CONCRETE COATINGS Why Coat Concrete A concrete surface

CONCRETE COATINGS

Why Coat Concrete ? A concrete surface is porous, subject to wear and tear, and frequently exhibits chemically induced weathering process. The durability of concrete can be enhanced by application of protective coatings. Surface treatment can also enhance the overall appearance of a concrete surface. There could be three main reasons for coating concrete structures: 1. To keep moisture in concrete during curing– coatings to assist curing of concrete( application of curing compounds) 2. To keep moisture and other harmful agencies like chemicals from penetrating concrete during service of structures—protective coatings. 3. To impart beneficial surface characteristics like wear resistance and improved appearance of concrete surfaces. Note: This presentation will deal with coatings for curing of green concrete and protective coatings in the context of protection of Bridges

UNDERSTANDING THE PROCESS OF DETERIORATION OF REINFORCED CONCRETE STRUCTURES Some of the main reasons attributable to porosity of concrete for deterioration of reinforced concrete structures are: CHLORIDE ATTACK Ø Presence of chloride ions leads to corrosion of reinforcing steel. Chloride ions cause localised break-down of the passive film that forms on steel as a result of the alkaline nature of concrete Ø The harmful chloride ions can be originated from the contaminated mix constituents like aggregates, water and admixtures ; or Ø From the surrounding environment like coastal environment Ø Porosity of concrete helps in penetration of chlorides from external sources

CARBONATION OF CONCRETE Ø Carbonation is reaction of CO 2 with hydrated cement. Ø CO 2 is present in atmosphere – about 0. 03% in rural air and up to 0. 3% in large cities ØIn the presence of moisture , CO 2 forms carbonic acid( H 2 CO 3), which reacts with Ca(OH)2 to form Ca. CO 3 ØDecomposition of Ca(OH)2 results in lowering of PH value of concrete, in the process vitiating the protection of steel from corrosion ØCarbonation proceeds from the surface of the concrete but it happens extremely slowly ØThe actual rate of carbonation depends on the permeability of concrete, its moisture content, and on the CO 2 content and relative humidity ( highest at about 50 -60% of RH) of the surrounding medium ØThe extent of carbonation can be easily determined by treating a freshly broken surface with phenolphthalein – the free Ca(OH)2 is coloured pink while the carbonated portion is uncoloured

SULPHATE ATTACK Ø when hardened concrete is exposed to sulphates from external sources, it leads to formation of calcium sulphate( gypsum)-sulphates reacting with Ca(OH)2 - and calcium sulphoaluminate ( ettringite)- sulphates reacting with hydrated C 3 A, both products occupying a greater volume than the compounds. Ø This results in expansion and disruption of hardened concrete ØThe extent of sulphate attack depends on its concentration and on the permeability of concrete

LEACHING AND EFFLORESCENCE Ø If the concrete is very permeable , so that water can percolate right through its thickness, Ca(OH)2 will be leached out. Evaporation at the surface of the concrete leaves behind deposits of Ca(CO)3, formed by the reaction of Ca(OH)2 with CO 2 ØThis deposit of whitish appearance is known as efflorescence ØEfflorescence is generally not harmful , however, extensive leaching of Ca(OH)2 will increase porosity so that concrete becomes progressively weaker and more prone to chemical attack ØCrystallisation of other salts also cause efflorescence

• As can be seen from above discussion, the main property of concrete leading to deterioration due to various types of chemical attacks is the Permeability. • Thus concrete should be produced to achieve minimum permeability. • Permeability tests on concrete should be performed as per provisions of Concrete Bridge Code for Railway Bridges • Protective coatings can de employed with benefit to slow down the deterioration of concrete due to chemical attacks. Such coatings can be justified on the basis of life cycle costing of important structures like Railway Bridges.

CONCRETE COATINGS FOR MOISTURE RETENTION (CURING COMPOUNDS)

What is Concrete Curing Compound? • Curing is essential for the hydration of cement in concrete making. So, to maintain required moisture content, some precautions are necessary. • Concrete curing compound helps to prevent the loss of moisture content from the concrete. So, concrete is properly cured which results in full development of strength of concrete. • Maintaining moisture in green concrete also helps in avoiding plastic shrinkage cracks. • Liquid curing compounds are applied by rolling, brushing or spraying • Curing compounds normally cover about 7 -7. 5 m 2/L for Type I and about 5 m 2/L for Type II compounds • The coatings work by allowing water vapor to pass through them at a controlled rate determined by the chemistry and thickness of the application

TYPE OF COATING REQUIREMENTS COMPOUNDS & GENERAL • In absence of IS code for concrete coatings (membrane forming compounds), ASTM C 1315 -19 can be referred. • The following types of liquid membrane-forming compounds are included in ASTM C 1315 -19 Type I—Clear or translucent, and Type II—White pigmented • Type I coatings shall be clear or translucent and have a minimum of 25 % solids by mass. • Type II coatings hall consist of finely divided white pigment and vehicle integrally ready mixed for immediate use as is and have a minimum of 25 % vehicle solids by mass. The membrane-forming compound shall present a uniform white appearance when applied uniformly to a new concrete surface at the specified rate of application • A minimum vehicle solids content is specified in order to provide an approximately 0. 025 mm thick dry film at the specified rate of application. This film thickness is considered necessary to achieve the desired characteristics.

• White-pigmented concrete curing compound provides a reflectant film that not only retains water but also reflects sunlight to help keep the concrete surface cooler and prevent excessive heat build up, which can cause thermal cracking • Curing compounds shall be of such a consistency that they can be readily applied by spraying, or by brushing or rolling, to form a uniform coating at temperatures above 4°C. • For uniform application in the field on vertical concrete surfaces, the specified rate of application may be achieved by two coats, each applied at one half the normal rate with approximately 1 h drying time between coats or in accordance with manufacturer’s recommendations. • Curing compounds shall not react deleteriously with concrete. Deleterious reactions are detected by scratching the surface of a mortar specimen used for the water-retention test with a knife or screwdriver, not less than 72 h after application, and comparing with the surface hardness similarly determined on a similar specimen that has been moist cured for one half the time. Any softening of the treated surface shall be considered sufficient cause for rejection of the compound

• Testing for deleterious reactions is not needed on a routine basis. However, it must be done when testing compounds of a new or unknown composition • Curing compounds shall be storable for at least 6 months without deterioration • The volatile portion of liquid membrane-forming compounds shall be of materials that are neither toxic 9 nor have flash points of less than 38°C • Commercially available compounds by reputed manufacturers like FOSROC, BASF, SIKA and PIDILITE are generally as per ASTM standards

On BASIS OF CHEMICAL COMPOSITION CURING COMPOUNDS CAN BE CLASSIFIED AS: Synthetic Resin Concrete Curing Compound • Synthetic resins will seal the concrete by forming membrane. If we want to provide plastering, the membrane can be removed by washing it with hot water. Acrylic Concrete Curing Compound • Acrylic is made of polymers of acrylic acid. It also seals the concrete in good manner. It is having property of adhesion to the subsequent plaster. No need to wash the surface of acrylic with hot water if we want to provide plastering Wax Concrete Curing Compound • Wax compound have similar properties like resin compound. The wax membrane will lose its efficiency with time. The wax type curing compound is suitable for use on concrete that will not subsequently be painted, tiled or treated in any manner. The wax contained in the material remains on the concrete surface and hampers the adhesion of future paint or mastic Chlorinated Rubber Curing compound • Chlorinated rubber type curing compound will form thick layer when applied. It seals the concrete tightly and also fills the minute pores present

Properties of Concrete Curing Compound • Following properties may be tested to deicide the quality of concrete curing compound namely (quality tests to be performed as per ASTM C 1315 as no IS code is available) 1. Water retention : Loss of water should not be more than 0. 4 kg/m in 72 hrs 2. Reflectance: for white pigmented curing compounds the daylight 2 reflectance should not be less than 65% 3. Drying period: Liquid membrane-forming compounds shall dry to touch in not more than 4 h.

Uses of Concrete Curing Compound • If wet curing is not possible, then curing compound can be used to cure the concrete surface. • For larger areas of concrete surfaces which are opened to sunlight, wind etc. curing is big task. But with the presence of curing compound it is easier. • A suitable liquid membrane curing compound is capable of maintaining a minimum of 95 percent of the original moisture content in a concrete mix • Curing of concrete pavements, run ways, bridge decks etc. can be cured to reach their maximum strength. • Maximum durability of structure will be developed. • Curing compound can be used for curing of canal linings, dams • Columns, beams, slabs can also be cured with curing compound • ASTM C 309 provides specifications and testing of concrete curing compounds. • https: //www. youtube. com/watch? v=ynn 9 ua. U 7 b. JQ (Short Video on Comparing Curing Compounds)

Process of Applying Concrete Curing Compound • Curing compound is applied when the finishing is completed and free water present on the surface gets disappeared. The curing compound is applied through spraying nozzle with constant rate of pressure. • Generally, one litre of curing compound can be sprayed for 4 5 m 2 surface area of fresh concrete. The sprayer pressure is usually 0. 5 – 0. 7 MPa. In small areas, we can also use brushes or paint rollers to apply curing compounds. • Curing compound should not be applied on surfaces which receive additional concreting. • Concrete curing compounds form a membrane when applied to fresh concrete. This member does not allow the inside moisture to come out of concrete hence, curing occurs. • The curing compounds possess waxes, natural resins, synthetic resins and solvents of high volatility. Generally white or grey colour appear when we apply curing compound on fresh concrete.

CONCRETE COATINGS FOR DURABILITY (PROTECTIVE COATINGS)

NEED FOR PROTECTIVE COATINGS : • Although concrete is very durable under sunlight and potable water, it is subjected to several types of chemical attack that can degrade the surface or cause loss of structural integrity • Concrete is a porous material and can absorb water, sodium chloride, carbon dioxide, acid precipitation, and other chemicals, in liquid or gas states from the surrounding environment that cause concrete degradation. • The service environment may also produce mechanical damage such as impact, wear, and erosion. • The type of service environment (location) is a key parameter to establishing coating performance criteria. The temperature range, humidity, and contaminants found in the surrounding environment are all considerations • The prevailing service environment, the expectation for the life-cycle cost of the structure and the aesthetics are all key parameters in selecting coating systems.

• Concrete bridges are subjected to a wide range of exposure conditions dependent upon their location, and the specific corrosion proneness of the structure. • Water can penetrate naturally through the capillary pore structures of concrete. Carbon dioxide reacts with calcium hydroxide in the pore liquid of the cement matrix and deposits as calcium carbonate, which results in carbonated concrete • Chlorides come from both de-icing salts that may be used in winter, or from salt water in marine environments. The expansion of free water in the capillary pores of concrete during freezing conditions result in internal stress • Concrete can spall and crack in areas that become carbonated, where there is high chloride content in contact with steel reinforcing bars, or due to the stresses associated with temperature cycling • There are several corrosivity categories to consider for the various elements of bridge structures. For example, the piers may be in a water or soil corrosivity category, or in an atmospheric corrosivity category. There may also be special system corrosivity categories such as splash zone areas. As such, there are challenges related to the selection of coatings for these structures.

• Coating’s ability to prevent water, carbon dioxide, and chloride ingress is necessary for adequate protection of bridge structures • In addition to water resistance, staining resistance from water that has puddled is also needed to prevent discoloration. • Coating products intended for concrete structures should also be blush resistant, which is often accomplished by keeping the particle size small, using specifically formulated polymerizable surfactants and keeping the level of other hydrophilic materials (such as surfactants) low.

Exposure Conditions The general environment to which concrete will be exposed during its working life is classified in three levels of severity MODERATE: Protected against weather or aggressive condition. sheltered from severe rain or freezing while wet. Concrete continuously under water. Concrete in contact with non aggressive soil/ground water SEVERE: exposed to severe rain or alternate wetting and drying or occasional freezing, severe condensation. Exposed to aggressive sub-soil water or coastal environment EXTREME: exposed to sea water, corrosive fumes, severe freezing condition, exposed to abrasive action, surfaces of members in tidal zone.

Testing concrete coating system performance • The only global standard that covers the performance characteristics and key criteria for the selection of protective coating products for use on concrete is EN 1504 -2. • This European standard outlines the performance characteristics that provide protection against ingress, moisture control, and provide physical and chemical resistance. The standard includes performance characteristics that are intended for all concrete structures and is not specific to bridge structures.

Components of Coatings : All organic coatings consist of three basic components: • Solvent • Resin • Pigment.

• 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. • 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.

Coating Types: Coating types can be broadly classified under two 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.

Functional Requirements Coatings have two main functions: (1) providing protection against harmful agents thereby increasing the durability, and providing aesthetic appearance to the structure. (2) To achieve these functions, coatings should have the following attributes • Good adhesion to the surface to be coated • Resistant to alkalis, as the coatings are applied on alkaline concrete • Resistance against CO 2, 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.

LINKS TO SOME USEFUL VIDEOS • https: //www. youtube. com/watch? v=GH 2 Lxb. Ne 1 f. M (FOSROC Deckguard) • https: //www. youtube. com/watch? v=Lci. NLVw. Qio 8 ( Carbonation in Concrete) • https: //www. youtube. com/watch? v=Stmj. B 7 CUgy 4 • https: //www. youtube. com/watch? v=5 KKj 11 p. Hn. O 8

Codal Provisions : Curing Copounds Concrete Bridge Code § Para 8. 4. 2 Curing Compound: Ø Approved curing compounds may be used in lieu of moist curing with the permission of the engineer. Such compounds shall be applied to all exposed surfaces of the concrete along with stripping of form work. Ø Tests shall be done to ascertain : 1. Loss of moisture in concrete with and without curing compound. 2. Cube strength of concrete with moist curing and curing compound. 3. Permeability of concrete. IS 456 § Para 13. 5. 2 Membrane Curing : Ø Approved curing compounds may be used in lieu of moist curing with the permission of the engineer-incharge. Ø Such compounds shall be applied to all exposed surfaces of the concrete as soon as possible after the concrete has set. Impermeable membranes such as polyethylene sheeting covering closely the concrete surface may also be used to provide effective barrier against evaporation.

COATING OF CONCRETE (CS 2 para 5. 4. 7) • To provide adequate resistance against corrosion of steel embedded in RCC structures ( CS no 2) • Recommended coatings are as under : Non-aggressive Aggressive Environment ( Severe and Extreme) Environment ( Moderate) SUPER STRUCTURE SUBSTRUCTURE ( In affected part only) ALL STRUCTURES Epoxy-phenolic IPN ( Interpenetrating Polymer Network) coating ( CBRI) or CECRI integrated four coat system Coaltar epoxy coating No coating required

CORROSION PREVENTION FOR REINFORCEMENT ( para 7. 1. 5) • CS No 7 • Following measures are prescribed : • a) removal of loose mill scales, rust and dust from surface of R/F bar shall generally be sufficient • b) In extreme cases, say up to 30 km from coastal areas and in unavoidable circumstances , Stainless Steel R/F bars conforming to IS 16651: 2017 may be used.

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