Laboratory investigation of reinforcement corrosion initiation and chloride threshold content for self-compacting concrete

Time-to-corrosion (T_i) of reinforcement in concrete and chloride threshold content (C_th) are important service life determinants for reinforced concrete structures in chloride-laden environments. In this study, the two determinants of a series of self-compacting concretes (SCC) and regular concretes were experimentally investigated. A new sampling approach for C_th determination (milling powder from corrosion active site at the rebar/concrete interface) was adopted to accurately express chloride content resulting in corrosion occurrence. It is found that the T_i and C_th follow the 3-parameter Weibull distribution. The results indicate that the corrosion initiation of rebar in concrete slabs depends upon both cement alkalinity and superplasticizer. Rebar, embedded in high alkalinity cement SCC, exhibits better corrosion resistance as indicated by the longer T_i, higher C_th and larger Weibull modulus, m. A larger Weibull modulus indicates that anti-corrosion performance of rebar in slabs is more stable and less scattered. The effects of specific superplasticizer on rebar corrosion resistance are discussed from the viewpoint of air void amount and size distribution at the rebar/concrete interface.     

Characteristic service life for concrete exposed to marine environments

A statistical treatment has been applied to a deterministic service life model of concrete structures in marine environments. The chloride ingress model based on Fick's second law of diffusion was assumed. The quality of concrete was quantified in terms of three factors, namely, an apparent diffusion coefficient at 1 year (D_a), surface chloride concentration (C_s) and a critical chloride level (C_cr). The standard deviation of service life can be estimated from standard deviations of the four factors, namely, C_s, D_a, C_cr and cover depth. The effect of the severity of environment on service life was also demonstrated. With data from the literature and an approximation of the inverse error function, sensitivity analyses were carried out. Service life was found to be more sensitive to cover depth than the diffusion coefficient, and more sensitive to surface chloride concentration than the critical chloride level. Characteristic service life of a range of normal Portland cement (NPC) concrete grades was evaluated as a function of 28-day strength and cover depths for a nominated confidence level. Such characteristic service life can be readily used and appreciated by design engineers.     

The apparent and effective chloride migration coefficients obtained in migration tests

The apparent (D_app) and effective (D_eff) migration coefficients obtained in chloride migration tests are investigated in this study. The presented D_app profiles in concrete show that the apparent migration coefficient is strongly concentration-dependent. As demonstrated, the binding of chlorides during the migration tests is very low at low free-chloride concentrations and therefore the chloride penetration front progresses throughout the concrete only slightly retarded by the binding. The diffusion flux during migration tests is shown to be insignificant compared to the migration flux. The D_RCM obtained from the Rapid Chloride Migration (RCM) tests are found to be equal to the computed D_app at the locations of the chloride penetration fronts, which gives an indication that the D_RCM represents only the migration coefficient at the front. A linear correlation is found between the D_RCM obtained from the traditional RCM model and the D_eff obtained from the chloride transport model which includes non-linear chloride binding and concentrations in non-equilibrium.     

Nitrite diffusivity in calcium nitrite-admixed hardened concrete

The apparent diffusivities (D_app) of nitrite in concrete were estimated by monitoring time-dependent concentrations of nitrite leached into water from calcium nitrite-admixed hardened concrete specimens. Experiments were conducted with five different concrete mixes and with deionized water (DI), limewater, or synthetic seawater as the leaching agents. The D_app in Type II Portland cement concrete for long curing times and a w/c ratio of 0.40 was ∼1.7×10⁻⁸ cm²/s when leached at 22 °C with limewater. The D_app was relatively insensitive to nitrite dosage and to DI or limewater as the leaching agent, but an increase in the w/c ratio to 0.49, or an increase in temperature by ∼14 °C, increased D_app by ∼50%. A 20% Type F fly ash cement replacement reduced the apparent diffusivity by ∼60%. The D_app decreased with concrete curing time. The magnitude of the D_app and its dependence on concrete and exposure parameters were comparable to those observed in the transport of chloride ions in concrete.     

Comprehensive modeling of chloride ion and water ingress into concrete considering thermal and carbonation state for real climate

This article presents a comprehensive modeling of temperature, carbonation, water and chloride ions transport in cover concrete using the transport model “TransChlor”. The TransChlor transport model employs weather data and chloride ion concentrations present on the concrete surface to predict the temporal and spatial evolution of the presence of chloride ion concentrations in the cover concrete pores. The main features of the TransChlor model are presented and validated.The TransChlor model has been calibrated using experimental data on liquid water movement in concrete of different permeabilities under realistic microclimatic conditions. Chloride ion transport is validated by means of experimental results obtained from a newly developed chloride ion optical fiber based sensor.     

New method to measure the rapid chloride migration coefficient of chloride-contaminated concrete

The apparent chloride diffusion coefficient, D_app, which is obtained by fitting chloride profiles as the result of time-consuming immersion tests can be substituted in a model on chloride ingress by the rapid chloride migration (RCM) coefficient of concrete, D_RCM, which is determined under electrically accelerated conditions. Until now, it was not possible to measure D_RCM of chloride-contaminated concrete, as already inherent chlorides interfere with the common colorimetric indicator used for penetration depth measurements. Furthermore, carbonation may also interfere with the penetration depth reading. To overcome these problems, the regular test has been modified by using iodide as penetrating ion and iodate-starch acetic acid as indicator. A strong linear relationship between the regular RCM test and the newly developed Rapid Iodide Migration (RIM) Test was found. Carbonated specimens can be tested using the RIM test without additional interference. Thus, the new method enables the quantification of the actual concrete quality sampled from existing structures during a condition assessment.     

Melt differentiation and crystallization of clinker minerals in a CaO-SiO2-Al2O3-Fe2O3 pseudoquaternary system

In the CaO-SiO₂-Al₂O₃-Fe₂O₃ pseudoquaternary system, the solid solutions of Ca₃SiO₅ [C₃S(ss)], Ca₂SiO₄ [C₂S(ss)], Ca₂(Al_xFe_1 − x)₂O₅ with 0.40 ≤ x ≤ 0.57 (ferrite) and Ca₃Al₂O₆ [C₃A(ss)] were crystallized out of a complete melt with 52.9 mass% CaO and Al₂O₃/Fe₂O₃ = 0.70. When the melt was cooled from 1673 K at 80 K/h, the crystals of ferrite with x = 0.40, C₃S(ss) and C₂S(ss) would start to nucleate from the melt at 1630 K. During further cooling, the x value of the precipitating ferrite would progressively increase and eventually approach 0.57 at 1613 K. The resulting ferrite crystals showed a zonal structure, the x value of which successively increased from the cores toward the rims. Actually, the x values of 0.43 and 0.52 were confirmed for, respectively, the cores and rims by EPMA. As the simultaneous crystallization of zoned ferrite, C₃S(ss) and C₂S(ss) proceeded, the coexisting melt would become progressively enriched in the Al₂O₃ component. After the termination of the ferrite crystallization, the C₃A(ss), C₃S(ss) and C₂S(ss) crystallized out of the differentiated melt. The end result was the four phase mixture of ferrite, C₃A(ss), C₃S(ss) and C₂S(ss), being free from the nucleation of Ca₁₂Al₁₄O₃₃ solid solution.     

Temperature and concentration dependence of diffusion coefficient in dilute solutions

Temperature and molecular weight dependence of k_D in D(C) = D(O) [1 + Ck_D], where D(C) is the diffusion coefficient for the density fluctuations in a dilute polymer solution, is investigated by first expressing D(C) as a function of the static structure factor S(q,C) within the framework of the Kirkwood-Riseman theory. The continuous transition of k_D from negative values under theta conditions to positive values in good solvents is calculated using various models for the intermolecular interaction potential and the results are presented graphically as function of a reduced variables $\text{S}\text{?}\text{R}{}_{\mathrm{H}}$ that combines both molecular weight and temperature effects. It is shown that the negative value of k_D at the theta temperature can be explained at least partially, in terms of an increase in the chain dimensions of two overlapping molecules. The concentration dependence of the self-diffusion coefficient is also discussed.     

Characterization by solid-state NMR and selective dissolution techniques of anhydrous and hydrated CEM V cement pastes

The long term behaviour of cement based materials is strongly dependent on the paste microstructure and also on the internal chemistry. A CEM V blended cement containing pulverised fly ash (PFA) and blastfurnace slag (BFS) has been studied in order to understand hydration processes which influence the paste microstructure. Solid-state NMR spectroscopy with complementary X-ray diffraction analysis and selective dissolution techniques have been used for the characterization of the various phases (C₃S, C₂S, C₃A and C₄AF) of the clinker and additives and then for estimation of the degree of hydration of these same phases. Their quantification after simulation of experimental ²⁹Si and ²⁷Al MAS NMR spectra has allowed us to follow the hydration of recent (28 days) and old (10 years) samples that constitutes a basis of experimental data for the prediction of hydration model.     

Finite element simulation of deep bed filtration

In this paper, the numerical model for separation efficiency and transport in periodic porous media is studied. Finite element method was used to simulate the development of a predictive model of behavior of porous media during injection of particles. This paper describes the effects of injected particle size, Reynolds number and particle drag coefficient. The numerical results show that the separation efficiency increased with injected particle size increase. The separation efficiency is found to increase with increasing Reynolds number. For the effect of drag force, C_D, in porous media, numerical results show that for C_D<10 and C_D>100, the separation efficiency is not affected by drag coefficient in the range of drag coefficient from 10 to 100, and the separation efficiency significantly depends on the Reynolds number.     

Steady-state fluorescence and NMR study on self-association behavior of poly(methacrylamides) bearing hydrophobic amino acid residues

Self-association properties of poly(N-methacryloylphenylalanine) and poly(N-methacryloyltryptophan) (pMPhe and pMTrp, respectively) were investigated by several characterization techniques, including steady-state fluorescence and NMR. These characterization data revealed similarities and distinctions of their self-association properties.The pH dependencies of association properties of pMPhe and pMTrp are practically the same. Apparent pK_a values for pMPhe and pMTrp were determined to be 5.7 and 5.8, respectively, by potentiometric titration. Steady-state fluorescence measurements at varying pH using pyrene as fluorescence probe indicated that, at pH≈5 (< apparent pH), hydrophobic microdomains were formed, while, at pH≈7 and 9 (> apparent pK_a), hydrophobic microdomains were not formed significantly. ¹H NMR spectra for both the polymers measured in D₂O exhibited that a significant fraction of aromatic rings in amino acid residues were located close to the polymer main chain, and that, at pH≈5, the mobility of the polymer main chain and the aromatic ring was extremely restricted.The polymer concentration (C_p) dependencies of association properties of pMPhe and pMTrp at pH≈5 are distinct. Steady-state fluorescence data at varying C_p indicated that pMPhe was more hydrophobic microscopically than pMTrp. Dynamic light scattering data indicated that pMTrp had a stronger tendency for interpolymer association than pMPhe did at pH≈5. It is concluded that the distinction in C_p dependency of the self-association properties of pMPhe and pMTrp is due to the differences in the bulkiness and the hydrophobicity of the substituents of amino acid residues.     

Water transport in densely crosslinked networks: A comparison between epoxy systems having different interactive characters

Water transport in a densely crosslinked epoxy network, TGDDM-HHPA, was investigated by coupling time-resolved FTIR spectroscopy and gravimetric measurements. Vibrational analysis yielded information on the different water species present in the system and allowed a quantitative estimation of their population at sorption equilibrium.The transport of water molecules was found to follow a Fickian behaviour characterized by an effective diffusion coefficient which increases with total water concentration. The dependence of the effective diffusion coefficient, D_eff(C), on concentration, as obtained from integral sorption curves, was interpreted by introducing a concentration dependent term, D(C), and an “interaction factor”, ?, which accounts for the effect of the molecular interactions which effectively slow down the diffusion process.The results were compared with those obtained on another epoxy network, TGDDM-DDS, characterized by the presence of stronger interacting sites. Differences and similarities of the spectroscopic and gravimetric results are critically discussed in the light of the structure of the two networks.     

Validation of computation method to predict aerodynamic diameter of particles with rough surface

Two instruments, namely the Aerosizer LD and the Particle Size Distribution Analyser (PSDA) 3603, were used to measure aerodynamic diameters of spray dried bovine serum albumin (BSA) particles with different surface corrugation (D_S). The measured aerodynamic diameters were compared with the values calculated using the theoretical model previously developed by our group. The model-calculated aerodynamic diameters have been shown to agree with the measured values for corrugated and less cohesive BSA particles with D_S varying between 2.18 and 2.41. For smoother and more cohesive BSA particles with D_S of 2.06 the measured and calculated values differed by 27% (compared to the value measured by PSDA 3603) and 115% (compare to the value measured by Aerosizer). This variation resulted from the difficulty to completely disperse the more cohesive particles during measurement. Hence, the model can be used to estimate aerodynamic diameter in situations when direct measurement fails due to unavailability of equipment, limitations or difficulty in dispersing powders into their individual particles in the aerosol.     

Modelling the post treatment process of model implants prepared by in situ polymerized poly(ε-caprolactone) using a BF3-glycerol catalyst system

The post treatment process of a poly(ε-caprolactone) (PCL) model implant prepared using a boron trifluoride (BF₃) catalyst and glycerol initiator by in situ polymerisation process for craniofacial and maxillofacial treatment is modelled using a ‘moving-boundary’ diffusion model. A numerical method was used to solve a system of diffusion equations of the model. The variable diffusion coefficient (D) was correlated with crystallinity (x_c) of the polymer which is a function of its molecular weight (M_w) and its degradation rate constant (k_d), D=f(x_c(M_w,k_d)). The post treatment time and the molecular weight retained after post treatment can be obtained using this model. The modelling results show that the process is potentially suitable for manufacturing thin model implants of complex shape.     

Structures and stabilities of C36-rings

C₃₆-rings with D_nh symmetries have been investigated using the semi-empirical molecular orbital method (AM1). The ring structures are beneficial to the stability of the C₃₆ systems. An analysis of several factors, such as the change in strain energies due to the distortion of C₃₆ cages, the type of the bonded carbon atoms, the size of retained aromatic domains and the shared pentagon-pentagon double bonds, is given for their contributions to stabilities of the C₃₆-rings. The electronic properties are also discussed and compared with those of C₆₀-rings.     

Formation of single-crystalline BaTiO3 nanorods from glycolate by tuning the supersaturation conditions

We have studied peculiarities in the formation of single-crystalline barium titanate (BaTiO₃) nanorods from a glycolate-mediated complex via a single-step hydrothermal process under different supersaturation (S_R) conditions. X-ray diffraction (XRD) showed the formation of pure BaTiO₃ with an S_R of above 19. The tetragonality for the BaTiO₃ (c/a) reached 1.013 at S_R = 19–29 and dropped to 1.010 for S_R = 39. According to the transmission electron microscopy (TEM) and XRD analyses, the rod-shaped particles exhibited single crystallinity and crystal growth along the [001] plane. With scanning electron microscopy (SEM), the morphological evolution from a plate-shaped intermediate precursor (S_R = 6–9) to a rod-shaped product with an aspect ratio of 6–9 (S_R = 19–29), and to non-polar material with an irregular structure (S_R = 39), was observed. The negative slope, linear dependence of the particles’ width and length on the supersaturation level in the range S_R = 19–39 was established for the first time. The replacement of the prevailing crystallization mechanism from in-situ topotactic transformation into dissolution-precipitation above S_R = 19 was observed. It was shown that with a simple regulation of the S_R, the structural and morphological characteristics of the obtained BaTiO₃ nanoparticle can be effectively tuned.     

Calculation of chloride ion concentration in expressed pore solution of cement-based materials exposed to a chloride salt solution

To investigate the relationship between the chloride ion concentration of the expressed pore solution (C_e) and the exposure solution (C_es) of cement-based materials exposed to a chloride salt solution, a calculation of the C_e, based on an electrical double layer (EDL) model was proposed. The C_e values were calculated using the proposed equations, applying the measured parameters: relative potential (δψ₀), dielectric constant (ε), porosity (P), bulk density (D), volume percentage (p_i) of pores with diameter d_i. The calculations explain why C_e does not always equal to C_es. The results indicate that the expression method does not test the concentration of free chloride ions in the bulk solution (C_Cl⁻_b) or the average concentration of the transportable chloride ions (C_ttr.a) in the pores. However, the ratio (C_r) of C_e to C_Cl⁻_b and the relationship between C_e and C_ttr.a can be calculated to more accurately measure chloride ion migration in cement-based materials.     

Oxygen permeation studies on alternative proton exchange membranes designed for elevated temperature operation

Kinetic and mass transport properties were investigated for the oxygen reduction reaction in Nafion 117 and a sulfonated poly (arylene ether sulfone) membrane (SPES-40, 40% sulfonated groups/repeat unit) under 1 atm oxygen pressure, 100% relative humidity in a temperature range of 303-343 K using a solid-state electrochemical cell. Kinetic parameters were obtained using slow-sweep voltammetry while mass transport parameters, the diffusion coefficient (D) and solubility (C), were obtained using chronoamperometry at a Pt (microelectrode)/proton exchange membrane (PEM) interface. Oxygen reduction kinetics was found to be similar for both Nafion® 117 and SPES-40 membrane at the Pt microelectrode interface. The temperature dependence of O₂ permeation parameters showed same trends for both the membranes studied, there was an increase in D and a concomitant decrease in C. Despite lower equivalent weight and hence higher water content SPES-40 exhibited relatively close values of D with Nafion® 117. The results are discussed in the context of their different microstructures. Values of C showed a closer relationship to water content and the percent volume of aqueous phase in the respective membranes. The values of overall oxygen permeability were significantly higher in Nafion® 117, with a higher positive slope in its variation with temperature.     

Novel poly(diphenylacetylene)s with both alkyl and silyl groups as gas permeable membranes: Synthesis, desilylation, and gas permeability

Diphenylacetylenes having a dimethyloctylsilyl group and an alkyl group at para positions [Me₂n-C₈H₁₇SiC₆H₄CCC₆H₄R; R = H (1a), i-Pr (1b), t-Bu (1c), n-Bu (1d)] and having only an alkyl group [PhCCC₆H₄R; R = i-Pr (1B), t-Bu (1C)] were synthesized and then polymerized with TaCl₅/n-Bu₄Sn catalyst to provide the corresponding poly(diphenylacetylene)s (2a, 2b, 2c, 2d, 2B, and 2C). The formed polymers afforded tough free-standing membranes by casting from toluene solutions. Desilylation reaction of Si-containing membranes (2a-d) was carried out with trifluoroacetic acid to give the desilylated membranes (3a-d). The permeability of these membranes to O₂, N₂, and CO₂ were determined. All the Si-containing membranes exhibited almost the same gas permeability. The desilylation of Si-containing membranes of 2a-c resulted in large increase of gas permeability. No apparent increasing of gas permeability was observed in the desilylation of 2d. To clarify the effects of desilylation, CO₂ diffusivity (D(CO₂)), CO₂ solubility (S(CO₂)), and fractional free volume (FFV) of the polymer membranes were investigated. The S(CO₂) values of desilylated membranes were much larger than that of Si-containing counterparts. The D(CO₂) and FFV of membranes of 2a-c increased through desilylation. The desilylated membrane of 3d had small D(CO₂) value and almost the same FFV compared with 2d. Further, the comparison of the permeability between three types of membranes with the same chemical structure revealed that the microvoids were not generated by the desilylation of membranes of poly(diphenylacetylene)s containing alkyl groups.     

Deuterofullerenes

We describe the results of investigations of the products obtained in the results of the interaction of solid C₆₀ and D₂ (P=2 MPa) at different temperature. It was shown that at first with increasing temperature the amount of deuterium absorbed by fullerite increases, then at above 723 K it begins to decrease. This decreasing content of absorbed deuterium is accompanied by deuteromethane evolution that has been found experimentally at 773 K. This points to the destruction of the closed carbon cage at least in some of the C₆₀ or C₆₀D_x molecules.