SURFACE PREPARATION OFBUILDING SUBSTRATES

A Durability + DesignCollection

Surface Preparation ofBuilding Substrates

A Durability + Design Collection

Copyright 2013 byTechnology Publishing Company2100 Wharton Street, Suite 310

Pittsburgh, PA 15203

All Rights Reserved

This eBook may not be copied or redistributed without the written permission of the publisher.

Introduction

Surface Preparation: A Primerby Jayson L. Helsel, KTA-Tator, Inc.

Peeling Away the Years: Taking on the Job of Getting Old Coatings Offby Deborah Slaton, Wiss, Janney, Elstner Associates

Building a Formula for Removing Coatings from Masonry Surfacesby Kenneth A. Trimber, KTA-Tator, Inc.

Six Key Points You Should Know about Concrete Surface Preparationbefore Coating Applicationby Fred Goodwin, BASF Construction Chemicals

Department of Defence: Protecting Wood Exteriorsby Jayson L. Helsel, KTA-Tator, Inc.

A Firm Foundation, Part 2: Surface Preparation and Test Methodsby Charles H. Holl, Dayton Superior Corp.

Cleaning Exotic Woodsby Diane Calabrese and Brett Martin

Contents

ii

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IntroductionThis eBook consists of articles from Durability + Design (D+D),durabilityanddesign.com, the Journal of Architectural Coatings (JAC),and Paint and Wallcovering Contractor (PWC) on the subjects of coatingsremoval and surface preparation of building substrates. More articleson this topic may be found online at durabilityanddesign.com.

Photo courtesy of Sponge-Jet Inc.Cover photo courtesy of istock

Getting It Right1

Surface Preparation: A Primer

urface preparation, together with the related process of removing existing coatings, alwaysranks as one of the most critical steps in successful coatings application. The objectives of theseprocesses are twofold: to clean and roughen the substrate according to the specified require-

ments. The methods used to prepare surfaces for coating application may clean and roughen simultaneously—

as with abrasive blast cleaning. At other times, these steps must be performed separately, as with chemi-cal stripping. In either situation, cleaning and roughening must be treated as two distinct acceptance criteria.For example, there may be situations where the level of cleaning is adequate, but the roughness may beinsufficient or excessive to facilitate proper adhesion of newly applied coatings. Alternatively, the surfaceroughness may be adequate, but the level of cleaning may be inadequate.

Editor’s Note: This article apeared inJAC in October/November 2008. SBy Jayson L. Helsel, P.E.KTA-Tator, Inc.

Various removal,

cleaning methods

are tailored to

meet challenges of

substrate type and

condition, service

requirements

2

Sur face-preparation methodsSurface-preparation methods can range from simple solvent cleaning to hand and power-tool cleaning;from dry and wet abrasive-blast cleaning to chemical stripping; and from water jetting to other, more non-traditional methods, such as sponge jetting and cryogenic blast cleaning with dry ice pellets.

The degree of cleaning required for a given project specification depends on the service environment (theenvironment in which the coating system must provide sufficient performance), the composition and prop-erties of the coating system, and the intended service life of the installed coating.

Hand-tool cleaningHand-tool cleaning, typically employed in relatively minor touch-up repairsfor maintenance painting activities, is done with wire brushes, scrapers, andother tools that do not depend on electric or pneumatic power to operate.Hand tools are only intended to remove loosely adhering materials; when thesubstrate is steel, this includes corrosion products, old paint, and mill scale.Hand tools used on steel are not intended to roughen the substrate and pro-duce a surface profile or “anchor” pattern to facilitate adhesion of newly ap-plied coatings. For softer substrates such as wood, hand tools may producesufficient roughening for paint application.

Power-tool cleaningPower-tool cleaning is typically performed with grinders, pneumatic chisels,needle scalers, rotopeen tools, and other mechanisms that require an electricor pneumatic power source to operate. Most of these tools can remove bothloosely and tightly adhering corrosion products, paint, and mill scale fromsteel surfaces. Some of these tools can also produce a small anchor pattern onsteel surfaces. Additionally, these tools can be equipped with vacuum portsand hoses for attachment to filtered vacuums so that the fine, airborne parti-cles that are created during surface-preparation activities are collected at thepoint of generation. Power-tool cleaning is typically employed for maintenance repairs of small

areas, but can also be used when methods such as abrasive blast cleaning arenot an option.

Dry abrasive blast cleaningBlast cleaning with dry abrasive media is one of the most common methods for preparing a surface for coat-ing. Abrasive blast cleaning can be used to roughen an existing coating for subsequent overcoating, or tocompletely remove everything from the substrate, including the existing corrosion products, coating, andmill scale.

Abrasive blast cleaning is the most productive of all surface-preparation methods, in volume terms. Thou-sands of square feet of surface can be prepared for coating in a single work shift. The hardness and massof the abrasive media, combined with the velocity of the abrasive as it exits a nozzle at high speed, gen-erates high levels of energy. As the abrasive media impact a surface, they can remove existing coating lay-ers, corrosion, and mill scale, while simultaneously generating a surface profile or anchor pattern.

The level of cleanliness that is achieved is ultimately determined by the distance that the nozzle is heldfrom the surface and the “dwell time” that the operator employs. The depth and shape of the surface pro-file is determined by the type and size of the abrasive media employed, as well as the hardness of the sur-face being prepared. Therefore, selection of the correct type and size of abrasive media is critical. Selectingan abrasive size that is too small will generate a surface profile that is too shallow, and selecting a too-largeabrasive will create a surface profile that is too deep. The abrasive type and size ultimately selected shouldbe tested before production to verify that the specified surface profile depth and shape can be achieved.

Coatings-removal and surface-preparation processes are tailored to theneeds of a given project. Abrasive-blast methods employing proprietarysponge-type media were used for the coatings removal projects shown inthe photos on this and previous page.Previous page and above: Exterior of a brick building before coatings removal(above) and after removal (previous page).Below: Coatings removal in progress.Photos courtesy of Sponge-Jet Inc.

Typically, harder abrasives—those designed to achieve complete coating removal and an adequate pro-file—include steel grit, steel shot, and mineral abrasives such as coal slag and garnet. Softer abrasives,which are typically used to remove loose coatings and other loose material from a substrate, include alu-minum/magnesium silicate, corncobs, walnut shells, limestone, or some mineral sands.

Another mineral abrasive—usually used in the form of beads—is glass. Although not frequently em-ployed, glass-bead blasting can be used to clean and prepare certain glass, plastic, rubber, and metalsurfaces. The manufacturers of glass-bead abrasives should be consulted for specific applications suitablefor their materials.

An alternative to blasting with traditional dry abrasives is “sponge blasting,” which uses proprietaryblast media and equipment manufactured by Sponge-Jet Inc. According to Sponge-Jet, the sponge media“is an open-celled, water-based polyurethane impregnated with abrasives.” A variety of sponge abrasivematerials are available to achieve various levels of cleaning. The nature of the sponge material can allowfor greatly reduced levels of dust and airborne contaminants during the blasting process.

Shot blasting for concrete floorsAnother variation of dry abrasive blast cleaning is shot-blast cleaning with self-contained centrifugal wheelblast units. Such units employ a vacuum system to contain the abrasive and debris, and are well suited foruse on large, horizontal surface areas such as concrete floors. The machines prepare a strip of surface rang-ing from several inches up to 30 inches in width in one pass. This method of surface preparation can re-move any existing coatings, in addition to roughening the concrete surface.

Pressurized water cleaningPressurized water cleaning is a good method for cleaning surfaces, but it cannot etch a profile into the sur-face. Water jetting, however, can restore an existing surface profile. The common levels of cleaning are:• LPWC: Low-Pressure Water Cleaning (up to 5,000 psi);• HPWC: High-Pressure Water Cleaning (5,000–10,000 psi); • HPWJ: High-Pressure Water Jetting (10,000–30,000 psi); and • UHPWJ: Ultra-High-Pressure Water Jetting (>30,000 psi).

The degree of cleanliness that can be achieved with water varies widely, depending on the pressure ofthe water used. Selecting the water pressure is dependent on the adhesion of the existing coating to the sur-face, the desired level of cleanliness, and the project specification requirements. Surface rust can be re-moved using water jetting, but this will not remove intact mill scale; this requires mechanical removal orselection of a coating system that can be applied to intact mill scale and still perform adequately in the serv-ice environment.

Low-pressure water cleaning (LPWC) or “pressure washing” is often specified for projects where “over-coating” is possible—where the existing coating is salvageable and is left in place to serve as part of themaintenance coating system for the structure. LPWC can be very effective in removing dirt, chalking, birddroppings, and other contaminants from the surfaces, although mechanical agitation of the surface duringLPWC is often required to ensure adequate removal.

The other three levels of water jetting, all involving high pressure, are used primarily to remove coatings.Waste generation is minimized when the water is captured, filtered and reused. The debris that is createdis limited to the materials removed from the surface. This is desirable if the coating to be removed containstoxic metals.

It must be noted that, because of its use of water, water jetting will cause steel surfaces to flash rust.Therefore, the flash rusting must either be accepted as part of the process—with coatings selection focusedon materials that are tolerant to this condition—or the use of a rust inhibitor may be considered, taking careto ensure that the inhibitor is compatible with the coating system to be applied to the prepared surfaces.

When low-pressure water cleaning is used to clean and remove loose materials from substrates such aswood and masonry, adjustments in water pressure (generally lower), stand-off distance, and the type andsize of tips for the cleaning wand, are necessary to avoid damaging the substrate.

3

Water jetting is frequently employed to remove graffiti.Photo courtesy of Wiss, Janney, Elstner Associates Inc.

Chemical strippingChemical stripping, like water cleaning, does not generate a surface profile and will not remove rust or millscale from a steel surface. Therefore, mechanical methods of surface preparation may be required after thecoating has been removed with chemicals. Chemical stripping is often chosen where other methods, suchas abrasive blast cleaning, are not possible due to the surrounding environment or where damage to a softersubstrate (e.g., wood) would be likely.

The proper selection and use of a chemical stripper is important for successful removal of existing coat-ings. Stronger strippers, which typically contain methylene chloride, will remove almost any coating, butare the most hazardous in terms of environmental and worker exposures. These materials, often known as“aircraft” strippers, have been used in the commercial aircraft industry to remove coatings from the exte-rior of aircraft fuselages.

Other strippers are typically caustic materials (high pH) that break down and soften the coating’s resinto facilitate removal. Multiple applications can be required, depending on the coating system and thickness.Neutralization of the surface following removal of the stripper, however, is required for proper coatingapplication and adhesion.

A variety of “environmentally friendly” chemical strippers are available that are characterized byneutral pH and little odor. These materials may work more slowly on thicker films and also require severalapplications to remove all coating layers. A neutralizing step may also be needed with these strippers.

Industry standardsAll of the surface-preparation methods discussed here are the subject of applicable industry standards orguidance that defines their use (see table above). It should be noted that some methods are covered by mul-tiple standards, such as processes involving power-tool cleaning and abrasive-blast cleaning.

As suggested in this discussion, a good understanding of the various surface preparation methods andtheir applicable uses is needed when determining the best approach for a coating project. In addition,referencing applicable industry standards is recommended.

4

Surface Preparation Method Industry Standard/Guidance

Hand Tool Cleaning SSPC-SP 2, Hand Tool Cleaning

Power Tool Cleaning SSPC-SP 3, Power Tool Cleaning

SSPC-SP 11, Power Tool Cleaning to Bare Metal

SSPC-SP 15, Commercial Grade Power Tool Cleaning

Dry Abrasive Blast Cleaning SSPC-SP 5, White Metal Blast Cleaning

SSPC-SP 6, Commercial Blast Cleaning

SSPC-SP 7, Brush-Off Blast Cleaning

SSPC-SP 10, Near-White Metal Blast Cleaning

Pressurized Water Cleaning SSPC-SP 12, Surface Preparation and Cleaning of Metals

by Waterjetting Prior to Recoating

Chemical Stripping SSPC Technology Up-date No. 6, Chemical Stripping of

Organic Coatings From Steel Structures

JAC

ecause properly selected and applied coating systems deliver a long service life, considera-tion is not always given to the reality that coatings may be destined for eventual removal. Evena coating that is appropriate for a given application will likely be removed at some point. Forexample, in situations where many applications of a coating system have taken place over

time, product manufacturers typically recommend that the existing layers be removed before recoating toensure effective performance and appearance results. Aged and deteriorating coatings need to beremoved to provide a sound substrate for new coating application.

In some cases, it may be desirable to remove coatings and leave the substrate uncoated and exposedfor aesthetic reasons. In other situations, existing coatings may need to be removed because they containhazardous components such as lead or other heavy metals. In other cases, the existing coating may not beappropriate for the substrate, such as the situation where non-breathable coatings applied to masonrysubstrates must be removed to permit moisture that enters the masonry to escape.

Finally, unwanted coating or marker application in the form of graffiti must be removed, sometimes asan ongoing or cyclical maintenance procedure.

Coatings-removal methods and materialsThe process of coating removal is closely linked to that of facade cleaning. Currently, these cleaningsystems can generally be classified in four categories: water, abrasive, chemical, and laser tech-niques. Systems in each of the first three categories can be considered for coating removal, while lasertechniques may be used for cleaning of painted finishes.

Depending on the nature and condition of the substrate and the coatings to be removed, one or acombination of systems may be necessary. Chemical techniques are most often used, but water andmicroabrasive methods can sometimes also be effective.

Peeling Away the Years: Taking on the Job of Getting Old Coatings Off

5

By Deborah Slaton,Wiss, Janney, Elstner Associates Inc.

Preservation/Restoration

Editor’s note: This article appeared inJAC in March/April 2007. BRemoval techniques

vary, depending on

the condition of

existing finishes

and substrate

6

WaterCleaning methods employing water include soaking, intermittent or continuous mist spraying, and brush-ing the surface, followed by rinsing to soften or dissolve loosely bonded dirt, soil, or coatings. These tech-niques can sometimes also be effective on poorly bonded coatings. In preservation projects, water pressuresused for prewetting, cleaning, and rinsing generally range from very low (less than 50 psi), to low (100 to200 psi), to medium (200 to 400 psi). Moderate- to high-pressure water washing (400 to 800 psi) is gen-erally safe for use on some stones, and higher pressures can be used on concrete. At the other end of thepressure continuum, hydro-blasting at pressures greater than 30,000 psi can be used in removing coat-ings from sound, high-strength concrete.

SteamRemoval techniques with steam offer the advantage of consuming less water and are useful if the volumeof water used in the cleaning process needs to be limited, as in the case of porous substrates. These meth-ods have become more readily available during the past decade by means of equipment that providesbetter control and improved worker safety.

Where film-forming coatings are not well bonded, the use of water combined with scraping or scrubbingwill generally remove the coating. Removing any loose coating material by hand scraping (avoiding theuse of metal scrapers) before applying water results in a more effective process overall. Steam will oftenhelp remove an existing coating where cold water will not; however, steam can also partially dissolve somecoatings, which can result in a difficult clean-up process if the loosened coating then bonds to other areasof the substrate or adjacent surfaces.

If the substrate is porous or contains open joints or cracks, the amount of water used may need to be lim-ited. For masonry substrates, water cleaning needs to be limited in cold weather as damage can occur tomasonry substrates from cyclic freezing and thawing of entrapped moisture. In addition, the water usedshould be checked to confirm that it is free from impurities or minerals that can stain the substrate.

AbrasivesAbrasive and microabrasive systems clean by abrading or eroding the surface, and therefore can effec-tively remove existing soil or coatings. But the extent to which the coating is removed without damage tothe surface varies widely, depending on the nature of the substrate and the aggressiveness of the systemused. Mechanical cleaning methods using high-pressure sand or grit blasting are no longer in use forcleaning or coating removal from masonry because of damage caused to the substrate, but they are stillused for removing coatings from metal, and for preparing metal for recoating. When used with proper con-trols, newer microabrasive systems, which clean by directing fine particles at the wall at very low pressures,can remove contaminants without significantly damaging the substrate.

Abrasive and microabrasive systems may be wet or dry. They vary in terms of special nozzles and otherapplication equipment required and the type of particulates used. An example of a microabrasive mediumis crushed dolomitic limestone particles that are less than 90 microns in diameter.

Although useful in cleaning applications, microabrasive systems are often not effective in removing coat-ings from masonry, primarily because the particulate will rebound from some types of applied coatings with-out removing them. Microabrasive techniques, however, are successfully used to remove some coatingsfrom metals in both shop and field applications.

One medium used in microabrasive cleaning—sodium bicarbonate (baking soda)—has been used to re-move graffiti from exterior masonry in urban areas, but has been found to be problematic in many cases.If application pressures are too high, the graffiti is removed but a shadow of the image of the graffiti is leftin the etched or eroded substrate. The use of this technology has also created additional problems in someapplications because baking soda and water form a paste that is difficult to remove. In addition, sodiumbicarbonate is a salt; if not thoroughly removed by rinsing, it can damage masonry.

(Previous page): Graffiti applied to this caststone using spray-paint, or in this case markers,is very difficult to remove as the solvent-bornecoating penetrates into the pores of the sub-strate.(Top): A previous attempt to remove the graffitiby pressure application of sodium bicarbonate(baking soda) resulted in etching of the caststone substrate. Also note the residue of bakingsoda on the surface after rinsing.(Bottom): Careful, repeated applications of achemical coating-removal product were requiredto remove the graffiti without damage to thesubstrate.Photos courtesy of Joshua Freedland, WJE

7

ChemicalsChemical coatings-removal products react with a substrate or a coating to dissolve orremove the coating (or, for facade cleaning, the soil deposits). Chemical cleaning com-pounds include alkaline or acidic cleaners and organic solvents, and may be suppliedin liquid, gel, or poultice form. Within this broad range of chemical cleaning products,a variety of specialized coating-removal systems are offered. These systems may be al-kali based—containing calcium hydroxide, sodium hydroxide, or potassium hydrox-ide—or solvent based. The solvent-based products may contain aliphatic and aromatichydrocarbons such as petroleum distillates, mineral spirits, naphtha, benzene, andtoluene; chlorinated hydrocarbons such as methyl chloride, ethylene dichloride, andtrichloroethane; N-methyl or 2-pyrrolidone (NMP); citrus terpenes; esters such as di-methyl adipate, dimethyl glutarate, and ethyl acetate; alcohols; or ketones.

In situations where coatings are intact and tightly adhered to the surface, chemicalcoating-removal systems are usually most effective in removing coatings from masonry.A combination of chemical coating removal followed by water or steam rinsing can re-sult in good overall results in many cases. Different products are more effective on cer-tain coatings than others, so research, queries to the manufacturer’s representative, andespecially test samples are necessary to select the right product.

Environmental, safety issues with chemicalsIt is important to note that each chemical coating-removal product carries specific en-vironmental and safety limitations that should be thoroughly understood before the prod-uct is specified or used. As with any chemical product, some of these componentspresent a significant safety hazard for users and others in the work environment. Vehi-cles and buildings adjacent to the work area can be damaged by coating-removal prod-ucts spread by wind drift. Special collection and disposal procedures may be requiredto control runoff from chemical-coating removal, both because of the chemicals in theremoval product and certain components of the coatings being removed. Precautions involving the use of coating-removal products should be followed, and if

a relatively benign removal system will work effectively, products that are more ag-gressive should be avoided. Manufacturer’s literature and the more detailed informa-tion about product composition listed in the material safety data sheet (MSDS) shouldbe carefully reviewed prior to selecting a chemical coating-removal product. It is alsovery important to review up to date product information, as formulations change overtime.

Some proprietary chemical coating-removal systems include a paper or fiber masksupplied by the manufacturer. These removal products are typically provided in pasteor gel form, and adhere to the wall. The coating-removal product is applied to the sub-strate, covered with the mask, and allowed to dwell for a prescribed period, after whichthe mask is removed together with the remnants of stripper and removed coating. Somesystems also come with special bags for disposal of the paper, cleaner, and debris.These proprietary systems are particularly useful in situations where the coating debrismust be contained for special disposal.

A few caveatsSelection of the most appropriate system for coating removal depends on the natureand condition of the substrate, the type of coating to be removed, and whether the ex-isting coating is well bonded or has penetrated the substrate. The success of the coat-ing-removal process depends on proper selection and application of the product, carefultrial sampling, and quality control throughout the removal process.

Failed, peeling coatings can be removed by scraping; care must betaken to avoid damaging the substrate. Multiple coating layers were

present on this calcium silicate brick wall of a 1920s building.

Removal of localized areas of coatings by scraping revealed remnantsof prior coatings and graffiti. The wall had been repainting many timesto the height a worker could reach from grade to cover repeated incidencesof graffiti. Photos on this page courtesy of David S. Patterson, WJE

Here, following removal of loose coatings by scraping, trial samplingwas carried out with several chemical coating-removal products.

8

On historic-preservation projects, it is often important to document existing coatings prior to removal, andto retain samples for future reference. A special consideration with the removal of coatings from historicstructures (to be addressed in a later column) is the need to remove overlying coatings to reveal previouscoating layers that represent historically or artistically significant finishes.

If toxic substances such as lead or other heavy metals are present in the existing coatings to be removed,special removal, containment, and disposal methods are required. Testing performed before selecting andspecifying a coating-removal system will help identify whether special procedures are required. In somecases, such as restoration of wood or metal windows, coating removal in the shop rather than in the fieldis more efficient.

Finally, for historic buildings, as with other structures, it is important to ensure that the methods used donot result in damage to the existing substrate. Trial samples and mock-ups should always be performed be-fore initiating full-scale work to evaluate the effectiveness of the coating-removal system, its effect on thesubstrate, and the condition of the substrate concealed beneath the existing coating.Joshua Freedland, Senior Associate and Conservator with WJE, contributed to this article.

The selected product, a hydrogen peroxidebased chemical coating removal system, wasapplied with a low pressure water rinse.

Photos this page courtesy of James Lawless,Streamline Powerwashing, Inc.

JAC

Frequently during the life of a building constructed of concrete masonry units (CMU) or brick,it is no longer feasible to refurbish the exterior by pressure washing and applying more coatsof paint (Photo 1, next page). This occurs because:

• The existing coating becomes too thick or too poorly adhered to support the weight and curing stressesof another coat, or• The number of coats causes the permeability of the system to be too low to allow moisture vapor to passthrough, or• The desired repaint material is incompatible with the existing coating.In these cases, some or all of the existing coating needs to be removed and replaced.

Removal of the coating from CMU and brick can prove challenging, and a number of decisions need tobe made at the design stage in order to select the optimum paint-removal method for the job. Questionsthat need to be considered include:1. Does the existing coating have to be completely removed or just the loose coating? Cost and complex-ity increase significantly if all of the coating must be removed.2. Can heavy roughening of the block or brick face be tolerated, including some damage to mortar joints?Note that for historic preservation, even very slight roughening may be unacceptable.

Building a Formula for RemovingCoatings from Masonry Surfaces

9

F

Good TechnicalPractice

Editor’s note: This article appeared in D+Donline in November/December 2010. Awebinar on this subject, presented by theauthor, can be viewed by performing asearch for “Paint Removal from MasonrySubstrates” in the archived webinars atwww.durabilityanddesign.com/webinars/

By Kenneth A. Trimber,KTA-Tator, Inc.

Removal of coatings from masonry can prove challenging, anda number of decisions need to be made at the design stage inorder to select the optimum paint-removal method for the job.Below: Chemical coatings-removal methods were employed atthe historic Tivoli Student Union at the University of Colorado’sAurora campus (see Memorable masterpieces, published inJAC in October 2005). Photo courtesy of Dumond Chemicals

10

3. Can existing paint be allowed to remain present within the porosity of the block?4. Can large volumes of water be tolerated (environmentally, and in terms of potential water intrusion intothe block or brick)?5. Can airborne dust be tolerated?

The answers to these questions will help to identify candidate methods of surface preparation for theproject. The questions are presented again at the conclusion of this article, together with candidate meth-ods of cleaning based on the responses to each.

Industry standardsA few industry standards identify methods of surface preparation that can be used on CMU,brick, and other concrete surfaces (floors, poured walls, precast, etc.). Unfortunately, most ofthe standards focus on the preparation of floors or concrete substrates that are bare, rather thanpainted CMU or brick.

Also, the standards do not provide definitive acceptance criteria for judging the degree ofcleaning that is carried out. Most of the standards essentially state that sufficient material mustbe removed to achieve a sound concrete surface free of laitance, glaze, efflorescence, and in-compatible curing compound.

Accordingly, while the methods described may be suitable for use on CMU and brick, thelanguage in the standards does not specifically provide acceptance criteria when removingexisting paint from these substrates.A few of the standards are described below, produced by SSPC (Society for Protective Coat-

ings), NACE (National Association of Corrosion Engineers), ICRI (International Concrete RepairInstitute), and ASTM International (formerly American Society for Testing and Materials).

SSPC-SP13/NACE No. 6, Surface Preparation of Concrete. This standard briefly describesmethods that are available for cleaning a variety of concrete surfaces, and frequently references otherASTM and NACE standards. It addresses the following methods of surface preparation:• Dry-abrasive blast cleaning, wet-abrasive blast cleaning, vacuum-assisted abrasive blast cleaning, andcentrifugal shot blast cleaning with references to ASTM D4259 (described below). These methods will re-move existing coating, but possess the potential to significantly roughen or damage the surface of the blockor brick and mortar joints.• High-pressure water cleaning or water jetting according to SSPC-SP12/NACE No. 5 or ASTM D4259(described below). These methods will remove poorly adhered coating, allowing intact coating to remain,or completely remove existing coating given that adequate pressures and dwell times are used. They willbe less damaging to the surface of the block and brick than the abrasive blast-cleaning methods, but someroughening will occur.

Note that SSPC-SP12/NACE No. 5 is titled Surface Preparation and Cleaning of Metals by Water Jet-ting Prior to Recoating. While the methods described in the standard are suitable for use on concrete, theacceptance criteria for the degrees of cleaning are based on metallic substrates.• Impact power tools according to ASTM D4259 (described below). These methods include needle gun-ning and rotary peening, which will remove existing coating but will fracture and remove concrete at thesame time. These tools are not commonly used on CMU or brick because of the potential damage created.• Power grinding, sanding and wire brushing according to ASTM D4259 (described below). While coat-ings can be completely removed by grinding or sanding, these methods are more commonly used for theremoval of loose coating and for feathering edges.• Acid etching according to ASTM D4260. Acid etching is used on horizontal surfaces to etch bare con-crete; it will not remove existing coating.• Flame (thermal) cleaning. This method is used to extract organic contaminates from concrete, but is notsuggested for coating removal.• Methods used for cleaning contaminants from the surface prior to painting are also addressed. The meth-ods are vacuum cleaning, air-blast cleaning, water cleaning, detergent water cleaning, and steam clean-ing, all according to ASTM D4258.

Photo 1: Aged coatings often become too weakand too thick to be overcoated

Photos: KTA-Tator Inc. unless otherwise noted

ICRI Guideline No. 310.2-1997 (formerly No. 03732), Selecting and Specifying Concrete Surface Prepa-ration for Sealers, Coatings, and Polymer Overlays. This guideline addresses methods of surface prepa-ration used on concrete. While the focus of the guideline is on concrete floors, some of the methods aresuitable for use on CMU and brick. It also includes nine (9) concrete surface-profile (CSP) coupons that arereplicas of the type of profile (surface roughness) created by the various methods of surface preparation(Photo 2).

The coupons range in texture from very smooth, typical of acid etching (CSP1), to veryrough, typical of heavy scarification (CSP 9).Methods addressed in the ICRI Guideline that are capable of removing coatings from

CMU and brick are listed below. Note the similarity to the methods listed above in SSPC-SP13/NACE No. 6.• Abrasive blasting (will remove coatings, but has the potential to significantly roughenor damage the surface of the block or brick as well as mortar joints).• Scarifying (will remove coatings, but is extremely aggressive to the substrate)• Needle scaling (will remove coatings, but is very aggressive to the substrate)• High/ultra-high-pressure water jetting (can remove coatings and minimizes damageto the substrate, but some roughening will occur)• Low-pressure water cleaning (will typically remove only loose coating)

Methods in the Guideline that will not totally remove coating or are not suitable for use on CMU and brickare:• Detergent scrubbing (will not remove coating)• Acid etching (will not remove coating—used on bare horizontal surfaces)• Grinding (will remove coating, but equipment addressed in the Guide is limited to floors)• Steel shotblasting (will remove coating, but equipment in the Guide is limited to floors)• Scabbling (will remove coatings but is extremely aggressive to the substrate and best used on floors)• Flame blasting (can remove coating, but is not recommended due to the hazards involved with burningthe paint)• Milling/rotomilling (will remove coating and top layer of concrete, but used on slabs and not suitable forCMU/brick)

ASTM D4258, Standard Practice for Surface Cleaning Concrete for Coating. This practice briefly de-scribes methods of cleaning that will remove surface contaminants such as grease, dirt, and loose mate-rial, but will not remove coatings. These methods are used in conjunction with other coating-removalmethods to assure that the surface is free of contamination prior to painting. The methods addressed arebroom cleaning, vacuum cleaning, air-blast cleaning, water cleaning, detergent water cleaning, and steamcleaning.

ASTM D4259, Standard Practice for Abrading Concrete. This practice addresses methods for removingmaterial, including coatings, and roughening the surface. The methods addressed are the following.• Mechanical abrading; these methods involve power tools, both impact and grinding. The power tools arecapable of removing coating, but are better suited for localized use. The impact tools can also cause dam-age to the substrate.• Water blast cleaning; this method involves high-pressure water blasting, but the pressures are not defined.Given adequate pressure and dwell time, coatings can be removed.• Abrasive blast cleaning; these methods include dry blasting, wet blasting, and use of a self-contained ap-paratus that reuses the abrasive. These methods will remove coating, but can significantly roughen the sub-strate in the process.

ASTM D4260, Standard Practice for Liquid and Gelled Acid Etching of Concrete. This practice addressesthe preparation of bare concrete by acid etching, but will not remove existing coatings.

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Photo 2: ICRI Concrete Surface Profile Coupons.

ASTM D4261, Standard Practice for Surface Cleaning Concrete Masonry Units for Coating. This prac-tice addresses the same surface-cleaning methods covered in ASTM D4258, with the addition of mechan-ical tool cleaning for the removal of mortar spatter and efflorescence. The standard is not applictble to theremoval of existing coatings.

Production ratesProduction rates for some of the methods are provided in the Painting and Decorating Con-tractors of America (PDCA) Estimating Guide Volume 2, Rates and Tables. (www.PDCA.org),as well as ICRI Guideline No. 310.2-1997 described above (www.icri.org).

Coating-removal methods for CMU and brickAs is apparent from the review of the standards, only a few of the methods are suitable forwholesale removal of existing coating from CMU and brick, and even then, there are risks as-sociated with damage to the substrate or the potential for excessive water intrusion. Also, aneffective method of coating removal—chemical stripping—is not addressed in the above stan-dards.

With any of the methods, it is advisable to do test areas before undertaking wholesale re-moval in order to perfect the removal process and to confirm the acceptability of any rough-ening of the substrate that may occur.

Brief summaries of methods that could be used on CMU and brick are provided below.

Dry abrasive blast cleaningCompressed air is used to propel abrasive particles against the surface at high velocity. The im-pact of the abrasive fractures and dislodges the paint (Photo 3).

While abrasive blast cleaning can efficiently remove paint, when used on smooth and split-faced block, all traces of paint within the irregularities of the surface will not be removed with-out significant erosion of the block surface (Photo 4). Abrasive blast cleaning also has thepotential to cause damage to mortar joints if lengthy dwell times are required to remove thepaint.

Coatings such as elastomerics can also be difficult to remove by abrasive blast cleaning be-cause the abrasive tends to bounce off of the coating, rather than cut into it. In these cases, theoperator must attempt to “work” the coating off the surface by blasting beneath it at an angle,but if the adhesion is good, even this process may not be successful.

Blast-cleaning abrasives include a variety of slag byproducts (boiler slag, copper slag, nickelslag), garnet, crushed glass, sponge, walnut shells, and others.

The operation is very dusty, caused by the breakdown of the paint, breakdown of the abra-sive as it impacts the surface, and the erosion of the CMU or brick. If the dust cannot be toler-ated due to ambient air regulations or the location of the building, ventilated containmentsystems that are maintained under negative pressure using dust-collection equipment can beused, but they are costly to install and maintain.

Guidance on containment systems can be found in SSPC Guide 6, Guide for ContainingSurface Preparation Debris Generated During Paint Removal Operations.

Wet abrasive blast cleaningWet abrasive blast cleaning is a variation of dry abrasive blast cleaning described above withthe same potential for damage to the substrate (Photos 4 and 5). The principles of operationand coating removal are the same except that water is introduced into the compressedair/abrasive stream to significantly reduce the generation of airborne dust. Water can be in-troduced using a special nozzle that mixes the water with the abrasive prior to exiting the noz-zle, or through a water collar that is attached to the nozzle, which introduces water to theabrasive as it exits the nozzle.

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Photo 3: Dry abrasive blast cleaning

Photo 4: Abrasive blast cleaning can significantly erode thesubstrate when used in attempting to remove all coating, orleave patches behind when an objective is minimizing damage

Photo 5: Wet abrasive blast cleaning

Systems are also available that create a water/abrasive slurry that is pumped through the blast hose usingcompressed air.

Sodium bicarbonate blast cleaningSodium bicarbonate blast cleaning is similar to wet abrasive blast cleaning, except that the abrasive con-sists of baking soda, but with larger particle sizes than the baking soda found at home. The sodium-bi-carbonate abrasive is propelled to the surface using compressed air. Water is typically added to theabrasive stream at the nozzle to control the dust. Without the water, the operation is extremely dusty.

The process can remove paint, but it is best used to clean the surface (remove chalking, graffiti, and con-tamination) prior to painting. It can also be used on bare brick for the removal of streaks of efflorescenceprior to the application of seal coats. It is important to flush all residues of sodium bicarbonate from the sur-face before painting or sealing the surface.

Pressure washing (low-pressure water cleaning (<5,000 psi), and high-pressure water cleaning (5,000to 10,000 psi))SSPC-SP12/NACE No. 5, Surface Preparation and Cleaning of Metals by Water Jetting Prior to Recoat-ing, divides pressurized water systems into two broad categories: Water Cleaning (less than 10,000 psi)and Water Jetting (greater than 10,000 psi).

Despite the word “metals” in the title of the standard, water cleaning-and water-jetting equipment areused for the preparation of CMU and brick, although the definitions for the degrees of cleaning in the stan-dard are written for metallic substrates. Water cleaning is discussed below; water jetting is discussed in thefollowing section. The SSPC/NACE standard indicates that flow rates from 1 gal./min. to14 gal./min. arecommon for the water-removal methods.

Water Cleaning is divided into two categories: Low-Pressure Water Cleaning (less than 5,000 psi) andHigh-Pressure Water Cleaning (5,000 to 10,000 psi). Althoughthe methods are labeled as “water cleaning,” they can effec-tively remove loose coating (Photo 6). Pressure-washing meth-ods are also capable of removing intact coatings, especially atthe higher pressures and when a zero-degree rotating tip isused.

When removing all coatings, the downside to these methodsis that a large volume of water is introduced to the surface (gen-erally 5 to 10 gals./min.) which, when combined with thelonger dwell times required to remove all coatings, creates thepotential for the water to permeate through the block to the in-side of the wall (Photo 7).

The efficiency of paint removal can be increased by injectingexpendable abrasive such as slag into the pressurized waterstream.

High temperature (250 F), pressurized water (generally<5,000 psi) can also be used to remove coatings. The highertemperature generally improves the cleaning efficiency whentotal coating removal is required (Photo 8).

Water jetting (high-pressure water jetting (10,000 to 30,000 psi) and ultra-high-pressure water jetting(>30,000 psi)Similar to water cleaning (described above), water jetting is also addressed in SSPC-SP12/NACE No. 5,Surface Preparation and Cleaning of Metals by Waterjetting Prior to Recoating. High-pressure water jet-ting (10,000 to 30,000 psi) and ultra-high-pressure water jetting (>30,000 psi) can efficiently removecoatings to the bare substrate (Photo 9).

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Photo 6: Pressure washing effectively removesloose coating

Photo 7: Interior surface exhibits penetrationof water (5,000 psi) due to the long dwelltimes and volume of water needed to removethe exterior coating

Photo 9: Ultra-high-pressure water jettingsuccessfully removed all coating from threedifferent block types (smooth face, split face,and scored CMU) on a single wall.Photo courtesy of Final Coat Painting.

Photo 8: High-temperature water (250 F) at3,000 psi softens and strips the coating tothe substrate

The water-jetting units typically use less water than the pressure-washing units, reducing the potential forwater intrusion into the block. Water-jetting units are also available with vacuum assemblies that surroundthe nozzles to capture and vacuum the debris and water as it is generated (Photo 10). The vacuumed watercan either be recycled or containerized for disposal.

Power-tool cleaningPower-operated hand tools include power sanders, power grinders, and impact tools such as roto-peensand needle guns. Power tools remove coating by wearing it away (sanding/grinding, Photo 11) or by im-pact to fracture it from the surface (roto-peens/needle guns).

The impact tools are not commonly used on CMU or brick due to the potential for damage to the sub-strate. The use of power tools is labor intensive, and they will not remove all traces of coating from theporous block surface. They are best used for localized removal of loose coating, or in the case of powersanding, for feathering the edges of intact coating to create a smooth transition between coats or to the baresubstrate (Photo 12).

Chemical strippingWater-based chemical strippers effectively remove multiple layers of old coating without damaging thesubstrate. The water-based strippers do not contain methylene chloride or caustic materials, and arebiodegradable. The strippers are applied to the surface by brush, spray, or roller.

After application, the dwell time prior to removal depends on the coating type, coating thickness, andambient temperature, but the typical duration is overnight, followed by removal by pressure washing (PhotoNo. 13, next page). For small areas, removal can be accomplished by scraping, and bucket and sponge.Depending on the number of layers to be removed, a second application may be necessary.

Coating-removal guidel ines, in summaryA few industry standards address the preparation of concrete substrates for painting, but the focus is onfloors, bare concrete, and poured concrete. In addition, definitive acceptance criteria for determining theadequacy of cleaning are not provided.

As a point of comparison, when preparing steel surfaces, SSPC/NACE specifications explicitly define theacceptance criteria (e.g., Near-White Blast, Commercial Blast, etc.). No such guidance is available forcleaning masonry surfaces, especially CMU and brick. The industry would be well-served if such standardswere developed.

Despite the limitations of the standards in terms of definitive acceptance criteria and the focus on floorsand bare and poured concrete, many of the methods are applicable to the removal of existing coatings fromCMU and brick. These methods include:• Dry abrasive blast cleaning• Wet abrasive blast cleaning• Sodium bicarbonate blast cleaning• Pressure water cleaning (low pressure <5,000 psi; high pressure 5,000 to 10,000 psi)• High temperature pressure water cleaning (<5,000 psi)• Water jetting (high pressure, 10,000 psi to 30,000 psi; ultra-high pressure, >30,000 psi)• Power tool cleaning• Chemical stripping

As indicated at the beginning of the article, when formulating a paint-removal project, the review of afew questions will help the facility owner, architect, engineer, and contractor prescreen the various meth-ods of coating removal. These questions are repeated below, together with typical methods of surfacepreparation that are suitable for each.

Note the methods suggested for each question are based on the author’s experience, rather than an in-dustry standard, and should not be considered as absolute. The attributes of the various methods of sur-face preparation as reflected by the questions are also summarized in Table 1 (page 16).

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Photo 10: Ultra-high-pressure water jettingwith vacuum shroud collects water and paintdebris at the point of generation.Photo courtesy of Final Coat Painting.

Photo 11: Power tools are best used forlocalized coating removal.

Photo 12: Edges of paint feathered by powersanding

Whenever possible, test areas should be prepared on site to confirm the appropriateness of the method(s)before engaging in wholesale cleaning.1. Does the existing coating have to be completely removed or just the loose coating?

a. Methods typically suitable for partial coating removal: sodium bicarbonate blast cleaning, pressure water cleaning <10,000 psi), high-temperature pressure water cleaning (<5,000 psi), water jetting (>10,000 psi), and power-tool cleaning (sanding, grinding).b. Methods typically suitable for total coating removal: dry abrasive blast cleaning, wet abrasive blast cleaning, high-temperature pressure water cleaning (<5,000 psi), water jetting (>10,000 psi), and chemicalstripping.

2. Can heavy roughening of the block or brick face be tolerated, including some damage to mortar joints?a. Methods that will typically result in little to no roughening: sodium bicarbonate blast cleaning, pressurewater cleaning <10,000 psi), high-temperature pressure water cleaning (<5,000 psi), water jetting (>10,000 psi depending on pressure and dwell time), power-tool cleaning (sanding, wire brushing), and chemical stripping. Note that for historic preservation, with the exception of chemical stripping, the roughening created by the above methods may be unacceptable.b. Methods that will typically result in heavy roughening: dry abrasive blast cleaning, wet abrasive blast cleaning, water jetting (>10,000 psi depending on pressure and dwell time), and power-tool cleaning (impact tools).

3. Can existing paint be allowed to remain within the porosity of the block?a. Methods that will typically leave only a slight amount paint in the porous surface: high-temperature pressure water cleaning (<5,000 psi), water jetting (>10,000 psi), and chemical stripping.b. Methods that will typically leave more than traces of paint in the porous surface: abrasive blast cleaning,wet abrasive blast cleaning, sodium bicarbonate blast cleaning, pressure water cleaning (<10,000 psi), and power-tool cleaning.

4. Can large volumes of water be tolerated (environmentally, and in terms of potential water intrusion intothe block or brick)?

a. Methods that typically use moderate to no water: dry abrasive blast cleaning, wet abrasive blast cleaning,water jetting (>10,000 psi if vacuum shrouded), power-tool cleaning, and chemical stripping (if removed by scraping and bucket/sponge, rather than pressure washing).b. Methods that typically use a large volume of water: sodium bicarbonate blast cleaning, pressure water cleaning <10,000 psi), high-temperature pressure water cleaning (<5,000 psi), water jetting (>10,000 psi without vacuum recovery), and chemical stripping (removed by pressure washing).

5. Can airborne dust be tolerated?a. Methods that generate little to no airborne dust: wet abrasive blast cleaning, sodium bicarbonate blast cleaning, pressure water cleaning (<10,000 psi), high-temperature pressure water cleaning <5,000 psi), water jetting (>10,000 psi), power-tool cleaning, and chemical stripping.b. Methods that generate a great deal of dust: abrasive blast cleaning.

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Photo 13: Existing paint on split-faced blockcoated with stripper and being removed bypressure washing

1—This table represents the practical application of the various methods when used under normal operations based on the au-

thor’s experience, but it is not absolute. For example, abrasive blast cleaning can partially remove existing coating from CMU or

brick, but it is not commonly used for this purpose.

2—Slight to heavy roughening can occur with water jetting, depending on dwell times and pressures.

3—Roughening created by power-tool cleaning is dependent on the tool being used, ranging from slight roughening with power

sanding to heavy roughening with power impact tools.

4—Vacuum shrouding water jetting will significantly reduce the volume of water. Without vacuum shrouding, the volume is

greater, but typically not as high as pressure washing at <10,000 psi.

5—A large volume of water is used if the stripper is removed by pressure washing. Much less water is used if removed by

scraping and sponge/water, but this is only practical for small localized areas.

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Table 1: Practical Cleaning Attributes of Surface Preparation Methods1

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Six Key Points You Should Knowabout Concrete Surface Preparationbefore Coating ApplicationBy Fred Goodwin,

BASF Construction Chemicals

Editor’s note: This article appeared inD+D in July/August 2012 as part of aSpecial Report on Concrete Maintenanceand Protection.

oncrete is perceived as a common, hard, durable, and relatively unexciting materialsurrounding us in our homes, highways, bridges, airports, workspace, sewers, mines, andmonuments. Coating concrete is the most common method to enhance its appearance and

improve its durability. Proper surface preparation is one of the most important stages in achieving successfulcoating installation. The most important factors in surface preparation of concrete are:• project objectives,• concrete surface quality,• compatibility of the surface preparation with job site conditions and coating system,• contamination of the concrete,• moisture content and movement in the concrete, and• concrete surface profile.

Project objectivesThe project objectives must be determined; otherwise, they can become a moving target. These include thesubstrate conditions, coating requirements, owner requirements, and application conditions, all of whichmust be well thought out together.

Define with the owner and other interested parties what success means for this project. Mockups arevery helpful in deciding what can be done and can serve as a test bed for different techniques, materials,and cost vs. performance results. Decide in advance what happens if the results are less than expected—who pays, what penalties are assessed, and who can arbitrate disputes?

Also, agreement needs to be reached about the project “tolerables,” such as how to mitigate the side ef-fects of the construction process (e.g., noise, dust, vibration, fumes); what to do with debris (especially if ahazardous material); what, if any, utilities (power, ventilation, water, etc.) are available for the needed pro-cedures; what kind of protection from weather and traffic is possible for the project area; and how will theenvironment around the project be protected from the construction activity.

C

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Concrete sur face quality Not all concrete is created equal. With varying degrees of success, coatings are applied to all types of con-crete, such as ”green” fresh concrete, fully cured virgin concrete, contaminated concrete, and previouslycoated concrete. Concrete’s appearance doesn’t change much, but the properties can vary widely basedon the water-to-cement ratio, aggregates, admixtures, curing, age, orientation, service history, and whatis and has been in contact with the concrete.

The concrete’s exposure to different conditions can also influence the extent of surface preparation andthe techniques. Concrete can be oriented horizontally both on- and above-grade, sloped, vertically, andoverhead. Different surface preparation techniques are more suitable for different orientations.

ACI 201.1, 201.2, 311.1, and 364.1 are especially useful for evaluating concrete to receive coatings.SSPC-SP 13/NACE 6 is also a useful reference for characterizing the concrete surface and qualifying theprepared surface (Tables 1 and 2). Cracks must also be addressed, both to prevent further cracking in theconcrete as well as reflective cracking in the coating system (ACI 224.1).

Table 1: Typical Surface Properties of Finished Concrete

* From SSPC-SP 13/NACE No. 6

(A) These surface properties are based on similiar concrete mix, placement, and vibration and are prior to surface preparation.(B) Strength depends on application and cure.(C) Shotcrete may be refinished after placement, which would change the surface properties shown in this table.

Method Profile Porosity(A) Strength(A) Problems

Formed concrete Smooth to medium Low to medium Medium Voids, protrusions,release agents

Wood float Medium Medium Medium

Metal trowel Smooth Low High

Power trowel Smooth Very low High Very dense

Broom finish Coarse to very coarse Medium Medium

Sacking Smooth Low to medium Low to high(B) Weak layer if notproperly cured

Stoning Smooth to medium Low to medium Low to high(B) Weak layer if notproperly cured

Concrete block Coarse to very coarse Very high Medium Pinholes

Shotcrete(C) Very coarse Medium Medium Too rough for thincoatings

Sur face preparation and coating system compatibi li tyEach coating type requires minimum substrate conditions to assure compatibility. Sealers require surfacepreparation mainly to promote penetration of the concrete, as any visible defects or profile will be unaf-fected by the sealer. Surface polishing is an enhancement for concrete surfaces that are usually either in-tegrally colored or then stained and sealed. Thin-film coatings may be formulated to mask very minordefects and surface discolorations. Thicker coatings such as self-leveling materials, polymer overlays, top-pings, and high-build coatings have much in common with thin-film coatings regarding the relationship be-tween surface profile and dry film thickness, moisture sensitivity, and wear; however, the thicker coatinglayer can fill larger defects, allow non-skid surfaces through surface texture, and provide longer servicelife than materials applied in thinner layers. Existing cracks will tend to telegraph through an applied coat-ing. The use of impact, pulverization, and other mechanical methods of surface preparation can cause“bruising” or microcracking, creating a surface layer weakened by partially interconnected cracks in theconcrete. It is very unlikely that any coating system will have satisfactory performance over a bruised sub-strate. Surface preparation methods are described in Table 2.

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Table 2: Summary of Surface Preparation Methods, Applicability, Equipment, Mechanisms and Resulting Surface Texture and CSP Ranking*

*CSP (Concrete Surface Profile) profile range according to ICRI 310.2 (Goodwin)

Method Equipment Mechanism Surface Texture Achieved CSP RankingDetergent Scrubbing Mop and Pail, Floor Emulsification No change 0-1

ScrubberLow Pressure Water Pressure Washer Emulsification (if soap Removal of loose debris 0-1Rinse in water), Erosion

(of loose particles)Acid Etching Acid, Mixing, Container, Reaction Light profile, removal of concrete paste, 1-3

Neutralizing Agent discolorationDry Grinding Dry Grinder Erosion Smooth surface, dust, debris to remove, pattern 1-3Wet Grinding Wet Grinder Erosion Wet, smooth surface, slurry, debris to remove, 1-3

patternDry Abrasive Dry Sand Blast Pulverization, Erosion, Dusty substrate, light profile (depending on 2-4Blasting Expansive Pressure media, size, pressure, time) debris to removeRecuperative Vacuum Recovery Pulverization, Erosion, Light profile (depending on media, size, pressure, 2-4Abrasive Blasting Sand Blasting Expansive Pressure time)Wet Abrasive Wet Sand Blast Pulverization, Erosion, Wet substrate, light profile (depending on media, 2-4Blasting Expansive Pressure size, pressure, time) debris and slurry to removeShot Blasting Shot Blast Unit Pulverization, Impact Dust free substrate, some pattern, depth 2-8

Erosion dependent on shot size, sustrate hardness, equipmentScarifying Scarifier Impact Dusty substrate with striated pattern, bruising 4-9

likely, debris to removeNeedle Scaling Needle Scaler Impact Similar to shot blasting, striated pattern, debris 5-8

to removeScabbling Scabbler Impact Dusty substrate, irregular pattern, fractured 7-9

aggregate, bruising likely, debris to removeHydrodemolition, High- and Ultra-High- Erosion, Expansive Saturated substrate, debris to remove, profile 6-9Hydroblasting, Water Pressure Water Blast Pressure dependent on substrate hardness, equipment,Jetting pressure, timeFlame Blasting Special Oxy-acetylene Expansive Pressure, Irregular chipped surface, hot, charred debris to 8-9

Torch, Saturated Reaction remove, bruising possibleSubstrate Helpful

Rotomilling Rotomiller Impact Dusty substrate (unless water used to suppress 9dust), grooving, tool marks, fractured aggregate,bruising likely

Liquid Surface Etchant Specialty Chemical, Reaction Exposed aggregate, green wet concrete with 3-9Fresh Concrete debris to remove using pressure wash, curing

still required, no bruising, depth dependent onretarder chemistry, curing rate, length of exposure

Table 3: Suggested Performance Tests for Concrete Following Surface Preparation*Property Test Method Light Service (A) Servere Service(B)

Surface tensile strenght See Paragraph A1.6 1.4 MPa (200 psi) min. 2.1 MPa (300 psi) min.Surface profile Visual comparison Fine (150) abrasive paper min. Coarse (60) abrasive paper min.Surface cleanliness Visible dust No significant dust No significant dustResidual contaminants Water drop 0° contact angle 0° contact anglepH ASTM D 4262 (pH of rinse water) -1, +2(C) (pH of rinse water) -1, +2(C)

Moisture content(D) ASTM D 4263 No visible moisture No visible moistureMoisture content(D) ASTM D 1869 15 g/24 hr/m2 (3 lb/24 hr/ 15 g/24 hr/m2 (3 lb/24 hr/

1,000 ft2) max. 1,000 ft2) max.Moisture content(D) ASTM D 2170 80% max. 80% max.

*From Table 1 in SSPC-SP 13/NACE No. 6

(A) Light service refers to surfaces and coatings that have minimal exposure to traffic, chemicals, and changes in temperature.(B) Servere service refers to surfaces and coatings that have significant exposure to traffic, chemicals, and/or changes in temperature.(C) The acceptance criterion for ASTM D 4262 is as follows: The pH readings following the final rinse shall not be more than 1.0 lower or 2.0 higher than the pH of the rinsewater (tested at the beginning and end of the final rinse cycle) unless otherwise specified.(D) Any one of these three moisture content test methods is acceptable.

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Primers can also be considered as a form of surface conditioning following surface preparation: Theybridge dissimilar materials (i.e., concrete and coating); seal the substrate from outgassing (i.e., the emis-sion of gas or vapor from the substrate usually due to displacement from the absorption of the coating ortemperature changes in the substrate); encapsulate dust; and increase coating coverage by limiting the ab-sorption of the liquid from the coating.

ContaminationOne description for concrete is ‘a hard, wet sponge.’ Like a sponge, concrete can have residual contami-nants from previous treatments or absorb contaminants from the environment that can inhibit bond, re-duce durability, and create other problems with applied coatings. Residual contaminants include incidentaldust, form release transfer, curing compounds, existing membranes, adhesives, water repellants, and coat-ings.

Residual contaminants, usually present on the surface, can be removed during surface preparation; how-ever, elastomeric materials may respond poorly to some types of surface preparation such as grinding,shot blasting, and abrasive blasting.

Absorbed contaminants can be carbonation, water-soluble materials, and solvent-soluble materials suchas oils that all penetrate the concrete and may chemically react to create additional issues in the concrete.A listing of the effects of many contaminants can be found in ACI 515.1R or PCA IS001, but individual con-sideration of the consequences of the contamination must be given, depending on the nature of the con-taminant, depth of penetration, and any reactions that may have occurred. If deleterious materials arefound to interfere with the coating system, many can be removed by excavating the concrete during sur-face preparation to their penetration depth.

Moisture content and movement in the concreteMoisture in concrete can come from many sources, such as the water used for placing or curing the con-crete, maintenance or usage (e.g., washing a concrete floor or process water leaks), exterior infiltration frompoor drainage or weather protection, or even the selected method of surface preparation (Table 3). Forsome cementitious materials, moisture in the concrete is not a problem, and bonding performance actuallycan be improved using a “saturated surface dry (SSD)” concrete substrate. For moisture-sensitive materi-als, drier concrete is better. Achieving sufficiently low moisture emissions is often one of the greatest diffi-culties encountered with concrete substrates and requires evaluation and considerations discussed in ACI302.2 and elsewhere (Kanare), but no completely satisfactory solution exists for all situations. ICRI has atechnician certification program for several tests of moisture in concrete.

Profi leDifferent surface preparation methods produce a wide range of profiles, best defined by ICRI 310.2 Con-crete Surface Profile (CSP) replica chips. Achieving bond to smooth surfaces is usually more difficult thanto textured surfaces. Rougher surfaces have greater surface area, will tend to hide defects, but requirehigher build coatings with lower coverage rates. Usually, a particular coating system will specify the re-quired profile. ASTM D7682-10 is a new test method that produces a permanent replica of the concretesurface, which can then be compared to visual profile standards (e.g., ICRI CSP’s) or evaluated quantita-tively for profile depth.

SummaryWhen concrete is the substrate and the coating is what is visible, the difference between success and fail-ure lies in between. Concrete is a durable material, and coatings can help maintain and in some cases im-prove that durability. To achieve that purpose, the concrete must be sound, repairs must be performed asnecessary, the surface must be properly prepared, the appropriate coating must be selected, and the coat-ing system must be correctly installed. Surface preparation is one of the most important and frequently neg-lected factors in achieving successful coating of concrete.

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References1. ACI 201.1R-08, Guide for Conducting a Visual Inspection of Concrete in Service, 2008, ACI International,Farmington Hills, MI. 2. ACI 201.2R-08, Guide to Durable Concrete, 2008, ACI International, Farmington Hills, MI.3. ACI 311.1-R07 ACI SP-2(07), Manual of Concrete Inspection, 2007, ACI International, Farmington Hills, MI.4. ACI 364.1R-07, Guide for Evaluation of Concrete Structures before Rehabilitation, 2007, ACI Interna-tional, Farmington Hills, MI.5. SSPC-SP 13/NACE No. 6, Surface Preparation of Concrete, 2003, The Society for Protective Coatings,Pittsburgh, PA. 6. ACI 224.1R-07, Causes, Evaluation, and Repair of Cracks in Concrete Structures, 2007, ACI Interna-tional, Farmington Hills, MI.7. ICRI No. 310.2–1997, Selecting and Specifying Concrete Surface Preparation for Sealers, Coatings, andPolymer Overlays (formerly No. 03732), 1997, International Concrete Repair Institute, Des Plaines, IL.8. Goodwin, F., “An Overview of Preparing Concrete for Coatings; What to Ask, What to Do,and Where to Find Help,” JPCL May 2009, pp. 40-48. 9. ACI 515.1R-79 (reapproved 1985), Guide to the Use of Waterproofing, Dampproofing, Protective, andDecorative Barrier Systems for Concrete, HIS Services, http://www.ihs.com.10. PCA IS001, Effects of Substances on Concrete and Guide to Protective Treatment, 2007, Portland CementAssoc., Skokie, IL.11. ACI 302.2R-06, Guide for Concrete Slabs that Receive Moisture-Sensitive Flooring Materials, 2006,ACI International, Farmington Hills, MI. 12. Kanare, H. M., “Concrete Floors and Moisture,” 2005, Portland Cement Assoc., Skokie, IL.13. ASTM D7682-10, Standard Test Method for Replication and Measurement of Concrete Surface Pro-files Using Replica Putty, 2007, ASTM International, West Conshohocken, PA.

D+D

22

Department of Defence: Protecting Wood Exteriors

Getting ItRight

Editor’s Note: This article appeared inJAC in June/July 2009.

By Jayson L. Helsel, P.E.KTA-Tator, Inc.

ood is a versatile material that is commonly used in residential and commercial con-struction, with typical uses that include various types of siding and shingles, in additionto framing for doors and windows.

When used in exterior exposures, wood generally needs to be painted or stained to provide protectionfrom the effects of weathering. When wood is selected for exterior uses, it is important to realize that morefrequent maintenance of the coating or stain system is typically required than is the case with steel or con-crete substrates. This makes recognition of common problem areas and specification of the proper coat-ings all the more critical.

Moisture issuesThe most common coating problems related to wood can be attributed to excessive moisture content or adegraded wood surface. New or “green” wood generally contains moisture on the order of 20% or greater,which is too high for successful application of paints or coatings; many coatings manufacturers recom-mend a moisture content of 15% or less before painting. The moisture content can be determined in mostcases using a moisture meter that measures the resistivity between two probes held on the wood surface.It is important to check any specific guidelines a coatings manufacturer provides for moisture content andinstrumentation recommendations.

WSuccessful strategies

are built on attention

to substrate condition,

selection of coating

material

23

Once wood has initially dried to sufficient moisture levels for painting, environmental sources of watercan continue to be a problem. Wood, being an absorbent material, will readily allow water to penetrateif given the opportunity. In this respect, the design of a structure must allow for proper drainage of rain-water or other sources of moisture. For example, if wood siding is installed on a building, moisture mustnot be allowed to seep behind the wood planks. Equally important is taking care to apply paint or coat-ings to all exposed sides of the wood surface such as edges. Even if the edges of wood-siding planks arebutted against one another, the smallest gap can allow water to penetrate the wood at these edges.

Edges are particularly vulnerable, since the ends of the wood grain are typically exposed, allowing waterto travel into the wood along the grain pattern. Even the back or unexposed side of a wood plank or sid-ing section may need to be painted or primed as recommended by the coating manufacturer.

Effects of moistureOnce moisture gets into the wood, regardless of the source, it becomes aproblem for any applied coating. Just as the coating film prevents penetra-tion of the substrate by water from the exterior environment, the coatingalso can prevent any absorbed moisture from escaping from the wood. Thismoisture entrapment eventually causes a failure of the coating due to blis-tering, peeling, cracking, and general loss of adhesion.

Most moisture problems can be avoided if the moisture content of thewood is sufficiently low before proceeding with painting.

Wood degradationAside from moisture, the other major problem area encountered whenpainting wood is a degraded wood surface. Wood will degrade relativelyquickly when left exposed to sunlight. Although the effects may not be im-mediately obvious, degradation due to ultraviolet exposure from sunlight oc-curs in some types of wood (e.g. red cedar) in as little as one week,according to studies by the U.S. Forest Service Forest Products Laboratory.

Degraded wood creates a weak surface layer that eventually detachesfrom an underlying sound wood layer. How quickly this may occur dependson how severely the wood surface was degraded, the exposure conditions,and the type and thickness of coating(s) applied. Regardless of other fac-tors, failure is likely to occur in a relatively short period if the wood surfaceis unable to tolerate the curing stresses of the coating. In other words, theprocess of drying and curing of the coating can impose sufficient stress tocause a failure of the system at its weakest link, the degraded wood surface.

The only solution to a degraded wood condition problem is removing theweak surface layer prior to coating application. If the problem has alreadyresulted in a coating failure, then all poorly adhered coating—along withthe degraded wood layer—must be removed. This situation can be prob-lematic in that it may be difficult to remove all of the existing coatings andat the same time avoid damage to the wood surface.

Preparation methodsPressurized water cleaning of the wood or existing painted surface is thetypical method of preparation prior to application of paints, coatings, orstains. Low pressures (less than 2,000 psi) should be used, and care mustbe taken to avoid damage to the wood substrate by adjusting the nozzle,water pressure, and stand-off distance as needed. Caution is also neededto avoid forcing water into any gaps that may exist in the structure thatwould cause intrusion into the interior walls. The cleaned wood surfacesmust be thoroughly dry before applying any new paint.

Previous page: Specification of the right coat-ings is pivotal in formulating initial painting andmaintenance-repainting programs for buildingswith exteriors that include wood elements.Photo courtesy of Cabot Stains.

Excessive moisture in the wood and a degraded wood surface are two primary con-tributors to coating failure. Photo courtesy Duffy Hoffman, Preservation Specialist.

If coating failure is apparent, all poorly adhered coating and the degraded wood layer must be removed prior to application of new paint or coating.

Photo courtesy of Duffy Hoffman,Preservation Specialist.

Additional preparation by sanding may also be needed to further remove existing coatings that are not welladhered, or to further prepare bare wood surfaces. If older wood substrates show too much damage, such ascracking, the surface will need to be repaired or possibly replaced. Wood-repair materials recommended bycoatings manufacturers for application of specific coating products should be used. Many wood-repair mate-rials are epoxy based and must be painted relatively quickly when exposed to exterior weathering conditions.

Coatings choicesA variety of coatings can be successfully applied to wood substrates; selection depends in part on the ap-pearance traits desired for the finished surface. Coatings for wood surfaces can be generally categorized intopenetrating finishes, such as water-repellent products and pigmented semitransparent stains, or film-formingfinishes such as paints, solid-color stains, and varnishes.

The protection and service life that a coating (system) provides is related to factors such as whether the ma-terial is opaque and how thickly it is applied. Since water repellents are generally clear materials with little mea-sureable film build, they are known to possess the shortest service life (often just 6 to 12 months), and willrequire the most frequent maintenance. Other penetrating products, such as pigmented stains, will provideprotection for a much longer period before maintenance is required.

The most durable penetrating finish products are semitransparent stains that potentially can rival the servicelife of typical paint products. Depending on formulation, the pigments in semitransparent stains can absorbmuch of the ultraviolet radiation that causes degradation in wood. Penetrating finishes are typically applied inmultiple thin coats.

Film-forming finishes, as the name suggests, result in a thicker, measurable coating film upon curing. Theseproducts by nature offer better protection since they are generally opaque and provide a much thicker film buildover the substrate than penetrating finishes. Generally, acrylic paints are judged to be the best-performingcoatings for wood, with the coating system typically consisting of a primer and one or two finish coats, result-ing in a total dry film thickness of 6 to 9 mils. An acrylic system can deliver a service life of up to 10 years.

Typical paint systems for wood include acrylic and alkyd (oil-based) products. Alkyd paints are usually sol-vent-based products that result in a harder and less flexible (more brittle) paint film as compared to acrylicpaints. Alkyds generally offer moderate to good exterior weathering characteristics, although various modifi-cations to alkyd formulations can be made to improve certain properties. One such modification involves theaddition of polyisocyanates to improve drying time and chemical and abrasion resistance. These resins are com-monly called uralkyds (“ur” from urethanes), and they generally exhibit excellent gloss and color retention.Uralkyds may also be used for interior floor coatings, where these types of products may be referred to as“polyurethanes.”

Acrylic or latex paints are water-based materials characterized by a more flexible paint film, compared toalkyd coatings. Acrylic coatings also generally provide better exterior weathering characteristics, which trans-lates to better color and gloss retention. Any exterior “acrylic” products should be identified by the manufac-turer as containing “100% acrylic” resin versus an acrylic/vinyl or polyvinyl acetate resin.

With wood, per formance is built on fundamentalsAs emphasized in this review, the moisture content and substrate condition of wood surfaces must be properlyevaluated prior to application of paints, coatings, or stains. The selection of these products should be based onappearance objectives, while also taking into account the maintenance frequency required for various prod-ucts recommended for use on wood.

24

JAC

25

ost concrete surfaces will require repair, resurfacing, topping, overlayment, or coating atsome point during their service life. The overall success and performance of protectivecoatings, linings, or patching materials applied to concrete or masonry substrates are

highly dependent on the quality of the cleaning and surface preparation performed. Here, in Part 2 of our review of industry guidelines and standards for cleaning, preparation, and test

procedures for concrete, we focus on surface preparation and test methods.As emphasized previously, referencing and following the appropriate guides and standards published

by recognized industry organizations are strongly recommended, as is following cleaning and surfacepreparation instructions from the manufacturers of relevant coatings and repair materials. These industryguides and standards are listed on page 41.

A Firm Foundation—Part 2: Surface Preparation and Test Methods

Editor’s Note: This article appearedin JAC in January/February 2008.The first part of this article appearedin the October/November 2007 JAC.

By Charles H. Holl, Dayton Superior Corp.

M

Industry standards

and guidelines set

the bar for cleaning,

preparation, and

test procedures

for concrete

Photos courtesy of Dayton Superior Corp.

26

Preparation methods Chemical stripping is a wet method of surface preparation, generally using alkaline chemicals based onmethylene chloride, hydroxide, citrus, and soy to dissolve or soften cured coatings for subsequent me-chanical removal. Chemical stripping of coatings and contaminated films from concrete is usually confinedto small areas that cannot be prepared more effectively by other means. Chemical stripping requires ad-ditional cleaning and surface preparation before applying a coating. It should never be used as the solecleaning method, because contaminants in the form of the removed material and/or the high-pH stripperchemical itself, are always left behind. To verify that the alkaline chemicals have been neutralized, pHshould be tested per ASTM D 4262.Shot blasting (ASTM D 4259; ASTM D 4258; SSPC SP13/NACE No. 6; and ICRI Guideline No.

03732). Centrifugal shot blasting is a very effective, clean, and dust-free method for removing contami-nation from hardened films and for texturing horizontal concrete without using water or chemicals. (Note:this is the method most commonly recommended by coatings manufacturers).

Shot blasting involves impacting the surface with high-velocity steel shot abrasive. The shot-blastingmedia, available in a range of sizes and shapes, are directed against the concrete from an enclosed, high-velocity, rotating paddle wheel. The abrasive, dust, and contaminants are then removed by a separate dustcollector. The cleaned steel shot is then recycled to the blast wheel, where the cycle repeats.

Shot blasting provides a clean, physically sound substrate with a rel-atively uniform texture, ranging from fine granular to a coarse sand-paper finish. Shot blasting is particularly useful and cost-effectivewhen used on large, unobstructed, horizontal surfaces. Preparingmost concrete surfaces using shot blasting generally removes up to amaximum of 3 mm (0.125 in., or 1/8 in.) of the surface per pass. If only laitance is to be removed, or if the surface is to be prepared

for thin (<890 micron [35mil]) coatings, fine-grade steel-shot abrasiveshould be used. The profile made by this type of equipment can showthrough thin-film coatings as a track line. Using fine shot and/or fasttravel speeds on the machine will produce a light etch, or “brushblast” on the surface, and can minimize the tracking lines. This brush-blast etch or surface profile breaks open the slick concrete surface tofacilitate mechanical adhesion of coatings, toppings, or overlayments.

Although shot blasting is primarily used on horizontal concretefloors, specialized machines can be used for shot blasting vertical sur-faces as well.

If a thick-film topping or overlay is to be installed, a much deeperetch must be achieved. This will require the use of coarse steel shot orsetting the travel speed of the machine at slow, which will produce asurface that exposes the top of the coarse aggregate in the concrete.In some cases, multiple layers of thin-film materials require the use ofangular steel grit. Grit punctures rather than fractures the coatingwithout adversely affecting the substrate. The use of angular steel grit,however, increases the wear and tear on the equipment.

So called “‘working mixes” consisting of approximately 60% shotand 40% grit will provide an aggressive removal method and reducethe wear factor on equipment, as compared to using 100% angulargrit. Care must be taken, however, to avoid contaminating the shot orgrit abrasive with oil, grease, and dirt, or exposing it to water. Steel-shot blasting is not very effective for removing rubbery, elastomericmaterials and some aliphatic urethane coatings; however, in somesituations it can be used for this purpose.

Abrasive blast cleaning, or sand blasting (ASTM D 4259, D 4258, and D4285; SSPC SP13/NACE No. 6;and ICRI Guideline No. 03732). This type of blast cleaning is a method for preparing and texturing con-crete surfaces by means of impact with a high-velocity stream of fine mineral-aggregate abrasives propelledby clean, compressed air. The blasting media usually consists of hard, angular mineral aggregates of a sizerange selected to be most effective. Blast cleaning with this method produces a textured, physically soundsubstrate that is free of surface contamination and fines.

The actual surface hardness of concrete should determine whether it is best to prepare it by abrasive blastcleaning. Test areas should be treated, using the same equipment, air pressure, hose lengths, nozzle size,and abrasive being considered for the job. Production rates, dusting, and cleaning profiling (roughening)effects should be noted for bidding purposes. Generally, larger abrasive sizes are used for preparing con-crete than are used in blasting steel surfaces. Sand abrasives having a diameter of 2,000 to 1,600 microns(8-12 mesh) size are recommended for heavy cleaning. If only the removal of concrete laitance is required,a diameter of 840-350 microns (20-40 mesh) sand gradation is sufficient.

Mineral abrasives should have a sharp angular shape and be at least a 6.5 on the Mohs Mineral Hard-ness Scale, where talc is a 1.0 and diamonds are a 10.0. Mineral abrasives having a Mohs hardness ofless than 6.5 are too soft to clean at high production rates. Softer abrasives also are characterized by avery high breakdown rate, fracturing on impact and creating excessive dust and reduced efficiency.

Abrasives (sands) containing free silica should be avoided, as they can cause the lung disease silicosis.Wet abrasive blast cleaning and equipment may be used when dust abatement is required. Abrasive blastcleaning is not generally effective on rubbery, elastomeric materials, but it does lend itself to both horizontaland vertical concrete surfaces. The compressed air used in the blast cleaning must be checked to make surethat it is oil free; ASTM D 4285 is the recommended test method to determine this.Bush hammering (SSPC SP13/NACE No. 6 and ICRI Guideline. No. 03732). Bush hammering is an im-

pact method for roughening a concrete surface using a hammer device with a serrated face, with rows ofround or pyramidal, hardened steel points. This tool can be used on either vertical or horizontal concrete.Bush hammering produces a much-roughened surface by removing the top layer of concrete; concretemortar paste and aggregates are also removed. Bush hammering while removing deteriorated concrete canresult in fracture damage to the sound concrete below and at the exposed surface. Concrete that has beenbush hammered should be checked afterwards for its tensile strength in accordance with ASTM C 1583. Scarifying (SSPC SP13/NACE No. 6 and ICRI Guideline. No.03732). Scarifying is a method for re-

moving heavy concentrations of deteriorated concrete, surface contamination, or other substances fromconcrete floors. Scarifiers are heavy machines equipped with hardened-steel cutter wheels vertically alignedand arranged on a large horizontal cylinder that rotates at a high rate of speed. Scarification will removethe concrete surface in closely spaced parallel lines. Up to ½ inch of concrete can be removed in one pass.High and ultra-high water blasting and jetting, and hydro demolition (ICRI Guideline No.03732; ASTM

D 4259 and SSPC SP13/NACE No. 6 and No. 5). This method employs water that is sprayed at pressuresof 35–300 Mpa (5,000–45,000 psi). This method can be used to remove heavy dirt and loose, friable ma-terials, and to remove concrete and some coatings. This preparation method is usually more difficult inpractice than abrasive blast cleaning, producing a more irregular surface profile. Water blasting or jetting,however, may be preferable if it is imperative to avoid airborne sandblasting media, cement particles, anddust.

High-pressure water produces a surface profile of varying degrees, free of all contamination. It is oneof the most efficient methods for selectively removing and preparing concrete. Hydro-demolition water jet-ting does not produce the bruised, fractured layer in the concrete work area that other impact methods cancreate. Consideration must be given, however, to what methods will be used to control and dispose of thespent water.Power tool methods (ASTM D 4259; SSPC SP13/NACE No. 6). These methods include circular grind-

ing, sanding, and wire brushing, and may be used to remove existing coatings, laitance, weak concrete,fins, and protrusions.

27

Preparation methods such as blasting withwater or abrasives play a major role in thesuccess of coating application to concretesurfaces. Photo courtesy of Dayton Superior

Corp.

Floor grinders such as the machine shownhere are used to remove surface laitance and

contaminants, and reduce waviness in concrete floors.

Photo courtesy of Blastrac Corp.

Etching with acid (ASTM D4260; ASTM D4258; SSPC SP13/NACE No. 6 and ICRI Guideline.No.03731). This method is applicable to horizontal concrete floors only. Acid etching works fairly well onnon-surface-hardened floors, but is difficult or impossible on vertical surfaces. (Note: This method is not rec-ommended by most coatings manufacturers.) Acid etching does roughen the surface, but does not removelaitance or other loose material.

Acid etching often employs a 5-10% solution of muriatic (hydrochloric) acid in clean water. The concreteshould be pre-wet, and all oil, grease, paint, sealers, gum, tar, and any other foreign materials must be re-moved to assure uniform etching of the surface.

Many products containing surfactants are readily available at commercial contractor centers for de-greasing the surface of concrete/masonry substrates. Scraping or freezing, then impact scraping can re-move gum, tar, and other thick and soft contaminants. After the surface is free of contaminants, one gallonof acid solution is spread on 5 to 7 m2 (50 to 75 sq. ft.) of concrete and allowed to stand for two to threeminutes. The surface should be immediately rinsed with fresh water to avoid the formation of salts. This pro-cedure should be repeated until the concrete exhibits the texture of medium sandpaper. It is advisable,whenever possible, to use a pressure-wash rinse with a minimum pressure of 13,800 KPa (2,000 psi),using a 15° tip for good results.

Pressure washing forces the fines out of the pores in the surface and assures removal of the acid-weak-ened and etched surface layer of the concrete; thus, pressure washing will greatly reduce the potential foradhesive bond failure of any subsequent coating system. In environments where pressure washing is notpossible and rinsing may not be thorough enough, it is advisable to use a neutralizing agent dispersed inwater to ensure that the acid has been neutralized. A diluted ammonia solution of 3–5% in water, mixedat a rate of one quart concentrate to five gallons water and flooded over the surface of the concrete willgenerally be sufficient for neutralizing.

Baking soda can also be used to neutralize acid. Here, the powder is broadcast onto the wet concreteand scrubbed in with a synthetic bristle broom. This is left to dwell for 10 minutes, then flushed completelywith clean rinse water. Baking soda is often the preferred method because it eliminates the strong odors as-sociated with ammonia.

After the neutralizing solution is left to soak for about five minutes, the surface is rinsed with clean water.If it is not possible to rinse to drains, all standing water is vacuumed and the rinse is repeated. This proce-dure must be repeated until no particulate is visible in the rinse water. After rinsing, the surface should bechecked with pH paper in accordance with ASTM D 4262 to assure that it has been neutralized. Acid etch-ing should only be used when no other methods of preparation are possible. Again, acid etching is onlysuitable for horizontal concrete or floor slabs.Vacuum cleaning/air blast cleaning (ASTM D 4261; ASTM D 4258; ASTM D 4285 & SSPC SP

13/NACE No. 6). Vacuum cleaning or air blow-down with clean, oil-free compressed air is a final clean-ing step used to remove loose dust or dirt on a prepared surface immediately before applying a coatingor patching material. Vacuum cleaning is preferable to air blow-down when the dispersion of dust must becontrolled and limited.

Field testing and inspectionBefore, during, and after cleaning and surface preparation, tests and inspections may be conducted, if de-sired, to help establish the quality and general acceptability of the substrate for the application of coatings.Some of these tests are described as follows. ASTM C 1583 (Test Method for Tensile Strength of Concrete Surfaces and the Bond Strength or Tensile

Strength of Concrete), and ACI 503R. This test method, sometimes referred to as the pull-bond or pull-outtest, enables the in-situ evaluation of the tensile strength of concrete and/or the adhesive bond strength ofapplied toppings, mortars, and coatings over concrete. A detailed description of how to conduct the test isprovided in ACI 503R, Appendix A.

This field test facilitates the following assessments.• Evaluation of the in-place bond strength between a repair overlay and the concrete substrate

28

• Evaluation of the in-place tensile strength of the repair mortar and the concrete itself• Evaluation of the effect of surface-preparation methods on the tensile strength of the concrete substratebefore applying a repair material, overlay, or coating

This test is conducted by epoxy gluing a disc to the prepared concrete testing surface. The disc is thenpulled off after a partial core is cut around the disc. The pull-off force is then measured and provided inpsi units. General industry consensus holds that pull-off values or tensile strength of the system should bearound a 200 psi average. It should be taken into consideration that the tensile strength of concrete usu-ally falls between 8% to 12% of the concrete’s compressive strength.Moisture (determining moisture in concrete by the Plastic Sheet Method ASTM D 4263). Here, a heavy-

gauge, 102-micron- (4-mil)-thick plastic sheet approximately 156 cm2 (18 sq. in.) in size is taped to theconcrete surface around its perimeter. The plastic film acts as a moisture barrier and traps moisture mi-grating through the concrete. If the concrete appears dark, damp, or wet under the sheet, the presence ofmoisture in the concrete is indicated. If this is the case, additional drying or curing time may be required,and the test procedure can be repeated. Sheets of test film should be placed at various locations through-out the surface area to be coated, particularly in areas that are likely candidates for moisture presence, suchas below-grade, low-spot sections, inside corners, and lower-wall areas where “rising damp” may occur.The test film should be completely sealed using approximately 51mm (2-in.)-wide duct tape and left inplace from eight (8) to sixteen (16) hours. A minimum of one test every 46 sq. m, or 500 sq. ft., should beconducted. Determining pH of chemical rinse water on the concrete surface (ASTM D-4262). With this test, a strip

of pH test paper should be dipped in the rinse water and remain on the surface. After the paper changescolor, it should immediately be compared with the color chart accompanying the paper to determine acid-ity or alkalinity. The pH reading of the final rinse water should also be taken. The pH reading from the rinsewater on the concrete surface should not be more than 1.0 pH unit lower or 2.0 pH units higher than thefresh rinse water; if it is, the surface should be further neutralized with fresh water and re-tested until thepH is acceptable. Two readings should be taken on random sections of every 50 square meters (about 500sq. ft.) of concrete, and in corners, along the (wall to floor) interface, and in areas that are difficult to prop-erly flush and rinse. This pH testing is appropriate for gauging the neutrality of concrete that has been acidetched or cleaned with an alkaline detergent.Oil in air supply (ASTM D 4285). This test method is used to check for the presence of oil in the com-

pressed-air supply used for abrasive blast cleaning, air blow-down, and coating operations. This methodemploys a white blotter paper that the compressed air source is directed at for a continuous, one-minuteperiod; the paper is then is checked for the presence of oil contamination.Oil, grease, and gum. Oil on the concrete surface may be detected by conducting a water break test.

Here, clean, potable water should be lightly sprinkled of sprayed (fine mist) onto the surface. If the waterwets and spreads out instead of beading up, the surface may be considered relatively free of oil and grease.Gum may take on the appearance of an oil spot and will cause problems in the form of coating delami-nation if not properly removed. Dust. Dust or airborne fallout may be detected by wiping the concrete surface with a clean, dry cloth and

inspecting the cloth for any contamination. A black cloth works best for showing up dust and fine-particu-late dirt. A more critical dust test involves the use of transparent tape, which is applied to the surface, re-moved, and then visually inspected for dust pick-up.

Summing upMost concrete surfaces will require repair, resurfacing, topping, overlayment, or coating at some point dur-ing their service life. The first steps taken in carrying out these functions is extremely critical. The best, state-of-the-art materials, correctly mixed and applied, can nonetheless be doomed to failure unless the concretesurface is properly cleaned and prepared. Unsound concrete and deleterious surface contaminants mustbe removed and the surface roughened and properly cleaned.

Many different techniques, methods, and equipment can be used to effectively clean and prepare con-crete. With this review, we have covered some of the more common cleaning and preparation techniques

29

and procedures that are designed to ensure successful application and maximize the service life of coat-ings and patching materials applied to concrete and masonry. Any shortcomings in these critical cleaningand preparation methods will compromise the performance of even the highest-quality coating, topping,or patching system.

Referenced documentsDocuments referenced in this article can be obtained from the following organizations.• The American Society of Testing and Materials (ASTM), 100 Barr Harbor Drive, West Conshohocken, PA19428—tel: 610-832-9500 • American Concrete Institute (ACI), 3800 Country Club Drive, Farmington Hills, MI 48331—tel: 810-848-3700 • International Concrete Repair Institute (ICRI), 1323 Shepard Dr., Suite D, Sterling, VA 20164-4428—tel:703-450-0116 • National Association of Corrosion Engineers (NACE), P.O. Box 218340, Houston, TX 77218-8340—tel: 713-492-8254 • The Society for Protective Coatings (SSPC), 40 24th St., Pittsburgh, PA 15222—tel: 412-281-2331

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Important standards and guidelines

A listing of the appropriate standards and guidelines for concrete surface cleaning, preparation, andtesting should include the following.

American Society for Testing Materials (ASTM) Standards for Cleaning. Sur-face Preparation and Testing. � D 4258 Surface Cleaning Concrete for Coating � D 4259 Abrading Concrete � D 4260 Acid Etching Concrete � D 4261 Surface Cleaning Concrete Unit Masonry for Coating � D 4262 pH of Chemically-Cleaned or Etched Concrete Surfaces� D 4263 Indicating Moisture in Concrete by the Plastic Sheet Method� D 4285 Indicating Oil or Water in Compressed Air� D 5295 Standard Guide for Preparation of Concrete Surfaces for Adhered (Bonded)

Membrane Waterproofing Systems� C 1583 Test Method for Tensile Strength of Concrete Surfaces and the Bond Strength or

Tensile Strength of Concrete

American Concrete Institute (ACI) Guidelines� ACI 515.1 R-79—A Guide to the Use of Waterproofing, Damp proofing, Protective and

Decorative Barrier Systems for Concrete (currently being revised)

� ACI 201.2R—Guide to Durable Concrete� ACI Guide 224.1 R—Causes, Evaluation and Repair of Cracks in Concrete Structures� ACI Guide 301—Specifications for Structural Concrete for Buildings� ACI Committee Report 503R Appendix A—Test Methods

International Concrete Repair Institute (ICRI) Guidelines� ICRI Guideline No. 03730—Surface Preparation Guidelines for the Repair of Deteriorated

Concrete Resulting from Reinforcing Steel Oxidation� ICRI Guideline No. 03732—Selecting and specifying Concrete Surface Preparation for

Sealers, Coatings, and Polymer Overlays.

NACE International Guidelines� RP0591-91 Coatings for Concrete Surfaces in Non-Immersion and Atmospheric Service� TCR6G166 Surface Preparation of Concrete for Coatings� Publication No. 6F166 Recommended Practices for Inspection of Linings on Steel and Concrete.

SSPC: The Society for Protective Coatings � Joint Surface Preparation Standard SSPC-SP 13/NACE No. 6 Surface Preparation of Concrete

JAC

or all the intrigue evoked by the word “exotic,” cleaning non-native species of wood is no differ-ent from cleaning any other substrate: Know the substrate, establish the goal, and then find an en-vironmentally compatible method to get the job done.

The problem is the mysterious nature of certain exotic woods. The names applied are not always scien-tifically accurate. Mahogany, for instance, has become a catchall moniker.

Therefore, any contractor who wants to capitalize on the expanding market of exotic woods must be ableto identify species.

And the market is growing. “It’s definitely increasing,” says James Foley, who owns Diamond Jim’s Pres-sure Cleaning LLC, in Waterbury, Conn. “Ever since problems with pressure-treated lumber, people haveswitched” to exotics when possible.

Deck choicesThe idea is that the natural constitution of some woods allows them to withstand the vagaries of the en-

vironment, especially assault by insects. So the hunt for tough species extends to tropical forests, and namesget muddled during harvest and transport.

Ipe, for example, has become “quite popular in the last 10 years,” says Rick Petry, owner of WindsorWoodCare in Plainsboro, N.J. Hard, heavy and dense, ipe can withstand the harshest outdoor exposures—even sea spray.

The proportion of exotic woods species used for exterior construction, particularly decks, varies with ge-ography. In the Garden State, Petry sees about 65 percent pressure-treated Southern Yellow Pine (SYP), 20percent cedar (primarily Western red cedar), and 15 percent mahoganies (actually a mix of species).

Along the shores of the Nutmeg State, Foley sees decks that are largely either exotic woods (on high-endhomes) or composites (on more affordable homes).

31

Cleaning Exotic Woods

Editor’s note: This article ap-peared in PWC in July/August2010.

By Diane M. Calabreseand Brett Martin

F

Southern Pine Council

Pressure-washing decks

can be a great niche. But

watch out for shifting

names and lookalikes.

32

What’s in a name?Definitions of “exotic wood” often hinge more on prevalence than origin. “[In] the Midwest, you see a

lot of cedar,” says Petry. “[On the] West Coast, you see a lot of redwood.” However, when those woodsare seen in other regions, they may be considered exotic, so the term “is hazy at best.”

Instead of putting the emphasis on hardwoods from far-flung places, it’s best to take each species on itsmerits. The importance of determining the type of wood before devising a cleaning method applies to allwood.

Even treated SYP is not all the same, explains Petry, because the formula for treating the wood haschanged over the years.

Ipe is a popular choice for exotic decks in Northern Virginia, says Steve Chapman, owner of Elite Pres-sure Cleaning, in Bristow, Va. “We are seeing more [exotic woods] on higher-end homes. They’re gorgeous.The projects are really impressive.”

The drawback for deck owners is that exotic woods require more frequent cleaning and staining than doother wood species. “The wood is so dense, no stain will penetrate the wood and it grays really quick,”Chapman says.

On the other hand, that’s good news for contractors like Adrian B. Carrier, owner of ABC Pressure Wash-ing & Deck Rescue in Houston, who is cleaning more exotic decks than ever—partly because homeownershave not maintained them.

“Some exotic species require re-sprays every six months to maintain their beauty,” Carrier says. “We arealso seeing exotics with our other homeowners as well, due in part to renovations and folks simply keep-ing their homes a little longer.”

Washing warningsSo what is the role of using water under pressure to clean exotic woods?“Pressure washing in and of itself is not harmful,” says Petry. “Some species like ipe can take a lot more

pressure.”Caution comes at the boundary between cleaning and removing coatings. “Pressure itself shouldn’t be

used to strip wood,” says Petry. Notes Foley: “What you’re getting rid of [with power washing is] dirt and pollen.” Washing hardwood

like ipe with pressure between 500 and 1,000 PSI is fine. Then, light oil can be applied.But ipe-type hardwoods also present a choice, Foley adds. “Either they’re going to last a lifetime and turn

gray,” because stains do not penetrate and must be repeated. Or, if no stain is used, “they have to becleaned every other year.”

Cleaning basicsCleaning prevents soil and spores from getting between wood fibers. That infiltration can set up condi-

tions for a micro-community of organisms that can grow and compromise the wood’s integrity. Foley usesa solution of bleach and surfactant with water.

Cleaning will also brighten wood, says Petry. Again, the choice is whether to just clean or to follow witha stain. For example, if the decision with ipe is to stain it, “you’re going to have to do it every year.”

As for cleaning, it is a two-step process, explains Petry. Sodium hydroxide or sodium bicarbonate isused, and then citric acid is applied to neutralize the basic solution. The sodium bicarbonate or similarcleaning agent is applied and then lightly washed off with water under low pressure.The goal of wood maintenance is longevity. Graying may be an acceptable aesthetic choice in hard

woods like ipe. But less dense woods may be vulnerable to warping if they are not sealed. And thosechanges in shape, in turn, can contribute to cracks, protruding nails and other safety issues.

Speaking of speciesSorting out the species of exotic woods in a universal way is close to impossible. A few examples serve.Ironwood, ipe and mahogany are all common names that cut across taxonomic groups established by

botanists. Ironwood is generally used to refer to the hornbeam plants in the genus Carpinus, but many

stevecoleccsAlways know the wood species before devising acleaning method.

33

deck builders call ipe an ironwood. In doing so, they reference its strength ahead of its phylogenetic al-liances.

Ipe itself is Tabebuia impetiginosa, a species native to Mexico and Argentina. Mahogany includes treesin the genera Swietenia from the tropics of South America, Khaya from Africa, and Eucalyptus or Australianred mahogany.

Such confusion is reason enough for contractors to emphasize to customers the importance of buyingwood that is certified as a particular species. Do not recommend wood that cannot be verified by speciesand supported by proper import permits.

Nor should you rely exclusively on architectural specifications to identify wood, says Foley. The specs canbe misleading, because of the generic use of names for wood species.

Difficult jobsAs noted previously, water under pressure should not be used to remove coatings. Removal is typically

a job for mechanical devices. Foley, for example, uses a tungsten carbide abrading machine.On the other hand, mistakes by one contractor can lead to a good contract for another who can step in

and fix the problem. Foley has been taking care of red cedar doors in a multi-dwelling residence since hewas able to go in and correct another contractor’s error. (“It all happened because they put the finish ontoo soon.”)

Resolving issues for customers is gratifying, says Foley. Figuring out what the wood is and how to carefor it is professionally satisfying and typically leads to long-term maintenance contracts.

Petry, too, thrives on resolving problems. A few years ago, for example, he was called on to deal withnatural redwood—a rarity, since most redwood used in decks now comes from plantations.

“The redwood was 24 years old,” says Petry. “It was just loaded with its own resins.” Those resins, com-bined with a sealer recommended for redwood, had turned the color black.

The lesson, say Petry and other contractors: Testing first is a must, for coatings and pressure settingsalike. For example, Port Orford cedar that comes out of Canada is relatively soft but can take the pres-sure, says Foley. (Port Orford cedar, Chamaecyparis lawsoniana, is also known as Oregon cedar and Law-son cedar.)

General adviceIt is important to understand that “exotic wood degrades differently,” says Foley. “The first finish is the

most important.” “You’re stripping a lot in the beginning,” he says. Then, after the wood is fortified to resist invasion of

particles and organisms, the key becomes the contractor’s knowledge of coatings.“As far as cleaning exotic woods, it has to do with understanding sealers,” says Foley. That means more

than following manufacturers’ instructions. It also demands keeping careful track of results and making ad-justments as necessary.

Mahogany and cambara (Erisma uncinatum) are “easier to take care of as they age,” says Foley. “Theyopen up more, and product can go into them.” At the same time, some alkali finishes are not suitable forthose types of wood, because they get in too deep.

Minding the detailsTo keep all of this straight, record keeping is essential. Foley keeps a portfolio of photos to show customers

what they can expect. Petry used to carry wood samples to show customers. Now, he also uses photos. Developing experience with, and a systematic approach to, exotic woods can make contractors experts

beyond the traditional spheres of cleaning and maintenance. For example, customers now ask Foley to recommend what wood to use in deck replacements and new

construction. “You really get to know exotic wood when you’ve done it a few times,” he says.Adapted with permission from Cleaner Times: The Journal for High Pressure Water Applications (Vol. 20,

No. 5). Brett Martin also contributed to this article. pwc

34

its environment and sometimescan reposition itself due to theloose grip of the hardware used,”he says. “It is not uncommon to re-place nails or screws either due torust or grip failure. I generally usegalvanized wood screws as it re-pels rust and premature aging.” The best finish for pressure-treated decks offers ultraviolet(UV) protection, repels water, andstops mildew growth. Applyingpenetrating stains to CCA-treateddecks at least once a year mayeven reduce potential exposure to

arsenic, the U.S. Environmental Protection Agency has said.Stains and sealers may be water- or oil-based. Each has

pluses and minuses, and even these may be in the eye of thebeholder, depending on your priorities. Oil-based productsgenerally penetrate deeper, give a richer color, and last longer.But the oils typically are also harder to work with, harder toclean up, less environmentally friendly, and take longer to cure.Moreover, each product is unique, and even the same prod-

uct can perform differently under differentconditions, depending on climate, wear,moisture and other factors. In the end, yourbest bet is to talk to the manufacturer’s rep,store reps and other contractors about aproduct. Remember to read the entire labelbefore you start the job.And, the pros say, avoid paint, since itforms a film over the wood.“I wouldn’t recommend a solid paint overP.T. wood, as it will not allow the wood tobreathe and will dry it out, causing prema-ture cracking and warping,” Carrier says.“Oil will keep a soft wood soft. Semi-trans orsemi-solid oil finishes will also allow you tosee the beauty of the wood grains.”

Brett Martin has worked in the construction and remodelingindustries. Contact him at dee-brett@msn.com.

By Brett MartinPressure-treated wood has been

a favorite for outdoor projects forgenerations. Infused with preserva-tives under pressure, P.T. wood canwithstand moisture, decay andground contact for years.Although pressure-treated wood

containing Chromated Copper Ar-senate (CCA) is no longer beingproduced for use in most residen-tial settings, much of it is still outthere. Whatever the preservative,pressure-treated wood requires adifferent approach than untreatedwood.Washing with low water pressure and then using a pene-

trating finish are the keys to maintaining a pressure-treatedwood deck. “You really don’t want to use high pressure, because once

you damage the wood, you have another problem to deal with,”says Colin McCown, executive vice president of the AmericanWood Protection Association (AWPA) in Birmingham, Ala.Adrian B. Carrier, owner of ABC Pressure Washing & Deck

Rescue in Houston, recommends no morethan 600 to 800 psi of water pressure to re-move mold, algae and the old finish. “Let your strippers and cleaning solutions

do the work for you,” he says. “High pressureonly damages the wood and guarantees thatyour next step will be the rental or purchase ofa walk-behind sander before applying yourfinish.” Steve Chapman, owner of Elite Pressure

Cleaning in Bristow, Va., uses a bleach-freebrightener and low water pressure to prep thewood. “After pressure washing, I use carbide

brushes to remove loose, soft fibers,” hesays. “That’ll make the stain last longer.”Because pressure-treated wood is a softwood, Carrier re-

sinks nails and screws in the deck. “Being so soft, it ‘gives’ with

Finishes for

pressure-treated

decks should have

UV protection,

repel water and

inhibit mold

growth.

When the Pressure is OnCleaning and sealing pressure-treated wood takes a special touch.

joebrandtPressure treated wood is still popular for its ability to resistwater and mold.