Non-fractured, non-fibrillatable short fibers, for reinforcing matrix materials such as concrete, have substantially uniform transverse cross-sectional areas along their length for maximum efficiency in pull-out resistance, and two different tapering characteristics along their lengths. Preferred bi-tapered fibers of the invention have a high modulus of elasticity in the range of 5-250 Gigapascal and are preferably modulated in both tapering dimensions. Matrix materials containing the fibers, as well as a method for making the fibers, are disclosed.