Degradation and deterioration of concrete structures occurs due to exposure to different environmental conditions and agents, such as carbon dioxide and chlorides, and methods that prevent such agents from permeating concrete surfaces are critical to maintaining the concrete’s durability.
The deterioration of reinforced concrete structures is caused by the penetration of external agents such as carbon dioxide or chlorides and there are several different methods of mitigating it, including cathodic protection, electrochemical-based techniques, migrant corrosion inhibitors, and surface treatments.
Cathodic protection is generally applied to concrete used in infrastructure and requires periodic inspections and appropriate design to avoid insufficient protection, overprotection, and adverse side effects such as hydrogen embrittlement and stress corrosion cracking.
Design techniques to prevent the damage of reinforced concrete foundations include proper selection of foundation depth, construction of a gravel layer below the foundations, construction of proper drainage and adequate moisture and water insulation, and the proper placing and curing of concrete.
During the concrete mixing process, it is important to consider the degree of exposure the concrete has to external substances, while various admixtures can be used in the concrete to enable water-tightness and frost resistance.
While electrochemical methods and migrant corrosion inhibitors have undergone several studies, they are still emerging developments and not widely available for protective concrete applications.
However, concrete surface treatments are widely used to delay the degradation of reinforced concrete structures, with studies over the decades confirming their effectiveness at preventing the ingress of external agents.
Concrete treatments include a variety of protective agents such as hydrophobic silicon impregnation chemicals, epoxy resins, bitumen sealing strips or self-adhesive strips, bitumen emulsions, synthetic membranes, hydro-insulation cement masses, bentonite barriers, studded membranes, and high-density polyethylene (HDPE).
Surface coatings include those that form a continuous film on the concrete, that impregnate soluble surfaces to create a protective barrier, and the hydrophobic impregnation of surface constituents to make water-repellent concrete.
As a film on the surface of concrete, the coating acts as a physical obstacle limiting the penetration of substances into the concrete, and are usually produced as either polymer-based or cementitious coatings.
Polymer-based coatings include epoxy resins, acrylics and polyurethanes, all of which form a dense polymeric film on the concrete surface with a thickness of about 0.1 to one millimetre.
Cementitious coatings are cement-based mortars manufactured with a huge number of polymers (epoxy, polyurethane or acrylate) that form as a low-permeability cement-based layer that is about two to 10 millimetres thick.
The addition of polymers improves the mechanical properties of the concrete, such as tensile strength, adhesion and resilience, as well as the chemical resistance and impermeability of the cement mortar.
In the Concrete Reinforcement Degradation and Rehabilitation publication, Professor Yuli Panca Asmara explained that concrete modification techniques, such as the use of geopolymer concrete and the application of inhibitors, offered alternative approaches to enhancing the performance and longevity of reinforced concrete structures.
Prof Asmara wrote: “Coatings applied to the concrete surface offer an additional layer of protection – thickening the concrete cover, the layer of concrete surrounding the reinforcement, is another effective strategy for reinforced concrete protection.
“By employing these various methods and techniques, reinforced concrete structures can be adequately protected, ensuring their longevity, structural integrity, and resistance to corrosion.”
Emerging graphene oxide protective applications
A recent innovation in the development of protective coatings is the use of nanomaterials, which have properties that enable superior penetration through the cracks and pores in concrete.
In particular, graphene oxide has shown great potential due to its characteristics such as strength, high surface area, and dispersibility in water, as well as being cheaper than comparable carbon nanomaterials.
Research published in Coatings in 2023 assessed the use of graphene oxide in protective coatings, with the objective of establishing the optimal amount of nanomaterial to use while keeping its degree of protection intact.
Key parameters examined were water transmission, resistance to freeze/thaw cycles, and resistance to carbonation, all within a defining parameter of concrete durability.
The study found that the surface concentration and amount of deposited graphene oxide was directly correlated to its protective efficacy, but the optimal amount required varied between parameters.
The authors explained: “If the concrete is going to be exposed to less aggressive environments, the application of a mild surface coating of graphene oxide is sufficient for its protection.
“If the concrete is going to be exposed to more aggressive environments, such as carbonation, [then] it is necessary to increase the amount of graphene oxide to maintain an adequate protection of the concrete.”
They also noted that graphene oxide dispersion was commercially widely available and relatively affordable, being the cheapest graphene derivative and mostly produced in Spain and elsewhere in Europe.
The authors added: “Taking into account the economic cost of this product, the optimisation studied and the protection it provides to concrete, graphene oxide can be accepted as an excellent surface treatment, which can be practical for large surfaces or specific applications due to the small amount used during its application.”
Another study examined graphene oxide’s protective effect, with a focus on its ability to prevent the penetration of water and other ions using three application methods: brushing, spraying, and submerging.
Water absorption and permeability tests showed that graphene oxide coatings reduced the volumetric and capillary intake of water in concrete samples by 40 per cent and 57 per cent, respectively.
Importantly, the researchers found that when the graphene oxide content of a surface coating is increased, both the volumetric and capillary absorption of concrete was lower.
Graphene oxide is also being investigated as a potential concrete additive, but has limitations as adding graphene oxide to cement-based materials results in decreased workability, due to its relatively large surface area that tends to absorb water.
However, research has shown that the inclusion of a small amount of graphene oxide (about 1 per cent by weight of the cement) enhances compressive strength, while the use of about 0.5 per cent graphene oxide increases both the compressive and flexural strength of concrete.
