A prerequisite for a sustainable and beautiful wood floor is a strong and dry screed. Therefore this screed has to be carefully accessed. The first source of information is the architect, engineer, or main contractor who will have all of the information about the build-up and nature of the floor. The second source is the screed contractor who can provide the screed details and if appropriate, any subfloor heating protocols. If, as so often in renovation projects most of this information is not available, there are a few simple methods that can be used to evaluate the screed.
Screed Evaluation Methods
Visual inspection and level survey
Is the floor level and uniform/even, are there any cracks and why, where are the joints and how have they been formed and / or treated?
Cross-hatch testing for surface strength
Is the screed surface strong enough?
The surface strength can be tested by scratching a 3 mm cross-hatched grid using suitable metal nail. If the surface flakes or chips-off, then the screed strength is insufficient and needs to be strengthened or in extreme cases, removed and levelled again.
Surface Wiping test for dusting
Is the screed surface free from loose and friable particles leading to dusting?
By simply wiping the surface by hand several times it is possible to assess if the surface tends to dusting or not. If it does then further investigation is required and the substrate surface probably needs to be consolidated with an impregnating primer.
Hammer-testing for screed soundness and bond to the subfloor
Is the screed sound and firmly bonded to the subfloor?
Tapping the surface with a small hammer will reveal if areas of the screed are not bonded firmly to the subfloor with a hollow sound. If there are hollow areas these must be marked and the screed needs to be removed and replaced. The UK’s BRE Drop Hammer and other similar equipment is also used for more formal testing of screed soundness.
Tensile strength: the “pull-off” test
Is the screed strong enough to install a wood floor?
With the tensile “pull-off” test according to EN 13892-8, the tensile strength of the screed is determined.
To be able to accept a bonded wood floor the screed must have a tensile strength of ≥ 1 N/mm2 and for wood block parquet flooring ≥ 1.2 N/mm2.
Moisture content testing
Is the screed dry enough to install a wood floor?
A screed can be considered as dry if it is not emitting moisture to the environment and this is also dependent on the environment it is exposed to. In warm and humid regions the amount of water in the subfloor is higher than in cold and dry regions.
Consequently there are several country specific standards defining the acceptable limits and how to measure the moisture content of a screed and interpret the results.
The interpretation of moisture content measurement results has to be done with great care and understanding of the screed. The equilibrium moisture content of the cured screed is the only relevant dimension, but this can vary significantly, especially for premixed or accelerated screeds. Always follow the instructions of the screed producers.
The free water content in the screed can be determined by destructive (direct) and nondestructive (indirect) methods:
- Destructive methods measure the water in the system;
- Non-destructive methods measure physical properties of the system that change according to the water content.
The following are commonly used methods of moisture content testing:
- Darr method (gravimetric moisture measuring method): Samples of the screed are extracted and weighed before and after being heated to 105°C (cement based) and 40°C (calcium sulfate / gypsum based). The result is the amount of free water in the samples. This method is considered the most reliable and is often used in many ‘Expert reports’ etc.
- Calcium Carbide Method (CM): In a closed test system the moisture (water) contained in the samples reacts with calcium carbide that is added to produce acetylene gas creating a constant pressure which is then measured. The results correlate well to the Darr humidity test done at 40°C instead of 105°C.
- Relative Humidity (RH): Holes are drilled into the screed to a certain depth and permeable plastic sleeves inserted. After an equilibration time of 72 hours the air humidity within the sleeve is measured with a hygrometer and tables correlate this to the screed moisture content.
- Capacitive method: The electrodes of a measuring device are pushed to the screed and the dielectric constant is measured. In addition to the water, the mineral salts content and the density of the material will have an impact on the results and therefore it is necessary to take measurements in several different places. The results are then converted into a CM or RH value by tables. This method is now the one that is most commonly used worldwide, due to its reliable consistency and easeof use. e.g. Sika Tramex.
- There are also non-destructive methods where an impermeable sheet or hood is placed and sealed onto the surface and sealed for an extended period of time before it is removed and surface measurements taken. These methods are described in several standards including ASTM F2170 & F2420 and BS 8201, 8203, 5325 Standard. Although very time consuming these methods are useful in the event of determining the relative importance for the floor finishes of continuous moisture transmission from below sue to leaking water supply or drainage etc., which may have to be repaired before any wood floor can be installed.
Dr. David Sager
Corporate Market Development Manager
Target Market Sealing & Bonding
Sika Services AG