Effects of polypropylene fibre type and dose on the propensity for heat-induced concrete spalling

Abstract

The term high-performance concrete (HPC) is typically used to describe concrete mixes with high workability, strength, and/or durability. While HPC outperforms normal strength concrete in nearly all performance criteria, it also displays a higher propensity for heatinduced concrete spalling when exposed to severe heating or fire. Such spalling presents a serious concern in the context of the historical approach to fire safe design of concrete structures, where structural engineers typically rely on concrete's inherent fire safety characteristics (e.g. non-combustibility, non-flammability, high thermal inertia). It has been widely shown that the inclusion of polypropylene (PP) fibres in concrete mixes reduces the propensity for heat-induced concrete spalling, although considerable disagreement exists around the mechanisms behind the fibres' effectiveness. This paper presents an experimental study on the effects of PP fibre type and dose on the propensity for heat-induced spalling of concrete. A novel testing method and apparatus, the Heat-Transfer Rate Inducing System (H-TRIS) is used to test medium-scale concrete specimens under simulated standard fire exposures. Results show (1) that although the dose of PP fibres (mass of PP per m 3 of fresh concrete) is currently the sole parameter prescribed by available design guidelines, both the PP fibre cross-section and individual fibre length may have considerable influences on the effectiveness of PP fibres at reducing the propensity for heat-induced concrete spalling; and (2) that current guidance for spalling mitigation with PP fibres is insufficient to prevent spalling for the HPC mixes tested.

A Heat-Transfer Rate Inducing System (H-TRIS) Test Method

“…The occurrence of heat-induced concrete spalling for specimens tested with H-TRIS was in reasonable agreement in terms of time-to-spalling (i.e. 72 min) with specimens cast from identical concrete mixes and tested during prior standard furnace tests [84]. This result provides further evidence of H-TRIS' ability to accurately replicate not only the in-depth time dependent temperature distribution experienced by concrete specimens during a standard fire resistance test (see Fig.…”

“…A complete description and analysis of the results from this experimental study are presented elsewhere [79,84]. The following observations were made based on the parameters varied between specimen sets:…”

“…Heat-induced explosive concrete spalling occurs when the exposed surface of concrete is heated, and it flakes away in a more or less violent manner [84]. As a consequence, the concrete cover to the internal reinforcement may be reduced, possibly resulting in rapid temperature increases in the internal reinforcement and within the core of the structural element in addition to a direct influence on load bearing capacity due to the loss of physical or effective cross sectional area.…”

“…8) was imposed until an incident radiant heat flux of 100 kW/m 2 was reached; beyond this point the incident radiant heat flux was maintained constant at 100 kW/m 2 . If no spalling occurred within 60 min of heating the test was halted, since heat-induced explosive spalling is unlikely to occur at such a late stage [84]. Fig.…”

“…highly optimised prestressed concrete slabs [84], lightweight structural concrete sandwich elements [85], concrete tunnel lining segment materials [86], intumescent coatings [87], cross laminated timber [88], non-metallic dowelled timber connections [89], and building insulation materials [90]). This section illustrates application of H-TRIS within a project studying heat-induced concrete spalling.…”