Coating materials are considered one of the most antique materials of human civilization; they have been used for decoration and the protection of surfaces for millennia. Concrete structures—due to their permanent exposure to different types of environments and contaminants—require the use of coatings that contribute to its preservation by reducing the corrosion of its components (steel and aggregates). This article intends to introduce the principal causes of concrete deterioration and the coating materials used to protect concrete structures, including a summary of the coating types, their advantages and disadvantages, and the latest developments and applications. Furthermore, this paper also assesses brief information about the potential challenges in the production of eco-friendly coating materials.
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In the last few decades, reinforced concrete (RC) has become one of the most used construction materials. Its versatility and adaptability offer infinite applications in the construction sector [1,2]. The construction industry has been looking for several methods to improve the durability of concrete structures; rehabilitation, restoration, and strengthening are the most common activities to extend an existing structure’s life cycle [3]. The durability of concrete structures embedded in soil and exposed to different types of contamination might be affected by two factors: deterioration from concrete components and chemical deterioration caused by external agents [4,5]. summarizes the factors involved in the decrease of the durability of structures exposed to contamination.
Alkali–silica reaction (ASR): It is one of the most concerning topics regarding the durability of concrete, leading to costly maintenance and rehabilitation works. ASR occurs when cement aggregates react with the alkali hydroxides in concrete, producing a hygroscopic gel that in the presence of water causes an expansion and thus the cracking of the concrete surface
Corrosion of steel bars: The corroded bars occupy a greater volume than the non-corroded ones, causing cracking and delamination of the concrete surface. Steel corrosion is caused by the presence of chloride ions or carbon dioxide.
Caused by external agents
i.
Chemicals that promote a rapid deterioration: Aluminum chloride, calcium bisulfite, hydrochloric acid, nitric acid, and sulfuric acid.
ii.
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Chemicals that produce a moderate deterioration: aluminum sulfate, ammonium bisul-fate, ammonium nitrate, ammonium sulfate, ammonium sulfide, and sodium bisulfate.
: It can be divided into two groups:
Volume changes: Freeze–thaw cycles, plastic and drying shrinkage, and thermal changes are the leading causes of volumetric change.
Construction, energy, mining, agriculture, and transport industries, are one of the primary sources of contaminants; according to Enshassi et al. and Zolfagharian et al. [10,11], these can be defined as solid and liquid waste, harmful gases, noise, water, soil, and air pollution. Even though the construction sector causes several impacts to the environment, this sector is also affected by the pollutants released by other industries, e.g., soil contamination due to agricultural and mining activities reducing the durability of structures embedded in the soil caused by the presence of chemical compounds, and air pollution produced by energy and transport sectors, where the emanation of chlorine oxides contributes to the accelerated corrosion [12,13]. For this reason, it is essential to develop processes that generate less contamination and allows the protection of construction elements exposed to contaminants.
Previous studies have focused on the durability, deterioration, and service life of concrete structures, including numerical models [14,15,16] and experimental studies [17,18]; however, these studies mainly focused on constructions located above ground level and ignored the impact of the different factors on the structures located below ground level. Wei et al. [19] investigated how acids coming from the atmosphere and retained in the superficial layers of the ground induce concrete degradation decreasing the compressive strength and increasing the corrosion coefficient of concrete; it was identified that the main reason for premature deterioration of concrete is due to the changes in temperature where the corrosion coefficient was increased about two times for samples exposed to 40 °C. However, the compressive strength results did not show any significant changes during the 90 days of exposition. Kozubal et al. [20] have proposed a numerical model that allows preventing structural damage of vertical elements exposed to a contaminated soil environment. This model permits design engineers in the decision-making process by ensuring the safety of concrete structures embedded in the soil. The mathematical model was proposed based on the deterioration of concrete Controlled Modulus Columns (CMC) exposed to different sediments in groundwater, evidencing the apparition of cracks due to chemical corrosion. Li et al. [21] presented an analytical approach to predicting the life span of reinforced concrete pipe piles that are constantly exposed to chloride contamination and are affected by the earth pressure causing deterioration of the elements by the diffusion of microcracking. Among the principal assumptions, it can be highlighted that the end of the service life of these structures is going to be reached once the elements present total transverse cracks allowing the penetration of chlorides into the concrete core; this method provides a genuine approach for the evaluation of service life of concrete pipe piles allowing the improvements of durability design and reducing the maintenance of this concrete elements.
Recently, different coating materials have been used to protect concrete structures in the construction industry. Among the most common ones, it is possible to find fire protection coatings used as a precautionary measure preventing buildings from collapsing during fire exposure [22] and waterproof coatings widely used in the protection of concrete against reinforcement corrosion, erosion, carbonation, silica reactivity in aggregates, and chemical attacks, such as acids, salts, alkalis, and sulfates [2,7,23]. The use of coatings also increases the structure’s lifetime by preventing the appearance of cracks and reducing the maintenance cost. shows the general classification of coating materials for different industries.
In the last few decades, research studies about the utilization of coating materials as protection for concrete elements exposed to different environments have increased due to the significant growth of this sector and the development of a large diversity of coating materials, varying not just raw ingredients but also the process of manufacture; among the most common techniques for the preparation of coating materials, it is possible to distinguish the solution casting method proposed by Sakamaki [34], the phase-transfer catalyst process, the taffy process, and the fusion process [30]. summarizes the historical milestones in the development of coating materials from prehistory until the present day.
Generally, coating materials are commonly used in concrete structures when they are exposed to contaminants. Zouboulis et al. [39] proposed the study of corrosion protection of concrete samples covered with six different coatings with magnesium hydroxide against contaminants contained in sewage systems. This study has been developed in a controlled environment in a laboratory simulating the biological contamination produced in an actual sewage plant using a sulfuric acid solution and using concrete type MC 0.45 simulating the concrete used in the sewage pipes, the grade of protection of the coating was evaluated with an accelerated degradation method by spraying H2SO4 in the surface sample, this process was performed until the coating’s degradation was evidenced visually. Among the results, it is possible to identify that the thick layer of the coating material is directly related to the durability time, samples with 0.002 g/mm2 presented double duration time than the samples covered with 0.001 g/mm2, also the XRD analysis showed that all samples obtained gypsum formations before the total degradation of the coating material, even though the coating material presented degradation, its superficial pH was constant in all cases, maintaining an average value slightly over 8. Aguirre-Guerrero et al. [40] evaluated the protection effectiveness of inorganic coatings applied to concrete exposed to chloride contamination by analyzing different properties, such as water absorption, resistance to chloride ion penetration, adhesion strength, and corrosion resistance. Among the results, it is important to mention that coated concrete has not performed well, presenting lower resistance to water penetration and an increment in their capillary absorption. However, all concrete samples protected with inorganic coating showed an increment in chloride penetration resistance compared to concrete samples without protection by reducing the penetration of chlorides from high to moderate and, in some cases, to low. Finally, the use of coatings prolongs corrosion and extends the time of cracking. Sakr et al. [41] studied how different coating materials protect concrete with different water–binder (w/b) ratios when exposed to constant salt attack. It is evidenced that acrylic emulsion, epoxy, and ethyl silicate successfully protect concrete surface from physical salt attack regardless of the type of concrete and salt concentration. At the same time, the protection capacity of coatings made with the addition of fly ash strongly depends on the concrete (w/b) ratio. In general, coating materials successfully protect concrete against different types of chemical aggressions extending the lifespan of concrete elements and reducing the maintenance of structures.
This review paper aims to review the most relevant and recent investigations related to the use of coatings materials for the protection of concrete exposed to different types of contamination, also it reviews the deterioration of concrete exposed to a contaminated environment by summarizing the relevant manuscripts published in the last five years, until 2021. and shows the statistical data of the resources used in this review paper, such as total of publications used per year and per country. The research gaps in the implementation of coatings materials and challenges for the future are identified and discussed.
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