Home Articles New Concrete Protection: Solution to Enhancing Service Life of RCC Structures against Corrosion

Concrete Protection: Solution to Enhancing Service Life of RCC Structures against Corrosion

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Concrete Protection
Samir Surlaker
Director, Assess Build Chem Private Limited

Introduction

RCC is one of the most versatile and widely used construction materials on account of its formability and designable durability. The concrete is said to be durable when it can withstand design exposure conditions, over its service life with little deterioration. Durability is a function of the quality and quantity of cover to the reinforcement. Considering practicality of construction coupled with completion deadlines, it becomes virtually impossible to cast specified cover in terms of quality and / or quantity. The outermost layer of concrete therefore becomes the weakest one. A combination of the porous nature of concrete and a weak cover leads to diffusion of detrimental materials like water, CO2, chlorides and SO2 into concrete. These materials or a combination thereof, lead to loss of passivity and corrosion of the steel and ultimately deterioration of the concrete, which leads to weakening of the RCC structure as a whole.

Durability of concrete can be greatly enhanced by use of apt protective coatings that can form a physical and a chemical barrier against ingress of water, chlorides, CO2 and SO2. Paints and other decorative coatings are designed for aesthetic purposes and normally not for protection of concrete against carbonation, chloride attack, UV radiations and for crack bridging properties. Using an appropriate concrete protection incorporating these properties will help in mitigating corrosion of reinforcement in RCC.

The Process of Corrosion of steel in Concrete / Loss in durability

Concrete is a material composed of different ingredients and is therefore non-homogenous. The cover concrete by definition has many non-visible micro cracks, which act as avenues for water and gas penetration. If the permeability of the formwork is not controlled, it is stated that, the water cement ratio is 0.10 more and the cement content is about 45 kg/m3 less compared to the original concrete mix and therefore the cover is most vulnerable to attacks.

In hardened concrete, the pH values in the matrix reach about 12.5. Thus, the reinforcement in the concrete remains in a passive condition and does not corrode. Once water, chlorides, CO2 or SO2 enter the concrete, the process of depassivation and corrosion begins. The change of concrete pH due to carbonation mechanism is given in Figure 1.

Figure 2 shows Tuuti’s 2-phase model of corrosion. Corrosion is characterized by Initiation and Propagation. In the context of protective coatings, if we can protect the structure, when new then the initiation stage can be substantially delayed. In case of old structures, where corrosion has initiated and is probably propagating, application of the correct protective coating will help stop the further propagation of the structure, thereby increasing its lifespan.

Corrosion Protection by use of Anti-Carbonation Coatings

To prevent corrosion, we need to remedy the concrete cover both in terms of quality and quantity. Concrete Protection can be carried out based on two principles:

Principle 1: – Set up an additional concrete cover that is sufficient in density and thickness (like jacketing) OR

Principle 2: – Leave the cover as it is or set up a minimum required cover and apply a surface protection system.

Figure 3 shows both the principles. The application of protective coating is an attempt to increase the effective cover of concrete in terms of both quality and quantity. By virtue of their formulations, anti-carbonation protective coatings provide the protection quotient of meters of concrete cover in a very thin layer. Figure 4 shows this principle with available materials. Therefore, the principle of equivalent cover is very valuable both in repair strategies and even to safeguard new structures that have a very long design life. Specialized protective coatings can be of many types:

  • Thick Coatings (1-2mm): Like Different Breathable waterproof cementitious or mineral based polymer modified coatings
  • Breathable, Elastic Elastomeric Crack bridging coatings (200-300-micron thickness)

Several benefits can be obtained by incorporating the surface treatments in a well-designed manner right from conception of project. Although the main function of any surface protectant is moisture ingress control either by physical barrier concept or conversion of capillaries to hydrophobic, the coatings can also be designed for resisting chemicals. Chloride ingress can be effectively controlled by surface protectants. Further, the diffusivity of carbon dioxide, sulphur dioxide and oxygen can be lowered for corrosion control. Root and vegetation growth can be prevented in concretes under damp conditions.

The challenge here is to choose the right coating for the right application. For e.g. marine structures, wastewater treatment structures, wet areas in bathrooms and building construction, terraces, interior walls of water tanks, etc. call for thick protective coatings. Exposed concrete structures like bridges, power plants, fair face finished building walls, external faces of water tanks, etc. call for special anti-carbonation coatings.

Properties Needed in Coatings for Concrete in General Applications

The coatings to be applied on concrete should not be selected solely for aesthetics but also on protection criteria. It is possible to have both aesthetic and protection judiciously combined in a single well-designed coating. Figure 5 shows the properties and requirements of general protective coatings for concrete and cover the concrete protection solutions in the section above. According to international literature the following properties, are most essential for an excellent concrete protection system.

 

 

Breathability

It is that property which enables the water vapor to move in and more importantly out of the concrete with the fluctuation of temperature and humidity. The water in concrete (or for that matter any porous media) tends to go from wetter to drier sections. Breathability is essential because, moisture movement in concrete may lead to osmotic pressure building up under non-breathable coatings and eventual debonding of the protective coating. Breathability also enhances drying of the internal concrete, arresting corrosion. Mineral based coatings or high-performance acrylic polymer modified anti-carbonation elastic elastomeric coatings, provide much higher breathing capacity than most resin-based coatings, and thus can protect structures longer.

Impermeability to Water, CO2 and other gases

Protective coatings should be permeable to water vapor, but impermeable to water. Though these properties look contradictory they must exist complimentarily. Most coatings do possess waterproofing characteristics but are not breathable. This is the balancing point separating protective coatings from normal paints. In addition to water, it prevents ingress of corrosion causing gases into the concrete. Overall breathability and impermeability combined should form a protective coating not very unlike human skin. Good protective coatings provide a sound physical and chemical protection barrier to prevent corrosion in concrete, ensuring durability. These properties can be verified by adequate testing to ensure quality of coatings.

Crack Bridging

This is an important property as the coating should be able to negotiate expansions and contractions in the structure due to temperature or dynamic loads. It should also effectively seal and bridge over cracks, to ensure no water or gases enter concrete through them. Even after extensive weathering, protective coatings should provide excellent crack bridging capacities in a variety of static or dynamic loading conditions, in varying exposure conditions and even on weathered concrete surfaces.

Ultraviolet (UV) rays resistance

For exterior application it is mandatory that the coatings should be UV stable. The effect of sunlight can cause certain polymers to become brittle and lose their physical and chemical properties. Exposure to sun can also lead to fading of pigments and differential colouring marring the beauty of the structures. Correctly designed protective coatings remain flexible even after exposure to sunlight without degrading.

Specifying the Protective Coating

Rather than specifying coatings by generic names and film thickness it is preferable to specify them by performance requirements. Some performance properties that can be normally specified are:

  1. Diffusion resistance of CO2
  2. Diffusion resistance against H2O vapour, when designated as breathable coatings.
  3. Resistance against water penetration
  4. Resistance to chlorides (ponding)
  5. UV Resistance
  6. Crack Bridging ability under weathered conditions

The ability of coating to resist the ingress of CO2 and inhibit the carbonation imparts the designation of Anti carbonation Coating. The diffusion is measured using Fick’s Law of Diffusion. For the sake of expression, diffusion is the extent a coating is impermeable compared to air. This is expressed as  CO2.Thereafter the equivalent air layer thickness “R” is calculated as:

R =  mCO2 x S                                                                     {1}

S is the film thickness in meter and here comes the involvement of film thickness. For different formulations, mCO2 can be very different. This is the main reason why dry film thickness alone cannot be criteria for coating selection and holistic specification should involve all three parameters.

DIN EN 1504 specifications state R should be greater than 50m. Taking an example of an acrylic protective coating  mCO2 = 3.02 x 106 for a Dry Film Thickness of 162m. The equivalent air thickness ‘R’ can be calculated as:

R = 3.02 x 106 x 162 x 10–6 = 489 m

which is > 50 m. under such conditions the specifier can reduce the film thickness to 125m and arrive at a value:

R = 3.02 x 106 x 125 x 10–6   = 377 m

which is still adequate. Therefore, the carbonation protection quotient depends not only on film thickness in isolation but are directly related to mCO2. Many a times for the sake of clear understanding of CO2 diffusion, this value can also be expressed as equivalent thickness of concrete layer or equivalent cover, which would have same resistance. Equivalent thickness of concrete expressed as

                                          (2)

For a concrete of 30 N/mm2 average m value is approximately 400. So, for the same acrylic coating mentioned in the above examples with a Dry Film Thickness of 125m  and R = 377 m

which means an equivalent cover of 95 cm. This is the correlation between the CO2 diffusion coefficient, equivalent air layer thickness, film thickness and equivalent concrete cover.

Further it is imperative that anti carbonation coating also has to be breathable, otherwise the internal moisture will lead to active corrosion in concrete but also due to fluctuation in temperatures, moisture trapped in concrete will damage the coatings with pore pressure of water in capillaries. This value is expressed as follows: g/ m2 / day or cm2/s as diffusion coefficient vapour translation rate. If expressed as equivalent air layer thickness for the same coating as indicated above for which m H2O is 12.8 x 103 considering Dry Film Thickness of 163 m, equivalent air layer thickness.

Sd = mH2O x S (in meters)

= 12.8 x 103 x 163 x 10-6

= 2.08 m < 4 m (requirement)

for a Dry Film Thickness of 125m, the calculation will be

Sd = 12.8 x 103 x 125 x 10–6 = 1.6 < 4 m

indicating that is adequately breathable. DIN EN 1504 Part 2 stipulates that the SdH2O < 4m.

In addition to these two characteristics, water absorption should be specified. Eventually when dealing with structures which vibrate like bridges the coating must vibrate with dynamic of structure and be elastic elastomeric with crack bridging capacity. Crack bridging capacity is a function of elasticity as well as film thickness and this must be also specified.

Specifying protective coating is one of the difficult function qualified architects and engineers have to perform. There are so many unknown factors to be considered, it becomes almost difficult to specify single coating. Some resort to generic raw material bases while others gives more weightage to wet and dry film thickness. This creates ambiguity, since a high-quality coating may perform better even at lower film thicknesses and when minimum thicknesses are specified, they become uneconomical while in reality they may be cheapest with respect to performance.

First and foremost, the specifier must clearly understand the protection requirements. It is therefore definite that specification will change form project to project. Specifications should also depend upon the life span, durability and protective quotient with respect to time. Cost calculations must be time dependent.

A lot of confusion occurs on specifying the dry and wet film thicknesses on concrete surfaces which has relatively high undulations. It is difficult to measure the Dry Film Thickness. There are instruments such as Paint Identification Gauges, etc. which can measure the Dry Film by cutting but accuracy is limited when coating thickness is in the range of 200 – 500m. The easiest way is to measure the Wet Film Thickness (WFT) of the coating. Knowing the WFT and solid contents by volume of protective coatings, the Dry Film Thickness (DFT) can be easily calculated.  The formulae stated {4 and 5} provide the basis of calculation for theoretical coverage rate, once volume solids and Dry Film Thickness are known:

Dry Film Thickness (DFT) in microns = Wet Film Thickness in Microns x % Solids by Volume ….{3}

           (4)

          (5)

Further referring to Table 1 in which relationship between solid contents by volume and dry and set films are stated, coverage can be calculated as under:

Example:  Volume solids 90% and required DFT is 125 µm

Therefore a 139 µm wet film thickness is required to achieve 125 µm DFT at 90% volume solids.

which gives a theoretical coverage rate of 7.2 m2/litre at a wet film thickness of 139 µm.

Rather than specifying coatings by generic names and film thickness it is preferable to specify them by performance requirements.  The generic name can be a stated as part of specification, viz. acrylic as it indicates that coating is resistant to UV radiations by virtue of its raw material base and it is proven to be the best with respect all the qualities as well as economy. The performance properties normally specified are diffusion resistance CO2 and Diffusion resistance against H2O vapour, when designated as breathable, anti-carbonation coatings. The equivalent air thickness ‘R’ in metre should be > 50 m and water vapour diffusion which is also expressed as Diffusion equivalent Air Layer thickness Sd which denotes breathability is normally expected to have a value < 4 metres. It is also expressed as g/m2/day in other specifications.

Conclusion

The protection of concrete against corrosion should start right at the time of design and strategies have to be decided for internal as well as external protection of concrete. External protection, which normally includes surface sealers, impregnations or anti-carbonation coatings, work on the principle of providing equivalent extra cover. The two major factors that induce corrosion are water, chlorides or CO2; therefore, the surface protection should primarily prevent their ingress. In addition, the coating should let the concrete breathe, as entrapped moisture in concrete trying to escape can offset the protection process. Only less permeable concrete, proper provision of cover and additional protection strategies can guarantee durable concrete and structures.

Studies reveal that protected concretes are definitely more durable than the unprotected concretes. The depth of carbonation is much lower in protected concrete for a period of time signifying with durability. Under given conditions of severe exposure to aggressive atmospheres, sunlight, rain and extreme temperatures, it seems that acrylic elastomeric coatings and mineral based breathable coatings ensure the best results.  The breathing capacity is the most important characteristic especially for newly laid concretes. For dynamically loaded structures, crack-bridging characteristics subjected to several test cycles is mandatory.

Consider internal moisture before application of protective coating to prevent corrosion damage or chloride contamination in concrete. If the internal relative humidity exceeds 50% one has to select the coating with a high vapor transmission rate. If the relative humidity is 85% it is preferable to avoid coating and to protect the concrete by penetrating sealers and impregnation. Excessive moisture interferes with proper bonding of elastomeric coatings and can cause blisters during high temperature variations. It is very important to control the wet thickness and dry thickness of the coated film as this factor influences crack bridging. The criteria for selection of coating should be primarily protection and secondly aesthetic.

Concrete protection treatments have both decorative and protective characteristics and periodical maintenance guarantees the durability of the structures. Performance Specification should replace prescriptive specifications to allow for comparisons with respect to protective quotient and economy.

Reference

  • Emmons, Peter H. Concrete repair and Maintenance – Problem analysis, repair strategy Techniques
  • Tracy, Robert G and Fling Russel S, Rehabilitation strategies, Repair and rehabilitation of concrete structures
  • Warner, James, Selecting Repair Materials, Concrete Repair and Restoration
  • Patch Repair of reinforced concrete, Technical Report No.38, Concrete Society, London
  • Polymers in Concrete, Technical Report No. 39. Concrete Society, London
  • Defects in building, Department of the Environment, Property Service Agency, UK
  • Stieker, Peter Puler, Corrosion damaged structures, SCIRIA, UK

Author’s Bio

Mr. Samir Surlaker is a Civil and Structural Engineer from VJTI having over 40 years of National and International experience and exposure in Concrete Technology, Construction Chemicals, Repair and Rehabilitation Strategies, Injection Systems, Waterproofing, Flooring, etc. in Germany, Europe, Middle East, Far East and India. He is currently Director of Institute for International Talent Development and is also Director at Assess Build Chem Pvt. Ltd. Prior to this he retired as the Managing Director of MC-Bauchemie (India) Pvt. Ltd., in March 2016.

Expertise in Materials Technology, Civil Engineering Applications and in Training and development. He served as Vice President of Indian Concrete Institute, Director of India Chapter of American Concrete Institute, Governing Council Member of Association of Consulting Civil Engineers, India and Past President of Construction Chemicals Manufacturers Association. He is currently advisor to CCMA and is a Hon. Seretary of IIT Arb. Maharashtra Centre.

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