Concrete is the most commonly used construction material. In the majority of applications (such as buildings, bridges, nuclear reactors, tunnels, offshore structures) it faces the risk of exposure to high temperatures and in this case its structural behaviour is jeopardised by one major problem, namely the likelihood of the occurrence of explosive spalling. Explosive concrete spalling at high temperatures results in serious economic and human safety problems.
Today, and in the future, the risk of explosive spalling of concrete at high temperatures will markedly increase by the increasing use of high performance, low permeability concretes in construction. While such concretes are indeed 'high performance' under normal service conditions, they become 'low performance' at high temperatures because their dense structure does not allow moisture and vapour to escape readily, thus leading to the build-up of high pore pressures, which contribute to explosive spalling. The main parameters influencing explosive spalling are thermal stresses associated with the build-up of pore pressures inside the material. Explosive spalling can, therefore, truly be described as the 'Achilles' heel' of concrete. The use of polypropylene fibres reduces and even eliminates this serious problem.
The use of polypropylene fibres has developed rapidly in European tunnelling and the NewCon (New Concrete) research and testing programme is under way to further develop the level of fire protection afforded to treated concrete. One of the project�s aims is to develop a software package that will enable civil engineers around the world to design fire resistant concrete for use in bridges, tunnels and high-rise buildings.
The fibres melt at about 160°C (new fibres have been produced which melt at 130°C) thus providing channels for the escape of water vapour during heating and avoiding the build-up of vapour pressure. The major problem in convincing industry to use this innovative new technique lies in the facts that little research has been carried out into this subject, the use of polypropylene fibres is far from being fully understood or optimized, concrete durability under service conditions has been questioned, and last but not least the toxicity of its products of combustion needs to be examined.
NewCon is an innovative research programme proposed with the main purpose of studying the use of new types of fibres, to optimise on the positive effects on the permeability increase and on the same time to minimise any negative effects. The main technological objectives of the project are the development of appropriate polypropylene fibres for use in concrete against explosive spalling at high temperatures; the development of adequate mixing and casting techniques for fibrous concrete; the assessment of the post heating properties and damage of fibrous concrete; the assessment of the durability of fibrous concrete before and after heating; and the design recommendations for the use of polypropylene fibres in concrete. The programme funded through the EU�s Eureka initiative amounts to EUR910,000 and its duration is 48 months, ending in July 2007.
By applying polypropylene fibres (normally used to limit plastic shrinkage cracking in green concrete) the permeability of concrete at high temperatures may be increased, due to melting and even burning of fibres, thus creating channels for the moisture to escape. Scientific research into the innovative use of polypropylene fibres is, however, limited. It is the challenging aim of this proposal to study the effect of adding polypropylene fibres to concrete in all stages of the structure's life, namely during construction (mixing and casting; service; heating; and post heating exposure).
The main participants are Fire Safety Design of the UK, Centre d'Etudes des Tunnels (CETU) of France, TNO (IBBC) of the Netherlands, Anglo Danish Fibre Industries (ADFIL) of the UK, and Prof. Carmelo Majorana of the Padova University of Studies. FSD�s projects include the thermal structural computer analysis of a whole building in fire, spalling and structural analysis in nuclear reactors, post-fire assessment in the Channel tunnel and Great Belt tunnel, and also the design against fire in two Heathrow tunnels, the CTRL tunnels between London and Folkestone and the Dublin port tunnel. An important part of the work is the protection of the tunnel concrete against explosive spalling in fire.
For several years, CETU has been developing a code devoted to the analysis of material structures. The CETU has been involved in concrete evaluations, for example with the Mont Blanc tunnel. Visit
www.cetu.equipement.gouv.frTNO is the only scientific institution in the Netherlands entirely dedicated to fire research. Visit
www.tno.nl ADFIL supplies the fibre material, a patented monofilament fibre called IGNIS, for all the test programmes and a significant amount of information and know-how on polypropylene fibres and their use in concrete in general and under fire conditions in particular. The NewCon research will establish the exact dosage of IGNIS necessary to allow concrete in tunnels to resist explosive spalling during various fire curves (i.e. RWS and ISO). The findings will ultimately improve efficiencies in construction, allowing lower volumes of fibres to be used without compromising the effectiveness of the passive fire protection system. Visit
www.adfil.co.ukProf. Carmelo Majorana heads the department of construction and transport at the Padova University of Studies. Visit
www.dic.unipd.it
