Feature: Concrete construction project round-up
By Chris Sleight09 December 2010
Growing demand for electricity is an issue in many parts of the world, and these days generating power in as sustainable way as possible is also a big factor in what gets built. This may explain the apparent rise in the number of hydro-electric power schemes going ahead in recent years. Of course such schemes often mean huge quantities of concrete being poured, placed and formed.
Having gained experience using a Power Curbers 5700-C to form electrical conduit sections on South Africa's Gautrain high-speed rail project last year, this year saw Gautang province's Exotic Concrete apply the machine to the Bedford Dam project near Ladysmith. The dam, which sits 1700 m above sea level, forms the upper reservoir for the Ingula Pumped Storage scheme.
The project comprises a 50 m high, 577 m long concrete facing supported by a rock filled embankment. The size of the dam is staggering, requiring 982000 m3 of rock fill and another 27000 m3 of conventional concrete. Behind these walls, the reservoir holds 22 million m3 of water.
Beginning at the base, Exotic poured a 400 mm x 760 mm trapezoidal curb the length of the dam using the Power Curbers curb & gutter machine. Behind the curb, rock backfill was placed and compacted. Following the backfill completion, the 5700-Super-B, running on the first pass, poured a second curb which partially overlapped the first.
And so it continued. After each section of wall was slipformed, the rock fill was trucked in and compacted prior to the next rise going being placed. Pass after pass, working up the dam, the Power Curber 5700-Super-B poured 42 linear km of concrete facing. The concrete and rock portion of the dam was completed in seven months.
The contractor ran into several on-site issues which made an already large undertaking a real challenge. "On average, we slip-formed 300 m per day and would have done much more if not delayed by the rock fill" reported Quentin Leppan, Director of Exotic Concrete. "We regularly emptied 6 m3 trucks in six to seven minutes."
In addition, the machine had to cope with temperature shifts in excess of 30° C during the course of the working day and wind speeds up to 110km/hour. "The Power Curber performed extraordinarily well under very harsh weather conditions. The 5700-Super-B performed with zero standing time. This was important due to the fact that we had a heavy standing rate penalty clause in the contract" explained Mr Leppan. "We definitely recommend the Power Curber for its ease of operation and maintenance."
Meanwhile in China the US$ 7.36 billion Xiluodu hydro-electric plant is currently being built on the Jinsha River - a tributary of the Yangtze. As well as generating 57 to 64 billion kWh of electricity per year - making it the third largest hydro electric plant in the world - , it will help with flood prevention and navigation of the river.
A double-curved arch dam should dam up a storage lake of 12.7 billion m3. The maximum height of the dam is 278 m and the overall length is 700 m. The total excavated material for this project amounts to 39.8 million m 3 of which 25.6 million m 3 is above-ground excavation and 14.2 million m 3 is from the construction of tunnels.
In total, 16.7 million m 3of concrete is required for this project and 6.6 million m3of this is required just for the body of the wall. The concreting work commenced in 2008. It is expected that the first generator sets will be installed in June 2012. The first electrical energy should be generated in June 2013 and the entire project should be completed by 2015.
Much of the concrete placing work is being carried out with a Putzmeister TB 110G Telebelt, a mobile conveyor that can be swung, lifted and extended for accurate placement of up to 1000 m3 of material per hour.
The complexity of the site means the Telebelt is always in demand for concrete pours, with it being used on construction of a range of concrete structures. In on one occasion, the Telebelt was used on five different construction sites within six days, during which time it continuously placed 4,800 m3 of concrete for the overflow tunnel, the cable crane platform and the underwater outlet.
The new power plant under construction at Embretsfoss in southern Norway currently ranks as one of the largest hydroelectric builds in Scandinavia, and Doka planned and supplied the entire formwork solution for this major infrastructure project.
By early 2012, more than 40000 m³ of concrete will have been cast using the formwork. One of the most complex areas is the 50 m long outlet, due to its geometry. The cross-section changes from circular at the turbines to oval in the radiused zone, transitioning into three rectangular chambers at the discharge.
The Doka Top 50 beam-formwork assemblies for this section were designed with the aid of a 3D model of the structure. Specialists from the Doka Ready-to-Use service then took over and built the units accurate to the millimetre and shaped perfectly to the radii of the finished structure's curvature.
The power plant's massive channel walls meanwhile are being built using the sturdy Doka MF240 climbing formwork carrying Top 50 assemblies. The curvature at the end faces of the channel walls again necessitated 3D modelling so that the climbing brackets would subsequently be positioned with the essential degree of precision.
Outside the power sector, concrete is of course an important material in other types of infrastructure, including road transport.
In the Swiss city of Bern, a Wankdorfplatz roundabout is being re-positioned almost 10 m under the ground which will transform a dismal road intersection into an attractive gateway to the country's capital. Formwork for the scheme is being supplied by Peri, including forms to build the massive beams, reinforced concrete slab and conical roundabout walls, which are to be formed in architectural concrete.
The customised formwork for the curved external walls of the roundabout were prefabricated by Peri based on the Vario GT 24 girder wall formwork system. And Trio panel formwork.
The construction of the 1.80 m wide, pre-stressed beams with varying heights of between 1.15 m and 2.70 m took place on a combined load-bearing and working level which angled upwards to the centre of the roundabout. This was formed using the flexible Multiflex girder slab formwork system with the help of high load bearing GT 24 lattice girders.
Peri's Multiprop system served as support with towers comprised of MRK frames connected to Multiprop aluminium slab props. Thanks to the free-running collar and the integrated measuring tape, these could be adjusted to suit the varying heights of between 5 m and 6 m.
Following this, four slab segments were to be constructed and connected to the beams. The tight construction schedule meant 1400 m2 of formwork - enough for the entire slab - was needed at once.
The up to 1.20 m thick reinforced concrete slab runs in a truncated cone with an approximate 3 % gradient on the upper side. In addition, the underside of the slab features five height offsets and a number of different inclinations.
The heavily trafficked Wankdorfplatz road junction had to stay open during the construction period, and this of course, affected site logistical operations. With limited storage space on site, Peri Switzerland was required to deliver the formwork on a just-in-time basis..
An innovative plastic slipforming system developed in Vietnam is being used for one of the tallest building projects in Ho Chi Minh City, the twin Saigon M&C Towers - featuring a 195m high commercial tower and a 170m high residential tower. The plastic formwork system has been developed by Vietnamese company Fuvi Mechanical Technology.
Sited in the heart of Ho Chi Minh City's commercial centre, the twin M&C Towers are valued at approximately US$ 200 million and consists of one residential and one commercial tower with 45 floors each. There is a five-level basement with car park, a five-storey shopping centre, and a helicopter pad on the roof. Construction is due for completion in the first half of 2011.
Fuvi has designed, manufactured and is operating two independent Fuvi slipform units incorporating EH plastic form faces, one on each of the two towers cores. For the towers' cores construction, the contractor is using approximately 270 m³ per 8 hour cycle using a concrete grade 750 (MPa); trucked to the site, and pumped into place through the placing boom. The contractor is achieving an average five days cycle to complete one storey on each of the towers.
The use of plastic forming panels is more cost effective than plywood, because whereas wood can be used only a maximum of three months in concrete, the Fuvi plastic panels can be used up to two years without any deterioration in quality.
Further for typical high-rise buildings, it is not necessary to exchange the Fuvi plastic form-face; this compares with wood or metal form-faces which require changing every 10 -20 floors respectively
The plastic panels are lighter than any other slipform panelling, weighing 7kg/m² as opposed to an approximate 10kg/m² for wood, 20kg/m² for aluminium, and 31kg/m² for metal.
Over the past 24 months Toronto, Canada-based contractor CanMar has completed more than 55741 m2 of garage slab restoration using the three Aquacutter robots from Aquajet Systems, as opposed to using the noisy and labour-intensive traditional method of breading out old concrete with jackhammers.
"Working a typical 10 hour day, we are able to achieve on average of 1,000 ft2/day (93 m2/day) using the Aquajet robot "said CanMar's hydrodemolition manager. "A single jackhammer can at best achieve 50 ft2/day (5 m2/day) so it would require at least 20 jackhammers to match the robots production; meaning a significant cost in labour costs."
Currently working on a three phase contract in downtown Toronto's Charles Street, an Aquacutter HVD robot is key to completing an underground three-level basement car park ahead of schedule. On completion, it will be CanMar's largest hydrodemolition contract to date totalling more than 84,000 ft2 (7,804 m2).
Constructed some 30 years, the 8 in (203 mm) thick garage slabs had deteriorated badly from exposure to de-icing chemicals. The salt-laden water had penetrated the concrete causing the rebar to corrode and expand, creating stress within the concrete, eventually leading to the concrete to 'spall' and break up.
To ensure that the garage remains operational throughout the removal and new concrete placement by CanMar, each floor was divided in half to allow working over an area of approximately 28000 ft2 (2601 m2). The Aquacutter robot is used to remove the first 4 in (152 mm) of the slab to expose the rebar and, with selective removal across the slab, undertake 'thru-removal' for the full 8 in (203 mm).
The key element of hydrodemolition is to pressurise and widen existing pores and micro cracks in the weakened concrete structure using high pressure water penetration. Material is easily removed as the build up pressure exceeds the tensile strength of the damaged or weakened concrete.
CanMar's contract includes the removal of the deteriorated concrete and replaced with a similar grade 35-mPA concrete. The complete slab is then treated with a thin mastic protective coating.
These applications and construction techniques all reflect the versatility of concrete as a construction material, and its enduring popularity, be it for civil engineering or building applications.