As a subcontractor to Bauest OÜ, Traveter Grupp carried out concrete repair, shotcrete and waterproofing works on filters F17–F19 in Building A of AS Tallinna Vesi’s Ülemiste Water Treatment Plant, as well as the renovation of the filter hall. The filters were built in 1927 in the historic Building A of the Tallinn Filter Waterworks, which is a listed heritage building, and are in direct contact with drinking water, so all materials used had to be certified for contact with drinking water.

The original plan

Based on the information available at the time of the tender, the plan was to remove the ceramic tiles and filter media, clean the surfaces of the filter chambers with ultra-high-pressure water jetting down to the load-bearing reinforced concrete, repair and level the concrete surfaces with repair mortar, and coat the filter walls with MasterSeal M 808 – an elastic, easy-to-clean waterproofing membrane designed for drinking water tanks.

The challenge revealed during the works

After ultra-high-pressure jetting of the walls of filters F17 and F18, it became clear that the concrete was in much worse condition than expected. When the filters were built in 1927, uncrushed natural gravel was used as concrete aggregate instead of crushed stone. Its rounded particles bond to the cement matrix considerably more weakly than crushed stone. The concrete was particularly weak from the bottom of the filter up to a height of 2 metres, where the filter media had previously been.

We carried out pull-off tests at eight test locations in each filter. The average bond strength was 1.16 and 0.83 N/mm² respectively, and a large share of the individual results were below 1.0 N/mm². The waterproofing manufacturer requires a substrate tensile strength of at least 1.0 N/mm², and the concrete repair standard EN 1504 requires at least 1.5 N/mm². In addition, no individual result may be below 1.0 N/mm². It was therefore not possible to continue with the original solution.

The solution

As the required bond strength could not be achieved, the technology had to be changed. Further removal of the weak concrete by hydrodemolition would have had an unpredictable duration and outcome, so we proposed to the Client a solution in which a layer of shotcrete is added to the existing walls and anchored to the substrate with reinforcement mesh.

The final scope of works included:

• removal of ceramic tiles and filter media and cleaning of all filter chamber surfaces with ultra-high-pressure water jetting;
• installation of 5/200/200 reinforcement mesh anchored to the existing walls;
• coating of the walls with Sakret TBD class R4 shotcrete;
• a 28-day curing period for the shotcrete before waterproofing;
• repair and levelling of concrete surfaces and sealing of penetrations;
• installation of a waterproofing system for drinking water tanks – MasterSeal P 770 primer and a two-coat elastic MasterSeal M 808 membrane;
• restoration of the reinforced concrete edges beneath the filter railings;
• refilling of the filter media – granite aggregate, quartz sand and anthracite;
• renovation of the filter hall walls, columns and ceiling, closing of window openings and installation of fire doors.

Throughout the works, we used ventilation and heating to maintain suitable temperature and humidity for applying the waterproofing, and separated the work zone with plastic sheeting to keep dust out of the operating water treatment plant.

Although the original plan had to be revised during the works, we completed the project successfully. Filters F17–F19 now have a strong, anchored concrete layer and drinking-water-compliant waterproofing that ensures their long-term reliability.