From JPCL, July 2012 Fred Goodwin
BASF Construction Chemicals
Fred Goodwin is a fellow scientist in research and development at BASF Construction Chemicals LLC (Cleveland, OH). He has 30 years of experience in the construction chemicals industry and is an active member of SSPC, ICRI, ASTM, ACI, SDC, NACE, and ISO. Goodwin is a chair on the ICRI Materials and Methods, the ACI 364 Rehabilitation, and the ASTM C09.68 Volume Change Committees. Goodwin and co-author Gail Winterbottom won a JPCL’s Editor’s Award for their January 2005 JPCL article, “Concrete Cracks: Causes, Correcting, and Coatings.” Goodwin received a JPCL Top Thinker award this year.
Concrete is perceived as a common, hard, durable, and relatively unexciting material surrounding us in our homes, highways, bridges, airports, workspace, sewers, mines, and monuments. 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 successful coating 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 Objectives
The project objectives must be determined; otherwise, they can become a moving target. These include the substrate conditions, coating requirements, owner requirements, and application conditions, all of which must be well thought out together. Define with the owner and other interested parties what success means for this project. Mockups are very 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 effects of the construction process (e.g., noise, dust, vibration, fumes); what to do with debris (especially if a hazardous material); what, if any, utilities (power, ventilation, water, etc.) are available for the needed procedures; what kind of protection from weather and traffic is possible for the project area; and how will the environment around the project be protected from the construction activity.
iStockphoto Concrete Surface Quality
Not all concrete is created equal. With varying degrees of success, coatings are applied to all types of concrete, such as ”green”
fresh concrete, fully cured virgin concrete, contaminated concrete, and previously coated concrete. Concrete’s appearance doesn’t change much, but the properties can vary widely based on the water-to-cement ratio, aggregates, admixtures, curing, age, orientation, service history, and what is and has been in contact with the concrete. The concrete’s exposure to different conditions can also influence the extent of surface preparation and the techniques. Concrete can be oriented horizontally both on- and above-grade, sloped, vertically, and overhead. 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 the prepared surface (Table 1 and 2). Cracks must also be addressed, both to prevent further cracking in the concrete as well as reflective cracking in the coating system (ACI 224.1).
TABLE 1
Concrete Finishes Producing Different Profiles, Strength, and Issues*
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 not properly cured Stoning Smooth to medium Low to medium Low to high(B) Weak layer if not properly cured Concrete block Coarse to very coarse Very high Medium Pinholes
Shotcrete(C) Very coarse Medium Medium Too rough for thin coatings
* 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
(B) Strength depends on application and cure.
(C) Shotcrete may be refinished after placement, which would change the surface properties shown in this table.
TABLE 2
Suggested Performance Tests for Concrete Following Surface Preparation*
Property Test Method Light Service (A) Servere Service (B) Surface tensile strength 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 dust
Residual contaminants Water drop 0° contact angle 0° contact angle
pH 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 moisture Moisture content (D) ASTM D 1869 15 g/24 hr/m2(3 lb/24 hr/ 1,000 ft2 )
max
15 g/24 hr/m2(3 lb/24 hr/ 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 rinse water (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.
Surface Preparation and Coating System Compatibility
Each coating type requires minimum substrate conditions to assure compatibility. Sealers require surface preparation mainly to promote penetration of the concrete, as any visible defects or profile will be unaffected by the sealer. Surface polishing is an enhancement for concrete surfaces that are usually either integrally colored or then stained and sealed. Thin-film coatings may be formulated to mask very minor defects and surface discolorations. Thicker coatings such as self-leveling materials, polymer overlays, toppings, and high-build coatings have much in common with thin-film coatings regarding the relationship between surface profile and dry film thickness, moisture sensitivity, and wear; however, the thicker coating layer can fill larger defects, allow non-skid surfaces through surface texture, and provide longer service life than materials applied in thinner layers. Existing cracks will tend to telegraph through an applied coating. 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 the concrete. It is very unlikely that any coating system will have satisfactory performance over a bruised substrate. Surface preparation methods are described in Table 3. Primers can also be considered as a form of surface conditioning following surface preparation: They bridge dissimilar materials (i.e., concrete and coating); seal the substrate from out-gassing (i.e., the emission of gas or vapor from the substrate usually due to displacement from the absorption of the coating or temperature changes in the substrate); encapsulate dust; and increase coating coverage by limiting the absorption of the liquid from the coating.
Contamination
One description for concrete is ‘a hard, wet sponge.’ Like a sponge, concrete can have residual contaminants from previous treatments or absorb contaminants from the environment that can inhibit bond, reduce durability, and create other problems with applied coatings. Residual contaminants include incidental dust, form release transfer, curing compounds, existing membranes, adhesives, water repellants, and coatings. Residual contaminants, usually present on the surface, can be removed during surface preparation; however, 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 such as 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 consideration of the consequences of the contamination must be given, depending on the nature of the contaminant, depth of penetration, and any reactions that may have occurred. If deleterious materials are found to interfere with the coating system, many can be removed by excavating the concrete during surface preparation to their penetration depth.
Moisture Content and Movement in the Concrete
Moisture in concrete can come from many sources, such as the water used for placing or curing the concrete, maintenance or usage (e.g., washing a concrete floor or process water leaks), exterior infiltration from poor drainage or weather protection, or even the selected method of surface preparation (Table 3). For some cementitious materials, moisture in the concrete is not a problem, and bonding performance actually can be improved using a “saturated surface dry (SSD)” concrete substrate. For moisture-sensitive materials, drier concrete is better. Achieving sufficiently low moisture emissions is often one of the greatest difficulties encountered with concrete substrates and requires evaluation and considerations discussed in ACI 302.2 and elsewhere (Kanare), but no completely satisfactory solution exists for all situations. ICRI has a technician certification program for several tests of moisture in concrete.
TABLE 3
Summary of Surface Preparation Methods, Applicability, Equipment, Mechanisms and Resulting Surface Texture and CSP Ranking*
Method Equipment Mechanism Surface Texture Achieved CSP
Ranking Detergent Scrubbing Mop and Pail, Floor
Scrubber
Reaction Light profile, removal of concrete paste, discoloration
1-3
Dry Grinding Dry Grinder Erosion Smooth surface, dust, debris to remove, pattern 1-3 Wet Grinding Wet Grinder Erosion Wet, smooth surface, slurry, debris to remove,
pattern
1-3
Dry Abrasive Blasting Dry Sand Blast Pulverization, Erosion, Expansive Pressure
Dusty substrate, light profile (depending on media, size, pressure, time) debris to remove
2-4
Wet Abrasive Blasting Wet Sand Blast Pulverization, Erosion, Expansive Pressure
Wet substrate, light profile (depending on media, size, pressure, time) debris and slurry to remove
2-4
Shot Blasting Shot Blast Unit Pulverization, Impact Erosion
Dust free substrate, some pattern, depth dependent on shot size, sustrate hardness, equipment
2-8
Scarifying Scarifier Impact Dusty substrate with triated pattern, bruising likely, debris to remove
4-9
Needle Scaling Needle Scaler Impact Similar to shot blasting, striated pattern, debris to remove
5-8
Scabbling Scabbler Impact Dusty substrate, irregular pattern, fractured aggregate, bruising likely, debris to remove
Irregular chipped surface, hot, charred debris to remove, bruising possible
8-9
Rotomilling Rotomiller Impact Dusty substrate (unless water used to suppress dust), grooving, tool marks, fractured
aggregate, bruising likely
9
Liquid Surface Etchant Specialty Chemical, Fresh Concrete
Reaction Exposed aggregate, green wet concrete with debris to remove using pressure wash, curing still required, no bruising, depth dependent on retarder chemistry, curing rate, length of exposure
3-9
*CSP (Concrete Surface Profile) profile range according to ICRI 310.2 (Goodwin) Profile
Different surface preparation methods produce a wide range of profiles, best defined by ICRI 310.2 Concrete Surface Profile (CSP) replica chips. Achieving bond to smooth surfaces is usually more difficult than to textured surfaces. Rougher surfaces have greater surface area, will tend to hide defects, but require higher build coatings with lower coverage rates. Usually, a particular coating system will specify the required profile. ASTM D7682-10 is a new test method that produces a permanent replica of the concrete surface, which can then be compared to visual profile standards (e.g., ICRI CSP’s) or evaluated quantitatively for profile depth.
Summary
When concrete is the substrate and the coating is what is visible, the difference between success and failure lies in between.
Concrete is a durable material, and coatings can help maintain and in some cases improve that durability. To achieve that purpose, the concrete must be sound, repairs must be performed as necessary, the surface must be properly prepared, the appropriate coating must be selected, and the coating system must be correctly installed. Surface preparation is one of the most important and frequently neglected factors in achieving successful coating of concrete.
Sources Consulted
ACI 201.1R-08, Guide for Conducting a Visual Inspection of Concrete in Service, 2008, ACI International, Farmington Hills, MI.
ACI 201.2R-08, Guide to Durable Concrete, 2008, ACI International, Farmington Hills, MI.
ACI 311.1-R07 ACI SP-2(07), Manual of Concrete Inspection, 2007, ACI International, Farmington Hills, MI.
ACI 364.1R-07, Guide for Evaluation of Concrete Structures before Rehabilitation, 2007, ACI International, Farmington Hills, MI.
SSPC-SP 13/NACE No. 6, Surface Preparation of Concrete, 2003, The Society for Protective Coatings, Pittsburgh, PA.
ACI 224.1R-07, Causes, Evaluation, and Repair of Cracks in Concrete Structures, 2007, ACI International, Farmington Hills, MI.
ICRI No. 310.2–1997, Selecting and Specifying Concrete Surface Preparation for Sealers, Coatings, and Polymer Overlays (formerly No. 03732), 1997, International Concrete Repair Institute, Des Plaines, IL.
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.
ACI 515.1R-79 (reapproved 1985), Guide to the Use of Waterproofing, Dampproofing, Protective, and Decorative Barrier Systems for Concrete, HIS Services, http://www.ihs.com.
PCA IS001, Effects of Substances on Concrete and Guide to Protective Treatment, 2007, Portland Cement Assoc., Skokie, IL. ACI 302.2R-06, Guide for Concrete Slabs that Receive Moisture-Sensitive Flooring Materials, 2006, ACI International, Farmington Hills, MI.
Kanare, H. M., “Concrete Floors and Moisture,” 2005, Portland Cement Assoc., Skokie, IL.
ASTM D7682-10, Standard Test Method for Replication and Measurement of Concrete Surface Profiles Using Replica Putty, 2007, ASTM International, West Conshohocken, PA.
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