Modelling of load-deflection of concrete beams internally prestressed with unbonded CFRP tendons

A theoretical model was developed for predicting the flexural load-deflection of simply supported concrete beams internally prestressed with unbonded carbon fibre reinforced polymer (CFRP) tendons under four-point loading. At each state in the load-deflection curve (initial cracking state, initial yielding state (if any) and ultimate load state), three equivalent rectangular curvature blocks, which can closely simulate the beam rotations at the supports and the midspan beam deflection, were determined by iteratively correcting the strain value of the unbonded tendon. The validity of the model was demonstrated by comparing its predictions with the test results of four rectangular beams and two T-beams with different prestressing reinforcement ratios, initial prestressing and type of auxiliary bar. The developed model was used to examine the effects of different parameters on the first cracking and ultimate strengths of the beam and the additional strain of the unbonded tendon to the effective strain.