sting Testing time for cladding
The impetus to start the Centre for Window and Cladding Technology (cwct - see box, page 44) some 10 years ago came from a perceived quality problem throughout the cladding industry - in design, specification, manufacture and installation. The defining moment for Roger Blundell, a director of Taywood Engineering - one of cwct's founding organisations - was Broadgate. There, in the mid 1980s, cladding companies from the us with a more thorough approach and a culture of testing showed by example that uk practice should be significantly better.
Since then, progress has been solid rather then dramatic. For example, around 90 per cent of full-size cladding samples tested by Taywood still fail the common exposure tests first time. This is not necessarily bad. Such testing is usually part of a product development process running for another three-to-six months; by the time development is complete the cladding systems normally pass. Rather, the concern is that many at least partially bespoke systems are not tested. Is this really because everyone is confident these systems would pass first time if tested?
Stephen Ledbetter, director of cwct, sees an increasing demand for assured cladding performance among repeat clients for buildings. While the new client may worry whether the building stands up, the repeat client is much more focused on the reality of disruption caused by leaking facades. Added to this, Ledbetter expects the currently modest interest in life- cycle performance to grow, fostered by pfi. There is a broadening of focus from 'will it leak on day one?' to a wider concern with durability.
Latham and now Egan also bring pressures for change. Paradoxically, cladding has not been a particularly good advertisement for the idea that getting assembly off the site and into the factory improves overall quality. But in practice one significant weak link has often been site assembly of the factory-made components. cwct is currently developing a registration scheme for site installers. Its other training initiatives include developing courses for technical designers in cladding companies, a range of ad hoc training courses and a masters degree at Bath University in facade engineering. Despite the growth of facade engineering groups in major consultancies such as Arup, Ledbetter sees a general shortage of facade consultants.
It is interesting that Taywood now presents itself as a consultancy that the client and architect could employ to assure cladding quality is delivered, not just as a test house working for the cladding supplier. Partly this can be seen as a piece of commercial positioning by Taywood, trying to make people aware that as the biggest private research organisation in the industry it has broad-based expertise to offer. But this business move is also based on the assumption that as an industry we won't reliably get it right first time; that however tightly the cladding package is hedged around with standards and warranties, today there may still be significant shortcomings.
Architects have a role here, of course. As an organisation with members from all parts of the industry, including clients, cwct has an overview of what architects could do. Ledbetter suggests the following priority areas.
Integrated design - integrating facade, services and structure.
Cladding life-cycle - Like the rest of the industry, the focus of design, specification and testing has tended to be on getting the building built successfully. Increasingly the focus should include a lifetime view, with design for durability and the ability to offer the client options with different maintenance and component-replacement cycles.
Better specification - Could do better. (cwct itself has put a lot of effort into developing specification documents.) Glass is a particular area of concern. The material itself may be understood, but specifying the framing performance is equally important, especially for the more 'live' support structures such as atrium roofs.
Interfaces - If the architect (and main contractor) do not consider the interfaces, say between walling, roofing or doors and the cladding, who will? Cladding test samples often include an area of the surrounding construction - the interfaces between cladding and other materials are often one location where exposure testing fails.
Buildability - Ideally, a cladding supplier is involved early in the project to inform buildability; in practice clients often want late competitive tendering. Architects could at least research the industry better to get more feel for suppliers' varying capabilities to deliver systems large or small, more or less bespoke, etc. And avoid specifying cladding-system customisation unless systematic product development and testing will follow.
Testing - Where needed, agree a programme of testing that is specific to the job. Not all parameters are covered by standard tests. Improvisation may be needed. And where standards exist, the choice of test regime may still be problematic. Testing may be of integrity (rain, air-tightness, etc) structure (wind, safety, etc) and durability. The notes below indicate current thinking on tests of cladding samples.
There are standards for weather testing and guidance on their use from the likes of cwct. The question is more what test pressures and other factors to use to simulate the site's particular exposure. Generally, on site, the highest winds are not usually associated with the heaviest rain. And typically, suction loads are greater than positive pressures.
Care is needed in relating results from weather tests to results from wind tunnel simulations.
The on-site hosepipe test provides a follow-up check for installed cladding, although there are some questions about the appropriateness of the test regime and consequent interpretation of results. Many tests can be performed on site as a last resort, by creating a box around a sample of cladding. But obviously this is not recommended, as failures disrupt site progress.
The dynamic aero-engine test developed in the us can give significant kinetic energy to simulated rain and be combined with rapid pulsing effects by rapid pressurisation and depressurisation of the back-box. While there are some questions about precisely what is being simulated, cladding that passes the test is expected to work in practice.
Test regimes cover normal attempts at airtightness. Work is still needed on standardising test regimes for open-jointed cladding such as rain screens and patent glazing.
Structural tests for wind load, cyclic fatigue, self weight and similar are normally straightforward. The impact and security tests sometimes used were not developed for cladding (for example domestic security tests), so there is a question of how far the test regime simulates possible field conditions.
Structural tests may need to address racking in support framing as well as in the cladding. Light framing can be particularly affected, such as in atrium roofs and light staircases at the ends of buildings.
Movement should be taken up in the joints. Achieving structural robustness suggests using wide joints; aesthetics usually the opposite.
Structural tests could lead to some permanent deformation. Weather testing probably needs repeating to check if integrity has been maintained.
Typically, durability tests are on components rather than complete cladding assemblies, such as accelerated tests of salt-spray corrosion of metals, uv effects on colour-fastness and deterioration of gaskets and sealants.
Since interpreting accelerated test results is inevitably a matter of judgement, such tests may be more convincing when used comparatively. For example, did the previous sealant, of known durability, do better or worse than the new one in the same tests?
In principle a cladding system can last for ever, as long as each component can be replaced as it wears out. So part of durability testing may be checking the interchangeability of components and controlled dismantling.
Cladding samples for testing are usually built on site by the supplier's operatives rather than the tester's. This provides the chance to monitor buildability in a controlled way.
Including some of the surrounding structure, such as brickwork, allows assessment of buildability as well as of the integrity of its interfaces with the cladding.
For further information see the internet sites at www.cwct.co.uk and at www.taywood.co.uk