The Central Sensitization Inventory (CSI) is a screening tool designed to detect symptoms related to Central Sensitization (CS) and Central Sensitivity Syndromes (CSS) by measuring the degree of related phenomena. The objective of this study was to create a German, culturally-adapted version of the CSI and to test its psychometric properties.A German version of the CSI (CSI-GE) was developed, culturally-adapted, and pretested for comprehensibility. The psychometric properties of the resulting version were validated in a clinical study with chronic pain and pain-free control subjects. To assess retest reliability, the CSI-GE was administered twice to a subgroup of patients. Structural validity was tested using factor analyses. To investigate construct validity a hypotheses testing approach was used, including (1) correlations between the CSI-GE and several other well-established questionnaires as well as (2) an investigation of the CSI-GE discriminative power between different subgroups of participants believed to have different degrees of CS.The CSI-GE showed excellent reliability, including high test-retest characteristics. Factor analyses confirmed a bi-factor dimensionality as has been determined previously. Analysing construct validity 6 out of 11 hypotheses (55%) were met. CSI-GE scores differentiated between subgroups according to expectations. Correlations between CSI-GE scores and other questionnaires suggested that none of the correlated constructs was identical, but there was overlap with other questionnaires based on symptom load. Several correlations did not fit with our current understanding of CS.The CSI-GE appears to be a reliable tool for measuring CS/CSS-related symptomatology. Whether this implies that the CSI-GE measures the degree of CS within an individual subject remains unknown. The resulting score should be interpreted cautiously until further clarification of the construct.The online version contains supplementary material available at 10.1186/s12891-021-04481-5. No gold standard for the diagnosis of CS exists, so it is difficult to assess its presence and magnitude [Chronic Pain is often related to a multitude of underlying factors, which can trigger, contribute to, and maintain it , 2. Receagnitude , 9, 10. agnitude , includiagnitude  and imagagnitude . Howeveragnitude , 13.Yunus  postulatMayer et al.  developeIn this project, the CSI was first translated into the German language, pretested for comprehensibility, and culturally adapted. Then its psychometric properties were tested, including internal consistency, dimensionality, construct validity, and discriminative ability in a group of subjects with a broad spectrum of chronic musculoskeletal pain disorders and a separate group of pain-free control subjects.This study consisted of two parts. First, the original version of the CSI was forward (English to German) and backward (German translation back into English) translated, and cross-culturally adapted into German by an expert translation committee following the multistep approach recommended by the American Association of Orthopaedic Surgeons Outcomes Committee , 21. A pSecondly, a multicentre clinical study was conducted to assess the psychometric properties of the CSI-GE with four recruiting partners: the multidisciplinary pain clinic at the University Medical Center Göttingen; the pain clinic at Rotes Kreuz Krankenhaus Bremen; the MVZ Endokrinologikum (an outpatient rheumatology practice) based in Göttingen; and an outpatient practice for pain medicine, also located in Göttingen. The multicentre study design followed the COSMIN recommendations , 24 and For each involved institution ethical approval through the responsible ethics committee was provided: Ethics Committee of the University Medical Center Göttingen (15/09/2017) (including the pretests), Ethics Committee of the Ärztekammer Bremen (Antrags-Nr. 666–11/04/2019), and the Ethics Committee of the Ärztekammer Niedersachsen (Grae/055/2019).The validation study was registered at the German Clinical Trials Register (DRKS-ID: DRKS00015252). All participants signed informed consent.Patients with diverse musculoskeletal pain disorders were included in this study. Inclusion criteria were chronic pain for at least 3 months, sufficient physical and cognitive ability to participate, sufficient knowledge of the German language, and age older than 18 years. Exclusion criteria were psychiatric disease with pain as the main symptom , initial or unstable phase of a rheumatological disease, or a primarily neuropathic pain component. All patients with chronic pain seeking routine care during the recruitment period were considered for participation and screened for eligibility by the responsible physicians of the individual case and at least one member of the study team. Eligible patients were informed about the study and asked for consent to participate.In addition, a healthy control group (HC) was recruited from the general population via local contacts. For that purpose, members of a local choir were asked to participate in the study. To broaden the age range for the control group, students from the University Göttingen and acquaintances of the study team were asked to participate as well. The primary inclusion criteria were that they reported neither acute nor chronic pain. This was determined by an additional questionnaire asking for the presence of other than every day kind of pain  or painTo determine the appropriate sample size, we followed the recommendations for a factor analysis (FA), suggesting 4 to 10 participants per item  and moreThe fibromyalgia (FMS) subgroup included patients with fibromyalgia as the primary diagnosis. Fibromyalgia was determined by the preliminary 2011 ACR criteria, including a full clinical assessment .Multisite chronic pain (MCP) included patients with a diverse spectrum of chronic pain disorders, with pain in at least three body regions.Regional chronic pain (RCP) included patients with a localized pain disorder in one well-defined body region .Chronic back and/or neck pain (CBNP) included patients with a chronic pain syndrome in the back and/or neck region.In addition, patients with Rheumatoid arthritis in remission (RAR) were included if their rheumatologist clinically determined they were in disease remission at the time of their consultation. The level of CS in this subgroup was presumed to be lower than in the multisite chronic pain and fibromyalgia subgroups. All patients within this (RAR) subgroup were recruited at the outpatient rheumatological practice and evaluated by both the rheumatologist and one member of the study team (MK).The chronic pain group was classified into five subgroups as described below. The subgrouping was based on clinical reasoning and on the main pain-related diagnosis and distribution of pain as used for classification in KEDOQ-Schmerz, a German pain register project for chronic pain based on the data provided by the widely-used German pain questionnaire , 28. TheThe CSI consists of two parts (A and B). Part A includes 25 items which assess typical symptoms associated with CS/CSS. Patients rate the degree of these symptoms on a five-point Likert scale ranging from never to always . The summation of all single item scores results in a total score ranging from 0 to 100. Part B inquires about 10 previously-diagnosed disorders in the patient’s medical history, including seven common CSSs and three other conditions linked to CS/CSS. Part B of the CSI only aims to provide additional information and is not scored , 18. We The PSQm is a condensed version of the PSQ which assesses individual subjective sensitivity to pain. The instrument includes 7 questions rated between 0 and 10. The mean rating of the 7 questions is used as the resulting score. The PSQm has previously demonstrated high correlations with sensitivity to experimental pain in healthy controls  and chroThe DASS includes three scales which measure symptoms of depression, anxiety, and stress. The instrument contains 21 items . All items can be rated from 0 to 3, higher scores represent more severe symptoms. The German version was validated in 2015 and has demonstrated acceptable psychometric properties for screening for depression, anxiety, and stress in patients with chronic pain .The PHQ15 is a self-administered instrument that contains 15 items which assess the severity of somatic symptoms, indicating the individual degree of somatization. It has been translated into the German language and validated . We usedThe painDETECT is a screening tool to detect a neuropathic pain component in chronic pain patients . It has The PCS focuses on the quantification of catastrophizing attitudes and thoughts towards pain . It contThe FSQ is a self-administered tool to identify FMS in survey research without a physical examination. It has been validated for the German population  and consThe MFHW is a short Questionnaire capturing perceived general wellbeing by addressing positive thoughts in 7 questions. The total score ranges from 0 to 35. Higher scores indicate a higher degree of habitual well-being .The VR 12  measuresVon Korff et al.  introduc.The MPSS by Gerbershagen distinguishes three increasing degrees of pain chronicity by its associated features  . ThroughIn addition to the questionnaires, we included a question about the duration (in months) of each patient’s chronic pain status.Study participants had the choice to complete the questionnaires during a hospital admission, outpatient visit, or at home (postage-prepaid envelope provided). All returned questionnaires were checked for missing items and participants were contacted to provide the missing answers. Participants were instructed in a standardized manner to complete missing items including the option not to answer.n = 50 was determined adequate, assuming an ICC (intraclass-correlation-coefficient) of 0.8 with a 95% CI of ±0.1 [n = 56) received a second questionnaire for completion of the CSI-GE 2 weeks after the initial study visit. They were provided with a prepaid envelope and reminded after 3 weeks if the envelope had not been received by then. The test-retest time interval of 2 weeks, which has previously been recommended by de Vet et al. [To analyse test-retest reliability, a time-interval of 2 weeks between two measurement occasions and a sample size of  of ±0.1 . A subgrt et al. , seemed To assess the psychometric properties of the CSI-GE part A, both reliability and validity were investigated. Reliability was tested by analysing internal consistency and test-retest reliability. For assessment of the structural validity, both exploratory and confirmatory factor analyses (FA) were used to analyse the dimensionality of the questionnaire. To assess the construct validity, the relationship with other well-established clinical variables and questionnaires was analysed. In addition, to assess the discriminative validity, the ability of the CSI-GE to differentiate among different patient subgroups, believed to have different levels of CS (as described above) was investigated. The normality and data distributions were assessed by the skewness, kurtosis, histograms, Q-Q Plots, and Kolmogorov-Smirnov as well as Shapiro-Wilk tests. Demographic variables are described by means and standard deviation. Overall sex differences within the different subgroups were analysed using Fisher’s-Exact-Test. Age differences were analysed using a Kruskal-Wallis-Test followed post hoc analysis for every specific subgroup comparison.Floor and ceiling effects were determined by examining the prevalence of participants scoring the lowest and highest possible score on the CSI-GE sum score. As proposed by McHorney and Tarlov  the effe2,1  [difference/ Internal consistency was evaluated using Cronbach’s α. To evaluate test-retest reliability, the intraclass-correlation-coefficienteasures)  was calceasures)  ICC valueasures) . SEM wasp-values from the respective significance tests. An ANOVA-type χ2 difference test between the nested models was performed.Structural validity was analysed using FA. Exploratory factor analysis (EFA) using Maximum-Likelihood-Extraction with Promax-Rotation was carried out with data from the chronic pain sample to assess whether there would be a different factor structure than described in previous validation studies. Confirmatory factor analyses (CFA) were performed to assess the fit of three previously published models to our dataset, including both subject groups (CPP and HC). We assessed the fit to the original model by Mayer et al.  with fouTo assess construct validity 11 different hypotheses Table  were forThe discriminative power of the CSI-GE was assessed by examining differences among the six subgroups  using a Kruskal-Wallis test and Dunn-Bonferroni post-hoc-tests for nonparametric data. Bonferroni-Correction for multiple comparisons was used. A conservative non-parametric statistical approach was chosen for this analysis due to the violation of normal distribution and heterogeneity of variance between the different subgroups.r > 0.1, medium for r > 0.3, and large for r > 0.5. To use these categories for the differently scaled Kendall’s τ, we translated these to the scale of Kendall’s τ, using the formula given by Kendall [Pairwise correlations between total scores on each questionnaire and on the CSI-GE were calculated using Kendall’s τ as a non-parametric correlation coefficient. Only data from the CPP were used, and pairwise deletion of missing cases was applied. Tests against the null hypothesis of no correlation were performed. Correction for multiple testing was done using Holm’s procedure. Following Cohen , correla Kendall . ConsequThe significance level was set to alpha = 5% for all statistical tests. The pairwise correlations with other questionnaires and the CFA were performed with the statistic software R version 3.6.1  using th. The allocation of the 310 datasets to subgroups and the corresponding demographics are shown in Table At the end of the recruitment phase, 346 individuals had been recruited to the study. Thirty-six datasets could not be included for different reasons listed in Table 1p = 0.01). However, post hoc comparisons showed that this age difference was only significant between the HC and RAR group. Seventy percent of the individuals were female, with 50.8% in the HC group and 94.6% in the FMS Group. Fisher’s exact test (p = 0.002) indicated that overall, sex was not distributed similarly within all subgroups. Correlating the CSI-GE sum score with age  and sex  showed only a weak correlation with age (p = n.s.) and weak correlation with sex. The mean CSI-GE total score in the total CPP sample was 43.6 and in the HC sample was 18.4. The FMS group had the highest mean CSI-GE score (54.9) and the HC group had the lowest. The FMS group also reported the highest number of summed responses (3.6) on CSI-GE part B, and the HC group reported the lowest number of responses (0.3).As shown in Table No floor (sum score = 0) or ceiling effects (sum score = 100) occurred with total CSI-GE scores. The lowest score was 4 points by two participants , and the highest score was 77 points by one participant .2,1 of 0.917  for the overall sum score of the CSI-GE, with a mean test-retest time interval of 18.42 (min. 14/max. 32) days. The limits of agreement can be observed in Fig. Reliability analysis of the CSI-GE yielded a Cronbach’s α of 0.928, which can be considered high . Excellep < 0.001) showed an overall good [The Kaiser-Meyer-Olkin criterion (0.88) and Bartlett’s test .x2(269) = 553.09, p < 0.001. As expected, all items showed significant positive factor loadings, with standardized coefficients ranging from 0.438 to 0.861. As demonstrated in Table Original 4-factor model proposed by Mayer et al. : The modB).p < 0.001. All items showed significant positive factor loadings, with standardized coefficients ranging from 0.407 to 0.848.The 1-factor model proposed by Cuesta-Vargas et al. : The modC).x2(250) = 430.6, p < 0.001. All items showed significant positive factor loadings for the general factor, but not all factor loadings were significant or positive for the other four latent factors, with standardized coefficients ranging from − 0.251 to 0.816.The Bifactor model proposed by Cuesta-Vargas et al. : The modx2(19) = 115.1, p < .001) than the original 4-factor model, while the original 4-factor model fit the data significantly better than the 1-factor model (x2(6) = 136.9, p < .001).Comparing the different models used in the CFA, the bifactor model fit the data significantly better (p < 0.001) in CSI-GE total scores among the subject subgroups. Post-hoc analyses found that the control group scored lower than all the other groups except patients with only regional pain. Patients with multisite or FMS-related pain were not significantly different from each other but scored higher than the other groups  and FSQ-SSS (τ = 0.56), and low correlations with the FSQ-WPI τ = 0.47), CSI-GE-Part-B (τ = 0.45), DASS-Anxiety (τ = 0.45), DASS-Stress (τ = 0.43), PainDETECT-subscore (τ = 0.43), DASS-Depression (τ = 0.41), VonKorff (τ = 0.36), VR12-MCS (τ = − 0.35), MPSS (τ = 0.32), general well-being (τ = − 0.28), PCS (τ = 0.28), pain intensity (τ = 0.27), PSQm (τ = 0.23) and negligible or no significant correlations with pain duration and VR12-PCS  ranged from 0.31 [The objective of this study was to create a culturally-adapted German version of the CSI and to test its psychometric properties. Internal consistency of different CSI translations has been examined in most international studies , but demonstrated limited construct validity with respect to hypotheses more closely related to the construct of central sensitisation itself. Nevertheless, only the PHQ and SSS can be considered comparator instruments, the other questionnaires measured potentially related yet different constructs than the CSI. Subsequently, individual hypotheses, as well as the explorative correlations are further discussed.The CSI-GE construct was supported by its ability to differentiate between subgroups believed to have different degrees of CS. The FMS and MCP groups scored significantly higher than all other groups except each other. This was expected as those groups were believed most likely to have the highest degree of CS. Also, the HC group differed significantly from all pain groups except the RCP group, which was believed to most likely have lower levels of CS. These finding are supported by Knezevic et al. , who fouHowever, correlations with other instruments were not as unequivocal. The highest correlations with somatic symptom severity may partly be explained by overlapping items between instruments. FMS patients, known to score highly on these instruments and to embody features of CS , were thAll three scales of the DASS showed a low, positive correlation with the CSI-GE, demonstrating an increasing negative affective/emotional burden with increasing scores in the CSI-GE. Chiarotto et al.  showed pThe correlation between the CSI-GE and the painDETECT showed a small association with neuropathic pain components. Rehm et al.  found thThe degree of chronification (MPSS) and severity (Korff) of chronic pain both correlated poorly with the CSI-GE. The slightly higher correlation with the Von Korff scale is possibly explained with pain-related disability, which is considered within the score. This is supported by findings of Kregel et al.  who showThe PCS correlated to a small degree with the CSI-GE. This was unexpected as we believed that the concept of catastrophizing is an amplifying factor for CS, and previous CSI studies have reported higher correlations with the PCS , 64. CorThe CSI-GE demonstrated no association with the duration of pain. This finding is in line with other CSI studies , 61. OneOur results support the negative correlations between mental quality of life and the CSI scores that have been found in previous studies , 64, 69.After the responses on CSI part B were summed, the FMS group reported the highest number of CSS-related diagnoses (3.6) and the HC group reported the lowest number (0.3). These results are in line with previous studies that have assessed CSI part B , 63. ThoAs with all studies of this kind, the results are based on a limited sample of subjects within two regions of Germany, so the findings may not generalize to other populations. More women than men were recruited as in previous CSI validation studies , 58, whiAs there was no formal a priori definition of the hypothesis, the selected cut–off points (like high or medium correlation) remain somewhat arbitrary and of limited validity. The possibility of treatment effects, prior to data collection, on CSI scores needs to be considered. This highlights a potential limitation in ours, as well as other CSI validation studies, as they include little information about previous treatments such as psychotherapy, interdisciplinary multimodal pain therapy, or drug therapies that could affect questionnaire responses , 73. OneThe CSI-GE demonstrated robust psychometric properties as well as solid reliability. Based on the results of our factor analysis, it thus seems justified to compute one overall sum score. Our construct validation assessment suggests that the questionnaire reflects a dimension that no other tool that we compared has captured in the same way, although high symptom load was a prominent overlapping feature. Some of the correlations were unexpected within the current understanding of CS. It remains uncertain to which degree the CSI-GE captures CS and quantifies its symptoms. Combining our findings with previously published research regarding the CSI, we can conclude that interpretation of the total CSI score is made more difficult because definitions of CS are diverse; symptoms are broad and overlapping in a variety of conditions and may indicate polysymptomatic distress; new concepts such as nociplastic or chronic primary pain may be insufficiently considered; and no gold standard exists for the clinical or experimental quantification of CS. Experimental studies and studies examining the responsiveness to interventions in well-characterized patient groups may help to better define the scope of the instrument.In conclusion, we recommend using the CSI-GE in clinical practice only with caution and primarily as a screening for symptoms that may be related to CS. The concept of CS requires further clarification within a research context. There is currently no established clinical diagnostic or treatment pathway in case of a positive screening result using the CSI-GE.Additional file 1: Supplement 1. Different international CSI validation studies in different languages.Additional file 2: Supplement 2. The German translation and cross-cultural adaptation of the Central Sensitization Inventory (CSI-GE).Additional file 3: Supplement 3. CENTRAL SENSITIZATION INVENTORY German version (CSI-GE)_V_19.09.2020.Additional file 4: Supplement 4. Visualized pairwise correlations between the CSI-GE part A sum score and each questionnaire.


The amounts of the components in a microcapsule self-healing system significantly impact the basic performance and self-healing performance of concrete. In this paper, an orthogonal experimental design is used to investigate the healing performance of microcapsule self-healing concrete under different pre-damage loads. The strength recovery performance and sound speed recovery performance under extensive damage are analyzed. The optimum factor combination of the microcapsule self-healing concrete is obtained. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) are carried out on the concrete samples before and after healing to determine the healing mechanism. The results show that the healing effect of self-healing concrete decreases with an increase in the pre-damage load, and the sound speed recovery rate increases with an increase in the damage degree. The influence of the sodium silicate content on the compressive strength and compressive strength recovery rate of the self-healing concrete increases, followed by a decrease. The optimum combination of factors of the microcapsule self-healing system is 3% microcapsules, 30% sodium silicate, and 15% sodium fluosilicate. The results can be used for the design and preparation of self-healing concrete. Self-healing concrete is used to prevent concrete deterioration and improve durability ,2,3. SinSince White et al.  first deOther studies focused on the diversity of healing conditions and mechanisms of self-healing concrete with microcapsules. Sun et al.  preparedHowever, the amounts of the components in the microcapsule self-healing system and the components’ influences on the basic properties and self-healing properties of concrete remain unclear. In this paper, a microcapsule system is used in cement-based composites, and an orthogonal test is used to analyze the healing performance for different pre-damage loads. The strength recovery performance and sound speed recovery performance under extensive damage are analyzed using an orthogonal test. The optimum factor combination of the microcapsule self-healing cementitious composite system is obtained. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) are conducted on the concrete samples before and after healing to determine the healing mechanism of the microcapsule self-healing concrete.A Wj-3 extrusion spheronizer  was used to prepare the microcapsules. The main steps included preparation of the wet material, extruding the strips, rolling the balls, spray coating, and hot-air drying. The first three steps focused on forming the pellets, and spray coating was used to combine the core and the wall. An appropriate coating solution was used. Due to the relatively high viscosity of the ethyl cellulose solution used in the capsule wall, the dissolving agent has volatile components. Thus, that material is prone to thermosetting, and hot-air drying was needed to prevent mutual adhesion. The preparation of the wet material was the first step and determined the composition of the repair material in the core material. Portland cement and sodium silicate, which react with calcium hydroxide, were added to the self-healing microcapsules, representing the core materials. This composition facilitates the comparison with subsequent experiments.Concrete specimens (100 mm × 100 mm × 100 mm cubes) were prepared. The fixed water binder ratio was 0.54, the fineness modulus of the sand was 3.0–2.3, and the particle size of the gravel was about 3–5 mm. The cement properties are listed in A three-factor, three-level experimental design was adopted; the orthogonal experimental design is shown in R on the compressive strength of concrete is 60%. The F-value of the proportion of sodium fluorosilicate is significantly higher than that of the other two factors, and the confidence level exceeds 95%.F-value and B has a higher F-value for the concrete cured for 28 d, indicating that the strength of the microcapsules may have a larger influence on the concrete aged for 28 d. In addition, the significant decrease in the F-value of factor C is due to the retarding effect of sodium fluosilicate. An increase in its dosage results in a more pronounced retarding effect for the concrete cured for 7 d and a decrease in the retarding effect for the concrete cured for 28 d.The order of influence of the factors on the compressive strength at 28 d is B > C > A, and the influence of factor B is similar to that of factor C. Compared with the concrete aged for 7 d, factor A has a lower K-values in A comparison of the The recovery rate of the compressive strength according to the ultimate compressive strength of the cube specimens before and after healing is listed in K-values  and was low at a concentration of 70%. Therefore, the optimum factor level of the sodium silicate content is 50%.The The influence of factor C (sodium fluosilicate content) on the recovery rate depends on the curing time. At 7 d of curing, the recovery rate increases with an increase in the sodium fluosilicate content due to its retarding, resulting in a substantial reduction in the concrete strength aged for 7 d and an increase in the recovery rate in the later period. At aging for 28 d, the optimum factor level of C2 is 15%. The reason is the same as for factor B, i.e., the healing effect at a low dosage cannot compensate for the cracks, but the damage is similar at a high dosage.F-values of the compressive strength recovery rate after healing at aging for 7 d and 28 d are listed in F-value is obtained and vice versa. This finding demonstrates the effectiveness of the self-healing performance of the microcapsule self-healing concrete system.The F-values of the factors are greater than 1, and the confidence level is greater than 50%, indicating that all three factors affect the self-healing efficiency of concrete. The influence of the sodium fluorosilicate content is higher under a lower damage load because there are fewer broken microcapsules, and the sodium silicate may dissolve in water. The more sodium fluorosilicate is added, the higher the reaction rate is, and the greater the strength recovery is.The variance analysis shows that in most cases, the K-values of the compressive strength and compressive strength recovery rate indicate that the optimum factor combination in the microcapsule self-healing concrete system is A2B2C2, . This combination provides the optimum self-healing effect of the concrete without reducing the concrete strength under different damage loads.The variance analysis and As the effect of the sound velocity of concrete is negligible at damage loads of 80% and 70%, the ultrasonic wave velocity was only tested for concrete specimens with 100% and 90% pre-damage. The test results are listed in F-values are obtained for the microcapsule content, which is consistent with the concrete healing rate under a high damage load. The confidence level for the microcapsule dosage is more than 75%, whereas that of the other two factors is almost zero. K-values of factors B and C are relatively gentle, and the maximum sound speed recovery rate is obtained at 3% dosage; thus, the optimal factor level of the sound speed recovery rate of factor A is 3%.For the sound speed recovery rate, the largest The most important factor affecting the sound speed recovery rate is the degree of crack filling or shrinkage of the concrete after cracks have formed, which leads to the shrinkage of internal cracks and holes, restoring the propagation speed of the ultrasonic wave. As this study focuses on concrete with extensive damage, the most important factor affecting the sound speed recovery rate is the amount of the healing material, i.e., the microcapsule content. Sodium silicate is an adhesive that bonds with concrete and affects the extension and expansion of concrete cracks. The concrete also shrinks in the natural environment; thus, a change in the sodium silicate content will have an effect, especially under low damage loads. Sodium fluorosilicate is a curing agent of concrete. Although sodium fluorosilicate and sodium silicate have a curing effect, the main focus of this study is the concrete strength. The bonding effect of sodium fluorosilicate is the same at high and low dosages, but different curing effects have different influences on the concrete strength. Therefore, the influence of the sodium fluosilicate content on the sound speed recovery rate is negligible.There are some differences between the recovery rate of the compressive strength of the specimens and the sound speed recovery rate. For example, the other two factors have only a negligible influence. Only the sodium silicate content under a 90% damage load has a low confidence level. The characterization of acoustic properties has distinct advantages and disadvantages compared with the compressive strength recovery rate. An ultrasonic velocity test can be carried out without destroying the specimen, and it is highly sensitive to cracks, and the strength recovery is be affected by the concrete strength. There are also disadvantages. Due to the strong randomness of natural fractures in concrete , crack eSome fragments of the completely damaged specimen are shown in The images of the concrete specimens with 100% damage after immersion healing are shown in The micromorphology of the damaged microcapsules in the concrete is shown in The EDS analysis of the specimens  and the 2NaF. Finally, as shown in 4 gel is required for the reaction.Sodium fluoride is soluble and sodium carbonate is easily soluble in water, resulting in sodium loss. In addition, In this paper, the strength recovery and acoustic performance of microcapsule self-healing concrete were used as evaluation indices to assess the damage to the microcapsule self-healing system. An orthogonal experimental design with three factors was used. The optimum factor combination of the system and the best mix proportion of the microcapsule self-healing concrete were determined. The following conclusions were drawn.(1)The evaluation indices showed that the self-healing performance was better for the microencapsulated self-healing concrete than the ordinary concrete. The healing effect of the self-healing concrete decreased with an increase in the pre-damage load, and the sound speed recovery rate increased with an increase in the damage degree.(2)All three factors affected the healing rate. In contrast, the proportion of sodium silicate and the sodium fluosilicate dosage had a negligible influence on the sound speed recovery rate at a high damage degree.(3)The microcapsule content was the highest under a high damage load and vice versa. The influence of the sodium silicate content on the compressive strength and compressive strength recovery rate of the self-healing concrete increased, followed by a decrease. The curing agent had a more significant effect on the concrete at 7 d than at 28 d of curing.(4)The optimum combination of factors of the microcapsule self-healing system was 3% microcapsules, 30% sodium silicate, and 15% sodium fluosilicate.


IC) and fracture energy (GF) of SFHSC specimens were studied. The results show that an increase in the volume fraction of steel fiber (fρ) added to high-strength concrete (HSC) significantly improves the fracture behavior of HSC. As compared to milled and sheared corrugated steel fibers, cut bow steel fibers significantly improve the fracture behavior of SFHSC. The effect of incision depth changes on the KIC and GF of SFHSC and HSC for the comparison group has no common characteristics. With an increase in incision depth, the values of KIC of the SFHSC specimens decrease slightly. The GF0.5/GF0.4 of the SFHSC specimens show a decreasing trend with an increase in f.ρ According to the test results, we propose calculation models for the fracture behavior of SFHSC with different strengths. Thus, we present a convenient and accurate method to calculate fracture parameters, which lays a foundation for subsequent research.The behavior of steel fiber concrete, which is the most widely used building material, has been widely examined. However, methods for calculating Fracture parameters differ by fracture behavior of SFHSC with different strengths. In this study, the fracture behavior of steel-fiber-reinforced high-strength concrete (SFHSC) was -investigated using three-point bending tests. A total of 144 notched concrete beams with a size of 100 mm × 100 mm × 515 mm were tested for three-point bending in 26 groups. The effects of the steel fiber volume ratio, steel fiber type, and relative notch depth on the fracture toughness (K With the development of modern building technology, numerous varied complex structures have emerged with cement concrete performance requirements; therefore, methods for improving the performance of concrete are always of interest to researchers ,2. CurreTo enhance the strength of the concrete matrix and to reduce the toughness, investigators have trialed different treatments. Studies have found that merging fibers into concrete can enhance the tensile strength, fracture toughness, dynamic mechanical properties, and the durability of concrete ,7,8. Fibnet) of the material, then the material can be considered to have notch sensitivity . The peak load, peak deflection, and ultimate deflection all increase to varying degrees with an increase in fρ.Steel fiber can more effectively improve the G(3)IC values of the BF are 3.62 and 7.41 times those of the MF and SF, respectively. The KIC increment ratios of the BF are 3.17 and 7.77 times those of the MF and SF, respectively. The GF values of the BF are 6.57 and 5.85 times those of the MF and SF, respectively. The GF increment ratios of the BF are 7.06 and 5.67 times those of the MF and SF, respectively.The BF is most effective for improving the fracture behavior of SHFSC. The K(4)IC of HSC specimens. The KIC of SFHSC specimens decreases slightly with an increase in the incision depth.A change in notch depth has no significant effect on the K(5)Based on the statistical analysis of the test results, the calculation models for the fracture parameters of SFHSC with different strengths are obtained. The fracture toughness calculation model is expressed as Based on the TPBT of the notched concrete beam, the influence of the type of steel fiber, the volume fraction of steel fiber, and the relative notch depth on the fracture behavior of SHFSC was studied. In actual large-scale structural design projects, the fracture performance of the selected SHFSC can be measured and judged based on the conclusions and formulas offered in this study. The main conclusions are as follows:In this paper, the fracture behavior of SHFSC is only studied through TPBTs, which can be further verified by comparative tests. The research on SHFSC materials is developing rapidly. Future work should explore the bridging role of fibers at a deeper level. The research will use concrete tests and construct a micro finite element model. Future work will also propose a model suitable for concrete design and application. Finally, a model suitable for concrete design and application will also be proposed.


The aim of this study was to reveal the macroscopic and mesoscopic deterioration behaviors of concrete under the coupling effect of chlorine salt erosion and the freezing–thawing cycle. The rapid freezing–thawing test was carried out in a 5% chlorine salt environment. The macroscopic characteristics of concrete were analyzed by testing the mass, the relative dynamic modulus of elasticity, and the compressive strength of concrete under different freezing–thawing cycles. Using CT scanning technology and three-dimensional reconstruction technology, the pore structure, CT value, and surface deviation of concrete before and after freezing–thawing were analyzed. Based on the changes of solid volume, pore volume, and solid CT value of concrete, the calculation method of relative CT value was proposed, and the damage model was established with relative CT value as the damage variable. The results demonstrate that the mass loss rate decreases in the beginning and then increases in the process of chlorine salt erosion and freezing–thawing, and the smaller the concrete size, the greater the mass loss rate. The relative dynamic modulus of elasticity decreases gradually, slowly at the initial stage and then at a faster rate, and the compressive strength loss rate increases gradually. The pore quantity, porosity, and volume loss rate of concrete increase in a fluctuating manner, whereas the relative CT value decreases. The comprehensive analysis shows that the chlorine salt frost resistance of concrete is negatively related to the water-cement ratio when the freezing–thawing cycle is fixed. The damage model could better reflect the freezing–thawing damage degree of concrete with different water cement ratios, and the damage evolution process is well described by the Weibull function. With the advantages of simple production technology, easy availability of raw materials, wide range of strength grade, and low cost, concrete has become one of the most extensively used building materials . It has − spreads continuously in concrete. On the one hand, Cl− induces chemical reactions with hydration products in concrete, loosening the internal structures of concrete materials [− induces erosion of rebar, thus causing structural damages [Chlorine salt erosion and freezing–thawing damage are important factors that influence concrete durability, thereby significantly affecting structural performance and service life . In chloaterials . On the  damages . Influen damages . Under t damages ,11. Ther− were analyzed, and the durability was evaluated. Referring to the variation laws of concrete mass and compressive strength, Mangi et al. [Currently, a great deal of research on concrete durability under the influences of salt erosion and/or freezing–thawing cycles has been done. Under chlorine salt erosion, Li et al.  charactei et al.  discussei et al.  establisi et al.  analyzedi et al. . Liu et i et al.  have disSome scholars have studied concrete durability under the coupling effect of chlorine salt erosion and freezing–thawing cycles. For example, An et al.  carried Hence, CT technology could characterize the evolutionary characteristics of concrete’s internal materials and pores after being influenced by the external environment in a more straightforward and precise manner. This study analyzed the relationship between microscopic characteristics and macroscopic indexes with CT technology as a mesoscopic characterization method, combined with the macroscopic deterioration law of concrete under the chloride salt erosion and freezing-thawing. Based on the changes of volume, pore structure, and CT value, the relative CT value was used to characterize the damage of concrete, and the degradation mechanism of concrete with different water cement ratios under the chlorine salt erosion and freezing–thawing cycle was comprehensively analyzed.3, and a crushing index of 6%. The fine aggregate was medium sand, with an apparent density of 2630 kg/m3, a mud content of 1.1%, and a fineness modulus of 2.7. The material parameters of gravel and sand were obtained according to the test method of Chinese Standard JGJ 52 [In the test, the ordinary Portland cement P·O 42.5 from Jidong Cement  was used. The physical and mechanical properties are listed in d JGJ 52 . Local tConcrete specimens with three different water–cement ratios were designed in the test. All specimens were divided into groups A, B, or C according to different water–cement ratios, and each group included two sizes of specimens. There were three 100 mm × 100 mm × 400 mm specimens in each group, and the samples were used to test the dynamic modulus of elasticity. Thirty-nine 100 mm × 100 mm × 100 mm specimens in groups A and B and twenty-seven 100 mm × 100 mm × 100 mm specimens in group C were used to test the compressive strength of concrete, and three specimens in each group were taken out every 25 freezing–thawing cycles. Mixing ratios and mechanical properties are listed in Standard for test methods of long-term performance and durability of ordinary concrete (GB/T 50082-2009) [The freezing–thawing test was conducted using the fast-freezing method in 82-2009) . In the The concentration of salt solution has an influence on the frost resistance of concrete. Yang  showed tFirst, three groups of specimens were immersed in 5% (mass fraction) NaCl solution for 4 d, and the freezing–thawing cycle test was performed in 5% NaCl solution after attaining saturation. The NM-4B nonmetallic acoustic wave tester  was used to test the wave speed of concrete prismoid, after which the RDME was calculated. The Brilliance iCT 256 hierarchical spiral topspeed CT machine  was used for chip scanning of cube specimens. Scanning parameters were set as follows: slice thickness: 1 mm, display matrix: 512 × 512, voltage: 120 kV. CT tests were conducted under saturation conditions of concrete. To avoid liquid evaporation, specimens were wrapped with plastic film. After obtaining CT images, the specimens were rebuilt by VGStudio Max 3.2 . After setting unified coordinates, the pore structures, CT values, and other mesoscopic indicators were obtained through threshold segmentation method. The major test process is shown in Morphologies of concretes with different water–cement ratios after different salt freezing–thawing cycles are shown in As can be seen, after 100 salt freezing–thawing cycles, Group A had small surface damage, whereas Groups B and C began to develop mortar peeling on the concrete surface. After 200 salt freezing–thawing cycles, the surface damage of concrete became intensified. Group A showed evident surface mortar peeling at the corners. Group B showed serious surface mortar peeling on the surface and exposed aggregates. Group C showed the most serious damages and a large aggregate exposure range accompanied by aggregate falling at the corners. Moreover, one specimen from Group C developed rupture failure. After 300 salt freezing–thawing cycles, the surface mortar peeling range of Group A was expanded, and aggregates at the corners fell. The surface mortar peeling of Group B was intensified accompanied by evident aggregate falling at the edges and corners. Furthermore, Group B could not maintain the integral morphology. Obviously, the apparent damage of concrete, which was greatly influenced by the water–cement ratio, was intensified gradually with the increase in salt freezing–thawing cycles. The higher water–cement ratio led to the large and rapid development of apparent damage. The salt freezing-induced damage of concrete mainly presented the following phenomenon: surface mortar peeling in the early stage of salt freezing, progressive inward damage and aggregate falling with increased salt freezing–thawing cycles, and finally rupture failure.The mass variation law of concrete can be reflected by solution absorption and peeling degree during salt freezing–thawing cycles. The mass loss rates of different groups of prismoid samples are shown in The mass loss rates of cube specimens are shown in Long-term Performance and Durability Test Method of Ordinary Concrete (GB/T 50082-2009), the judgment reference of failure occurs when the RDME decreases to 60%. In the current study, Group C first broke at 200 salt freezing–thawing cycles and Group B broke at 250 cycles.The different curves of the RDME of different concrete groups under salt freezing–thawing cycles are shown in As shown in Variations of compressive strength loss rate of different groups under salt freezing–thawing cycles are shown in The reduction of concrete strength was introduced as follows. Chlorine crystal pressure and the expansive force of solution acted on the micropores in the concretes. When the pore wall pressure exceeded the compressive strength of concrete, cracks initiated inside the concrete . With thTo study the evolution rules of the internal microstructure of concrete with different water–cement ratios under salt freezing–thawing cycles, the CT section images of concrete after salt freezing–thawing cycles were analyzed in this study. Limited by the article length, only Group B was analyzed. Three sections were chosen for display . AggregaEvolution of internal structures could be analyzed by observing the sections. However, it is difficult to judge the variation laws of compaction degree in different regions of concrete. To observe the evolution of the compaction of concrete section more intuitively, the distribution diagrams of CT value hotspots on different sections are shown in In this study, three-dimensional (3D) reconstruction of CT scanning data was conducted by using VGStudio Max software. The microstructural evolution rules of concrete after the salt freezing–thawing cycles were analyzed through variations in meso-structure parameters, such as pore structure, rebuilt volume, CT value, and surface deviation. Let us take Group B as an example. The morphologies and 3D reconstruction model after 0, 100, 200, and 300 salt freezing–thawing cycles are shown in As shown in After 3D reconstruction, the quantity of pores, porosity, volume, and CT value of concrete could be ascertained, thus facilitating the analysis of the evolutionary rule of concrete microstructure. The selection method is mainly determined by the practical situations of the chosen concrete, and the reasonable threshold range is acquired through repeated debugging. The chosen pore threshold range in the current study was –1024 to 1250 Hu, and the threshold range of solid phase materials was 1250 to 3701 Hu.The pore structures of concrete according to the chosen threshold ranges are shown in According to the pore extraction results, the quantity and porosity of pores of concrete after the salt freezing–thawing cycles was analyzed. The evolutionary rule of the pore quantities and porosity of different concrete groups is shown in As shown in The evolutionary rules of the concrete porosities of different groups based on the number of salt freezing–thawing cycles were analyzed b. It canAfter 3D reconstruction, the volume information of scanned concrete can be gained. Based on the above morphological analysis results, concrete shows great material loss after the salt freezing–thawing cycles. The volume loss rates of the concrete specimens were analyzed to conduct a quantitative study on concrete volume loss of different groups under salt freezing–thawing cycles .With the increase in the number of salt freezing–thawing cycles, the volume loss rate of different groups changed greatly. After 100 cycles, the volume loss rates of Groups A, B, and C were 0.09, 0.82, and 1.42%, respectively. After 200 cycles, the volume loss rates of Groups A, B, and C were 1.80, 2.18, and 4.46%, respectively. After 300 cycles, the volume loss rates of Groups A and B were 2.14 and 32.12%, respectively. In summary, the volume loss rate of concrete increased gradually with the increase in the number of salt freezing–thawing cycles. Given the same salt freezing–thawing cycles, overall volume loss rate of concrete was positively related with the water–cement ratio.As shown in the results, concrete experienced changes in compaction degree in addition to changes in pore structures and volume under salt freezing–thawing cycles. Among all parameters measured from the CT test, CT value is the parameter that reflects the compaction degree of materials. Therefore, the variation laws of the compaction degrees of concrete specimens were analyzed by CT values. The evolutionary rules of solid CT values of concrete are shown in As shown in After 3D reconstruction, the apparent state of concrete after the salt freezing–thawing cycles and the initial apparent state were compared. Apparent deviations of concrete before and after the salt freezing–thawing cycles were discussed, in which a greater deviation indicated the higher material loss depth of concrete. The nephograms of the deviations of Groups A, B, and C after 300, 300, and 200 salt freezing–thawing cycles are shown in As shown in For the quantitative analysis of apparent deviations, the relation curve between concrete surface deviation and proportion of concrete surface was plotted . After tAs shown in Generally, after chlorine salt erosion and freezing–thawing cycles, concrete can develop damage due to volume loss and changes in pore structure and compaction degree. The intensifying damage can significantly influence the mechanical properties of concrete. The damage variable is a state variable in materials, and its determination is the primary task of damage model construction. With respect to concrete durability, relevant scholars often used RDME as the damage variable. However, this method cannot evaluate damages from the perspective of microstructural evolution. Thus, to determine the evolutionary rules of the internal microstructure of concrete under salt freezing–thawing cycles, the compaction degree was used as the damage variable to analyze the damage evolution of concrete by combining with the CT test.I could be gained. Through the reconstruction of computer images, the CT images composed of m × n attenuation coefficients could be gained. Through the linear normalization of attenuation coefficient relative to water, the compaction degree of concrete can be expressed by the CT value as follows [H is the CT value of concrete (Hu), μ is the attenuation coefficient of concrete, and wμ is the attenuation coefficient of water.At CT scanning, concrete was divided into m × n individual elements so that m × n values of  follows :(1)H=100i material component, and i of the material.If the measured substance is a mixture, the volume percentages of the constituent materials are there is :(2)μ=∑i=sV, lV, and gV are the volume of solid, liquid, and air phases in concrete (mm3); V is the overall volume of concrete (mm3); and sH, lH, and gH are CT values of solid, liquid, and air phases in concrete, respectively (Hu).The overall attenuation coefficient of concrete represents the average value of each component material. From the perspective of material characteristics, the composition of internal materials of concrete is divided into three states: solid, liquid, and air. Therefore, the overall attenuation coefficient is mainly composed of three parts, which can be expressed as:n salt freezing–thawing cycles (Hu); n salt freezing–thawing cycles (Hu); n salt freezing–thawing cycles (mm3); n salt freezing–thawing cycles (mm3); n salt freezing–thawing cycles (mm3); 3); and 3).Under salt freezing–thawing cycles, CT value can only represent the compaction degree of concrete at a moment after volume loss of concrete, but it cannot reflect the degree of variation of the compaction degree. For comparison between compaction degree after salt freezing–thawing cycles and initial value, the concept of relative CT value was introduced. When the concrete was in the drying state, it was hypothesized that material loss of concrete is filled by air. Therefore, the relative CT value of concrete after a salt freezing–thawing cycle can be expressed as:From Equation (6), the relative CT values of concrete (nD), expressed below.It can be seen from n is number of salt freezing–thawing cycles.It has been proposed  that theBased on the fitting of test data, the Weibull damage evolutionary equations of groups A, B, and C after different numbers of salt freezing–thawing cycles could be gained using Equations (9)–(11). The comparison between the calculated results of the damage model and test value is shown in As shown in 2 in the chlorine salt environment, as its solubility increases, thereby increasing porosity in concrete and inducing concrete damages. Furthermore, Cl− can react with C3A in concrete to produce C3A·CaCl2·10H2O. As the volume of reaction products is higher than the original structure, a swelling stress is produced in concrete, thus changing the microstructure and finally causing concrete damages. The major chemical reactions are shown below.Under the coupling effect of chlorine salt erosion and freezing–thawing cycles, concrete bears chemical erosion and physical erosion simultaneously. Chemical erosion is mainly caused by chemical reactions between chlorine salt and hydration products found in the concrete . In hydrPhysical erosion mainly involves frost heaving and permeability. In freezing–thawing cycles, volume swells upon icing of water in the concrete pores. When swelling stress exceeds the compressive strength of concrete, cracks initiate in the concrete. With the increase in the number of freezing–thawing cycles, cracks propagate gradually and connect mutually, after which a failure surface is formed, thus causing falling of mortar and aggregate . In a chIn the salt freezing–thawing cycle test, concrete with different water–cement ratios showed different frost resistance levels. The frost resistance has been shown to be lower when the water–cement ratio is higher. This could be interpreted based on the permeability and energy of concrete . On the (1)Concrete surface experienced three stages of microcrack initiation, mortar peeling, and aggregate falling during the salt freezing-thawing cycles. The mass loss rate decreases in the beginning and then increases in the process of chlorine salt erosion and freezing–thawing, and the smaller the concrete size, the greater the mass loss rate.(2)The relative dynamic modulus of elasticity decreases gradually, slowly at the initial stage and then at a faster rate, and the compressive strength loss rate increases gradually. Comprehensive analysis showed that the frost resistance of concrete is negatively related to the water-cement ratio when the freezing–thawing cycle is fixed.(3)The evolution of concrete micro-structure during the salt freezing–thawing cycles can be quantified by CT technology. The pore quantity, porosity, and volume loss rate of concrete increase in a fluctuating manner, whereas the relative CT value decreases. The peeling depth of the concrete surface increased gradually. The higher the water-cement ratio is, the greater the change of micro-structure will be.(4)The damage model was established with relative CT value as the damage variable, and the model could better reflect the freezing–thawing damage degree of concrete with different water cement ratios, and the damage evolution process could be well described by the Weibull function.The macro–meso damage evolutionary rules of concrete under the coupling effect of chlorine salt erosion and freezing–thawing cycles were analyzed comprehensively in this work. The following conclusions are drawn:


Concrete members are typically designed so that flexural failure initiates with steel yielding and ends with concrete crushing in compression in order to take advantage of the yielding property of steel that allows for large deformations prior to any fracture of the material. On the other hand, if a large percentage of steel or linear elastic non-yielding reinforcement  is used, the member flexural failure typically initiates and ends with concrete crushing in compression. These members are known as compression-controlled members and typically exhibit brittle behavior. This study proposes a new approach in improving the flexural behavior of over-reinforced members through concrete confinement using carbon fiber reinforced polymer (CFRP) grid tubes in the compression zone. The concept was experimentally tested using rectangular beams. Beam 1 (control beam) had no grid reinforcement and beam 2 (tube beam) had two 152 mm grid tubes embedded in its compression zone. Experimental results indicate improvement in the ductility of the tube beam compared to the control beam of approximately 20–30% depending on the criteria used. Considering the low amount and mechanical properties of the CFRP grid, the improvement is significant, which shows that the proposed approach is valid and improves the ductility of compression-controlled members. Flexural failure of over-reinforced concrete members with steel tension reinforcement typically initiates and ends with concrete crushing in the compression zone. Therefore, such members lack the ductility that is associated with yielding of the reinforcement. This is typical not only for compression-controlled members with steel tension reinforcement but also for members utilizing linear-elastic non-yielding tension reinforcement, such as fiber-reinforced polymer (FRP) composite bars. Concrete as a material does not behave in a ductile manner. Concrete may exhibit limited ductility , but this ductility varies depending on the aggregate material and the strength of the concrete used. High-strength concrete is typically more brittle than low-strength concrete and therefore, for high-strength concrete, the pseudo-ductility tends to diminish. International design codes ,2 do notIn order to change the behavior of over-reinforced members, the properties of one of the main materials must be altered. The members must be designed in such a way so as to initiate and end failure in the material with the ability to undergo some plastic deformation. Since changing the properties of mild steel in an over-reinforced section will have no effect on the behavior (failure is taking place in concrete), the focus of improving member flexural behavior (ductility) should be placed on concrete rather than the tension reinforcement. One way to alter concrete material behavior is through confinement. If the concrete in the compression zone of a flexure dominated member is confined, then degradation of the compression zone at capacity is delayed. This results in a more ductile response.Although the carbon fiber-reinforced polymer (CFRP) composite grid has been used as confinement reinforcement before, this was in the context of evaluating the concrete properties under axial compression in concrete standard cylindrical specimens ,4 or in Carbon fiber-reinforced polymer composite grids have been used to reinforce concrete decks and beams for both strength and crack control ,9,10,11,Several attempts have been made by various researchers to improve the ductility of over-reinforced concrete members. The success of the methods used varied. In some cases, helix reinforcement was used ,15,16,17f/As, had a significant influence on the flexural performance of hybrid FRPRC beams. The effect of the hybrid reinforcement ratio on the flexural performance of concrete beams was studied for both under- and over-reinforced scenarios using three-dimensional finite element models. The results showed that for a preferable strength and ductility performance to be obtained, an appropriate design of the hybrid reinforcement ratio is necessary for optimizing strength and ductility performance. It is obvious that the main objective in all of these studies with the implementation of a number of different techniques is the improvement of flexural capacity and stiffness. The aim is to prevent brittle failures in over-reinforced flexural members.Other findings on the installation of short stirrups as confinement reinforcement were also reported . For exaThe application of a carbon FRP composite grid as confining reinforcement for concrete is investigated in this study. In order to verify the proposed concept, experimental testing was conducted on beams using a commercially available CFRP grid. The grid was formed into tubes and was embedded in concrete as confinement reinforcement in the compression zone. The beams were tested in a simply supported four-point bending configuration. The aim of the study was to change the behavior of the over-reinforced members under flexure and subsequently improve member ductility. The results indicate an improvement in strength, ductility (ductility index), and energy absorption.Applications of the CFRP grid in over-reinforced members under flexure to improve member flexural behavior and produce a more ductile response are not available in the literature, and it appears they have not been attempted yet. Therefore, the data from this study will serve as the starting point for further studies in this research area. The CFRP composite grid is fabricated from carbon fibers embedded in an epoxy matrix. It is available in the form of rolls of 1.041 m wide and 274.32 m long see . The strThe tensile properties of the grid were obtained through testing of strands from both the longitudinal and transverse directions . The speThe strength of each specimen was taken as the ratio of the peak load over the cross-sectional area and the modulus was determined by linear regression of the stress–strain data .Although the number of tests may be considered as insufficient to determinate any statistical significance, the relatively high coefficient of variances (COV) for strength (24%) and tensile modulus (18%) indicate significant variability between the CFRP strands. The average grid strand strength was 696 MPa and the average modulus was 64 GPa. These values were significantly lower than the typical average values found in the literature for carbon epoxy unidirectional composites. The properties of the strands of this particular grid were approximately 2.5 times lower than the typical properties in the literature.To evaluate the level of confinement provided by the CFRP grid, a test program involving the axial compressive testing of cylindrical specimens was conducted. Cylindrical specimens with and without the CFRP grid were tested . The speExperimental results from the cylindrical specimens are listed in The control specimens failed in a typical manner without very significant post-peak behavior. Concrete crushing for the control specimens took place at a strain of approximately 0.0031. A control cylinder after testing can be seen in Grid specimens reached higher peak loads and accommodated larger displacements compared to control specimens. This is evident in egt) was calculated based on the following expression:ln is the number of CFRP grid layers, gb is the width of the grid strands, gt is the thickness of the grid strands, and gs is the CFRP grid strand spacing.When the CFRP grid is used as confinement reinforcement, the confining pressure and confinement effectiveness are less than that of a fully wrapped system. Therefore, only light confinement is achieved. Models developed using data from highly confined concrete may not be adequate. Several existing models were investigated to model the behavior of CFRP grid confined concrete. The grid strands in the circumferential direction only cover part of the area. One approach is to determine an equivalent full-coverage thickness for the circumferential grid strands. The equivalent grid thickness (cE) was calculated based on the recommended expression used in Eurocode 2 [2.The secant modulus of elasticity of concrete (ruf) was determined using simple pressure vessel mechanics. The equilibrium condition requires that the force from the confining strength be equal to the force in the FRP encasement. The force from confinement is equal to the confining strength times the diameter of the enclosed concrete, and the force in the encasement is equal to the strength of the encasement times twice the thickness of the FRP encasement. By rearranging the equation, the confinement strength (ruf) was found:ruf is the confinement strength, gd is the diameter of the CFRP grid tube, and guf is the ultimate strength of the CFRP grid strands.The confinement strength fcc″=The confinement effectiveness coefficient for concrete confined by steel is usually taken between 2.8 and 4.1. Campione and Miraglia  found thcoε) was determined based on available expressions in the literature [The axial strain of unconfined (Equation (5)) and CFRP grid confined (Equation (6)) concrete at the peak stress εccε is the concrete strain at a specific stress.The modified Hognestad stress–strain equations for the pre-peak (Equation (7)) and post-peak (Equation (8)) regions are as follow:The material properties of the CFRP grid strands were used to develop the stress–strain curve of the CFRP grid confined concrete. The average strength of the control cylinders was taken as the strength of unconfined concrete .The unique application of the CFRP composite grid as confining reinforcement in the compression zone of compression-controlled beams was investigated with the construction and testing of two full-size beams. Beam 1 (control beam) had no CFRP composite grid tubes and beam 2 (tube beam) had two CFRP composite grid tubes placed in the compression zone. Details of the dimensions, amount, and location of steel reinforcement and CFRP composite grid tubes can be found in The mechanical properties of the CFRP composite grid were discussed in detail in previous sections. The CFRP composite grid was formed into a circular tube (or coil) using a 152 mm diameter plastic pipe, and was held in place by a thin string wrapped around the tube along its length See a.The concrete used in both beams was ready-mix concrete with limestone aggregates. It had a 28-day design strength of 35 MPa, and maximum aggregate size of 9.5 mm. Mild steel reinforcement with minimum yield strength of 420 MPa was used for both tension and shear reinforcement.Both beams were cast on the same day. Concrete was placed in both beams continuously and was vibrated as seen in Both beams were tested in a simply supported four-point bending configuration, as shown in The beams were loaded at a constant rate of approximately 0.5 kN/s. The rate was selected in order to reach concrete crushing strain in approximately the same the time it took when the concrete cylinders were tested. The load was applied monotonically until the beam could no longer sustain a significant load, which for the purposes of this study was set to approximately 60% of the peak load.A series of 15 displacement transducers (LVDTs) were placed at 305 mm intervals to measure the displacement of the beams at different locations along their length see . The LVDFoil strain gages were attached on the sides of the beams at mid-span a and on The concrete cylinders were tested on the same day as the beams in order to have an accurate value of the unconfined concrete strength in the beams. The results are listed in Both beams failed due to concrete crushing in the constant moment region, as it can been seen in The load-displacement curves for both beams can be seen in As it can be seen from The strains measured from the strain gages installed on the two beam sides (east and west) were approximately the same for each specific location. This was typical for all strain gages. The strain at each location was calculated by taking the average value recorded by the two strain gages corresponding to each location. For example, the strain at 25 mm from the top of the beam (S1) was taken as the average of SE1 and SW1. The same approach was used for all other locations. The maximum average strain at peak load recorded during the test for the control beam was at the S1 location and was equal to 0.00284. The maximum average strain recorded at peak load for the tube beam was also at location S1 and was equal to 0.00255, approximately 10% lower than the strain recorded for the control beam. However, these were not the maximum values since it is known that maximum compressive strains occur at the extreme fibers, in this case located at the top of the beam. The compressive strain at the extreme concrete fiber can be redetermined by linear interpolation of the available experimental strain data. It should be noted that cracking at the location of the bottom strain gages prevented the recording of valid data by those gages, and that is the reason they were excluded from the plotting of the strain profiles. Assuming a linear distribution of strain through the beam cross-section, a straight line can be fitted, and the equation of the fitted line can be used to determine the strain at the extreme concrete fiber. The fitted lines were in very good agreement with the experimental data, as seen in The strain at the extreme compression fiber (top) at peak load for the control beam was 0.00317, and 0.00301 for the tube beam. This was expected since limestone aggregate concrete exhibits higher brittleness compared to concretes with other types of aggregates. The ACI 318 concrete design code assumes a concrete crushing strain of 0.003 . The conThe moment curvature (M-Φ) diagrams of the beams were developed using the available experimental data. It is well-known that the curvature can be determined by taking the second derivative of the deflection equation, as follows :(9)Φ=d2y2 = 0.997). By taking the second derivative of the deflection equations, the curvature equations for each load were developed and used to calculate the curvature values. The moment at midspan was calculated for each load and was plotted against the curvature from zero load up to a post-peak load equal to 80% of the peak load. The curves for the mid-span location are shown in Using the displacement values from the LVDTs at various loads and fitting a polynomial line through the deflection profile across the length of the beam, the deflection equation was developed at various increasing loads. The polynomial used was of the third degree since it is well-known that the displacement equations for point loads are third degree polynomial equations with respect to distance. An example is shown in The ductility, in the form of ductility factors, was calculated to investigate the structural behavior of the tested beams. The ductility of the specimens was calculated using two ductility factors, the deformation and curvature ductility factors. The deformation ductility factor was defined as the ratio of the axial deformation at 20% drop of axial load and axial deformation at yield load .u is the axial deformation at ultimate load, and δy is the axial deformation at yield load.For the purpose of this study, the yield load definition suggested by Pessiki and Pieroni  was usedu is the curvature at ultimate load, and Φy is the curvature at yield load.Similarly, the curvature ductility index can be calculated as:The energy absorption capacity (W) of the beams was calculated as the area under the load versus the deformation curve for the mid-span location. The ductility factors and energy capacities for the beams are listed in Both ductility indices showed similar improvements. The deformation ductility index of the tube beam was improved by 20% compared to the control beam, while the curvature ductility index indicated 24% improvement. The energy absorption capacity of the tube beam was 1.34 times higher compared to the control beam.Section analysis of the beams was conducted using a simple fiber model that incorporated the concrete model described in Some of the assumptions employed in this fiber model include perfect bond between concrete and the reinforcing bars (strain compatibility), and plain sections remain plain (linear strain distribution). The area below the neutral axis was considered cracked and was ignored in the force and moment calculations.Each area of the compression zone was divided into thin strips, for which the compressive force was calculated as the product of the area of the strip and the average stress. The average stress was determined based on the stress–strain models developed in eff is the effective concrete modules, as defined by Eurocode 2 (7.4.3 (6)), and Ieff is the effective moment of inertia of the section, as defined by Eurocode 2 (7.4.3 (3)) [The peak curvature was calculated as follows :12)Φ=MEΦ=ME(12)ΦThe model predictions on the peak load and curvature at peak load are shown in In order to evaluate the effect of the CFRP grid on the curvature ductility index, a small parametric study was performed. The percentage of steel reinforcement was varied from 1% to 7%, and using section analysis, the moment and curvature were calculated. It should be noted here that the curvature was calculated as the ratio of the strain in the extreme concrete fiber in compression over the depth of the neutral axis. Then, curvature ductility indices were calculated in the same manner described at the beginning of From the parametric study results, it seems that as the reinforcement ratio increases, the ductility index is reduced for both the control and grid beams. No significant change in the ductility index was observed beyond the 6% reinforcement ratio. The improvement in ductility index between the control and grid beams follows the same trend. There was a larger improvement for the lower reinforcement ratios compared to the higher reinforcement ratios. The tension-controlled beams had higher ductility compared to the compression-controlled beams, which was in line with design codes. It should be noted that the tested beams had a reinforcement ratio of approximately 6%. The model predicted a ductility index improvement of 34%, while the experimental ductility index was improved by approximately 25% for the tube beam compared to the control beam. This represents a difference of approximately 9% between the model prediction and the experimental data. This difference was considered acceptable.The CFRP grid provides light confinement to concrete with the post-peak portion of the stress–strain curve having a descending branch similar to the results obtained by other researchers for lightly confined concrete.The confinement model developed predicted the behavior of concrete confined by CFRP grids with a high degree of accuracy.The results from the two beams indicate that the CFRP grid tubes provided confinement to the concrete. The use of a series of tubes rather than a large one was effective, and confinement was provided. This was evident from the small plateau that was formed when the load-displacement or moment-curvature curves of the tube beam were plotted.There was a nominal increase in the peak load of the tube beam of approximately 3% compared to the control beam. The load increase was considered minor.The ductility indices of the tube beam were 20–24% higher compared to the control beam. Similarly, the energy absorption capacity of the tube beam was improved by 35% compared to the control beam. This was in line with other compression-controlled specimens employing the same grid as confinement reinforcement for concrete tested by Michael . The impThe model used to predict moments and curvatures for the beams was fairly accurate since it predicted the peak moment with high accuracy and the associated curvatures with relatively good accuracy. It also predicted the improvement in the ductility index for the beams.The parametric study indicated the obvious, that confinement of the compression zone improves ductility indices for the beams for which failure was initiated through steel yielding. The combination of the additional capacity for crushing strain due to confinement and steel yielding resulted in very significant improvements. The improvement reduces as the percentage of steel reinforcement increases. However, improvement was observed even with high percentages of steel reinforcement in compression-controlled members due to the confinement of concrete from the CFRP grid tubes.The CFRP grid used in this study had relatively low mechanical properties. It is anticipated that if grids with superior properties are used, the confinement effect will improve. If that is also combined with larger amounts of the CFRP material, significantly larger improvements can also be expected. Further investigation using such CFRP grids is needed since this type of application has not been investigated by many researchers.In this study, a model predicting the confinement effect of a particular CFRP grid was presented along with the test results of compression-controlled concrete beams using CFRP grid tubes in the compression zone as concrete confinement reinforcement. Based on the findings presented, the following conclusions were drawn:


Palm oil clinker (POC) aggregates is a viable alternative to the naturally occurring sand and gravel in the manufacturing of concrete. The usage of POC aggregates assists in the reduction of solid waste and preserves the consumption of natural resources. Although researchers investigated the mechanical response of POC-containing concrete, limited research is available for its torsional behavior. In general, the torsional strength depends on the tensile strength of concrete. This research investigates the compressive, tensile, and torsional response of concrete with various ratios of POC-aggregates. Five batches of concrete were casted with POC-aggregate replacing granite at ratios of 0, 20, 40, 60, and 100%. The selection for the mixture proportions for the various batches was based on the design of experiments (DOE) methodology. The hard density, compressive strength, splitting tensile strength, and flexural strength of concrete with a 100% replacement of granite with POC-aggregates reduced by 8.80, 37.25, 30.94, and 14.31%, respectively. Furthermore, a reduction in initial and ultimate torque was observed. While cracks increased with the increase in POC-aggregates. Finally, the cracking of concrete subjected to torsional loads was monitored and characterized by acoustic emissions (AE). The results illustrate a sudden rise in AE activities during the initiation of cracks and as the ultimate cracks were developed. This was accompanied by a sudden drop in the torque/twist curve. Researchers have investigated the recycling of waste products to enhance the efforts of sustainable development . In the missions ,15 and pIn tropical countries, agricultural waste in the production of cementitious composites is a viable alternative ,16. As a−3 MPa/kg·m−3, representing an increase of 63% when compared to normal weight concrete (NWC) [3, while granite has a bulk density of 1431.7 kg/m3. This could increase cost savings and provide flexibility in structural design. Ahmmad et al. [Researchers investigated the effect of replacing the natural coarse aggregate with POC at various ratios on the compressive, tensile, and flexural strength of concrete. Results indicate that full replacement natural coarse aggregate with POC results in a reduction in compressive strength, splitting tensile strength, flexural strength, and modulus of elasticity of POCC by 40, 32, 39, and 26%, respectively [te (NWC) . The higd et al.  illustrad et al. ,20,21,22Limited research has investigated the torsional behavior of lightweight concrete ,24,25. IThe evaluation of the formation and growth of cracks is essential to predict the structural integrity and performance of the POCC. The location of cracks, fractures, and delamination can impact the condition of the structure. The acoustic emission (AE) method is a nondestructive evaluation (NDE) technique that can detect and assess fractures by capturing the energy released during the formation and growth of cracks . AE is nThis research investigates the use of AE methodology to detect the initiation, propagation, and localization of initial and ultimate torsional cracks in POCC subjected to torsion. Two AE parameters including the absolute energy and cumulative signal strength (CSS) were analyzed in reference to the torque/twist response. The analyses focused on the detection and identification of the damage process during crack formation, propagation of the crack, and ultimate crack in the concrete specimens. Furthermore, the AE signals were used for mapping cracks. The captured cracking profile was compared to the results of visual inspection of the POCC specimens.2/g and 3.14 g/cm3, respectively.The materials used included cement, water, sand as fine aggregate, and granite and palm oil clinker as coarse aggregates. The cement used for all concrete batches was ordinary Portland cement (OPC)   type 1—4Potable tap water with a pH value of 6.20 was used for the concrete mixture and for curing the concrete specimens. Two types of aggregates, crushed granite and palm oil clinker, were used as coarse aggregates. Crushed granite  was locally procured from the crushing plant and had a size in the rage of 4.75–20 mm. The POC coarse aggregate was the byproduct of the palm oil industry  and was crushed and sieved to the required size  as illustrated in A total of six batches were casted with various proportions of POC coarse aggregate and crushed granite. The control batch (POC0) had 100% granite as coarse aggregate. The five remaining batches had 20, 40, 60, 80, and 100% of the coarse aggregate as POC. For identification, the batches are identified by POC0, POC20, POC40, POC60, POC80, and POC100. Initially, POC coarse aggregates were 100% saturated  by soaking the aggregate in water for 24 h. This is essential to ensure the proper water-to-cement ratio is maintained as POC is highly porous when compared to granite. After soaking, the POC coarse aggregate was air-dried to attain the saturated surface dried (SSD) condition. The design of experiment methodology was implemented for ensuring that the control batch is consistent with grade 40 MPa concrete and a slump between 60 and 180 mm. For POCC specimens, the SSD POC coarse aggregate, crushed granite, and river sand were dry-mixed for 2 min to obtain a homogenous mass. The OPC and 70% of water were added and mixed for another 5 min. Finally, the remaining water was added to obtain the required slump value. Three layers of the fresh concrete was casted in the molds, and a vibrating table  was used for the compaction. After 24 h, the specimens were extracted from the molds, and the specimens were cured in water for 28 days and then tested.3 were casted for investigating the splitting tensile strength and flexural strength of the specimens, respectively [Cubes of 100 mm were fabricated to investigate the compressive strength, water absorption, and ultrasonic pulse velocity (UPV) ,39,40. Tectively ,42.3 were casted to investigate the torsional response. An automated torsion machine  shown in Furthermore, three prism specimens with dimensions of 500 × 100 × 100 mmTo identify the cracking behavior of the specimens subjected to torsional load, four AE sensors with a resonant frequency of 400 kHz were mounted on the specimens. The sensors were attached using electron wax coupling agent at the two-perpendicular sides with a separation distance of 230 mm, as shown in The water absorption of concrete increased as the POC ratio increased as shown in The UPV value of the concrete reduced with the increase in the ratio of POC coarse aggregate. The maximum reduction in the UPV value was observed at 19.64% for POC100 specimens as compared to POC0 specimens. This is due to the porous structure of the POC coarse aggregate, as shown in The splitting tensile strength was reduced with the increasing ratio of POC. A maximum reduction in splitting tensile strength of 30.94% was observed for POC100 specimens as compared to the POC0 specimens. The reduction in strength is due to the failure of the POC aggregate and at the aggregate-matrix interface . This fi3, except for POC100 specimens as illustrated in The flexural strength of concrete was reduced with the increasing ratio of POC. A maximum reduction of flexural strength at 14.31% was observed for POC100 specimen when compared to POC0 specimens. This can be attributed to the low aggregate crushing strength of the POC as compared to granite . Aslam e2 when compared to all POCC specimens. The control specimens had the maximum cracking torsional stiffness due to cracking at the smallest angle of twist. A maximum reduction in cracking torque and torsional stiffness of 28.42 and 38.97%, respectively, was experienced by POC100 specimens when compared to the control (POC0) specimens. It is worth noting, that POC60 specimens sustained a high cracking torque of 47.18 Nm at the maximum twist of 0.0060 rad/m.In general, the cracking torsional stiffness of concrete is dependent on their tensile strength. The torsional failure occurs by the development of tensile stresses . Table 3The initial torsional toughness decreased with the increasing ratio of POC. The maximum reduction in initial torsional toughness of 31% occurred for POC100 specimens when compared to control (POC0) specimens. However, the twist of POC100 that resisted the cracking torque was almost 8% smaller than the twist of POC0 specimens at the cracking torque. The torsional stiffness is not dependent on the type of reinforcement of the concrete . TherefoThe torsional capacity of concrete depends on the concrete strength; however, due to the absence of reinforcement, the cracks initiated suddenly and propagated rapidly . The ultThe ultimate twist decreased upon increasing the POC ratio from 0 to 40%. Beyond a 40% ratio of POC, the specimens had an increase in the ultimate twist. A large ultimate twist of 0.039 and 0.034 rad/m, was observed for POC0 and POC100 specimens, respectively. This indicates that POC0 and POC100 specimens have high resistance to ultimate cracks due to homogeneity and the existence of a single type of coarse aggregate . However, POC100 specimens resist the lowest torque, while POC0 specimens exhibit the maximum ultimate toque. This can be attributed to the low impact and crushing strength of the POC aggregates as compared to granite.All initial cracks or cracking torque of the mixes occurred in the range of 14–25% of the ultimate torque. The POC100 specimens had an initial crack torque of 25% of the ultimate torque. POC0 and POC100 had similar initial and ultimate twists. All specimens failed at ultimate torque and did not experience torque failure. This is due to the brittleness of concrete. Furthermore, all specimens experienced a sudden drop in the torque/twist response at ultimate cracking and the maximum fracture energy was released.The AE methodology is extremely sensitive for the detection of crack initiation and propagation. Furthermore, it can identify and detect microcracks that are induced in at early stages . ShahidaThe quantifiable measurement energy that consists of all events and AE hits is known as absolute energy . The torThe initial cracks from the absolute energy and torque/twist graph for the specimens are shown in In general, the absolute energy reduced after the formation of the initial cracks. Post the initial cracking of the specimens, low AE activities can be attributed to the propagation of the existing crack. In addition, microcracks were observed on the surface of the concrete specimen as the applied torque increased. When new cracks developed, a jump in the absolute energy graph was accompanied by a drop in torque. This is due to the release of high fracture energy as cracks are formed. The ultimate crack is formed at the maximum AE activities, accompanied by the maximum torque. The specimens failed at the ultimate crack, and a drop in the torque/twist curve was observed. The area covered by the voltage signal of AE over the period of the waveforms is known as signal strength . In addiAt the formation of new cracks, a hike in the CSS graph was observed. Also at the ultimate torque, a high fracture energy was released, which in turn increased the CSS. The same phenomena were seen in the signal strength-time response that increased suddenly at the ultimate crack . POC0 anIn addition, the history of the AE amplitude was investigated for all specimens as illustrated by The initial crack of POC0 and POC100 specimens initiated from 232 mm of the x-axis from the fixed end of the machine and then spread on the y- and z-axis, as shown in The initial crack of POC60 started at a higher position, i.e., at 242 and 21 mm on the x-axis and y-axis, respectively, as illustrated in Maximum reductions of 18.83, 37.25, 30.94, and 14.31% in hard density, compression strength, splitting tensile strength, and flexural strength of concrete, respectively, occurred when POC was fully substituted for granite in the concrete mix. Meanwhile, the water absorption of the POC100 specimens was 1.78 times higher than that of POC0 specimens.The initial and ultimate torque decreased by increasing the amount of POC and showed a reciprocal relationship with the initial and ultimate cracks. POC40 specimens had the maximum initial torque among all POCC specimens. In addition, POC40 specimens had high initial torsional stiffness but had the lowest angle of twist when compared with all specimens.POC100 specimens sustained the initial and ultimate torque at a large angle of twist and was comparable to the control (POC0) specimens. However, the initial torsional stiffness and ultimate torsional toughness of POC100 were 1.64 and 2 times smaller than the control (POC0) specimens, respectively.The absolute energy was found to be a useful AE parameter for the detection of both initial and ultimate torsional cracks. On the other hand, the AE amplitude and CSS parameter were found to be capable of identifying the growth of cracks and the ultimate torsional cracks. High-intensity AE events were recorded, and high-energy dissipations were observed for POC0 and POC100 specimens. The initial crack for the tested specimens occurred almost in the range of 14–25% of the ultimate crack.The failure pattern of all tested specimens had a skewed bending failure. In addition, the width of the crack increased with the increase in the replacement ratio of granite with POC. POC80 and POC100 specimens broke into two and three pieces, respectively.The following conclusions were drawn from the analysis and discussion of the experimental results of this researchFurther work can be carried out to study the mechanisms of torsional crack initiation and propagation, supported by microstructural analyses, which will greatly contribute to the understanding of the torsional resistance of POC concrete and other lightweight concretes.