Moisture in walls can affect the performance of coating and water-repellent systems. While ASTM publishes standards for determining the moisture content of cementitious substrates, their primary focus is concrete floors. Until this year, little guidance was available for the measurement of moisture content in walls, either from ASTM or other organizations.
Similarly, tolerance of pinholes in coatings applied to concrete and masonry is often not discussed before a job begins, but is questioned after the fact when pinholes are visible. Unfortunately, guidance was not previously available for specifiers and contractors to use in classifying the frequency of pinholes and establishing acceptance criteria and a common understanding of expectations.
The SSPC Commercial Coatings Committee recognized the need to fill a void in both areas.
As seen in Durability and Design, November 2017
SSPC’s Commercial Coatings Committee was formed at a November 2011 planning meeting in Pittsburgh and held its first working meeting three months later in February 2012 at the SSPC National Conference in Tampa, Florida. At the 2012 meeting, the group developed the scope and content of a new moisture detection guide for building walls. A first draft was reviewed by the committee in January 2013 at the SSPC National Conference in San Antonio, Texas, and in 2017, it was finalized and published as SSPC-Guide 23, Field Methods for the Determination of Moisture in Concrete and Masonry Walls and Ceilings, EIFS, and Stucco to address the need.
During the January 2013 meeting, the committee also agreed to develop a standard for classifying the frequency of pinholes in coatings applied to concrete and masonry. The first draft was reviewed in February 2014 during the SSPC National Conference in Orlando, and is currently in the process of being finalized as Standard for Visual Evaluation of Pinholes in a Concrete or Masonry Coating in response.
This article will examine both new publications.
SSPC-GUIDE 23: FIELD METHODS FOR THE DETERMINATION OF MOISTURE IN CONCRETE AND MASONRY WALLS AND CEILINGS, EIFS, AND STUCCO
The scope of Guide 23 is described in Section 1.1:
1.1: This Guide describes common field methods for the determination of moisture content in painted and unpainted exterior concrete and masonry walls (CMU, brick, stone [manufactured and natural], poured-in-place, pre-cast, and tilt-up), Exterior Insulation Finishing Systems (EIFS), stucco, and concrete and masonry ceilings (panels, planks and cast-in-place). Many of the ASTM test methods cited in this Guide were developed for use on floors, but the methods are also suitable for use on walls and ceilings. References to relevant industry standards and recommendations for selecting test locations, frequency of testing, and acceptance criteria are provided. Methods other than those included here may also be suitable for use.
SSPC-Guide 23 describes the use of five different test methods for detecting moisture:
- Method 1: Plastic Sheet Method
- Method 2: Electrical Impedance Moisture Meter
- Method 3: Radio Frequency Moisture Meter
- Method 4: Electrical Conductivity (Resistance) Moisture Meter
- Method 5: Relative Humidity Probes
Method 1: Plastic Sheet Methodmo
Method 1 is a qualitative method for determining the presence of moisture. Use of the plastic sheet is described in ASTM D4263, Standard Test Method for Indicating Moisture in Concrete by the Plastic Sheet Method, and in Appendix X2.2 of ASTM F710, Preparing Concrete Floors to Receive Resilient Flooring.
Method 1 requires taping the perimeter of a sheet of polyethylene to the surface, allowing it to remain in place for a minimum of 16 hours, and examining the underside of the sheet and the surface of the substrate for evidence of moisture.
A variation of the test involves determining the moisture content of the test area with a moisture meter (a meter from Methods 2, 3 or 4 below) prior to attaching the plastic sheet, then measuring it again upon removal of the sheet. An increase in the readings is an indication that moisture is present.
The plastic sheet is 4 mils (0.1 mm) thick or heavier, measuring approximately 18 x 18 inches (457 x 457 mm). A different shape is also permitted if it provides an equivalent minimum surface area. When testing a rough surface like split-faced block, the shape of the plastic should match the mortar joints (for example, stair step or cross pattern) so that a continuous, tight seal can be created along the joints, rather than across the face of the block.
Plastic sheet cut to match mortar joints to achieve a tight seal on split-face block.
Method 2 – Electrical Impedance Moisture Meter
Method 2 involves the use of a moisture meter based on electrical impedance to non-destructively determine the moisture content of approximately the top 1 inch (25 mm) of the substrate. Use of the instrument for floors is described in ASTM F2659, Standard Guide for Preliminary Evaluation of Comparative Moisture Condition of Concrete, Gypsum Cement and Other Floor Slabs and Screeds Using a Non-Destructive Electronic Moisture Meter.
If the substrate is painted, it is recommended that the paint be removed before taking a reading, but the guide includes a procedure to determine whether the paint is affecting the results. If not, the paint can remain. Moisture content with electrical impedance instruments usually provide results as percent moisture, typically up to 6 percent.
Method 3 – Radio Frequency Moisture Meter
Method 3 utilizes radio frequency to non-destructively determine the relative moisture content of approximately the top three-quarter inch (19 mm) of the substrate. Use of the instrument for floors is described in Appendix X2 of ASTM F2659.
The instrument can detect moisture in the substrate beneath paint, so there is no need to remove existing coating. The presence of moisture is displayed on a relative scale from dry to damp, rather than by percent moisture.
Method 4 determines relative moisture content by measuring the electrical conductivity of the substrate between two electrodes, approximately a half inch (12 mm) apart, located on the instrument housing or on a separate instrument probe. Surface measurements are taken non-destructively, but subsurface measurements require driving two masonry nails into the substrate approximately one-quarter inch (6 mm) deep and touching the electrode to the nail heads. Deeper measurements can be made by drilling holes into the substrate, placing nails in the holes and touching the electrode to the nail heads. Use of the instrument for floors is described in Appendix X2.4 of ASTM F710.
Existing paint does not have to be removed, provided the paint is non-conductive and the tips of the electrode can be pushed through the paint to make intimate contact with the substrate. The presence of moisture is displayed on a relative scale from dry to damp, rather than by percent moisture.
Method 5 – Relative Humidity Probes
Method 5 destructively measures the relative humidity within the substrate. It requires drilling a hole into the substrate, generally three-quarters inch (19 mm) in diameter (diameter varies by manufacturer). The depth of the hole is based on the thickness of the substrate. A probe assembly is placed into the hole, capped and allowed to equilibrate for 72 hours before taking a reading. Provisions are included for taking readings in shorter periods of time, including only a few hours after placement, if allowed by the manufacturer. Use of the instrument for floors is described in ASTM F2170, Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes.
This method is typically only used if the temperature and relative humidity of the surrounding air are in balance or can be controlled. The presence of moisture within the wall is displayed as percent relative humidity.
Suitability of the Test Methods for Various Surfaces
SSPC-Guide 23 includes a table that compares the suitability of each test method for use on different substrate types and textures, from poured concrete to textured CMU, EIFS and stucco.
Test Frequency and Test Locations
SSPC-Guide 23 recommends a specific number of tests for Method 1 (plastic sheet) and Method 5 (relative humidity probes) based on the size of the area that will be painted in a given day. The number of tests generally range between three and 10. For Methods 2, 3 and 4 (moisture meters), the guide recommends taking a moisture reading at three different heights for every 25 to 35 linear feet of wall that will be painted in a given day. The guide also indicates that test frequencies for troubleshooting or forensic investigations may deviate from the recommendations.
The guide notes that the environmental exposures along a wall can vary (for example, locations protected by a canopy compared with the portions of the wall that are exposed), and that all exposures should be tested. It also indicates that localized spikes in moisture content can occur in certain locations such as grouted cells, bond beams and mortar joints. Higher moisture content is also possible adjacent to control joints and around leaking gutters, downspouts and windows. All the conditions should be tested as applicable.
Interpretation of Results
SSPC-Guide 23 provides suggestions for conducting comparative testing to determine if a surface is adequately dry for painting. For example, if the surface will be prepared by pressure washing, it recommends taking initial readings in areas that are free of failure — areas that do not exhibit any evidence of moisture problems. After cleaning, the surface should be allowed to dry to the point that the readings are no higher than the initial baseline readings. Coating manufacturers can also provide moisture limitations for their products based on the instruments that are used. The guide cautions that since the methods and instruments assess the presence of moisture at different depths in the concrete and provide results in different units, the results cannot be directly compared, but there should be consistency in trends.
Laboratory Moisture Saturation Testing
A project might require that samples of the substrate from a building be removed to determine moisture retention properties at various levels of relative humidity. This is often required in historic structures constructed of non-modern materials because moisture detection instrumentation can react differently on these materials, irrespective of the amount of moisture that is present. Appendix 1 of SSPC-Guide 23 describes a procedure for collecting samples from buildings and conducting percent saturation analysis in the laboratory.
It also describes a procedure for establishing a correlation between moisture meter readings and the percent saturation of the various substrate types. This is done so that field assessments of certain substrate materials can be accomplished quickly using moisture meters, rather than removing samples for laboratory analysis each time.
STANDARD FOR VISUAL EVALUATION OF PINHOLES IN A CONCRETE OR MASONRY COATING
Two relevant paragraphs from the scope of the “pinhole standard” (1.1 and 1.3) are as follows:
1.1: This standard is intended for evaluation of pinholes in the cured finish coat of a single or multi-coat system. It establishes frequency categories for classification of the quantity of pinholes that occur in protective coating systems after application to concrete or masonry substrates. It provides a procedure for defining areas of applied coatings to be inspected for pinholes to assign the appropriate frequency category. Acceptance criteria for various frequency categories as determined by the service environment of the coated area are also included.
1.3: This standard is not intended to be used to assess the frequency of craters or other surface imperfections, nor is it intended for evaluation of coatings that are aggregate-filled, stippled-finish, or intentionally porous.
Classification of Pinhole Frequency
The standard establishes the frequency of pinholes per “evaluation spot,” which is defined as an area comparable in size to one CMU block, approximately 144 square inches or 1 square foot (0.09 square meters). Table 1 shows the classifications based on pinhole frequencies:
|Classification||Number of Pinholes per Evaluation Spot|
|Low||1 to 10 pinholes|
|Moderate||11 to 20 pinholes|
|High||> 20 pinholes|
Pinhole Acceptance Criteria
The standard provides acceptance criteria based on service environment but notes that, if pinhole criteria is provided in the project specification, it will prevail, or if the manufacturer’s restrictions are more stringent, they should be followed.
|None||Interior atmospheric coatings applied to areas such as medical, food, drug or pharmaceutical clean rooms, or areas frequently wet or washed down|
|Moderate or less||Atmospheric coatings applied to interior surfaces in general areas|
|Atmospheric coatings applied to exterior surfaces in general areas|
Pinhole Inspection Procedure
When examining a wall for pinholes, the first step is to establish an “evaluation zone.” An evaluation zone is an easily defined area — for example, the entire north wall, or the first floor of the south wall. The number of square-foot evaluation spots to be examined in each evaluation zone is based on the total surface area of the zone, ranging from one evaluation spot per 100 square feet (10 square meters) to three evaluation spots per 1,000 square feet (100 square meters). The inspections are conducted without magnification at approximately 12 inches (30 cm) from the surface.
If one of the evaluation spots fails the acceptance criteria, the standard provides a procedure for conducting additional inspections at 3-foot (1-meter) increments in each direction (0, 90, 180, 270 degrees) around the nonconforming area to designate the extent of the nonconforming area.
More paint is applied to structures considered to be architectural, commercial or institutional as compared with industrial, and yet there are far fewer guidance documents and standards available to support the work.
The SSPC Commercial Coatings Committee is helping to fill the void. The recent publications on moisture detection and classification of pinholes represent the type of support that is being developed. Special thanks to Kevin Brown of KTA-Tator Inc. for chairing the task group that developed SSPC-Guide 23, and Sam Scaturro of Alpine Painting & Sandblasting Contractors for chairing the task group that is developing the pinhole standard. The documents are available at SSPC.org.
ABOUT THE AUTHOR: Kenneth A. Trimber, president of KTA-Tator Inc. has more than 40 years of experience in the industrial painting field. A NACE-certified Coating Inspector and SSPC Protective Coatings Specialist, he holds a bachelor’s degree from Indiana University of Pennsylvania. A past president of SSPC, Trimber is chairman of the association’s Commercial Coatings Committee, Surface Preparation Committee, and Containment Task Group, as well as a member of the Standards Review Committee. He is a past chairman of ASTM D1 on Paints and Related Coatings, Materials, and Applications and author of The Industrial Lead Paint Removal Handbook.