Bituminous trowel applied thick film coatings are very popular and reliable products, used widely in much of Europe for external waterproofing of basement areas. The products are applied by trowel in 2 thick, waterproofing and crack bridging layers. The products originated in and are still primarily produced in Germany. They come under the realm of the German waterproofing standard Din 18195. These type of products are becoming increasingly popular in the UK, mainly due to their reliability and ease of application.
Thick Film Coatings can be applied by specialist spray equipment or by trowel. For most infrastructure tanking projects in the UK trowel application is the favored method. The contractor can carry out the waterproofing work without heavy investment in equipment or having to engage another specialist contractor.
The trowel application of the thick film waterproofing is usually carried out in two layers. However the way in which the two layers are applied is subject to some disagreement between different contractors.
Some contractors are using the traditional Method, whereby a notched trowel is used to distribute the thick film waterproofing over the substrate. This notched profile is then immediately smoothed off using the flat side of the same trowel. Once this first coat has firmed, the second coat is applied in the same way; a notched trowel to distribute the material at the correct thickness then a flat trowel to smooth. The steps taken are as follows: –
First Coat applied at 6mm with notched trowel.
First Coat smoothed down to 3mm with flat side of trowel.
Second Coat applied at 6mm with notched trowel.
Second Coat smoothed down to 3mm with flat side of trowel, total applied thickness 6mm, with two separate waterproofing layers.
The alternative technique favored by some contractors is what I call the open technique. This differs from the tradition in that the first coat is not smoothed down, but left to cure as a notched finish giving 50% surface coverage at 6mm thick. Once the 1st coat has hardened the second coat is applied with a flat trowel, infilling the gaps in the 1st coat. The steps taken are as follows: –
First Coat applied at 6mm with notched trowel.
First Coat left to cure (substrate is still open).
Second Coat applied with a flat trowel to infill gaps in first coat.
Second Coat dries to give a total applied thickness of 6mm.
In order to determine which is the preferred technique, it is simply a matter of going back to the basic principles of waterproofing. It is a fundamental principle of most waterproofing systems that it is better to have 2 coats of a material than one. In most cases one coat would theoretically be enough to achieve a waterproofed structure. However there is always a possibility of imperfections in the concrete or in the application of the waterproofing that can lead to small misses or pinholes. The second coat is there to make sure that a total seal is achieved under site conditions. With careful application the chances of a pinhole occurring at exactly the same place in two consecutive layers of waterproofing are remote.
With the traditional trowel application of thick film sealants we achieve two separate and complete waterproofing layers. With the open technique the possibility of voids forming between the 1st and 2nd layers is increased. There is also quite a high possibility of the second layer not fully filling the depth of the first layer, leaving pockets in the coating.
The other aspect to consider is curing time and rain risk. It is my experience that the 1st coat in both techniques will take about the same amount of time to cure. Although the open technique has a thicker layer which would normally be slower to cure, it is also open at the sides which leads to a higher surface area which will speed up the cure, so these two points considered together cancel each other out.
Where there can be more of a problem is if it rains between the first and second coat, particularly on a horizontal application sloped to falls. Most Bituminous thick film coatings will resist a light rain shower (not standing water) after a couple of hours curing. In the case of the traditional method, as long as the deck is well drained the rain will pass over the part-cured coating with no ill effect, water can not penetrate the concrete. With the open technique, rainwater can still penetrate the concrete, possibly getting in underneath the partially cured coating leading to reduced bond and slowed final cure. It is also likely that the open notches will retain water to a greater extent than the traditional method. This can not be avoided without all the notches run towards the falls.
For the above reasons I firmly believe that the traditional method represents best practice. It has been used for many years in Germany and has real advantages over the open technique which only has the advantage of being slightly quicker to apply. For further information or advice please visit our website by clicking on the following link: Basement Waterproofing