Concrete bond failures triggered by differential solar gain were the root cause of the partial collapse of a multi-storey car park under construction at Eindhoven airport in the Netherlands earlier this year.
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This was the conclusion of the recently published report from Netherlands research organisation TNO.
No-one was hurt when the collapse occurred on the evening of 27 May. “All the site staff had left for the day, so they were very lucky” says TNO senior scientist Huibert Borsje.
“It was the hottest, sunniest day of a very hot and sunny week, and this high level of solar gain was the trigger for the collapse.”
Progressive partial collapse
A 60m by 15m section of the top floor of the four storey structure failed first, initiating a progressive partial collapse of the three floors below it. The floors were constructed with the innovative biaxial BubbleDeck system, which uses spherical recycled plastic void formers to save weight. (See box).
“Unfortunately this means the finished slab is a good heat insulator,” Borsje says. “So the top surface would get very hot in the sun, while the soffit remained relatively cool.
“These temperature differentials would increase stresses in an area of the slab that was already highly stressed.”
BubbleDeck construction begins in the precast yard. A reinforcement cage that surrounds the void formers is set up on spacers in a shutter and a shallow depth of self-compacting concrete is poured. This has to be deep enough to securely locate the reinforcement and eliminate the risk of the void formers floating up during a full depth concreting operation. The layer of precast concrete then acts as a permanent shutter and as an important structural element.
In the case of the Eindhoven car park, each prefabricated unit measured 10m by 3m, and the precast layer was 70mm deep. The void formers were 360mm in diameter, at 400mm centres. On arrival on site the elements were craned into place by main contractor Bam and supported on temporary falsework.
One of the main benefits claimed for the BubbleDeck system is that it needs no downstand beams or walls for support, spanning in both directions between columns. This reduces floor to floor height as the resulting flat soffit maximises headroom. Service installation is also simplified.
Car park (flattened)
At Eindhoven the supporting columns were on a 15m by 10m grid. Away from the column line, where the slab was under normal bending stresses, 16mm diameter link bars 1.5m long were inserted on top of the precast sections to transfer bending stresses in both directions and tie all the elements together into a monolithic biaxial slab. A C35/45 concrete with a nominal 32mm aggregate was then poured 380mm deep, creating a 450mm deep monolithic deck.
Failure occurred on floor four at the longitudinal joint between central precast elements, an area that had been poured on 1 February this year, later identified as the area of maximum positive bending moment. Floor four had been self-supporting for nearly three months before the collapse.
Borsje says TNO investigators were surprised when they surveyed the debris pile. “We discovered that large areas of the failed section from the fourth floor had finished up upside down beneath the section from the third floor – which was upside down below the upside down section from the first floor.”
It was the hottest, sunniest day of a very hot and sunny week, and this high level of solar gain was the trigger for the collapse
Huibert Borsje, TNO
This unlikely event was a result of the progressive collapse sequence. Further analysis showed that the link bars should have been able to cope with the stresses caused by the differential solar gain – assuming the bars were adequately bonded into the slab, once the insitu concrete had set and hardened. This also assumed that the insitu concrete and the precast section were also fully bonded.
TNO concluded that without such a bond the shear resistance of the slab would have been seriously compromised. Achieving an effective bond between fresh and fully hardened concrete can be challenging. TNO noted that the upper surface of the BubbleDeck precast section had not been roughened in any way. The TNO report does not comment on ambient conditions during the 1 February pour, but winter concreting always requires extra care.
“There was another factor,” Borsje adds. “The orientation of the individual units was not ideal.
In a “standard situation” the long sides are erected at right angles to the column line, with the end joints at the column lines. In this situation only secondary positive bending moments occur in the long sides, and no positive bending moments occur in the end joint.
“In Eindhoven, however, the long sides were parallel to the column line, with rather high, primary, positive bending moments in the joints.”
The orientation of the individual units was not ideal
Huibert Borsje, TNO
Recommendations in the report include increased emphasis on achieving an adequate bond between the insitu concrete and the precast section, and the importance of reducing bending moments at the joints. TNO carried out the investigation on behalf of Eindhoven airport. Car park contractor Bam also commissioned an investigation by consultant Hageman, that came to much the same conclusions.
Bam Bouw en Techniek director Jaap Hazeleger says: “Fortunately an event like this is highly exceptional. We are very grateful that no personal accidents have occurred. With the investigation results in hand, we will engage in further talks with Eindhoven Airport to determine the most appropriate follow-up steps.”
BubbleDeck UK director Paul Harding stressed that while it was an unfortunate incident, it was unconnected with the BubbleDeck invention.
“BubbleDeck takes safety extremely seriously and we would undertake site inspections of any project in the UK in which we are involved,” he says.
“This includes ensuring the connecting reinforcement between the precast and insitu concrete and between each elements is accurate and meets our engineering standards.”
Explainer: Lightweight slabs
Concrete’s inherent low tensile strength has always limited its ability to span long distances. Prestressing is one solution: forming voids in the concrete is another. BubbleDeck, invented by Danish structural engineer Jorgen Breuning in the 1990s, is the latest in a number of attempts to develop what has been dubbed a biaxial voided slab.
A simply supported concrete slab will develop compression in the upper section and tension in the lower, in response to dead and live loads. In between the higher stressed outer zones there will be a central core of “dead” concrete around the neutral axis, which is lightly stressed. This contributes little positive to the structural performance of the slab, and its dead weight increases the tensile and compressive stresses in the outer sections.
Eliminating as much of this dead concrete as possible offers many theoretical advantages. For many years “hollow core” pretensioned prestressed precast concrete “planks” were virtually ubiquitous on multi-storey projects in northern Europe. These, however, only spanned in one direction and had to be supported by substantial beams or walls.
Biaxial slabs spanning in two directions also offered significant advantages, especially if dead concrete was minimised. Waffle slabs were one approach: experiments with expanded polystyrene void formers achieved only limited success.
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Lightweight void formers will inevitably try to float up as fresh concrete is poured around them. In the BubbleDeck system the recycled polypropylene spheres, which are available in standard sizes ranging from 180mm to 360mm diameter, are fixed between top and bottom layers of mesh reinforcement. These in turn are linked by lattice girders.
Uplift is countered by concreting in two phases. A first pour, typically 70mm thick, locks the reinforcement cage and the spheres into position without creating destabilising uplift. In most cases this will be left to harden, transforming it into a precast permanent shutter.
A fully insitu installation is also possible. The first pour again will be around 70mm: it will then be left to gain enough strength to resist the uplift from the second pour, which should then achieve a good bond with the lower layer.
Precast elements can be linked together by tie bars top and bottom, or mesh might be used at the top. Extra shear reinforcement may be needed at columns or walls, or the bubbles might be omitted in high stress areas.
Edge reinforcement will also be needed in most cases. One of the advantages claimed for the BubbleDeck system is that it can be used for oddly-shaped slabs, unlike hollow core units and the like.
Other claimed advantages include a 30% to 50% reduction in concrete use, allowing significantly reduced foundations. Spans up to 50% longer than possible with “traditional” structures are also claimed. BubbleDecks as much as 1.6m deep are said to be available to special order, and the system may also be combined with post-tensioned prestressing.
There have been a number of applications of BubbleDeck in the UK since its first use in 2004, including its use on the Arup-designed £150M National Automotive Innovation Centre, currently under construction on the campus of the University of Warwick.