Coat it to float it
How the team in charge of Dubai's floating bridge is keeping it above water.
One topic on everyone's minds in Dubai is the traffic. Currently, crossing the Creek has been limited to three bridges and a tunnel, and these have been stretched beyond capacity as the emirate's population growth and car-use have skyrocketed. But the floating bridge, which is due to open at the end of June, will add a fifth crossing, allowing the Dubai-Sharjah traffic to make a smoother transition from Al Ittihad Road into Bur Dubai.
Waagner Biro Gulf won the US $42 million (AED155 million) main contract last October. The Roads and Transport Authority (RTA) is the client, Parsons is the consultant and Clement is the project manager. A joint venture of Nukote Coating Systems Middle East and Horizon Metal Products Coating Company won the coating contract earlier this year - a total of 25,000m2 of Nukote CG coating and Polyprime primer will be used to form an impermeable layer on each of the 92 'pontoons', which will form the base of the structure. A three-lane road (in each direction) will then be laid on top. So far, much of the construction has taken place on land, as each of the pontoons is cast, coated and prepared for launch.
According to Amir Adib, managing director, Horizon Metal Products Coating, the techniques involved in coating each pontoon are similar to those used for floating marinas.
"We've done other pontoons for floating docks but not floating bridges," he says.
"There are very few firms that offer this coating and even fewer that have the experience that we have. We specialise only in this type of coating - nothing else.
"Also, we are associated with Nukote, the manufacturer, for building projects, so we tendered for the project as a conglomerate - we had one other competitor that had a similar product."
The coating used is a Polyurea - a 100%-solid elastomer coating - which is spray-applied using plural component equipment.
"Once the two components touch each other then you have about five or 10 seconds before they actually gel, and within 30 seconds to a minute, it can be touched tack-free. Because of this, we are able to launch it into the water immediately," says Adib.
The pontoons are 20m-long and 6m-wide hollow concrete blocks cast around a polystyrene centre. After the formwork is in place, the rebar framework is put in, followed by the pieces of polystyrene. After casting, there is a 12cm wall of concrete on the outside and a 15cm wall on the two ends.
"The pontoon is cast, and then it has to stay inside the mould for three days," adds Adib. "Once that is done it can be popped off the mould. It has to cure for a few more days, and then we can start our surface preparation. So we are looking at a period of seven days before we can touch one."
On the first day of working on a pontoon, the coating team carries out surface preparation to ensure a smooth application of the coating, and on the second day, it does the actual spray-on process of the Polyurea.
"To prepare the surface we get rid of the small holes and surface damage, and then we do a rendering - an epoxy - to fill all the voids, pin holes and imperfections in the concrete surface," says Adib. "Once this has dried and cured, we put a Polyprime primer on top, which dries in 20 minutes - and we can spray our coating right on top of that. So these two areas take time - the rendering may take half a day per pontoon."
The coating team is working to complete the contract in under three months, which entails completing two to three pontoons per day - a surface area of 680m2. Although the process is structured and repetitive, the team has faced logistical challenges on site due to the process of coating the base of each of the structures, which at 95 tonnes each, requires a crane to lift it off the casting pedestal.
"The only problem has been to coat the bottom; we actually have to lift the pontoon, which is the slow part. Once it has been lifted, it only takes another three hours, and then the deck is ready to go into the water," says Adib.
"Within the rebar mould, it's all just polystyrene. And what happens is that the bottom of the polystyrene is actually exposed - so what we are doing is putting a coating onto the bottom of the polystyrene prior to casting and then again when it is finished to encapsulate the block completely."
But the process of coating the polystyrene base prior to casting was only carried out after the team realised the damage that was being done to the first pontoons on the production line.
"The first 15-20 blocks were cast without this coating of Polyurea on the base of the polystyrene, so they were damaged. It has taken us a lot of extra work and materials to get it all covered up - so now we are doing the base coating before the block is made," adds Adib.
Once the base has been sealed, a total 440kg of coating will have been added to each pontoon. Each one is able to be put in the water only 20 minutes after application of the coating.
At the time of going to press, the 92 pontoons have been cast, coated and are in the water. Work on the backbone is now being carried out before the road is cast on top.
And work on the access roads each side of the bridge is now well underway. When completed, it will connect the intersection near Deira City Centre and Dubai Creek Golf and Yacht Club, and will end at the intersection to be built on Riyadh Street between the Dubai Courts and Creek Park. An additional track between Khalid bin Al Waleed Road and Oud Metha Road from one side, and Al Ittihad Road and Deira from the other, will also be provided. The bridge is scheduled to be completed by the end of June, in line with the introduction of Salik, the new road toll system in Dubai.