Ion Erosion Research Articles

Effect of Sulfate Concentration on Chloride Diffusion of Concrete under Cyclic Load.

The existence of chloride ions, sulfate ions, and vehicle dynamic loads may lead to a shortened service life and premature failure of the road and bridge structures in northwestern China. Immersed in a dual-salt solution while simultaneously applying cyclic flexural loads, the free chloride ion concentration and erosion depth in concrete specimens were measured. The influence of the sulfate concentration on the apparent surface chloride concentration (Cs) and apparent diffusion coefficient (Dapp) was studied. An exponential model was used to fit the Cs, and the influence of sulfate concentration on the Cs was analyzed. The result showed that cyclic loading and solution concentration were two primary factors affecting chloride diffusion. Meanwhile, compared with the emersion conditions, dynamic loading would induce significantly accelerated chloride ion penetration. Under the coupling effect of sulfate and dynamic loading, as the sulfate concentration increased, the chloride ion concentration and erosion depth were both decreased. The existence of sulfate ions improved the chloride ion penetration resistance of concrete. The results provide insight in designing concrete in regions where multiple salt ingression (sulfate and chloride) is a major durability issue of the structures.

Stratified damage characteristics of high‐water quick‐setting material exposed to sulfate under dry–wet cycles

AbstractThe stability of the grouting material under the coupling effect of sulfate attack and dry–wet cycles is essential to engineering safety. As a novel grouting material, the stability research of high‐water quick‐setting material (HWQSM) under different corrosive environments is in the initial stage. Moreover, the stratified damage characteristics (SDC) of HWQSM under sulfate attack were not fully explored. To this end, the strength degradation and SDC of HWQSM exposed to three kinds of sulfate solutions (i.e., 10% MgSO4 solution, 10% Na2SO4 solution, and hybrid solution of 5% Na2SO4 and 5% MgSO4) and dry–wet cycles were analyzed by using ETM‐C mechanical testing machine, scanning electron microscopy (SEM), XRD, and AFM. The results show that the strength reduction rate, the thickness of the corrosion layer, and roughness between the corrosion layer and the transition layer of HWQSM are all positively correlated with the concentration of Mg2+ in the erosion solution for the increase in dry–wet cycles. Under the combined action of ion erosion and carbonization, the pores and cracks of the corrosion layer expand, which would reduce the permeability resistance but increase the erosion damage rate. This research furnishes new views into the SDC of HWQSM under sulfate attack and dry–wet cycles, which would deeper our understanding to this special material undoubtedly; the research results are expected to furnish theoretical guidance of HWQSM resistance to different sulfate solutions.

Study on the bond behavior of steel bars embedded in concrete under the long-term coupling of repeated loads and chloride ion erosion

Twenty-seven reinforced concrete beam specimens were cast to study the bond properties between steel bars and concrete under the coupling of chloride ion erosion and repeated load (0, 0.25, 0.45, and 0.65 of ultimate load) for 210 days. The bond strength and slip of steel bars embedded in concrete were measured by the flexural bond test. Based on the test results, the higher the level of repeated load alone, the lower the bond strength. The bond strength of beam specimens under the coupling of repeated load and chloride ion erosion is lower than that under repeated load alone or chloride ion erosion alone. The bond strength decreases gradually when the corrosion ratio exceeds 1.5%. The slip between steel bars and concrete decreases with the increase of repeated load level under the coupling of chloride ion erosion and repeated load when splitting failure occurs. Based on the test results, the bond strength-slip constitutive relationship was proposed.

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.

Carbonation Characteristics and Bearing Capacity Attenuation of Loaded RC Beam Coupled with Chloride Erosion

The durability of a concrete bridge structure is a systematic problem composed of material, structure, natural environment, and service environment. Various factors are coupled, which affect each other, and single‐factor research cannot fundamentally solve this problem. In this paper, the carbonation characteristics of RC beams with different loading states under the coupling action of carbonation and chloride erosion are studied. Through the experiment, the author tries to find the influence of stress state and chloride ion erosion on the carbonation of concrete and analyze the failure mode and the attenuation rules of the flexural and shearing capacity of the corroded RC beams under the coupling action. For this purpose, five groups of experiments under different working conditions were designed, including chloride ion erosion and carbonation experiments without external load of the cubic blocks, and chloride erosion and carbonation coupling experiments of RC beams under different stress states and stress levels. The carbonation rate of concrete can be reduced by 56%∼60% under the coupling action of chloride salt. Different loading states and stress levels have an obvious influence on carbonation and chloride ion corrosion, which further affects the corrosion rate of steel bars. Under a low corrosion rate, the bending and shear failure modes of the corroded beams are similar to those of the noncorroded beams, and the section strain distribution still approximately conforms to the plane section assumption. The relationship between the relative ultimate shear strength or the relative ultimate flexural strength and the average section‐corrosion rate of the reinforcement is approximately linear.

Study on the Time‐dependent Reliability of Corroded Reinforced Concrete Bridge Structures due to Ship Impact

The cumulative damage caused by chloride ion erosion to coastal bridge structures reduces the ship impact resistance of channel bridge structures and leads to changes in reliability of bridge structures on ship impact. It is of great theoretical and practical significance to study the performance degradation change law of corroded bridge structures and to evaluate the ship impact time‐dependent reliability of in‐service corrosion‐damaged bridge structures. Based on the durability decay model of reinforced concrete structures in a chloride ion erosion environment, the time‐dependent resistance analysis of bridge structures is carried out considering the reduction of yield strength of longitudinal reinforcement and hoop reinforcement in pile sections and the decay of compressive strength of protective layer concrete. Based on the study of the probabilistic model and parameters of random variables affecting the time‐dependent reliability of ship‐bridge collision, the typical damage modes of bridge structures under ship impact are analyzed, the time‐dependent reliability analysis model of bridge structures under ship impact is established based on the structural damage criterion, the ship‐bridge crash limit state functional function is given, and the time‐dependent reliability analysis of ship‐bridge collision is carried out based on the response surface method.

Experimental Study on the Influence of a Cementitious Permeable Crystallization Admixture (CPCA) in Improving Concrete Durability

The corrosion of steel reinforcement in reinforced concrete is a serious problem affecting the durability of concrete structures. And the steel corrosion caused by chloride ion erosion is the most serious damage to reinforced concrete structures, which affects the safety and service life of the structure. In order to prevent steel corrosion caused by chloride, it is necessary to pay more attention to chloride ion penetration damage. At present, the most commonly used method is to use additives in concrete structures, so as to improve the durability of concrete by reducing its permeability coefficient. Cement‐based permeable crystalline admixtures can effectively improve the impermeability and mechanical properties of concrete by complex crystallization reactions between reactive chemicals and cement gels. In the paper, the Penetron admix 803 was selected as a typical CPCA in the experimental study, and the rapid chloride migration (RCM) experiments and mechanics experiments were carried out for four concrete structures with different dosages of Penetron admix 803 (PA8). Compared with plain concrete, the concrete structure containing PA8 makes it difficult for water and other liquids to enter by filling micropores and shrinkage cracks by scanning electron microscope test, so its impermeability is greatly improved. And concrete structure containing permeable crystalline admixture can reduce chloride ion permeability effectively. The experimental results showed that with the increase of PA8 content, the diffusion coefficient of the concrete structure decreased rapidly from 45.8 × 10−12 m2/s to 17.7 × 10−12 m2/s, while the service life of concrete structures increased continuously. And the mechanism of the chemical reaction between PA8 and the internal components of concrete structure was revealed, which was mainly due to the catalytic reaction between the reactive chemicals of PA8 and the cement hydrates of concrete structure forming insoluble crystals filled with concrete micropores. Experimental and theoretical results indicate that the permeable crystalline admixture can greatly improve the chloride‐related durability and mechanical properties of the concrete structure.

Experimental investigation on the corrosion and corrosion fatigue behavior of butt weld with G20Mn5QT cast steel and Q355D steel under dry–wet cycle

Corrosion and corrosion fatigue experiments were conducted on a butt weld constructed of G20Mn5QT cast steel and Q355D steel in a dry–wet cycle with 3.5 wt% NaCl solution. The microstructural characterization was analyzed using SEM. The butt weld suffered from pitting corrosion in a dry–wet cycle. Chloride ion erosion and defects (welding defects and casting defects) have important influences on pitting corrosion. Internal defects and corrosion pits are the main cause of corrosion fatigue failure of the butt weld specimens. Drying time accelerates pitting corrosion and corrosion fatigue progression without changing the corrosion and corrosion fatigue mechanism in 3.5 wt% NaCl solution. The three-parameter Weibull model has good applicability for evaluating the corrosion fatigue behavior of G20Mn5QT cast steel and Q355D steel butt welds in simulated seawater.

Effect of different gypsum dosage on the chloride binding properties of C4AF hydrated paste

To reveal the mechanism of the high chloride-binding capacity for high ferrite Portland cement, ferrite phase (C4AF) was prepared and the effects of different gypsum dosages (0%, 5%, 10%, 20%, 30%) on the chloride binding performance, phase composition, microstructure and pore structure of C4AF hydrated paste were investigated by XRD, TG-DTA, SEM-EDS and nitrogen adsorption analysis. The results show that the pure C4AF hydrated paste had a strong chloride binding ability, in which the chemical binding chloride ion ability reached 28.3 mg/g. When the gypsum dosages increased from 5% to 30%, the chemical binding chloride ion ability decreased by 7.4%, 13.4%, 52.2% and 74.5%, respectively, compared with the pure C4AF hydrated paste. With the increase of gypsum dosages, the content of C3(A,F)H6 in the C4AF hydrated paste decreased, while a large amount of monosulfate (AFm) was generated and the specific surface area of hydration products was greatly reduced. During chloride ion erosion, mainly C3(A,F)H6 reacted chemically with chloride salt to generate Friedel’s salt, and the chemical chloride binding ability showed a linear correlation with the content of C3(A,F)H6 in the C4AF hydrated paste.

Chloride erosion resistance of calcium formate incorporated cement mortar under chloride attack

Calcium formate has been widely added in concrete as an accelerator to obtain a high early strength in civil engineering, such as shotcrete and concrete repairment. However, its effect on chloride ion erosion and steel bar corrosion under marine environments is elusive. In this study, rapid chloride migration and chloride ion titration methods were employed to investigate the effect of calcium formate on the chloride ion erosion in cement mortars. Electrochemical impedance spectroscopy and the loss weight ratio of rebar were measured to quantitatively characterize the corrosion extent of the steel bars. Furthermore, the relationship between chloride ion penetration and internal structures of the composite was established based on its pore structures characterized by means of mercury intrusion porosimetry. The crystalline phases and microstructures of the cement pastes were examined by XRD and SEM analyses, respectively. The results showed that calcium formate increased the formation of C2AH8 resulting in a significant increase in porosity and reducing the amount of chemical bound chloride ions inside, increasing the diffusion coefficient and content of chloride ion. Moreover, the increase in the number of internal pores allowed chloride ions to migrate easier in mortar and readily trigger corrosion of steel bars.

Multi-scale study water and ions transport in the cement-based materials:from molecular dynamics to random walk

The transport properties of water and ions play a key role in the durability of cement-based materials. However, little work has been done to study the diffusion behaviors of water and ions in a multi-scale view. Therefore, the current paper presented a multi-scale molecular dynamics fed random walk (MD-RW) simulation scheme for the purpose of obtaining the diffusion coefficients of water and ions in the calcium-silicate-hydrate (C–S–H) gel. At nanoscale, MD simulations were used to obtain the diffusion coefficients of water and ions in the C–S–H with nanopore size ranging from 0.5 nm to 5 nm since pore size has a crucial effect on the diffusion behavior of water and ions. Then these results were utilized as the input parameters for random walk simulation to scale the confining effect up and derive the effective diffusion coefficients of water and ions in the C–S–H gel at mesoscale. The results acquired from mean square displacement, atomic intensity distribution function and radial distribution function show that water molecules and Cl ions would form Ca–Cl, Ca–O ion-clustering and hydrogen bonds with C–S–H substrate which will restrict the transport of water molecules and Cl ions. In addition, with the size of gel pore increases from 0.5 nm to 5 nm, the diffusion coefficient of water increases from 0.016 to 1.85 × 10−9 m2 s−1 and the diffusion coefficient of Cl ions increases from 0.002 to 0.75 × 10−9 m2 s−1. In the random walk simulation, the diffusion coefficients of water and Cl ion were derived as 1.01 × 10−12 m2 s−1 and 1.2 × 10−13 m2 s−1 in the FCC packing model and 1.55 × 10−11 m2 s−1 and 1.77 × 10−12 m2 s−1 in the BCC packing model respectively, which reaches reasonable agreement with the previous studies results. Unlike traditional reverse methods which back-calculate these coefficients from macroscopic experimental results and hypothesized microstructure, this study presents the first bottom-up method deriving the transport properties of C–S–H gel directly from the physicochemical characteristics and microstructure, which has relevant significance for the study of concrete resistance to ion erosion and provides a basis for designing durable concrete.

Study on the bond behavior of corroded steel bars embedded in concrete under the coupled effect of reciprocating loads and chloride ion erosion

This work presented an experimental investigation on bond behavior of corroded steel bars embedded in concrete under the action of three different levels of reciprocating loads (25%, 45% and 65% of ultimate load) and sustained chloride attack using 51 RC beam specimens with dimensions of 100 mm × 150 mm × 550 mm. In the test, the steel is corroded by the method of energized accelerated corrosion (theoretical corrosion ratio of steel is set as 2%, 4% and 6%). Test results show that the effect of reciprocating load will cause the bond strength between uncorroded steel bars and concrete to decrease in the chloride environment, but the effect is not significant. Under the coupled effect of reciprocating loads and energized accelerated corrosion, when the corrosion ratio of longitudinal reinforcement is constant, as the increase of the reciprocating load level, the bond strength between the steel bar and concrete of the specimen decreases. When the theoretical corrosion ratio of steel bars is small, the bond strength between steel bars and concrete gradually increases with the increase of the corrosion ratio; as the corrosion ratio of steel bars further increases, the bond strength shows a downward trend. The theoretical corrosion ratio of steel bars reaches a peak of about 2%. In this paper, the mathematical model of bond strength-slip has been proposed. This work provided an important theoretical basis for correctly assessing the durability of buildings or structures subjected to reciprocating loads in coastal areas.

Study on the bearing capacity and chloride ion resistance of RC structures under multi-factor corrosive environment and continuous load

In this paper, the bearing capacity and chloride resistance of reinforced concrete (RC) structures under multi-factor corrosive environment and continuous load are investigated, and the correlation between the bearing capacity and chloride resistance is also studied. First, different corrosion environments and compressive stress conditions were simulated by freeze-thaw (F-T) cycles test, carbonization test and continuous loading test. Among them, carbonization test and F-T cycles test constitute corrosion cycle test. Then, according to the test results of bearing capacity, under the action of corrosion cycle test, the bearing capacity of RC beam decreases with the increase of corrosion cycles. The bearing capacity of RC beams have a tendency to rise first and then drop with the increase of compressive stress under the multi-factor corrosive environment. In addition, according to the test results of chloride ion erosion, the equation for calculating the corrosion resistance of chloride ion is established, and the resistance to chloride ion erosion increased with the increase of corrosion cycle. With the increase of compressive stress, the ability to resist chloride ion erosion displays the tendency rising up at the beginning and declining in late. Finally, through grey correlation analysis, the correlation between corrosion cycles and yield load is greater than the correlation between chloride ion erosion and yield load, while the correlation with ultimate load is opposite. Based on the results of correlation analysis, the bearing capacity decay equation is established.

Feasibility Evaluation of Replacing River Sand with Copper Tailings

This study aims to realize the resource regeneration application of copper tailing (as fine aggregates for partial replacement of natural fine aggregates), which avoid environmental pollution due to many landfills of copper tailings. The compressive strength and durability (dry shrinkage and sulfate attack) tests were carried out to evaluate the effect of copper tailings replacement on the performance of mortar. The results show that the mortar with copper tailings has higher compressive strength than the one with natural sand. More than 14% improvement in compressive strength can be achieved by adding copper tailings with no more than 40% replacement level. The dry shrinkage of mortar was increased with the copper tailings due to the increase of micro pores in mortar by using copper tailings. Compared with the mortar with natural sand, the dry shrinkage can be reduced by adding copper tailings with no more than 20% replacement level. The sulfate attack resistance is improved by using copper tailings, when the replacement rate is more than 20%. In fact, the micro-aggregate filler effect of copper tailings effectively refines the pore structure and forms more stable, uniform and fine interface micro pores, which is of vital significance for mortar to resist external forces and sulfate ion erosion. However, copper tailings, as a porous material, have water release characteristics in cement mortar. This characteristic is not conducive to the filler effect, which decreases the filling rate in later hydration, leading to higher porosity of copper tailings mortar. More importantly, mortar can solidify heavy metals in copper tailings, which prevents loss of heavy metal such as Cu, Zn, Sr, Zr, As, Ga due to environmental problems.

Study on Underwater Anti-washout Concrete in Sediment Environment for Offshore Engineering

The close packing theory is used to optimize the cementitious material system of ordinary underwater anti-washout concrete, and the performance of the prepared concrete is verified through experiments. The test results show that the tightly packed underwater anti-washout concrete with C40 mix ratio is cured under the condition of seawater curing., 7d compressive strength of 31.4MPa, 28d compressive strength of 55.3MPa, 56d compressive strength of 68.7MPa, electric flux of 860C, meeting the requirements of marine engineering concrete’s mechanical properties and resistance to harmful ion erosion.

Study on the bond behavior of steel bars embedded in concrete under the coupling of sustained loads and chloride ion erosion

The 27 reinforced concrete beam test specimens of 550 mm × 150 mm × 100 mm with initial cracks were used to investigate the bond properties of steel bars embedded in concrete under the action of three different levels of sustained loads (25%, 45% and 65% of ultimate load) alone, chloride attack (NaCl solution with a concentration of 5%) alone and coupling of sustained loads and chloride attack for 180 d. Based on the test results, the influence of three working conditions on bond behavior between steel bars and concrete has been compared and analyzed. Subjected to the same load level, the bond stress between steel bars and concrete under chloride attack was higher than that without chloride attack, the bond strength of the specimen subjected to the coupled effect of sustained load and chloride ion attack was higher than that under the single factor. As the load level increased, the bond strength of steel bars embedded in concrete gradually decreased whether there was chloride ion attack or not. When the corrosion ratio of the longitudinal bars was small (corrosion ratio ≤ 0.14 without chloride ion corrosion and corrosion ratio ≤ 0.28 with chloride ion corrosion), the bond strength between steel and concrete gradually increased with the increase of the load level; with the further improvement of the corrosion ratio of steel bars, the bond strength showed a decreasing trend. The free end slip of specimens is smaller than the load end. Based on the test results, the reinforced concrete bond-slip curves and its constitutive relationships under different conditions have been proposed.

Effects of Sodium Sulfate Attack on Concrete Incorporated with Drying‐Wetting Cycles

It has been widely observed that sulfate attack can damage the durability of concrete. This research investigated the mass loss and damage degree of concrete under sodium sulfate attack incorporated with drying‐wetting cycles. The impact factors, including water‐binder ratio, solution concentration of sodium sulfate, fly ash content, curing time, and drying‐wetting cycle system, were observed to influence the sodium sulfate attack by the mass loss rate and damage degree at regular time intervals. Also, the hydrates of sulfate‐attacked samples were analyzed using X‐ray diffraction. Results indicated that a high water‐binder and high‐concentration sodium sulfate solution could accelerate the transportation of sulfate ion inside the concrete and the deterioration degree of concrete. Appropriate fly ash and longer curing time can effectively improve the internal pore structure of concrete to reduce the sulfate corrosion damage. The sulfate ion erosion and deterioration degree of the concrete are synchronously intensified along with the increase of the baking‐immersing time ratio. The trend of the predicted life for concrete is basically consistent with the damage evolution result, indicating the feasibility of the Weibull distribution model to predict the service life of concrete under sodium sulfate attack incorporated with drying‐wetting cycles.

A Research About One-dimensional Chloride Ion Erosion In Concrete Under Drying-wetting Cycles

Reinforced concrete structures are now widely used in China, and with the exploitation of marine resources, many concrete structures are in a relatively harsh service environment. Concrete in wave and tidal stream areas is in a dry and wet cycle state. When unsaturated concrete is in a dry and wet cycle state, chloride ions will invade the concrete by diffusion and convection, which accelerates the erosion of the reinforcement within the concrete and degrades the concrete performance. Therefore, the study of chloride ion erosion in concrete under dry and wet cycles is particularly important. Fick’s law is a good predictor of the diffusion of chloride ions in saturated concrete with stable boundaries, but it is difficult to obtain satisfactory results for concrete under dry and wet cycles. There are many difficult parameters in the microscopic model that make programming the calculations more difficult. In this paper, we propose to use the radial basis function matching point method to solve the problem. It is found that the error is within acceptable limits and can be used to calculate the one-dimensional erosion of chloride ions under dry and wet cycles through solving the validation algorithm.

The Effect of Freeze‐Thaw Damage on Corrosion in Reinforced Concrete

The existing studies of the corrosion of reinforced concrete have mainly focused on the interface area and chemical ion erosion, ignoring the specific service environment of the reinforced concrete. In this study, the effect of freeze‐thaw damage was investigated via corrosion experiments under different freeze‐thaw cycle conditions. Steel reinforcement corrosion mass, ultimate pull‐out force, corrosion rate, and bond slippage were chosen as characteristic parameters in the experiments, and scanning electron microscopy (SEM) analysis was used to explain the mechanism of action of freeze‐thaw damage on corrosion. The results showed that, under identical corrosion conditions, the mass of steel reinforcement corrosion and corrosion rate increased by 39.6% and 39.7% when comparing 200 freeze‐thaw cycles to 0 cycles, respectively. The ultimate pull‐out force and bond slippage after 200 freeze‐thaw cycles decreased by 73% and 31%, respectively, compared with 0 freeze‐thaw cycles. In addition, SEM analysis indicated that microstructure damage caused by freeze‐thaw cycles accelerated the corrosion reaction and decreased cementitious properties, leading to decreasing ultimate pull‐out force and bond slippage. The effect of freeze‐thaw cycles and steel reinforcement corrosion on the macro mechanical properties of concrete is not a simple superposition.

Effect of Ion Corrosion on 517 Phase Stability

The main hydration product and source of strength of magnesium oxysulfate cement is 5Mg(OH)2·MgSO4·7H2O (known as the 517 phase). Hardened pastes containing 92.38% of the 517 phase were synthesized in this study, and the influence of different types of chloride solutions on the stability and compressive strength of the 517 phase was investigated. X-ray diffraction and the Rietveld method were used to investigate the 517 phase transition in chloride solutions. Ion chromatography and inductively coupled plasma spectrometry were used to analyze the ion concentrations of the chloride solutions. Scanning electron microscopy and mercury injection porosimetry were used to investigate the effect of ion erosion on the microstructure and pore size distribution. The results showed that the crystal structure of 517 phase remained stable upon immersion in chloride solutions (except for the CaCl2 solution) up to 28 days, and there was no discernible attenuation in the compressive strength of the hardened pastes. Immersion of the 517 phase in CaCl2 solution for 28 days caused Ca2+ ions to combine with SO42− groups to generate CaSO4·2H2O, thereby decomposing the 517 phase. An increase in the concentration of magnesium and sulfate ions in the immersion solutions confirmed the decomposition of the 517 phase. Gel-like Mg(OH)2 was observed in the microstructure of the decomposed 517 phase, and the decomposition of the 517 phase increased the porosity of the hardened pastes.