Cemex lays giant transfer slab in 24-hour pour

The construction industry is either praised or vilified for its impact on the built environment. There is no hiding place for the end results of construction activity, whether temples to mankind's ingenuity or 'carbuncles' on the face of modern society.

Yet the structural components that allow these creations to exist in the first place are often themselves overlooked, lost in the interior of the finished article. This is a particular shame when it is these elements that are the unique and innovative aspects of a building, rather than the seemingly anonymous retail block or railway station that they have facilitated.

The new retail block and Docklands Light Railway station at Woolwich Arsenal are a case in point. Attractive, functional buildings though they may end up being, any real claim to greatness they possess comes from their structure rather than their form. In particular, it is the concrete skeleton - the transfer slab that at the same time provides the roof of the station and the base of the four storey shopping development above - that is unique. Just how unique is explained by the man in charge of providing the concrete for the project, Cemex technical manager Andy Bourne.

"I've been involved in the industry for 26 years and this is the single most difficult and complex job I've ever done, in terms of mix design, in terms of logistics and in terms of planning," he says. "It took up nearly four months of my life and I was on site for 24 hours during the pour."

The reason for this complexity becomes clear when you look at the statistics of the job. The aim was to lay a single, homogenous transfer slab measuring 200m2 by 3.3m deep. This required the delivery of 583m3 of concrete (85 mixer loads) over 20 hours and use of what Bourne describes as one of the longest-boom concrete pumps in Europe (58m, extending over the main road next to the site).

Bespoke mix

It also required the development of a bespoke concrete mix, based on the company's evolution self-compacting product. The basic recipe for this particular mix was to create a C32-40, self compacting concrete, though there was also a lot more besides.

First came the requirement to incorporate polypropylene micro-fibres into the first 1.5m of the depth of the slab to help prevent fire damage in roof of station. about 1kg/m3. "Concrete contains water which, when heated such as during a fire, expands and breaks the concrete up very energetically," explains Bourne. As well as exploding concrete being a risk to fire fighters, it can also compromise the building's ongoing structural integrity.

This is resolved by the incorporation of polypropylene fibres. "These melt at 130C," he adds. "In doing so they create a void for water vapour to expand into and this allows the concrete to maintain its structural integrity."

Another challenge was to ensure a maximum temperature difference within the concrete slab of just 20C to avoid cracking. "Normally concrete goes off quickly and heats quite rapidly with quite a range in temperatures across the pour," says Bourne. "We used PFA (40%) and admixtures to delay this heating so the same amount of heat was released but over a longer period of time to avoid peaks and therefore cracks."

The use of PFA not only helped the sustainability credentials of the concrete but retarded the setting of the concrete, another crucial element of the job. "This and the use of chemical admixtures meant it was retarded for 12 hours," he continues. This in turn allowed some leeway in delivery, an important concern in its own right given the possibility of finding an unexploded WWII bomb in the marine aggregates supplied to Cemex's Angerstein Wharf production centre ("We found one on the day of one of the trials and had to call the bomb disposal experts in. They cleared the site for 3 hrs," recalls Bourne).

More importantly still, it allowed the concrete to remain sufficiently fluid to cope with flowing around the 138t steel inside the slab. "That's just an unreal amount of steel," winces Bourne. "It had to flow all around that without leaving any voids. What's more, there were four pour points that we used several times each so we had to make sure that by the time you'd gone through all four the first pour point hadn't started to go off as the next layer wouldn't then knit in with it. If that had happened it would in fact have become a series of slabs and wouldn't have worked structurally."

This was the essential element of the Cemex mix, without which Bourne says the slab could never have been laid. There other benefits as well, though such as the fact there was no need for either placing or vibration of the concrete. "There were only ever two guys moving the pump. There was no noise either - just the noise of the pump and the slosh of the concrete if could have done it with a traditional mix, it would have needed 20-30 people working all day and night that's a big saving."

A saving that's worth thinking of even though it's now all-but invisible to all those who benefit from it.

Going underground

The pour of the transfer slab wasn't the only demanding aspect of the job as Cemex provided the concrete for the entire DLR extension from King George V Dock to Woolwich for main contractor AMEC.

The company also provided the materials for the tunnel segments running 1.2km under the Thames. "This was slightly unusual as we were a Readymix supplier providing material for a pre-cast product," says Bourne. "Amec set up a factory to manufacture the segments, we supplied mix, which was a very high spec mix, this time a very quick, high strength concrete."

Cemex also provided the track slabs in the tunnel, for which the key issue according to Bourne was the difficulty of access inside the tunnel. "We had to design a mix we could pump into the tunnel. We couldn't let it go off in the pipe and it had to be workable for long enough once it arrived in the tunnel to be placed, which was complex as the slabs were never horizontal, they were always on a camber or a bend, and they had to incorporate and flow around the rail fixings as well."

The project is on target to finish in February 2009 as planned.