NCE's concrete special opens with a report from Saudi Arabia on Riyadh's answer to London's Canary Wharf tower, where construction is past the 200m mark and the summer heat can top 45degreesC. Words and pictures by Dave Parker.
Current concrete technology operates right at the limits during the Saudi Arabian summer. Daytime temperatures can top 45degreesC, fresh concrete is at permanent risk of flash-setting and sophisticated concrete admixtures behave unpredictably. Achieving acceptable concrete quality on even the most routine structure is a constant struggle. So the challenge posed by the £190M Al Faisaliah project, with its 267m high central tower and vast column-free banqueting hall, is colossal.
'Until Al Faisaliah, buildings in Riyadh were limited to 15 occupied floors,' structural designer Buro Happold project manager Peter deBoeck points out. 'But the client, the King Faisal Foundation, convinced the authorities that by putting the office space into a 30-floor tower they would free up space for a central, publicly accessible plaza - a rare feature in Riyadh.'
This plaza sits 7m above street level. Under it lurks what turned out to be the biggest challenge to the structural engineers on the project - a banqueting hall with a 59m clear span capable of housing a wedding with 2,000 guests or a conference with 3,000 delegates.
DeBoeck says this originally was a straightforward auditorium, and was changed during the final design process in response to the booming Saudi wedding market. Buro Happold had to draw on bridge-building technology to solve the problem, eventually deciding on a bowstring arch design with an 80m nominal span and a rise of 8m. Post-tensioned tendons within the chamber's floor slab would take care of the outwards arch thrust - but to make it work a C60 concrete would be needed.
'We originally thought this would be too much for the local limestone aggregates and that granite aggregates would have to be imported from Jeddah,' says deBoeck. 'But by using Master Builder's Glenium superplasticising admixture to achieve a water/cement ratio of 0.32 with a cement content of 415kg/m3 we were able to meet the specified characteristic strength with ease.'
For the tower an admixture-free C40 mix would do the job. All major pours would take place at night - to minimise traffic disruption to the deliveries of readymixed concrete as well as to keep concrete temperatures down - but for the 6,000m3 tower basement C40 sulphate-resisting concrete pour even more stringent precautions were needed.
Says deBoeck: 'Cement comes from a works just outside Riyadh, and can still be pretty hot when it arrives at the readymix depot.
'For the big pour we had to insist it was stockpiled for a month beforehand to let it cool down enough.'
Eight concrete pumps were mobilised for the 4m deep foundation pour, which took 17 hours and is believed to be the largest ever in Saudi Arabia. All went smoothly. The use of crushed ice at the batching plants helped keep fresh concrete temperatures down below 32degreesC, and workability was retained throughout.
Similar success attended the C60 bowstring arch pours. Here the main concern was the post-tensioning operation, carried out by specialist sub- contractor VSL. As Buro Happold senior structural engineer Graham Worsnop explains, this has to be carried out in four stages.
'The first stage of compression was put into the floor slab when the abutments were completed, the second when the arches themselves were finished and arch action was developed.
'There's a lot of structure sitting on the arches and when this was completed we tensioned for the third time.' At this point the supporting falsework was removed from the arches - they dropped an average of 20.5mm as against the 24mm predicted, Worsnop reports - and landscaping of the plaza above began. Once this is complete the final stage of prestressing will begin.
Post-tensioning of the maximum 800mm thick, 5m span floor plates can take place once the concrete has reached a cube strength of 20N/mm2 - usually after three days. The contractor opted to use the C60 mix for the central core, to speed early strength gain. Formwork specialist Doka developed a customised climbing form system, and with its aid, progress rates of a floor every five days have been achieved.
A combination of tandem pumps for placing the 'horizontal concrete' and the core, and crane and skip for the columns kept the concrete moving. Table forms and access platforms were another successful combination, deBoeck adds
Worsnop says considerable effort has gone into calculating short and long-term structural deflections. On the tower, the objective was to balance out the shrinkage of the core and the perimeter columns so that the floor plate finished up as near dead level as possible.
It was estimated that the corner columns, which taper from 2400mm x 1800mm at the base to 1000mm x 600mm at the top, would shorten by a maximum of 134mm over time, under the influence of load, creep and drying shrinkage.
The core, with wall thicknesses tapering from 800 to 400mm would shorten by only 85mm. These calculations are particularly relevant at the moment as the massive chilled water pipes running up through it are being installed and the allowance for shrinkage must be pre-set.
By comparison, the low rise buildings around the plaza are relatively straightforward. Insitu normally reinforced construction is used for the hotel, a combination of normally reinforced frame and post-tensioned floors for the retail malls and apartments.
Perhaps the trickiest problem was achieving a consistent smooth finish to the precast cladding panels to the retail and hotel facades. Casting them face down on plate glass turned out to be the answer.