Dr Amin Emami from 3A Composites looks at why aluminium composite materials are popular with architects for rainscreen cladding, and at the robustness of testing regimes in the UK post-Grenfell
Aluminium composite panels (ACP) or alternatively aluminium composite material (ACM), are flat panels consisting of two thin coil-coated aluminium sheets bonded to a non-aluminium core. These cores can be of combustible, fire retardant or non-combustible material. ACMs are often used to clad the external facades and soffits of buildings, as well as insulation and signage. ACMs are classified as lightweight materials. This is an important advantage when it comes to handling in both workshops and installation, as well as reducing transportation weight. In comparison with other metal-based building materials, panels created using this sandwich construction and manufacturing process are exceptionally smooth and flat, qualities which make them of particular interest to architects. Another major advantage of the process is that the thin metal outer layers can be ‘coil coated’ in a wide range of precisely reproducible coatings/lacquers. As well as being lightweight, flat, and durable, the material also offers a wide selection of surface finishes. This makes for easy manual bending to create freeform and three-dimensional shapes and geometry, by using special routing and folding techniques.
What are the different types of ACM?
It is essential to distinguish between different aluminium composite materials. According to MHCLG (Ministry of Housing, Communities and Local Government), ACM can be divided into three main categories:
• Category 1: Calorific potential ≤3 MJ/kg => limited combustibility core
• Category 2: Calorific potential > 3 MJ/kg and ≤35 MJ / kg => fire retardant core
• Category 3: Calorific potential > 35 MJ/kg => unmodified polyethylene core
The individual classification is derived from the differing reaction to fire of the materials. An ACM with a polyethylene core can significantly contribute to fire propagation and the side effects of fire (e.g. smoke production). A mineral-filled polymer core fulfils significantly more stringent fire requirements. Most responsible ACM manufacturers do not advocate polyethylene cored ACMs for architectural applications, and many countries prohibit the use of polyethylene cored ACMs in facade specification, especially for high-rise buildings.
Key considerations for rainscreen facades
Load bearing is not the only issue which plays an important role in rainscreen facades – building physics and fire protection must be looked at in detail. In recent years, the focus on fire protection has grown steadily in different countries. Various rules (building standards) have been established for fire safety in buildings. These standards specify which materials are permitted in the construction of a building and which materials may be used for the interior and exterior cladding of walls and ceilings. The aim of these standards is to ensure safe evacuation of people from the building in case of fire, but these safety codes present a major challenge. Fire behaviour is tested according to different criteria and test methods in each country, and the disparities between the individual member states mean that evaluating a product’s fire behaviour is very complicated. Harmonised classifications in regard to reaction to fire were laid down in BS EN 13501-1 in order to resolve such ambiguities and to put an end to certain national reservations. Harmonisation should ultimately lead to all country-specific classification systems being replaced by EU regulations. The European classification standards include a much wider range of classes and combinations than some national classifications. In addition to fire behaviour, the side effects of fire such as smoke production/development and burning droplets are taken into consideration, and divided into classes for the first time. The BS EN 13501-1 standard not only requires testing of individual materials for fire behaviour, but also system tests (small sections of facades). The aim of these tests is to aid evaluating the different materials in conjunction with each other. Experience has shown that material considerations alone are not capable of evaluating fire behaviour sufficiently, but that the system, and interaction between various materials, plays a significant role. Combatting the chimney effect for fire spread is one key measure in reducing the spread of fire. In order to understand this better, it is important to analyse the chimney effect which is due to the ventilation gap present. Some European countries are in the process of introducing large-scale fire tests as additional means of testing for critical systems. In the UK, BS 8414 is currently an important test, offering an accredited way of assessing how facade materials interact with each other. According to the requirement of BR135, one of the most significant criteria is that during the test, the temperature at level 2 (about seven metres) must not exceed 600°C within 15 minutes.
Since the terrible Grenfell Tower tragedy, numerous tests have been conducted, both in and outside the UK. When introduced, these test series were very valuable and useful; however, the real usefulness of many of the experiments can be questioned. For many test series or system combinations, clear and unambiguous results proving compliance with the (BS 8414) test method are already available. Tests have shown that ACM A2 fire performance in combination with non-combustible insulation is entirely safe. 3A Composites carried out its own BS8414 test series with the BRE in Watford in 2016, and this has led to the positive results being taken into consideration in the MHCLG test series. 3A Composites’ experience with numerous different international large-scale fire tests and the results of the MHCLG test have shown that an ACM with mineral filled core (A2 version) in combination with non-combustible insulation can be judged entirely safe. The individual components – both the facade panels and the insulation – make no special contribution to the spread of fire. Even an ACM with mineral filled polymer core in combination with non-combustible insulation provides adequate safety. Practical examples confirm our experiences and recommendations. 3A Composites recommends A2 in conjunction with non-combustible insulation for high-rise buildings in order to ensure safety. Mineral filled polymer core can also be recommended in conjunction with non-combustible insulation for non high-rise buildings. These material combinations prevent fire spreading for a sufficient length of time until the fire-fighting operations begin. For all combinations, it is important that fire barriers are used in compliance with current regulations.
Dr Amin Emami is head of technical department & technology centre at 3A Composites