The increasing annual emissions of iron ore tailings have proved a great threat to the natural environment, and the shortage of natural river sand, as well as the pursuit of sustainable development materials, provides motivation to reuse iron ore tailings as a fine aggregate in concrete. Due to the significantly different properties of iron tailings sand compared with natural river sand—such as the higher density, higher content of limestone particles smaller than 75 μm and its rough and angular shape—concretes prepared with iron tailings sand show remarkably higher shrinkage. This study presents the shrinkage characterization and shrinkage-reducing efficiency of three different methods on iron tailings, sand concrete and river sand concrete. The internal humidity was also monitored to reveal the shrinkage-reducing mechanism. The obtained results indicated that the autogenous and total shrinkage of iron tailings sand concrete were 9.8% and 13.3% higher than the river sand concrete at the age of 90 d, respectively. The shrinkage reducing agent (SRA) was the most effective shrinkage reducing method for river sand concrete, while for iron tailings sand concrete, super absorbent polymer (SAP) and controlled permeable formwork liner (CPFL) it worked best on autogenous shrinkage and drying shrinkage, respectively. Furthermore, the shrinkage mitigation strategies worked earlier for the drying shrinkage behavior of iron tailings sand concrete, while no such condition could be found for autogenous shrinkage. In recent years, the worldwide mining of ferrous and other metallic ores has been increasing to answer the ever-growing need for various alloys used in daily life. With the rapid development of iron and steel industry in recent years all around the world, iron ore tailings, which result from ore dressing, account for an increasing proportion of industrial solid waste. It is estimated that for each tonne of beneficiated iron ore, 400 kg of tailings are produced . It was Recent researches have shown that iron tailings sand has potential to produce concrete by replacing river sand. Ugama et al.  investigThe application of iron tailings sand in normal or high-performance concrete was also investigated. Zhang et al.  investigThe majority of research on iron tailings sand for concrete infrastructures focuses on mechanical and durability performance, while experimental studies on shrinkage properties are rarely found. However, shrinkage properties are vital to the service life of concrete structures, since high shrinkage may induce cracks, which would intensify the intrusion of aggressive substances ,16,17. I2/kg, P·O 42.5, confirming to Chinese standard GB175-2007 was obtained from Chongqing Fuhuang company. Ground granulated blast furnace slag (GGBFS) with a specific surface area of 430 m2/kg was also provided by this company. Fly ash was supplied by the Chongqing Luohuang power plant. The chemical composition of cement, GGBFS and fly ash were characterized by the X-ray fluorescence (XRF) method, and is shown in Portland cement with a Blaine fineness of 350 m3.The coarse aggregate used was crushed limestone with two grades (5–10 mm and 10–20 mm), mixed with a ratio of 4:6 by mass. One of the fine aggregates was river sand with fineness modulus of 3.10, and its sieving curve is reported in 3 and 3.10, respectively. Its sieving curve is presented in 2O3 content as high as 16.96%, which resulted in a significantly higher bulk density than river sand. The X-ray diffraction (XRD) spectrograms  of the iron tailings sand in Iron tailings sand was provided by a mining group in Shangluo, Shanxi province. The bulk density and fineness modulus of iron tailings sand were 3.03 g/cm2-CH(CO2Na)]. All the particles passed the sieve of 200 mesh. Each gram of SAP can absorb about 300 mL of water. The SRA was obtained from Sobute New Materials company , and its recommended dosage is 0.6~1.2% (percent mass of the binder content). The CPFL used in this study was provided by Huimin, Shengzhen, China. The appearance and properties of CPFL were shown in The SAP  was a commercial product supplied by a materials company in Jiangsu province. The main composition was cross-linked sodium polyacrylate [-CHThe compressive and flexural strength of concrete were tested at ages of 7 d and 28 d, and the dynamic modulus of the elasticity of concrete was also measured at 28 d. The samples for the compressive strength test are cubes with a dimension of 100 mm × 100 mm × 100 mm. All those results were the average of three concrete samples.The drying shrinkage of concrete was determined according to Chinese standard GB/T 50082-2009. Iron tailings sand and river sand concrete was cast in 100 mm × 100 mm × 515 mm steel molds and vibrated on a vibrating table until no air bubbles appeared on the surface, followed by covering with plastic film to avoid water evaporation. The concrete specimens were cured in a room with a temperature of 20 ± 2 °C and a relative humidity (RH) of 95%. After 1 d, concrete samples were demolded and subsequently cured in the same environment for another 2 d. Then, the specimens were transferred into a room with a temperature of 20 ± 3 °C and an RH of 60 ± 5%. The length changes along the longitudinal axis were measured immediately by a dial gauge.The autogenous shrinkage followed the same procedure with the drying shrinkage test, while concrete samples were sealed by Teflon film after 3 d of standard curing, and the initial length was also recorded after the specimens were transferred into a room with a temperature of 20 ± 3°C. The shrinkage value of autogenous and drying shrinkage of concrete were the average of three samples.3, KCl, NaCl and Mg(NO3)2) with a known constant RH in the range of 55–100% RH. The accuracy of the sensors was within ±1.8% RH. The average of two replicate measurements was reported.An internal RH test was carried out on 100 mm cubic samples by using a humidity sensor . Before Rapid chloride penetration tests were carried out according to ASTM C1202. Cylindrical samples with a diameter of 100 ± 1 mm and a height of 50 ± 2 mm were used to determine the resistance to chloride penetration by total charge passed through method at the age of 28 d.3) is larger than the river sand (2.68 g/cm3); it could be calculated that the volume ratio of fine aggregate in concrete decreased from 27.8% to 24.0%, when the river sand was replaced by the iron tailing sand of the same mass. Therefore, it results in a higher shrinkage for iron tailings sand concrete, since sands are dimensionally stable. The second reason is that the cement paste content was higher in concrete made by iron tailings sand due to its lower volume ratio, which could also account for the higher shrinkage deformation of iron tailings sand concrete. The third is a higher content of particles smaller than 75 μm, which was identified as calcite in the XRD spectrograms of the iron tailings sand. It was reported that limestone could provide a lower nucleation energy barrier compared with quartz during the nucleation process of C-S-H, and also provide additional nucleation sites, thus accelerating the hydration rate of cement [The autogenous shrinkage of concrete includes volume deformation caused by chemical shrinkage and self-desiccation ,27. The f cement ,29,30. TThe influence of SAP and SRA on the autogenous shrinkage of river sand concrete and iron tailings sand concrete are shown in The impact of shrinkage mitigation strategies on the total shrinkage of two types of concrete was presented in The total shrinkage includes the autogenous shrinkage and the drying shrinkage. The reduce of total shrinkage caused by the application of SAP and SRA could mainly be ascribed to the decrease of autogenous shrinkage. SAP shows a superior desorption capacity, which can release water during cement hydration, such that this water can compensate water consumed by hydration, and thus increase the internal RH of the concrete and reduce the autogenous shrinkage ,32. The The development of internal RH is a result of dissolved salts in the pore solution and curvature of the fluid/vapour menisci ,39. IonsFor the drying environment, apart from the water consumed by cement particles hydration, the water exchange with the environment would also cause moisture loss ,43. The Since SAP was added in powder form, it could absorb water during concrete mixing. As the internal RH dropped, water was released from SAP’s reservoir, thus the internal RH decrease would be delayed. The SRA could also improve the internal RH by slowing down the evaporation rate of water. The application of CPFL was cable of enhancing the quality of concrete in the cover region and improve the permeability of concrete, thus reducing water loss and increasing the internal RH.The impacts of shrinkage-reducing methods on 28 d compressive strength is presented in The sieving curves and fineness modulus of the iron tailings sand were similar with the river sand, and it can be utilized as a complete replacement of the river sand to produce concrete with comparable workability, and a higher mechanical strength and elasticity modulus. Meanwhile, the utilization of iron tailings sand can eliminate its negative effects on the environment, and protect the river channels through the preservation of the river sand. However, the shrinkage of iron tailings sand concrete was significantly larger than the river sand concrete, which may cause a high risk of cracking and shorten the service life of buildings. Therefore, it is necessary to study strategies which can mitigate the autogenous and drying shrinkage of iron tailings sand concretes. This study presents the results and analysis conducted to investigate the effects of the application of SAP, SAR and CPFL on shrinkage characteristics of the iron tailings sand concrete. From the results obtained, the following conclusions can be drawn:1. The river sand was replaced by the iron tailings sand of the same mass in this study. Under this condition, the autogenous and total shrinkage of the iron tailings sand concrete were 9.8% and 13.3% higher than the river sand concrete at the age of 90 d, respectively, which could be ascribed to the accelerated hydration rate brought by the small calcite particles, lower volume ratio and poor water-retaining property of the iron tailings sand. 2. Approximately 30% and 18% reduction in the autogenous shrinkage of iron tailings sand concrete was obtained at 90 days by incorporation of SAP and SRA, respectively. For total shrinkage of iron tailings sand concrete with SAP and SRA, a similar reduction (25% and 21%) was obtained at 90 days, while a much higher reduction (41%) was obtained using CPFL. Comparing shrinkage behaviors of the river sand and iron tailings sand concrete, it could be found that the SRA was the most efficient shrinkage-reducing method for the river sand concrete, while for the iron tailings sand concrete, SAP performed better on autogenous shrinkage and CPFL had the most prominent reducing effect on total shrinkage. Furthermore, the shrinkage mitigation strategies did not make any noticeable difference for the total shrinkage behavior of river sand concrete before 14 days, while far more obvious effects could be observed in iron tailings sand concrete after 3 days.3. The application of SAP, SRA and CPFL was effective in reducing the autogenous and total shrinkage of iron tailings sand concrete, and they had no negative impacts on the mechanical property and chloride penetration resistance of concrete. Therefore, through implementing some measures, concretes with properties no worse than river sand concrete could be produced by using iron tailings sand.


According to the study findings, 55.7% of respondents prescribed a 5-day antibiotic course. Moreover, Amoxicillin 500 mg was the first-choice antibiotic for 55.1% of the respondents, followed by Clindamycin 600 mg (18.4%). For patients allergic to penicillin, 61.4% of respondents prescribed Clindamycin. Statistically significant differences emerged only in relation to acute apical abscess with systemic involvement, whereby dentists aged 46–55 were least likely to prescribe antibiotics in these clinical situations (p = 0.04). Analyses further revealed that recommendations for safe antibiotic prescribing practices were not always followed, as in certain cases, patients were given antibiotics even when this was not indicated. These findings highlight the need for additional education on responsible antibiotic use to prevent bacterial resistance.The study goal was to provide an overview of antibiotic prescribing practices of Serbian dentists when treating endodontic infections and to disseminate the current ESE (European Society of Endodontology) recommendations to the study participants. A link to an online questionnaire was sent to 628 Serbian dentists whose email addresses were publicly available on the Internet, 158 of whom responded to the survey, resulting in a 25.16% response rate. The significance of possible associations was assessed via the Chi-squared test and Cramer’s V measure of association, with  Excessive antibiotic use and consequent bacterial resistance are significant global problems . Given tIt is well known that antibiotics are ineffective in reducing pain or swelling of odontogenic origin in the absence of systemic signs of infection . An amplhttps://docs.google.com/forms/d/14J0BNQ6UWILc1cQCQoP4FSqIDmBLJ_Dd6xglALKfm-0. As the Law on Personal Data Protection  prohibits use of the Serbian Dental Chamber records for identifying study participants, only dentists whose email addresses were publicly available on the Internet were contacted and invited to take part in the survey. This resulted in an initial sample of 628 potential respondents. The survey items were adapted from previously published questionnaires with the permission of their authors [® free online survey was utilized as it facilitated direct input of the survey data into a spreadsheet, thus aiding in subsequent analyses. Prior to its application, the questionnaire . To obtain the data necessary for meeting the study objectives, an online questionnaire was developed and was made available to Serbian dentists via the following link:  authors . The queionnaire  was revi® 25.0 . Chi-squared test of independence was conducted to examine the potential relationships between nominal variables, with p < 0.05 considered statistically significant. In addition, Cramer’s V coefficient was calculated to assess the magnitude of correlations between nominal variables and as a measure of effect size when the Chi-squared test yielded statistically significant results.Statistical analyses were performed using SPSSp = 0.009), which was expected, as specialization in any branch of dentistry is a time-consuming process. With regard to the duration of antibiotic treatment they recommended to their patients, most of the participating dentists stated that they prescribed a 5-day antibiotic course  completed questionnaires. According to surveysystem.com, this corresponded to a confidence interval of 6.75% at a 95% confidence level. The sample comprised of 92 female and 66 male dentists. Nearly half (48.7%) of the respondents were aged 36–45 years, while those aged 56–65 were the least represented (10.8%). When asked to indicate their educational attainment, 50.6% of the respondents indicated that they possessed a Doctor of Dental Medicine (DDM) undergraduate degree, 13.3% held a Master’s degree in Endodontics, and the remaining 36.1% held a master’s degree in other branches of dentistry. Further analyses revealed a positive relationship between academic degree and age  of the initially contacted dentists (628) responding to the survey, this sample size ensured a confidence interval of 6.75% at the 95% confidence level [This was the first study in which Serbian dentists’ antibiotic prescribing practices for endodontic infections were examined quantitatively. As indicated earlier, pertinent data were obtained via an online survey comprised of survey items developed by other authors, which were replicated or adapted with their permission ,15. Accoce level . Unfortuh 158 25.% of the While the relatively small sample size is a notable limitation of this study, we were legally not permitted to reach out to the full population of Serbian dentists through the Serbian Dental Chamber. A further limitation stems from the self-reported nature of the data, as its veracity could not be established. Consequently, all study findings need to be interpreted with caution.The majority of the respondents held a Doctor of Dental Medicine (DDM) undergraduate degree in dentistry, with a much smaller percentage of those having a master’s degree (either in endodontics or other branches of dentistry). As specialization in any branch of dentistry requires at least three years of additional education, as expected, older participants were more likely to hold a postgraduate degree.In the treatment of endodontic infections, use of antibiotics is indicated in a small number of clinical situations, namely for treating acute apical abscesses in medically compromised patients, acute apical abscesses with systemic involvement, progressive infections, and persistent infections . ConsideStaphilococcus aureus, it is often prescribed in combination with clavulanic acid. In addition, β-lactam antibiotics  are recommended by the European Society of Endodontology as the first option in the treatment of endodontic infections in non-allergic patients [Amoxicillin was the first-choice prescription for patients without an allergy to penicillin for 55.1% of the respondents, irrespective of age or academic qualifications . This fipatients . In SerbFusobacterium and Prevotella lineages from dentoalveolar infections were resistant to these antibiotics [According to the survey results, Clindamycin (600 mg) was the first-choice antibiotic in the treatment of endodontic infections in patients who are allergic to penicillin, irrespective of the dentist’s age and qualifications . This pribiotics .The present survey also inquired into the dentists’ antibiotic prescribing practices for various pulpal and periapical conditions . AccordiEven though it is known that antibiotics are ineffective in treating pulp necrosis , 3.2% ofIn cases of acute apical periodontitis with spontaneous pain, pain on percussion and biting, and widening of periodontal space, systemic antibiotic treatment is not required . HoweverSimilar percentage (11.5%) was reported by Bolfoni et al. , while SEven though acute apical abscesses with no systemic involvement characterized by localized fluctuant swelling do not require systemic usage of antibiotics, but rather root canal treatment , 31% of locus minoris resistentiae), antibiotic use is indicated because systemic diseases result in impaired immunologic function [In acute apical abscesses in medically compromised patients (immunocompromised patients, patients with function . In the function .Antibiotics are essential in the treatment of acute apical abscesses with systemic involvement ; hence, According to the survey findings, 79.7% of the respondents utilized systemic antibiotic therapy when treating progressive infections , and thiPersistent infection, manifesting as chronic exudation that is not resolved by regular intracanal procedures and medications, also requires antibiotic treatment . HoweverFor patients experiencing postoperative pain, antibiotics are not indicated; however, they are prescribed by 6.3% of the survey respondents . PostopeIt is encouraging that, according to this survey, the majority of dentists in Serbia prescribe antibiotics in clinical situations in which this is warranted. It is also noteworthy that a small percentage of respondents prescribed antibiotics for treating irreversible pulpitis, pulp necrosis, acute and chronic apical periodontitis, and acute apical abscess with no systemic involvement . On the Finally, it is reassuring to note that only 1.3% of our survey participants routinely prescribed antibiotics for patients undergoing endodontic treatment, irrespective of the diagnosis .Hence, raising the awareness of Serbian dentists of correct antibiotic use in the treatment of endodontic infections is an important step in the global fight against AMR. Such information dissemination should start during their undergraduate training and regular updates should be made available to all practicing dentists. In undergraduate education, the severity of the AMR phenomenon should be emphasized and the guidelines for appropriate antibiotic use should be frequently reiterated. Similarly, conferences and seminars, along with online repositories, are ideal platforms for ongoing education of dental practitioners at all levels.Despite a relatively small sample size and the self-reported nature of the data analyzed as a part of this investigation, it can be tentatively concluded that a significant percentage of Serbian dentists prescribe antibiotics responsibly. Unfortunately, recommendations are not always followed, and in certain cases, patients are given antibiotics even when this is not indicated. The disparity between the actual and recommended prescribing practices supports the need for additional education on responsible antibiotic use. We hope that by distributing the ESE guidelines to the study participants, we have contributed to this ongoing endeavor.


Considering that the ultrasonic method is applied for the quality evaluation of concrete, this study experimentally and numerically investigates the effect of inhomogeneity caused by changes in the moisture content of concrete on ultrasonic wave propagation. The experimental results demonstrate that the propagation velocity and amplitude of the ultrasonic wave vary for different moisture content distributions in the specimens. In the analytical study, the characteristics obtained experimentally are reproduced by modeling a system in which the moisture content varies between the surface layer and interior of concrete. As many infrastructures deteriorate at an early stage, long-term structure management is required. If structural deterioration can be detected as early as possible, the scale of repair can be reduced, decreasing maintenance costs . In receThe propagation properties of elastic waves in a material vary depending on the density and elastic modulus ,16, and The enhancement of the measurement accuracy of the ultrasonic method, which has a relatively high detection accuracy, has been extensively researched ,19,20. IIn this study, mortar specimens without coarse aggregate were prepared and ultrasonic testing was performed. The objective of this study was to understand the effect of the water content of the specimen on the propagation characteristics of ultrasonic waves by storing the specimen in a thermostatic dryer and conducting ultrasonic tests during the transition from the saturated to dry state. In addition, by simulating the propagation of ultrasonic waves through finite element analysis with the numerical values obtained from ultrasonic testing, the effect of the internal and boundary conditions of concrete on the propagation characteristics of ultrasonic waves was investigated, and applicability of this method to concrete was examined.3) and crushed sandstone (2.60 g/cm3) were mixed at a volume ratio of 1:3.8 and W/C of 70%. To investigate the effect of internal voids on the ultrasonic propagation velocity and vibration reception waveform, the airflow of the sample was adjusted with an air-entraining agent, and the air content was set to 5%, 10%, and 14.5%, respectively. Concrete is a composite material comprising cement hydrates, aggregates, and voids, which affect ultrasonic-wave propagation. In this study, a mortar sample without coarse aggregate, which affects ultrasonic-wave propagation, was used mainly to investigate the relationship between the ultrasonic-wave propagation velocity and water content of the sample. The aim of this experiment is to understand the ultrasonic propagation properties of concrete with varying moisture content. Therefore, ultrasonic measurements were performed during a process where the specimen changed from wet to completely dry. The specimens were initially placed in water for at least a week, and the mass and ultrasound were measured after the specimens were wet. Further, the specimens were placed in a constant temperature drying oven at 40 °C, and ultrasonic measurements were performed periodically for a week until the specimens were completely dry. The moisture content of the specimens was calculated under wet and absolutely dry conditions. After removing the specimens from the constant-temperature drying oven to prevent the specimen temperature from affecting the ultrasonic measurements, the specimens were placed in a constant-temperature oven at 20 °C for 24 h, and measurements were performed after the specimen temperature was constant. Each ultrasonic wave propagation test was repeated twice.The measuring device shown in As shown in t per step of the analysis. Considering the computer performance and analysis time, we used the minimum number of steps without affecting the analysis accuracy. Based on the results of various investigations, we selected 0.2 µs per step. To simplify the problem, the attenuation of ultrasonic waves was not considered. In this study, we did not focus on reproducing the experimental data but on understanding the sensitivity of the effect of porosity and water content on elastic-wave propagation in concrete.The wave propagation characteristics in an inhomogeneous field are validated through numerical analysis. In this study, the analytical model for concrete is regarded as an elastic body. The numerical integration method in which the equation of motion is numerically integrated at small time intervals to obtain the solution, and the mode polymerization method in which the linear response of a multiple degree-of-freedom system is expressed by the superposition of each order of the natural vibration mode are typical methods for analyzing the wave response. In this study, a numerical integration method was used. The Newmark-β method, which is most extensively used in seismic response analysis, was used for numerical integration. The finite element model a and theρ and wave velocity LC, and the most commonly used assumption of a Poisson ratio ν = 0.2.The Young’s moduli and densities of the specimens were calculated based on the experimental results, and applied in the numerical analysis. In this study, even though the experimental results indicated that the different air content in the specimen affected the wave propagation velocity in concrete, the data of the specimen with 10% air content was used as a representative value in the analysis because the different air contents showed the same velocity reduction and amplitude increase/decrease trends. In addition, because mortar was used in this experiment, the effect of fine aggregate was assumed to be negligible with respect to the size of the finite element mesh, and parameters with the same density and Young’s modulus were used. The applied analysis parameters are listed in Further, In this section, we develop an analytical model that considers the change in the moisture content distribution with material drying. Previous studies have indicated that the Young’s modulus decreases with the decrease in water content during ultrasonic testing. Therefore, the elements of the analytical model are classified into two types: wet (mortar 1) and dry (mortar 2), and the analytical parameters are set as shown in From saturated conditions, structural concrete gradually dries from the center of the specimen to the surface. Therefore, this two-phase model does not simulate the continuous change of the moisture content in concrete. The investigation of this two-phase model intended to clarify the wave propagation behavior in an inhomogeneous field and is a model with no significant differences from actual concrete.The Young’s modulus and density in the wet condition for mortar 1 and in the dry condition for mortar 2 were set based on the experimental results  at 10% aThe propagation velocity, arrival time, and amplitude of the first wave obtained through analysis are shown in The moisture content of the analytical model was approximated and compared between the ultrasonic test and numerical analysis. For the analytical model, the moisture content was calculated as the ratio of the area of mortar 1 to a saturated moisture content of 10.7%, which was determined through ultrasonic testing when all the elements were in mortar 1 (wet state). The results of ultrasonic testing confirmed that the elastic-wave velocity tended to decrease with the increase in the air content of the specimen. In addition, the elastic-wave velocity exhibited a decreasing trend as the moisture content of the specimen decreased with drying.As the moisture content of the specimen decreased, the amplitude of the vibration waveform changed. The amplitude decreased with the decrease in moisture content and reached a minimum value at approximately 1% moisture content; however, the amplitude exhibited an increasing trend at lower moisture content.The numerical analysis reflected the decrease in the elastic modulus with the decrease in moisture content in the analytical model, and the results reproduced the increasing and decreasing trends of the amplitude obtained through ultrasonic testing. It is considered that ultrasonic waves were attenuated at the boundary between regions with different parameters, and that the amplitude initially decreased as the boundary increased with the expansion of the dry region; however, this reverted to an increasing trend as the boundary decreased when the dry region exceeded a certain range.The ultrasonic method is applied for the quality evaluation of concrete. In this study, the effect of inhomogeneity caused by the change in the moisture content of concrete on ultrasonic-wave propagation was investigated experimentally and numerically. The following conclusions were obtained:Although the present study was carried out in using mortar, it is necessary to consider the effect of the presence of coarse aggregate on the reflection and diffraction of waves in concrete for the study in actual structures. However, the understanding of wave propagation phenomena in concrete with inhomogeneous water content, as in the present study, is essential for actual field measurements for precise prediction of parameters such as concrete strength and permeability, and will be necessary in the future.


This article reviews the literature related to the performance of fiber reinforced concrete (FRC) in the context of the durability of concrete infrastructures. The durability of a concrete infrastructure is defined by its ability to sustain reliable levels of serviceability and structural integrity in environmental exposure which may be harsh without any major need for repair intervention throughout the design service life. Conventional concrete has relatively low tensile capacity and ductility, and thus is susceptible to cracking. Cracks are considered to be pathways for gases, liquids, and deleterious solutes entering the concrete, which lead to the early onset of deterioration processes in the concrete or reinforcing steel. Chloride aqueous solution may reach the embedded steel quickly after cracked regions are exposed to de-icing salt or spray in coastal regions, which de-passivates the protective film, whereby corrosion initiation occurs decades earlier than when chlorides would have to gradually ingress uncracked concrete covering the steel in the absence of cracks. Appropriate inclusion of steel or non-metallic fibers has been proven to increase both the tensile capacity and ductility of FRC. Many researchers have investigated durability enhancement by use of FRC. This paper reviews substantial evidence that the improved tensile characteristics of FRC used to construct infrastructure, improve its durability through mainly the fiber bridging and control of cracks. The evidence is based on both reported laboratory investigations under controlled conditions and the monitored performance of actual infrastructure constructed of FRC. The paper aims to help design engineers towards considering the use of FRC in real-life concrete infrastructures appropriately and more confidently. A concrete structure may be exposed to a variety of environmental conditions throughout its service life. The durability of a concrete structure is therefore defined by its ability to withstand these exposure conditions without major repair or rehabilitation. It has long been believed that concrete is an inherently durable material, which can last many decades or even centuries with no or very little maintenance. However, the relatively low tensile strength and (quasi)-brittle behavior in tension necessitate the use of reinforcement  in most structures. While steel rebars are still predominantly applied, the use of fibers has seen much development in the past decades. Various types of micro and macro fibers have been used as a secondary reinforcement in concrete.Fiber reinforced concrete (FRC) is a special class of concrete which incorporates fibers in the formulation to enhance its resistance to (tensile) loads. Different classes of FRC with varying advantages have been developed. Fibers with different cross sections  have been used. Traditionally, fibers have been classified in terms of materials: commonly used are metallic, glass, synthetic, and natural fibers. A further distinction can be made based on the fiber size: we differentiate between micro  and macro fibers . Physical properties of commonly used fiber types are summarized in Continued development of fibers has resulted in many classes of advanced FRC’s. Engineered, strain hardening cementitious materials (ECC/SHCC) ,2,3 are The optimum fiber content in FRC depends on the FRC class, and the fiber type and aspect ratio. For micro-fiber such as polyvinyl alcohol (PVA) and polyethylene (PE), the optimum content was found to be in a range of 1–2% in SHCC ,12. In tDue to its numerous advantages—good tensile strength, ductility, fatigue resistance—FRC’s have been used in a wide range of applications, including pavements, industrial floors, tunnel linings, slope stabilization, and impact resistant structures, among others . The appFRC is also used for strengthening of structures sensitive to earthquakes . The higFor practical use of fiber reinforced concrete, it is important to understand its long-term performance in different environmental conditions. Compared to plain concrete, the field of fiber reinforced concrete is less mature. Consequently, significant research efforts have been made in recent years to increase our understanding of the durability of FRC. This article aims to review the existing literature related to durability of FRC in terms of different deterioration mechanisms, including chloride and carbonation-induced corrosion, freeze/thaw cycles, and alkali-silica reaction. Some practical applications are also discussed. This review can form a basis for the practical use of FRC in infrastructures, as well as provide researchers with background before embarking on research in the field of FRC durability.In general, deterioration of concrete may be caused by a variety of physical, chemical, or mechanical processes which commonly act in combination . One of 3A) and tetra-calcium alumina ferrite (C4AF) together with the bound chloride ions form Friedel’s salt, which has a less porous structure and slows down the transport of chloride ions [3A, C4AF, tri-calcium silicate (C3S), water/cement ratio, of which C3A has the most dominant effect [As described, chloride ions can cause corrosion of reinforcing steel in concrete. While sometimes present in the concrete mix—as part of contaminated aggregate or used as set accelerators in the past—typically chlorides from the environment penetrate the concrete cover and slowly reach the steel. The main consequence of chloride ingress is reinforcement corrosion. Pitting is a typical form of chloride induced corrosion . Since aide ions . Overallt effect . TherefoAs already mentioned, issues related to chloride ingress may be even more pronounced in the presence of cracks. Therefore, in this section, the focus is mostly on studies related to performance of FRC under coupled effects of cracking and chloride ingress.Chloride resistance of uncracked steel fiber FRC and RC specimens was investigated by Abbas et al.  through Excellent crack bridging capacity and formation of multiple fine cracks in SHCC result in good resistance against chloride penetration, which can delay the corrosion initiation process in reinforced concrete ,44. FiguResistance to steel corrosion in high-performance fiber-reinforced cementitious composite (HPFRCC) made with 1.5% by volume PE fibers with bending cracks was studied by Miyazato & Hiraishi . In theiCorrosion resistance of HPFRCC as both patch repair and surface coating material was investigated by Kobayashi et al. . PolyethIn terms of practical applications, at present there is no agreement on the acceptable crack width threshold for chloride-induced corrosion limitation in FRC. Bernerd  concludeIt must be noted that the deterioration process (chloride ingress and corrosion) in FRC is significantly influenced by the fiber type. Some authors suggest that, while steel fibers themselves are certainly susceptible to corrosion, they have higher corrosion resistance due to the presence of millscale on the surface . The durIn general, the improved performance of FRC can be ascribed to the formation of more uniform chloride diffusion in multiple-cracked steel-reinforced FRC elements and structures, and a more uniform formation of smaller anode/cathode reaction compared to normal RC . HoweverCarbonation causes chemo-mechanical changes in the concrete, in particularly changes in strength, permeability, pore size distribution, and chemistry. Furthermore, it is well-known that carbonation may cause shrinkage and thereby, potential cracking of the concrete . It is aFrom the literature, it is evident that the appropriate amount of fibers can delay the rate of carbonation in FRC compared to concrete without fibers. Since carbonation is related to the matrix porosity, pore structure and permeability, the optimum fiber content plays an important role in carbonation. The role of different percentages of steel fibers  on the carbonation depth of uncracked FRC was investigated by Wang et al. . They re2 penetration. Although no relation was found between the crack width and corrosion initiation time, a beneficial effect of fiber addition on carbonation induced corrosion rate was found [2 and the RH of 60% for 4 days and then a subsequent wet environment for 10 days with 90% RH to complete one cycle. In this way, a total of 6.5 cycles (about 91 days) were completed for specimens before measuring the carbonation depth. At lower w/c (0.30), carbonation depths in both uncracked HPFRCC and normal mortar specimens were almost the same (nearly zero). However, at higher w/c (0.6), the carbonation in HPFRCC specimens was more than double that of mortar specimens. An opposite trend was reported for cracked specimens. In this case, the penetration depth in mortar specimens was found to be three times higher than in the HPFRCC specimens. It must be noted that the residual crack width in mortar specimens was 0.3–0.4 mm, while in HPFRCC specimens it was 0.10 mm or less. Also, in mortar specimens, only a single crack was formed in contrast with the HPFRCC specimens where multiple cracks were formed. Therefore, the higher carbonation depth of mortar specimens can be attributed to the presence of deeper and wider cracks compared with HPFRCC specimens. Typically, carbonation develops faster at the cracked region and corrosion in the steel rebars at cracked regions is higher [The performance of uncracked FRC specimens made with steel fibers subjected to carbonation was reported to be good, but some studies also reported severe corrosion damage in fiber at the bridging cracks which caused significant reduction in residual tensile strength of FRC ,83. In cas found . Similaras found . Note thas found . All spes higher . Also, is higher . The locs higher . Neverth2. Also, if the cementitious matrix rich in Ca(OH)2, it may limit the decrease in pH and repassivate the anodic area [The dense and uniform fiber-matrix interface of uncracked FRC creates a uniform coating surrounding the fibers, which may limit the access of oxygen and thus separate the steel rebars electrolyte (limiting the ion diffusion on rebar surface). This may ultimately delay the carbonation process at the steel and concrete interface . In cracdic area .Alkali-silica reaction (ASR) is a swelling reaction occurring between the highly alkaline cement paste and amorphous silica which may be present in the aggregates . For ASRSeveral researchers investigated the effect of fiber inclusion in controlling the ASR induced cracking of concrete ,94,95. MEffective ASR control in FRC was also reported at low volume content of steel fibers (1–2%) . After sIn general, concrete reacts to high temperatures relatively well due to its incombustibility and low thermal diffusivity . However2 expansion [The compressive strength of FRC with steel fiber gradually increases when the material is heated up to 200–300 °C, but starts to decrease as temperature further increases . This isxpansion . Dependixpansion .In another study, FRC was made with three different types of fiber  at 0.5 and 1.0 vol.% . Prior t2 as recommended by the RILEM Committee TC 117-FDC45 [The effect of fiber addition in concrete on reducing the freeze-thaw degradation in FRC is discussed in this section. The acceptable mass loss of concrete specimens (size 150 × 140 × 50 mm) in CDF  test after 28 freeze-thaw cycles is <1500 g/m17-FDC45 . Similar17-FDC45 ,119. It Excellent resistance of FRC with synthetic fibers to freezing and thawing has also been reported in many studies . Yin et Compared to synthetic, steel fibers appeared to be more effective for freeze-thaw resistance. Also, straight short fibers have shown better efficiency than longer hooked ends fibers . This caThis section discusses the existing models that deal with FRC especially in the area of corrosion. In a study by Wang et al. , time tou). In another study [The expected service life of FRC specimens made from different percentages of softwood fiber  with and without applied compressive stress was predicted for the chloride diffusion coefficients as shown in er study , the serStudies were also devoted to develop service life models of FRC for temperature and freeze-thaw actions and, in both cases, increased life for FRC compared to plain concrete was found ,134. UndTraditional FRC applications have seen a distinction between use of micro-fiber and macro-fiber FRCs. Micro-fiber FRCs have been widely used for plastic shrinkage control ,138, impThe dominant conventional FRC application was reported to be slabs on grade, or so-called ground slabs in the form of industrial floors and road pavements , which c3). However, steel bar reinforcement was incorporated in the strips spanning between columns to address the risk of progressive collapse. At the time of publication, no reports were received on the performance of these slabs from the authors. However, successful and routine use of shorter span (3–5 m) suspended FRC (only) slabs for a period of 15 years has been reported [2. As reported in In suspended slabs, the fibers contribute to crack width reduction and crack spacing control, which improves serviceability and durability . Howeverreported . In the While FRC tunnel linings were introduced in the 1980s , the firAging of infrastructure, damage by earthquakes and premature corrosion of reinforcement in coastal and regions where de-icing salts are used have led to use of FRC strengthening and retrofitting interventions on highway infrastructure. The performance of retrofitted infrastructure for improved resistance to ingress of water and deleterious substances by the use of high density FRC, as well as where greater deformability and ductility have been provided, is reported here.Ultra-high strength (180–200 MPa compressive and 7–15 MPa tensile strength) UHPFRC with low water permeability has been applied in partial replacement of deteriorated decks and slabs of several road and railway infrastructures and buildings since 2004 . The higTo improve earthquake resistance and reduce maintenance requirements of movement joints in highway bridges, a slab connecting system has been developed ,161. TheIn another retrofit to improve infrastructure durability with FRC, Ishikawa et al.  developeA main durability enhancing feature of FRC is crack control which limits the rate at which deleterious substances such water, chlorides, and carbon dioxide ingress into structural elements, thereby prolonging the service life of the structure.There is evidence that also the propagation period is prolonged in R/FRC compared with RC. This applies to most dominant deterioration processes in infrastructures, which are carbonation and chloride-induced corrosion, and ASR-deterioration.Fibers in concrete may alter and improve resistance to other deterioration mechanisms. Corrosion induced by stray current in rail-infrastructure is reduced or prevented by the dispersed, non-interconnected fiber, compared with continuous steel bar reinforced structures. Polymeric fiber in FRC reduces of prevents explosive spalling in situations of fire, by melting and freeing up void space for expansion.As in RC, wide cracks in FRC or R/FRC lead to high deterioration rates. A typical threshold crack width is 0.1 mm. Crack widths in excess expose crack surfaces to ingress and subsequent high deterioration rates. Apart from low ingress and deterioration rates for fine cracks, self-healing of cracks up to this threshold may occur, further reducing deterioration processes.It is possible and essential to combine improved durability of FRC with enhanced mechanical integrity. Structural systems like link/connecting slabs on bridge infrastructure are excellent examples of efficient use of customized FRC design, combining sufficient deformability and crack control by fiber bridging toward medium- and long-term cost-efficiency by avoiding regular maintenance and repair interventions and safety in seismic events.In this paper, the properties of different types of FRC exposed to various deterioration processes are reviewed. The mechanisms of improved durability brought by the addition of fibers are elaborated, as are pitfalls causing negative impacts on concrete durability. The authors believe that this review may bring insight and perspective to researchers and designers on the durability potential and performance of FRC, and of FRC used in combination with reinforcing steel bars, enabling them to select these materials for appropriate applications confidently. The following conclusions are drawn from the review:Deterioration mechanisms in FRC are not yet fully understood or characterized. The quantification of deterioration rates in actual structures in the field and appropriate limit states of durability behavior are of crucial importance. Even for RC, durability design that includes both initiation and propagation phases of reinforcement corrosion is not yet established, although more information is available. An added complexity is significantly reduced crack spacing potential with particular FRC classes, for which there is evidence that deterioration processes are altered and their rates reduced, for instance by predominant micro-cell corrosion in multiply and finely cracked FRC, as opposed to high-rate macro-cell corrosion in wider, localized cracks in RC. To fully exploit the durability and service life preservation potential by appropriate FRC material and structural design, research effort towards fully understanding these mechanisms, increased data pools, and standardization are required. Finely, standardized durability tests are imperative, that enable characterization of the durability performance of structures in the field. Long duration durability tests in the field are vital, to enable not only improved durability of infrastructures, but improved predictability toward reliable service life design.


The topics of serious leisure and subjective well-being have been discussed extensively in previous research. It is generally acknowledged that people prefer to experience deeper satisfaction and happiness through serious participation in leisure-time physical activities. However, it is essential to examine the relationship between serious leisure and subjective well-being in an urban setting as well as the mediating effect of leisure satisfaction. Data were collected from 447 recreational runners at the 2018 Wuxi International Marathon event in China. The study results showed that serious leisure was positively associated with leisure satisfaction and subjective well-being, that leisure satisfaction was positively associated with subjective well-being, and that leisure satisfaction completely mediated the relationship between serious leisure and subjective well-being. Running group membership significantly affected the path from serious leisure to leisure satisfaction, while other demographic variables  did not moderate any paths. These results help explain the intricate relationship between serious leisure and subjective well-being and offer theoretical and managerial implications for serious leisure. Due to rapid economic development and improvements in living conditions, leisure has become an essential part of life for urban residents in China. People are no longer satisfied with experiencing short-term, monotonous rewards; instead, they prefer pursuing the long-lasting and multidimensional benefits achieved as they progress in leisure participation . PreviouThe concept of serious leisure (SL) was first proposed by The existing literature provides sufficient evidence supporting the role of SL on leisure satisfaction (LS) and subjective well-being (SWB). SL has been defined as “The systematic pursuit of an amateur, hobbyist, or volunteer activity that is sufficiently substantial and interesting for the participant to find a career there in the acquisition and expression of its special skills and knowledge” . StebbinExtensive evidence indicates that endurance sports are connected to the qualities of SL. Distance running , which is considered an endurance sport, fits the definition of SL because it requires a time commitment for training, the acquisition of knowledge and skills, and is usually challenging in nature . PreviouSWB is defined as an individual’s overall state of subjective wellness and usually includes two primary components: cognitive and affective . Over thSeveral researchers have recognized SL as a significant predictor of SWB. A quantitative study revealed that demographic variables and the qualities of SL can positively predict SWB, and the significant indicators included level of education, self-enrichment, self-expression, and self-gratification (enjoyment) . AnotherHypothesis 1: Serious leisure is positively associated with subjective well-being.Studies of LS began in the 1980s and aimed to examine individuals’ positive perceptions of leisure participation and overall satisfaction with leisure experiences. Existing studies have examined the differences in LS among participants with different degrees of leisure involvement. For example, Hypothesis 2: Serious leisure is positively associated with leisure satisfaction.Some scholars have made great effort to explore the relationship between LS and SWB, and their results have shown that LS is significantly positively associated with SWB. For example, Hypothesis 3: Leisure satisfaction is positively associated with subjective well-being.Previous research has provided an important background for exploring the relationship among SL, LS, and SWB. Previous research suggests that LS significantly improves as the degree of leisure involvement increases and that SWB improves as LS increases. However, LS may play an important role in the relationship between SL and SWB, and the relationship among the three variables must be re-examined, especially based on the context of a distance running event in China. To address these gaps in the existing literature, this study proposes a mediating model to explore the influence of LS on SL and SWB. Thus, the following hypothesis is proposed see :Hypothesis 4: Leisure satisfaction can mediate the relationship between serious leisure and subjective well-being.The data analyzed in this study were collected at the Wuxi International Marathon Event held on March 25, 2018, in Wuxi, a medium-sized city in Southeast China. This event was certified as a gold label event by the China Athletics Association (CAA) in 2015 and has successfully attracted nearly 30,000 participants since 2014. During the questionnaires collection process, the participants were informed about the consent and purpose of the survey, and ethical approval was not required according to the local legislation and institutional requirements. In total, 480 paper questionnaires were randomly  delivered to half marathon (21.095 km) and full marathon (42.195 km) runners near the finish area. In order to ensure the seriousness of the marathon runners, this study investigated marathon runners who reported running more than three times a week, had run in the past 6 months, and had run more than 5 km once, based on the operational definition suggested in existing studies . As a reIBM SPSS 22.0 was first used to calculate the descriptive statistics , Cronbach’s alpha (CA), and correlation analysis. Then, confirmatory factor analysis (CFA) was conducted using AMOS 21 to evaluate the reliability and validity of each construct in the hypothesis model. Third, a linear regression was used to examine Hypothesis 1 and Hypothesis 3, and SL was measured by the Serious Leisure Inventory and Scale (SLIM) proposed by LS was measured by a scale developed by Based on research conducted by Concerning the applicability of the measures above in the Chinese context, first, the measures in English were translated into simplified Chinese by one researcher. Then, another researcher translated the measures back into English. The original meaning of the measures was retained based on a comparison of the two English versions. Second, the final measures were perfected before the formal investigation based on suggestions by three professors who mainly focus on leisure sport studies.Previous studies have confirmed that SL, LS, and SWB are associated with related sociodemographic variables . ConcernThe respondents included 131 female runners (29.3% of the sample). Most marathon runners were married (61.3%) and ranged in age from 20 to 44 years (76.9%). Most respondents held a college or postgraduate degree (84.4%), and most respondents had an annual income greater than US $7,500 (72.9%), which is higher than the average income level in the region . Moreover, most runners were members of a running group (57.9%) see .M = 4.29; SD = 0.69) and personal effort  were the qualities on which the marathon runners agreed the most, and the average score of the SL was M = 4.11, SD = 0.62. Similarly, the marathon runners exhibited high scores on LS, ranging from 4.13 to 4.33. The relaxation  and physiological  dimensions were the two dimensions on which the marathon runners agreed the most, and the average score of LS was M = 4.22, SD = 0.53. SWLS 2  and SWLS 3  were scored relatively high, and the average score of SWB was M = 5.33, SD = 1.03  verified that all observed variables could be effectively used to evaluate the latent construct in the hypothesized model. Previous research suggests that indicators with factor loadings higher than 0.70 are acceptable , with a r = 0.79, p < 0.01) and SWB ; LS was positively correlated with SWB ; SL, LS, and SWB were correlated with most of the demographic variables, except for the correlations between gender and SL , LS , and SWB , between marriage and LS  and between income and LS  and SWB .P < 0.001) and age  were positively associated with SWB; LS , age , and marriage  were positively associated with SWB. Thus, hypotheses 1 and 3 were supported in this study.Step 1 and Step 3 in P < 0.001) and RG  were positively associated with LS. LS , age , and marriage  were positively associated with SWB. However, SL  was not associated with SWB. Concerning the results of step 1, step 4 indicates that LS had a complete mediating effect between SL and SWB. The value of the mediating effect was 0.357 . Thus, hypothesis 2 and hypothesis 4 were supported.Step 2 and Step 4 in This study further examined the effects of demographic variables  on the model path through a series of z-tests. This study mainly examined the interrelationships among SL, LS, and SWB through a regression analysis and Process macro V3.3 for IBM SPSS. First, involvement in a distance running event had a positive influence on the individuals’ life satisfaction . The resConsistent with the hypotheses in this study, SL was positively associated with LS in marathon running. This result filled the gap between SL and LS and expanded the quantitative findings reported by Another finding from this study is that the LS of marathon runners positively influenced their SWB. The current study enriches the previous literature  by examiThe results of Process macro V3.3 indicated that LS had a mediation effect on the relationship between SL and SWB. This finding indicates that SL is more likely to influence individuals’ LS with running, thereby increasing their SWB. This finding extends the existing literature  by introIn addition, this study examined the impact of demographic variables  on the path of the theoretical model. The study results indicated that marathon runners differed significantly on the path from SL to LS in the model based on their membership in a running group. As This study further extends the application of the SL framework to the leisure sports context among Chinese distance running event participants. A recent study confirmed that SL qualities were positively associated with marathon running participation behaviors, such as the number of running years, running frequency per week, and longest marathon event . Based oFour research hypotheses were proposed in this study. First, consistent with previous studies , the finThis study has several limitations. First, this study examined the relationship among SL, LS, and SWB based on marathon running in China. Future research should be applied to other leisure sports activities and should further examine the hypothetical model. Second, this study verified the mediation effect of LS on the relationship between SL and SWB. Other variables  should be explored via the mediation model in subsequent studies. Third, this study examined the mediation effect of LS using the total SL, LS, and SWB scores; future studies should explore the relationship among the subdimensions of the three variables and consider the mediation effect of other variables  in the model in this study.These results have theoretical and practical implications for the field of leisure activities. First, many variables may mediate or moderate the influence of SL on SWB. Scholars should focus on exploring these variables through both quantitative and qualitative methods to discover the potential framework that affects participants’ well-being. Second, organizers of marathon events should cater to the various needs of participants  and provide high-quality service in various aspects, such as security, energy supply, and route design. The more LS marathon runners acquire, the higher their SWB, and the more likely they are to participate in marathons again in the future. Third, government agencies  should provide more support to people, such as perfecting the running route, strengthening the construction of running groups, and optimizing relevant policies. These changes could promote individuals’ serious participation in running activities and offer a deeper degree of LS and SWB.The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.Ethical review and approval was not required for the study on human participants in accordance with the local legislation and institutional requirements. Written informed consent from the participants was not required to participate in this study in accordance with the national legislation in the host country and the institutional requirements.HT wrote this manuscript. YQ was responsible for research design and ideas. YL was responsible for research revision. WZ and CF were responsible for collecting questionnaire and analyzing the data. All 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.


An experimental setup of restrained slabs was utilized to induce the shrinkage cracks, and the geometry characteristics were quantified with image analysis technology. The results indicated the depth–width scaling  Steel corrosion induced by chloride is one of the major deterioration issues for reinforced concrete structures worldwide. Moreover, the chloride diffusion in cover concrete, a key point for the deterioration, is associated with increased importance to the design of concrete structures and studied by a great many investigators. Most studies are with sound (uncracked) concrete ,4,5,6,7.Different methods were employed to generate cracks in concrete in laboratory studies. Overall, the cracks could be distinguished into two groups: traversing cracks and non-traversing cracks. To induce cracks more realistically, the Brazilian or wedge splitting method is usually used to create traversing cracks, by which the cracks generated have a uniform length and depth . FurtherBecause the shape can be relatively well-controlled by mechanical expansion, multiple works have been performed on the permeability properties of concrete with traversing cracks. In those researches, the traversing cracks are commonly simplified as parallel in hypothetical models ,12, and Meanwhile, the “threshold crack width” ,18 is inThis paper proposes to take a further step in understanding of the characterization of real shrinkage cracks, especially the crack depths and its effects on chloride ion diffusion into concrete. To simulate the dry-shrinking and induce various shrinkage cracks, an external restraining test method was developed in slabs. With image processing and analysis technology, the crack width, depth, and their correlation are all quantified statistics, and their influence on the chloride diffusion is studied by combining a theoretical model, analysis, and experimental methods. The availability of this research is indispensable to accurately evaluate the influence of shrinkage cracks on the concrete permeability.D is the diffusivity of chloride [m2/s], ΔE is the potential applied [V], I is the current [A], H is the concrete specimen (disc) thickness [m], A is the cross-section area of concrete specimen [m2], and k is a constant that determined by the test method.When chloride diffusion is measured by a steady state migration test under electric field acceleration, the saturated concrete can be seen as an electrical conductor, and the method of a series-parallel connection of resistors in electricity can be used to study the diffusion process in cracked concrete. So, the total current passing through the concrete can be expressed as follows:l × w × h (m3), the diameter of cylindrical cross section specimen is b (m). Then, the total potential drop of cracked concrete sample can be divided into two parts which are installed in series, as shown in Equation (2) and Since the angle of the crack tip is extremely small, the crack is idealized as a straight channel. If the crack size length × width × depth = Replacing Equation (1) in Equation (2), one finds Equation (3):uI = dI = I, and Equation (3) may be rewritten as:eqD, ΔtE, I refer to the equivalent diffusion coefficient, the total potential drop, and the currents of cracked concrete (m2/s). uD, ΔuE, uI refer to the diffusion coefficient, the potential drop, and the currents of upper part concrete, respectively (m2/s). D0, ΔdE, and dI are the diffusion coefficient, the total potential drop, and the currents of down part uncracked concrete (m2/s), respectively.For each part, the charge conservation equation leads to The resistance of upper part concrete with crack can be divided into two parts which are installed in parallel, as shown crI and sI are the currents through the crack and sound upper part concrete [A], respectively. Replacing Equation (1) in Equation (5), one finds Equation (6):crD is the diffusion coefficient through the crack (m2/s), while crA and uncrA are the superficial areas of crack and uncracked upper part concrete (m2), respectively. Since uncrA/A ≈ 1, combining Equation (7) with Equation (4) and solving for eqD, yieldsThen, the total current through two parallel parts can be given as follows:A = πb2/4. Equation (8) may be rewritten asFor disc specimens, h = 0 or w = 0 or l = 0, eqD = D0, and traversing crack state h = H, eqD = uD. Equation (9) satisfies this.Boundary condition check: uncracked state Usually, the length of cracks in engineering is much larger than its width. To simplify the analysis, a section can be taken along the crack length for discussion. Equation (8) may be rewritten ascr/DD0 is far greater than 1 for concrete  as a function of the crack relative depth (h/H) and width (w) is illustrated in According to previous research results, i et al. ). Here, h/H < 1, e.g., the non-traversing cracks, the equivalent permeability coefficient has a nonlinear relationship with the crack width, and eqD/D0 increases with w but the growth rate is decreasing. When h/H = 1, e.g., the traversing cracks, the relationship evolves to be linear, which conforms to the results recently reported by Djerbi et al. [As can be seen in this figure, when i et al. .h/H increases. This essentially means that the influence of crack depth will be relatively more important than crack width on the equivalent diffusivity for cracked concrete. It is therefore important to understand the depth characteristics of the shrinkage crack. Existing literature has revealed that, for opening-mode cracks, the depth and the width are related [The curves of  related ,22, suchThe mix proportion used in the present study is listed in A concrete mould of the external restrained slab was developed to generate shrinking cracks. The dimension of slabs is 500 × 500 × 50 mm. In order to simulate the shrinkage cracking of concrete slab in terms of practical engineering, using the maximum size of aggregate, we refer to the suggestions of Reference  and Chinϕ 100 mm cylindrical were obtained from the cracked parts of those slabs and cured for 3 months.Four slabs were casted. After casting, the slabs were drying for 3 days to induce cracks, exposed to a constant relative humidity of 25% ± 5% and at a constant room temperature of 35 °C inside a humidity-controlled chamber. Then, the slabs were cured with external restraint for 28 days in water, which could allow the concrete to hydrate further and release the uneven residual stress caused by drying shrinkage to prevent the crack from healing. The diamond wire saw cutting was used to reduce the extra damage to concrete, and samples of The concrete conductivity test method ,25,26, sThe measured parameters were the crack length, the width and the depth. The crack length and width information were gathered by image analysis. First, we imaged the cracked specimens using a digital camera with 20 MP (megapixels). Then, we convert the RGB color image into a 256 gray level image and define the crack pixels as 1 and the background pixels as 0 to get a binary image. After that, we rebuilt the image with a smooth filter to suppress noise and remove small objects, and the geometrical shape of cracks in concrete was distinguished . Based o3 solution was injected into the cracks to keep show of their outlines and then the specimens were split into two slices at the crack. In the end, the crack depth was measured at locations from the visible white silver chloride precipitation, as shown in The crack depth was measured after the diffusion test. A 0.1 mol/L AgNOThe scaling of depth to width is defined as follows:First, we need to ensure the validity of the measured data before analysis, and this can be checked by a consistency test because of the randomness of shrinkage cracks. We use the chi-square method for checking. Through the experimental test, 64 sets of effective crack data were obtained, and which satisfies the general statistical requirements (>30). These data were statistically analyzed using SPSS software, and its statistical parameters are listed in The Chi-square statistical result is shown in Then, a statistical analysis of collected Further, we calculate the h, Equation (12) essentially shows that h and w are related. After analyzing the reasons for this, we believe it can be related to the constraint of the bottom of the concrete slab. In the crack developing process, the bottom of the concrete slab is always constrained, and this constraint will limit the development of the crack. The greater the crack depth, the greater the constraint applied, and the more difficult crack is to develop.Since For shrinking-cracked concrete cylinder specimen, the correlation between depth and width established in eqD/D0) is shown by the solid line in w increases, eqD/D0 increases more rapidly. The dotted line in The variation of equivalent diffusivity of shrinking-cracked concrete (D0 = 1.49 × 10−12 m2/s). The results indicate that the coefficient eq/DD0 of cracked specimens is on the rise with the increasing crack width. In addition, their relationship is nonlinear, which is in accordance with the law of Equation (13) or The test results of chloride diffusion through shrinking-cracked concrete are shown in crD may be obtained. w, crD/D0. crD/D0 is increasing approximately squared with the increasing of crack width from 0.06 to 0.2 mm  is substituted with the data in λ increases from 42.55 to 54.53, the crack permeability coefficient is greatly reduced by 13%. Further, it is easy to know that the diffusivity at the crack is about 0.36–0.87 times of that in free solution (2.032 × 10−9 m2/s at 25 °C), which is larger than those reported by Djerbi et al. [It should be noted that The “threshold crack width” for diffusion is 0.08 mm found by Jang et al.  and Djer= 48, crD/D0 = 1020, and the equivalent permeability coefficient can be calculated by Equation (9), of which the change rate does not exceed 20%. For the load crack, refer to Zhang et al [= 67), and crD/D0 = 700 found by Djerbi et al. [Considering that, in actual engineering, concrete cracks may be generated by shrinkage or load, and cracks generally do not run through the protective layer, the limitation on crack width should be discussed separately. Assume that the thickness of the protective layer of concrete structure is 40 mm. For the shrinkage crack, based on the conclusions of the present study, when the crack width is 0.2 mm, ng et al , the craBecause of the difficulty in crack shape control, we cannot get a series of shrinkage cracks with different widths that we wanted. In order to obtain as many width shrinkage cracks as possible, only one mix proportion is designed for four concrete slabs in this paper. To assess whether the conclusion about eference  for an aeference , mortar The main conclusions of the present study are:A series-parallel composite model was derived to evaluate the diffusivity of concrete with non-traversing cracks, and it shows that the equivalent diffusivity of cracked concrete is a function of crack depth and width for a crack, and it was relatively effected by the crack depth more than the crack width.A drying-shrinking method on external restrained slab is proposed to generate shrinkage cracks, and the statistical analysis shows that the method can well simulate the generation of shrinkage cracks in concrete engineering.The correlation between depth and width is obtained, which can be used easily to get the depth information (that is difficult to measure) by simply measuring the crack width, and is important for the prediction of chloride diffusion in shrinking-cracked concrete. The depth to width scaling of shrinkage crack increases with the crack width and was almost constant when the crack width was approximately 0.3 mm or more. Its average value is 44.90, and the crack tip angle is about 1–2 degrees.The present study shows that the variation of equivalent diffusivity of shrinking cracked concrete presents a nonlinear rising growth trend with crack width, of which the correctness is verified by present test data.λ significantly affects the diffusion coefficient through concrete cracks. As the ratio decreases with crack width, the diffusion coefficient increases. While the ratio increases and crack width is essentially constant, the permeability coefficient decreases. Therefore, attention must be paid to the width and λ characteristics of cracks for the permeability analysis of cracked concrete.The ratio The calculation of chloride diffusivity in concrete is performed for shrinking crack and loading crack, respectively, which shows that a crack width of 0.2 mm has little effect on the permeability of concrete, and it is not necessary to impose a stricter limit on the concrete crack width.


Pore structure development in Portland cement, fly ash, or/and ferronickel slag (FNS) was investigated using mercury intrusion porosimetry and X-ray CT tomography. The progress of hydration was observed using X-ray diffraction (XRD) analysis and compressive strength while durability of concrete was monitored by chloride penetration resistance and chloride profiles. Mercury intrusion porosimetry (MIP) results suggested that the blended cement had a higher porosity while lower critical pore size. The major reason to this increased porosity was the formation of meso and micro pores compared to ordinary Portland cement (OPC). In terms of chloride transport, replaced cement, especially ternary-blended cement had higher resistance to chloride transport and exhibited slightly lower development of compressive strength. X-ray CT tomography shows that the influence of pore structure of ternary-blended cement on the ionic transport was strongly related to the pore connectivity of cement matrix. Concrete is a nonhomogeneous and porous construction material that includes pore network of different sizes and shapes. Its physical and chemical properties are strongly influenced by this pore structure. For instance, properties such as strength, corrosion resistance, durability, and permeability are closely related to the characteristics of the pore structures for cement-based materials . Therefo2SiO4) and its major oxides are SiO2, MgO, Fe2O3, CaO, and Al2O3. When mixed with ordinary Portland cement (OPC), FNS clinker reacts with Ca(OH)2 to create secondary C-S-H gel, thereby contributing to the long-term strength development [Ferronickel slag (FNS) is an industrial by-product obtained after nickel ore and bituminous coal are melted at high temperatures (between 1500 °C and 1600 °C) and thereafter separated from ferronickel ,4. The pelopment . Howeverelopment  showed tRecently, it has been reported that the use of FNS has many technical advantages through comparative analysis with conventional cementitious materials as pozzolanic materials, but there is no verification of its reactivity with other existing binders. In that sense, the use of industrial by-products as cementitious materials may reveal different properties compared to conventional materials . TherefoPrevious study mentioned above have focused on the investigation of characteristics of FA and FNS as cementitious materials. In order to study the effect of it as an admixture, OPC with binary and ternary-blended mix designs using OPC, FA, or/and FNS were used. The investigation of hydration progress, pore structure refinement, and penetration resistance were also conducted. Since no chemical admixture was used to exclude possible reactions, the experiment was conducted with a specimen that had been cured for a longer period, i.e., one year.2, MgO, and Fe2O3. The CaO contents of FNS was counted for only 6.28% which is lower than OPC but much higher than FA. FA is a by-product generated when using coal as the raw material in thermal power plants. Its chemical composition showed that it is principally composed of silicon with minor amounts of aluminum oxide. Therefore, FA used in this study can be classified as class F, according to ASTM C618 [The materials used in this study were OPC, FA and electric arc furnace FNS powder produced by Korean company “P.” The particle size distribution is shown in STM C618 .2) and belite  are the main clinker while other clinkers such as gypsum, periclase, brownmillerite (4CaO∙Al2O3∙Fe2O3) accounted for lower quantities. For FA, XRD patterns mostly consisted of high silica contents, while for FNS the composition mainly accounted for forsterite (Mg2SiO4) and fayalite (Fe2SiO4) which have crystalline nature. Most of the MgO peaks detected from XRF analysis indicated forsterite and it originally shows very late hydration which takes about 2 years for complete reaction [The reaction .3 and fineness modulus of 2.73 was used. In the case of gravel, crushed aggregate with specific gravity of 2.62 g/cm3 and maximum size of 25 mm was employed.The mix proportions of the specimens are shown in In this study, the hydration behavior and properties of ternary blends incorporating FNS and conventional binders are evaluated. For this, paste specimens were used in accordance with X-ray diffraction pattern (XRD) and mercury intrusion porosimetry (MIP) tests while concrete was used for compressive strength, rapid chloride ion permeation tests (RCPT), and water permeability to observe pore structure characteristics in terms of durability. The prepared specimens were cured up to 365 days, because there was no addition of chemical admixtures to eliminate compound effects on the cement matrix. Except for the compressive strength test, the experiments were conducted by a single specimen or samples without replications. In order to examine the hydration products, cement paste specimens were mixed with 0.45 of a total water-binder ratio (W/B). After mixing of the dry powder with distilled water for 5 min, the pastes were placed into a cubic mold (50 × 50 × 10 mm), which was in turn cured at room conditions  for 24 h. Then, the specimens were demolded and stored in a water bath (20 ± 2 °C) for specified periods. After it, the hardened paste specimens were fragmented or ground off for its application to each microscopic examination. To mineralogically determine hydration products, XRD analysis using MDI JADE package of software  was emplTo investigate the porosity and pore size distribution of the paste samples, mercury intrusion porosimetry was applied by using AutoPore IV . At the age of curing, crushed paste samples were placed in acetone to stop further hydration and then dried to remove residual water in an oven at 50 °C for 24 h. Then, the scraps were placed in a tube filled with mercury. The obtained samples were initially evacuated to about 50 µm mercury and the low pressure was generated up to 0.20 MPa by nitrogen gas. The specimen was subsequently intruded by mercury at a filling pressure ranging from 0.0037 to 413 MPa and the mercury intrusion volume was recorded at each corresponding pressure. The pore diameter was derived from the pressure using the Washburn‘s equation as given in Equation (1) .(1)d =-Cylindrical concrete specimens (Ø100 × 200 mm) were fabricated according to ASTM C192  and cureThe RCPT was carried out in accordance with ASTM C1202 [S is the surface chloride concentration (%), x is the depth (m), D is the apparent diffusion coefficient (m2/s), and t is the time of exposure (s).The resistance of chloride transport in concrete was evaluated by measuring the chloride profile using cylindrical concrete specimens (Ø100 × 50 mm). One end for each specimen was coated by epoxy resin, and the rest of the surface was uncoated to induce one-direction penetration of chloride ions through concrete. Then, the specimens were immersed in a 4 M NaCl solution for 365 days. Samples were ground from the surface with 5.0-mm increments using a diamond blade grinder for chloride profiling. The collected dust sample was diluted in a nitric acid solution to extract chloride ions; then the diluted solution was taken to measure the concentration of chloride ions by cover depth using an ion-selective electrode. Once the chloride profile was obtained, the diffusion coefficient and surface chloride concentration were determined by solving Fick’s 2nd law as given byAfter curing for 365 days, the concrete specimens (Ø100 × 200 mm) were cut to 50 mm in height and 100 mm in diameter and kept in ambient temperature for 1 h stabilization of mass. Then, samples were placed in CT scanning. The details of equipment and operation are given in After scanning, the obtained images are filtered to remove the noises and segment the pores using OTSU  . Then, r2) and ettringite (Ca6Al2(SO4)3∙32H2O). As for the mix of OPC with FA (OFN0), hydration products mainly consisted of portlandite, calcite (CaCO3), and unreacted particles , while OFN15 was indicative of the presence of identical hydration products except for anhydrate olivine crystalline 2∙SiO4), presumably being originated from raw FNS powder. In fact, it is notable that there was no further formation of hydration products in FNS-mixed ternary-blended cement paste. A formation of calcite in OPC, OFN0, and OFN15 may be attributed to long-term curing age. A reduction of the peak intensity for portlandite was observed in the XRD curves for OFN15 and OFN0 paste unlike OPC, reflecting that a partial replacement of supplementary cementitious materials in binder would be more likely to form C-S-H gel, together with the depletion of CH in the cement matrix.To determine the hydraulic reactivity of ternary-blended cement containing OPC, FNS, and FA, the XRD examination was performed for pastes at 365 days, as given in To characterize the pore structure of FNS-mixed ternary-blended concrete, the pore distribution was preliminarily examined by MIP. Paste was used for the MIP to allow the pores to be formed in the cement matrix rather than at the interfacial zones between paste and aggregate; thus, the impact of distribution of aggregate was removed. Fundamental properties of ternary-blended cement containing FA and FNS were investigated in terms of compressive strength as given in The charge passed through concrete specimen is given in −11, 1.78 × 10−11, and 1.39 × 10−11 m2/s, respectively. The further reduced value of the diffusion coefficient for OFN15 may arise from unreacted crystalline FNS clinkers, which leads to pore refinement , and lower diffusion coefficient. In fact, FA was effective in pore refinement, leading to a dense pore structure, but only a marginal reduction of the chloride diffusivity was achieved. However, incorporation of FNS had an effect of the pore-blocking in inner pores, so that the chloride ingress was decreased at the inner depth (from 10–15 mm).The resistance of concrete against ionic/molecular transport was evaluated by chloride profiling of concrete which was submerged in saltwater. Because chloride transport was driven by only one directional diffusion, the apparent diffusion coefficient and surface chloride concentration were simultaneously determined by using chloride ingresses at different depths for a certain duration of immersion. As seen in The smallest pore diameter is mainly detected in OPC concrete, however, macro pores that are not interconnected were detected in OFN15. The distribution of smallest pore was relatively detected in OFN0, but interrelated network was also detected. As seen in (a)From XRD analysis, a weakened intensity of portlandite peak was observed in blended cement compared to OPC because of the pozzolanic reaction in the cement mix. When it comes to the porosity, due to the pore refinement of FA or/and FNS, blended cement showed an increased porosity while decreased critical pore diameter compared to OPC. An incorporation of FA or/and FNS as a binder in concrete resulted in decreased compressive strength compared to OPC concrete, however, the difference was not significant.(b)As for the resistance to chloride penetration, the OPC concrete indicated higher level of chloride penetration while the concrete containing other binder was equally ranged in “very low” range.(c)−11.The diffusion coefficient and concentration of concrete were obtained by profiling of chloride concentration at each depth after immersed in salt solution. The surface chloride of OPC, OFN0, and OFN15 ranged from 1.30 to 1.52% while the diffusion coefficient was in the range of 1.39–1.92 × 10(d)The pore distribution of concrete measured by X-ray CT analysis indicated that pore connectivity marginally decreased because of the incorporation of binder. The apparent pore refinement was detected in OFN15 derived from increased amount of C-S-H gel, which was attributed to lower reactivity of FNS induced by increased water to clinker ratio.In this study, the pore structure characteristics of blended cement were investigated by microscopic analysis , compressive strength, chloride transport and X-ray CT. Detailed conclusions from this investigation are as follows: