HOW IS THE ADHESION OF A CEMENT-BASED GLUE MEASURED?
Technically, cement-based glues are also called cement-based adhesives.
The adhesion of an adhesive is defined as the tensile strength of that cement under the action of a continuous and direct force.
Adhesion is not a single value, but will depend on the test conditions to which the tiles under study have been subjected.
The test standard EN 1348 defines four different test conditions:
a.- initial adhesion
b.- adhesion after immersion in water
c.- adhesion after heat ageing
d.- adhesion after freeze-thaw cycles
Depending on the application, one of the above four adhesions will be more or less relevant: are we building a kitchen, a swimming pool, a façade, …?
Currently, the adhesion value is expressed in N/mm2. For example: an adhesion of 0.5 N/mm2 is equivalent to 5 kg/cm2 in the old unit of measurement.
At this point, we can easily calculate the gripping force to which a 100 x 100 mm tile glued with an adhesive with an initial adhesion of 0.5 N/mm2 will be subjected:
F = 0.5 N/mm2 x 100 mm. x 100 mm. = 5,000 N (= 500 Kg.)
In plain English: we would have to exert a force of 500 kg to be able to pull a single 10×10 cm piece off its surface, provided that the surface did not give way beforehand.
As we can see, it is always assumed that the contact between the cement and the back of the tile is 100%. Any adhesion value expressed in N/mm2 assumes full contact.
So why do we have to lay all ceramic pieces in a solid form?
If we stick to common sense, we can say that “tile adhesive that does not touch the tile is certainly not gripping it”.
For this reason, it is generally recommended that the contact between cement-based glue and ceramic tile should be at least 70%, although in special installations such as swimming pools or façades this percentage should be increased to 100%. For this purpose, it is recommended to use the double gluing technique: cement glue is applied to the surface, but also to the back of the ceramic pieces.
