Crazing and/or Cracking of the Joint Filling

Visualisation

Formation of longitudinal and transverse cracks in the tile-to-tile joint filling material, without this affecting the adhesion to the tile flanks, soon after completion of the grouting operation.

Defect assignment

Aside from what has already been remarked concerning the ‘grout slurries’ for which there is no performance or quality assurance, the grouts envisaged in standard EN 13888 display a maximum shrinkage of 2 mm/m for the cementitious grouts (CG 1 and CG 2) and 1.5 mm/m for the reaction resin grouts (RG). These shrinkages are measured under standard laboratory conditions and they may be expected to vary more greatly when the material is subject to weather conditions that favour rapid water evaporation. Excess water in the preparation of the grout will also cause excessive shrinkage and, consequently, be a factor that encourages cracking of the joint.

The formation of longitudinal and transverse cracks in the tile-to-tile joints, usually soon after those joints have been installed, may have different causes, related to the preparation and application of the grout:

• Environmental conditions of temperature and moisture that produce rapid and non-uniform evaporation of the mixing water
• Excess water in the mixing of the material or water additions after mixing or during the grouting operation
• Mistaken selection of the grout, as regards the ratio of the aggregate size in relation to the joint width

In the course of time, different types of cracks may also develop as a function of the stiffness of the grout. They are usually the result of tensile and compressive stresses stemming from movements of a thermal or hygrometric origin of the ceramic tiles and the fixing surface.

Cracking may also occur in permeable grouts as a result of the crystallisation of salts or frost/thaw cycles. In the former case, these crystallisations will be accompanied by whitish stains, unevenly distributed on the joint surface. In the later case, the frost cracks will be accompanied by detachment of material in the form of flakes.

In the grouts prepared in situ, the shrinkage will obey all the factors already indicated for fixing or levelling mortars: quantity of binder, water/cement ratio, size and distribution of the aggregate, mix uniformity, etc. Since, in the joint, it is necessary to combine adhesion to the tile flanks (depending on the water absorption capacity of the ceramic), mechanical strength, impermeability, and the lowest possible stiffness, it is advisable not to prepare mixes at the building site, even when prescription mortars of proven efficiency and good performance are involved.

Prevention

• Selection of grouts with low shrinkage and scarce water absorption/suction, which are if possible also deformable [CG 2 S1, CG 2 S2]
• Observance of the mixture proportion and instructions of the manufacturer
• Performance of the grouting operation under favourable weather conditions, also without direct exposure to sunlight