Highlighting safety concerns ... the Al Nasr building in Doha on fire.
Solid panels offer better results: Pohl
Fred-Roderich Pohl, senior consultant at the Germany-based economic development consulting firm Pohl Consult International, compares the performance of solid aluminium cladding sheets with aluminium composites in building construction.
01 November 2007
Solid aluminium sheets have existed since the invention of aluminium around a century ago.
Different alloys were developed and the mechanical properties were optimised to suit the application and/or the fabrication process. Aluminium sheets have always had an inherent problem of an uneven surface, which results from the hot rolling process followed by the cold rolling process.
When anodised sheets were first introduced, the comparatively high price of the product was another challenge. Aluminium composite panels (ACPs) – produced from two 0.5 mm pre-painted (coil-coating) exterior sheets plus a 3 mm plastic core – were developed as a solution to issues. At present, ACPs are an apt solution for interior decorations, sign-boards, exhibition booths and related applications.
They have been also used for external façade claddings for high-rise buildings – where however the technical problems of this product are overlooked. It is estimated that there are presently around 100 manufacturers of ACP worldwide, but only a few are capable of supplying high-quality products. The increase in the number of suppliers has led to a drop in prices, which again led to compromises on quality standards on the aluminium alloys and polyethylene core used, for instance the thickness of the exterior metal sheet has now has been reduced from 0.5 mm to an incredible 0.3 mm in an attempt to cut costs.
Four technical criteria must be examined in order to make a fair comparison of the performance of solid aluminium sheets versus ACPs. These include fire behaviour that entails combustibility, dripping and de-lamination as well as toxic gas and smoke development; mechanical properties, particularly after bending (notching stress and the principle of the “light construction grade,”; the recycling of production scrap at the end of its life span; as well as insulation and sound-deadening, which are detailed below:
Fire behaviour
Most ACPs consist of plastic cores with an “aluminium veneer” on top, and what appears to be solid aluminium is in fact 75 per cent of combustible plastic. So called “official tests” can often be misleading or are even fake.
However, they cannot match solid aluminium, which remains at the top of the ‘A1’ classification for non-combustible materials. Its tendency to produce toxic gas and smoke may be even more relevant to fire fighters and the victims of fire catastrophes.
Mechanical properties
ACPs normally have two 0.5 mm aluminium sheets on either side made from AA 3000 series alloys. The plastic core adds to the rigidity only when the sheet is not bent by V-grooving on the reverse side – which is necessary when forming cassettes.
This process, which leaves only a 0.5 mm thick aluminium at the bent edges, is easy, cost-effective and achieves sharp bending radii. However, this is a potential fatigue breakage area because of notching stress, particularly under constant wind pressure and suction load.
This “pre-determined breaking point” can only be compensated by fixing aluminium angles into the cuts on the reverse side for support, which is often omitted because of the additional work and the substantial cost involved.
By engineering standards 0.5 mm aluminium or even 0.3 mm thickness can be considered to be “wall-paper” in mechanical performance. To support the loads of large panels, the whole cassette would again need to be stiffened and supported by an expensive stiffening substructure – which contradicts its so called “light construction” grade.
Bending of solid sheet leads to a further stabilisations, providing a strong “frame” along with an optimum stability and flatness of the cassettes.
Recycling
Recycling of ACP material is becoming an issue. As the panels are designed to stick together as a sandwich throughout its lifespan, the manufacturers choose the right glue and heat treatment to achieve an adhesion to avoid de-lamination. However, de-lamination is required for the recycling process and although possibilities were found for the recycling process, cost reasons as well as transport and logistic problems associated with ACPs would render the composite scrap of no value.
Insulation
Although ACPs claim to have insulation (U-value) this is only academically true, as a 4 mm ACP is equivalent to 0.2 mm standard insulation. Their sound-deadening characteristic is a major advantage. However, with the invention of sound-deadened solid aluminium (with trade name of Daempfaluminium) the noise problem of solid sheets has been eliminated.
An alternative technical approach would be to compare ACPs with pre-painted 2 mm and 3 mm thick solid aluminium sheets such as the alloy AA 5754, H 42, PVdF 80/20 wet paint system.
Solid pre-painted aluminium has the following advantages when compared to ACPs:
• No fire hazard, since it has a non-combustible “A1” ranking according to EN 13501, and produces no toxic gas or smoke under fire;
• Mechanical properties (permissible loads) of up to approximately 30 per cent stronger than regular ACPs. When formed/bent, the bent corners will further stabilise the panels. An economical and technical option can be achieved with 3 mm solid sheets that can have a width of up to 2,000 mm and length of 6,000 mm; and
• Easy recycling with substantial scrap value.
With solid aluminium panels, architects, consultants and owners of high-rise buildings should be aware that they have an interesting technically superior and competitive alternative, even for high rise buildings. Solid pre-painted aluminium sheets are an ideal for decorative, high-tech façade claddings, instead of a “vulnerable” sandwich system.
A proper evaluation of the advantages and disadvantages is usually not done, as solid sheets in some areas have not been widely used yet. For several reasons, one of the most successful applications of solid sheets in high-rise buildings can be found in Greater China. In other countries, solid sheet has become an option in the aftermaths of recent disasters with composite panels catching fire such as Astana in Kazakhstan, or more recently in Doha in Qatar, and several similar fire accidents across the globe.
