With the rapid development of urbanization, many new buildings are erected, and old ones are demolished and/or recycled. Thus, the reuse of building materials and improvements in reuse efficiency have become hot research topics. In recent years, scholars around the world have worked on improving recycle aggregates in concrete and broadening the scope of applications of recycled concrete. This paper reviews the findings of research on the effects of recycled fine aggregates (RFAs) on the permeability, drying shrinkage, carbonation, chloride ion penetration, acid resistance, and freeze–thaw resistance of concrete. The results show that the content of old mortar and the quality of recycled concrete are closely related to the durability of prepared RFA concrete. For example, the drying shrinkage value with a 100% RFA replacement rate is twice that of normal concrete, and the depth of carbonation increases by approximately 110%. Moreover, the durability of RFA concrete decreases as the RFA replacement rate and the water–cement ratio improve. Fortunately, the use of zeolite materials such as fly ash, silica fume, and meta kaolin as surface coatings for RFAs or as external admixtures for RFA concrete had a positive effect on durability. Furthermore, the proper mixing methods and/or recycled aggregates with optimized moisture content can further improve the durability of RFA concrete. With the rapid economic and social development of the 21st century, the urbanization process has produced a large amount of construction waste. According to statistics, China produces more than 3 billion tons of construction waste each year, and waste concrete accounts for 50–60% of all construction waste . TherefoCurrently, the mechanical properties and durability of recycled aggregate concrete (RAC) have been studied in depth. A comprehensive review of the mechanical properties of RAC has been conducted, and the results show that degradation of the mechanical properties of RAC is mainly related to the high water absorption of the aggregates and the weak areas formed by the mortar adhering to the old interface ,20. The Recycled aggregates can be divided into recycled coarse aggregate (RCA) and recycled fine aggregate (RFA). Compared to recycled coarse aggregate concrete (RCAc), recycled fine aggregate concrete (RFAc) has been less studied, but RFA is also important in replacing sand in the preparation of concrete. The application of RFA has a positive impact on the environment . HoweverIn this paper, the durability of RFAc is reviewed in terms of impermeability, drying shrinkage, carbonation resistance, chloride penetration resistance, acid resistance, and freeze–thaw resistance. Understanding the effect of RFA on the durability of concrete can provide a theoretical basis for the future large-scale application of RFA as a substitute for sand in practical projects.2, and sulfate can enter concrete, or other external factors can cause damage to the concrete, making the service life of concrete greatly affected and thus becoming a global concern [Impermeability is a characteristic of the durability of concrete and is closely related to other durability characteristics. For example, water-soluble chloride ions, CO concern ,34,35. T concern ,37,38.The substitution rate of RFA is closely related to its physical property index. The permeability of the concrete prepared by replacing RFA with different physical properties is also different. Compared with natural fine aggregates, the physical properties of RFA are poor. The higher the substitution rate, the worse the performance of the prepared RFAc. The poor grading of RFA particles with a large number of voids on the surface and micro cracks attached to the old mortar renders the prepared RCAc less dense, so the level of RCAc impermeability decreases with the increase in the RCA substitution rate ,40. In sThe water–cement ratio is an important parameter in the preparation of concrete, which will affect the permeability resistance of concrete. A water–cement ratio of 0.65 has a lower impermeability rating than RFAc with a water–cement ratio of 0.55, indicating that the higher the water–cement ratio, the worse the impermeability of RFAc, because the higher the water–cement ratio, the slower the hydration of the RFAc cement and the more pores it produces, which leads to the poor compactness of RFAc ,40,44.Due to its volcanic ash nature, fly ash (FA) has been widely used as a replacement for silicate cement in engineering construction . The addThe water absorption rate of RFA is much higher than that of natural fine aggregate. RFA absorbs water when freshly mixed with concrete, which has an effect on the mechanical properties and durability of concrete, so the water content of RFA before mixing should not be neglected. Therefore, the effect of RFA in three states of air-drying (AD), oven-drying (OD), and saturated surface drying (SSD) on the permeability of the prepared RFAc was investigated, and it was found that the permeability of these states at the same RFA replacement rate was ordered as follows: SSD > AD > OD. This finding was different from that of RCA. The permeability resistance of concrete prepared from RCA with different moisture contents was ordered as follows: SSD < AD < OD . The reaThe addition of RFA has a serious effect on the permeability of concrete. An increase in the RFA replacement rate and the W/C ratio will lead to a decrease in the permeability resistance. However, the use of saturated surface-dried RFA, optimized mixing methods, and the addition of fly ash can improve the impermeability of RFAc. In addition to fly ash, silica fume, metakaolin, and other mineral admixtures also have zeolite properties, so the impermeability of RFAc mixed with silica fume and higher mineral admixtures is also beneficial. Drying shrinkage requirements are already an important part of project specifications . The evaThe drying shrinkage value increases as the RFA substitution rate increases, and when the substitution rate is 100%, the drying shrinkage value of RFAc is twice as high as that of normal concrete . The dry3, RFA2: 2326 kg/m3), indicating that RFA1 contains a higher amount of old cement paste than RFA2; and (2) the water absorption is lower than that of RFA1 . These two differences in physical properties indicate that the water content of RFAc prepared by RFA1 is higher than that of RFAc prepared by RFA2 and that the high water content has a low binding effect on concrete [Different sources of RFA also have an important effect on RFAc, and the study of RFAc prepared from different sources of RFA has great significance for the application of waste concrete in practical engineering. RFA from recycled laboratory-produced concrete and RFA from recycled precast concrete, both of which have similar strengths, were studied for drying shrinkage values of RFAc under both recycling routes and were found to be similar . Howeverconcrete  , produced during the smelting process of the ferrosilicon industry, has a silica content of up to 75% and is an effective volcanic ash material . Due to Different mixing methods also have an effect on the performance of concrete. RFA is one of the important reasons for the performance of concrete due to its excessive water absorption; however, the use of moisture in the two-part addition, i.e., the two-stage mixing method (TSMA), can effectively prevent the excessive water absorption of RFA . The perIn summary, the drying shrinkage values of the concrete prepared by the RFA substitution of sand became larger with the increase of the RFA substitution rate. In order to reduce the effect of RFA on the drying shrinkage value of concrete, the drying shrinkage value can be effectively reduced by adding admixture (SP) and mineral admixture, and the drying shrinkage value of RFAc with TSMA is also smaller than that of RFAc with a normal mixture. Drying shrinkage is a long-term performance feature of concrete, which is of great significance to concrete in the process of use, but RFAc is significantly affected by RFA, so it is necessary to consider an optimal replacement rate of RFA when preparing RFAc. Therefore, an optimal replacement rate of RFA needs to be considered when preparing RAFc, and the effect of RFA on the drying shrinkage of concrete should be reduced by other means.2 in the air is about 0.03%. The pores present in concrete allow CO2 to diffuse within it and react with hydration products to form a carbonation process. Carbonation is mainly a chemical reaction between carbon dioxide (CO2) and calcium hydroxide (CH), hydrated calcium silicate (C-S-H), or bentonite (AFt) [The amount of COte (AFt) . Determi2 curing is one of the means of treating RFA, and CO2 curing can fill the pores and cracks on the surface of the aggregate and improve the performance of the old and new ITZ [2-cured recycled coarse and fine aggregates; it was found that the treated RFAc had improved carbonation resistance compared to the untreated one, with the maximum carbonation resistance depth of the former being about 42.5% lower than that of the latter  addition had a lower carbonation depth than both without plasticizer and with SP1 (Sikament 400 plus) RFAc at each prophase, mainly because the addition of SP2 decreased the effective w/b of RFAc. Thus, the internal porosity was lower, and the gas permeability was lower . The res2 curing of RFA and the addition of mineral ex-dopant. CO2 curing is one of the methods that can be used to improve the quality of RFA, and mineral ex-dopant coating and microbial carbonate precipitation treatment can also improve the quality of RFA and thus the anticarbonation performance of RFAc. The particle size of RFA affects the carbonation resistance of RFAc, and the increase of the RFA substitution rate and W/C can negatively affect the carbonation resistance of RFAc. The anticarbonation performance of RFAc can be improved by the CO2 curing, and mineral admixtures.Buildings in harsh marine environments are subjected to long-term chloride ion corrosion, leading to the degradation of reinforced concrete structures, which can seriously affect the service life of buildings ,81. TherThe chloride ion diffusion coefficient and electric flux of RFAc are much higher than those of control groups and show a linear growth trend with the increase of the RFA substitution rate. Reasons include the following: (1) a large amount of old mortar attached to the surface of RFA leads to high porosity, which provides a channel for chloride ion transmission; (2) the old mortar on the surface of RFA absorbs water and reaches the saturation surface, which prevents the transition area of the old and new mortar interfaces from becoming dense, causing it to form chloride ion transmission channels; and (3) additional water cannot be completely absorbed by RFA, resulting in a water–cement ratio that is higher than the effective water–cement ratio and thus a low compactness in the RFAc ,84. The Curing is an important stage in the early stages of concrete. Reactive powder concrete (RPC) was cured using two curing conditions, normal curing and thermal curing, and the resistivity of RPC was found to be higher under thermal curing conditions, which is due to the improved volcanic ash reaction of RPC under thermal curing conditions, producing a denser CHS gel that fills the pores .2-cured RFA and untreated RFA were compared to study the effect of both on the chloride ion permeability of RFAc, and it was found that the chloride ion permeability of RFAc prepared from CO2-cured RFA was higher at all substitution rates  recycled fine aggregate and recycled coarse aggregate mixed together (R1) and (2) fully recycled fine aggregate (R2), and it was found that the resistivity of RFAc mixed with R2 was higher than that mixed with R1 due to the improved physical properties and the lower water absorption of R2 . CO2-curon rates , and theted zone .2 and has high volcanic ash properties [2 content and volcanic ash effect [2 and CSH in GRHA impeded the movement of chloride ions [The chloride ion resistance of the concrete added to RFA was generally reduced, and it was found that the chloride ion resistance of RFAc could be improved by adding an external admixture. The addition of 10% silica fume (SF) to self-compacting RFA concrete had a lower chloride ion permeability than the control group for all prophases, and this was because SF acts as a microfiller, increasing the density of the SSC interface transition zone . A high operties . RHA is h effect ,94,95. Ih effect . The duride ions . The duride ions ,97.Surface coating treatment is one of the methods used to enhance the performance of RFA. The RFA was coated with fly ash, slag, and PVA , and 25% of the natural sand was replaced with the coated RFA. It was found that the resistivity of the slag-coated RFA was maximal, regardless of the w/b, and increased with the thickness of the coating [The addition of SP can improve the durability performance of concrete, and the addition of SP in RFAc can increase the density of the concrete, so the durability performance of RFAc can be improved. However, it was found that there was no significant difference in chloride ion penetration between RFAc with SP and those without .2 was detrimental to the chloride ion permeation resistance of RFAc, but the coating-treated RFA was positive against chloride ion permeation, and the addition of mineral admixtures enhanced the chloride ion permeation resistance of RFAc. It is impossible to avoid the reduction of the chloride ion permeation resistance of concrete by adding RFA, so we should consider upgrading the quality of RFA itself or adding mineral admixtures and SP to mitigate the reduction in its performance. The resistance of RFAc to chloride ion permeation was decreased with the increase of the RFA substitution rate. The treatment of RFA with COx) and sulfur dioxide (SO2) [Industrial production causes a large amount of sulfur dioxide emissions, as well as an increasing number of nitrogen oxides from vehicle exhaust, which are the main components that contribute to the production of acid rain (NOde (SO2) . The conde (SO2) .Ca(OH)2The acid resistance of concrete depends mainly on its percentage reduction in mass and compressive strength in acid solutions. The reduction in mass loss and compressive strength in a 3% sulfuric acid solution was discussed by adding RFA with different substitution rates to the activated concrete, which was considered from two sources, one from normal concrete and the other from activated powder concrete. It was found that the mass loss and compressive strength of the activated powder concrete prepared from both sources of RFA subjected to sulfuric acid attack increased as the substitution rate of RFA increased, but both had good acid resistance . This isDifferent types of concrete supplemented with RFA showed mass loss and compressive strength reduction in acid solutions. The more significant the mass loss and compressive strength reduction was, the more the replacement rate of RFA increased. The incorporation of natural volcanic ash into SSC incorporated with RFA improves the acid resistance. An appropriate RFA substitution rate should be considered for RFAc in acidic environments, and a certain amount of natural volcanic ash can be considered to improve the acid resistance.In the UK, freeze–thaw cycles have been identified as one of the most significant deleterious factors affecting the life of concrete structures ,105,106.The freeze–thaw resistance of RFAc was investigated by considering three groups of RFAc: plain RFAc with a water–cement ratio of 0.53 (C); high-strength concrete (HC) without SP and with an aerator (AEA), with a water–cement ratio of 0.35; and high-strength concrete (HCAE) with SP added but without an AEA with a water–cement ratio of 0.35, at replacement rates of 0, 20, 50, and 100%. Three hundred freeze–thaw cycles were tested for the three groups of RFAc, and it was found that the mass loss of C was much higher than that of HC and HCAE . This reInternal curing is the internal water supply to concrete to achieve maintenance. Internal curing can reduce water consumption, improve the homogeneity of concrete, and reduce the cracking of concrete surfaces. Studies dedicated to internal curing found that internal curing may have an effect on the durability of concrete structures . RFA hasDifferent substitution rates of RFA also have an important effect on the freeze–thaw resistance of RFAc. By studying the performance of SSC with added RFA, it was found that the instability and the weak ITZ of SSC increased with the increase of the RFA substitution rate, and the compressive strength of 30% of the SSC specimens with added RFA after 28 and 56 days of freeze–thaw cycles decreased to less than 350 MPa .In summary, the freeze–thaw resistance of RFAc is significantly affected by the substitution rate of RFA, but reducing the water–cement ratio can enhance that resistance. In addition, using RFA as a source for internal maintenance and using RFA with a suitable moisture content both have positive effects on this freeze–thaw resistance. RFA itself suffers from a high water absorption, but using it as a source for internal maintenance can be reasonably applied to enhance the resistance of RFAc to freeze–thaw cycles. (1)The durability of RFAc is closely related to the quality of the RFA, which is mainly influenced by the content of the old mortar, specifically, the higher the content of the old mortar, the higher the porosity and water absorption. The average water absorption of natural sand is about 1.1%, while the average water absorption of RFA can reach about 8.4%.(2)The impermeability, drying shrinkage, resistance to chloride ion penetration, carbonation, acid resistance, and freeze–thaw cycle resistance of RFAc decrease as the RFA replacement rate improves. For example, the drying shrinkage value at a 100% RFA replacement rate is twice that of normal concrete, and the depth of carbonation increases by approximately 110%.(3)The impermeability, carbonation resistance, and freeze–thaw cycle resistance of RFAc decrease as the effective water–cement ratio increases. For example, when the water-to-ash ratio is reduced from 0.7 to 0.5, the mass loss of RFAc under freeze–thaw cycles is reduced. Meanwhile, the drying shrinkage is less affected by the change in the water–cement ratio.(4)RFA with appropriate moisture benefits the permeation resistance and freeze–thaw cycle resistance of RFAc.(5)2 has a positive effect on the anticarbonation performance of RFAc. However, it is unfavorable to the resistance to chloride ion penetration.The treatment of RFA with CO(6)Advanced mixing methods focused on improving the ITZ have been reported, and they can partially compensate for the poor performance of recycled concrete fine aggregates. Thus, the relative durability of RFAc can be optimized.(7)Existing studies report an optimized triple mixing method, where the SP is added together with additional gelling material. This addition promotes the dispersion of that gelling material, which, in turn, leads to a higher content of additional gel material in the ITZ, a better volcanic ash effect at a later stage, and a reduction in the porosity of the RFAc, thus allowing for an impermeability rating of 8 for the RAC using this method. In addition, because a water-reducing agent is added early on, together with the additional gelling material in this method, a thin layer of additional gelling material can be formed, even when the mixture is insufficient.This paper focuses on the durability of RFAc. The efficiency of RFA with appropriate water content, pretreated RFA, concrete mixing methods, and mineral admixtures on the durability of RFAc is summarized. Some important points are highlighted here:The replacement rate of RFA has a serious impact on the durability of concrete. Therefore, when the RFA replacement rate is 100%, 10% FA can be added to improve the impermeability. In terms of drying shrinkage values performs, the best RFA replacement rate of the concrete is 30%; as for how the resistance to chloride penetration performs, the best RFA replacement rate is 25%. The durability of RFAc is closely related to the quality of RFA. In addition to the methods mentioned in this paper to improve the quality of RFA, there are also thermal–chemical, thermal–physical, and microbial precipitation methods; thus, the durability of concrete prepared from RFA by different treatment methods is also worth studying in future research. In addition, other work will be focused on the properties of acid resistance and freeze–thaw cycle resistance. The performance of concrete in harsh environments should also be considered.


This paper reports an evaluation of the properties of medium-quality concrete incorporating recycled coarse aggregate (RCA). Concrete specimens were prepared with various percentages of the RCA . The workability, mechanical properties, and durability in terms of abrasion of cured concrete were examined at different ages. The results reveal insignificant differences between the recycled concrete (RC) and reference concrete in terms of the mechanical and durability-related measurements. Meanwhile, the workability of the RC reduced vastly since the replacement of the RCA reached 75% and 100%. The ultrasound pulse velocity (UPV) results greatly depend on the porosity of concrete and the RC exhibited higher porosity than that of the reference concrete, particularly at the transition zone between the RCA and the new paste. Therefore, the sound transmission in the RC required longer times than that in the reference concrete. Moreover, a predictive equation relating the compressive strength to the UPV was developed. As the global population grows at a fast rate, there is a great demand for concrete to develop infrastructure, urbanization, and accommodation, leading to the depletion of raw materials and natural aggregates for the production of concrete and cement manufacturing. Consequently, population growth raises concerns about greenhouse emissions and energy consumption issues. In addition to the depletion of natural aggregates and raw materials for contemporary constructions, the construction and demolition waste (CDW) existing in old structures is an imperative problem. The amount of CDW is approximately 850 million tons in the European Union, while it is 250 million tons in the USA ,2. The CThere have been many attempts to produce RCA and recycled fine aggregate in the concrete industry. The properties of concrete containing ceramic waste aggregate and coal waste powder have been investigated ,6. TheseLofty and Al-Fayez  found thThe influence of the RCA on the RC properties was investigated in previous studies. However, more experimental investigations are still needed at different ages. Moreover, un-destructive tests must be conducted on the RC and evaluations of the compressive strength. Therefore, this paper reports evaluations of the mechanical properties and durability in terms of the abrasion of medium-quality concrete incorporating RCA at different ages. Concrete specimens were fabricated with various percentages of RCA .The specifications of the materials used in this study were tested by the authors. Ordinary Portland Cement (OPC), specified by ASTM C 150 , was useAll concrete mixtures used river sand as the fine aggregate, while RCA replaced the NCA either partially or fully. The used NCA was crushed river gravel with a grain size distribution, as shown in The source of the used RCA was old reinforced concrete buildings and barriers, as shown in The laboratory program included the design of five concrete mixtures, as listed in The concrete ingredients were mixed by a mechanical mixer for about 4 min until a homogenous mix was obtained. Concrete specimens were prepared according to ASTM C31/C31M . The numAll experiments were conducted in the Department of Civil Engineering Labs at Baghdad University, Iraq. Three specimens were tested for each group and the average results were adopted.c’), as shown in A standard compression machine was used to conduct the compression and splitting tensile tests according to ,32, respThe abrasion resistance of the RC surface was investigated using the standard test method according to . The abrNon-destructive testing of specimens at the age of 28-days was performed by passing an electronic wave through the concrete specimens according to . UltrasoThe RC specimens were exposed to sustained axial compressive loads to determine the effect of creep according to . RC cyliThe RCA influence on the RC density was examined according to . ConcretThe influence of the content of the RCA on the water absorption of the RC was explored according to . RC cubeThe workability, mechanical properties, and durability in terms of the abrasion of cured concrete  were examined at different ages.o is the slump in mm of the CC mix and Sf is the slump in mm of the RC mix. The workability of concrete depends mainly on the water content and grading size distribution of the used aggregate. The effect of the grading size distribution can be minimized and eliminated in this study since the RCA was prepared so as to have the same distribution as the NCA. Therefore, the slump loss of the RC could be attributed to the effective water content that was left after absorbing into the capillary pores of the coarse aggregate. As presented in The workability of the fresh RC was examined using the slump flow test according to ASTM C143 . Figure The average compressive strength of the RC with various RCA contents is exhibited in The failure shapes were well-formed cones for both ends for concrete with the NCA and RCA (25% and 50% replacements). Whereas, the failure mode was cone shapes at the bottom ends with vertical cracks for the other replacements.The presented data are the average results of three concrete cylinders tested for each group at a certain age with a standard deviation range of 1.50% to 2.15%. The RC showed a lower elastic modulus relative to the reference concrete, as presented in The difference in the elastic modulus between the reference concrete and the RC in this study was lower than that observed in previous studies ,9,45. ThMechanical degradation of concrete due to vehicular movement or pedestrian traffic causes loss of concrete mass, which is defined as abrasion. The abrasion wear of the RC and reference concrete increased at an early age, which led to reductions in the concrete thickness. Notably, the concrete thickness loss was significantly high at the age of 7 days—the specimens lost approximately 5.0 mm of the thickness see . HoweverThe measured results of the ultrasound pulse velocity (UPV) tests are shown in 2). Whereas, the second approach was based on the graphical analysis of the residuals (diagnostic plots). The Minitab statistical software V. 17 was used for the model development. Two groups of experimental results were used for this target. The first group consisted of the 30 data points that represent the original test results. The second group included an additional 15 data points  to be adopted for the proposed model justification. The model equation was as follows.c is the compressive strength at the age of 28 days in MPa and U is the UPV (km/s100). The validation process to assess the adequacy of the proposed prediction model and measure the error or accuracy of the prediction is exhibited in A regression analysis was conducted to develop a predictive equation that relates a criterion variable to a predictor variable. The regression analysis is useful to estimate the compressive strength of the recycled concrete using a non-destructive test (UPV test). In this research, the criterion variable included the compressive strength at 28 days and the predictor variable was the ultrasonic wave velocity. Two approaches were used to assess the adequacy of the proposed statistical regression model. The first approach was based on examining goodness of fit measures and the coefficient of multiple determinations . The workability, mechanical properties, and durability in term of abrasion of cured concrete were examined at different ages. According to the experimental results, the following findings were concluded.The high demand for water of the RCA significantly affected the workability as well as the durability of the RC. The workability of the RC was reduced by 8% to 38%.The compressive strength was reduced by 5.0–9.3%, in comparison to the reference concrete. More reductions in the compressive strength of the RC were obtained as the RCA replacements increased.For 100% replacement of the RCA, a 10.7% difference of the tensile strength was observed between the reference concrete and the RC at the age of 7 days. Whereas, this reduction decreased to 8.3% at an age of 28 days and over. The residual cement mortar in the RCA provided weakened points, which caused reductions in the tensile strength of the RC.The concrete thickness loss was significantly high at an early age (age of 7 days), where the thickness of the specimens decreased by approximately 5.0 mm. However, the thickness loss was insignificant after 28 days.The provided regression analysis is useful to estimate the compressive strength of the recycled concrete using a non-destructive test with a goodness of fit of 0.93.For 100% replacement of the RCA, the creep strain increased by 67.5% relative to the reference concrete. However, only a 16% increase was obtained for the case of 25% replacement. The higher volume of cement paste in the RC compared to that of the reference concrete caused higher losses of the physically absorbed water under the sustained stresses and, subsequently, led to this increase in the creep strains.Using the RCA as a replacement for the NCA led to an increase in the water absorption of the RC and a linear proportion was observed between the content of the RCA and water absorption. For 100% replacement of the RCA, the water absorption increased by approximately 7%.


This paper presents the results of an experimental program aimed at the assessment of the freeze–thaw (F–T) resistance of concrete based on the evaluation of fracture tests accompanied by acoustic emission measurements. Two concretes of similar mechanical characteristics were manufactured for the experiment. The main difference between the C1 and C2 concrete was in the total number of air voids and in the A300 parameter, where both parameters were higher for C1 by about 35% and 52%, respectively. The evaluation of the fracture characteristics was performed on the basis of experimentally recorded load–deflection and load–crack mouth opening displacement diagrams using two different approaches: linear fracture mechanics completed with the effective crack model and the double-K model. The results show that both approaches gave similar results, especially if the nonlinear behavior before the peak load was considered. According to the results, it can be stated that continuous AE measurement is beneficial for the assessment of the extent of concrete deterioration, and it suitably supplements the fracture test evaluation. A comparison of the results of fracture tests with the resonance method and splitting tensile strength test shows that all testing methods led to the same conclusion, i.e., the C1 concrete was more F–T-resistant than C2. However, the fracture test evaluation provided more detailed information about the internal structure deterioration due to the F–T exposure. Concrete belongs to the most common building materials used in various civil engineering applications. The world produces about 4.4 billion tons of concrete annually, of which a substantial portion is consumed for the construction of transportation networks with strict requirements for their durability. This means that, in addition to basic strength and deformation parameters, the characteristics related to water, air permeability, and cracking tendency are strictly monitored under different weather conditions.The alternation of positive and negative temperatures (freeze–thaw cycles) is considered one of the most destructive processes that substantially influence the durability of concrete structures . It can There are various testing approaches and procedures (direct and indirect) for the assessment of the F–T resistance under laboratory conditions, which are adjusted to the actual weather conditions of a particular world region or country, and they mainly differ in the length of the F–T cycle, temperature range, monitoring intervals, monitored characteristics, and the limit number of F–T cycles or the limit value prescribed for a decrease in the monitored characteristic . The relThere are reasonable arguments for why the fracture characteristics should be suitable for the assessment of the rate of F–T deterioration. They arise from the essence of the fracture theory which deals with the resistance of the material to crack initiation and propagation . FracturThe F–T resistance of concrete depends on the quality of components and the overall composition of the fresh concrete, as well as on the quality of compaction and curing after its placing. In the hardened state, the F–T resistance depends on the quality of the cement matrix, aggregate, and interfacial transition zone (ITZ) between the aggregate and cement matrix. The air-entraining admixture is commonly used for enhancing the F–T resistance of concrete . In thisThe main objective of this paper was to present the specifics, advantages, and disadvantages of the fracture tests performed in the context of F–T damage to the professional and general public. A wide range of fracture parameters were evaluated in the paper to discuss the sensitivity of a particular parameter to the extent of concrete deterioration due to F–T cycling. In addition, the results of acoustic emission (AE) measurements during the fracture tests, which is a tool for nondestructive monitoring of active changes in progress during the loading of concrete specimens, are presented. These results were compared with the results of the commonly used testing approaches, i.e., changes in fundamental resonant frequency, mass, compressive strength, and tensile strength.3 is given in Two types of ordinary concrete, C1 and C2, with natural aggregate were designed for the experiment. The composition of both concretes per 1 mThe ready-mix concretes were prepared in a concrete plant and supplied to the laboratory for molding the test specimens. A combination of two plasticizing admixtures Sika ViscoCrete-4035 (superplasticizer with a fast effect) and Sika ViscoFlow-25 (plasticizer with delayed effect and stabilizing character)  was usedThe F–T tests were performed according to the standard ČSN 73 1322  valid foAn automatic freeze–thaw cabinet KD 20  was used for the experiment, see ErL and dynamic Poisson’s ratio µr of concretes during the F–T test. All specimens were measured before the start of the F–T test. The specimens subjected to the F–T action were measured at regular intervals (after each 25 F–T cycles) throughout the F–T test. The natural frequency of longitudinal and torsional vibrations was measured using a Handyscope HS4 oscilloscope equipped with an acoustic sensor. The readers are referred to [ErL and µr were calculated in compliance with ASTM C215-19 [ErL is the dynamic modulus of elasticity, L is the length of specimens, W and B are cross-section dimensions, m is the mass of specimens, and fL is the fundamental longitudinal frequency.µr is the dimensionless dynamic Poisson’s ratio, and Gr is the dynamic modulus of rigidity, calculated asR is the shape factor (1.183 for a square cross-section prism), and ft is the fundamental torsional frequency.A nondestructive test based on the resonance method see  was emplerred to  for more C215-19  as folloThe AE method is a tool for the nondestructive monitoring of active changes in a material produced during the loading of concrete specimens. The principle of the method consists of the continuous monitoring of the acoustic response caused by crack initiation and propagation during the loading of the specimen. To analyze the extent and progress of the specimen deterioration, it is very important to define an appropriate method for AE signal identification even before the start of the measurement. The most widely used approach is based on the setup of a signal threshold to distinguish failure. This specific approach presumes that each signal exceeding this threshold indicates a certain type of material disruption. The extent of material deterioration can be determined as a function of the number of AE events (counts) or the time of signal duration . HoweverIn the experiment presented herein, the monitoring of the AE activity was done using a double-channel unit DAKEL ZEDO with the following input parameters: the threshold value for counts was 0.561 mV, the threshold value for individual AE hits was 56 µV, the sampling frequency of AE hits was set to 10 MHz, and the cutoff frequency of the low-pass filter was set to 800 kHz. The total gain was 59 dB (pre-amplifier 34 dB and amplifier 25 dB). The AE sensors were attached to the specimens with beeswax in a thin layer. The extent of specimen deterioration was expressed by the cumulative value of RMS calculated for specific load intervals.F‒d) and load–crack mouth opening displacement (F‒CMOD) diagrams. In all cases, the test was terminated at a deflection of the specimens of at least 0.6 mm . The surfaces, especially those near the artificial notch, of all test specimens were inspected using a digital microscope with a magnification of 250× to verify the presence of microcracks just before the start of the test.Before testing, all test specimens were provided with an artificial notch with a depth of approximately one-third of the specimen height using a diamond blade saw. The test specimens were subjected to three-point bending tests (span length was 300 mm) with a constant displacement increment of 0.02 mm/min. This allowed obtaining the whole record of the load–deflection , an inductive sensor with a measurement range of 2 mm, a strain gauge, and a Quantum data-taker were used for testing. This apparatus allowed a precise setup of the test and a continual record of all measured quantities. The inductive sensor was used for the measurement of specimen deflection. It was mounted on the front of a special frame bedded on the upper surface of the specimens during the test see . The fraAll fractured specimens were further used for the determination of the compressive and splitting tensile strength on specimen fragments.F‒d and F‒CMOD diagrams were processed using GTDiPS software before their evaluation [All the recorded aluation   is a function of geometry [a is the initial notch depth.The fracture toughness value was determined using the linear elastic fracture mechanics approach for brittle fracture. This parameter is related to the stress field near the tip of the crack. The fracture toughness value  follows :(1)KIc=6geometry , and a iα = a/W is the relative notch depth.In this case, a geometry function for three-point bending configuration proposed by Brown and Srawley (1966) was used .(2)Yα =ae > a. The effective crack length ae is calculated from the secant stiffness of the concrete specimen corresponding to the maximum load Fmax and matching midspan deflection Fdmax. The value of ae for the prismatic specimen with a central edge notch tested in the three-point bending configuration was determined according to [E is the static modulus of elasticity calculated from the initial part of the recorded F−d diagrams according to Stibor [q is the self-weight of the specimens per unit length, S is span length, andαe = ae/W is relative notch depth, and Y(x) is the function of geometry shown in Equation (2), where α is replaced by αe. Since the effective crack length ae is expressed in Equation (4) as the argument of integral, the problem is solved using an iterative method.Several adaptations of linear elastic fracture mechanics have been proposed to cover the nonlinear behavior of a material. One of them is the effective crack model (ECM) , which irding to  from theo Stibor , q is thKIce value was calculated using a linear elastic fracture mechanics formula (Equation (1)), where α was replaced with eα in compliance with [Subsequently, the effective fracture toughness nce with .F−d diagrams, including their post-peak parts, were employed to determine the work of fracture WF value, which was given by the area under the diagram. In this case, WF was calculated according to Stibor [GF value was determined according to the RILEM method [The complete o Stibor , where tM method .(5)GF=WGF,1 considers the area under the F−d diagram up to the maximum load Fmax, and the second part GF,2 considers the remaining area under the F−d diagram.The value of fracture energy was also investigated when the area under the curve was divided into two parts see . The firF‒CMOD diagrams to determine selected fracture parameters. This model allowed the calculation of the parameters describing different phases of the fracture process. The unstable fracture toughness KIcun is defined as the critical stress intensity factor corresponding to the maximum load Fmax, and it represents the phase of unstable crack propagation. This parameter is of similar meaning to the effective fracture toughness used in the ECM by Karihaloo [ac was determined from the following equation [FCMODmax is the CMOD corresponding to maximum load Fmax, andcα = (ca + H0)/(W + H0); H0 is the thickness of blades fixed on the bottom surface of the specimens between which the strain gauge was placed.The double-K fracture (2K) model  was usedarihaloo . The equequation :(6)CMODFca is known, KIcun was determined according to Equation (1), where ca was substituted by a, and the geometry function in this case was expressed as follows [When the equivalent elastic crack length  follows :(8)YacW=The important parameter for nonlinear fracture mechanics calculation is the relationship between the stress and crack opening displacement see .GF is a derivative parameter of this relationship, which represents the area under this curve (softening function). There are two methods to obtain the parameters of the softening function. The first is based on the experimental determination of GF from the uniaxial tensile strength test with deformation-controlled loading. The GF is then calculated as the area under the σ‒COD diagram. However, it is quite hard to perform such a test in a stable way for concrete specimens, i.e., to also record the post-peak branch of the diagram. The other method consists of an indirect method of determination of CODc. In this case, GF and ft, determined experimentally from the 3PBT and uniaxial tensile test, respectively, and a suitable shape of the softening function are the input parameters [KIcc, which can be interpreted as an increase in the resistance to crack propagation caused by the bridging of aggregate grains and other toughening mechanisms in the fracture process zone (FPZ) [The fracture energy rameters . In the ne (FPZ) .GF and ft obtained by inverse analysis [σ(cCTOD) at the tip of an initial notch at the critical state could be then obtained from this softening function.ft is the tensile strength, c1 = 3 and c2 = 6.93 and are the material constants, which were taken from , and cCTOD is the critical crack-tip opening displacement according to Jenq and Shah [In this paper, the nonlinear softening function according to Hordijk , and GF analysis  were useand Shah  [Jeng 19cCOD is the critical crack opening displacement calculated according toGF and tensile strength ft were obtained by an inverse analysis based on an artificial neural network using the FraMePID-3PB Software [F‒d diagram responses to those obtained during real-time specimen loading. It is presumed that such strength is very close to the uniaxial tensile strength.The values of fracture energy Software . The priσ(x) along the length of the equivalent elastic crack can be formulated as follows:Subsequently, the linear function for the calculation of cohesive stress KIcc is determined as follows:U = x/ca is used, and F is determined according to [When this relation is known, the cohesive toughness rding to  [Xu 1999KIcini:KIcini represents the phase of stable crack propagation.The following formula based on the formerly obtained parameters was used to calculate the initial cracking toughness Fini, which expresses the load at the outset of stable crack propagation from the initial notch, was determined according toMS is the section modulus , S is the span length, and Y(α) is the geometry function (Equation (8)), where α = a/W is used instead of ca/W.Lastly, the load level In this section, the results of the performed experiments are presented in tables and figures. Fmax) was also very similar. Similar results could also be observed in the values of selected fracture characteristics, such as crack strength, fracture toughness, unstable fracture toughness, and effective crack extension. The difference between these parameters for C1 and C2 was up to 5%. The highest difference is recorded in the value of fracture energy (GF), which was about 14% higher for concrete C2. The values of GF,1 and GF,2 suggest that this difference was especially caused by the different post-peak behavior of investigated concretes; GF,2 was about 16% higher for C2, whereas GF,1 was almost the same for both concretes. A similar difference was recorded in the values of splitting tensile strength, which was about 15% lower for C2 compared to C1, but the variability for C2 was more than twofold higher. Similarly, the values of Fini , initial fracture toughness, and critical crack opening displacement (c)COD could not be simply compared because of the high differences in variability recorded for each concrete, which was about twofold higher (more than threefold for cCOD) for C1 compared to C2.nRV is a relative value of a particular material characteristic determined for n F–T cycles , nP is an average value of the set of specimens determined for a particular material characteristic after n F–T cycles, P0 is an average value of the set of non-frost-attacked specimens determined for the particular material characteristic .The results of the F–T tests are presented in figures below. All parameters (except the changes in mass and dynamic Poisson’s ratio) are displayed as the relative values of the results obtained for frost and non-frost-attacked specimens of particular concrete as follows:nRSD is a relative value of the standard deviation of a particular material characteristic determined for n F–T cycles , nSSD is a sample standard deviation of the set of specimens determined for a particular material characteristic after n F–T cycles, and nCoV is a coefficient of variation of the set specimens determined after n F–T cycles.The error bars represent the relative standard deviation of the results for a particular set of specimens.The decrease in mass and almost constant value of dynamic Poisson’s ratio see  implies ErL was about 5% and was quite stabilized after reaching 25 F–T cycles for C1. No decrease in the compressive strength was observed for C1. In both cases, the results exhibited low variability. The situation differed for concrete C2; a gradual decrease in ErL and compressive strength up to about 20% was observed upon reaching 100 F–T cycles, after which the values of both parameters started to grow. The final decrease was about 15% and 4% for ErL and compressive strength, respectively. The long-term experience of the authors with the utilization of the resonance method as a nondestructive technique for monitoring of the F–T damage in concrete suggests that a decrease in ErL of about 15% indicates a decrease in the flexural or splitting tensile strength of at least about 25% [Although none of the concretes showed visible disruption, there were differences in the development of the dynamic modulus of elasticity and compressive strength see  for C1 about 25% ,33. ThisK model. The assessment of fracture tests herein was based on an evaluation of F−d and also F−CMOD diagrams. The results based on the evaluation of F−d diagrams are presented first.The figures below display an advanced evaluation of the F–T damage based on the fracture mechanics of quasi-brittle materials. Note that the results of fracture tests are often accompanied by a high variability, which may in some cases reach more than 20%. The reason for this variability can be found in the test method itself. The method is based on the very slow loading of specimens; therefore, all microdefects are reflected in the resulting parameters. Another reason results from evaluation approaches, especially when the evaluation of the results is to a large extent based on the theoretical hypotheses, as is, e.g., the case in the double-GF  in the value of GF,1 for C1 after 50 F–T cycles, while almost the same value was recorded for C2 throughout the F–T test. According to the tensile behavior of concrete, as reported by Wardeh [y Wardeh , the preGF,2 exhibited almost the same trend for both concretes with a different value of decrease at the end of the F–T test. The value of GF,2 increased by about 20% and 10% for C1 and C2, respectively, after 50 F–T cycles and was almost the same as before the start of freezing for C1, while a decrease of about 14% was observed for C2 after 200 F–T cycles.The energy KIc, determined according to the linear elastic fracture mechanics approach  and decreased for C2 (of about 20%) throughout the F–T test .Unstable fracture toughness transfer . If the CTODc of more than 60% for C1 after 200 F–T cycles. This indicates an increase in the fictitious crack width before the failure due to F–T exposure. An increase of about 34% followed by a steep decrease of about 50% was recorded for C2 after 50 F–T cycles. This indicates a gradual increase in brittleness of C2 with an increasing number of F–T cycles.KIcc), unstable (KIcun), and initial fracture (KIcini) toughness after 50 F–T cycles, after which all toughness components were already stabilized during the remainder of the F–T test. In the case of concrete C2, the trend of development slightly differed for the three toughness components. No decrease in KIcc was recorded after 50 F–T cycles, while a gradual and steep decrease was recorded for KIcun and KIcini, respectively. A maximum decrease of about 11% and 17% followed by an increase in the value of cohesive and unstable fracture toughness, respectively, was observed after 100 F–T cycles. The final value of cohesive fracture toughness was about 8% higher compared to the value before the start of freezing.The above results correspond to the results of fracture toughness obtained using the 2K model. As shown in The increase in cohesive fracture toughness recorded for C1 reflects the action of the cohesive forces in the FPZ, which led to softening of the material , and it KIcun, KIcini, and KIcc indicates higher deterioration of C2 accompanied by more brittle failure due to the F–T action compared to C1 concrete.The descending trend of Fini/Fmax ratio for both concretes during the F–T test. The results show a gradual increase of up to 27% for C1 after 100 F–T cycles. The final value after 200 F–T cycles was about 16% higher than before the start of freezing. A gradual decrease of up to 15% was recorded for C2 after 200 F–T cycles. The increasing load ratio indicates an extension of the linear part of the diagram, which expresses the later onset of the stable crack propagation, i.e., the resistance to the crack onset increased during the F–T exposure for C1 concrete until reaching 100 F–T cycles.All these parameters together indicate an enhanced resistance of C1 concrete to brittle fracture.Fini and Fmax. The changes in loading force at the selected load levels are displayed separately in Fini were recorded for both concretes after 50 F–T cycles. After that, the value of Fini was almost stabilized for both concretes. Concerning Fmax, a gradual decrease was observed for both concretes up to 100 F–T cycles, after which the value of Fmax increased for C1 and C2. The variability in the loading forces was reflected in the evaluation of the RMS values, which was determined for the region of the average value of Fini ± standard deviation. The same procedure was applied for the load level Fmax and region 0–Fmax.AE signals were continuously recorded throughout the fracture tests to evaluate the extent of the deterioration of the concrete specimens due to exposure to F–T cycles. The changes in the RMS value were evaluated for selected load levels It is important to emphasize that AE results are influenced by two different loading processes which are concurrently in progress. The first is the F–T action, which proceeds continuously, and the second one is the fracture test performed after a selected number of F–T cycles. Considering that the F–T cycles act as a continuous loading and unloading process, failure occurs due to the progressive damage of the internal structure of concrete, which is reflected in a decrease in the acoustic signal amplitude or a shifting of the dominant frequency, as reported in . This irFini (expected crack initiation) after 200 F–T cycles. Almost the same descending trend of RMS for C1 was recorded when evaluated for the load range of 0 up to the Fmax. The increasing deterioration of C2 was reflected in a steep decrease in RMS, the value of which was negligible compared to the non-frost-attacked specimens after 200 F–T cycles. Although the resonance test showed a resistance of C1 concrete to the F–T action, the AE measurement revealed a gradual degradation of internal structure, as reflected in a decrease in RMS value of about 40% after 200 F–T cycles. This proved the evolution of the microcrack network during the pre-peak loading phase, which was reflected in the increase of the fracture toughness, crack extension, and fracture energy.The results of AE show a gradual decrease in RMS values for both concretes at all investigated load levels see . HoweverThe presented results are only a small part of an extensive experimental investigation focused on the F–T damage of concrete tested in the laboratory and on-site. The results of the performed experimental–numerical analysis indicate the high potential of employing fracture mechanics as a tool for the assessment of F–T damage, especially if the double-K model is also employed. The performed investigation also investigates the issue of methods for inverse analysis which might be adjusted to the pre-cracked material (due to the F–T action) submitted to the fracture tests. This novel approach provides a wider range of evaluated fracture parameters than usually presented in this context; thus, it is not possible to fully support the discussion with the literature. However, we believe that it could build a base for further investigation and a comparison of results with other research groups.300, which were both higher for C1. It is important to emphasize that both concretes did not exhibit macro-defects throughout the F–T test duration, i.e., no surface scaling or macrocracks were observed.This paper presented the results of an experimental program aimed at the assessment of F–T resistance of concrete based on fracture test evaluation, accompanied by acoustic emission measurement. Two concretes of similar mechanical characteristics were manufactured for the experiment. The main differences between the C1 and C2 concrete were in the total number of air voids and in the parameter AF–d and F–CMOD diagrams using two different approaches: linear fracture mechanics completed with the effective crack model and double-K model. It was observed that both approaches gave similar results, especially if the nonlinear behavior before the peak load was considered.The evaluation of the fracture characteristics was performed on the basis of experimentally recorded The changes in the root-mean-square (RMS) of the acoustic emission signal were evaluated from the continuous AE measurement.It can be supposed that C1 concrete exhibited better resistance to the F–T action compared to C2. All fracture parameters together indicated an enhancing resistance of C1 concrete to brittle fracture during the F–T test.It can be stated that the continuous AE measurement is beneficial for the assessment of the extent of concrete deterioration and suitably supplements the fracture test evaluation.The results showed that the F–T damage was more reflected in the fracture toughness parameters than in the fracture energy.F,1, and GF,2 separately for pre- and post-peak load phases. The presence of microcracks led to an increase in the pre-peak fracture energy GF,1 , labor intensiveness, and the process of evaluation, which limit its wider utilization in standard practice.According to the results, the following conclusions can be drawn:


This review article offers an overview of the differences between traditional energy integrating (EI) X-ray imaging and the new technique of X-ray photon counting spectral imaging (x-CSI). The review is motivated by the need to image gold nanoparticles (AuNP) in vivo if they are to be used clinically to deliver a radiotherapy dose-enhancing effect (RDEE). The aim of this work is to familiarise the reader with x-CSI as a technique and to draw attention to how this technique will need to develop to be of clinical use for the described oncological applications. This article covers the conceptual differences between x-CSI and EI approaches, the advantages of x-CSI, constraints on x-CSI system design, and the achievements of x-CSI in AuNP quantification. The results of the review show there are still approximately two orders of magnitude between the AuNP concentrations used in RDEE applications and the demonstrated detection limits of x-CSI. Two approaches to overcome this were suggested: changing AuNP design or changing x-CSI system design. Optimal system parameters for AuNP detection and general spectral performance as determined by simulation studies were different to those used in the current x-CSI systems, indicating potential gains that may be made with this approach. Radiotherapy is a staple component of modern cancer therapies due to its ability to be targeted to volumes of interest and to damage cancer cells more readily than their healthy counterparts. The targeting capability in particular allows both for localisation of radiation to tumours that cannot be accessed surgically  and whole organ coverage to prevent recurrence, without the systemic toxicity associated with chemotherapy .The radiotherapy dose that can be delivered to a patient’s tumour is constrained by concerns regarding the effects of incidental radiation dose on healthy tissues. Whilst careful treatment planning and the use of modern multileaf collimators mean that the dose delivered to healthy tissues can be kept many times lower than that delivered to the tumour, concerns regarding side effects and the inducement of secondary tumours limit the radiation dose that can be delivered to tumours. This is particularly problematic in cases where the therapeutic index  is low, or where radiosensitive healthy structures are located near the treatment site. Therapeutic indices are often reduced where hypoxic regions of the tumour are present , and incVarious approaches have been suggested for improving the therapeutic index by sensitising cancer cells to radiotherapy by reducing hypoxia ,5, delivThere is extensive literature on tailoring the synthesis of AuNPs to produce a wide range of shapes and sizes ,12, as wPhysical enhancement mechanisms are those that improve radiotherapy response of cells due to physical interactions between incident radiation and the AuNPs themselves. These can include increased energy deposition (due to the higher Z-number of gold producing a higher probability of interaction with the incident radiation beam) and production of more ionising radiation types when irradiated . These Chemical enhancement mechanisms are those that improve radiotherapy response of cells due to an increase in the number of reactive oxygen species (ROS) that are produced in the presence of AuNPs compared to in their absence . These eBiological enhancement mechanisms are those that alter cell behaviour or metabolism in such a way as to predispose the cells to radiotherapy damage. These range from molecular changes such as inhibition of DNA repair , to cellOnce accumulated, AuNPs can increase the susceptibility of cancer cells to radiotherapy by delivering a range of different RDEEs. These RDEE mechanisms can be broadly divided into three major categories:It is worth noting that there is much discussion in the literature as to where the line between chemical and biological mechanisms is drawn; however, what is not debated is that the effects mentioned above exist ,22,23,24Crucially to this review, the use of AuNPs in a clinical setting will require that their distribution within a patient can be imaged and quantified. Initially this ability will be used to confirm the accurate targeting and localisation of AuNPs within tumours or other targets of interest and to refine functionalisation approaches that improve tumour selectivity in humans. In routine clinical practice, quantitative images of AuNPs will again be needed so that the spatial distribution of dose enhancements can be calculated, and the effective dose delivered to organs at risk properly calculated and managed. Whilst the physical properties of AuNPs are often cited as suiting them to optical imaging techniques such as photoacoustic spectral imaging ,28, it iAu seeds are already used as fiducial markers in X-ray imaging , and mucx-CSI approaches are still under development, though clinical systems are available, such as the Extremity 5X120 scanner by MARS Bioimaging  and the I, can be related to a linear summation of the formn is the linear attenuation coefficient of the nth component material and nl is the total path length through that material, traversed by the X-ray beam traveling from source to detector. Importantly, µn is a function of incident X-ray energy, and this fact can be used to differentiate otherwise similarly absorbing materials. The X-ray intensity reaching the detector can then be converted into an electrical signal for reading out. Most modern X-ray detectors use either scintillation or direct conversion to perform this conversion.X-ray imaging works by assessing variations in radiopacity to differentiate different materials and structures. Consider an X-ray beam passing through a biological sample composed of N different materials before impinging on a pixelated X-ray detector. If we ignore scatter and system noise for the moment, the intensity recorded by each pixel, Scintillators, such as sodium iodide (NaI) or caesium iodide (CsI), convert X-rays into a pulse of photons at visible (or near ultraviolet) wavelengths. These optical photons are then detected and amplified by photomultiplier tubes, or detected using traditional photodiodes, to produce an electrical signal. In contrast, direct converters, such as silicon (Si), gallium arsenide (GaAs), or cadmium telluride (CdTe), convert the absorbed X-ray directly into free charge carriers within the material. These carriers are then collected by applying an electric field across the sensor material to produce the electrical signal measured.In both approaches, the final electrical signal can be related to the energy of the X-ray photon absorbed in the sensor material. Compared with direct converters, the multiple steps involved in scintillation detectors  lead to more statistical noise in relating electrical signal and incident X-ray energy. The transition times for each conversion step and variable generation-to-collection path lengths for the optical photons caused by their isotropic emission act to further degrade the temporal and spectral resolution of scintillator approaches in comparison with direct converters. For these reasons, and due to the high temporal resolutions required, PC systems almost exclusively are designed using direct conversion techniques. The remainder of this review will thus concern itself with this type of detection technology.Traditional X-ray systems are energy integrating (EI), which differ from PC systems in the way that they use the electrical signals from the sensors to measure X-ray interaction. In general, EI systems integrate the signals generated by multiple X-ray photons to estimate the total X-ray energy deposited in the sensor, whereas PC systems seek to count the number of X-ray photons that are absorbed by the sensor. Each approach has its own advantages and limitations.act. Variations between pixels can then be used to determine the differential attenuation of the incident X-ray beam by the material in the source-to-pixel path.EI systems relate the signal intensity of a given pixel to the total energy deposited in that pixel over the acquisition time, act. Additionally, due to the simpler electronic readouts, lower data outputs , and the statistical advantages of averaging shot noise and other random processes over many thousands of photons, EI detectors are comparatively simple and reliable to manufacture consistently and at scale.EI approaches have several advantages over the newer photon counting techniques. As they integrate signal over multiple photons, they do not experience detector paralysis, nor do they require particularly fast refresh rates in the electronics, though they should refresh at least as fast as 1/E1 and one photon of energy E2, where E1 = 2E2. This introduces a problem in some clinical tasks, as it means that higher-energy photons are overrepresented in the output signal compared with the lower-energy photons. This is particularly problematic for soft tissue discrimination tasks, as their low effective atomic number, effZ, means that soft tissues are primarily differentiable in their differing attenuation of lower-energy X-ray photons, being largely transparent to the higher energies used.Despite these advantages and their widespread use in clinical settings, EI techniques have inherent limitations that constrain future developments. Firstly, we can consider spectral response. As they are only sensitive to total energy deposited, EI systems cannot differentiate the arrival of two photons with energy n varies as a function of X-ray energy. This information can be diagnostically useful, allowing for better delineation of tumours, for example n and µn are the concentration and the attenuation coefficient, respectively, of the nth material comprising that volume. µn is a fixed and well-defined function of energy that is unique for each component material: µn ≠ µm ∀ m ≠ n. By sampling A at N different energies, we can construct a series of N linear equations with a unique solution, so long as the system is critically or overdetermined (N ≥ n). In this way, we can quantify the concentrations of N materials. Due to their short wavelengths, X-rays interact with individual atoms rather than molecules, and there are significantly more than two elements present in human bodies, with most soft tissues comprising H, N, and O as the major components and bone constituting significant levels of Ca. To overcome this, a set of “basis materials” with different atomic compositions are often defined to allow for a semi-quantitative analysis . Alternatively, the two channels can be used as fitting parameters to determine effZ and effρ, the effective atomic number and effective electron density, respectively. This allows the relative ratio of photoelectric and Compton interactions in the sample to be assessed, which will vary between tissue types [effZ and effρ, for most clinical applications material decomposition is preferred. This is due to the higher mathematical instability associated with determining effρ and effZ with the two channels in EI systems that contain significant overlap in the energy spectra used for generating each channel. Material decomposition approaches can also be extended to a third basis material where additional information can be incorporated into the reconstruction as constraints; however, these require task-specific algorithms and are time and computational resource intensive [As this review paper is focussed on X-ray imaging for AuNP quantification, it is useful to explore how spectral information can be used to provide this. The measured attenuation, ue types . The detntensive .effZ and effρ is of great interest in the field of oncology specifically, however, as estimations of dose deposition used routinely in radiotherapy planning require estimates of these parameters for accuracy. For proton therapy in particular, uncertainties in effZ and effρ could translate into clinically significant differences in dose profile to the patient given the relatively small volume over which proton beams deposit the bulk of their energy. Where effZ and effρ accuracy needs optimising or more than three basis materials are needed, dual-energy EI systems will need replacing with truly spectral (N > 2) x-CSI systems. Crucially for this review, quantitative AuNP imaging for informing radiotherapy treatment plans will require both of these.The estimation of N can be scaled (relatively) simply by increasing the number of thresholds used in the electronics, without the cost of adding additional sources/detectors or the decrease in temporal resolution associated with additional voltage switches.Though x-CSI systems use spectral information in a similar way to dual-energy EI systems, they gain this information in a different way, which adds significant advantages in both quality and flexibility to scale up. By excluding electronic noise from the counts and dividing a single X-ray beam into multiple bins rather than utilising multiple beams, x-CSI systems produce data sets that are intrinsically less noisy and contain less spectral overlap. This point is perhaps stated overly simplistically, and it should be noted that electronic noise is still present in x-CSI systems—it just manifests in a less damaging way . Spectral overlap also still occurs in x-CSI systems, due to noise sources, partial energy deposition, and charge-sharing effects . NeverthIt is hard to overstate the transformative potential of x-CSI over dual-energy EI for medical-imaging applications. x-CSI systems are intrinsically more versatile than their EI counterparts, and allow for new applications such as: simultaneous, multi-contrast agent decomposition ; improveX-ray imaging techniques have obvious advantages over other clinically available imaging modalities when it comes to imaging nanoparticles (NP). Compared with ultrasound or photoacoustic imaging, X-ray images are easily produced for whole patient volumes, regardless of depth, with less noise and less inter-user variability. Compared with positron emission tomography (PET) (or single photon emission computed tomography (SPECT) for radiolabelled NPs), X-ray scanners provide higher spatial resolution, faster imaging times, and lower ionising radiation doses to patients, and do not require specialist isotope production and nuclear chemistry. Compared with MRI, X-ray imaging is faster, less claustrophobic for the patient, cheaper, and suitable for patients with metal implants and/or cardiac pacemakers.Conventional X-ray imaging techniques are of limited use for in vivo AuNP imaging, however, due to a range of outstanding issues. These include blurring and streaking artefacts associated with metal implants/foreign bodies ; poor sow is the linear attenuation coefficient for water with respect to the whole monochromatic beam, and µa is the linear attenuation coefficient of the beam by air. HU is thus semiquantitative in nature, due to it being fixed by these two points; however, the HU values for various tissue types can vary significantly, even on the same machine, depending on the scanning protocol used  [In this equation, µd, etc.) . The aboIterative metal artefact reduction techniques that account for the spectral information from x-CSI provide unprecedented performance, allowing previously difficult tasks to be achieved with confidence, such as assessing in 3D the state of bone–metal interfaces for surgical screws or other metal implants ,56. As nN  allows dipix3RX  and the dipix3RX , and achdipix3RX . A simildipix3RX . This syZ-number NPs [The additional basis materials can also feasibly be used to enable molecular imaging capabilities in x-CSI by utilising functionalised high-mber NPs . This womber NPs ,63,64.The lower noise of the PC approach means that x-CSI is a more dose-efficient technique than EI. As a result, x-CSI images can be produced with similar quality but at a fraction of the dose to the patient . WhilstFinally, the exclusion of false counts from electronic noise allows x-CSI to use smaller pixel sizes than EI CT systems for a given SNR. x-CSI systems can thus produce higher spatial resolutions for a given scanner geometry.As already noted, x-CSI shares many disadvantages in common with other X-ray imaging techniques, though to a reduced extent in all cases. This section will focus on the considerations specific to x-CSI that limit the applications it is suitable for and the designs that can be achieved.dt, this could be achieved by ensuring the time required for the circuitry to collect the charge, increment any counters, and then reset, dr, is less than dt. This would be a trivial problem to solve if photon interactions were uniformly distributed in time; however, photon interactions are governed by Poisson statistics. This means that the probability of two photons arriving in the same pixel before the circuit can count and reset is non-zero no matter how small dr is. For a fixed X-ray flux, the proportion of events for which dt < dr gets smaller as dr is reduced. A minimum value of dr is set by physical constraints and electronics parameters. Sufficient time needs to be allowed for the charge carriers in the pixel to be completely collected, regardless of the depth of interaction, to avoid ballistic deficits that degrade the energy resolution of the system. Higher operational voltages can be employed to speed up the collection of charge from the pixel volume; however, there are limits to the voltages that can be used. The proportion of events for which dt < dr can also be reduced by decreasing the pixel size, increasing dt by reducing the per-pixel X-ray flux for a given per-unit-area X-ray flux. Both approaches are used in prototype x-CSI systems, with counting times of the order of tens of nanoseconds and pixel sizes down as far as 50 µm [As x-CSI systems attempt to determine the energy of each incident photon, they require a way to distinguish between different photons interacting with the same pixel in quick succession. Assuming that photon interactions with a pixel are separated by some short time interval, as 50 µm ,68.dt < dr for the majority of events places an upper limit on the per-pixel X-ray fluxes that can be used before spectral distortions from pulse pileup or lost counts due to detector paralysis become significant [Despite these efforts, keeping nificant . This liA second limitation of x-CSI systems arises from the way in which they separate “low”- and “high“-energy data sets. As only a single beam is used, the “low”- and “high”-energy photons are spatially and temporally coincident on the same pixels, increasing the risk of cross talk between the two. Two issues may arise in this setup: down-counting (where a higher-energy photon is counted in a lower-energy bin) or up-counting (where lower-energy photons are counted in a higher-energy bin). The extent to which each issue occurs depends on pixel size, with larger pixels favouring up-counting (due to multiple photons being summed together) and smaller pixels favouring down-counting (due to the photon depositing only part of its energy within a given pixel’s volume).Even in the case where a single photon deposits all of its energy within a single pixel volume and no additional photons are present within the count and reset time of the system, there are no guarantees that the full charge generated by the photon will be collected for analysis in that pixel. This is due to the presence of a range of charge-sharing effects (CSEs) that collectively act to distribute charge deposited in one pixel across neighbouring pixels . Fluoresdw. The assumption is made that dw << dt, such that the probability of two unrelated photons being detected in adjacent pixels within dw is negligible and so where this occurs it must be the result of a single photon interaction being shared across multiple pixels. The correction then applied can be either preservative or reconstructive. Preservative CSCAs simply remove the suspected CSE events from the system (prevent associated counters from incrementing) so that, ideally, CSEs are removed from the spectra. This is the simplest correction, though it does result in reduced dose efficiency, as counts are thrown away. In contrast, reconstructive CSCAs attempt to reconstruct the original photon energy by summing the charge found across the involved pixels before thresholding and incrementing counters as normal. Despite the involvement of multiple pixels, only a single pixel’s counters are incremented: usually the pixel with the highest share of the detected charge. Whilst this approach preserves the counts that would otherwise be lost, the reconstructed events may be shifted in energy slightly due to the additive effect of electronic noise from the various pixels.Pre-thresholding CSCAs involve linking neighbouring pixels together so that CSEs can be identified and corrected for. CSEs are identified where two events are detected in adjacent pixels within some small time window, dw << dt limits the acceptable count rates in a similar way to using larger pixels [P × P pixels of size Y µm × Y µm can maintain the spatial resolution of Y µm × Y µm pixels, its spectral performance is not as good as if the pixels were replaced with a single pixel of size PY µm × PY µm and no CSCA. This is due to a range of effects, including charge trapping in inter-anode streets [Pre-thresholding CSCAs can provide impressive gains in spectral resolution at small pixel sizes, though the requirement that r pixels . Selectir pixels . These s streets  and poorPost-thresholding CSCAs, though well studied ,74, are Whilst optimising pixel pitch and sensor thickness can be achieved relatively easily with standard simulation tools for other modalities ,78, the A final x-CSI-specific concern to note before closing this section involves data storage and management. The detailed images produced by x-CSI are significantly larger than those produced by their competitors. A whole body x-CSI dataset could be ~1 TB in size, compared with DICOM files of hospital CTs, which are usually measured in tens of MB in size , and evex-CSI is clearly more complicated to develop than EI; however, its lower imaging noise, superior soft tissue contrast, and broader spectral capabilities make it an area of great interest to the scientific community. In particular, the next section will look at the potential of x-CSI in implementing AuNP-mediated RDEE by reviewing achievements in AuNP quantification with x-CSI.Given the specific focus of this review, it is helpful to understand the sensitivities desired of an x-CSI system if it is to be of use in clinical AuNP-mediated RDEE tasks. Comparisons of AuNP concentrations used in imaging and RDEE applications are not simple, in no small part due to the inconsistencies in reporting concentrations found in the AuNP-mediated RDEE literature. Units of mass percentage (mg/kg) , mass coCalculating the volume of the unit cell  that is Determining the volume of AuNPs used in the study;Dividing the answer from 2 by the answer from 1 to calculate the number of Au atoms per AuNP;Multiplying the answer from 3 by the reported molarities of AuNPs  to determine the effective number density of Au atoms;Multiplying the answer from 4 by the atomic mass of Au (197) to yield the Au atom density in g/L .iρ is the imaging density, NPM is the molarity of AuNPs used, and NPr is the radius of the AuNP used. This equation is suitable for nanospheres; however, similar equations can be derived for other nano shapes where needed.The above steps lead to the standardisation equationThis approach assumes that AuNPs share the same unit cell as bulk gold  and that the mean volume of a sample of AuNPs can be related to the volume of an AuNP with a diameter equal to the sample’s mean diameter. The approach was validated using the subset of studies that reported both molarity and mass density values and it was found that the “imaging density” agreed with the expected mass density to within 0.2%.In constructing At first glance, the studies considered indicate that measurable RDEEs have been demonstrated using Au concentrations ranging from tens of g/mL to a few tenths of µg/mL: eight orders of magnitude. The lowest limit reported to be effective  may actuThe next section of this review will consider the advances made in AuNP imaging with x-CSI to date. It should be noted that an x-CSI system specifically optimised for AuNP detection has not been developed, and so the results considered may well have room for improvement. Nonetheless, it is instructive to compare the achievements and requirements for this particular application so that the extent and nature of the outstanding work can be commented on.The use of AuNPs as contrast agents in x-CSI is well known and most physical x-CSI systems that exist have been tested using either AuNPs or aqueous Au solutions. Despite this, the relative sparsity of physical x-CSI detectors, let alone of full x-CSI systems, means that the total number of such studies is small. For this review, we will also consider studies performed on dual-energy PC systems more generally, as these are reasonably expected to perform close, but not superior to, full x-CSI systems. These results will therefore contribute to establishing minimum quantification capabilities that x-CSI can be expected to have when fully developed.Work by the MARS group using their pre-clinical small-animal-imaging scanner has shown that x-CSI techniques can distinguish contrast agents with as little as 4 keV between their K-edges . They weAuNP concentrations down to 1 mg/mL have been reported by other groups using x-CSI systems based on modified Phillips Healthcare scanners . This stIt is apparent that detection and quantification of AuNP concentrations as low as a few mg/mL have been achieved with x-CSI systems. Crucially, this has been shown to be possible in vivo, but the relevant question to this review is how these concentrations compare with the AuNP concentrations that will likely be used in AuNP-mediated RDEE therapies. 7 NPs/mL (~1.7 × 10−5 mg/mL when standardised as above). These data thus support the claim that RDEE effects are negligible below some minimum AuNP concentration, implying a lower detection limit needed for AuNP imaging for AuNP-mediated RDEE applications. Whilst it is reasonably inferred that AuNP design should affect this limit due to the many design parameters known to affect RDEE [AuNPs could be modified to reduce their toxicity/RDEE efficacy so that they do not produce appreciable effects at concentrations below those detectable with x-CSI. Chitthrani et al.  demonstrect RDEE ,117, varect RDEE . Furtherect RDEE ,119,120,ect RDEE ,122,123.An alternative approach is to improve the detection limits of x-CSI systems so that they can quantify the lower AuNP concentrations already used in the literature. Whilst it is true that the studies mentioned have not tried to push the detection limits lower, the uncertainties associated with the images that were obtained indicate these systems would struggle to accurately quantify AuNPs at the lowest concentrations reviewed. No x-CSI system to date has been specifically designed and optimised for the task of AuNP imaging, however. Simulation studies to determine optimal imaging parameters for this task have been performed ,76, and The technical requirements associated with producing such an optimised system are not significant given pixel pitches and sensor thicknesses both smaller and larger than the optimal ones are used already, and the CSCA parameters are already combined in some ASICS. Whilst the AuNP detection limits of such an optimised system have not yet been calculated, it is reasonable to assume based on the weight of the literature that at least an order of magnitude increase in sensitivity can be made.Though a gap of two orders of magnitude is a lot of ground to cover, the two approaches mentioned above are worth exploring, and it may be that clinical realisation initially requires a combination of them both in the short term, or the use of locally introduced and relatively immobile AuNPs so that an assumption can be made as to the maximum distribution of the AuNPs based on their initial placement. Such local immobilisation would limit some of the selective uptake benefits of AuNPs, but would allow for the collection of valuable data on how AuNP concentration and RDEE delivered correlate.The use of AuNPs to enhance the effectiveness of radiotherapy is a well-studied topic, though the corollary requirement of quantitative in vivo AuNP imaging is often taken for granted in the RDEE literature. This review introduced x-CSI techniques as a way to achieve this imaging and explained the fundamental differences between them and more conventional X-ray-imaging approaches.Whilst it is widely recognised that AuNPs offer an exciting new imaging contrast for x-CSI systems, promising molecular-imaging capabilities, the AuNP concentrations tested in practical systems (~1 mg/mL) are orders of magnitude higher than those likely to produce clinical RDEE effects. For the specific task of AuNP-mediated RDEE delivery, current x-CSI scanners are thus not yet suitable.Two paths for closing this sensitivity gap are proposed: modifying AuNPs to be less-effective RDEE agents at low concentration and optimising x-CSI scanners for AuNP detection. Specific advice from this review included the more regular reporting of negative results in RDEE literature (so that lower limits on AuNP concentration needed to induce appreciable RDEE can be established) and that x-CSI systems in future be tested using Au concentrations that overlap more significantly with those used in the RDEE literature, so that lower limits of x-CSI systems can be determined directly rather than inferred from extrapolation of data at much higher concentrations.The intrinsic trade-offs in optimising an x-CSI system for different tasks mean that instead of a single general-purpose scanner, task-specific scanners may be more common. It is hoped that the recent approval of a commercial x-CSI system by the FDA signals the first of many developments in these scanners, and that the high research interest in Au as a contrast agent is translated into a scanner optimised for Au imaging in the not-too-distant future.