A studt of flexural tensile strenght and explosive spalling behavior of high volume macro polypropylene fiber reinforced concrete

Explosive spalling of concrete is a phenomenon of high uncertainty with remarkable consequences on human’s lives as well as structures. Emergence of pore pressure and thermal stresses are known to be the predominant reasons for the explosive spalling of concrete when exposed to high temperature. Small addition of short polypropylene (PP) fibres into concrete is an effective method in controlling the explosive spalling of concrete. While there are numerous results on the successful application of low dosages of short PP fibres, with a length of 6-19 mm and a diameter of a couple of micro-meters to 200 µm, there seems that not much work has been done to study the effect of high dosages of macro PP fibres on the explosive spalling of concrete. As macro PP fibres impart very different mechanical characteristics to concrete in terms of higher deformation capacity in the post peak behavior, this work focuses on the effect of these type of fibres on tensile behavior of concrete through bending tests, and then evaluates the spalling behavior of macro fibre concrete exposed to high temperatures. In the present work, first the tensile flexural behavior of four mix designs is studied through a three point bending test on notched beams. One mix design contains 2 kg/m3 of fibres with a length of 8 mm and a diameter of 19.8 µm (Micro2), and three mixtures are cast containing 4, 8 and 12 kg/m3 of fibres with a length of 39 mm and a diameter of 780 µm (respectively Macro4, Macro8, and Macro12). The results obtained, emphasizes again on the very different behavior of micro and macro fibres in the post peak regime, while comparable results are obtained for strength values. Further, it is observed that increasing the content of macro fibers enhances the residual strength of concrete after cracking. In the study of explosive spalling behavior, concrete slabs of 600×600×100 mm3 made of plain, Micro2 and Macro12 mixtures are compared in three different series (S1, S2, and S3). In S1, slabs are tested in as cast condition, in S2 geometry of samples are slightly modified, and S3 has a more moderate fire exposure with biaxial compression in plane of the slabs, while the intrados of the slabs is exposed to fire. Further, some non-destructive tests has been done before and after the tests to compare the extent of decay of material properties in high temperature. While the micro fiber shows to be effective in elimination of spalling, it was unexpected to see that the macro PP fiber, not only do not reduce the spalling, but further increases the spalling risk. While the plain concrete does not show any spalling (which might be due to cracking), the Macro12 mixture shows extensive spalling.