Beams are loaded with short-term ultimate load, beam dimensions 15/28/300 cm, and I am analyzing ordinary strength C30/37 and high strength C60/75 concrete beams with steel fibers and conventional reinforcement.
The best procedure I can suggest you is reflected in ASTM C1609. This test method provides an accurate procedure for measuring the toughness of fiber reinforced concrete based on measuring concrete beams deflection under loads.
Alternatively, you can simply use LVDT device. It is broadly used by researchers, however, if it is set up in a different way from the procedure that ASTM C1609 describes, you may get inaccurate results. BUT still you can compare your results relatively, irrespective of the absolute value of deflection.