In swimming pool constructions, it has been common practice to install waterproofing membranes first, followed by the wall-levelling render, and then the tiling finish. The reasons given for this sequencing are:
However, this layering sequence of:
concrete substrate → waterproofing → levelling render → tiling installation
has given rise to some major problems. These are:
a) Delamination of the waterproofing/rendering/tiling assembly, partially or wholly, from the substrate. (Partial delamination is detected by listening to the hollow sounds generated by knocking on the tiles. Complete de-lamination leads to collapse of the whole assembly.)
de-bonding & collapse of waterproofing membrane & levelling render in swimming pool under construction (the waterproofing membrane was applied below the levelling render)
b) Efflorescence. This is a common occurrence. It does not happen as long as the pool is filled and the walls are under hydrostatic pressure. However, when the pool is emptied for cleaning the first time, unsightly whitish salt deposits start appearing on the tiles.
efflorescence appearing on walls of swimming pool emptied for cleaning
The reasons for these problems are explained below.
a) Delamination
Swimming pool structures, especially outdoor ones, are subjected to dynamic stresses caused by thermal movements, ground movements, emptying/filling of pools for cleaning, and vibrations in the environment. For this reason, the EN 14891 standard specifies that, for swimming pool installations, waterproofing membranes applied beneath ceramic tiling installed with adhesives must have a crack-bridging ability (under standard conditions) of ≥ 0.75 mm*. The crack-bridging property is a measure of the waterproofing membrane’s ability to deform and absorb stresses originating in the substrate.
*-Waterproofing membranes which satisfy this criterion include Mapelastic and Mapelastic Smart. MAPEI also recommends deformable adhesives for swimming pool installations e.g. Keraflex Maxi S1 (C2TE S1) and Kerabond T + Isolastic (C2E S2).
Consider what happens when the waterproofing membrane underlays the levelling render in addition to the tiling assembly. It is now carrying a much heavier load as the levelling render commonly varies in thickness from 20 to 60 mm (depending on how well the substrate was aligned to start with). Movements which originate in the substrate subject the waterproofing membrane to greatly increased stresses due to the significantly higher over-hanging load, thus increasing the risk of adhesion loss.
On the other hand, applying the thick wall-levelling render directly on the concrete substrate provides an obvious technical advantage for improving its adhesion i.e. by making it possible to pre-treat the surface with a spatter-dash coat. (This is obviously not possible with the waterproofing membrane which must be applied on an even substrate.)
b) Efflorescence
Three conditions are necessary for efflorescence to occur:
With reference to condition (1.), all OPC-based concretes and mortars contain hydrated lime, a byproduct of the cement hydration reaction. As the concrete substrate and the levelling plaster/screed are the heaviest components in the assembly, they are obviously the ones with the highest efflorescence potential.
Condition (2.) cannot be avoided for swimming pool structures.
With reference to condition (3.), even in the absence of defects such as cracks and voids, water finds its way into concrete and render/screed matrices through capillary pores. As long as the pool is filled, hydrostatic pressure keeps the water within the matrices. However, as soon as the pool is emptied, the water finds its way out carrying with it the hydrated lime. On the pool walls, the hydrated lime reacts with carbon dioxide in the atmosphere to form calcium carbonate, the hard, white, insoluble salt deposits we recognize as efflorescence.
For this reason, if control of efflorescence is an important requirement, it is strongly recommended that the waterproofing membrane, being impermeable to water, is placed as close to the surface as possible to stop water from infiltrating the matrices which are rich in efflorescence potential. Therefore, the recommended layering sequence is:
concrete substrate → levelling render → waterproofing → tiling installation
Conclusions
The practice of installing the waterproofing membrane below the wall-levelling render seems to be governed mainly by commercial considerations i.e. the manner in which the sub-contracts are awarded.
However, as the above explanations have shown, there are definite technical advantages to be gained from applying the levelling render first before the waterproofing membrane:
✓ It is possible to pre-treat the concrete substrate to improve adhesion of the levelling render.
✓ The waterproofing membrane does not have to carry the additional load of the levelling render and the increased risk of adhesion loss is avoided.
✓ Placing the waterproofing membrane outside the levelling render cuts off the pathways for leaching of efflorescence-causing compounds from a rich source (the levelling render itself).
The “obstacle” described in the beginning of this article of responsibility for protection of the waterproofing works can be overcome by either:
▪ appointing one sub-contractor to undertake both the waterproofing and tiling works, or,
▪ appointing a sub-contractor who is willing to take responsibility for protecting the waterproofing works to undertake the tiling works.
The technical advantages of carrying out the works in the right sequence are too important for them to be sacrificed for a customary practice which has proven to be problematic.
preferred layering sequence: installing the waterproofing membrane over the levelling render and as near to the water source as possible
Reference: relative weights of materials
| − Levelling render, 40 mm thick | : ~ 70 kg/m2 |
| − Waterproofing membrane, 2 mm thick | : ~ 4 kg/m2 |
| − Tiling adhesive, applied to 3 mm thick | : ~ 5 kg/m2 |
| − Tiles, 4 mm thick | : ~ 10 kg/m2 |
In swimming pool constructions, it has been common practice to install waterproofing membranes first, followed by the wall-levelling render, and then the tiling finish. The reasons given for this sequencing are:
However, this layering sequence of:
concrete substrate → waterproofing → levelling render → tiling installation
has given rise to some major problems. These are:
a) Delamination of the waterproofing/rendering/tiling assembly, partially or wholly, from the substrate. (Partial delamination is detected by listening to the hollow sounds generated by knocking on the tiles. Complete de-lamination leads to collapse of the whole assembly.)
de-bonding & collapse of waterproofing membrane & levelling render in swimming pool under construction (the waterproofing membrane was applied below the levelling render)
b) Efflorescence. This is a common occurrence. It does not happen as long as the pool is filled and the walls are under hydrostatic pressure. However, when the pool is emptied for cleaning the first time, unsightly whitish salt deposits start appearing on the tiles.
efflorescence appearing on walls of swimming pool emptied for cleaning
The reasons for these problems are explained below.
a) Delamination
Swimming pool structures, especially outdoor ones, are subjected to dynamic stresses caused by thermal movements, ground movements, emptying/filling of pools for cleaning, and vibrations in the environment. For this reason, the EN 14891 standard specifies that, for swimming pool installations, waterproofing membranes applied beneath ceramic tiling installed with adhesives must have a crack-bridging ability (under standard conditions) of ≥ 0.75 mm*. The crack-bridging property is a measure of the waterproofing membrane’s ability to deform and absorb stresses originating in the substrate.
*-Waterproofing membranes which satisfy this criterion include Mapelastic and Mapelastic Smart. MAPEI also recommends deformable adhesives for swimming pool installations e.g. Keraflex Maxi S1 (C2TE S1) and Kerabond T + Isolastic (C2E S2).
Consider what happens when the waterproofing membrane underlays the levelling render in addition to the tiling assembly. It is now carrying a much heavier load as the levelling render commonly varies in thickness from 20 to 60 mm (depending on how well the substrate was aligned to start with). Movements which originate in the substrate subject the waterproofing membrane to greatly increased stresses due to the significantly higher over-hanging load, thus increasing the risk of adhesion loss.
On the other hand, applying the thick wall-levelling render directly on the concrete substrate provides an obvious technical advantage for improving its adhesion i.e. by making it possible to pre-treat the surface with a spatter-dash coat. (This is obviously not possible with the waterproofing membrane which must be applied on an even substrate.)
b) Efflorescence
Three conditions are necessary for efflorescence to occur:
With reference to condition (1.), all OPC-based concretes and mortars contain hydrated lime, a byproduct of the cement hydration reaction. As the concrete substrate and the levelling plaster/screed are the heaviest components in the assembly, they are obviously the ones with the highest efflorescence potential.
Condition (2.) cannot be avoided for swimming pool structures.
With reference to condition (3.), even in the absence of defects such as cracks and voids, water finds its way into concrete and render/screed matrices through capillary pores. As long as the pool is filled, hydrostatic pressure keeps the water within the matrices. However, as soon as the pool is emptied, the water finds its way out carrying with it the hydrated lime. On the pool walls, the hydrated lime reacts with carbon dioxide in the atmosphere to form calcium carbonate, the hard, white, insoluble salt deposits we recognize as efflorescence.
For this reason, if control of efflorescence is an important requirement, it is strongly recommended that the waterproofing membrane, being impermeable to water, is placed as close to the surface as possible to stop water from infiltrating the matrices which are rich in efflorescence potential. Therefore, the recommended layering sequence is:
concrete substrate → levelling render → waterproofing → tiling installation
Conclusions
The practice of installing the waterproofing membrane below the wall-levelling render seems to be governed mainly by commercial considerations i.e. the manner in which the sub-contracts are awarded.
However, as the above explanations have shown, there are definite technical advantages to be gained from applying the levelling render first before the waterproofing membrane:
✓ It is possible to pre-treat the concrete substrate to improve adhesion of the levelling render.
✓ The waterproofing membrane does not have to carry the additional load of the levelling render and the increased risk of adhesion loss is avoided.
✓ Placing the waterproofing membrane outside the levelling render cuts off the pathways for leaching of efflorescence-causing compounds from a rich source (the levelling render itself).
The “obstacle” described in the beginning of this article of responsibility for protection of the waterproofing works can be overcome by either:
▪ appointing one sub-contractor to undertake both the waterproofing and tiling works, or,
▪ appointing a sub-contractor who is willing to take responsibility for protecting the waterproofing works to undertake the tiling works.
The technical advantages of carrying out the works in the right sequence are too important for them to be sacrificed for a customary practice which has proven to be problematic.
preferred layering sequence: installing the waterproofing membrane over the levelling render and as near to the water source as possible
Reference: relative weights of materials
| − Levelling render, 40 mm thick | : ~ 70 kg/m2 |
| − Waterproofing membrane, 2 mm thick | : ~ 4 kg/m2 |
| − Tiling adhesive, applied to 3 mm thick | : ~ 5 kg/m2 |
| − Tiles, 4 mm thick | : ~ 10 kg/m2 |
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