Abstract It is well known that waste tire chip modified concrete has very high toughness. However, its strength and stiffness are reduced significantly. Sometimes, the strength and stiffness are so low that the modified concrete cannot be used as structural materials. In this paper, waste tires were used in the form of fibers and waste tire fiber modified concrete was developed. A total of 42 ∅ 152.4 mm by 304.8 mm (6 in. by 12 in.) cylinders were prepared to conduct compressive strength, split tensile strength, and modulus of elasticity tests. Among them, six cylinders were prepared without waste tires, and six samples were prepared using waste tire chips. They were used as controls in this study. The remaining 30 samples were prepared using waste tire fibers with various lengths and stiffness. The test results show that, while the strength and stiffness of the concrete modified with waste tire fibers were still lower than those without waste tires, they were higher than those with waste tire chips. A two-phase composite model was also developed and a finite element analysis was conducted on this model. The analysis results show that the stress concentration in waste tire fiber modified concrete is lower than that in waste tire chip modified concrete. Using stiffer and thinner waste tire fibers can further reduce stress concentrations.