The object of this research effort was, upon request for
evidence from a building contractor, to compare the influence of various
amounts and types of fibers on crack widths, using a steel ring mold.
Comparisons were made between synthetic fibers (polyolefin) of 48 mm length, hooked-end steel fibers of
diameters 0.6 mm and 1.05 mm, both of 50 mm length. 10-liter samples were extracted from
concrete ready-mix truck batches at delivery sites, whereupon fibers were mixed
into the samples, layer by layer, by applying a drill-mounted mortar mixing
device. For each amount of fiber content, 4 rings were cast, and of the plain
concrete control samples, 5 rings were cast. After removing the outer steel
casting, strain gages were installed on the exposed outer concrete surface.
Strain values were continuously logged, and crack developments and crack widths
were measured daily. Sufficient data with statistically high significance were
obtained to indicate that: A
synthetic fiber content of 3 kg/m3 did not decrease crack-widths as compared to the non-fiber concrete samples.
Synthetic fiber contents of 5 kg/m3 and higher, did reduce crack widths on par with hooked-end steel fibers in the
amounts of 25 kg/m3 and above. Hooked-end steel fibers of aspect ratio 80 are more efficient with
regards to crack width reduction, yielding 33% narrower cracks, than hooked-end
steel fibers, at equal weight-contents, with aspect ratio 45.