Shani Wallis, TunnelTalk

What is spray-on waterproofing? How does it work? What does it look like when applied? What are the raw materials? What are the design and performance criteria and how are these specifications tested in the laboratory? The answers to all these questions, and more, were on offer at a live demonstration and seminar (deminar) session organised by ITAtech, the Committee on Technical Developments of the ITA, International Tunnelling and Underground Space Association, on Friday, 31 May, just before the start of the ITA World Tunnel Congress in Geneva.

Close up look to test if seeing really is believing

For those who missed the event, it was the most productive opportunity to investigate and come to know more about the different products and suppliers that support a technique that has been around for many years in surface civil engineering industries, such as bridge structure rehabilitation. It is only relatively recently, however, that the process has begun to find applications in the underground. The promise in the welcome address was that "seeing is believing", and the aim of the day was to "accommodate" just that.

The venue was ideal. Staged at the Hagerbach underground test facility and laboratory in Sargans, east of Zurich, the underground venue for lectures, live demonstrations and laboratory test set-ups, provided a real environment in which to showcase the products and their application.

The aim of ITAtech is to provide a platform for commercial suppliers and manufacturers to draw global experience and expertise together for the benefit of the whole industry, and to find ways of supporting the introduction of new techniques and products by collaborating on the development of design and performance criteria and preparing application of best-practice guidelines. For the Activity Group on Lining and Waterproofing, the first area of focus was spray-on waterproofing, a technique that has attracted substantial controversy, particularly from the sector of the industry that supports the use of sheet membrane waterproofing systems as the only way to ensure a waterproofed underground environment; and others who say that no system can guarantee a completely dry underground space.

Underground seminar venue at the Hagerbach facility in Sargans

Vested interests and passionate opinions aside, spray-on waterproofing as a technique has found traction in the industry and is being introduced and included in infrastructure designs by clients and design engineers around the world. Contractors are now also appreciating the alternative.

The savings potential of the technique, especially when used as part of a combined shotcrete final lining structure, is the most attractive promotion tool. A primary shotcrete lining engaged to work in tandem with a bonded spray-on waterproofing membrane and an equally bonded final shotcrete lining, can provide a lining that is half as thick as a system using a sheet membrane. Additionally, there are significant time savings, as well as the possibility of the contractors' own tunnelling crews applying the membrane using their own equipment and within their own scheduling programme.

The deminar was organised by four prime sponsors of the ITAtech Committee and Activity Group: (in alphabetical order) BASF; Mapei; Normet; Sterling Lloyd; and had the support of others including the Hagerbach Laboratory itself; Babendererde Engineers; Wacker Chemie; and Hydromat Ltd. In addition to the day-long deminar the Activity Group took the opportunity to launch and distribute to the delegates its 64-page Design Guidance for Spray Applied Waterproofing Membranes, which is also available as a free pdf download on the ITA website.

A group of about 75 delegates attended the deminar, ranging from clients, design engineers, contractors and representatives of shotcrete additive manufacturers. An audience, it was thought, that was receptive to the purpose of the day.

The lectures started off with an address that stated categorically that "spray membranes are NOT for every application. Spray is not competing with sheet membrane systems but rather complementing them. Sheet membranes are for high pressure, high volume water ingress situations, spray systems are not; spray systems are more appropriate for complex underground structures where sheet membrane is hard to apply; spray membranes are more appropriate behind shotcrete final linings and only possible for composite lining designs".

Concentrated attention in the laboratory

There was more splitting of opinion here as to whether shotcrete can be applied satisfactorily to sheet membranes, and expression of the growing opinion that concrete itself can be "watertight" without needing rebar to limit cracks to design specification. There was also acknowledgement of trials where the waterproofing polymers are being integrated into the concrete mixes itself with "good initial results".

The lectures continued with presentations that addressed many of the questions and preconceived ideas of the delegates. Among them: What exactly is "dry" in reference to an underground structure? We were sitting in an underground lecture room of the Hagerbach facility and it was pointed out that although the environment was very comfortable and felt "dry", there were in fact small buckets attached to the ceiling in one particular spot to catch water drips (see image of the lecture room above). It wasn't explained how the room had been waterproofed but the uneven surface confirms no in situ final lining. "Dry" is a specification that is set by the designer in accordance with his client's wishes and is therefore perhaps to be regarded as a non-specific criteria. It is the desire of ITAtech and its design guidance documents, however, to bring some degree of standardisation and uniformity to these specifications and definitions.

Lectures went on to discuss one coat or two? Hand applied or robot applied? Liquid base product from the factory, or dry base product with water introduced at the nozzle? One component or two for the base product? What are the requirements for supervision of applications and training for the nozzlemen?

It was explained that each producer is, at the moment, writing their own set of specifications and method statements, and that introducing uniformity and minimum standard requirements were a matter for the Activity Group and its members via its Design Guideline publication.

Leaving the lecture room, the group visited next the Hagerbach laboratory to witness testing. The following were explained and demonstrated, or illustrated; gauging the roughness of a shotcrete substrate; thickness of applied membrane; bond strength; and watertightness.

Gauging the roughness of the substrate was illustrated using dry, fine-grained sand worked into the substrate and then measured across the diameter, or by using a fine wire 'comb' type gauge.

Thickness was tested by cutting out a patch from the applied membrane.

A dolly glued to the membrane and pulled off by the gauge measured the failure point strength and the location of failure. A pull off strength of a minimum 0.5MPa is agreed and stated in the Guidelines. The test demonstration had the failure of the sample above 0.5MPa and the failure was in the concrete, not at the bond interface.

Test results of sample with and without a spray-on waterproofing membrane

Watertightness was demonstrated by breaking open samples that had been under a 5 bar pressure of water for a minimum 28 days. The penetration into the poor quality concrete, for demonstration purposes, after just three days, was deep, while penetration of water through the spray-on membrane under the same conditions and over the minimum 28 days, was zero.

More time could have been spent in the laboratory asking questions and having processes further explained but it was time to move on again to the live demonstrations. After getting properly attired with hard hats, boots and coveralls, crew members assembled to carry out the live demonstrations had the opportunity to explain all.

Of the four suppliers of the products, Sterling Lloyd was unable to be part of the deminar at the last moment, but live demonstrations of the BASF, Normet and Mapei systems were completed to the appreciation and deep interest of the gathered delegates. The liquid base product delivered from the factory by Mapei (Mapelastic TU); the single layer Masterseal product from BASF; and the two-tone, two-layer TAMseal product from Normet, were applied to the walls of the underground space. Delegates then had the opportunity to get up very close to touch the application, see its behaviour, and witness it being applied. This, by far, is most effective way of appreciating and understanding the application of new products and processes.

As part of the deminar delegation TunnelTalk also had a turn at operating the spray-on robot and was told that it wasn't a bad job for a first-time effort. The live demonstration was another part of the programme on which more time could have been spent. Although the representatives of the companies had seen it all before, there was so much to know and understand, query and have explained, by visitors several of whom were seeing the process and products for the first time.

Application of the final permanent shotcrete lining

The well-organised day of lectures and demonstrations was thoroughly appreciated by everyone in the group and after a final discussion session in the lecture room, there were a few things that were not discussed in the forum. These included: What exactly is a pinhole? How detrimental is a pinhole to the integrity of the sprayed membrane? What are the case study experiences of spray waterproofing membranes in the underground to date? What are the case study experiences of sheet membrane applications in the underground to date?

These questions and so many more surround the whole issue of groundwater control in the underground environment. In an aside to TunnelTalk one delegate said pre-excavation grouting is the only way to get to grips really effectively with controlling water long-term in the underground, and there opens up an additional long list of questions, queries, opinions and vested interests. Along with many other topics, the art of pre-excavation grouting could be next on the list for ITAtech focus. Its work is vital and some would say long overdue, although others are asking is it not too commercial and should the ITA be putting its stamp on commercial reports? The work and the lively debate goes on.

References

Design Guidance for Spray Applied Waterproofing Membranes - ITAtech, April 2013
(Free pdf download from the ITA website)

UK applies spray-on waterproofing - TunnelTalk, March 2010
Finding real applications for spray-on waterproofing - TunnelTalk, August 2008

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