M. N. Ramesh, Director, Talrak Construction Chemicals Pvt. Ltd., Bangalore

Corrosion of steel reinforcement in concrete
Corrosion of steel reinforcement in concrete

Introduction

Reinforced concrete is a composite material. Its structural performance is realised only when concrete and steel act in unison during the service life of the structure. The compressive and tensile loads are carried by concrete and steel respectively. Steel protects concrete from cracking under tensile loads and concrete protects steel from corrosion by providing an alkaline environment around it. So long as this happens, the reinforced concrete structures perform satisfactorily. Even though the concrete can be a very strong material, it is also subject to deterioration. Concrete can be porous so that chemicals can penetrate the pores and attack the paste. The paste and aggregate can also be worn down by physical impact and abrasion. Water can penetrate concrete, freeze and expand inside it when the temperature drops, and ultimately weaken the concrete from within. In addition, if the concrete has reinforcing steel bar (rebar) to impart additional strength and other properties, the rebar can corrode if moisture, oxygen and chloride ions penetrate the concrete. Corrosion or rebar contributes to the deterioration of concrete.
 

Various external hostile environmental substances, such as, water, carbon dioxide, oxygen, chlorides, sulphides and biological organisms are transported from the atmosphere into the concrete and attack steel and concrete in different mechanisms causing premature deterioration of reinforced concrete, challenging its durability and resulting in premature failure of the structures. The entry of the harmful agents can be restricted and or avoided by providing the barrier surface coatings on the concrete.Thus conservation of structural integrity can be achieved during its service life.

Corrosion of reinforcing steel due to carbonation
Corrosion of reinforcing steel due to carbonation

Deterioration Mechanism


There are various mechanisms of concrete deterioration. But the present discussion on the topic of protective coating focuses only on the factors influenced by the atmospheric exposure of reinforced concrete. Among them, the very common ones are carbonation, chloride attack and attack by sulphides.
 

Protective Coatings

Sulphate Attack for Concrete
Sulphate Attack for Concrete
Protection of concrete should actually begin at the conceptual stage and meticulous strategies be adopted for protecting the concrete from both internal and external environments. Various coating materials and application methods for concrete surface repair and strengthening have been developed. However, selection criterion for these materials has not been established yet. Selection procedures of concrete coating materials must focus on deteriorating mechanisms diagnosed carefully by the conditions of the target structures. For instance, in case of salt damage, repair policy should consider corrosion environment and deteriorating condition to determine symptomatic indications such as (1) removal of permeated chloride ions (2) penetration block of chloride ions moisture, and oxygen (3) derusting of rebar (4) corrosion-control method (coating or potential control). However, it is not still clear to determine which is the best material and coating system, because there is not enough durability data to estimate.

Components of Coatings

All organic coatings consist of three basic components:
  • Solvent
  • Resin
  • Pigment.
Figure 1: Components of Coating
Figure 1: Components of Coating
Not all coatings contain solvent and pigmented components. There are solvent-free (100 per cent solids) coatings and clear, pigment-free coatings, but not resin-free coatings. Coating chemical formulators commonly group solvent, resin, and pigment components into two general categories. The first category combines the solvent and the resin together. The solvent portion is called the “volatile vehicle,” and the resin portion is called the “non-volatile vehicle.” The combination of the solvent and the resin, where the resin is dissolved in the solvent, is called the “vehicle.” The second category is the pigment. Pigments are additives that impart specific properties to the coating and are subdivided into two general categories: (1) colour and (2) inert and reinforced. Figure 1 illustrates the relationship of these components. When a coating is applied, the solvent evaporates during the curing process, leaving only the resin and the pigment components on the substrate. The remaining resin and pigments are sometimes called the “coating solids,” and they form the protective film for corrosion protection.

Solvent

Organic solvents are formulated into coatings to perform three essential functions:
  • Dissolve the resin component
  • Control evaporation for film formation
  • Reduce the coating viscosity for ease of application.
Solvents will also affect dry film adhesion and durability coating properties. In general, resins that are less soluble will require either more solvents or stronger solvents to dissolve the resins. The terms “solvents” and “thinners” are often used interchangeably, but there are distinctions within and between the two terms. The term “solvent” can imply two different usages: (1) the solvent or solvent blends in the coating formulation at predetermined concentration levels; or (2) cleaning solvents in strong concentration strength for cleaning brushes, rollers, hoses, and other equipment. The usage of the term “thinner” (a thinner is a solvent) is most often associated with the coating applicator adding a thinner to a coating container (normally about 475 ml thinner to 3.75 litres of coating) to reduce the viscosity for ease of application. Adding thinner to a coating in the field is often called “field thinning.”

Resin

The resin (frequently called binder) is the film forming component of a coating. Resins are typically a high molecular weight solid polymer that forms large repeating molecules in the cured film. The primary purpose of the resin is to wet the pigment particles and bind the pigment particles together and to the substrate (hence, the term “binder”). The resin imparts most of the coating properties The various types of resins formulated in a coating will display distinct properties. These properties are:
  • Mechanism and time of curing
  • Performance in service exposure type
  • Performance on substrate type
  • Compatibility with other coatings
  • Flexibility and toughness
  • Exterior weathering
  • Adhesion
  • Breathability (water vapour transmission- ability)
No single resin can achieve a high degree of success in meeting the above coating properties with wide variations associated with each property. Therefore, generic coating types are generally classified by the primary resin type used in the coating formulation. Typical resins are acrylics, alkyds and epoxy polymers.

Pigment

Pigments are insoluble and are the heavier solid portion of a coating that typically settles to the bottom of the container. Pigments are additives to the coating formulation that impart specific properties to achieve the desired film properties. Pigments impart colour and aesthetics to the coating.

Coating Types

Coating types can be classified under three systems. Unlike coatings for steel substrates, protective coatings for concrete do not in most cases require or include inhibitive or sacrificial pigments to provide protection. Coatings applied to concrete are typically barrier coatings. They provide protection by becoming a physical barrier, or shield, isolating the concrete from its immediate environment. A barrier coating must prevent aggressive liquids and gasses from passing through it and reaching the concrete.

Barrier —A coating that forms a barrier between the concreter surface and prevents the entry of harmful agent into the concrete body by many of the transport mechanisms such as absorption, capillary suction, diffusion etc. are essentially film forming agent on curing. Examples are acrylics, epoxies, coal tar epoxies etc. One of the important properties of a barrier coating is the permeability. The permeability of a barrier coating’s film depends on its moisture vapour transmission (MVT) rate. The MVT rate is determined by how fast water molecules pass through and move around the spaces between the resin molecules. The effectiveness of a coating in preventing permeation depends on how closely and tightly bound the molecules of the resin are to one another. The coating’s effectiveness also depends on the type of resin molecule and the amount and type of pigment. Cross-linking is a measure of the degree of intense bonding of coating resins. The lower the permeability of a barrier coating, the more protective the coating is. Basically, the higher the degree of the coating resin’s crosslinkage, the lower the permeability, the better the adhesive bond of the coating to the surface, and better the overall protective barrier. These intermolecular spaces between the resin molecules are much larger than the water molecules and should not be confused with physical holes (pinholes) in the coating film. Pinholes in the coating film are generally considered defects and should be repaired. Spaces between resin molecules are not defects. The barrier properties of coatings can be improved by adding reinforcement fillers to the resin. Fillers come in a variety of forms, such as silicate aggregates (sand), glass ormica flakes, fibers, ad woven fiberglass (incorporated as a mat in the resin system as it cures). The addition of fillers physically increases the length of the path that the intruding liquid or gas molecules must take to penetrate the coating. Flake materials form layers of overlapping platelets, parallel to the concrete surface, somewhat like shingles on a roof. Fillers and fiberglass mat can also be added to improve the barrier coating’s physical properties, such as impact and abrasion resistance.

Inhibitive - A penetrant or a primer that is slightly soluble in water or a solvent that forms a chemical inhibitor and effectively coats the walls of the capillaries in concrete. They generally impart properties like hydrophobicity for the concrete but allow the water vapour transmission through them. Examples are Silane –siloxanes etc.

Galvanic - Zinc-rich primer coatings that provide galvanic or cathodic protection to ferrous metal (zinc sacrifices itself to protect the ferrous metal). Galvanic coatings are effective only if applied directly to bare metal. They prevent formation of incipient anodes in the reinforcing steel in a typical patch repair situation.

The following generic coatings and general descriptions are typically specified by the consultants.

Acrylics - In water-borne acrylic coatings, the resins are dispersed in water to form a water emulsion.

Water - borne acrylics are specified for atmospheric exposures as a primer or top coat and have excellent colour and gloss retention. Acrylics cure by coalescence. They are breathable and UV stable and a good barrier for carbon dioxide.

Alkyds - Alkyds are normally natural oils (soya, tung, styrene) that have been chemically modified to improve cure rate, chemical resistance, and hardness. Phenolic modified alkyds are specified as a primer, and silicone alkyds are specified as the topcoat for atmospheric service exposures primer, as well as the topcoat for atmospheric service exposures especially for metals. They are not suitable for alkaline (concrete or masonry) surfaces or environments. Alkyds cure by air oxidation of drying oils.

Bituminous - Bituminous coatings are heavy-bodied materials applied with a cutback solvent. They have good moisture barrier resistance, and fair to good chemical resistance but are not resistant to solvents. Commercial bituminous products are specified on a limited basis by consultants for protection of aluminum surfaces in contact with cementitious material or steel and copper cable weld connections. Bituminous coatings cure by solvent evaporation.

Epoxy, Amine - Amine epoxies are two-component coatings that are catalyzed (hardened) by an amine curing agent to produce a hard, tightly bonded, chemical resistant (alkali, acid, and solvent) product, but they are moisture and temperature sensitive during application. They are specified for burial and immersion service exposures, but they will fade and chalk in direct sun light. Amine epoxies cure by chemical reaction.

Epoxy, Polyamide - Polyamide epoxies are two component coatings that are catalysed by a polyamide curing agent to produce superior resistance to water and salt solutions, but they do not provide the chemical resistance of the amine epoxy. Polyamides have a greater flexibility than the amine epoxies. They are specified for burial and immersion service exposures, but they will fade and chalk in direct sun light. Polyamide epoxies cure by chemical reaction.

Epoxy, Coal Tar - Coal tar epoxies are generally an amine or polyamide epoxy modified with coal tar pitch resin to produce a high-build film that has good chemical resistance and excellent water resistance. They have a tendency to become brittle with age and delaminate between coats or beneath repair patches. They are specified for burial and immersion service exposures, but they will fade and chalk in direct sunlight. Coal tar epoxies cure by chemical reaction.

Epoxy, Fusion-Bonded - Fusion bonded epoxies (commonly called powder coatings) are complete coatings in powder form. There are two application methods, fluidized-bed and electrostatic. In the fluidized-bed method, the metal items are preheated to a fusion temperature and immersed in the powder-epoxy solution. In the electrostatic method, the epoxy powder particles are charged with high voltage, and the metal item is then sprayed. After spraying, the item is placed in an oven to cure at about 180°C to 350°C. Fusion bonded epoxies are specified for reinforcing steel, but they become brittle and fail to protect steel in the long run. They act as a barrier to the steel getting direct contact with the alkaline concrete and deprive the reinforcing steel to have natural protection by the alkaline concrete.

Inorganic Zinc Primers – Inorganic zincs are primers that incorporate a high loading (kg per litre) of metalliczinc for pigmentation (hence, the term “zinc-rich”) and are either solvent or water-based. Depending on the solvent and resins used, the coating may be a zinc-rich epoxy or urethane. These coatings are exclusively primers because they provide galvanic or cathodic protection to steel substrate. Inorganic zincs are specified for atmospheric and immersion service exposures, but they can be top coated to extend their service life. Suitable topcoat material selection is required to prevent out-gassing from the inorganic zinc that produces small pinholes in the top coat. Suitable for steel bars and structural steel section buried in concrete contaminated with chlorides.

Polyurethane - Technically, polyurethane is a subclass of urethane. Two-component polyurethane is created by chemically combining a polyisocyanate and a polyol to produce an isocyanate that has a two mode cure mechanism of solvent evaporation and chemical reaction. Generally polyurethanes are specified for top coating compatible (i.e. same manufacturer) amine and poly amide epoxies to protect against direct sun light or UV and to provide specific colours. Polyurethanes are specified for atmospheric and partial or fluctuating immersion service exposures.

Urethane - Urethane coatings vary widely in formulations for specific service environments and application requirements. Many times, single-component; moisture-cured urethanes are specified. They cure from moisture in the atmosphere and can be applied to damp surfaces that do not have free moisture present. These urethanes are formulated with various pigmentations and are specified in several combinations to suit the intended service exposure. These urethanes are specified for atmospheric, burial, and immersion exposures.

Coatings That Breathe

As discussed earlier, the curing of new concrete often results in the release of substantial quantities of water. If this water is trapped between the coating and the concrete, it can cause the coating to lose adhesion or form blisters. It is sometimes necessary, therefore, to use coatings that “breathe.” These coatings allow water vapour (the gas form of liquid water) to pass through them. However, care should be taken when selecting a more permeable coating to ensure that the service conditions are not beyond the range of the coating. The higher the permeability, the lower is the resistance in preventing water or other chemicals from the outside environment from passing through the coating. It is the coating manufacturer’s and the specifiers responsibility to select and furnish the coating with the right degree of “breathability” and “permeability rating” for the intended service (use) of the coated concrete.

Functional Requirements

Coatings have two main functions: providing protection against harmful agents thereby increasing the durability, and providing aesthetic appearance to the structure. To achieve these functions, coatings should have the following attributes: Table.1 shows the properties of different resins available in the market.

Comparison of Performance of Various Resin Systems
  • Good adhesion to the surface to be coated
  • Resistant to alkalis, as the coatings are applied on alkaline concrete
  • Resistance against CO2, sulphate and chlorides to provide barrier property
  • Good flexibility as the structural members undergo dimensional variation due to cyclic loads
  • Excellent weathering resistance
  • Breathability should allow water vapour transmission through the coating to avoid blistering of the coatings; a durability requirement.
  • Resistance to UV exposure is a durability requirement
  • Low susceptibility to staining
  • Good resistant against growth of fungi, algae moss etc.
Typical Causes of failure of Coating Systems for Concrete
However, it should be noted that all resin materials are not totally resistant and impermeable to all aggressive agents and do not provide a total protection. Chemical/physical degradation of resins and de-bonding of coatings are the major phenomena affecting the durability of surface protection. The mechanisms of destructive processes in such heterogeneous materials as resin composites are complicated and not completely understood. Degradation of resins mainly involves swelling, dissolution and scission of molecular chain bonds. A wide variety of reactions is possible for resin degradation. The transport of gases and liquids aggressive to substrate into or through the coating is the major problem of its delamination. Various mechanisms of deterioration of resin composites and coatings are summarized in Table 2. There are many parameters that influence the deterioration process of coatings, such as chemical agents, temperature, solar radiation, pressure, abrasion, cyclic temperature-moisture changes etc. All these parameters can occur simultaneously or they can be complementary to one another.

The barrier may be subjected to continuous exposure or intermittent contact occasioned by splash, spray, or accidental wetting with aggressive substances. Usually, chemical/physical degradation and de-bonding of coatings is the major problem of deterioration leading to their cracking and delamination. On the basis of the degradation processes the basic requirements for protective coatings of concrete structures in aggressive environments can be formulated. They are as follows:
  • Resistivity to chemical/physical actions
  • Low permeability to water, solutions and gases
  • Good bond to concrete
  • Sufficient flexibility to avoid cracking caused by thermal or mechanical movements
  • Similar physical properties of the overlay material and underlying concrete
  • Adequate abrasion or skid resistance.
  • Resistivity to chemical action of concrete and humidity in concrete.
  • Bridging of fine cracks in concrete

Principles of Protective Coatings (Design)

The design of an appropriate protective system for new or existing structures is a complex process involving:
  • Identification of service environment of the particular structure in the original design
  • Identification and assessment of the condition state and deterioration (if any) of the existing structure
  • Selection of the appropriate prote- ction system
  • Definition of coating parameters: type of binder, formulation, covers thickness
  • Anticipated time between periodic recoating.
The performance/efficiency of any coating depends on its chemical/ physical resistance to disintegration, permeability, extensibility, mechanical resistance (example abrasion, punching), and adhesion to concrete. Traditionally codes and recommendations contain requirements for structural design in terms of resistance – load format. Similarly to that design concept, the design of coatings must be developed on the basis of deterministic or probabilistic analysis taking into account the aggressive environment as an action and coating performance as resistance. In particular, aggressive actions as well as material and geometric properties of coatings may vary substantially. Limit state functions can be represented in resistance or lifetime format:

g(t) = R(t) − S(t) = R0 R (t)θR − S(t)θS ≥ 0, g(t) = tθt − td ≥ 0, for all 0 < t ≤ td

Where
g(t) is the margin of safety with
g(t) > 0 denotes safe and
g(t) ≤ 0 denotes failure;
R0 is protective barrier capacity in the un-degraded (original) state;
R(t) – degradation function;
θ − uncertainty of the calculation models and errors in data observation and recording;
t – the time of assessment;
td − the design or target service life.

Once the limit state functions have been developed, the reliability of coating can be evaluated. The reliability verification of coating in relation to a given mode of failure in a given period of time may be defined as:

P{t} = P{g(t) ≥ 0} = P{R0, R (t)

θR ≥ S(t)θS} ≥ Pt arg, for all 0 < t ≤ td, (29)

Where, Pt arg is an acceptable level of structural reliability. The service life of the coating is defined when the reliability falls below an acceptable level. Different resins respond in a different way to the influence of aggressive environments. The time dependent monotone decreasing degradation function R(t) can be expressed in different forms (linear, parabolic, quadratic, etc.) with the following boundary conditions.

at t = t0, R(t0) = 1,0, at t = td, R(td) = min.

The design of resin coatings requires checking of their performance in general using the following four conditions:
  1. The condition determining the chemical/physical resistance with R(t)
    = R0φR(c0;t) and S(t) = Rmin, where, R0 and Rmin are initial and minimum acceptable resistance of coating, respectively; φR(c0, t) – degradation function of coating in a given exposure c0 after time t;
  2. The condition determining the penetration through the coating with R(t) = ccr and S(t) = c(dpc,t), where ccr and c(dpc;t) are the critical and expected concentration of aggressive substances, respec- tively, on the surface of the concrete;
  3. The condition determining the cracking of the coating with R(t) = fpt(t) [εpt(t)] and S(t) = σmax (εmax), where fpt(t) [εpt(t)] is tensile strength (strain) of resin and σmax (εmax) is maximum stress (strain) in coating;
  4. The condition determining the delamination (separation) of the coating with R(t) = τcon [KIc(t)] and S(t) = τmax [KIcor(t)] or R(t) = Dcr and S(t) = D(t), where KIc(t) and KIcor(t) are critical initial and after exposure in aggressive environment stress intensity factor, respectively; Dcr and D(t) is critical and expected degree (area or %) of delamination, respectively.

Concluding Remarks

  1. Concrete is a porous material having high gas, vapour and liquid permeability leading to deterioration of reinforced concrete structures. One of the ways to protect RC structures from corrosion is to use protective coatings. Frequently, the coating is the main option to protect the concrete structures in service. Many coating materials are not totally resistant and impermeable to all aggressive agents. It is necessary to well understand the mechanism of degradation of coatings materials to enable design of coatings with required barrier properties.
  2. The degradation of polymers is a complex interaction of physical and chemical processes leading to breakdown of its chemical structure as well as cracking and debonding of protective coatings. Classification of coating degradation has been done based on the nature of action.
  3. The mechanisms of degradation of coatings caused by aggressive actions are well understood and predictive models for the deterioration over time have been developed which can be applied to design of surface polymer coatings, to preserve concrete structures against deterioration.
  4. Design of coatings is based on deterministic or probabilistic analysis in the resistance load format aggressive environment as an action and coating performance as resistance. Such a design of protective coatings is available in a simple form for engineering design purposes.
  5. There are a large number of materials and systems in the market that claim a variety of properties. Experience shows that the processes of coating deterioration and the loss of protection ability are very complicated. The physical and chemical reactions for each protective system in particular environments have to be determined experimentally.
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