Chloride Migration Coefficient Research Articles

Relationship between chloride migration coefficient and pore structures of long-term water curing concrete

The chloride migration coefficient is a useful index for quality control and durability design of reinforced concrete. This paper explored the relationship between chloride migration coefficient and pore structures of long-term water curing concrete. Low-field nuclear magnetic resonance (NMR) and water absorption were used to probe the total porosity and pore sizes distribution. An appropriate dosage of supplementary cementitious materials reduced the total porosity and increased the proportion of capillary pores in the concrete. Fly ash, slag and silica fume effectively increased the capillary pores in the concrete. The capillary pores of 0.35 water to cement ratio concrete with 25% slag was 20% higher than that of the 0.35 water to cement ratio plain concrete at 415 days. The capillary pores of 0.45 water to cement ratio concrete with 25% slag was 32% higher than that of the 0.45 water to cement ratio plain concrete at 415 days. The big pores in the concrete were significantly reduced. According to our analysis, pores were divided into four parts: 0 ∼ R12, R12 ∼ R23, R23 ∼ 50 μm, ≥ 50 μm. R12 and R23 were the sequential minimum distribution frequency pore sizes in the pore size distribution figures tested by the Low-field NMR method. The chloride migration coefficient was dependent on the porosity which included the pore size over R23 and the pore radius R3. R3 was the biggest pore size among several peak frequency pore sizes in the figure of pore size distribution. Their relationship was non-linear. This meant the controlling factors of chloride migration in the concrete were larger capillary pores, voids and the porosity made up of these large capillary pores and voids. The persistent evolution of pores determined the permeability of concrete in late age.

Waste glass as a precursor in alkali‐activated materials: Mechanical, durability, and microstructural properties

AbstractThe long‐term characteristics and durability of alkali‐activated mortars based on waste glass are crucial to better understand their performance in aggressive environments. In this regard, the performance of waste glass as a building material and its characteristics such as mechanical (compressive, flexural, and tensile strengths), durability (chloride migration coefficient, sulfate resistance, and drying shrinkage), and microstructural properties [x‐ray diffraction (XRD), Fourier transform infrared (FTIR), and scanning electron microscopy (SEM) with energy dispersive x‐ray analysis (SEM/EDX)] were evaluated. For this purpose, the specimens with different amounts of silica modulus and Na2O content were prepared. The specimens were cured at 95°C for 20 h and then kept at a relative humidity of 50% at room temperature until testing. The specimen with 10% Na2O and a silica modulus of 1.5 demonstrated the highest compressive, flexural, and tensile strengths. The specimens illustrated a lower chloride migration coefficient and lower expansion than the Portland cement one. By increasing both silica modulus and Na2O contents, the drying shrinkage of the specimens increased due to the presence of more free water. The microstructural results indicated that amorphous gels such as sodium aluminum silicate hydrate (N‐A‐S‐H) and sodium (calcium) silicate aluminum hydrate [N‐(C)‐A‐S‐H] were formed. The results of this study signify the utilization possibility of waste glass as an eco‐friendly material with desirable characteristics.

Determination of the chloride migration coefficient for interfacial transition zone in cement-based material with fly ash replacement

Because the properties of the interfacial transition zone (ITZ) are difficult to quantify, this study provides a method for measuring the chloride penetration depth of the ITZ and calculating the migration coefficient (the diffusivity of chlorides under an electric field) of the ITZ. To obtain a continuous ITZ, a mortar specimen was added with a rod-shaped aggregate (φ19mm × 50 mm). The migration coefficient of the mortar specimen with fly ash replacement was determined through the rapid chloride migration test. The results indicated that the migration coefficient of the ITZ was 1.627–2.651 times that of the matrix for specimens with various water-to-binder (w/b) ratios. Adding fly ash improved the ITZ; the degree of improvement of ITZ was up to the 2.5 times that of the matrix. On the basis of the relationship between the migration coefficient and w/b ratio, the w/b ratios of the matrix and ITZ were determined.

Influence of serpentinite aggregate on the microstructure and durability of radiation shielding concrete

Concrete mix design for radiation shielding is mainly based on selection of aggregate materials of specified elemental composition. Serpentinite aggregate could be a desirable component of concrete for mixed-radiation attenuation and for high temperatures, however its durability performance needs further attention. The objective of the study is to reveal the influence of serpentinite aggregate on the microstructure and transport properties of baryte-serpentinite concrete. Mix design was developed for low-heat Portland cement and slag cement by using a combination of serpentinite aggregate and baryte aggregate. The compressive strength, chloride migration coefficient, carbonation resistance were experimentally investigated. The durability tests were compared with open porosity, mercury intrusion porosimetry measurement and examination of thin section of concrete by optical microscope. The results revealed increased porosity of contact zone of serpentinite aggregate with cement matrix. Open and capillary porosity of concrete was found to increase with increase of serpentinite aggregate content. Consequently, an increase of the chloride migration coefficient up to 2.5 times, the rate of carbonation up to 4 times were observed.

Modeling and experimental study on chloride migration of interfacial transition zone based on its pore structure

In this work, chloride migration coefficient of the interfacial transition zone (ITZ) between mortar and coarse aggregate was investigated. A three-phases-in-parallel model was employed to determine the chloride migration coefficient of ITZ (DITZ) from the migration coefficients of bulk mortar (Dmor) and concrete (Dcon). The ITZ porosity distribution and microscopic pore structure were measured by backscattered electron image (BSE) and mercury intrusion porosimetry (MIP), respectively. A porosity-dependent model was proposed to predict the DITZ. The results indicate that the ITZ thickness ranges from 20 to 40 µm and DITZ is almost 25 to 55 times larger than Dmor. Addition of fly ash remarkably decreases the ITZ thickness and DITZ, and also slightly decreases the ratio of DITZ to Dmor。The ratio of DITZ to Dmor predicted by the model ranges from 25 to 60, in good agreement with the experimental results. MIP validates that addition of fly ash decreases the total porosity primarily by lowering the pore volume of pores >10 nm. Sensitivity analyses indicate that the Dcon is more sensitive to the variation of pores between 10 and 100 nm compared with other pores.

Efficient Use of Graphene Oxide in Layered Cement Mortar.

Graphene oxide (GO) has been found to be an attractive nanomaterial to improve the properties of cementitious composites. However, the use of GO in the industry is limited by its high cost. To achieve a higher cost/performance ratio, GO can be strategically applied in certain parts of cementitious composites structure according to the principle of functionally graded materials. In this study, graded distribution of GO in cement mortar was achieved by sequentially casting a fresh GO-incorporated cement layer on another cement mortar layer. The mechanical properties, especially flexural strength, of layered cement mortar were found to be dependent on the GO content, the delay time, and the interface formed due to layering fabrication. With the GO incorporated in the tensile region only (30% of the total depth), the flexural strength of the layered beam attained 90.91% of that of the beam, with GO uniformly distributed throughout the sample. Based on the results of rapid chloride migration tests, when 12 mm GO-incorporated cement mortar layer was used, the chloride migration coefficient was reduced by 21.45%. It was also found that the measured chloride migration coefficient of layered cement mortar agreed with the series model. The present investigation provides an efficient approach to use GO in cement-based materials from the perspective of mechanical and durability properties.

Concrete admixtures - sustainable concrete

Durability of concrete, an important part of the sustainability theme, was, is, and will be one of the main topics for large infrastructure projects like the Gotthard base tunnel. For such important structures a service life of more than 80 years is expected. The prevailing underground conditions like water ingress, aggressive waters etc. will influence the choice of the concrete system and mix design. Therefore, a focus is on the potential damage mechanisms on the concrete like chloride ingress or sulfate attack. A well-designed concrete will be durable concrete which can withstand various external attacks and achieves or even extends the designed service life of the concrete structure. Based on the knowledge that the main damaging mechanisms for concrete are based on water penetration into the concrete, various measures can be taken to achieve concrete structures with significantly reduced permeability. These measure ranges from reduction of the water/binder-ratio, increasing the density of the concrete using silica fume, to the use of special admixtures in order to increase the durability of concrete. In an extensive research program, the influence of mix design components e.g. different admixture types, water/binder-ratio on hardened concrete properties like chloride resistance (chloride migration coefficient), sulfate resistance and water conductivity have been tested and analyzed.

Facile preparation of robust superhydrophobic coating on concrete surface through “all-covalent” strategy

PurposeThe purpose of this study is to prepare a robust superhydrophobic coating on concrete substrate with remarkable chemical and mechanical durability through “all-covalent” strategy.Design/methodology/approachAmino-modified silica nano/micro-particles were prepared through two synthetic steps. “All-covalent” strategy was introduced to prepare a robust superhydrophobic coating on concrete surface via a “all-in-one” dispersion and a simple spraying method. The successful construction of the products was confirmed by Fourier transform infrared spectroscopy, water contact angles (WCA), X-ray photoelectron spectroscopy (XPS) and scanning electron microscope (SEM). The concrete protective properties were verified by solution immersion test, pull-off test and rapid chloride migration coefficient test. The mechanical durability was tested by falling sand impact.FindingsHierarchical structures combined with the low-surface-energy segments lead to typically superhydrophobic coating with a WCA of 156° and a sliding angle of 1.3°. The superhydrophobic coating prepared through “all-covalent” strategy not only improves chemical and mechanical durability but also achieves higher corrosion and wear resistance than the comparison sample prepared by physically blending strategy. More importantly, the robust superhydrophobic coating showed excellent adhesion and protective performance of concrete engineerings.Practical implicationsThis new “all-covalent” superhydrophobic coating could be applied as a concrete protective layer with properties of self-cleaning, anti-graffiti, etc.Originality/valueIntroduction of both silica nanoparticles and silica microparticles to prepare a robust superhydrophobic coating on concrete surface through “all-covalent” strategy has not been systematically studied previously.

Chloride ion penetration resistance and microstructural modification of concrete with the addition of calcium stearate

Hydrophobic modification is a conventional method of promoting the durability of cementitious materials located in ion erosion environments. In this study, calcium stearate was used to modify the hydrophilicity of concrete. The mechanical strengths, chloride diffusion coefficient, contact angle and pore structures of concrete with different amounts of calcium stearate addition were investigated and the underlying mechanism of calcium stearate on the resistance of concrete to chloride penetration was discussed. The results revealed that the water absorption of concrete decreased with increasing calcium stearate content, the compressive strength was substantially reduced with increasing calcium stearate, and the rapid chloride migration coefficient, electric flux and natural chloride diffusion coefficient first increased and then decreased with the addition of calcium stearate. The optimal amount of calcium stearate was 4% because the concrete with 4% calcium stearate had the strongest resistance to chloride penetration. The improvement in the resistance of concrete to chloride penetration by calcium stearate was indicated by an increased in the contact angles, and then the rise in hydrophobicity hindered the connection of capillary water channels in the mortar matrix. Additionally, the water absorption and connected chloride ion transport channels were both reduced, and finally, the chloride concentration in the concrete decreased.

Mechanisms and Critical Technologies of Transport Inhibitor Agent (TIA) throughout C-S-H Nano-Channels.

The critical issue of the durability of marine concrete lies in the continuous penetration and rapid enrichment of corrosive ions. Here a new ion transfer inhibitor, as TIA, with calcium silicate hydrate (C-S-H) interfacial affinity and hydrophobicity is proposed through insights from molecular dynamics into the percolation behavior of the ion solution in C-S-H nano-channels and combined with molecular design concepts. One side of the TIA can be adsorbed on the surface of the cement matrix and can form clusters of corrosive ions to block the gel pores so as to resist the ion solution percolation process. Its other side is structured as a hydrophobic carbon chain, similar to a door hinge, which can stick to the matrix surface smoothly before the erosion solution is percolated. It can then change into a perpendicular chain shape to reduce the percolation channel’s diameter and thereby inhibit the percolation when ions meet the inhibitor. Therefore, once the erosion solution contacts TIA, it can quickly chelate with calcium ions and erosion ions at the interface to form clusters and compact pores. In addition, the water absorption, chloride migration coefficient, and chloride content of concrete samples decreased significantly after adding TIA, proving that TIA can effectively enhance the durability of cement-based materials. The structure–activity relationship of ion transfer that is proposed can provide new ideas for solving the critical problems of durability of cement-based materials and polymer molecular design.

Chloride diffusion models for plain and blended cement concretes exposed to laboratory and atmospheric marine conditions

Chloride-induced corrosion of steel in a reinforced concrete structure compromises its safety and may decrease the designed service-life. The initiation and the rate of reinforcement corrosion mainly depends on the diffusion of chloride ions to the surface of the steel bars. As such, it is important to determine the coefficient of apparent chloride diffusion (Da) for the type of concrete to be used in a structure, which is time-consuming. Therefore, the development of a short-term and simple procedure to predict Da is highly desirable. This study was conducted to develop a correlation between the short-term chloride migration coefficient (Dnssm) and long-term Da for Type I, Type V and pozzolanic cement concretes. Two groups of concrete specimens, each consisting of five concrete mixtures designed with a w/b ratio of 0.4, were prepared. The first group was exposed to a marine exposure site for 10 years and Da was evaluated after 1, 2, 5 and 10 years. The second group was tested according to NT BUILD 492 under laboratory conditions to determine the short-term Dnssm. Correlation models between the field and laboratory results were developed and statistically validated by comparing the predicted values with the measured ones. The predicted Da values are within ±7% of the measured values. Further, the Dnssm of pozzolanic cement concretes is 2.20–4.40 and 3.55 to 7.30 times less than that of Type I and Type V cement concretes, respectively. The developed models can be utilized to assess the long-term Da by determining the short-term Dnssm.

Testing the Bromide penetration resistance of concrete: substitution of NaCl by NaBr in Rapide Chloride/Bromide Migration Test (RCM/RBM)

The chloride migration coefficient of concrete describes the resistance of concrete against chloride ingress. This important material variable can be determined with the rapid chloride migration test (RCM). During the test, chloride ions penetrate concrete by means of an applied electrical field. If the concrete has already been exposed to chlorides before the test starts, the test set up has to be adapted to achieve reliable results. Due to repeated exposure to the same salt (e.g. NaCl), there is no direct possibility to distinguish the chloride ions from the test procedure and the exposure before this, within the initial test procedure. Therefore, the RCM-test setup has been adapted by replacing NaCl through NaBr in the same molar amount. Consequently, this adapted test setup is called the rapid bromide migration test (RBM). The indicator silver nitrate AgNO3 forms sparingly soluble silver salts (AgCl, AgBr) in the presence of halogen ions (e.g. Cl-, Br-), which form a white precipitate in areas with Cl- or Br- ions. The penetration depths are measured at two split cylinder halves. In case the RBM-specimen has previously been exposed to NaCl, an analyzation technique has to be available for the two elements Cl and Br to enable a differentiation. As the white precipitate areas - a result of the formation of AgCl and AgBr - have a quite similar visual appearance, a differentiation cannot be achieved and other analytical techniques have to be considered. With laser-induced breakdown spectroscopy (LIBS), it is possible to determine the depth-dependent ion quantity (wt.-% of Cl or wt.-% of Br). In this study, it is shown by test results that it is possible to exchange NaCl by NaBr in the RCM test, since the penetration behavior of both ions (Cl-, Br-) is very similar in the migration test.

Influence of Bentonite on Mechanical and Durability Properties of High-Calcium Fly Ash Geopolymer Concrete with Natural and Recycled Aggregates.

In this study, bentonite (a naturally occurring pozzolana) was incorporated as a partial replacement (up to 20%) for high-calcium fly ash (HCFA)-based geopolymeric natural aggregate concrete (GNAC) and geopolymeric recycled aggregate concrete (GRAC). The mechanical (compressive strength and splitting tensile strength), durability (chloride migration coefficient, water absorption, and acid attack resistance), and rheological properties (slump test, fresh density, and workability) were investigated. The results revealed that incorporation of bentonite (10 wt % with ordinary Portland cement) showed appreciable improvement in the strength and durability of both the GNAC and GRAC, though its effect is more significant for GRAC than the GNAC.

Novel Nano-Precursor Inhibiting Material for Improving Chloride Penetration Resistance of Concrete: Evaluation and Mechanism.

Durability improvement is always important for steel–concrete structures exposed to chloride salt environment. The present research investigated the influence of a novel nano-precursor inhibiting material (NPI), organic carboxylic acid ammonium salt, on the mechanical and transport properties of concrete. The NPI caused a slight reduction in the strength of concrete at later ages. NPI significantly decreased water absorption and slowed down the speed of water absorption of concrete. In addition, the NPI decreased the charge passed and the chloride migration coefficient, and the results of the natural chloride diffusion showed that the NPI decreased the chloride concentration and the chloride diffusion coefficient. The NPI effectively improved the resistance of chloride penetration into testing concrete. The improvement in the impermeability of concrete was ascribed to the incorporation with the NPI, which resulted in increasing the contact angle of cement pastes. The contact angle went up from 17.8° to 85.8° for 0% and 1.2% NPI, respectively, and cement pastes became less hydrophilic. Some small pore throats were unconnected. Besides, the NPI also optimized the pore size distribution of hardened cement paste.

Transport Properties and Resistance Improvement of Ultra-High Performance Concrete (UHPC) after Exposure to Elevated Temperatures

Ultra-high performance concrete (UHPC) has a high self-healing capacity and is prone to bursting after exposure to high temperatures due to its characteristics. This work evaluates the damage and improvement of UHPC with coarse aggregates through mechanical properties (compressive strength and ultrasonic pulse velocity), transport properties (water absorption and a chloride diffusion test), and micro-properties such as X-ray diffraction (XRD), Mercury intrusion porosimetry (MIP), and Scanning electronic microscopy (SEM). The result demonstrates that polypropylene (PP) fibers are more suitable for high temperature tests than polyacrylonitrile (PAN) fibers. The result shows that 400 °C is the critical temperature point. With the increase in temperature, the hydration becomes significant, and the internal material phase changes accordingly. Although the total pore volume increased, the percentage of various types of pores was optimized within 400 °C. The mass loss gradually increased and the ultrasonic pulse velocity gradually decreased. While the compressive strength first increased and then decreased, and the increase occurred within 25–400 °C. As for the transport properties, the chloride migration coefficient and capillary absorption coefficient both increased dramatically due to the higher sensitivity to temperature changes. The results of the property improvement test showed that at temperatures above 800 °C, the compressive strength recovered by more than 65% and the ultrasonic pulse velocity recovered by more than 75%. In terms of transport properties, compared to the results before self-healing, the chloride migration coefficient decreased by up to 59%, compared with 89% for the capillary absorption coefficient, after self-healing at 800 °C. With respect to the enhancement effect after exposure to high temperatures, the environment of a 5% Na2SO4 solution was not as good as the clean water environment. The corresponding changes in microstructure during the high temperatures and the self-healing process can explain the change in the pattern of macroscopic properties more precisely.

Autogenous Healing of Cracked Mortar Using Modified Steady-State Migration Test against Chloride Penetration

Concrete is a popular building material all over the world, but because of different physiochemical processes, it is susceptible to crack development. One of the primary deterioration processes of reinforced concrete buildings is corrosion of steel bars within the concrete through these cracks. In this regard, a self-healing technique for crack repair would be the best solution to reduce the penetration of chloride ions inside concrete mass. In this study, a rapid chloride migration (RCM) test was conducted to determine the self-healing capacity of cracked mortar. With the help of the RCM test, the steady-state migration coefficient of cracked and uncracked specimens incorporating expansive and crystalline admixtures was calculated. Based on the rate of change of the chloride ion concentrations in the steady-state condition, the migration coefficient was calculated. Furthermore, bulk electrical conductivity tests were also conducted before and after the migration test to understand the self-healing behavior. It was evident from the test results that the self-healing of cracks was helpful to reduce the penetration of chloride ions and that it enhanced the ability of cracked mortar to restrict the chloride ingress. Using this test method, the self-healing capacity of the new self-healing technologies can be evaluated. The RCM test can be an acceptable technique to assess the self-healing ability of cement-based materials in a very short period, and the self-healing capacity can be characterized in terms of the decrease of chloride migration coefficients.

Facile fabrication of superhydrophobic coatings on concrete substrate

PurposeThe purpose of this study is to prepare a chemically stable superhydrophobic coating with remarkable mechanical properties and concrete protective properties.Design/methodology/approachOne synthetic step was adopted to prepare superhydrophobic coating. The process and product were analyzed and confirmed by fourier transform-infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), water contact angle (WCA), transmission electron microscopy (TEM), scanning electron microscope (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The mechanical properties were confirmed by tensile test. The concrete protective properties were confirmed by solution immersion test and rapid chloride migration coefficient test.FindingsMSiO2 nanoparticles (NPs) were chosen to enhance the hydrophobicity of fluorosilicone coatings. With a 4:1 mass ratio of fluorosilicone resin and MSiO2 NPs, the coatings show superhydrophobicity with a WCA of 156° and a SA of 3.1°. In addition, the tensile mechanical property was improved, and the chloride ion diffusion coefficient was decreased significantly after the addition of MSiO2 NPs.Practical implicationsThis new fluorosilicone coating hybrid by MSiO2 NPs could be applied as a concrete protective layer with properties of self-cleaning, antifouling, etc.Originality/valueIntroduction of MSiO2 NPs hybrid to prepare fluorosilicone coating with superhydrophobicity on concrete surface has not been systematically studied previously.

Investigation on Electrical Resistance of Chloride Penetration of Alkali Activated Slag Concrete

Alternating-current method for measuring chloride penetration resistance of concrete, test method for coulomb electric flux and rapid chloride migration coefficient (RCM) were applied to evaluate the resistance of chloride penetration in alkali-activated slag concrete in this paper. At the same time, the applicability of the above three electrical parameters test methods to the alkali slag concrete was discussed. The results show that NaOH activated slag concrete behaves higher resistance to chloride penetration than water glass activated slag concrete. Blend of fly ash increases the porosity of alkali-activated slag concrete and weakens the resistance of chloride penetration. Correlation coefficient between chloride migration coefficient and AC electrical resistivity is 0.99. There are good correlations among the evaluation results of three electrical parameters test methods, and all of them behave sound applicability to alkali-activated slag concrete.

Effect of steel fiber on impact resistance and durability of concrete containing nano-SiO2

Abstract Impact drop weight tests, rapid chloride migration coefficient tests, single-sided freeze–thaw tests, and mechanical property tests were performed to investigate the effect of the steel fiber (SF) content on the impact resistance and durability of concrete containing nano-SiO2 (NS). A fixed NS content of 3% and six SF contents in a range of 0–2.5% by volume were used. The impact resistance was measured based on the number of blows (N1, N2) and the impact energy. The durability of concrete includes its freeze–thaw resistance and chloride ion penetration resistance, which were appraised by the chloride ion diffusion coefficient (CDC) and relative dynamic elastic modulus (RDM), respectively. The ductility ratio was used to predict the impact resistance of concrete containing NS with different SF contents, and a linear relation between this ratio and the impact energy (R 2 = 0.853) was found. The experimental results indicated that SF could greatly improve the impact resistance of concrete. The addition of 2.0% SF increased N1 and N2 by 106 and 169%, respectively. In addition, an appropriate SF content significantly improved the durability of the concrete, including its frost resistance (especially in the middle and late freezing–thawing cycles) and chloride ion penetration resistance. An SF content of 1.5% was the optimum, decreasing the CDC of nano-concrete by 17.1% and minimizing the RDM loss. Moreover, the 1.5% SF content increased the compressive strength of concrete containing NS by 18.5%, whereas an SF content of 2.0% increased the splitting tensile strength and flexural strength by 77 and 20%, respectively. Furthermore, when the SF content exceeded a certain value, the improvement effect on these properties began to decrease and even became negative.

Laboratory assessment of early-age durability benefits of a self-healing system to cementitious composites

Shrinkage of cement composites (paste, mortar, grout, or concrete) due to moisture loss during the curing period is a challenge for their durability. In this laboratory study, urea-formaldehyde (UF) microcapsules and polyvinyl alcohol (PVA) microfibres were utilized as a self-healing system to improve the early-age durability of cement mortars by mitigating their total shrinkage during the curing period. Experimental results revealed that the admixed UF microcapsules/PVA microfibres together could mitigate 25% of the total shrinkage during the curing period of 35 days. In addition, this self-healing system could reduce the gas permeability of the mortars by over 75%. The UF microcapsules and PVA microfibres could interfere with the formation of some crystalline hydration products and modify the microstructure of hydrated cement mortar, resulting in slight reductions in the compressive strength (less than 12%) and at some dosages significantly reduced the chloride migration coefficient of the mortars (i.e., 1% 1100+PVA, 2% 700, and 1% 1100). The UF microcapsules mainly affected the pores with a radius between 10 nm and 1000 nm, whereas the PVA microfibers mainly affected the larger pores at the interfacial transition zone.