The invention concerns the use of organic fibers having a melting point
lower than 300.degree. C., an average length l more than 1 mm and a
diameter .O slashed. not more than 200 .mu.m, in ultra high performance
concrete for improving the concrete fire resistance, the amount of organic
fibers being such that their volume ranges between 0.1 and 3% of the
concrete volume after setting and the concrete having a compressive
strength at 28 days of at least 120 Mpa, a bending strength of at least 20
Mpa, and a spread value in non-hardened state of at least 150 mm, the
values being for a concrete preserved at 20.degree. C., the concrete
consisting of a particularly hardened cement matrix wherein metal fibres
are dispersed.
