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Foamed Concrete Integral To Gerrards Cross Tunnel Rebuild

 

In what is thought to be the largest lightweight foamed concrete project in the UK, Propump Engineering was approached by main contractor Costain in early 2009 to supply 375kg/m³ foamed concrete for use as a bulk fill material over the concrete tunnel on the Gerrards Cross project.

To reduce the loading on the tunnel, piled walls were installed either side of the tunnel, with foamed concrete being used over the top of the arch to form a level land base.

“We had produced 500kg/m³ density materials many times before, however at 375kg/m³ (25% lower) our first thoughts were cautious, as this material is rarely produced anywhere in the world and certainly not as a general construction supply material,” commented Trevor Ansell of Propump Engineering.

The list of requests however grew from not just requiring a very low density material but also a huge volume, with some 26,000m³ placed in total. In what was the largest foamed concrete pour undertaken since Canary Wharf in the 90’s, it quickly became apparent that congestion on site would require Propump to mobilise and place concrete on a day to day basis in restricted areas. The team had to coordinate deliveries along a 400 metre long single track which would also be the main artery for several other trades working at the same time.

With work undertaken through the winter into the summer months, and a temperature range of minus 5°C at night to as high as 27°C in the summer, Propump had to supply a mix which would tolerate extremes of temperature and equipment capable of providing such a low density material all year round.

Due to the progression of the works, the placement of material became critical with volumes peaking at close to 1000m³/day with up to three locations each day along the 400m tunnel.

Finding the right mix

Propump undertook a number of laboratory trials using different materials and mix designs. These were evaluated using a 1m tube 200mm diameter vertically filled with the test material. The sample was evaluated in the wet state for stability and bleed, then in the solid state it was cut and inspected for bubble size and uniformity. Samples of the material were then taken from the top, middle and bottom and weighed. The final mix design was chosen when top to bottom variation is negligible and the target density at 28 days is within the range of 345-405kg/m³ (375 +/-30kg) with cubed tests returning 0.6Mpa at the same age.

Work on the project commenced in November 2009 with the foamed concrete being produced through one of the continuous inline pumping systems. Pre-checks on the supply of base material were undertaken on every delivery with density of base material, temperature and flow all being recorded. Each of the checks undertaken had tolerance limits and any that did not meet the criteria were refused.

Delivering the goods

The initial pours were 2-300m³ with bays being covered with a large canopy constructed from scaffolding and tarpaulin to protect the material from rain. At such a low density, rain would severely damage the foamed concrete, the threat of rain in the winter and spring required a gang of men to assemble and disassemble canopies the size of tennis courts on a daily basis. This work was being undertaken while other trades were waterproofing the concrete arch prior to the placement of the foamed concrete. At the same time as this, the main 600mm thick continuous arch was still under construction.

After the first few pours, it became apparent the heat generated by the foamed concrete was accelerating the set of the fresh foamed concrete poured over the top. This meant the bays needed to be divided into manageable sizes. “We decided that pour volumes of 100m³/bay would be manageable as this equated to 4 deliveries.” added Trevor. “With such a low density of material being required each delivery would net over 25m³ and we were confident we could place this volume in less than 2 hours.”

Work progressed steadily using this method of 100m³ bay sizes with strategic placement of material to evenly load the tunnel. The depth of the pours was limited to just 500mm so Propump could ensure a uniform foamed concrete density was placed at all times. A further testing procedure of taking sample blocks from the foam concrete was adopted the following day. The blocks were accurately cut and measured and then weighed and the density of the block recorded.

The result of this information showed the wet density could be placed at 425Kg/m³ which was within the acceptable range of 395-455Kg/m³. The final dry density of the foamed concrete would be 375+/-30kg/m³ at 28 days.

Neal Carter, Costain’s Senior Engineer responsible for the works, said “the work we asked Propump to undertake was pioneering and pushing the boundaries of what had been completed before. Concerns were expressed by many that the target density could not be achieved, however after extensive testing and development we achieved our goal. Propump used their extensive experience combined with a dedication and philosophy to work with Costain’s team to deliver a thoroughly successful solution.”

Increasingly popular with designers and engineers, foamed concrete is perfect for filling voids where access is difficult or when a lightweight mix is needed to meet restricted loading limits. Its most common applications include bridges, floor and roof screeds, pre-cast panels, sewers and tunnels.

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