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Knowledge Base
Calcium Chloride Test
ASTM F1869 - Why Test Concrete for Moisture?
Before installing an Aspartech resin flooring system, it is important to establish the moisture condition of the concrete substrate. Concrete can appear completely dry at the surface while moisture continues to migrate through the slab from below. Excessive moisture vapour can place pressure on an impermeable or low-permeability resin flooring system and, depending upon the substrate and flooring specification, may contribute to problems including:
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Poor adhesion
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Blistering or bubbling
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Delamination
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Osmotic blistering
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Discolouration or blooming
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Reduced long-term durability
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Moisture testing therefore forms an important part of the assessment and preparation of concrete prior to the installation of an Aspartech resin floor.
What is the Calcium Chloride Test?
The Calcium Chloride Test is a quantitative method of determining the Moisture Vapour Emission Rate (MVER) from the surface of a concrete floor.
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The recognised test method is:
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ASTM F1869 – Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride.
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Unlike an electronic surface moisture meter, the test does not simply provide an instantaneous indication of moisture. Instead, a known quantity of dry anhydrous calcium chloride is exposed to the concrete beneath a sealed enclosure for a controlled period.
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Calcium chloride is highly hygroscopic, meaning that it readily absorbs moisture from its surroundings. Moisture vapour leaving the concrete is absorbed by the calcium chloride, increasing its weight.​ By accurately weighing the calcium chloride before and after the test, the amount of moisture emitted from the concrete can be calculated.
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ASTM describes the method as providing a quantitative measurement of moisture vapour emission from bare concrete and notes that the result represents the condition of the concrete surface at the time of testing.
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How is the Test Performed?
The test should be carried out on bare, properly prepared concrete representative of the surface onto which the Aspartech system will be installed.
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The general principle is:
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1. Prepare the test area
Any coatings, adhesives, curing compounds, sealers or other materials that could interfere with moisture transmission must not be present within the test area. The surface is prepared in accordance with the test method.
2. Weigh the calcium chloride
A dish containing a known quantity of anhydrous calcium chloride is accurately weighed before exposure.
3. Position the test
The dish is placed onto the prepared concrete surface.
4. Seal the test area
An impermeable cover is installed over the dish and sealed to the concrete, creating a controlled test area.
5. Allow moisture to be absorbed
The calcium chloride is left exposed to moisture emitted by the concrete for the period prescribed by ASTM F1869.
6. Reweigh the calcium chloride
At completion, the dish is immediately resealed and accurately weighed. The increase in mass represents moisture absorbed during the test.
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7. Calculate the MVER
The weight increase, test duration and exposed area are used to calculate the Moisture Vapour Emission Rate.
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Understanding the Result
ASTM F1869 traditionally expresses the result as:
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lb of water / 1,000 ft² / 24 hours
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For example:
3 lb / 1,000 ft² / 24 h means that the measured emission rate is equivalent to approximately 3 pounds of water being emitted through 1,000 square feet of concrete during a 24-hour period. For UK and European projects, this can also be expressed approximately in metric units:
There is no single universal MVER value at which every resin flooring system becomes unsuitable. The recommended maximum MVER for the Aspartech product range is:
3 lb/1,000 ft²/24 h (approximately 146 g/m²/24 h) when tested in accordance with ASTM F1869
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Results exceeding this value do not necessarily mean that an Aspartech resin floor cannot be installed. However, elevated moisture vapour emission should be investigated and an appropriate moisture-tolerant primer or moisture-control system may be required before installation.
Why Moisture Tolerance Matters
The presence of moisture does not automatically mean that a concrete floor cannot receive a resin coating.
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Different resin chemistries have significantly different tolerances to damp substrates. Some systems require extremely dry concrete, while appropriately formulated moisture-tolerant primers and resin systems can accommodate considerably more challenging substrates. This is an important characteristic of the Aspartech system, which has been developed with moisture tolerance in mind. However, moisture tolerance should not be confused with an ability to overcome uncontrolled hydrostatic pressure, standing water or a defective/missing damp-proof membrane.
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Where unusually high moisture readings are obtained, the cause should be investigated before proceeding.
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An Important Limitation of the Calcium Chloride Test
The Calcium Chloride Test measures the rate at which moisture is leaving the surface of the concrete at the time of testing. It does not directly measure the moisture condition throughout the entire depth of the slab.
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ASTM specifically cautions that the result represents the concrete floor surface at the time of the test and may not indicate future conditions.
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This distinction is important. Environmental conditions, heating, ventilation, groundwater, the presence or absence of a damp-proof membrane and changes caused by subsequently covering the concrete can all influence the moisture behaviour of a slab. For critical installations, calcium chloride testing may therefore be used alongside other assessment methods, particularly in-situ relative humidity (RH) testing.
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The Test Must Be Performed on Bare Concrete
ASTM F1869 applies to bare concrete floors. The current standard specifically states that it should not be used to determine moisture emission through existing coatings, reactive penetrants, patching materials or levelling compounds.
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This makes the timing of testing particularly important when specifying an Aspartech flooring system.
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Where possible, moisture assessment should be completed after the concrete has been exposed and appropriately prepared but before Aspartech PRIME, LEVEL-FIX, BASE or another resin layer is installed.
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Testing over an existing coating would measure the behaviour of the coating as well as the concrete and would therefore not provide a valid ASTM F1869 assessment of the underlying slab.
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More Than One Test May Be Required
Concrete moisture is rarely completely uniform across a floor.
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Areas close to external walls, construction joints, drainage, previous repairs or changes in slab construction can behave differently from the remainder of the floor. A single result should therefore not automatically be assumed to represent a large floor area. For larger installations, multiple test locations should be selected to provide a representative assessment of the substrate.
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Moisture Testing is Only Part of Surface Assessment
A satisfactory moisture result does not by itself mean that a concrete floor is ready to receive an Aspartech resin system.
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The substrate should also be assessed for:
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Surface strength and integrity
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Contamination by oil, grease or chemicals
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Existing coatings and curing compounds
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Laitance
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Cracks and movement joints
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Surface profile
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Temperature
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Ambient relative humidity
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Dew point
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Presence and condition of the damp-proof membrane
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The performance of any resin flooring system ultimately depends upon the condition and preparation of the substrate to which it is applied.
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Important
A Calcium Chloride Test should be regarded as a measurement of the moisture vapour emission rate at the time and location of the test, rather than a guarantee of the future moisture condition of the concrete.
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Test results should always be considered alongside the construction of the slab, environmental conditions, presence of an effective damp-proof membrane and the requirements of the specified Aspartech flooring system.
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Where the origin or extent of moisture is uncertain, further investigation should be undertaken before installation.
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