TECHNICAL | SAFETY/FIRE
SPALLING RISK REDUCTION
Narrower diameter PP fibres where shown earlier this year in fire test to reduce concrete spalling risk, and for lower dosages of the new Ignis product. Adfil’s Jeroen Smet, Technical Director, discusses the R&D results
INTRODUCTION Polypropylene (PP) fibres help improve the passive fire resistance of concrete in tunnels by the network of voids created under heat, so providing safe release of otherwise potentially explosive internal moisture and steam pressures that could trigger spalling and expose steel reinforcement. It has become, therefore, an important part of the potential range of fire safety solutions provided for underground environments. Over the last four decades, Adfil has been developing
and supplying, and improving, such monofilament PP micro fibre technology for passive fire resistance in concrete products. The company has owned the patent for 18µm diameter PP microfibres, branded as Ignis and which has been extensively used in tunnels worldwide – in precast concrete linings such as for segmental rings in bored tunnels, sprayed linings for conventional excavation, or cast in-situ concrete for the final, permanent, linings to tunnels or caverns. From the many tests Adfil was involved with it was
observed that the finer the diameter of the PP fibre, the better the fibre performed. Important experimental work on the role of the diameter of monofilament micro fibres in the explosive spalling of concrete in fires was undertaken in 2010 by Professor Gabriel Alexander Khoury.1
The research then showed that the amount of
spalling of structural concrete decreases by lowering the diameter of the PP fibres in the mix design.
Earlier in 2023, as part of its latest R&D programme,
Adfil successfully performed key fire tests of a new, slightly slimmer size of PP fibre, helping to add a further product size to the Ignis range on offer. The latest R&D adds a narrower PP fibre – 17µm
diameter – which can achieve effective performance at lower dosages in a concrete mix. The results achieved in fire tests were exceptional, according to the OBV norm. The product was immediately launched, on all
markets.
SPALLING RISK AND ITS MITIGATION Explosive spalling of concrete is a serious concern in tunnel fires. It refers to the violent ejection of concrete fragments caused by the rapid heating and release of entrapped moisture and steam within the concrete. This phenomenon not only compromises the structural integrity of the tunnel but also poses a significant risk to the safety of tunnel users and emergency responders. The Ignis products have been extensively tested and
proven to effectively eliminate explosive spalling in concrete exposed to fire. Test facilities such as TNO in Holland and the BRE in the UK have conducted rigorous experiments that demonstrate the prevention of explosive spalling, pointing to inclusion of the narrow PP fibres in a concrete mix as a way to help drastically reduce spalling risk.
Right:
Explosively spalled areas as percentage of fire exposed areas, and maximum SOURCE: ADFIL
90 80 70 60 50 40 30 20 10 0
Explosive spalling area (%) Maximum depth of spalling (mm)
12 microns 15 microns 17 microns 32 microns No fibres
20 | August 2023
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