Concrete joints cause hassle and hassle costs money, according to Reinforced Structures proprietor David Johnson. Use of Peri's Vario GT 24 formwork has enabled the sub-contractor to cast the 11m-high walls for a new pumping station inside full height shutters. As well as cutting the number of joints this has brought other benefits.
The system has given better access for fixing reinforcement, dispensed with internal scaffolding and dramatically reduced the number of people needed to set up the shutters.
Castle Donington power station - once the biggest in Europe - used to stand on the site. Dean & Dyball is main contractor for a £10M project to provide the infrastructure needed for owner Wilson Bowden to turn the area into a hub for warehousing and distribution.
A 57 week contract is on programme for completion in September this year.
Future businesses operating from the new East Midlands Distribution Centre will depend on the pumping station whenever there is heavy rain. Almost the entire 1.6km by 0.8km area will become hard standing, putting considerable demands on the drainage. 'In a 100 year flood it will generate 5m 3 of water a second, ' says Dean & Dyball contracts manager John McGinty.
Together the pumping station and drainage account for 40% of the infrastructure cost (see box).
Drainage sub-contractor Wilmat Construction has been installing a graduated mainline storm outfall sewer which reaches 2.2m diameter at the pumping station. From there, water will be discharged into the River Trent.
The pumping station is a reinforced concrete structure 13m by 11m in plan, built inside a sheet-piled cofferdam. Final concrete sections are now being finished off, but the real achievement was a few weeks ago when all the concrete was placed for the external walls.
These stand between 10m and 11m high with a thickness of 600mm. At these heights, the traditional approach would have been to break each wall into two pours, McGinty points out.
Indeed, the original intention at tender stage had been to cast the building in eight pours, with two lifts for each wall.
Instead, ubcontractor Reinforced Structures has been using Peri's V wall formwork system made up to the full height. The entire building was shuttered as two Ushaped sections with the same components were reused for the second half. This dramatically simplified construction. 'One benefit that this shuttering system provided was the reduction of the programmed pours from eight to two, ' says McGinty.
Peri carried out the calculations to determine the configuration needed for such a high wall. The Vario system uses vertical timber lattice girders which have a high flexural stiffness. These can be adjusted in small increments and spliced together if necessary to achieve the required height. In conjunction with horizontal steel walers spaced at about 1.5m the system can withstand a pressure of just under 80kN/m 2. Adjacent panels are connected with a wedge system and the two plywood faces are joined by ties.
Each U-shaped section contains some 180m 3 of concrete, which was delivered by a concrete pump over a distance of 32m, the pump typically discharging 18m 3 an hour. The concrete had a large percentage of the cement replaced with pulverised fuel ash (PFA), which produced a mix with a low initial strength gain to keep the crack width down. Time needed for a pour depended on temperatures on the day, as a change of just a degree or two can make a big difference, McGinty explains.
The average lift was about 1m in an hour, with the pump moved to a new position around the U once it had dispensed a full wagon load. One of the U-sections was cast in single full height pour: only low temperatures prevented the same for the other. A cold snap slowed concreting and it had to be finished the next day.
Use of the Vario system brought a number of other advantages, says Johnson. The equipment lent itself to a safe and efficient method for fixing the reinforcement. First, the formwork for the internal face was lifted in and supported using raked push-pull props.
Reinforced Structures could then work from external scaffolding to fix the reinforcement while the inside shutters acted as a guard to prevent anyone or anything falling through. The external shutters were then lifted in and secured to the internal ones with Dywidag tie rods.
The use of relatively substantial 20mm diameter ties cut down the number required, which saved time both in fixing and in plugging the holes afterwards. Another feature that proved useful was the ability to fix walkways to the formwork.
Johnson was pleased to be able to dispense with internal scaffolding. A more traditional approach would have meant using scaffolding inside as well as out, which would have created a logistical nightmare, he says.
He had another reason for liking the Vario system: Peri was able to deliver it ready for use. Fully assembled 11.425m by 2.4m panels arrived on site, taking up the full length and width of a trailer. Instead of needing 20 or so people to make and fix the shutters, it took just two joiners and two labourers.