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 resistant. They are often used to prevent the ingress of moisture 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 developed 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 surface, 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 coating 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 temperature-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 applied 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 essential that the surface of any existing cured coating is abraded and thoroughly 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 or chemically can be applied to damp surfaces. Other resistant membrane as appropriate formulations have excellent abrasion resistance or exhibit enhanced crack- Is the bridging properties. solution cost No effective? (including maintenance requirements)

Yes

Apply coating system

Fig. 5: Considerations for coating a deteriorating concrete structure

Fig. 6: Anti-carbonation coatings provide durability and improved appearance.

PCE March 2000 53 Copyright ©2000, Technology Publishing Company Polyurethane coatings are easily recoated or repaired, as measures must be employed during their use to prevent the new coat tends to partially dissolve the surface layer of chemical burns, dermatitis, and other skin irritations. the old coat and thus allows an intimate bond to be formed. Some formulations require the application of a Surface Impregnations special bond coat prior to application of the new coat. It is As an alternative to coating, surface impregnations can be essential to thoroughly clean any surfaces to be recoated applied to impart a certain degree of resistance to moisture. in order to achieve a good bond. These materials generally work by impregnating the pores in the outer layer of the concrete with a water-repel- Modified Elastomeric Bitumen Coatings lent material. This prevents the ingress of water into the These coatings consist of long polymer chains dissolved in concrete. Thus, the movement of soluble aggressive a strong solvent. The strong solvent is necessary to prevent species is dramatically reduced. the long polymer chains from “tangling” and causing the Examples of the ingredients in impregnating materials chains to stiffen. are silanes4 and stearates. Aluminium stearate dissolved in They can be applied by airless spray or brush. After ap- white spirit, a solvent, is frequently used, although its wa- plication, these coatings dry as the solvent evaporates. No ter-repellent properties are variable on siliceous substrates. chemical reactions take place, and the rate of drying de- Also, at temperatures below 10 C, the mixture tends to be- pends on the rate of solvent loss. Therefore, it is faster at come viscous and separate. higher temperatures. Some bituminous coatings are mois- Most silanes are clear and colourless, and some require ture tolerant during application. in-situ dilution with solvent before application. In some They can be removed by redissolving them in the same cases, it is helpful to know what type of impregnating ma- strong solvent used initially. Recoating is relatively simple terial has been used, but detecting the presence of silanes as the surface of the original layer is dissolved and an in- after application is difficult. Therefore, careful monitoring timate bond is created with the new coating layer. is required during the site stage. Since these coatings are elastomeric, they have excellent Thin barrier coatings also can be used as surface im- crack-bridging properties. They also are very moisture tol- pregnations to seal the surface. These are typically low- erant, which enables them to be used on the exterior of viscosity epoxies that can penetrate the concrete surface buried pipe or on submerged surfaces such as pilings and prior to curing and then seal the surface when they cure. jetties. However, they are relatively soft, and this can re- sult in poor abrasion resistance. Degradation Processes There are a number of factors that can lead to the degra- Modified Cementitious Coatings dation and failure of a coating film. Examples of degrada- These are generally anti-carbonation coatings that can tion include chalking, swelling and softening, peeling, and double as barriers against sulphate and chloride ingress. pinholing. Cementitious coatings are relatively brittle even after polymer modification, and they generally are not capable of Chalking bridging active cracks. When exposed to ultraviolet (UV) light, many polymer Application can be by brush, roller, or airless spray. The coatings will chalk. This is caused by fragmentation of the concrete surface should be dampened with potable water polymer surface. Chalking results in loss of decorative before application. The presence of sea water in the struc- and/or protective properties. In situations where exposure ture to be coated may affect the quality of the bond; ad- to UV light is likely, the coating should be selected to be vice should be sought from the manufacturer should this UV-resistant. situation arise. An example of where such a coating material could be Swelling and Softening used is shown in Fig. 6. If solvent-borne physical and oxidative drying coatings Modified cementitious coatings traditionally are made (and in some cases chemically curing coatings) are ex- with water-borne acrylic. Unlike the coating systems already posed to solvents, they will often swell and soften. This at mentioned, they do not contain solvents. Therefore, they do best causes a deterioration in the properties of the coating; not usually require the same level of safety precautions re- at worst it causes total failure. Tests that expose the coat- quired for use of solvent-borne materials. However, being ce- ing to its working environment can be used to determine mentitious, they are highly alkaline, and appropriate safety whether it will swell and soften.

54 PCE March 2000 Copyright ©2000, Technology Publishing Company Peeling monitored to ensure they fall within acceptable limits for Pull-off or cross-cut adhesion tests can provide data on the the coating system being applied. Dry or wet film thick- adhesive quality of the coating. Similar adhesion tests can nesses should be monitored to ensure the correct amount of be carried out after immersion tests in a specific environ- coating is being applied, and adhesion tests should be car- ment to determine the effect of the environment on the ried out at intervals to ensure a consistent quality of bond. coating adhesion. Any areas of disbonding or blistering on Once the coating is in service, it should be regularly in- a coated surface should be removed, and the surface spected for any signs of deterioration. It is difficult to di- should be recoated after the reason for the disbonding has rectly monitor the rate of degradation of the coating other been discovered and corrected. than by visual inspection. Cores can be taken to observe any changes in the condition of the coating and the con- Pinholing crete below it, although this results in damage to the coat- Pinholing describes tiny holes in the coating that could al- ing and concrete that require repair. low moisture or aggressive contaminants to penetrate the coating and come into contact with the substrate. Once a Summary coating has been applied, it should be visually checked for As with any coating application process, the successful pinholes, and if any are found, they should be repaired. coating of concrete requires quality control at each stage, On concrete substrates, spark testing can be used to de- from initial selection, through surface preparation, to ap- tect pinholes in non-conductive coatings. This is done by plication of the coating to trial areas. If careful considera- passing a high-voltage probe (commonly 5–10 kV) over the tion is given to each stage of the process and regular in- coating. If a pinhole is present, a spark will pass from the spections are undertaken, then numerous benefits can be probe to the substrate, indicating its presence. achieved by applying coating systems to concrete. This method of pinhole detection is commonly used on sheet polymer linings. Care should be taken to avoid using Acknowledgements excessively high voltages, because there is some evidence The authors gratefully acknowledge the contributions of that they can produce sparks that can induce pinholes in Dr. Paul Lambert and Dr. Darrell Leek, both of Mott Mac- the coating. Donald. References Testing and Monitoring Criteria 1. “Selecting and Specifying Concrete Surface Preparation Before applying a coating system to a structure, it is gen- for Sealers, Coatings, and Polymer Overlays,” Internation- erally advisable to perform a trial application on which al Concrete Repair Institute Guideline No. 03732 (1997) tests can be performed to determine the suitability of the Sterling, VA, USA. coating for the task. 2. “A Guide to the Use of Waterproofing, Dampproofing, Pull-off or cross-cut tests should be performed to deter- Protective and Decorative Barrier Systems for Concrete,” mine the quality and nature of adhesion of the coating to Manual of Concrete Practice 1R-1-43 (Farmington Hills, MI, the substrate. Three types of failure can be encountered: USA: American Concrete Institute, 1986), p. 515. • adhesive failure at the coating/substrate interface, 3. M.A. Shields, D.S. Leek, and P. Lambert, “Coatings for • cohesive failure within the coating, and the Protection of Structural Materials,” Construction Main- • cohesive failure within the concrete substrate. tenance and Repair, March/April 1992, pp. 2–9. Of these, adhesive failure is the worst case. It indicates 4. P. Lambert and M.A. Shields, “Silane-Based Treatment that the coating is either unsuitable for its intended service of New and Existing Bridgeworks,” National Seminar on or has been applied incorrectly. Provided the trial applica- Surface Protection Treatment of Concrete Structures, Uni- tion has been applied on a sound substrate, cohesive fail- versity of Dundee, September 1992. ure of the substrate shows the coating is bonded strongly to the concrete; therefore, it is the most desirable of the Editor’s note: An article titled “Update on European Stan- three test results. dards for Concrete Coatings” was published in the Decem- The test area should be checked for excessive pinholing ber 1999 issue of PCE, pp. 51–54. and any other defects that may compromise the performance of the coating. During the actual coating application, environmental factors, such as temperature and relative humidity, should be