*4.4. Coating Materials—Current State, Challenges, and Perspectives*

4.4.1. Research Gaps in the Use of Coating Materials

The rapid and continuous growth of different industries, the lack of control in the production of materials, food, and poor waste management, can eventually increase air and soil pollution, as evidenced in previous chapters, decreasing the service life of structures exposed continuously. Different materials have been implemented over the years to protect concrete elements by reducing corrosion at an early age. As Table 6 demonstrates, most of the researches are focused on analyzing the concrete and steel mechanical properties behavior in structures located above the ground when exposed to different types of contamination, either organic and non-organic. However, the use of coating materials is not evidenced for the protection of concrete elements embedded in the soil, taking this into account, it is crucial to invest in the research of coating materials that can be applied in concrete elements embedded in contaminated soil that allow the preservation of structures exposed to different types of contamination at various degrees.

#### 4.4.2. Current Status and Future Challenges

Protective coatings are present in most of the surfaces around us, used from the simple protection of food to the complex protection of steel and concrete. Nevertheless, most of these coatings go through a manufacturing process that generates contamination. Some of them use nonrenewable materials, such as bitumen obtained from petroleum refining, causing several environmental problems. In addition, some of the coatings use organic solvents that emit volatile organic compounds, producing air pollution that affect human health. Therefore, it is necessary to continue developing eco-friendly coating materials that contribute to environmental preservation without sacrificing the main properties of the materials, e.g., high durability, toughness, adhesion, strength, etc. Table 9 contains the main advantages and disadvantages of some of the most common coating materials used in the construction industry to protect concrete structures. Figure 3 shows the key aspects and challenges in the production of coating materials.

**Figure 3.** Critical aspects in the field of challenges of production of coating materials used for protection of concrete elements.


**Table 9.** Comparison of the most popular coatings for concrete elements [25,31,68–76].

4.4.3. Characteristics of Coating Materials According to Polish–European and American Standards

Coating materials must follow the specifications stated in the European Standards (Eurocode) regarding coating adhesion to the substrate, absorption, and permeability, among others. Table 10 contains the most imperative standards describing the physical and mechanical properties of coating materials intended to protect different surfaces, such as wood, steel, concrete, plastic, and glass. For this paper, the PN–EN standards based on the European Standards will be taken as a reference.

Table 11 summarizes the laboratories that performed evaluations of the physical and mechanical properties of coating materials used in the construction industry according to the academic articles used in the search methodology.


**Table 10.** Standard procedures for the determination of mechanical and physical properties of coating materials.

**Table 11.** Summary of laboratories for coating materials.


Approximately 54% of the articles reviewed applied Polish–European standards (PN-EN), 38% used the American Society for Testing and Materials (ASTM), and just 2% used local test methods approved by the ITB (Building Research Institute) in Poland. The most common procedures among the literature were adhesion, tensile stress, and resistance to freeze/thaw cycles tests. In addition, the article analyses focused on the reduction and control of the carbonation process and on the proposal of new coating materials for the protection of concrete exposed to contaminated environments.

During the last decades, different raw materials have been used to produce new protective coatings materials intended to improve the concrete properties. Elnaggar et al. [61] presented a novel protective material based on different ratios of isocyanate chemical groups (NCO) and a mix of 80% asphalt and 20% polyester. The asphaltic polyurethane (As/PU) coating was tested on concrete cubes; according to the results, an increment in the dry film thickness was shown, from 86μm to 98μm, in samples with a 1:4 ratio of NCO, which can be attributed to the density of the (As/PU) coating. Similarly, adhesion strength showed an increase of 145% in the samples with a 1:4 ratio of NCO, an effect that can occur due to the interaction between ACO groups and ANH. Finally, it was concluded that both dry film thickness and adhesion strength improved with the increase in NCO/OH ratio.

Francke et al. [62] proposed a new coating material modified with cementitious mortar to perform waterproof and chemical protection. Based on polymer–cement products, this

coating material effectively performed the functions of concrete carbonation protection by reducing the carbonation depth by 24% and increasing in 7% the adhesion strength in frost and storm environments. However, in freeze–thaw cycles with the addition of sodium chloride solution (salt), a decrease was evidenced in the bonding strength of about 40% with respect to the sample without environmental exposure.

Improving the protection of concrete structures is one of the most critical objectives in manufacturing new coating materials. Significant results have been evidenced by applying protective (As/PU) layers showing a reduction in the chloride penetration of about 75% with respect to the control sample. Even though the immersion of both coated and noncoated samples in sulfuric acid and NaCl solutions show a decrease in the compressive strength of the concrete samples, it can be evidenced that the coated samples present a reduction in the compressive strength between 22% and 27% and a decrease of 50% in the non-coated material. Finally, it can be concluded that the protection with asphaltic polyurethane (As/PU) coating improves the mechanical and physical properties gradually when the ratio is increased with respect to the samples without coating.

According to Baba et al. [63], to minimize the corrosion caused by carbonation, concrete surface protection can be performed with three different coating materials: penetrants for surface improvement, non-cementitious for finishing layer, and cementitious for finishing layers. In the research conducted by Lo et al. [64], eight non-cementitious coatings, emulsions, and synthetic paints were used in concrete prisms to analyze their impacts on the reduction of carbonation depth; four of them were tested for interiors and the rest of the coatings for the exterior. In addition, an accelerated carbonation test method was implemented, exposing the samples to a constant *CO*<sup>2</sup> flow in a chamber for 56 days, the deep carbonation was measured by exposing the samples to phenolphthalein solution. Results showed that for exteriors coatings, the C25 concrete samples obtained a reduction in deep carbonation of about 60% and 45% for interiors coatings, decreasing from 16.40 mm to 6.58 mm and 8.93 mm, respectively. For C35 concrete samples a reduction in the deep carbonation for exteriors coatings of 56% (3.78 mm) was also evidenced, and for the interior coating it was 40% (4.23 mm). Based on this it can be concluded that there is a significant reduction in the corrosion caused by carbonation using these coating materials.

The authors mentioned above evaluated the benefits of coating materials in structures exposed to contamination. It was evidenced that coating materials effectively reduce the impact caused by different chemical attacks, and are able to extend the lifespan of concrete structures and reduce corrective maintenance costs.

#### **5. Conclusions**

This literature review was prepared to give an overview of the causes of corrosion of concrete elements exposed to different types of contaminants and the procedures proposed and used by some researchers to protect these elements. Different coating materials have been proposed, varying from naturals sources, such as bituminous coatings, to synthetic productions, like acrylic coatings. Among the results, in all cases where concrete samples were subjected to contamination either by exposition to chemical or natural contaminants, the compressive and flexural strengths showed a significant reduction. In addition, galvanized and carbon steel bars embedded in concrete samples showed an increase in corrosion, potentially leading to a premature corrosion of the bars and premature cracking and deterioration of the concrete elements. Even though several investigations have been carried out on how different types of contamination affect concrete, there is not much evidence yet on how coating materials can protect concrete elements embedded in contaminated soils.

#### **6. Research Limitations**

This review paper was limited to Spanish and English articles found in the journals mentioned in Section 2, "Search Methodology", which excludes literature published in other languages and was limited to academic publications. It does not consider the results from industrial practice.

**Author Contributions:** Conceptualization, supervision, and validation: M.N. and H.B., formal analysis, investigation, and writing—original draft: G.P.M.R., methodology, and visualization: G.P.M.R. and H.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** This paper did not receive any specific grants from funding agencies in the public, commercial, or not-for-profit sectors.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **Abbreviations**



#### **References**

