Search Results (83)

Liquid calcium aluminate cement (LCAC) is an innovative material technology with significant potential for varied applications in civil engineering. However, despite its promising results, a significant gap remains in the direct application of LCAC as a concrete binder. The primary catalysts for LCAC are sodium hydroxide (NaOH) and potassium hydroxide (KOH). Therefore, it is crucial to investigate the effects of sodium and potassium ions on alkali–aggregate reactions in concrete structures. This study evaluated the durability of liquid calcium aluminate cement concrete catalyzed using four different concentrations of NaOH (0.5%, 1.0%, 1.5%, and 2.0%) as experimental variables, incorporating a control group of traditional concrete with a water–cement ratio of 0.64. The findings indicate that NaOH catalysis in the concrete significantly trigger alkali–aggregate reactions, leading to volume expansion. Furthermore, it increased chloride ion penetration and porosity in the concrete. These effects were more notable with the increase in NaOH concentration. The results suggested that NaOH catalysis can enhance certain chemical reactions within the concrete matrix; however, its concentration must be carefully controlled to mitigate adverse effects. The NaOH dosage should be limited to 0.5% to ensure optimal durability of the concrete. This study emphasizes the crucial importance of precisely balancing catalyst concentration to maintain the long-term durability and performance of liquid calcium aluminate cement concrete in structural applications. Full article

Electrochemical treatment by means of direct current (DC) is usually used as a measure for steel rebar corrosion protection, e.g., cathodic protection (CP), electrochemical chloride extraction (ECE), and re-alkalization (RA). However, the passage of an electrical charge through the pore system of concrete or mortar, coupled with the migration of ions, concentration changes, and resulting phase changes, may alter its chloride penetration resistance and, subsequently, the time until rebar corrosion activation. Porosity changes in hardened Portland cement mortar were studied by means of mercury intrusion porosimetry (MIP) and electrochemical impedance spectroscopy (EIS), and alterations in the mortar surface phase composition were observed by means of X-ray diffraction (XRD). In order to innovatively investigate the impact of DC treatment on the properties of the mortar–electrolyte interface, the cathode-facing mortar surface and the anode-facing mortar surface were analyzed separately. The corrosion of steel coupons embedded in DC-treated hardened mortar was monitored by means of the free corrosion potential (E_oc) and polarization resistance (R_p). The results showed that the DC treatment affected the surface porosity of the hardened Portland cement mortar at the nanoscale. Up to two-thirds of the small pores (0.001–0.01 µm) were replaced by medium-sized pores (0.01–0.06 µm), which may be significant for chloride ingress. Although the porosity and phase composition alterations were confirmed using other techniques (EIS and XRD), corrosion tests revealed that they did not significantly affect the time until the corrosion activation of the steel coupons in the mortar. Full article

When the chloride ion concentration within concrete reaches a certain threshold, it triggers corrosion of the reinforcing steel bars, severely compromising the durability of reinforced concrete structures. Accurately assessing how the chloride ion concentration in concrete evolves over time is crucial for ensuring structural safety and evaluating the remaining service life. This work first analyzes the advantages and disadvantages of several existing time-dependent models for chloride ion diffusion coefficients. Based on this foundation, a new time-varying model is proposed to more accurately predict the variation of chloride ion diffusion coefficient with service time. The newly proposed model can be regarded as a variant of the square-root model, incorporating only two fitting parameters. It can be readily transformed into a linear regression model for solving the fitting parameters, rendering it highly convenient to use. Using 11 sets of experimental data from the existing literature as examples, the new model consistently demonstrates the lowest mean fitting error and the highest coefficient of determination across all scenarios, showcasing its superior generality. This new model likely reflects the fundamental physical law governing the temporal variation of chloride ion diffusion coefficients. Full article

Structural crack seepage in concrete is a common condition in engineering applications. Under the combined effects of multiple factors such as water pressure and load, cracks are more likely to occur inside the concrete structure, thus aggravating the water seepage problem. To simulate the chloride ion erosion of structural cracks, an independent test system that can simultaneously consider the coupling effect of multiple factors was developed. Three typical factors—water pressure, vertical load, and erosion time—were selected and designed using the orthogonal test method to analyze the effect of factors on the chloride ion concentration. The results revealed that the vertical load is the least influential factor, water pressure and erosion time are the most noticeable factors, and the factors influencing the diffusion of chloride ion in concrete are, in order of magnitude, water pressure (0.86), erosion time (0.66), and vertical load (0.36). Nonlinear surface fitting, with an R-squared value exceeding 0.95, was used to characterize the relationship between chloride ion concentration, water pressure, and erosion time. Full article

The performance degradation of concrete structures directly impacts their safety. As such, accurately predicting their remaining service life is critical for effective operation and maintenance management. This paper reviews the key factors influencing the degradation of concrete structures, providing a comprehensive summary of current research on deterioration mechanisms, steel corrosion, crack propagation, and chloride ion penetration. It also compares the advantages and limitations of physical, empirical, statistical, and machine learning models used for life prediction. A critical aspect highlighted in this paper is the importance of model validation based on real-world field data, which can more effectively determine the applicability of prediction models in actual engineering practice. Model validation incorporates evaluation metrics like sensitivity analysis to gauge how fluctuations in input parameters, such as temperature, influence life prediction models and thereby reveal the uncertainties inherent in complex engineering environments. Currently, life prediction models are widely applied to infrastructure projects like bridges and tunnels. By incorporating environmental factors such as chloride ion concentration, temperature, and humidity, as well as real-time monitoring data, these models effectively predict the remaining service life, aiding engineers in developing optimized maintenance strategies. However, current models still face challenges in terms of data requirements and accuracy. Future research should focus on the integration of hybrid models and intelligent technologies. By leveraging the combined strengths of physical and data-driven approaches, hybrid models can enhance prediction accuracy. Additionally, intelligent technologies and real-time monitoring will help dynamically update and optimize models, resulting in more precise and efficient life predictions. Full article

The Qinghai–Tibet Plateau features a high-altitude, cold, and arid climate, with harsh environmental conditions. It is also one of the regions in China where chloride-rich salt lakes are abundant. These circumstances pose significant challenges to the durability of concrete. This study explored the impact of recycled fine powders (RFP) on the resistance of concrete to chloride ion erosion. To evaluate this, a 3.5% sodium chloride solution and Qarhan Salt Lake brine were employed as erosion media. The depth and concentration of chloride ion penetration, the free chloride ion diffusion coefficient (D_f), and the microstructure of the concrete were measured. The results demonstrated that when the replacement rate of RFP was 20%, the concrete displayed excellent resistance to chloride ion erosion in both the sodium chloride solution and the Salt Lake brine. XRD analysis and SEM images revealed that the addition of RFP enabled the concrete to bind more Cl⁻ to form Friedel’s salt, which filled the pores of the concrete and reduced the diffusion of Cl⁻ within the concrete. Moreover, as the soaking time extended continuously, the erosion and damage effects of the Salt Lake brine solution on the concrete were more severe than those of the sodium chloride solution. Full article

Salt lakes and the surrounding saline soils distributed across northwestern China and Inner Mongolia impose severe physicochemical corrosion on cement-based concrete. Understanding the corrosion products and mechanisms are crucial scientific and technological factors in ensuring the durability and service life of concrete structures in these regions. In this study, various analytical techniques—including X-ray diffraction, thermogravimetric–differential thermal analysis, X-ray fluorescence, and scanning electron microscopy coupled with energy-dispersive spectroscopy—were employed to systematically analyze the corrosion products of ordinary Portland cement (OPC) and high-performance concrete (HPC) specimens after eight years of field exposure in the Qarhan Salt Lake area of Qinghai. The study provided an in-depth understanding of the physicochemical corrosion mechanisms involved. The results showed that, after eight years of exposure, the corrosion products comprised both physical corrosion products (primarily sodium chloride crystals), and chemical corrosion products (associated with chloride, sulfate, and magnesium salt attacks). A strong correlation could be observed between the chemical corrosion products and the strength grade of the concrete. In C25 OPC, the detected corrosion products included gypsum, monosulfate-type calcium sulfoaluminate (AFm), Friedel’s salt, chloro-ettringite, brucite, magnesium oxychloride hydrate 318, calcium carbonate, potassium chloride, and sodium chloride. In C60 HPC, the identified corrosion products included Kuzel’s salt, Friedel’s salt, chloro-ettringite, brucite, calcium carbonate, potassium chloride, and sodium chloride. Among them, sulfate-induced corrosion led to the formation of gypsum and AFm, whereas chloride-induced corrosion resulted in chloro-ettringite and Friedel’s salt. Magnesium salt corrosion contributed to the formation of brucite and magnesium oxychloride hydrate 318, with Kuzel’s salt emerging as a co-corrosion product of chloride and sulfate attacks. Furthermore, a conversion phenomenon was evident between the sulfate and chloride corrosion products, which was closely linked to the internal chloride ion concentration in the concrete. As the chloride ion concentration increased, the transformation sequence of sulfate corrosion products occurred in the following order: AFm → Kuzel’s salt → Friedel’s salt → chloro-ettringite. There was a gradual increase in chloride ion content within these corrosion products. This investigation into concrete durability in salt-lake ecosystems offers technological guidance for infrastructure development and material specification in hyper-saline environments. Full article

Reinforced concrete (RC) structures in coastal atmospheres commonly suffer the penetration of chloride ions, which can lead to the corrosion of reinforcements and, thus, a reduction in their structural performance under earthquakes. In recent years, time-dependent seismic fragility analysis has been widely used as an effective tool to represent the deterioration in the seismic performance of aging RC structures. However, few studies have considered the influences of varying chloride ion exposure environments due to the different distances of structures from a coastline. In light of this, this study performs a time-dependent seismic fragility analysis for aging RC frames, considering varying distances of the buildings from the coastline. To conduct this, a time-dependent reinforcement corrosion rate model that can consider the effect of the distance of a building from the coastline is established by combining a concrete surface chloride ion concentration model, an initial corrosion time model, and an electrochemical corrosion rate model. By integrating material deterioration models for reinforcements and concrete, the seismic fragility relationships for structures with different degrees of corrosion damage can be developed. A corrosion deterioration factor is then proposed to quantify the relationship between the seismic fragility function parameters and the corrosion rate. Subsequently, time-dependent fragility functions considering the effect of the distance from the coastline can be established. A nine-story RC frame designed according to the existing Chinese codes is used for illustration. The time-dependent seismic fragility relationship of the structure is developed considering different distances of buildings from the coastline. The results show that the effect of the distance of a building from the coastline varies under different categories of environment. The seismic fragility results for a structure under a III-a environment are more significantly influenced by the structural distance from the coastline compared to those for a structure under a II-a environment. Full article

The corrosion of steel reinforcements substantially degrades the longevity of reinforced concrete structures, particularly in marine settings. This investigation introduces a comprehensive model that simulates the processes involved in moisture and chloride ion transport, rebar corrosion, and the consequent cracking of concrete. The model reveals that the transport dynamics of chloride ions are primarily dictated by their penetration rates through the solution. The sensitivity of the steel to corrosion is a function of the concentrations of water and chloride ions, whereas the rate of corrosion predominantly depends on the availability of oxygen at the corrosive site. Oxygen diffusion is the rate-limiting step in the entire process of the electrochemical reactions of the rebar. And the peak corrosion rates are observed at the interface between the solution and the gas phase. The model calculates the stress and strain in the concrete resulting from volumetric expansion due to oxidization of the steel bars. By accurately reproducing the progression of corrosion-related damage, this model provides crucial insights for predicting the service life of offshore concrete structures and enhancing durability against aggressive environmental conditions. Full article

Carbonation and chloride ingress are the most important damaging mechanisms for steel-reinforced concrete. The combination of these two corrosion processes accelerates the destruction of concrete, leads to extensive structural repairs, negatively impacts durability, and significantly reduces the service life of the structure. One possible and effective way to reduce chloride diffusion through the concrete pore system is through the use of crystalline materials. An experimental study focused on the ability of an applied crystalline coating to increase the chloride resistance of carbonated concrete is presented in this paper. Carbonated concrete specimens treated with a crystalline coating were exposed to a sodium chloride solution for various periods of time, and a water-soluble chloride ion content analysis was performed on powder samples taken from the tested specimens. Chloride profiles presenting the chloride ion concentrations at selected depths are presented for multiple types of concrete at various ages to show the effect of crystalline technology on the chloride resistance of concrete. The results of this study confirm the impact of carbonation on chloride ion ingress through concrete and show that crystalline coatings can improve the chloride resistance of concrete. Using crystalline coatings on carbonated concrete can, from a long-term perspective, significantly reduce the chloride ion content in concrete placed in an aggressive environment. The crystalline coatings were functional even after 28 days, when the concentration of chloride ions was below the critical concentration. The crystalline coating was able to reduce the concentration of chloride ions by 68% under the surface of the concrete and by 65% at depths of 20–25 mm after 180 days of immersion, compared to the untreated concrete. Crystalline coatings reduce the depth of critical chloride ion concentration, effectively protect the concrete reinforcement against corrosion and extend the service life of the structure. Full article

Marine concrete frequently experiences performance degradation due to the combined effects of chloride ion (Cl⁻) erosion and carbonation. While many studies have examined the separate effects of Cl⁻ erosion and carbonation, their combined impact on concrete is still debated. Investigating the interaction mechanisms between Cl⁻ erosion and carbonation is crucial for improving the durability of concrete structures. This study utilizes a method where concrete specimens are immersed in artificial seawater with NaCl concentrations of 5%, 10%, and 15% prior to carbonation, with carbonation depth serving as a key indicator for analyzing the impact of Cl⁻ erosion on carbonation. Both carbonation-treated and standard concrete specimens are immersed in 5% artificial seawater to evaluate the impact of carbonation on chloride erosion, with the free chloride content in the concrete serving as the assessment criterion. Scanning electron microscopy (SEM) is employed to examine the microstructure of the concrete, elucidating the interplay between Cl⁻ erosion and carbonation. This study reveals that (1) Cl⁻ erosion hinders concrete carbonation as NaCl crystals and Friedel’s salt in the pores limit CO₂ penetration, with this effect intensifying at higher artificial seawater concentrations; (2) carbonation has a dual impact on Cl⁻ erosion: in fully carbonated areas, carbonation products block pores and restrict Cl⁻ diffusion, while at the interface between carbonated and non-carbonated zones, carbonation depletes Ca(OH)₂, reducing Cl⁻ binding capacity, increasing free Cl⁻ content, and promoting Cl⁻ diffusion. Full article

Chloride ion concentration significantly impacts the durability of reinforced concrete, particularly regarding corrosion. Accurately assessing how this concentration varies with the age of structures is crucial for ensuring their safety and longevity. Recently, several predictive models have emerged to analyze chloride ion concentration over time, classified into empirical models and machine learning models based on their data processing techniques. Empirical models directly relate chloride ion concentration to the age of concrete through specific functions. Their primary advantage lies in their low data requirements, making them convenient for engineering use. However, these models often fail to account for multiple influencing factors, which can limit their accuracy. Conversely, machine learning models can handle various factors simultaneously, providing a more detailed understanding of how chloride concentration evolves. When adequately trained with sufficient experimental data, these models generally offer superior prediction accuracy compared to mathematical models. The downside is that they necessitate a larger dataset for training, which can complicate their practical application. Future research could focus on combining machine learning and empirical models, leveraging their respective strengths to achieve a more precise evaluation of chloride ion concentration in relation to structural age. Full article

To address the durability challenges faced by traditional concrete in marine environments, this study focuses on polyoxymethylene (POM) fiber-reinforced ultra-high-performance concrete (PFUHPC) and, for the first time, systematically investigates the inhibitory effects of POM fibers on microstructural degradation and mechanical performance deterioration of ultra-high-performance concrete under various erosive environments. The results indicated the following: (1) The mass loss rate and compressive strength degradation in PFUHPC under different erosive environments initially increased and then decreased, demonstrating that the inclusion of POM fibers delayed corrosion and significantly improved the durability and stability of the material’s performance. (2) Compared to the natural environment, after 180 days of immersion in different erosive environments (seawater immersion, wet–dry cycles in seawater, chloride salt immersion, sulfate salt immersion, and complex salt immersion), the compressive strength degradations were observed to be 4.8%, 9.7%, 6.8%, 11.7%, and 10.7%. (3) Microscopic analysis after 180 days revealed that the main corrosion products were gypsum, ettringite, and Friedel’s salt (calcium chloroaluminate). Under the environments of seawater immersion and cyclic wetting and drying, the low concentrations of chloride and sulfate ions resulted in fewer corrosion products and a denser matrix. The primary corrosion product under the chloride salt immersion was Friedel’s salt, which led to surface cracking and microporosity, while under the sulfate immersion, gypsum and ettringite were predominant, resulting in more porous and loosely bound hydration products and more severe corrosions. Full article

This paper aimed to evaluate two self-healing mechanisms of concrete exposed to chloride ions and carbon dioxide environments using chemical and bacterial solutions, contributing to understanding the real scenarios of concrete structures application. Expanded perlite (EP) impregnated with chemical and bacterial solutions with the aid of either a vacuum chamber or immersion was used in partial substitution of fine natural aggregate in ratios of 10%, 20%, and 30%. Samples were characterized by a compression strength test. Healing efficiency was evaluated with high precision in stereo zoom microscopy. Further characterization of the samples was obtained from SEM/EDS, and mineral content was determined from XRD. Samples impregnated with a chemical solution formed healing products identified as C-S-H, CaCO₃, and SiO₂ across and overflowing the fissure. Samples impregnated with the bacterial solution presented a maximum continuous healing region of 1.67 mm and an average of 0.514 mm. A comparison of submersed and wet curing yielded an equal number of results between the techniques. Overall, the products formed were mostly calcite (CaCO₃) and C-S-H, while the presence of CO₂ and Cl⁻ corrosives did not affect healing, with concentrations of 5% and 3%, respectively. Full article

Highly alkaline hydrogels are gaining increasing attention in building materials research. Specifically, cationic alkaline hydrogels based on diallyldimethylammonium hydroxide (DADMAOH) as the monomer have been effectively used to seal water-bearing cracks or serve as coupling media for electrochemical chloride extraction. However, the residual halogen content and challenges in scaling up monomer production have hindered broader application. Attempts to use a commercially available cation-selective membrane for ion exchange achieved up to 90% chloride-to-hydroxide switch, but the approach proved ineffective due to significant monomer decomposition during the process. By contrast, neutral gels and gel particles can be readily prepared from diallyldimethylammonium chloride (DADMAC) in large quantities and with a wide range of compositions. It is demonstrated here that these neutral gel particles undergo inverse static anion exchange when suspended in NaOH solution, generating DADMAOH particles with residual halide contents of <0.3%, without the need for ion-selective or dialysis membranes. This corresponds to an up to 100-fold reduction in residual chloride content compared to particles produced directly from alkaline monomer solutions, thereby significantly enhancing the efficiency of hydroxide ion release. The swelling behaviour of the particles is primarily influenced by the initial monomer concentration, while conductivity remains largely unaffected, indicating that charge transport occurs mainly along the particle surface. Despite the pronounced increase in swelling with decreasing particle radii, the specific conductivity of 2.8 Ω⁻¹ m⁻¹ is still sufficient for their use as coupling media in concrete applications. In summary, the alkaline particles prepared via inverse static anion exchange meet all necessary requirements for building materials applications, offering a broader range of tuneable properties and greater ease of production compared to gels or particles derived from DADMAOH. Full article