The post-cracking behavior of concrete reinforced with macro synthetic polypropylene fibers was investigated in this work. Three series of concretes, normal strength, high strength, and high strength with fly ash, were reinforced with fibers 54 mm long and 0.34 mm in diameter at two different percentages by volume: 0.6% and 1.0%. A series of classic experiments on 11x22 cm cylinders determined the basic mechanical properties and 3-point bending tests were performed on pre-notched 10x10x40 cm prisms. The specimens were continuously recorded throughout the flexural testing, and the Force-Crack Mouth Opening Displacement (F-CMOD) curves were assessed using the Digital Image Correlation (DIC) technique. An inverse analysis method established a tri-linear stress-crack opening (-w) law for each concrete series and reinforcement ratio. The EN 14651 standard defined the model parameters as a function of the standard residual bending parameters fR1 and fR3, and the results were also compared with the constitutive models proposed by the 2010 Model Code. As per the results, the fibers have a minor effect on the elasticity and compressive strength modulus. However, they greatly enhance the tensile and flexural strengths, particularly when the fiber content is 1%. The results revealed that the fR1 values at CMOD 0.5 mm ranged from 55% to 62%, and the fR3 values at CMOD 2.5 mm from 51% to 74% across the three concrete classes. These results meet the Model Code 2010 and EN 14889 standards.