Development, Challenges, and Applications of Concrete Coating Technology: Exploring Paths to Enhance Durability and Standardization

Abstract

Concrete coating technology is a key measure that enhances the durability of concrete structures. This paper systematically studies the performance, applicability, and impact of different types of anti-corrosion coatings on concrete durability, focusing on their resistance to chloride ion penetration, freeze–thaw cycles, carbonation, and sulfate corrosion. The applicability of existing testing methods and standard systems is also evaluated. This study shows that surface-film-forming coatings can create a dense barrier, reducing chloride ion diffusion coefficients by more than 50%, making them suitable for humid and high-chloride environments. Pore-sealing coatings fill capillary pores, improving the concrete’s impermeability and making them ideal for highly corrosive environments. Penetrating hydrophobic coatings form a water-repellent layer, reducing water absorption by over 75%, which is particularly beneficial for coastal and underwater concrete structures. Additionally, composite coating technology is becoming a key approach to addressing multi-environment adaptability challenges. Experimental results have indicated that combining penetrating hydrophobic coatings with surface-film-forming coatings can enhance concrete’s resistance to chloride ion penetration while ensuring weather resistance and wear resistance. However, this study also reveals that there are several challenges in the standardization, engineering application, and long-term performance assessment of coating technology. The lack of globally unified testing standards leads to difficulties in comparing the results obtained from different test methods, affecting the practical application of these coatings in engineering. Moreover, construction quality control and long-term service performance monitoring remain weak points in their use in engineering applications. Some engineering case studies indicate that coating failures are often related to an insufficient coating thickness, improper interface treatment, or lack of maintenance. To further improve the effectiveness and long-term durability of coatings, future research should focus on the following aspects: (1) developing intelligent coating materials with self-healing, high-temperature resistance, and chemical corrosion resistance capabilities; (2) optimizing multilayer composite coating system designs to enhance the synergistic protective capabilities of different coatings; and (3) promoting the creation of global concrete coating testing standards and establishing adaptability testing methods for various environments. This study provides theoretical support for the optimization and standardization of concrete coating technology, contributing to the durability and long-term service safety of infrastructure.

1. Introduction

Concrete, as a core material in modern construction and infrastructure, is used globally at a rate of approximately 250 million tons per year [1], making it one of the most cost-effective and widely used building materials [2,3]. However, problems with corrosion and performance degradation because of intrinsic flaws in the material and the impact of harsh climatic variables have increased worries about the durability of concrete structures. Across the world, the degradation of concrete has caused large financial losses in recent years. It is estimated that corrosion-related losses account for approximately 2% to 4% of the global GDP, with marine corrosion losses contributing to about one-third of that total [4]. In China alone, the direct and indirect economic losses caused by concrete corrosion reach as high as USD 310 billion annually, which is approximately 3.34% of the country’s GDP [5]. Furthermore, more than 50% of Europe’s annual construction budget is allocated to the renovation of aging concrete structures [6], and a study conducted in the United States over two decades ago on the corrosion costs of road infrastructure is still applicable to Europe today [7]. The global lack of regular preventive maintenance for concrete has significantly reduced the expected service life of infrastructure from 100 years to 30–50 years [8]. This not only results in catastrophic accidents but also leads to massive economic losses.

The corrosion and durability deterioration of concrete are complex and prolonged processes [9]. Several factors, including the structural properties of concrete (such as porosity and construction quality), environmental factors (such as chloride ions, sulfate ions, carbonation, and freeze–thaw cycles), and steel reinforcement corrosion expansion, affect these processes [10]. Generally speaking, steel reinforcement in concrete forms a stable passivation film within its alkaline environment, effectively preventing corrosion-causing ions (such as chloride ions, sulfate ions, etc.) from corroding the reinforced concrete [11,12]. In addition, concrete exposed to soil environments also faces the risk of severe erosion. Corrosive ions such as sulfates and chlorides in the soil can penetrate the concrete’s surface and internal pores through moisture in the soil, further damaging the concrete’s structure. Acidic soil can reduce the alkalinity of the concrete surface, accelerating the breakdown of the passivation film on the reinforcement. Furthermore, frequent wet–dry cycles and freeze–thaw cycles in the soil can cause microcracks or even large-scale cracking on the concrete surface, thereby accelerating structural deterioration [13,14]. However, due to the inherent characteristics of concrete (such as its surface porosity and internal channels) and the fact that most reinforced concrete often sustains cracks during its actual use, as well as the fact that engineering structures like bridges, water projects, and harbors are often exposed to saline environments, corrosion-causing ions can diffuse through surface pores or cracks or react with water to form corrosive solutions, which further penetrate into the concrete and come into contact with the steel reinforcement [15]. This has a negative effect on reinforced concrete’s durability. Furthermore, freeze–thaw cycles cause pore water to expand upon freezing, creating an internal pressure that gradually expands microscopic fissures in the concrete and causes them to connect [16]. Carbonation lowers the pH level inside the concrete through ${\mathrm{CO}}_2$ diffusion, further destroying the protective film on the steel reinforcement [17]. These corrosion mechanisms often work together in real-world engineering environments, accelerating the degradation of concrete [10]. Therefore, the invasion of moisture and corrosive ion solutions is considered the primary cause of the shortened lifespan of concrete, and preventing this is crucial.

To address the above issues, scholars have proposed various technical measures to delay the corrosion of concrete structures. For example, using high-performance concrete [18] or incorporating mineral admixtures and corrosion inhibitors significantly improves the density and impermeability of concrete [19]. At the same time, the development of high-performance concrete materials (such as alkali-activated concrete and ultra-high-performance concrete) has provided new directions for improving the durability of concrete [20]. Research has shown that these new materials perform excellently in terms of their chloride-ion-binding capacity and resistance to corrosion [21]. However, these measures are mostly limited to the construction phase and often come with high costs. Moreover, their long-term performance still requires further verification, especially in extreme service environments [22]. Unlike traditional measures that improve the internal performance of materials, surface coating technology has gradually become a key focus of research and has been applied for concrete corrosion prevention due to its convenience, low cost, and wide applicability [23]. This technology forms a protective layer on the surface of the concrete, effectively preventing the intrusion of external media and thus protecting the internal steel reinforcement from corrosion [24]. However, due to the wide variety of coating materials available and their differing application conditions, their actual performance still requires further evaluation, particularly in terms of their long-term durability in complex environments [25]. Surface coatings form a physical barrier that can effectively prevent the penetration of chloride ions, moisture, and other corrosive media [26,27,28]. At the same time, new types of silicone-based penetrating coatings show superior protective effects in saline and humid environments due to their excellent water repellency, anti-carbonation, and resistance to chloride-ion diffusion [29,30]. Research shows that compared to traditional surface-film-forming coatings, hydrophobic impregnation can more evenly cover the concrete surface and penetrate into its pores, providing longer-lasting protection [31]. In existing transportation infrastructure, the application of anti-corrosion coatings is becoming more widespread, especially in projects such as bridges and port piers, where they show good anti-corrosion effects [32]. With the diversification of coating materials and continuous progress being made in modification technologies, new coating materials are gradually replacing traditional materials in new projects.

Although surface coating technology has shown great potential in protecting concrete structures, numerous challenges and research gaps remain. Currently, the suitability of coating materials for actual service environments is insufficient. Different service conditions demand varying durability requirements of coatings, yet most coating selections still rely on experience rather than systematic design principles and guidelines. Additionally, significant differences exist in coating durability assessment methods across countries, such as variations in testing approaches, performance requirements, and service life evaluation systems. This lack of standardization reduces the comparability of studies, hindering the widespread application of coatings in engineering projects.

Moreover, most existing research on coating durability remains at the laboratory stage, lacking the performance monitoring and feedback mechanisms seen under long-term service conditions. This limitation makes it difficult to provide effective guidance for future engineering applications. Therefore, optimizing coating materials to enhance their durability in complex service environments and establishing a unified evaluation system for coatings have become crucial research topics in concrete protection technology.

To address these issues, this study aims to systematically summarize the types and properties of concrete coatings and the durability testing methods used on them, as well as to analyze the similarities, differences, and limitations of the durability standards in various countries. Furthermore, this paper explores the mechanisms by which chloride ions, sulfate ions, freeze–thaw cycles, and carbonation affect concrete durability and evaluates the effectiveness of coatings in different corrosive environments. Additionally, an optimized design strategy for concrete coatings and a durability assessment framework are proposed to provide a scientific basis for the engineering application of coating technology.

2. Coatings and Their Durability Performance

2.1. Classification and Characteristics of Coatings

Due to the complex and diverse scenarios under which concrete corrosion occurs, there are numerous factors that influence the degree of corrosion experienced. This has led to a wide variety of coatings being developed to protect concrete from harsh service environments and ensure its service life. Typically, coating protections are classified into three main types based on the environment in which they are used and their mechanism of action [33]: surface-film-forming coatings, pore-blocking agents, and hydrophobic impregnation, as shown in Figure 1. These differ as follows:

(a) The surface-film-forming mechanism primarily relies on polymer materials (such as fluorocarbon coatings and polyurethane coatings) to form a continuous and dense film on the concrete’s surface, creating a strong physical barrier. This barrier effectively blocks the penetration of moisture, chloride ions, sulfates, and other corrosive agents, thereby reducing the direct environmental erosion of the concrete.

(b) The pore-sealing mechanism functions through active components (such as silicates and polymers), which penetrate the capillary pores in the concrete surface and react in situ with cement hydration products (such as calcium hydroxide) to form insoluble solid compounds. These compounds fill and seal pores and microcracks, reducing the surface porosity of concrete, enhancing its impermeability, and improving its freeze–thaw resistance to some extent. Compared to surface-film-forming coatings, this mechanism retains a certain degree of breathability, allowing water vapor to diffuse rather than accumulate internally, thereby minimizing the potential damage caused by trapped moisture.

(c) The penetrating hydrophobic mechanism operates by making use of the penetration of silanes, siloxanes, or other organosilicon materials to form a hydrophobic molecular layer along the inner walls of concrete pores. This hydrophobic layer significantly reduces the water absorption capacity of concrete, thereby limiting moisture and chloride ion infiltration. Even under hydrostatic pressure, the water permeability of the concrete decreases significantly. However, unlike the surface-film-forming and pore-sealing mechanisms, this approach does not seal the pores but maintains the natural breathability of the concrete, allowing water vapor to diffuse freely and preventing internal moisture buildup that could damage the structure [34].

Within the large variety of coating materials available, each possesses unique performance advantages, which means their applicability and corrosion resistance effectiveness vary depending on the differences in their material properties. This presents a significant challenge when selecting coatings, as it is crucial to take into account both the coating’s flexibility and the unique corrosion properties of the concrete’s environment, ensuring that the correct coating material is selected to achieve the best protective effect. On the contrary, incorrectly choosing coating materials—especially using coatings that are unsuitable in particular environmental conditions—can lead to insufficient coating performance, thus failing to effectively prevent the penetration of external corrosive agents and resulting in poor protective effects. Furthermore, the coating material may gradually develop intrinsic problems like aging, peeling, and cracking, which further impair its protective properties and ultimately drastically reduce the concrete structure’s durability and service life. Therefore, coating design is a highly technical and experience-dependent task that requires rational selection based on the actual usage environment and material properties of the concrete. In engineering practice, the long-term monitoring of coating performance can help build up relevant experience, providing scientific guidance for determining the appropriate coating types for specific environments.

2.1.1. Surface-Film-Forming Coating

Surface-film-forming coatings create a continuous and dense protective film on the concrete surface through their curing reaction [35]. This film acts as a physical barrier, effectively blocking harmful external substances such as moisture, chloride ions, and pollutants from penetrating the concrete, thereby achieving corrosion protection. The coating not only reduces the intrusion of corrosive agents into both the concrete and its reinforcement but also lowers the concrete’s water absorption, preventing carbonation and chloride ion penetration and ultimately delaying reinforcement corrosion.

(1) Traditional Film-Forming Coatings

There are various types of surface-film-forming coatings, including acrylic, polyurethane, and epoxy resin coatings, the specific properties of which are shown in

Table 1. Properties of surface-film-forming coatings.

Coating Material	Advantages	Disadvantages
Epoxy Coating	Excellent adhesion, outstanding waterproofing capability, good corrosion resistance [45]	Low weather resistance, poor UV resistance, prone to aging [45]
Acrylic Coating	Good weather resistance, high UV resistance, strong adhesion, high mechanical performance [39]	Poor water resistance, low chemical corrosion resistance [39]
Polyurethane Coating	Good weather resistance, relatively high corrosion resistance, good permeability resistance, long service life, high impact resistance [28]	Poor deformation resistance, low UV aging resistance, reduced weather resistance in highly corrosive environments [46]

. Epoxy resin coatings have excellent adhesion and waterproofing capabilities [36] but, due to the high hardness caused by their benzene ring structure, they have poor weather resistance. As a result, they are typically used as base or intermediate layers in multilayer coating systems [37]. Acrylic coatings create good weather resistance and UV resistance [38], but they have poor water resistance and chemical corrosion resistance capabilities [39], so they are generally used as top layers in low-corrosion environments. Polyurethane coatings have good flexibility and corrosion resistance, and their molecular structure can be adjusted to meet different environmental requirements [40]. They also have good impact resistance and are used as top layers that help concrete resist external impacts. In addition to the three main types of organic coatings commonly used, other less common organic coatings include fluorocarbon resins [41,42], chlorinated rubber [43], and polyurea coatings [44], which are generally used as top layers and will not be discussed in detail here.

(2) New-Generation Modified Coatings

Single-component coatings typically focus on certain performance advantages, thus also exhibiting significant shortcomings. Currently, nano-material modification is a common method used for enhancing coating performance, with nano-${\mathrm{SiO}}_2$ and nano-${\mathrm{TiO}}_2$ being among the most widely used materials. These nanoparticles improve the density of organic coatings, reduce coating defects, and significantly enhance the strength, hydrophobicity, UV aging resistance, and chloride-ion-penetration resistance of modified coatings. However, experimental studies indicate that the beneficial effects of nano-materials on coating performance vary. For instance, their enhancement of carbonation resistance in coated concrete is more pronounced than their improvements in chloride-ion-penetration resistance. Additionally, nano-${\mathrm{SiO}}_2$ and nano-${\mathrm{TiO}}_2$ exhibit different effects on carbonation resistance, aging resistance, and chloride-ion-penetration resistance. Research has shown that incorporating nano-${\mathrm{TiO}}_2$ into polyurethane coatings yields better modification effects than nano-${\mathrm{SiO}}_2$. In contrast, for acrylic coatings, a nano-${\mathrm{TiO}}_2$ modification is more beneficial for enhancing waterproofing and chloride-ion-penetration resistance. As for other types of coating modifications, further experimental research is required to determine the optimal nano-material [47,48,49,50,51].

In addition to nanoparticle modification, other material modification techniques have also been explored. For instance, Huang et al. discovered that the mechanical qualities of coatings were improved by the addition of graphene oxide nanosheets [47]. Zheng et al. modified epoxy resin with graphene oxide, causing the coating’s hydrophobic angle to rise by 18.6° [52]. Kong et al. prepared an epoxy coal tar pitch coating (ECTPC) by modifying epoxy resin with coal tar pitch. This coating demonstrated excellent anti-corrosion performance in urban sewage environments, exhibiting good water impermeability, a resistance to water flow erosion, and antibacterial properties when used on concrete [53]. While these modified coatings have demonstrated promising protective performances in laboratory tests, they still lack sufficient results from real-world applications and require further research and validation.

Surface-film-forming coatings, by forming a physical barrier, can effectively prevent harmful substances from penetrating concrete. They are widely used for the corrosion protection of concrete structures. Each of the conventional coatings—polyurethane, acrylic, and epoxy resin—has advantages and disadvantages, making them suitable for different application scenarios. With the development of modification technologies, the addition of nanoparticles and other modifiers has significantly enhanced the overall performance of these coatings. However, their actual effectiveness in practical applications still requires validation through further engineering case studies and feedback.

2.1.2. Pore-Blocking Agents

Pore-blocking agents, through their permeability, allow a material or some of its active substances to penetrate into the pores of the concrete, where in situ reactions occur [35]. This procedure either fully or partially fills the concrete’s surface capillary pores, thereby protecting it. Compared to surface-film-forming coatings, pore-blocking agents “anchor” themselves inside the pores, effectively addressing the issue of coating peeling and flaking. Additionally, their penetration depth of several millimeters, combined with a surface protective layer, results in a total thickness far greater than that of surface-film-forming coatings.

Currently, the commonly used pore-sealing coatings are mainly water-based penetrating inorganic waterproof coatings and cement-based penetrating crystalline coatings [54], the specific properties of which are shown in

Table 2. Properties of pore-blocking agents.

Coating Material	Advantages	Disadvantages
Water-based penetrating inorganic waterproof coating	Enhances concrete surface density, improves mechanical properties [55]	Requires a long curing time, limited crack-sealing ability, loses waterproofing effectiveness when damaged [58]
Cement-based penetrating crystalline coating	Possesses self-healing ability, capable of repairing damaged concrete surfaces [56]	Cannot repair wide cracks, poor adhesion, ineffective crack repair in the absence of water [57]

. Water-based penetrating inorganic waterproof coatings utilize silicates and other materials to form insoluble silicates within concrete pores, effectively sealing them [55]. Cement-based penetrating crystalline coatings are rigid inorganic coatings composed of cement, sand, and an appropriate amount of active chemical substances [56]. These coatings contain active chemicals that, when exposed to water, can be repeatedly activated, catalyzing the ${\mathrm{Ca}}^{2+}$ in the concrete to form insoluble crystalline materials that fill surface pores. Since this mechanism of action relies on reactions with the concrete matrix to seal surface pores, these coatings are only suitable for use as base layers [57].

The research on the modification of pore-blocking agents is relatively limited, with only a few scholars working in this field and typically using nano-materials to enhance these agents’ performance. For example, Li et al. studied the modification of a water-based waterproof coating made of polymer emulsion, slag powder, and water glass [59]. Sun et al. created composite materials by combining cement-based crystalline penetration coatings with modified nano-silica [60]. By grafting hydrophobic alkyl groups onto the nano-silica’s surface, the composite coatings’ permeability resistance was greatly increased.

In addition to the application of crystalline penetration materials in coatings, studies have shown that these materials also have potential use in the smart monitoring of concrete. Liang et al. added CCCW materials to cement-based crystalline penetration materials and demonstrated the feasibility of using these coatings to monitor strain and cracks on the surface of concrete structures, providing a new technological means for structural health monitoring [61].

Cement-based crystalline penetration coatings exhibit excellent strength and environmental performance, but currently there is a lack of a unified environmental evaluation method or unified criteria. Environmental evaluation methods and standards for coatings urgently need to be improved in order for these to be used as a foundation for coating selection. The development of pore-blocking agents that use non-toxic, non-polluting reactive solvents or biotechnology to produce carbonates through bacterial precipitation will be a promising direction for future research.

2.1.3. Hydrophobic Impregnation

Hydrophobic impregnation coatings are primarily based on organic silicon materials, which typically have the molecular structures shown in Figure 2. The interaction between silane (RO-Si-OR) and the concrete surface involves three key stages. First, hydrolysis occurs when water is present, converting silane into silanol (HO-Si-OH) with hydroxyl (-OH) groups. Next, through condensation (cross-linking), silanol molecules react to form a stable cross-linked siloxane (Si-O-Si) structure. Finally, these hydroxyl-containing siloxane molecules chemically bond with the hydroxyl groups (-OH) on the concrete’s surface, creating a dense protective layer. This layer significantly enhances the water resistance, durability, and chloride-ion-penetration resistance of the concrete, improving its long-term performance. One of the most common types of materials used in this category is organic silicon penetrating protective agents, including butyl silane, octyl silane, isobutene silane, pastes, and silane oligomers. Silanes and siloxanes exhibit excellent permeability, allowing them to form hydrophobic molecular layers on the surface and inner walls of concrete pores [62]. This effectively reduces the water absorption or permeability of concrete under hydrostatic pressure, thereby preventing the infiltration of chloride ions [34] while maintaining the breathability and natural appearance of the concrete pores, thus avoiding the coating damage caused by gas accumulation. However, these coatings have poor carbonation resistance and are not effective at blocking the penetration of ${\mathrm{CO}}_2$ [63]. Consequently, hydrophobic impregnation coatings are commonly used for corrosion protection in coastal concrete structures, where they are expected to enhance permeability resistance under high-water-pressure and low-porosity conditions [64]. Additionally, when applied to concrete surfaces with low surface porosity, the penetration of organic silicon materials is hindered, leading to reduced corrosion protection [64].

Studies have indicated that the performance of silane-based composite coatings is considerably improved by the incorporation of nanoparticles. Boutamart et al. prepared coatings using polymerized silica and polydimethylsiloxane and demonstrated the coatings’ exceptional resistance to UV degradation and abrasion [65]. Ibrahim et al. incorporated nano-calcium carbonate and nano-clay particles into silane-based hydrophobic coatings, significantly improving the durability of the coatings during sulfate immersion [66]. Their mass loss was reduced by 76%, the angle of water contact increased from 114° to 145°, and their resistance to the penetration of chloride ions improved by 85%. Gu et al. developed a multilayer superhydrophobic coating (MLS), which, by altering isobutyl triethoxysilane with nano-silica, improved the durability of concrete in corrosive settings, particularly in terms of water and chloride ion penetration [67]. Sakr et al. demonstrated how concrete’s endurance in a salt-freezing environment might be effectively increased by nano-silica alterations. There was no mass loss in the coated concrete after 50 salt-freezing cycles [68].

Currently, penetrating hydrophobic coatings, which are primarily made of chlorideorganic silicon, efficiently lower concrete’s permeability and water absorption, preventing the ingress of chloride ions. The performance of these coatings has been greatly enhanced by the addition of nanoparticles, and especially the usage of modified materials like clay, calcium carbonate, and nano-silica, which improve the coatings’ durability and resistance to UV rays and chloride ion penetration. However, the carbonation resistance of these coatings still requires further research and improvement in order to enhance their performance in practical settings. Penetrating hydrophobic coatings are widely used for concrete corrosion protection in infrastructure such as bridges in coastal areas, and there have been attempts to use them as a protective base layer in coastal concrete structures. Future research could focus on improving the permeability resistance of organic silicon materials in high-pressure environments and low-porosity concrete, which could improve the application prospects of these coatings.

2.2. Durability of Coatings

Surface coatings are directly exposed to external environments and during their service life they can be harmed or lose their durability as a result of, e.g., physical abrasion, wet–dry cycles, freeze–thaw cycles, and UV deterioration. This degradation can affect their ability to protect the concrete from corrosion [69]. Therefore, it is essential to establish unified durability criteria and testing methods for coating materials. Various national standards and specifications have provided evaluation criteria for assessing the durability of coatings. As shown in

Table 3. Design standards for durability tests of coatings in different countries.

Performance Requirements	ASTM Standard	AASHTO Standard	ISO Standard	PN-EN Standard	JT Standard
Appearance	ASTM D610	AASHTO T327	ISO 4628	PN-EN 1338	JTJ 275
Adhesion strength	ASTM C1583	AASHTO T154	ISO 4624	PN-EN 1542	JTG/T 3310
Freeze–thaw resistance	ASTM C666	AASHTO T161	ISO 16546	PN-EN 12390-9	JTG/T 3310
Abrasion resistance	ASTM C779	AASHTO T96	ISO 5470-1	PN-EN 1338	JTG/T 3310
Water absorption	ASTM C642	AASHTO T258	ISO 7783	PN-EN 1062-3	JT/T 695
Chemical resistance	ASTM D1308	AASHTO T259	ISO 2812-1	PN-EN 13529	JTG/T 3310
UV aging resistance	ASTM G154	AASHTO T237	ISO 4892-3	PN-EN 927-6	JTG/T 3310
Chloride-ion-penetration resistance	ASTM C1202	AASHTO T277	ISO 1920-11	PN-EN 12390-11	JTS 153

, these requirements comprise eight indicators: tensile characteristics, chemical resistance, water absorption, adhesion, appearance, abrasion resistance, freeze–thaw resistance, UV aging resistance, and chloride-ion-penetration resistance. This table compiles the durability requirements and testing methods specified in the regulations of different countries.

The assessment of the performance of concrete coatings involves the ASTM, AASHTO, PN-EN, ISO, and JT standards. These standards provide clear specifications for key aspects of the coating’s performance, such as its appearance, adhesion, freeze–thaw resistance, abrasion resistance, water absorption, chemical resistance, UV aging resistance, and chloride-ion-penetration resistance. The primary testing methods used include pull-off adhesion tests, freeze–thaw cycle tests, electrical flux methods, and accelerated aging tests. A detailed comparison of specific performance indicators is presented in

Table 4. Comparative analysis of coating durability performance evaluation indicators from different standards.

Performance Requirements	ASTM Standard	AASHTO Standard	ISO Standard	PN-EN Standard	JT Standard
Appearance	After 500–1000 h salt spray test, only slight rust stains and blistering allowed.	After 2500 h UV test, no significant blistering, chalking, or adhesion loss.	After 1680 h comprehensive aging, only minor defects allowed.	After artificial climate aging, only slight discoloration allowed, with no significant defects.	After 1000–5000 h of artificial aging, no blistering, peeling, or chalking, with gloss retention ≥80%.
Adhesion Strength	Cross-cut test rating 4B or 5B and pull-off strength of concrete ≥1.5 MPa, with no significant decline after aging.	After 300 freeze–thaw cycles, adhesion strength ≥1.5 MPa, which remains stable after aging.	Initial adhesion ≥2.5 MPa (≥5 MPa for marine environments), with no significant decline after aging.	Pull-off strength ≥0.8–1.5 MPa, which remains stable after freeze–thaw aging.	Initial and aged adhesion ≥3 MPa; failure mode should be concrete substrate failure.
Freeze–Thaw Resistance	After ASTM C666 freeze–thaw cycles, no cracking or peeling, adhesion remains stable.	After 300 water freeze–thaw cycles, no significant damage, adhesion remains stable.	No cracking or adhesion loss after comprehensive aging (including freeze–thaw).	After 50 freeze–thaw saltwater cycles, no peeling and adhesion ≥1.5 MPa.	Based on concrete freeze–thaw test, coating must show no cracking, peeling, or degradation.
Abrasion Resistance	Taber abrasion test (1000 cycles): wear loss of less than 100 mg.	Road surface anti-skid coatings require low wear loss.	Taber abrasion test conducted as needed, with typical wear loss of 1000 mg.	H22 Taber wheel wear loss ≤3000 mg, with stricter standard for traffic zones.	Taber 1000-cycle wear test with strict limits, especially for bridge decks and driving lanes.
Water Absorption	After 24–48 h immersion, no significant changes, low water absorption.	Measures vapor permeability and immersion effects, coating blocks water but remains breathable.	Low water absorption, ≤$0.1\mathrm{kg}/({\mathrm{m}}^2·{\mathrm{h}}^{0.5})$, and moderate breathability.	Water absorption coefficient ≤$0.1\mathrm{kg}/({\mathrm{m}}^2·{\mathrm{h}}^{0.5})$ and moderate breathability.	After 24 h immersion, a lack of visible changes was qualitatively evaluated as low absorption.
Chemical Resistance	24–72 h immersion in chemical reagents with no significant changes in coating appearance.	Emphasis on resistance to deicing salts and alkaline environments, with no changes after long-term exposure.	No significant changes after 72 h immersion in oil, solvents, acids, or alkalis.	After long-term chemical immersion, performance loss ≤20%.	Resistant to alkalis (720 h) and acids (240 h) with no significant changes, and adhesion remains stable.
UV Aging Resistance	After 500–1000 h of UV aging, slight chalking and color change, with gloss retention ≥80%.	After 2500 h UV test, slight chalking and cracking, with gloss retention ≥70%.	After 480–720 h UV aging, no significant chalking or cracks, minimal color change.	After 2000 h xenon lamp aging, low chalking grade, minimal color change.	After 1000–5000 h of UV aging, coating remains intact, with gloss retention ≥80%.
Chloride-ion-penetration resistance	Rapid chloride ion test shows minimal charge transfer, with no significant chloride ingress after 90-day immersion.	After 90-day chloride solution column test, chloride content remains extremely low at rebar depth.	Indirectly assessed via salt spray test, no specific method.	Controlled through low absorption and high adhesion, indirectly limiting chloride ion penetration.	Chloride ion penetration rate strictly controlled (≤$1.5\times {10}^{-4}\mathrm{kg}/({\mathrm{cm}}^2·\mathrm{d})$).

.

When evaluating the accuracy of different standards in assessing coating durability, it is important to remember that each standard has its own characteristics.

• The ASTM (American Standard) focuses on individual tests such as salt spray, UV aging, and freeze–thaw tests, with clear but relatively independent indicators, resulting in a weaker comprehensive evaluation.
• The AASHTO (North American Transportation Standard) includes specific testing requirements for UV aging, freeze–thaw cycles, and chloride salt environments, making it particularly suitable for assessing the outdoor long-term exposure of coatings in cold regions, with strong practical applicability.
• The ISO (International Standard) uses comprehensive aging tests that cover a wide range of conditions, but some tests lack specific individual indicators, making them somewhat theoretical.
• The EN (European Standard) is highly comprehensive and strict, especially in evaluating freeze–thaw resistance, artificial climate aging, and adhesion stability, with clear and detailed requirements, making its application highly practical.
• The JT (Chinese Transportation Industry Standard) has the strictest comprehensive evaluation criteria, explicitly specifying results for UV aging (up to 5000 h), adhesion (≥3 MPa, requiring concrete failure), freeze–thaw resistance, abrasion resistance, and the chloride ion penetration rate. Notably, the chloride-ion-penetration and UV aging performance of coatings are clearly and strictly defined in this standard.

Therefore, in terms of overall comprehensiveness, indicator clarity, and strictness, the JT standards are the most accurate in assessing the actual durability performance of coatings, followed by the EN standards. AASHTO, ISO, and ASTM rank lower by comparison and are more suitable for basic or supplementary evaluations.

While the testing methods used across standards are similar, their specific requirements and evaluation criteria differ. As infrastructure construction and environmental protection standards become more stringent, coating performance standards will focus more on durability and environmental adaptability, potentially leading to the development of more intelligent evaluation technologies. Moreover, the coordination and harmonization of standards will become increasingly important to meet the growing demand for consistency in international engineering projects.

2.3. Coating Design

The design of multilayer coatings aims to overcome the disadvantages single-layer coatings have while preventing failures caused by scratches or wear. By layering multiple coatings, each layer can complement the shortcomings of the others, thereby enhancing the overall performance and durability of the coating system. In order to guarantee materials’ long-term durability and protection, coating design is essential. This typically involves determining the number of layers required, selecting the types and thicknesses of the coatings used, assigning functions to each layer, and choosing the appropriate construction process.

The number of layers directly influences the protective effect of the coatings and should be selected based on the corrosion environment of the concrete structure. The cost of these projects must be considered in terms of their required performance criteria. The coating types must be chosen according to the strength of the corrosion environment, and the appropriate thickness should be determined based on the type of coating used. The arrangement of layers should consider the characteristics of each layer fully. For example, the base layer should focus on adhesion and anti-corrosion performance, the middle layer should improve impermeability, and the top layer should provide weather resistance and UV protection. In order to guarantee the long-term stability and superior protective performance of the finished coating system, the correct construction process is essential. The application technique, thickness control, and curing time must all be carried out in accordance with the unique requirements of each layer.

In Chinese standards, different coating systems are recommended based on the intended usage scenario, including clear recommendations for the types of coating materials that should be used and the thicknesses of each layer. The coating’s design should be based on technical indicators and recommended coating systems for different application environments, as outlined in standards such as JT/T 695-2007 Appendix A and JTG/T 3310-2019. JTG/T 3310-2019 suggests referring to the concrete coating systems specified in JTS 153-2015, which recommends a three-layer system of epoxy sealing paint (base layer) + epoxy resin paint (middle layer) + polyurethane paint or acrylic resin paint or fluorocarbon resin paint or chlorinated rubber paint (any one of these options could form the top layer). The following two-layer system is recommended: acrylic resin sealing paint (base layer) + acrylic resin paint or chlorinated rubber paint (either of these options could form the top layer).

In the ASTM and AASHTO standards, there are no clear recommendations for multilayer coating systems for concrete like those seen in JT/T 695-2007 or JTG/T 3310-2019. For example, ASTM D412, D714, and D1654 test adhesion, blistering, and corrosion resistance, respectively. These standards are more focused on evaluating the durability and protective capabilities of coatings but do not specify the composition of multilayer coating systems. AASHTO M300 and R31 mainly target inorganic zinc-rich primers used for steel, rather than specific coating systems for concrete.

Multilayer coating design enhances the protective performance and durability of coatings by overcoming the limitations of single-layer coatings. In addition to increasing concrete’s impermeability and resistance to corrosion, a well-designed coating system successfully guards against coating failures brought on by wear and scratches. Currently, only Chinese standards provide clear recommendations for multilayer coating systems and their thicknesses, while international standards such as ASTM and AASHTO primarily focus on durability assessments. Therefore, related standards need to be further improved.

In addition, aside from in the transportation sector, Chinese standards have yet to provide specific coating system recommendations for different application scenarios in other industries. There is an urgent need to establish a comprehensive set of concrete coating technical standards for various industries to guide design and construction units.

In the future, as demands for infrastructure construction and environmental protection increase, the performance standards for concrete coatings will continue to become more stringent, particularly in terms of their durability and environmental adaptability. More intelligent coating performance evaluation methods may emerge that utilized advanced smart detection technologies to monitor changes in concrete performance in real time. Furthermore, environmental sustainability and green materials will become key focuses of revisions to existing standards. New standards may pay greater attention to reducing carbon footprints and improving material recycling. Additionally, as globalization progresses, the coordination and harmonization of standards will become increasingly important in enabling consistency in international engineering projects.

3. Effects of Coatings on Concrete Durability

Durability indicators are typically used to evaluate the improvement in the corrosion resistance of reinforced concrete after the application of a coating. Existing standards provide specific regulations on the required durability performance of concrete, and their corresponding evaluation indicators are presented in

Table 5. Concrete durability standards.

Durability Indicator	ACI (318/201.2R)	EN 206 / PN-EN 206	GB/T 50476-2019	CEB-FIB Model Code
Hydrophobicity	Encourages the use of hydrophobic agents and coatings to improve surface performance; no specific test methods or limits specified.	Recommends surface protection agents, allowing reduction in coating thickness; no specific test requirements or limits.	Suggests coatings or hydrophobic agents to improve resistance to chloride ingress and carbonation; no mandatory test limits.	Recommends surface protection measures; no mandatory test limits specified.
Water Absorption (Absorption Rate)	Recommends reducing absorption; no mandatory test, commonly references ASTM C1585.	Recommends controlling absorption rate; no universal limit, but national standards often require ≤5%–6%.	Encourages absorption control; common full-water absorption limit ≤5%.	Suggests controlling water absorption to reduce erosion risks; no standardized limit specified.
Chloride Ion Corrosion Resistance	No mandatory test, recommends RCPT (ASTM C1202), with 28-day charge typically passing ≤1000–2000 coulombs.	No unified limit but recommends rapid chloride migration (RCM) test; some national annexes set D ≤ 1–$5\times {10}^{-12}$ m2/s limits.	Specifies chloride ion diffusion coefficient limits, with D ≤ 1–$5\times {10}^{-12}$ m2/s for marine structures.	Recommends service life prediction using a chloride ion diffusion model, with migration coefficient D ≤ 1–$5\times {10}^{-12}$ m2s.
Freeze–Thaw (Salt Freeze) Resistance	Mandatory air entrainment, recommends ASTM C666 freeze–thaw test, with ≥60% relative dynamic modulus required after testing; mass loss 5% in severe conditions.	Mandatory air entrainment or verification via freeze–thaw tests (e.g., CDF method); mass loss recommended to be 3%–5%.	Specifies freeze–thaw resistance grades (e.g., F50, F100); for salt freeze conditions, mass of loss 5% and ≥60%–80% relative dynamic modulus are required.	Suggests air entrainment and anti-freeze admixtures, recommends freeze–thaw mass loss of 3%–5% and dynamic modulus of ≥75%.
Carbonation Resistance	No direct limits specified, recommends accelerated carbonation tests (e.g., phenolphthalein indicator) to verify coating effectiveness.	No explicit carbonation depth limits, recommends accelerated carbonation models; coating must exceed predicted carbonation depth.	Specifies accelerated carbonation test (20 °C, 70% RH, 5% ${\mathrm{CO}}_2$), with 28-day carbonation depth typically ≤5 mm.	Recommends carbonation modeling for prediction, ensuring carbonation depth does not exceed coating thickness.
Sulfate Corrosion Resistance	Mandatory sulfate-resistant cement (Type V), recommends ASTM C1012 expansion test: ≤0.10% at 180 days.	Mandatory use of sulfate-resistant cement in XA environments; recommends sulfate expansion test (similar to C1012) ≤0.10%.	Suggests sulfate-resistant cement + Mineral admixture; specifies sulfate expansion test requirements (≤0.10%–0.15% at 180 days).	Strongly recommends sulfate-resistant cement, recommends sulfate expansion of ≤0.10%–0.15%.

.

However, traditional durability testing methods have primarily focused on the concrete itself, with no dedicated assessments of the durability improvements seen after the application of a coating. The introduction of coatings has led to the development of emerging testing technologies which can effectively evaluate the enhancement of coated concrete’s durability, accurately predict coating degradation, and assess the service life of coated concrete structures.

Building upon this review of concrete durability indicators and standard requirements, this paper highlights advanced durability testing technologies that have emerged in the field of coated concrete over the past decade. The aim is to provide a comprehensive technical reference for enhancing the corrosion resistance of coated concrete.

3.1. Waterproof Performance

In harbors, concrete structures are often submerged in seawater for extended periods of time. Usually, water transport and dispersion allow harmful particles to enter the concrete’s core. Consequently, one of the most important factors in assessing the longevity of concrete is its waterproof performance. Two indications are typically used to evaluate the waterproof performance of coated concrete: the coating’s hydrophobicity and the concrete’s ability to absorb water both before and after the coating is applied.

3.1.1. Hydrophobicity

The hydrophobic performance of a coating is typically assessed using the water contact angle. The contact angle ($\theta$) reflects whether the coating is hydrophilic or hydrophobic by providing a quantitative measure of a liquid’s wettability on a solid surface. A water contact angle $\theta =90°$ is considered the critical point: materials are regarded as hydrophilic when $\theta \le 90°$ and as hydrophobic when $\theta >90°$. Research has shown that the larger the water contact angle, the stronger the hydrophobic performance of the coating, with a positive correlation between the two [47]. Currently, international standards such as ISO 19403-2:2017 and ASTM D7334 utilize the water contact angle to evaluate the hydrophobic performance of coatings. However, in China’s existing regulations, there are no explicit standards for water contact angles, and related guidelines require further improvement.

The specific method for performing a water contact angle test involves using specimens with dimensions of 100 mm × 100 mm ×100 mm. Following a curing period of 28 days, the specimens are dried for 48 h in an oven set to 40 °C. Following this, they are covered with an appropriate coating and left to stand for seven days. Subsequently, distilled water is randomly dropped onto five points on the specimen’s surface. The droplet shape parameters are measured using an electron microscope, and the water contact angle is calculated using Equation (1) [70]. This testing process is illustrated in Figure 3.

$$\theta =2arctan\left({\displaystyle \frac{2h}{d}}\right)$$

(1)

where

• $\theta$ represents the water contact angle.
• h is the distance from the contact surface to the top of the water droplet.
• d is the diameter of the circular contact area between the water droplet and the concrete interface.

Although current Chinese standards in the transportation and water conservancy industries do not explicitly use the water contact angle as an evaluation indicator, it has been proven to be a critical reference for determining the type of modified materials obtained and appropriate mixing ratios during the selection of coating materials. Therefore, it is recommended that the water contact angle be incorporated into relevant standards for hydraulic and harbor engineering. This inclusion would enable a more scientific evaluation of the hydrophobic performance of coatings, providing stronger evidence for optimizing coatings and their practical applications.

3.1.2. Water Absorption

For corrosion damage to occur in reinforced concrete structures, there must be voids and fractures present. The quantity of these defects is often indirectly represented by the water absorption rate, making this one of the indicators used for evaluating the corrosion resistance of concrete. Currently, standards for water absorption testing include China’s Standard for Test Methods of Basic Properties of Building Mortar (JGJ/T 70-2009), the Method for Determination of Water Absorption of Paint Films (HG/T 3344-2012), and the Normal Protocol 7/81–Water Absorption Test.

Full-immersion tests and capillary absorption tests are the two main categories of tests used to determine the water absorption rate of concrete specimens in these standards. The full-immersion test involves completely submerging the specimen in water, with the aim of measuring the water absorption performance of the entire concrete specimen under minimal or no water pressure. This method is suitable for simulating the water absorption conditions of structures that are submerged for extended periods of time, with the evaluation index being the water absorption rate.

On the other hand, the capillary absorption test determines how well concrete absorbs water under continuous water pressure by submerging one side of the specimen in water. The capillary water absorption rate is the assessment index used in this technique, which is appropriate for simulating the water absorption conditions of structures exposed to tidal flushing. The water absorption rate is calculated using Equation (2).

$$W_m={\displaystyle \frac{m_1-m_0}{m_0}}\times 100\%$$

(2)

where

• $W_m$ represents the water absorption rate (%).
• $m_1$ is the mass of the specimen after a certain period of water absorption.
• $m_0$ is the initial mass of the specimen.

In addition to differences in testing methods and evaluation indicators, the duration of different water absorption tests also varies. A statistical review of the existing literature on water absorption tests for concrete reveals that most tests adopt immersion durations within the ranges of 10, 20, and 40 min or 1, 2, 4, 8, 24, and 48 h [72]. However, some studies report much longer immersion durations, such as 168 h [73], or even up to 650 h [48].

The inconsistencies in the duration of the tests used in different standards and studies prevent engineers and researchers from adopting a unified method to objectively evaluate cracks and internal voids in concrete structures. This lack of standardization reduces the efficiency and comparability of this indicator when used for assessing the compactness of different concretes.

It is worth noting that statistical results from extensive research show that changes in the water absorption rate are most pronounced within the first 48 h of immersion [48,72,73]. Beyond 48 h, the increase in the water absorption rate becomes slower. Considering factors such as the test workload and time efficiency, 48 h could be proposed as a standardized immersion period for water absorption tests.

The statistical results of the water absorption reduction rate of 10 types of coated concrete examined in different studies are summarized in Figure 4. The findings indicate that surface-film-forming coatings and hydrophobic impregnation achieve water absorption reduction rates exceeding 75%, demonstrating good waterproof performances. In contrast, pore-blocking agents exhibit water absorption reduction rates of approximately 20%, indicating poorer waterproof performances.

Further investigation into the connection between the water absorption rate and chloride-ion-corrosion performance will be required for concrete buildings that incorporate new materials in the future. For example, in the case of recycled aggregate concrete, studies should focus on the migration patterns of moisture within the concrete. Furthermore, studies examining the impact of varying immersion times on variations in the water absorption rate of concrete should be validated from a microscopic standpoint.

3.2. Chloride-Ion-Corrosion Resistance

3.2.1. Mechanism of Chloride Ion Corrosion

The chloride ion corrosion of reinforced concrete is a complex and long-lasting process. Concrete is a typical brittle material with a fracture energy of approximately 20 J/m² that is characterized by porosity and crack-prone behavior [33], which facilitates the intrusion of chloride ions, abundantly present in the environment.

Typically, chloride ions penetrate reinforced concrete via various transport mechanisms, such as diffusion [75], permeation [76], capillary adsorption [77], and ionic transport [78]. This process is prolonged, and the migration mechanisms involved are complex, with specific transport modes depending on the characteristics of the water environment surrounding the concrete. The methods by which chloride ions migrate in reinforced concrete structures exposed to various environmental conditions are listed in

Table 6. Chloride transport mechanism in structures under various exposure conditions [20].

Chloride Ion Contact Method	Location and Environment	Chloride Ion Migration Mechanism
Water Soaking	Lower structures during low tide	Diffusion
	Basement exterior walls or underwater tunnel linings during low tide	Permeation, Diffusion
Tidal Action	Lower and upper structures in tidal zones	Capillary Absorption and Diffusion
Splashing and Spraying	Coastal high-rise buildings during high tide	Capillary Absorption and Diffusion
Soil Covering	Underground ancillary structures in inland northwest regions	ionic migration

.

The diffusion of chloride ions into concrete is the primary mechanism behind reinforcement corrosion, and the extent of this diffusion directly determines the chloride ion resistance of concrete structures. During concrete hardening, the highly alkaline environment ($pH\approx 13$) within the concrete enables the formation of a dense passivation film on the reinforcement surface, which is composed of ${\mathrm{Fe}}_3$${\mathrm{O}}_4$ (inner layer) and ${\mathrm{Fe}}_2$${\mathrm{O}}_3$ (outer layer) [79,80], effectively preventing corrosion. However, when chloride ions diffuse to the reinforcement surface, their destructive effects occur in the following stages.

(a) Passivation Film Breakdown Stage

The invading chloride ions partially react with the calcium aluminate hydrate (3CaO·${\mathrm{Al}}_2$${\mathrm{O}}_3$) in concrete, forming Friedel’s salt (3CaO·${\mathrm{Al}}_2$${\mathrm{O}}_3$·${\mathrm{CaCl}}_2$·10${\mathrm{H}}_2$O), while the remaining free ${\mathrm{Cl}}^{-}$ ions migrate and accumulate on the reinforcement’s surface. When the ${\mathrm{Cl}}^{-}$ concentration exceeds the critical threshold level (CTL) [81], the localized alkalinity of the passivation film is neutralized, causing the film to rupture and triggering the pitting corrosion of the reinforcement.

(b) Electrochemical Corrosion Stage

Once the passivation film is damaged, a galvanic cell forms on the reinforcement’s surface, where the pitting site acts as the anode and the non-corroded areas serve as the cathode. This process occurs in different ways depending on the surrounding environment:

• In a chloride-free environment [82], Fe releases electrons at the anode to form $F{e}^{2+}$, while at the cathode, electrons migrate and participate in either oxygen reduction (in oxygen-rich conditions) or water reduction (in oxygen-deficient conditions) (see Figure 5).
• In the presence of ${\mathrm{Cl}}^{-}$ [80], chloride ions adsorb onto the anode surface, catalyzing $F{e}^{2+}$ dissolution and accelerating the formation of Fe(OH${)}_2$. Since ${\mathrm{Cl}}^{-}$ ions act as a catalyst and are not consumed, they continuously aggravate the corrosion process (see Figure 6).

(c) Structural Failure Stage

As Fe(OH${)}_2$ precipitates, the expansion stress generated by the increasing volume of corrosion products exerts pressure on the surrounding concrete. When this stress exceeds the tensile strength of the concrete, microcracks form on the concrete surface, gradually growing into larger cracks and spalling [83]. Studies have shown that a corrosion depth of only a few tens of micrometers into the reinforcement can initiate concrete cracking, ultimately leading to a significant decline in structural stability [83].

In summary, pores, cracks, and chloride-containing aqueous solutions within concrete are the primary causes of reinforcement corrosion and structural degradation. Preventing chloride-laden moisture from penetrating the concrete and distancing chloride ions from the reinforcement can effectively mitigate chloride-induced corrosion and damage. Based on this principle, anti-corrosion coatings that firmly adhere to the concrete surface can effectively block the intrusion of aqueous solutions through surface cracks, prevent chloride ions from making contact with the reinforcement, and thereby enhance the chloride corrosion resistance of concrete structures.

3.2.2. Chloride-Ion-Corrosion Test

The evaluation methods used for chloride ion diffusion can be categorized into two types: those involving steady-state and non-steady-state chloride ion migration.

Using a thin specimen, the steady-state method—also referred to as the diffusion cell method—separates fluids that contain and do not contain chlorine. The concentration of chloride ions in the solution is periodically measured using a spectrometer, and the migration rate of the chloride ions is computed using Fick’s first law. This testing technique is rarely used to assess the diffusion of chloride ions in reinforced concrete structures since it is labor-intensive, slow, and only works with cement paste specimens.

The non-steady-state method, commonly referred to as the immersion method, evaluates the diffusion state of chloride ions in reinforced concrete. According to the testing procedure outlined in AASHTO T259, specimens are immersed in a NaCl solution for an extended period, and then the chloride ion mass fractions at different depths are measured [72]. The chloride ion diffusion coefficient $D_c$ is calculated using Fick’s second law [84]. The testing period is long; however, this method is particularly useful as it is appropriate for determining the chloride-ion-corrosion resistance of reinforced concrete under a variety of circumstances. Accelerated corrosion tests using applied electric currents are frequently utilized to reduce the length of the test.

The rapid chloride permeability test (RCPT) and the rapid chloride migration (RCM) test are frequently used to gauge concrete’s resistance to chloride ion corrosion. While the latter assesses chloride permeability based on the electrical charge that passes through the specimen, the former employs the chloride ion migration coefficient in non-steady-state situations. These methods follow the guidelines specified in the Standard for Test Methods of Long-term Performance and Durability of Ordinary Concrete (GB/T 50082-2009). For reinforced concrete, an electrochemical workstation and a three-electrode system must be utilized to test the polarization curve of the specimens, which can be used to describe the corrosion condition of the reinforcement and evaluate the corrosion resistance of the reinforced concrete [85].

Furthermore, several researchers have used different techniques to assess concrete’s resistance to chloride ion corrosion. Sadati et al. buried a set of specimens in soil within 10 m of seawater along a coast and exposed another set of specimens to air near the sea [86]. After 8, 36, and 88 months, they assessed the chloride ion concentration on the surface of these specimens and their chloride ion diffusion coefficients. The actual chloride ion corrosion of concrete in natural settings was directly evaluated in this experiment. However, this test takes too long for it to be used to assess coated concrete’s resistance to chloride ion corrosion in a laboratory setting. Tsouli et al. tested the rebar’s uniaxial tensile strength after placing specimens in a Vötch chamber that was kept at a steady temperature of 35 °C and continually spraying them with a 5-weight-percent NaCl solution for two months. The salt spray test is suitable for simulating humid air environments containing chloride ions, such as those encountered at hydraulic docks exposed to prolonged chloride salt corrosion [87]. However, it is not suitable for assessing the corrosion caused by deicing agents on road surfaces or other concrete structures within inland highway bridges.

Table 7. Chloride-ion-corrosion resistance of different coatings.

Reference	Coating Type	Test	Performance Improvement (%)
[72]	Silane	Soaking Test	82
[72]	Polymer-modified Cement Coating	Soaking Test	66
[72]	Nano-${\mathrm{TiO}}_2$-modified Silicone	Soaking Test	71
[72]	Nano-${\mathrm{SiO}}_2$-modified Silicone	Soaking Test	76
[88]	Acrylic–Epoxy Resin	RCPT	68
[48]	Nano-${\mathrm{TiO}}_2$-modified Polymer Cement Coating	RCPT	60
[48]	Nano-${\mathrm{SiO}}_2$-modified Polymer Cement Coating	RCPT	85
[70]	Tetraethyl Orthosilicate-modified Acrylic	RCPT	73
[28]	Lignin-Waterborne Polyurethane	RCM	25

provides a summary of the research on how various anti-corrosion coatings improve the resistance of concrete specimens to the penetration of chloride ions. The results show that coatings greatly increase concrete’s resistance to chloride ion penetration. Silane coatings exhibit the best performance due to their excellent hydrophobic properties. The performance rankings of other anti-corrosion coatings are as follows: polyurethane > fluorocarbon coatings > epoxy resin > acrylic > chlorinated rubber. As for modified single-coating materials, the improvement in the chloride-ion-penetration resistance of coated concrete exceeds 50%, which is significantly superior to the resistance provided by unmodified single-coating materials.

In summary, electrochemical testing methods are highly favored by researchers. There is comparatively little research on chloride ion diffusion in coated reinforced concrete, as most current studies concentrate on the transport of chloride ions in regular concrete. Furthermore, studies on chloride ion diffusion should align more closely with the various coated concrete structures used in engineering applications. Corresponding chloride ion diffusion models should also be established to enhance and refine the theory of chloride ion diffusion.

3.3. Frost Resistance

3.3.1. The Mechanisms Behind Freeze–Thaw Damage

Concrete constructions are vulnerable to the freeze–thaw cycles occurring in adverse conditions, such as at cold temperatures, which can cause surface flaking and severe cracking. Concrete typically experiences three stages of degradation from freeze–thaw cycles: water absorption, freezing, and failure. In the first step, water slowly fills the interior holes in the concrete through its surface pores. As the outside temperature decreases, the water in the pores freezes, beginning the second step. At this stage, the freezing water undergoes a 9% volumetric expansion [89], generating hydrostatic pressure against the pore walls and subjecting the concrete to internal fatigue stress, resulting in the creation of microscopic fractures. After multiple freeze–thaw cycles, the third stage is reached, where repeated freeze–thaw actions lead to the interconnection of cracks, severe surface scaling, and a gradual reduction in the concrete’s strength until failure occurs [90,91], as shown in Figure 7.

The current theories on salt freeze–thaw damage are based on the hydrostatic pressure hypothesis, osmotic pressure hypothesis, and salt crystallization pressure hypothesis and provide a good explanation of the mechanisms of salt freeze–thaw damage in concrete. However, there is still a lack of research on the impact of pore solution salinity on the severity of freeze–thaw damage, and the interaction between external loads, ion concentrations in the environment, and freeze–thaw cycles requires further investigation.

3.3.2. Freeze–Thaw Damage Tests

Coatings can considerably improve concrete’s resilience to freeze–thaw cycles and lessen the damage that these cycles produce. The main purpose of freeze–thaw resistance testing is to assess how long concrete will last following extended exposure to these cycles. Chinese standards (like GB/T 50082), American standards (like ASTM C666), and European standards (like CEN/TS 12390-9:2017) are frequently utilized in this testing.

GB/T 50082 and ASTM C666 specify conventional freeze–thaw testing methods, including both rapid freeze–thaw and slow freeze–thaw methods, that can be used to assess concrete’s freeze–thaw resistance. The rapid freeze–thaw method is widely adopted due to its efficiency and operational simplicity; however, as the freezing speed exceeds that of natural environments, the test results may deviate from actual engineering conditions. In contrast, the slow freeze–thaw method better simulates natural environments but has limited utility due to its long testing period, high workload, and the greater variability in its results.

In addition, freeze–thaw cycles in chloride-rich environments often cause more severe damage to concrete. The European standard CEN/TS 12390-9:2016 suggests a testing method that evaluates freeze–thaw cycles in deicing salt environments, focusing on the overall freeze–thaw behavior of concrete to simulate the effects of deicing salts on concrete during winter road use. On the other hand, the Chinese standard GB/T 50082-2009 offers a one-sided freeze–thaw method to assess the anti-scaling performance of concrete under chloride freeze–thaw cycles.

However, existing freeze–thaw testing methods often fail to fully address the impacts of coatings on concrete’s frost resilience. Researchers have created modified rapid freeze–thaw techniques to test coated concrete’s resilience to freeze–thaw in order to address this problem. This method immerses coated specimens in NaCl solutions of varying concentrations, subjecting them to 10 to 200 freeze–thaw cycles (each cycle lasting 2–4 h), and evaluates the performance of the coated concrete by measuring parameters such as corrosion depth and flaking volume. Long et al. immersed specimens in a 3.5% NaCl solution for 4 days before conducting freeze–thaw cycles and recorded the mass and dynamic elastic modulus of the specimens after 0, 50, 100, and 150 cycles to assess performance changes [92]. Similarly, Zhao et al. used a 5% NaCl solution for both their immersion (4 days) and freeze–thaw cycles, testing specimens after 25, 50, 100, 125, 150, and 175 cycles by measuring their mass loss and dynamic elastic modulus [93]. They also employed a one-sided salt-freezing cycle method to measure the spalling volume every 5 cycles, conducting a total of 30 measurements.

Although freeze–thaw testing is widely used to evaluate concrete’s freeze–thaw resistance, significant variations exist in key test parameters, such as the number and duration of freeze–thaw cycles conducted, and the evaluation indicators selected for freeze–thaw resistance. Therefore, there is an urgent need to standardize and unify testing methods for assessing salt-freezing cycles in coated reinforced concrete. Additionally, suitable evaluation indicators of salt-freezing cycle corrosion in reinforced concrete should be established to quantify and compare the anti-corrosion effects of coatings.

In summary, the rapid freeze–thaw method is more suitable for testing salt-freezing cycles in reinforced concrete. There is currently little research on the corrosion caused by salt-freezing cycles in coated concrete, and there are no forecast models for how long coated reinforced concrete will last in these conditions. Future studies should concentrate on the mechanisms through which salt-freezing cycles impact anti-corrosion coatings, enhance evaluation criteria and procedures, build predictive models of reinforced concrete’s service life in salt-freezing cycle conditions, and develop useful, accessible testing procedures that more accurately represent real-world circumstances.

3.4. Carbonation Resistance

3.4.1. Carbonation Mechanism

The physical and chemical reaction process known as “carbonation” occurs when ${\mathrm{CO}}_2$ from the atmosphere permeates a material through adsorption and diffusion. Atmospheric ${\mathrm{CO}}_2$ dissolves in pore water, forming carbonic acid, which reacts with hydration products such as $Ca{\left(OH\right)}_2$ and C-S-H [94], resulting in the formation of calcium carbonate ($CaC{O}_3$) and other salts. Concrete undergoes a number of modifications as a result of this process: the precipitation of $CaC{O}_3$ during carbonation reduces the porosity of concrete by filling some of its interior pores. Due to these changes in porosity, the water absorption and permeability of the concrete also decrease [95].

As a result, carbonation has a relatively minor impact on the material properties of concrete and may even enhance its strength to some extent [96]. However, carbonation significantly affects the durability of reinforced concrete. The reduction in pH caused by carbonation destabilizes the protective layer around the steel reinforcement, leading to accelerated corrosion of the steel surface, which eventually causes damage to the concrete [97]. The main mechanisms behind the transport and chemical reactions involved in the carbonation process are illustrated in Figure 8.

3.4.2. Carbonation Tests

Carbonation testing aims to evaluate the penetration depth of ${\mathrm{CO}}_2$ into concrete during its service life. The carbonation depth is determined 3, 7, 14, and 28 days after specimens are placed in a carbonation chamber for accelerated carbonation, in accordance with the Chinese standard GB/T 50082. The phenolphthalein indicator method is also recommended by similar international standards, including ASTM C1910/C1910M, EN 13295:2004, and ISO 1920-12:2015, for ascertaining the carbonation depth of concrete.

In practical research, commonly used methods involve placing concrete specimens in a climate chamber with a ${\mathrm{CO}}_2$ concentration of $20\pm 2\%$, a temperature of 25 ± 5 °C, and a relative humidity of $70\pm 5\%$. After 60 days of exposure, the carbonation depth is determined through a phenolphthalein colorimetric test. Kumar et al. used the phenolphthalein indicator method to measure the carbonation depth of concrete specimens exposed to a climate chamber with a 20% ${\mathrm{CO}}_2$ concentration, 20 °C temperature, and 70% relative humidity for 10 to 50 days [99]. Nashed et al. proposed using the BS EN 1062-6:2002 method to test the anti-carbonation performance of coated concrete [100]. Li et al. placed concrete specimens coated with tetraethoxysilane and acrylic materials in a carbonation test chamber. The specimens were removed and fractured at 3, 7, 14, and 28 days to determine the carbonation depth [70].

In studies on the impact of anti-corrosion coatings on concrete carbonation depth, comparative experiments are commonly conducted. The results consistently show that coated specimens exhibit significantly lower carbonation depths compared to uncoated control groups. Additionally, in order to explore the use of non-destructive testing methods in real-world engineering applications, some researchers have utilized X-ray imaging technology to study the carbonation process in cement. This method provides a novel non-destructive, intuitive, and quantitative approach for detecting carbonation. By optimizing X-ray imaging parameters and correlating X-ray attenuation coefficients with gray values and density changes, the distribution of carbonation can be accurately characterized. Experiments demonstrate that this method effectively reflects carbonation features and is consistent with thermal imaging analysis results [101].

In summary, carbonation testing is an effective tool for evaluating the durability of concrete. Effective service life prediction models are lacking, and research on coated concrete’s anti-carbonation mechanisms is still scarce, despite the material’s superior performance in terms of decreased carbonation depth. Future studies should concentrate on how coatings affect the carbonation of concrete, develop better criteria for evaluating carbonation, create models for predicting the depth of carbonation, and investigate the use of non-destructive testing techniques to increase the precision of concrete durability evaluations.

3.5. Sulfate Resistance

3.5.1. Sulfate Corrosion Mechanisms

The sources of the corrosive ions causing sulfate attacks are diverse and include marine environments, saline–alkali soils, and underground sewage pipelines. There are currently no consistent conclusions regarding the intricate and multifaceted mechanisms behind sulfate attacks on concrete. Based on the form of the damage, sulfate attacks can be broadly categorized into chemical corrosion and physical corrosion [102], as illustrated in Figure 9.

Chemical corrosion includes ettringite-type and gypsum-type attacks. Under low sulfate concentrations (<1000 mg/L) and high-pH conditions (>12), sulfates react with hydration products to form ettringite, which initially increases the strength of concrete but then eventually causes cracking due to significant volume expansion (as the same mass of ettringite is approximately 8 times the volume of $C_{3}A$). When the pH drops to <11.5, the ettringite decomposes into gypsum, further exacerbating the damage. At high sulfate concentrations (>1000 mg/L) and low pH levels (<10.5), sulfates form gypsum crystals, which undergo substantial volume expansion, leading to cracking. Such conditions are commonly observed in laboratory tests that use high sulfate concentrations.

Physical corrosion occurs when temperature changes cause sulfate solutions to become supersaturated, resulting in the precipitation of crystals, which exert pressure on pore walls, creating cracks. Theories such as solid-phase volume change, crystallization water pressure, and salt crystallization pressure all suggest that crystal expansion or pressure is the primary driving factor behind physical corrosion.

In marine or saline–alkali soil environments, concrete is further subjected to coupled chloride and sulfate attacks. Chloride ions diffuse into the concrete and form Friedel’s salt, which reduces the production of ettringite and thus delays the sulfate attack. However, chloride ions simultaneously accelerate the corrosion of steel reinforcements, leading to faster concrete degradation [35].

3.5.2. Sulfate Corrosion Test

Concrete’s endurance in sulfate-rich environments is the main focus of tests used to evaluate its resistance to this type of corrosion. Common methods include those in the Chinese standards (e.g., GB/T 50082 and JT/T 700), ASTM standards (e.g., ASTM C1012 and ASTM C267), European standard EN 12390-9, and ISO 1920-10. In order to assess the effects of sulfate corrosion on concrete, these experiments often entail submerging concrete samples in solutions of sodium sulfate (${\mathrm{Na}}_2$${\mathrm{SO}}_4$) or magnesium sulfate (${\mathrm{MgSO}}_4$) and monitoring changes in their mass, strength, and length.

In practical experiments, researchers often modify standard test methods to better simulate real-world corrosion conditions. For example, Liao et al. immersed specimens in a 5% ${\mathrm{Na}}_2$${\mathrm{SO}}_4$ solution for 30, 60, 100, 140, and 180 days, measuring sulfate concentrations at different depths to assess the impact of sulfate corrosion [104]. An expedited technique that adds additional cementitious ingredients, like Class F fly ash and metakaolin, to mortar and concrete was proposed by Mousavinezhad et al. [105]. This significantly reduced the time required for sulfate corrosion assessments while maintaining consistency with the results obtained from the standard ASTM C1012 method. Zhang et al. tested the efficacy of several corrosion inhibitors in sulfate-rich environments using wet–dry cycle tests and 5% and 10% sodium sulfate solutions. Their results demonstrated that densification techniques and hydrophobic treatments significantly improved the durability of concrete, particularly in high-sulfate environments [106].

Additionally, some studies have highlighted the importance of combined chloride and sulfate corrosion tests when evaluating the durability of concrete. For example, real-world corrosion processes can be accurately simulated by employing solutions that contain different concentrations of sodium sulfate and sodium chloride under wet–dry cycling conditions. This approach provides valuable guidance for selecting appropriate construction materials for complex environmental conditions [107].

In summary, the evaluation of concrete’s sulfate corrosion resistance aims to assess concrete’s durability in sulfate-rich environments. This is typically achieved by immersing samples in sulfate solutions of varying concentrations (e.g., sodium sulfate) or subjecting them to wet–dry cycles while monitoring changes in mass, strength, and length. Researchers have further optimized standard testing methods to better simulate actual environmental conditions.

4. Challenges and Future Research Recommendations

Even though concrete coatings have been shown to be successful in prolonging the concrete’s service life and considerably slowing down corrosion, there remains considerable room for development in testing methods, material performance, and engineering applications. A review of the literature reveals that coating technology faces several key challenges: inconsistencies in testing standards, insufficient awareness of material properties and environmental adaptability, and issues related to construction quality and maintenance in practical engineering applications.

Improving the durability of coated concrete and precisely forecasting its service life require addressing these outstanding concerns through focused studies and technological optimization.

4.1. Challenges and Solutions for the Standardization of Testing Methods

(1) The Background and Causes of Non-Uniform Testing Standards

The issue of non-uniform testing standards for concrete coatings is the result of several factors. Firstly, significant differences in engineering environments across countries and regions play a major role. For instance, coastal areas prioritize resistance to chloride ion penetration, while cold regions focus on freeze–thaw resistance. These differences lead to varying priorities during standard development, resulting in notable discrepancies in testing methods and evaluation criteria. Additionally, differences in technological traditions and development pathways across countries exacerbate this problem. Technologically advanced countries like the United States tend to adopt accelerated testing methods (e.g., the rapid chloride permeability test, ASTM C1202) to meet their fast assessment needs, while European countries prefer steady-state diffusion methods that simulate natural conditions. In China, the relevant standards include multiple testing methods, emphasizing practicality and flexibility. Furthermore, a lack of in-depth collaboration between international standardization organizations has led to significant differences in the content and goals of testing standards across regions. Existing standards often focus on evaluating single performance metrics and fail to comprehensively account for the multifactorial environmental conditions of complex real-world engineering situations, making them inadequate for use in global projects.

(2) The Manifestation of Non-Uniform Standards

The non-uniformity of testing standards is evident in the various key performance metrics and testing methods used. For example, in the testing of chloride-ion-penetration resistance, the United States uses the rapid chloride permeability test (ASTM C1202), China employs the rapid chloride migration (RCM) test, and European standards favor steady-state diffusion methods. These methods differ significantly in their testing conditions, equipment requirements, and interpretation of the results, making a comparison of these test results challenging. Similarly, the methods used for testing adhesion also vary: the United States employs pull-off testing (ASTM C1583), China favors shear strength testing, while Europe evaluates adhesion through uniaxial tensile testing (EN 1542). The differences between these methods complicate the alignment of standards in international projects. Similar discrepancies are observed in testing UV aging resistance and chemical corrosion resistance. For instance, the United States (ASTM G154) uses accelerated aging chambers, while China and Europe have different guidelines for environmental settings and the duration of these tests. As for chemical corrosion resistance tests, the United States emphasizes short-term chemical exposure, while China and Europe focus on long-term corrosion evaluations. These inconsistencies not only increase the complexity of aligning different technical standards but also limit the cross-regional promotion of coating technologies.

(3) Recommendations for Resolving Non-Uniform Standards

To address the issue of non-uniform testing standards and promote global coordination and development in engineering practices, it is imperative to strengthen international collaboration and optimize testing technologies. The International Organization for Standardization (ISO) should play a leading role in partnering with major standardization bodies such as ASTM, EN, and JT to establish a cross-regional, cross-disciplinary standard-coordination mechanism. Through this mechanism, standards from different regions can be harmonized and made compatible while gradually developing a unified global testing framework. This would reduce the discrepancies between standards and enhance the efficiency of technical coordination during global projects. To ensure the scientific validity and applicability of this framework, key performance metrics such as chloride-ion-penetration resistance, adhesion, and UV aging resistance can be prioritized as pilot areas for international joint testing and comparative studies to further validate and optimize unified testing methods. This approach will not only promote consistency in testing technologies across countries but also enable more scientific and precise engineering design, construction, and material selection on a global scale.

Furthermore, the promotion of standardized testing equipment and processes is a critical step towards achieving consistent test results. By fostering global technology sharing and equipment standardization, testing errors can be reduced and the efficiency of technical exchanges can be enhanced. Existing standards should be updated to incorporate the evaluation of environmental sustainability and smart materials, creating standards suitable for testing the durability and long-term performance of new coating materials. This adaptation will address the need for material innovation in engineering practices. Additionally, standards should be tailored to specific environmental conditions such as high-salinity coastal areas, cold climates marked by freezing–thawing cycles, and humid and hot environments by introducing categorized and graded guidelines and additional testing conditions. These measures will provide more accurate reflections of the performance of materials in diverse environments, enhancing the adaptability and scientific rigor of these standards.

The integration of real-world engineering data and intelligent evaluation tools is also key to achieving standardized testing. By establishing a global engineering monitoring data feedback mechanism, long-term service data can be combined with laboratory testing results to optimize testing methods and evaluation criteria. In the future, leveraging big data and artificial intelligence technologies to develop tools for the real-time monitoring of coatings could provide scientific support for updating coating standards. The durability and safety of concrete structures will eventually be improved by these activities, which will facilitate the promotion and use of coating technologies and increase the effectiveness of technical collaboration.

4.2. Engineering Applications and Performance Optimization of Concrete Coating Technology

(1) Performance Requirements for Coatings in Different Environments

Concrete structures are often exposed to complex and diverse service environments, each of which imposes different performance requirements on concrete coatings. In coastal areas, where chloride ion corrosion is the primary degradation factor, hydrophobic impregnation coatings are favored for their excellent chloride ion resistance. However, their penetration is limited in low-porosity or highly dense concrete, which reduces the protection they provide. Surface-film-forming coatings excel in carbonation resistance, but in northern regions with low temperatures and high humidity, they are prone to peeling and delamination due to freeze–thaw cycles and humidity changes, compromising the service life of their underlying structures. In hot and humid regions, the combined effects of high humidity and strong ultraviolet radiation demand superior weather resistance from coatings, which is where fluorocarbon and modified polyurethane coatings perform exceptionally well. Since actual environments often involve a combination of these factors, single coatings are often unable to meet all these performance requirements. As a result, composite coating technologies have gradually become the accepted solution to multi-environment challenges, combining the properties of different materials to enhance their overall protective performance.

(2) Practical Applications of Coatings in Engineering

In practical engineering applications, the performance of coatings is influenced not only by their material properties but also by their construction quality and post-application maintenance.

Table 8. Coating systems used in real-world engineering projects.

Year	Project Name	Location	Coating Type	Coating System
2015	Sunshine Skyway Bridge	USA	Hydrophobic impregnation	Epoxy
				Impregnating resin
2016	Anlaby Road Flyover	UK	Surface film-forming	Acrylic-based coating
				Acrylic protective coating
2017	Clydebank Flyover	UK	Pore-blocking	Modified cementitious polymer
2018	Hong Kong–Zhuhai–Macao Bridge	China	Surface film-forming	Epoxy
				Epoxy
				Polyurethane
2019	Songxia Over-Sea Grand Bridge	China	Hydrophobic impregnation	Silane impregnation
2021	Meizhou Bay Over-Sea Bridge	China	Hydrophobic impregnation	Silane impregnation
2022	Anhai Bay Grand Bridge	China	Hydrophobic impregnation	Silane impregnation
2022	Quanzhou Bay Over-Sea Bridge	China	Hydrophobic impregnation + surface film-forming	Silane impregnation
				Epoxy
				Epoxy
				Fluorocarbon resin
2022	Point-No-Point Bridge, New Jersey	USA	Surface film-forming	Zinc primer
				High-solids epoxy
				Urethane topcoat
2024	Minjiang Bridge, Fujian	China	Surface film-forming	Epoxy
				Epoxy
				Polyurethane

summarizes case studies of the coatings selected in actual engineering projects. The statistical results show that the use of fluorocarbon resin coatings is steadily increasing, although primarily in areas with low corrosion levels, such as dry-surface zones. In severely corrosive environments like wet-surface zones, surface-film-forming coatings are gradually being replaced by hydrophobic impregnation. A few projects have attempted to combine the two on concrete surfaces in highly corrosive environments. However, there are currently no relevant standards providing recommended coating systems for these composite applications. Moreover, many concrete bridges still lack protective coatings, leaving them exposed to corrosive environments without adequate defense.

(3) Challenges in the Application and Long-Term Performance of Coatings

Currently, the selection of coatings is largely based on short-term laboratory testing results, with a limited understanding of their long-term performance in real-world environments. For example, while penetrating hydrophobic coatings demonstrate excellent chloride ion resistance in laboratory experiments, their effectiveness in engineering applications may be significantly reduced due to insufficient application thicknesses or improper substrate preparation. Similarly, surface-film-forming coatings may fail to deliver adequate protection if the interface treatments between layers are poorly handled during multilayer application, leading to insufficient adhesion or interfacial delamination. Furthermore, the control of critical indicators during the construction phase, such as coating thickness, adhesion, and curing time, often relies on manual experience rather than systematic and standardized monitoring methods. This is particularly problematic in environments with high humidity or significant temperature fluctuations, where performance variability is more pronounced. After application, the lack of long-term monitoring and maintenance of coating performance exacerbates these issues. The timing and causes of coating failure are often difficult to pinpoint accurately. For instance, early signs of failure, such as aging or adhesion degradation, may not be apparent but could lead to a rapid decline in performance over time. The shortcomings in quality control during coating application and the absence of post-application performance monitoring not only shorten the service life of coatings but also significantly increase subsequent maintenance costs.

(4) Optimization Strategies and Solutions

To address these challenges, a systematic engineering monitoring and feedback mechanism must be established. Online sensing technologies could enable the real-time monitoring of critical parameters such as coating thickness, adhesion, and key durability metrics, allowing for the early detection of potential issues. A regular assessment system should also be implemented to combine laboratory testing with performance evaluations in actual engineering environments, facilitating the accumulation of long-term performance data. Additionally, leveraging big data analysis and intelligent technologies to integrate the results from various projects can support material improvements and the development of new technologies. For instance, analyzing the long-term performance of coatings across different environments can optimize material formulations and enhance targeted protective effects. Additionally, by offering a scientific foundation for the creation of standards and guidelines, this feedback mechanism can progressively enhance the technical standards for the design, application, and maintenance of coatings.

5. Conclusions

1. The selection of coating type based on application scenario: There are various types of anti-corrosion coatings for concrete, and the selection of an appropriate coating should be based on the specific protective requirements of its intended environment. Surface-film-forming coatings (e.g., epoxy resin and polyurethane) create a dense barrier that reduces chloride ion diffusion rates by more than 50% and enhances carbonation resistance, making them suitable for humid and highly corrosive environments. However, their durability is affected by the thickness and adhesion of the coating, with potential delamination issues at the interface. Pore-sealing coatings (e.g., silicate-based and crystalline coatings) fill the capillary pores in concrete, improving density and impermeability and exhibiting excellent freeze–thaw resistance, making them ideal for harsh chemical corrosion environments. However, their brittleness and adhesion still require optimization. Penetrating hydrophobic coatings (e.g., silane, siloxane) reduce water absorption rates by over 75%, effectively minimizing the ingress of moisture, which is beneficial for coastal and underwater structures. However, their effectiveness in low-porosity or highly corrosive environments still requires improvement. Future research should focus on developing high-durability, self-healing, and environmentally friendly coatings to meet the long-term protection demands of concrete structures in complex service conditions.

2. The optimization of coating systems and construction quality control: The selection of coating systems and construction quality control are crucial for ensuring the anti-corrosion effectiveness of coatings. In China, standards recommend a two-layer or three-layer coating system and specify the type and thickness of each layer, while international standards lack detailed guidelines for coating system selection. Studies show that multilayer coating systems can significantly improve chloride-ion-penetration resistance (reducing it by over 82%) and freeze–thaw resistance (reducing the mass lost by approximately 50%) and enhance long-term stability. However, due to differences in national standards, cross-border engineering projects lack comparable coating designs, limiting the widespread application of coating technologies. Therefore, it is essential to establish standardized coating system recommendations for different corrosion environments and to develop unified construction processes and quality control standards to enhance the applicability and reliability of these coatings.

3. Variations in coated concrete durability testing standards: Significant differences exist internationally in terms of the evaluation criteria used for assessing coated concrete’s durability, which impact the comparability of different studies and coating applications. Different countries adopt different testing methods and indicators for assessing chloride-ion-penetration resistance, freeze–thaw durability, carbonation resistance, and sulfate corrosion resistance. For instance, the US standard employs the electrical flux method (ASTM C1202), while China uses the rapid chloride migration (RCM) test and European standards prefer the steady-state diffusion method. These methodological differences make direct data comparisons difficult. Moreover, current durability tests focus on single performance metrics and fail to consider multiple environmental interactions (e.g., the combined impact of freeze–thaw cycles, chloride ingress, and carbonation effects). Therefore, international collaboration should be strengthened to unify the durability testing standards for concrete coatings and establish a comprehensive evaluation system that considers multifactor environmental influences, improving the suitability of coatings in diverse service conditions.

4. Challenges in assessing the application, long-term durability, and maintenance of coatings: While concrete coatings are widely applied in bridges, ports, tunnels, and other infrastructure, challenges persist in assessing their construction quality and long-term durability. Research shows that construction quality directly impacts a coating’s service life, with common causes of failure including insufficient surface preparation, improper coating thicknesses, and incomplete curing. Additionally, the lack of long-term monitoring mechanisms results in limited feedback on the effects of different environments on a coating’s performance, leading to uncertainties in its maintenance and service life prediction. To enhance the reliability of coatings, it is essential to strengthen the quality management of their construction, develop standardized quality acceptance systems, and implement sensor-based long-term monitoring systems to assess real-time performance changes. Furthermore, future efforts should focus on the development of functionalized modified coatings to improve durability and environmental adaptability while also promoting the use of eco-friendly coating materials for the sustainable protection of concrete structures.