Abstract: Fiber additives for addition to proportioned concrete comprise a plurality of thermoplastic fibers having a profile geometry defined by the equation, ##EQU1## wherein the amplitude a.sub.0 falls within a range of from about 0.1 d.sub.f to 2.0 d.sub.f and the period .lambda. falls within a range of from about 2 d.sub.f to 15 d.sub.f for fibers having a diameter d.sub.f of from about 0.5 to about 1 mm; the peak pull-out stress .sigma..sub.peak and the specific pull-out energy absorbed to a maximal displacement of 7.5 mm .psi..sub.peak, both increase linearly with a deformity factor defined by the equation, ##EQU2## where .alpha.=0.8 and .beta.=-1, such that the peak pull-out stress is defined by the equation, ##EQU3## and the specific pull-out energy is defined by the equation, ##EQU4## the fiber additives having an optimimum deformity,D.sub.optimum .apprxeq.(0.5 to 1)D.sub.critical,D.sub.critical being that deformity where the ultimate tensile strength of the fiber equals k.sub.1 D.sub.