Longitudinal Debonding of the Joint Material

Visualisation

The grouting material displays continuous cracks along the tile flank, customarily affecting multiple tiles.

Detailed inspection may provide information on the origin of the debonding from the tile flanks:

• If the debonding is widespread or abundant, and more or less randomly distributed, adhesion will have failed under tensile or compressive stresses.
• If it follows a longitudinal path, it is a manifestation of movement in the underlying layers
• If that path is laddered (in wall tilings), a rupture in the fixing background or an important movement is certain to have occurred.

Defect assignment

Standard EN 13888 does not envisage adhesion as a fundamental characteristic, taking it for granted that CG 1 and CG 2 materials display appropriate bonding to the tile flanks (a few millimetres on each side of the joint). Even when that general assumption is made, the stiffness of a cementitious grout may jeopardise that adhesion when tension and compression cycles follow one another as a result of tile dimensional changes caused by swings in temperature or variations in humidity.

The larger the tile size, the greater is the magnitude of the tensile and compressive stress, which may exceed the adhesive tensile strength of the material on the tile edge. It is important, therefore, to consider the deformability of the cementitious grouts intended for exterior coverings, especially for flooring with large tile sizes (S>=900 cm²). The selection of CG 2 S1 or CG 2 S2 materials may successfully counteract the longitudinal cracking of the tile-to-tile joints.

However, longitudinal cracking of a tile-to-tile joint can never be avoided when it extends across the entire tiling. Joint mortars are not designed to absorb movements of underlying layers and the appearance of that crack indicates that a movement joint should have been installed there.

Prevention

Sometimes, the appearance of this defect is unforeseeable from the tile installation side (the last two manifestations described). In general, it the following recommendations should be observed:

• Design a joint width proportional to the size of the tile and to the expected movements in the ceramic tiling, especially those of hygrothermal origin. In exteriors, it is recommended to use joint widths equal to or larger than 5 mm, or to observe the principle that the joint area should be equal to or larger than 5% of the total area of the wall tiling/floor tiling.
• Install movement joints at the changes of material in the substrate or fixing surface. Design and install perimeter and intermediate movement joints according to the location and characteristics of the wall or floor tiling.
• Do not use grouts that fail to conform to the compulsory characteristics of standard EN 13888. It is recommended to select CG 2 grouts in exterior wall and floor tilings
• If possible, select deformable cementitious grouts [CG 2 S1, CG 2 S2] with large sizes (S>1600 cm²), in floorings with heavy traffic or on unstable fixing surfaces.