The best substrates for ceramic tiling are hard, solid, and inflexible because tiles are hard, solid, and inflexible so perform best on that sort of substrate, such as concrete.There are two states of concrete that the tile fixer will be faced with:
fresh concrete (green concrete) where tiles are being applied on a new build project or;
existing concrete such as in a refurbishment job. Each application will be different and will be considered separately.
New concrete slabs/floors and walls
Although tiling performs well on concrete, there are areas to watch out for.The life of concrete is too often ignored by specifiers and contractors as well as the threat it poses to subsequent tiling.
When a concrete floor-slab is cast, and tamped or vibrated to compact it, there floats to the surface a mixture of the finest aggregates in the mix (dust), cement and water - this process is generally referred to as bleeding.This mixture, when dry is known as laitance and has a high water and fines content which means that it‘s weak and porous and does not adhere strongly to the mass of the concrete. This weak layer in some instances will have to be removed prior to tiling.
Exceeding the open time is a potential contributor to later problems such as an insufficient bond between the tile and adhesive. For secure tiling, the laitance must be removed by mechanical means, such as shot blasting (i.e. with Blastrac equipment) (see British Standard BS 5385: Part 3:1989) in order to cleanly expose the aggregate in the concrete.
It is also likely that concrete walls will have been treated with a mould release agent which, prior to tiling, must be removed with a high-pressure cleaner.
Preparation technique
Allow at least 6 weeks for new concrete to cure with air drying before applying a screed or directly bedded materials.This time may have to be extended in wet weather (refer to BS 5385: Part 3: 14.2.3 for curing standards). If this is not followed then when the concrete shrinks, the bond between the adhesive and the concrete will be broken.
If early fixing is required for fast-track projects, use an external reinforced decoupling matting system such as a Shlüter system, along with a polymer modified adhesive such as PCI’s Tilefast 6 RapidFlex.
For vinyl sheeting, where a 150mm-thick concrete slab is laid on to a suitable DPM (Damp Proof Membrane) is used, it can take up to a year to dry.
With new build projects, there is usually a damp proof membrane below the concrete which will prevent the moisture rising but if not, consider using a surface membrane such as SDPM (Surface Damp Proof Membrane).
Use a primer, such as PCI’s Primer G, if the concrete is dusting or the porosity is high.
Adhesive application
For new concrete use either Thin Bed, Thick Bed or Fast-Set adhesive or for external applications use Thin and Thick Bed Adhesive, such as PCI’s Tilefast 6 Rapidflex or, Floorfast 20 Rapid. For large format tiles.
When using a white grout, always use a white adhesive.
For light toned marble or natural stone tiles use a specialist adhesive, such as those from PCI’s Carra range, so as to avoid any discolouration.
When using porcelain or vitrified tiles, use PCI’s Tilefast 6 Rapid, Tilefast 6 Rapidflex or Floorfast 20 Rapid polymer modified adhesives to promote bond strength.
Grouting
Once the adhesive bed has firmed up, grouting may commence.
The type of grout will depend on the width of the joint.
Drying time will depend on the site conditions and type of tiles.
For porcelain or vitrified tiles, use PCI’s Groutfast 20 Rapidflex to promote the bond at the tile edges.
Movement Joints
Please refer to Special Situations Bulletin 1 due to the complexity of this subject.
Power-trowelled concrete
The dense, smooth and impervious nature of a power-trowelled concrete slab means that tiling has to be approached differently - such as a surface may be ideal for the floor of a warehouse but is not regarded as suitable for tiling. This same process of removing laitance as described at the beginning of this section maybe appropriate on power-trowelled concrete. Concrete cast in formwork does not suffer the problem of laitance, except on its top surface which will not usually be tiled, however, a skin of cement paste may have formed against the formwork. As a result of chemical retarders, it is essential to remove this in order to promote a secure bond for the new tiling. Although poured slabs can present a very smooth surface, they more frequently have a rougher finish than is ideal for tiling with normal Thin Bed adhesives. If this is the case, then specify an adhesive capable of utilising a greater bed thickness, such as one of the PCI Floorfast mortars.
Preparation technique
Make sure that the concrete is clean (free from dust, dirt, dust, grease and residual adhesives) and of sound construction - where possible, mechanically prepare the surface to ensure a strong bond for the new tiling.
If the surface of the concrete is poor then the tiling may fail because of the weak layer (laitance).
Use a primer, such as PCI’s Primer G (diluted 1:1,water:Primer G) if the concrete is dusting or the porosity is high.
If early fixing is required for fast-track projects, use a decoupling matting system such as a Shlüter system, along with a polymer modified adhesive such as PCI’s Tilefast 6 Rapidflex.
Adhesive application
Use for example; PCI’s Tilefast 6 Rapidflex to promote bond strength at the adhesive/substrate interface.
When using a white grout, always use a white adhesive.
For light toned marble or natural stone tiles use a specialist adhesive, such as those from PCI’s Carra range, to avoid any discolouration.
When using porcelain or vitrified tiles, use PCI’s Tilefast 6 Rapid or Tilefast 6 Rapidflex to promote bond strength.
Grouting
Once the adhesive bed has firmed up, grouting may commence.
The type of grout will depend on the width of the joint.
Drying time will depend on the site conditions and type of tiles.
For porcelain or vitrified tiles, use PCI’s Groutfast 20 Rapidflex to promote the bond at the tile and edge.
Movement Joints
Please refer to Special Situations Bulletin 1 due to the complexity of this subject.
For more information, contact PCI’s Techincal Services Department on 0161 794 7411