Sensitivity analysis of simplified diffusion-based corrosion initiation model of concrete structures exposed to chlorides

This paper presents the results of a sensitivity analysis of the diffusion-based corrosion initiation model for reinforced concrete structures built in chloride-laden environments. Analytical differentiation techniques are used to determine the sensitivity of the time to corrosion initiation to the four governing parameters of the model, which include chloride diffusivity in concrete, chloride threshold level of steel reinforcement, concrete cover depth, and surface chloride concentration. For conventional carbon steel, the time to corrosion initiation is found to be most sensitive to concrete cover depth, followed by chloride diffusion coefficient, with normalized sensitivity coefficients of about 2 and − 1. For corrosion resistant steels, the time to corrosion initiation is most sensitive to the surface chloride concentration and chloride threshold level followed by the concrete cover depth and chloride diffusion coefficient. The results of this sensitivity analysis are discussed in detail, including the variations in predicted time to corrosion initiation induced by variations of the four model parameters and their implications for the design and maintenance of concrete structures built in corrosive environments.     

Experimental investigation of the effect of temperature on the first desorption isotherm of concrete

In the framework of the radioactive waste management in France, interim storage concrete structures should be submitted to temperatures up to 80 °C and subsequent desiccation. The impact of temperature on the sorption properties of concretes has been poorly studied and results are scarce. An experimental campaign was thus carried out to characterize the first desorption isotherms of a modern concrete at 30 °C and 80 °C. The results show a significant influence of the temperature increase that will have to be accounted for the durability assessment of the long-term interim storage concrete structures. Investigating the causes of these modifications, it appeared that desorption induced by temperature might be the principal mechanism rather than microstructure alteration and water properties evolution.     

Lattice modeling of rapid chloride migration in concrete

Test methods which use external voltage are commonly used to assess resistance of concrete to chloride ion penetration. In order to facilitate fast chloride ingress, electrical voltage (typically 10–60 V) is applied across the concrete specimen. These methods have also been used on microcracked and cracked specimens in order to study the influence of cracking on chloride ingress. Chloride migration transport mechanism is fundamentally different from the diffusion process usually occurring in practice. To study the behavior during the test, a model is proposed, based on the transport lattice modeling framework. First, the accuracy and computational aspects of the proposed model are discussed. Then, the model is applied to study the transport in heterogeneous concrete (i.e. on the meso-scale). Also, chloride migration in microcracked, notched, and cracked concrete is simulated. The findings show that the proposed model can successfully reproduce experimentally observed behavior.     

Identification and quantification of cracks in concrete by optical fluorescent microscopy

Cracks in concrete structures can indicate major structural problems and can damage the appearance of monolithic construction. Cracking of concrete is a major factor affecting for the material strength and durability. The development of a crack pattern can contribute to increasing the permeability and the diffusivity of concrete, which is generally connected with a substantial reduction of its durability. This paper describes a method for identification and quantification of crack patterns in concrete by means of optical fluorescent microscopy and image analysis. Results obtained for undamaged and deteriorated specimens are presented. The range of investigation included several concrete mixes made in the laboratory. In order to induce cracks, concrete mixes were exposed to freezing action 0, 1 and 2 h after mixing. The concrete cubes of 100-mm and 150-mm size were frozen for 0 (reference specimens) and 2 days. Investigation of compressive strength, water permeability, chloride migration and analysis of cracks was made after 28 days. The low-temperature deteriorated specimens showed a significant reduction of compressive strength and resistance to water and chloride penetration in concrete. Correlations between density of cracks and compressive strength, depth of water penetration and depth of chloride penetration have been proposed.     

Influence of process parameters on the morphology,rheological and dielectric properties of three-roll-milled multiwalled carbon nanotube/epoxy suspensions

The influence of process parameters on the morphology, rheological and dielectric properties of multiwalled carbon nanotubes (MWCNT)/epoxy suspensions processed by three-roll milling has been investigated. Two apron roll speeds, 180 rpm (low) and 270 rpm (high); two different number of passes, 1 (low) and 5 (high); and two MWCNT loadings 0.01 (low) and 0.1 wt.% (high) were examined. Results obtained from rheological and dielectric measurements indicate that milling at the lower roll speed of 180 rpm leads to better enhancement of rheological and dielectric properties compared to milling at 270 rpm. The results also indicate that a higher number of passes is more favorable for improved rheological and dielectric properties. The relationship between the microstructure and the properties of the suspensions has been explained using the exfoliation-rupture MWCNT agglomerate dispersion mechanism. Milling at 180 rpm was observed to promote exfoliation of MWCNT agglomerates, whereas milling at 270 rpm promoted rupture. Agglomerate statistics carried out on micrographs of the suspensions provided circumstantial evidence that milling at the lower speed indeed promoted exfoliation over rupture, while milling at the higher speed promoted rupture. This technique of combining rheological and dielectric measurements could be an invaluable tool for monitoring MWCNT agglomerate dispersion changes, and hence tuning nanocomposite properties.     

Complete relaxation map of polypropylene: radiation-induced modification as dielectric probe

The molecular relaxation behaviour of isotactic polypropylene (iPP) exposed to gamma radiation in air to various absorbed doses (up to 700 kGy) has been investigated by dielectric loss (tan  d { \tan }\;\delta ) analysis; the polar (mainly carbonyl and hydroperoxide) groups that were introduced by radiation-induced oxidation were considered as tracer groups. All relaxation zones (α, β, γ and δ in the order of decreasing temperature), between 25 K and melting temperature, were studied in the frequency range from 1 kHz to 1 MHz. The changes observed in the dielectric relaxation spectra were related to the modifications in the structural and morphological parameters attributed to exposure of the iPP samples to radiation. Wide-angle X-ray diffraction was used to investigate radiation-induced changes in the crystalline structure and degree of crystallinity, since the α relaxation is connected with this phase. Infrared spectroscopy and gel measurements were used to determine the changes in the oxidative degradation and the degree of network formation, respectively. Conclusions derived using different methods were compared. This study reveals high dielectric and/or relaxation sensitivity of iPP to gamma radiation. Disappearance of the low-temperature dielectric (γ and δ) relaxations together with large changes in intensity, position and activation energy of the dielectric α relaxation are observed with gamma irradiation and are mainly connected with oxidative degradation in iPP structure.     

Dielectric properties of carbon nanofibre/alumina composites

Carbon nanofibre (CNF)/Al₂O₃ composites with concentrations between 1 and 9 vol.% of CNF were prepared by the traditional ceramic processing route followed by spark plasma sintering. The dielectric properties of these composites have been studied in a broad frequency range from mHz to the infrared range. Unlike conventional composites, the percolation threshold in this system is more complex depending on the particles topology. Positive and negative variations by several orders of magnitude in the low frequency AC conductivity have been detected for concentrations near the threshold at ∼2 vol.% of CNF. To explain these results, a modified percolation model has been proposed which takes into consideration the effect of the concentration of the filler on the microstructure of the composite.     

Microwave dielectric properties of Al2O3 ceramics sintered at low temperature

The sintering behavior, microstructure and microwave dielectric properties of Al₂O₃ ceramics co-doped with 3000ppmCuO₂+6000ppmTiO₂+500ppmMgO (Cu/Ti/Mg) have been investigated. The results show that 1 wt% Cu/Ti/Mg can reduce the sintering temperature of Al₂O₃ ceramics effectively. Samples with relative densities of ≥97% and uniform microstructure can be obtained when sintered at 1150 °C. Higher temperature can further increase the density of the sample, but it inevitably leads to abnormal grain growth. Meanwhile, the investigation results show that the low-firing Al₂O₃ ceramics have good microwave dielectric properties especially high Q × f value. A high Q × f value of 109616 GHz is able to be obtained for the 1150 °C sintered sample. The reason for the low temperature densification, abnormal grain growth behavior and the changing trend of the microwave dielectric properties are discussed in the paper.     

Microwave dielectric properties,low-temperature sintering mechanism,and defects behaviour of temperature stable (1-x)Li2Zn3Ti4O12-xSr3(VO4)2 ceramics

(1-x)Li₂Zn₃Ti₄O₁₂-xSr₃(VO₄)₂ (0.1 ≤ x ≤ 0.4) microwave dielectric ceramics were fabricated by solid-state sintering technology. The impact of SV addition on the microstructure, dielectric properties, sintering process, and defects behaviour was studied. The formation of SrTiO₃ and the glass phase were observed via XRD and TEM, and the latter resulted in a decrease in the sintering temperature. The variations in microwave dielectric properties were consistent with the empirical mixture rules calculated by XRD refinement, and a near-zero τ_f value was obtained. The Li, Zn and V elements of the glass phase and the liquid phase sintering model were deduced via DSC, TEM and Raman spectroscopy. Then, the defect behaviour, such as oxygen vacancies, Ti³⁺, and V⁴⁺, was investigated by XPS and complex impedance spectroscopy. It was found that the generation and migration of defects occurred much more easily in 0.7LZT-0.3 SV than in LZT, resulting in a higher dielectric loss. Finally, the 0.7Li₂Zn₃Ti₄O₁₂-0.3Sr₃(VO₄)₂ ceramic sintered at 900 °C exhibited excellent microwave dielectric properties of ε_r = 17.8, Q × f = 41,891 GHz, and τ_f = ?4.4 ppm/°C and good compatibility with silver electrode, showing a good potential application for LTCC.     

Experimental investigation and numerical modeling of chloride penetration and calcium dissolution in saturated concrete

Chloride penetration and calcium dissolution have been investigated for a saturated concrete after exposure to a 0.5 mol/L NaCl solution for a period of up to 3150 days. Simultaneous ion transport model (SiTraM) that allows the transport of chloride and calcium ions to be simultaneously simulated in a hydrated cement system has been used to verify the experimental results.Self-compacting concrete (SCC) with a water to cement ratio of 0.3 resulted in a limited chloride penetration depth while the calcium dissolution was also reduced within the near surface zone. Increased unit water content for normal concrete resulted in higher chloride penetration depth and larger dissolution front of Ca(OH)₂ regardless of having the same water to cement ratio.It was revealed that the SiTraM can predict the profiles of chloride and calcium for self-compacting concrete. It was also found that the primary factor to control chloride penetration front and the dissolution front of Ca(OH)₂ was the pore structure characteristic of concrete.     

Moisture transport in heated concrete, as studied by NMR, and its consequences for fire spalling

During the past 30 years concrete has developed enormously in both strength and durability. A drawback of these improvements is the increased risk of explosive spalling in case of fire. The moisture inside the concrete plays an important role in the spalling mechanism. In order to study the moisture migration inside concrete during intense heating, a dedicated nuclear magnetic resonance (NMR) setup was built. This setup can be placed inside a 1.5-T MRI scanner.With this setup one-dimensional moisture profiles can be measured while the concrete sample is heated up to 250 °C. Besides concrete, measurements were performed on fired-clay brick and calcium-silicate brick.The results show that water inside the concrete sample is superheated to a temperature of 170 °C, which results in an increased pressure inside the concrete. A model was developed to predict the movement of the observed drying front.     

Influence of solid loading on densification behavior,micromorphology and dielectric properties of Ba0.67Sr0.33TiO3 ceramics fabricated by a novel DCC-HVCI method

Ba_0.67Sr_0.33TiO₃ (BST) ceramics with highly improved dielectric performance were fabricated by a novel direct coagulation casting via high valence counter ions (DCC-HVCI) method. The influence of solid loading on densification behavior, micromorphology, and dielectric performance of the samples was investigated. With the increase of solid loading from 40 to 50 vol%, the maximum densification rate of BST ceramics increased from 0.090 to 0.122 s⁻¹, and the densification temperature decreased from 1424 to 1343 °C, which indicated that high solid loading could promote the densification behavior of samples during sintering. BST ceramics fabricated by the DCC-HVCI method showed uniform grain size and microstructure, which was beneficial for the dielectric properties of BST ceramics. Samples obtained from 45 vol% suspensions possessed the lowest dielectric permittivity (ε_r ≈ 2801), and the dielectric loss (tanδ≈0.0262) was about 1/10 of that of dry-pressed samples (tanδ≈0.301), which could be attributed to the composition homogenization.     

Exceptional dielectric properties of chlorine-doped graphene oxide/poly (vinylidene fluoride) nanocomposites

Herein, we report a facile method to significantly enhance the dielectric performance of reduced graphene oxide-based polymer composites. Addition of thionyl chloride into graphene oxide (GO) dispersion induces synergistic modifications of the structure, chemistry, charge carrier density and electrical conductivity of GO, as well as the interfacial interaction and phase of the surrounding matrix in the poly (vinylidene fluoride) (PVDF) composite. The composites reinforced with a very low reduced chlorinated GO (Cl-rGO) content of 0.2 vol% deliver an exceptional dielectric constant of 364 with a moderate dielectric loss of 0.077 at 1 kHz. These values are well contrasted with the corresponding properties of the neat PVDF polymer with a constant of 28 and a loss of 0.0029. Synergistic effects arising from chlorination are identified, including the much enhanced electrical conductivity of Cl-GO sheets by more than 3 orders of magnitude through introducing charge-transfer complexes, the improved interfacial interactions between the fillers and the PVDF matrix through hydrogen bonds, and the transformation of PVDF to β-phase with an inherently high dielectric constant due to dipolar interaction. The comparison with the literature data confirms superior dielectric performance of the present Cl-rGO/PVDF composites.     

Structural,morphological, dielectric and magnetic properties of Zn-Zr co-doping yttrium iron garnet

In this study, yttrium iron garnet co-doped with Zn and Zr atoms with a chemical formula Y₃Zn_xZr_xFe_(5−2x)O₁₂ (x = 0.0-0.3) has been successfully prepared by the solid-state reaction method. The effects of doping concentration on the microstructure, crystal structure, magnetic properties, and dielectric properties of Y₃Zn_xZr_xFe_(5−2x)O₁₂ were investigated. The microstructure analysis indicates that co-doping of YIG with Zn and Zr can effectively reduce the grain size of the ceramic. The crystal structure results reveal that the doping concentration of Zn–Zr has substantial influence on the lattice parameters of YIG, such as, increases the lattice constant, crystal cell size, and interplanar spacing. However, the second phase of ZrO₂ appears once x ≥ 0.15. Additionally, the dielectric properties of YIG ferrite can be regulated using this Zn–Zr co-doping method. Zn–Zr co-doping can improve the dielectric stability and reduce the dielectric loss at high temperature. The magnetization measurement shows that the saturation magnetization is stabilized at x < 0.15, and the magnetic loss is decreased with the increase in the doping concentration. Overall, the findings show that the ceramic with x = 0.1 exhibits better properties included high saturation magnetization (24.607 emu/g), low magnetic loss (0.0025 @ 1 MHz), and relatively low dielectric loss (496 @ 400°C).     

Microstructural characterization of ITZ in blended cement concretes and its relation to transport properties

The improvements in the overall performances of concrete with blended materials were often ascribed to the modification of its hardened paste in general. In this paper, the effects of limestone filler (LF) and slag (GGBS) on chloride migration and water absorption of concretes with systematically varied aggregate properties were evaluated from the view point of ITZ by using BSE image, EDS, and MIP analysis. It was found that the incorporation of moderate amount of LF and GGBS would compact the microstructure of both ITZ and bulk cement matrix. The reduction in the pore volume (> 100 nm) contributes to the largest decrease in total porosity. Additionally, incorporating GGBS avoids the build-up of Ca(OH)₂ within ITZ and provides a more uniform microstructure. The mechanism for the improvement in limiting water and ions penetration was found to be mainly related to the densification of bulk cement matrix rather than the modification of ITZ.     

Microstructure and electrical properties of Nb-doped SrTiO3-BiFeO3 based lead-free ceramics

In this work, Nb-doped 0.75SrTiO₃-0.25BiFeO₃ (ST-BF) lead-free ceramics are designed and synthesized using a conventional solid-state reaction method. The influence of Nb doping on the microstructure, dielectric, and electrical properties are systematically investigated. With the increase of Nb concentration, the crystal structure of ST-BF remains pseudo-cubic as exhibited in the X-ray diffraction patterns. The grain size is found to increase from 0.33 to 6.23 μm, and then decrease to 1.88 μm by Nb doping, along with a clear heterogeneous core–shell microstructure. A relatively low dielectric loss (∼0.1, at 1 kHz) and a stable dielectric constant (∼700, at 1 kHz) are obtained for the 0.03 Nb-doped ST-BF composition at room temperature. Impedance spectroscopy analysis shows that Nb doping in ST-BF increases the total resistivity, forming an electrically conductive core and a nonconductive shell, with enhanced activation energy. The results may provide a feasible approach to develop novel ST-based lead-free dielectric ceramics for capacitor applications.     

Modified BCZN particles filled PTFE composites with high dielectric constant and low loss for microwave substrate applications

The modified 0.7Ba (Co_1/3Nb_2/3)O₃-0.3Ba(Zn_1/3Nb_2/3)O₃ (BCZN) powders filled PTFE composites were synthesized by hot-pressing. The influences of BCZN content on the microstructure, dielectric, thermal, mechanical properties and moisture absorption were investigated systematically. The modified BCZN powders filled PTFE composites exhibited better microstructure and dielectric properties compared with untreated powders. Various mathematic models were utilized to predict the dielectric constant of different composites and the effective medium theory (EMT) showed perfect consistency with the experimental results. The modified BCZN/PTFE composites possess the best comprehensive properties at the powders content of 50 vol% with high dielectric constant (ε_r) of 7.7, low loss (tanδ) of 0.0014, acceptable temperature coefficient of dielectric constant (τ_ε) of −125.6 ppm/°C and temperature coefficient of resonant frequency (τ_f) of 29.4 ppm/°C at 7 GHz, low moisture absorption of 0.07% and low coefficient of thermal expansion (CTE) of 33 ppm/°C. All the results show modified BCZN/PTFE composites are the potential materials for microwave substrate applications.     

Good thermal stability,giant permittivity,and low dielectric loss for X9R-type (Ag1/4Nb3/4)0.005Ti0.995O2 ceramics

With the intense demand of the developing microelectronics market, the study of giant permittivity dielectric materials is being promoted. However, it is difficult to obtain suitable dielectric materials for such applications, especially due to high dielectric loss at low frequencies. In this work, Ag+Nb codoped TiO₂ ceramics were designed and fabricated in a conventional solid reaction by sintering at 1290-1340°C for 5-10 hours. The issue of how the microstructure and dielectric properties of (Ag_1/4Nb_3/4)_0.005Ti_0.995O₂ ceramics are affected by the sintering conditions was discussed. By optimizing sintering conditions, a dense microstructure, a high dielectric constant (ε_r ≈ 9410), and a low dielectric loss (tanδ ≈ 0.037) at 1 kHz were achieved. Most importantly, the temperature coefficient value of ε_r at different frequencies remained stable between −14.3% and 13.7% within the temperature range from −190 to 200°C, which has potential applications in X9R capacitor.     

Evaluation of localized corrosion in duplex stainless steel aged at 850 °C with critical pitting temperature measurement

Effect of aging at 850 °C on pitting corrosion of UNS S31803 duplex stainless steel was examined in chloride solution by potentiostatic critical pitting temperature (CPT) measurements. The quantitative metallography coupled with X-ray diffraction technique was employed to follow the microstructure evolution. Moreover, the initiation and propagation of pitting corrosion had been imaged in relation to microstructure variations using scanning electron microscopy (SEM). The results demonstrated that the corrosion behavior is strongly dependent on the microstructure, namely the presence of sigma phase. A deterioration of pitting corrosion resistance is found after aging 4 min, resulting in a drop in CPT. In particular, the metastable current transients during CPT test can clearly reflect the initiation of pitting process. Pitting nucleates preferentially in the austenite phase for the solution-annealed specimen, while the initiation of pitting corrosion takes place around sigma phase, in the newly formed secondary austenite for the aged specimen.     

Effects of material and environmental parameters on chloride penetration profiles in concrete structures

On the basis of the transport mechanism of chloride ion, a prediction model of chloride penetration into concrete structures has been developed. The model includes the diffusion of chloride and its dependences on temperature, age, relative humidity, chloride binding and chloride convection by moisture transport. The experimental program has been set up to verify the model developed in the present study. Several series of concrete specimens were immersed in 3.5% chloride solutions for 15 weeks, and the chloride profiles of the specimens were measured and compared to the predicted chloride profiles. In addition, field measurements have been also conducted. From 10-year-old bridge piers, the chloride profiles in concrete under tidal zone were measured and compared with the predicted chloride profiles. The effects of chloride binding, relative humidity, temperature, exposure condition, and age-dependence on the chloride penetration in concrete were clarified from the present analyses. It was found from the present study that all these variables affect greatly the chloride penetration profiles in concrete. The comparison of the laboratory and field test data with the present theory confirms that the proposed model can be realistically used to predict the penetration of chloride ions into concrete structures under sea environments. Further, these results may be efficiently used for the realistic assessment and design for durability of concrete structures.