Polypropylene Fibres and Micro Cracking in Fire Exposed Concrete

Abstract

Though, concrete in general is a non-combustible building material, modern High Performance Concrete (HPC) is very susceptible to violent explosive spalling during a fire attack. This requires protective measures for fire safety design of concrete structures. The current most worthwhile method to prevent explosive spalling is the addition of monofilament Polypropylene fibres (PP-fibres). However, since it has become common knowledge that PP-fibres are suitable for fire safety design, a variety of theories concerning the mode of action of PP-fibres have been suggested. The present article summarizes the most important hypothesis and presents an innovative method for the analysis of micro structural processes in heated specimens. The results show that due to the thermal decomposition of PP-fibres capillary channels are created. Simultaneously, a netlike micro crack formation occurs, which connects these capillary channels. This enables the relief of internal stresses (mechanical effect) and the formation of a permeable transport system for the escaping water vapour (permeation effect).

Dynamic Single-Fiber Pull-Out of Polypropylene Fibers Produced with Different Mechanical and Surface Properties for Concrete Reinforcement

“…PP fibers are also applied in concrete to mitigate spalling at high temperatures [ 6 ]. Due to their low melting temperature of circa 170 °C [ 7 ], PP fibers melt when exposed to higher temperatures and leave channels through which water vapor pressure is relieved, and in turn, spalling effects are reduced [ 8 , 9 ]. Furthermore, PP fibers are used in concretes and mortars to improve their mechanical resistance to impact loading by increasing their energy-to-fracture and impact resistance [ 10 , 11 , 12 ].…”

Fire spalling sensitivity of high-performance concrete in heated slabs under biaxial compressive loading

“…Many of the studies conducted so far clearly show that spalling phenomenon can be effectively reduced, or even avoided at all, if polypropylene fibre is added to the mix, typically in the range 1-2 kg/m 3 (& 0.1-0.2% by volume). Polypropylene fibre, in fact, melts at 160-170°C producing voids and microcracks in the cement paste [8,11,12]. As a consequence, smoother moisture gradients [13] and lower pressure peaks [3] are observed, and possible relaxation of thermal stress is expected [11,14].…”

Testing of Action of Direct Flame on Concrete

“…The amount of mixing water was almost identical in all concrete mixtures with the exception of CEM I - GR-PF. Firstly, dose of water of this concrete mixture had to be increased to achieve optimal consistency of fresh concrete; the reason was higher dose of polypropylene dispersed reinforcement, which worsens consistency of fresh concrete [ 16 ]. Secondary effect of the increased amount of water was to achieve lower strength properties of the mixture.…”