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Quality Control of Coating Work on Concrete

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Quality Control of Coating Work on Concrete Quality Control of Coating Work on Why coat concrete? Concrete terial. Matching the colour of the re- First of all, concrete is not the pair material to the colour of the orig- chemically inert material many peo- inal concrete is almost impossible, and ple believe it to be. It is alkaline, and so concrete can also be coated for aes- so any acidic materials can cause it By C.P. Atkins and J.A. Phipps thetic purposes (Fig. 1). New or intact to deteriorate. In addition, sulphate Materials and Corrosion Engineering Group concrete can be coated to provide it a ions can attack the cement hydrates, decorative finish. soluble salts can be leached out, and Mott MacDonald Ltd. Finally, the surface properties of steel reinforcement can corrode be- Altrincham, UK concrete may be insufficient. Coatings cause of either the presence of chlo- can then be used to enhance the me- ride ions or a reduction in the local chanical or physical properties of con- pH of the concrete. crete (e.g., to provide an improved Corrosion of the reinforcement can skidding resistance or to reduce the lead to rust staining of the concrete, level of dusting of the surface). delamination, and ultimately spalling This article presents various con- as the reinforcement corrosion prod- siderations in the selection of surface ucts increase in volume. Therefore, preparation methods and coating ma- the first reason to coat concrete is to terials for concrete surfaces. Epoxy, keep deleterious materials out and to polyurethane, modified elastomeric increase its lifespan. bitumen, and modified cementitious Since concrete is inherently coatings are discussed. It also deals porous, it is sometimes necessary to with various factors that can lead to make it impermeable by coating it the degradation and failure of a coat- to provide ingress control against ing film. Finally, testing criteria are moisture and chloride, sulphate, presented. etc. To repair concrete, it is often nec- Fig. 1: One reason for coating concrete Surface Preparation essary to remove the damaged con- is to hide patch repairs. For any coating system, one of the (Except where noted, photos are crete and replace it with a repair ma- courtesy of the authors.) most important requirements is surface 50 PCE March 2000 Copyright ©2000, Technology Publishing Company Table 1: Factors to be Considered in Coating Selection Feature Considerations Fig. 2: Inadequate surface preparation can leave blow holes Original substrate New or old concrete and cracks in the concrete. Sufficiently cured substrate Presence of existing coating Surface contamination preparation. As with steel, it is essential that a coating is ap- Surface profile and condition plied to a clean and sound substrate free from oil and grease in order to achieve good adhesion. Environment Tidal, submerged, or spray The method of surface preparation will depend on a Other aggressive chemicals number of factors, including Level of chloride and sulphate • the condition and age of the concrete to be coated, • whether the substrate has already been coated, and Performance Adhesion UV resistance • the type of coating to be used. Water resistance There are a number of surface preparation methods Chemical resistance used for concrete: detergent scrubbing to remove surface Impact resistance contaminants; shot blasting for surface roughening and re- Elasticity/crack-bridging properties moval of old coatings; or heavy milling to remove high- Film hardness build coatings and unsound concrete (although this may Abrasion resistance introduce micro-cracking into the surface of the concrete). Toxicity Since concrete can exhibit a number of surface imper- Dirt pick-up Colour retention fections, such as blow holes (Fig. 2), it may be necessary to apply a skim coat (or fairing coat) to the surface to fill Application Surface preparation requirements defects prior to application of the coating. The skim coat Brushing, spraying characteristics should obviously be compatible with both the concrete Tolerance to substrate moisture and the coating to be applied. Temperature dependence (application and curing) The International Concrete Repair Institute (ICRI) has Site access produced detailed guidance on selecting and specifying Cost Unit material cost surface preparation for concrete.1 This information con- Number of coats required siders the coating to be applied and the methods that can Film thickness be used to achieve the required profile. It describes avail- Labour able surface preparation techniques and discusses practi- Maintenance PCE March 2000 51 Copyright ©2000, Technology Publishing Company Epoxy coatings can be applied by brush, roller, or airless spray. They are generally strong, hard, and very chemically re- sistant. They are often used to prevent the ingress of mois- ture as well as chlorides and sulphates into concrete, such as in tunnel walls, which tend to be damp. These coatings also have good abrasion resistance, which can be further improved by the introduction of glass flakes into the for- mulation. Glass flakes dramatically reduce the rate of chloride diffusion through the coating. Epoxy coatings, being mostly hard and brittle, are not able Fig. 3: Samples of surface roughness profiles showing concrete prepared to cope with any significant movement in concrete, such as by grinding (left), medium shot blasting (center), and scabbling (right). the opening of cracks. Therefore, they generally are not suit- ICRI has nine such surface profiles; the others illustrate preparation of concrete by acid etching, light shot blasting, light scarification, medium able for coating existing structures that have “live” cracks. scarification, heavy abrasive blasting, and heavy scarification. Flexible epoxies have been developed, but this has usually (Courtesy of the International Concrete Repair Institute) been achieved by a reduction in other properties. Most epoxy coatings are not moisture tolerant during application cal aspects of each one. In addition, the institute has de- veloped a set of sample profiles to assess the surface roughness of in-situ structures. Examples of the sample New Construction profiles are shown in Fig. 3. Likewise, the American Concrete Institute (ACI) has Characterise current and published information that addresses such issues as sur- future service environments face cleanliness, pH, moisture content of a concrete sur- face, testing for excess laitance, and adhesion testing of applied coatings.2 No Do structural materials require additional Coating Selection protection? A number of organic coatings are capable of protecting concrete. The coating must be selected in accordance with Yes the specific requirements of the structure. Table 1 includes Characterise substrate surface and select Do Nothing various factors that should be considered. In addition, the appropriate cleaning methods amount of volatile organic compounds (VOCs) that a coat- ing contains also should be considered, since they affect Review performance requirements both environmental and health and safety concerns. for the coating Considerations for the selection of coatings for new and deteriorating structures vary. The approach to the selec- Select a surface coating or chemically tion can be summed up for each case in the form of a flow resistant membrane as appropriate chart, as shown in Figs. 4 and 5.3 Is the Epoxy Coatings solution cost effective? No These are chemically curing coatings consisting of a sepa- (including maintenance rate base and curing agent that are mixed in specific pro- requirements) portions. The base consists of molecules with a number of Yes reactive sites; the curing agent consists of short molecules with reactive sites at either end. When mixed, the coating Apply coating system cures by the molecules chemically bonding together to form a three-dimensional array. The rate of curing is tem- perature-dependent. Fig. 4: Considerations for coating a new concrete structure 52 PCE March 2000 Copyright ©2000, Technology Publishing Company Deteriorating Substrate or vapour permeable in service. There- fore, they should generally not be ap- plied to concrete structures that are re- Characterise current and future service environments quired to dry out. Due to the nature of the curing process, epoxy coatings can be very Establish cause and extent of deterioration difficult to repair or recoat. It is es- sential that the surface of any existing cured coating is abraded and thor- oughly cleaned with an appropriate Yes Is structural No solvent before the application of any integrity impaired? subsequent coating. Will a Polyurethane Coatings coating arrest Polyurethane coatings cure in a similar Replace member or undertake No deterioration or prevent structural repair structural integrity being manner to epoxies. They also can be impaired? applied by brush, roller, or spray. Un- like epoxy coatings, they can be highly Yes flexible. Therefore, they are capable of No Do structural materials require additional bridging “live” cracks in service. protection? Because of the combination of chemical resistance and flexibility Yes that they offer, polyurethanes can be used as lining materials, such as for Characterise substrate surface and Do Nothing secondary containment structures. select appropriate cleaning methods As with all coatings, many different types of polyurethane coatings are Review performance requirements available with widely differing proper- for the coating ties. Some polyurethanes cure when in contact with moisture and, therefore, Select a surface coating
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