78-year-old woman visited the dermatology department of our hospital with a complaint of multiple subcutaneous nodules on her face, upper arms, and back. A needle biopsy was performed on a subcutaneous mass on her back. The histological assessment showed a lymphoid-like structure with slight infiltration of IgG4-positive plasma cells, but no definitive diagnosis was made. However, blood tests showed elevated serum IgG4  and positive antinuclear antibody , while anti-SSA/Ro antibody was negative, suggesting IgG4-related disease. Hence, she was referred to the department of collagen disease for further checkup. A whole-body computed tomography (CT) revealed a thickening of the skin of the right breast . After initiation, the right brachial nodule reduced in size and other nodules gradually disappeared. She has been in remission for 6 months, with 15 mg/day of PSL to date.According to previous reports, half of the cases of IgG4-associated mastitis were asymptomatic, i.e., mass without pain or redness, while the remainder had breast induration and mastalgia , 5, 6. IThe symptoms of IgG4-associated mastitis in this case were minor, and as far as mastitis alone is concerned, treatment may not have been necessary. IgG4-associated mastitis is not thought to increase the risk of breast cancer , 8. HoweWe reached the diagnosis of IgG4-related disease based on a biopsy of the mammary gland allowing the patient to start treatment. This case was characterized by breast skin thickening, which is obviously different from inflammatory breast cancer. Mastitis with atypical symptoms should be carefully considered for biopsy given the possibility of this disease. We believe that the skin findings observed in this case should be noted as a finding of IgG4-related mastitis.


Fiber optic sensors (FOS) have been widely explored in recent years for concrete durability monitoring due to their advantages of high sensitivity, immunity to harsh environments, small size, and superior sensitivity. The purpose of this review is to summarize FOS development and its application in concrete durability monitoring in recent years. The objectives of this study are to (1) introduce the working principle of FOS, including fiber Bragg grating (FBG), long-period fiber grating (LPFG), surface plasmon resonance (SPR), fluorescence-based sensors, and distributed fiber optic sensors (DFOS); (2) compare the sensitivity, resolution, and application scenarios of each sensor; and (3) discuss the advantages and disadvantages of FOS in concrete durability monitoring. This review is expected to promote technical development and provide potential research paths in the future for FOS in durability monitoring in smart concrete.Concrete is the most commonly used construction material nowadays. With emerging cutting-edge technologies such as nanomaterials , advanced sensing , and artificial intelligence, concrete can now achieve self-sensing, self-healing, and ultrahigh performance. The concept and functions of smart concrete have thus been partially realized. However, due to the wider application location  and changing climate (temperature increase, more CO Concrete is the most consumed construction material in the world. A total of 4.1 billion metric tons of cement were utilized to build concrete structures including buildings, bridges, and tunnels every year from 2018 to 2020 [The durability of concrete refers to the ability to resist deteriorating factors. The harmful consequences would be reduced and even avoided if real-time information on concrete durability could be obtained for guiding daily maintenance and providing timely alarm . TherefoThe sensor is the key unit in the SHM system to provide condition-related data for analysis . ElectriThe purpose of this review is to systematically summarize the recent progress and potential applications of FOS in monitoring durability-related parameters of concrete. The pros and cons of each type of sensor are thoroughly analyzed. Potential research paths in the future are also pointed out. The objectives of this work focus on the durability monitoring for concrete including relative humidity (RH), temperature, chloride ion penetration, corrosion, and pH. High humidity will accelerate concrete carbonation and steel corrosion, which will decrease the service life of the concrete structures. Massive heat is produced in the process of concrete hydration, which leads to thermal cracks, especially for mass concrete. Thus, it is important to monitor the temperature of concrete during and after the pouring process to avoid thermal cracking or other effects on the hydration process. Chloride ions from de-icing salts or marine environments will activate the depassivation of the passive film and accelerate the corrosion process of the steel bar in the reinforced concrete ,9. Mass FOS can be embedded in or attached to the concrete to provide durability sensing capability for smart concrete. However, the sensing principle, packaging methods, sensing characteristics, and deployment of each sensor vary dramatically. Therefore, the novelty of this work is to categorize, compare and discuss the main FOS-based durability monitoring technology including fiber Bragg grating (FBG), long-period fiber grating (LPFG), surface plasmon resonance (SPR), fluorescence-based sensors, and distributed fiber optic sensors (DFOS). This review is supposed to provide recent technical developments and practical applications of durability monitoring for future smart concrete.FBG sensors are the earliest developed and most frequently used FOS in civil engineering because of their advantages of low cost, multiplexing capacity, small dimensions, and easy installation . There aAs shown in In an FBG sensor, the relationship between the reflected central Bragg wavelength ber core .The basic setup for FBG fabrication is shown in RH is the ratio of the partial pressure of water vapor in the air–water mixture at a given temperature, which can be defined as:RH. However, if a humidity-sensitive material can be coated on FBG, the material will swell or shrink with RH change, and then FBG could be utilized as an RH sensor because it can monitor the swell/shrink strain under different humidity conditions [RH environment and a commercial hygrometer is employed to detect the RH inside the box.Since all optical phenomena of FBG occur inside the fiber core, FBG itself does not have the capability of sensing nditions . Figure The developed FBG RH sensors are shown in FBG has been developed as a temperature sensor due to its small size and thermal invasion. aded FBG ,30 were Steel rebar corrosion is one of the most critical issues for the durability of reinforced concrete. Compared to the conventional strain gauge, immunity to chemical attacks and stability make FBG an ideal sensor for corrosion monitoring. The principle of the FBG corrosion sensor is to measure the corrosion-induced strain. There are three main methods to do so: one scheme is to deposit the FBG surface with a Fe-C film, which has similar corrosion properties to steel rebar. As shown in Even though many field applications of FBG-based concrete durability monitoring have been conducted and verified its effectiveness ,26,33,34(1) The FBG sensor itself can only do strain sensing, so all durability-related parameters need to be converted into strain. However, there are external loads on concrete structures that can cause strain, and the FBG sensor will also measure it, which affects the durability monitoring results. The cross-sensitivity among strain, temperature, and other durability parameters needs to be thoroughly considered in field applications.(2) Sensor design, installation, and protection of FBG is a critical issue for long-term concrete durability monitoring. Although optic fiber is made of high-purity silica, the packaging and coating material are usually not as durable as the fiber itself. The sensor service life for in situ tests must be considered.(3) The cost of the FBG sensor is acceptable with the advanced fabrication technology, but the interrogator is relatively expensive compared to other electrical-based data acquisition systems. Moreover, the FBG interrogator is usually fragile and has strict requirements on operation temperatures and RH, and another computer is needed for data storage. All these issues limit the application of FBG and other FOS.Compared to FBG, the LPFG sensor has a larger grating period ranging from 100 µm to 1 mm . The LPFth cladding mode can be obtained by the couple mode theory and phase matching condition, which could be calculated as [th cladding mode, which is partially determined by the RI of the surrounding medium, and As shown in lated as :(3)λ=nco2 laser is usually the laser source for grating, and OSA is utilized to monitor the LPFG fabrication process.As shown in The most commonly used experimental setup in the lab to investigate the performance of the LPFG humidity sensor is shown in Polyvinyl alcohol ,46,47, PHan  exploredTang , Bey 7676, and PTo monitor the steel bar corrosion inside the concrete in the incipient corrosion stage, an LPFG-based corrosion sensor has been proposed and developed in recent years. H. Liu  designedFe-C-coated LPFG-based mass-loss measurement is another scheme to monitor the corrosion process since the Fe-C coating has approximately the same chemical component ratio as the steel member or rebar embedded in concrete. Thus, the corrosion process of the Fe-C coating could be utilized to reveal the corrosion process if the SPR fiber sensor was coated with Fe-C and attached to the steel bar.Y. Huang ,84, Y. CThe high alkalinity of concrete prevents the embedded steel rebar from corrosion by forming a passive film on the rebar surface. The decrease in pH caused by chloride ion ingression, acid attack, and carbonation reaction may lead to steel corrosion and concrete spalling, which makes pH a significant parameter for concrete durability. The traditional pH measurements, including chemical indicators and in situ leaching methods , are desCompared to FBG, LPFG has the unique capability of sensing the RI of its surrounding medium, which is superior for durability monitoring. However, similar to FBG, LPFG also has limitations in cross-sensitivity, packaging, and interrogator cost. Furthermore, LPFG has its own issues that need to be addressed for smart concrete.(1) Compared to the phase mask fabrication method of FBG, LPFG fabrication is relatively complicated since it needs CO2 laser point-by-point inscription or acid etching. These fabrication processes usually are expensive in terms of equipment, and the grating accuracy and robustness are difficult to control, which limits LPFG production at the commercial level. A more accurate and efficient fabrication method for LPFG is in urgent need.(2) Different from FBG, LPFG sensing depends on the transmission spectrum, which means that two ports are needed for LPFG interrogation. This will increase the workload and installation difficulty for LPFG sensors, especially for deployment in concrete. This is also the reason that the above research has not applied the LPFG sensor in real concrete structures. Further study on LPFG packaging and in situ concrete durability monitoring needs to be conducted.SPR refers to the resonant oscillation of conduction electrons at the interface between negative and positive permittivity material stimulated by incident light. Light in optical fiber transmits with total internal reflection, which makes the SPR principle of prism coupling suitable for optical fiber . This coThe SMS structure is a widely used SPR sensor, which is produced by splicing a multimode fiber (MMF) between two singlemode fibers, as shown in The MSM structure is produced by splicing a singlemode fiber between two multimode fibers, as shown in The fabrication procedures of the SMS structure are as follows: the cladding of SMF is first stripped, the MMF is intercepted to the specified length, and the fibers are spliced in sequence with splicer . The fabVarious setups can be applied in the SMS humidity sensor to improve the sensitivity. J. An , X. WangThe SPR temperature sensor has attracted much attention and has been widely developed. Researchers adopted four methods including chemical etching, coating, changing shapes of fiber, and altering structures to enhance the sensitivity of SPR fiber sensors to temperature.It can be seen in pH-sensitive hydrogel coating was widely used in SPR pH sensors for providing an in situ and non-destructive manner to monitor pH. Even though SPR sensors have been used in biological monitoring, there are few in situ applications in civil engineering, such as embedding in concrete for durability monitoring. Similar to LPFG, one of the main obstacles impeding the SPR sensor to wider application is the complex fabrication procedure. The SMS or MSM structures make it hard to achieve standard and efficient production. That is why most research is still in the lab stage.The fluorescence-based fiber optic sensor is a simple but effective inspector in concrete durability monitoring because of the unique working principle and capacity of on-field and real-time monitoring . The widThe basic setup in the lab for the fluorescence-based sensor consists of a laser source, a Y-type optical fiber, an optical fiber spectrometer, and a detector containing fluorescent material, as shown in Fluorescence-based temperature sensors utilize a single fiber to transmit both excitation and emission signals. The advantage of self-calibrating  attracts148.8 °C , and theto 80 °C . As showto 80 °C  developeMany fluorescence-based pH sensors have been explored and developed in recent years. As shown in Compared to the previous FOS, the advantage of a fluorescent sensor is that it has ion selectivity, which means the sensor can differentiate chloride, sulfate, OH, and other ions. This is crucial for durability monitoring since different ions have different effects on concrete. However, the service life of fluorescence-based optical sensors is limited by the short life span of the fluorescence material, which might make it fail to satisfy the requirement of long-term concrete durability monitoring. The main reason is that the fluorescent material will deteriorate; thus, the fluorescent intensity will decrease with time.The sensing principle of distributed fiber optic sensing is sending a laser pulse along a fiber optic cable and then measuring the laser that is reflected or backscattered. There are three types of backscattering used for measurement: Raman, Brillouin, and Rayleigh .Raman backscattering is mainly used in DTS (distributed temperature sensing). A DTS instrument sends a short laser pulse along a fiber optic cable and measures the laser scattered back in the fiber cable. Most backscatter has the same wavelength as the original laser pulse. The other backscatter shows the effect of Raman backscattering. The DTS instrument measures the intensity of the two backscatters, Stokes scatter and anti-Stokes scatter, of which Stokes scatter has a longer wavelength and anti-Stokes scatter has a shorter wavelength. The intensity of the anti-Stokes scatter is sensitive to the temperature .Brillouin backscattering is a basic physical process representing the interaction effect between light and optical medium in a propagation medium. Brillouin optical time domain reflectometry (BOTDR) and Brillouin optical time domain analysis (BOTDA) are two commonly used distributed fiber optic sensing based on Brillouin backscattering .BOTDR is a single-ended access technology, and it is based on spontaneous Brillouin scattering (SpBS). When an incident light propagates forward, it will produce spontaneous Brillouin backscattered light along the fiber, as shown in BOTDA is a more complicated form of BOTDR, and it is based on the Stimulated Brillouin Scattering (SBS) effect. Optical frequency domain reflectometry (OFDR) operates based on Rayleigh backscattering, which is caused by the interactions between the fluctuations in the density of the fiber optic core and the light. The fiber cable extends or contracts with the change in temperature or strain. Then, the distance between the imperfections in the fiber cable changes and will cause a change in the frequency of the backscattered light .Distributed fiber optic sensors can provide information along the entire fiber length, which makes this technology suitable for monitoring corrosion in long lengths . In manyAs corrosion sensors, the Brillouin backscattering distributed fiber optic sensors are installed mainly in two methods. The first method is to tightly install a singlemode optical fiber around a steel bar ,147, as Apart from Brillouin backscattering, Optical frequency domain reflectometry (OFDR) is another widely used distributed fiber optic sensing technology used in corrosion monitoring. The OFDR technology is applied mainly in three methods. The first method is close to the application of Brillouin backscattering distributed fiber optic sensor in corrosion monitoring. In this method, a singlemode optical fiber cable is tightly attached to the surface of the steel bar to monitor the strain changes caused by the volume expansion of the corrosion product ,150.Cracks before the concrete gets loaded are mainly induced by the differential and excessive shrinkage . To gainDistributed fiber optic sensors have the unique capability of providing distributed strain measurements along the whole length of the fiber optic cable, which makes them ideal for monitoring the durability of large structures. Additionally, the distributed sensors are cheaper compared with FBG and LPFG since grating fabrication is not needed. Brillouin and Rayleigh are the main two backscatterings used for measurement in durability monitoring. The sensitivity and monitoring range can be increased with the optimization of the transfer coefficient between the volume expansion of corrosion products, shrinkage, or crack-induced strain and the fiber cable.In this review, the sensing principle, fabrication methods, durability monitoring capability, advantages, and disadvantages of five main types of FOS are comprehensively summarized. The limitations of each sensor are discussed. In general, different types of FOS have their own advantages in specific parameter monitoring. LPFG and SPR sensors showed superiorities in RH, chloride, and corrosion monitoring since they possess high sensitivity and a larger sensing range for RI. FBG sensors have been widely used in strain and temperature monitoring. In terms of ion and pH monitoring, the fluorescence-based sensor is a promising technology due to its unique property of ion selectivity. The DFOS sensor has a unique advantage in long-distance monitoring for large-scale concrete structures compared to other FOS, especially for corrosion, shrinkage, and crack monitoring.In recent years, lots of research has been carried out to develop a durability monitoring sensor using optic fiber. The concept of smart concrete has thus been proposed since the FOS can provide critical information for daily maintenance, increasing the service life and avoiding possible catastrophic accidents. To further achieve smart concrete, the FOS still has several issues that need to be addressed, and these might provide research insights for future study.(1) Fabrication: the FBG and LPFG sensors are fabricated through laser inscription. SPR sensors are fabricated through the splicing of single- or multimode fiber or side-polishing. A fluorescent sensor needs to synthesize the chemical material. So far, only the FBG sensor fabrication has achieved the commercial level for high-efficiency production. The other types of sensors are still fabricated in the lab in limited quantities. This is the reason that we only see FBG sensors embedded in concrete for in situ monitoring. The next step of FOS development needs to improve the fabrication efficiency to meet a large amount of sensor demand for smart concrete.(2) Packaging: even though FOS has advantages in size and sensitivity and is immune to electromagnetic interference, it is fragile under tension, bending, or impact. To make the sensor stable and durable, packaging is the most essential procedure before application. However, the packaging material itself also needs to be durable. Steel or plastic might not be suitable for harsh environments such as corrosion, UV light, and high temperatures. Ceramic has also been used for sensor packaging in recent years, but its ductility is relatively low. Therefore, an ideal packaging material is urgent for FOS. On the other hand, the packaging size also matters. The mechanical manufacturing of steel or plastic has difficulty when the required size is small or irregular. Three-dimensional printing is a very promising path in this field.(3) Multi-parameter monitoring: the durability of concrete involves many parameters such as crack width, moisture, pH, corrosion, and temperature. If the FOS can only monitor one parameter, the sensing system, connection wires, and data acquisition module will be so bulky that it is difficult for engineers to manipulate. By utilizing the difference in sensitivity, wavelength multiplexing, and advantage in small size, FOS has the potential to achieve multi-parameter monitoring, which can not only simplify the sensor installation and administration but also reduce the average cost of the entire system since one interrogator can provide more data.(4) Distributed sensing: the point FOS is so small compared to the concrete structures, especially long-span bridges, long-distance tunnels, and tall buildings. One way to address this issue is using a sensor array. By connecting lots of FOS in one loop, or fabricating hundreds, even thousands of sensors (grating or SPR) on one optic fiber, distributed sensing of large-scale concrete can be achieved. However, the sensor array path requires a high-performance interrogator for data acquisition. Another strategy is utilizing distributed fiber optic sensors (DFOS). As discussed in (5) Concrete embedment: to the best of our knowledge, not all types of FOS discussed in this work have been embedded in concrete for in situ monitoring. The main problem in real applications is that some sensors or optic fibers cannot survive during the concrete vibration and consolidation process. As discussed in the packaging section, part of the reason is that the packaging material cannot provide enough protection to the fragile FOS. Moreover, the normal communication optic fiber is commonly used for signal propagation due to its thin coating and small diameter. However, this kind of fiber is fragile to impact, squeeze, and shear force. Once the fiber is broken in one spot, the entire sensing loop is destroyed, even though the sensor itself might be functional. Some may argue that more rugged fiber cables can be used, such as an armored cable with steel or aluminum protection. However, the larger diameter of an armored cable may affect the performance of the concrete. When the fiber cable is attached to the steel rebar, the fiber cable may reduce the adhesion between the concrete and rebar. Therefore, even though FOS shows promising potential in concrete durability monitoring, the deployment of the sensor inside the concrete still needs more effort in both scientific and engineering considerations.