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Bridges special: Czech dry deck

The 650 year old iconic Charles Bridge in the heart of Prague is undergoing a major renovation to repair weather damage, wear from many thousands of tourists and the floods of 2002.

The Charles Bridge in Prague was originally the only route across the River Vltava and as such has played an important role in the growth of the city.

Today it is a major tourist attraction too. The current £7.7M refurbishment includes repairs to the upper bridge section, replacing the old concrete deck, installing a new waterproofing system and replacing stones in sills where required. New drainage is also being installed and pillars repaired.

Project engineer Mott MacDonald − working for client Prague Municipality − specified the use of Stirling Lloyd’s Eliminator waterproofing system to replace the old and leaking traditional waterproofing. The supplier’s project engineer is Mott MacDonald and the client Prague Municipality.

Minimised impact

The new waterproofing has a strong bond to the bridge deck. After testing it averaged a bond strength of 2MPa, bettering the minimum requirement of 1.6MPa for both the new concrete deck and the existing bohemian sandstone of the main structure.

Eliminator’s advanced methyl methacrylate (MMA) technology enables the waterproofing to be applied in a wide range of temperatures and climatic conditions, which meant that installation of the waterproofing could be carried out throughout the year.

It was imperative to limit disruption. Consequently the programme allowed for work to be carried out to just one 150m² area at any one time.

As the bridge is an important tourist attraction it was imperative to minimise the visual impact of the work and limit disruption. Consequently the programme allowed for work to be carried out to just one 150m² area at any one time.

Before the waterproofing is applied, random spot tensile adhesion tests were taken to confirm the high bond strength.

Stirling Lloyd contractor Helifix arrived on site in December 2007. Due to the need to keep the most of the bridge open at all times and to maintain its splendour for the visitors from all over the world, the refurbishment of the 4,500m², 516m-long deck was programmed to take two years.

Consistent methodology

Refurbishment of each 150m² section of the bridge follows the same methodology. Once the area is tented off and prior to the application of the new waterproofing, the granite paving is removed and each piece numbered to ensure that it is returned to its original position or replaced with a replica. The whole of the old concrete deck and previous waterproofing system is then removed.

The new concrete deck is then put in place and left to cure for seven days before being primed to enhance the adhesion of the membrane to the substrate and sandstone upstands. The waterproofing membrane is then spray applied in two coats.

Each coat is of a different colour, contrasting so that contractors can be sure the required thickness has been applied. This quality control procedure is backed up by thickness tests.

Each coat is of a different colour, contrasting so that contractors can be sure the required thickness has been applied. This quality control procedure is backed up by wet film thickness tests being carried out throughout the application process. These on site controls ensure that the minimum specified thickness of 1.2mm is applied for each coat.

When this has cured − typically in under an hour − a structural adhesive is applied, in spots.

While the adhesive is still tacky the drainage geotextile that is being used is placed. This is then covered with a 100mm-thick layer of concrete, on top of which the original numbered paving is placed in a layer of sand. The refurbishment is due to finish this December.

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