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But I Can’t Wait 3 Days

No time for standard concrete slab moisture evaluations? Try these accelerated methods.

Let’s face it; we live in an age where instant results matter. Assessing concrete floors for moisture is no different. Most owners, contractors and manufacturers agree that these assessments are necessary prior to installing flooring treatments to achieve quality results. Numerous resources exist to support these assessments. In fact, many specifications and product data sheets reference specific tests that should be undertaken to determine concrete moisture content.

While most agree that assessing and testing concrete for moisture is important, it is not always performed due to cost, disruption to operations, or insufficient time in the project schedule. These challenges are common since restoration projects are often completed in retail establishments that remain open for business during the work. Additionally, funding for testing is not always available.

Many test methods take 72 hours or longer to complete and can create a tripping hazard in a store that is open for business.

moisture content
Fig. 1: Calcium chloride test installed on floor with wood flooring. The test can cause a trip hazard in a facility that is open for business. Photos courtesy of KTA-Tator Inc.

Standard Test Methods for Determining Moisture Content

ASTM F1869 Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride (referred to as the calcium chloride test) is a common test method for obtaining the MVER (moisture vapor emission rate). The results are provided in pounds per 1,000 square feet (93 square meters) over 24 hours. The test can take five days to complete on covered floors.

The standard indicates that floors with a floor covering should have a 20-square-inch (129-square-centimeter) patch of covering removed at least 24 hours before the calcium chloride test. The calcium chloride remains in place for 60 to 72 hours, so a five-day window is required for performing a calcium chloride test on a covered floor.

Removing the floor covering in an open-for-business retail store can be a safety concern if the covering is tile, wood or has an underlayment. Figure 1 shows a calcium chloride test installed in a retail store with wood flooring removed in the test area. Unless the test area can be concealed, special barriers must be constructed over the test location to minimize the trip hazard.

Another common test for determining moisture content of concrete is ASTM F2170 Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes. ASTM F2170 recommends allowing 72 hours to achieve moisture equilibrium within a specially drilled and sealed hole in the concrete before making relative humidity (RH) measurements.

moisture content
Fig. 2 (Top): RH probe installed in floor prior to installing probe cap. Fig. 3 (Bottom): Defects in a floor covering provide clues that the concrete moisture content is high.

Waiting three days to obtain results is sometimes not feasible on its own, but this three-day equilibrium time also requires at least two trips to the jobsite — one trip to install the probes and one trip to obtain results — which increases testing costs. The relative humidity test creates less of a safety issue since the testing apparatus is installed subsurface (Figure 2).

When Time Is of the Essence

Despite the time requirements in the ASTM standards, contractors and owners often need same-day assessments of concrete flooring in commercial applications such as retail stores. They may need quick results to avoid undesirable impacts on traffic and to reduce testing costs.

This does not mean the ASTM testing procedures should be disregarded, nor is it implied that they are unnecessary. However, strict compliance with the procedures may not be feasible in many cases, so what is the alternative?

There are techniques and instruments that can provide meaningful data, that construction professionals can use during a jobsite visit of just a few hours to assess a concrete floor. These techniques work well for concrete set to receive coverings or treatments with higher tolerances for moisture, such as stains and dyes for polished concrete.

Some manufacturers indicate that concrete stains and dyes are acceptable for concrete with moisture test results up to 5 pounds per 1,000 square feet over 24 hours. Moisture above this value can impact the color stability of the stain or dye.

Some tile manufacturers produce modified adhesives that are more tolerant to moisture compared to standard adhesives.

Some of these “moisture tolerant” adhesives are acceptable for concrete floors that have moisture content up to 8 pounds per 1,000 square feet over 24 hours or 90 percent relative humidity.

Quick assessment techniques can normally distinguish floors that have significant moisture problems from those that do not. However, for floor coverings that have lower moisture tolerance such as 3 pounds per 1,000 square feet over 24 hours or 75 percent relative humidity, a more thorough investigation and compliance with the dwell times in the ASTM standards may be required.

Moisture testing is typically one of the most important tasks completed when assessing concrete. An assessment of moisture in concrete can be performed within a few hours by conducting a (1) visual assessment, (2) same-day RH testing and (3) obtaining readings using moisture meters.

Determining moisture in concrete should always include a variety of assessment methods. Reliance on just one method can provide misleading results and should be avoided.

Visual Assessment

Moisture problems in floors can sometimes be determined visually and can be evident via problems with floor coverings. Some floor coverings will crack, blister or delaminate on floors with high moisture content. They may also have a musty odor underneath the floor covering. The assessment should document both conditions.

Examine suspect areas by removing a small section of floor covering. Floors with high moisture content will have visible signs of moisture, as shown in Figure 3.

In addition, visually inspecting the borehole drilled to 40 percent of the slab depth for the RH testing can provide beneficial information on moisture in the concrete. If the results of the RH tests are high, you can drill a borehole to full depth to determine if a vapor control is installed underneath the concrete.

moisture content
Fig. 4: Illumination from a borescope shows that no vapor barrier was installed.

For slab-on-ground work, vapor controls or damp-proofing membranes separate the concrete from moisture sources. Drilling through full depth can also determine deficiencies in the slab thickness.

Figure 4 shows an RH test hole that was drilled through due to high moisture test results. Testers viewed the hole through a borescope and found no vapor control installed underneath the slab. In addition, they found the slab thickness to be 2.5 inches (6.35 centimeters), despite a specification requirement of 5 inches. (12.7 centimeters).

RH Testing

RH testing of concrete can be performed in one day if the RH equipment manufacturer agrees. RH equipment manufacturers such as Tramex and Wagner Meters indicate that RH test results can be obtained soon after the RH probes are installed in the floor; waiting 72 hours for results is not always required.

moisture content techniques
Table 1: +Based on meters from specific manufacturers. Green = dry. Yellow = intermediate. Red = wet.

For example, Wagner Meters indicates that its RH sensor will generally give a reading within 3 percent RH of the reading you would see after the ASTM-required 72-hour period*, one hour after installation. Same-day results are typically accurate enough for materials that can tolerate higher moisture content.

Although RH testing is the most expensive of the three moisture assessment methods discussed, it has several benefits. RH testing instruments are independently calibrated, and calibration results are traceable. RH test results provide a subsurface measurement of the concrete’s relative humidity, which cannot be determined by calcium chloride or other methods. The RH is accurately measured despite the type of concrete mix, water cement ratio or thickness. It is important to know the subsurface moisture as well as the surface moisture because each result is independent of the other.

Moisture Meters

Moisture meters provide a quick means of assessing floor moisture problems. Moisture meters can be used to nondestructively identify moisture “hot spots.” They also provide comparative moisture condition data from one area of the slab to another. It’s helpful to take readings next to RH test areas for comparison, although surface and subsurface moisture results will not always correlate.

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, provides a description and procedure for using moisture meters to conduct preliminary assessments.

There are three main types of moisture meters available commercially. They are the conductivity-, the electrical impedance-, and the radio-frequency meters. Each type has its advantages, and it is important to use at least two of the three types when making assessments. Table 1 provides a brief comparison between the three types of moisture meters.

For floors with coverings, the radio frequency meter provides an advantage because it can read through several different types of floor coverings (Figure 5). Conductivity and impedance meters typically need direct contact with the concrete for accurate readings.

Impedance is the only meter type that provides the result as a percentage of moisture content. The others provide results on a relative scale. Visual assessments, RH testing and moisture meter readings can be conducted in a single-day assessment, offering time and cost advantages on commercial projects.

Make It Quick: pH and Hardness

In addition to moisture assessments, it’s important to assess the pH and hardness of the concrete floor. You can also determine these in a single-day.

You can determine the concrete’s pH using pH strips or pH pencils. Acceptance criteria varies by flooring product manufacturer, but most manufacturers require the concrete’s pH level to be under 10 before product installation.

Surface hardness of concrete is important especially if you’re considering polished concrete. If the top surface of concrete is weak from excessive carbonation or other factors, it can influence the type of honing and polishing process used.

Fig. 6: Mohs Hardness Test is conducted on concrete.

One easy way to determine surface hardness is with the Mohs Hardness Test (Figure 6). This is a simple test conducted with eight metal/alloy brass rods with picks ranging in hardness on a scale of 2 through 9.

Determine the relative hardness of the concrete by scratching the surface with the appropriate pick. If the point leaves a deep, definite scratch in the surface, the concrete is softer than the value of the tool. Typically, concrete with hardness of 4 or above is suitable for polishing. To put that in perspective, a hardness of 10 is equivalent to diamond, while 1 is talc.

Efficient and Effective

Concrete assessments are important for proper design and execution of floor restoration projects. Time and funds for detailed assessments are often a challenge in commercial applications, and a need exists for abbreviated, yet reliable assessments.

With the right tools and techniques, you can approximate the results of more time- consuming, expensive and destructive assessment techniques.


* Wagner Meter Rapid RH 4.0 EX instruction guide, Step 4.

About the Authors

Kevin J. Brown is the Commercial Services Group manager at KTA-Tator Inc. He provides architectural/ commercial consulting and inspection services for all coating types as well as decorative concrete applications such as concrete polishing. Brown has worked with numerous commercial painting contractors, paint manufacturers and coatings consultants. He has a Bachelor of Science degree and Master of Business Administration from Gardner-Webb University in Boiling Springs, N.C., and is a CXLT (Certified XL Tribometrist) and RRO (Registered Roof Observer).

Danny Jones is a building assessment assistant in the KTA Commercial Services Group. He focuses on the delivery of services to clients in the architec-tural/commercial field, while ensuring objectives are consistent with and contribute to group and corporate goals and objectives. Jones is a certified MPI Architectural Coating Technologist (ACT) Level 1 – Essentials of Paint and Painting Technology and has a Bachelor of Science degree from Appalachian State University in Boone, N.C. D+D

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