THE EFFECT OF ADDITIONAL REINFORCEMENT ON TIME-DEPENDENT BEHAVIOUR OF PARTIALLY PRESTRESSED CONCRETE

Moustapha M. CHOUMAN

The investigation reported in this thesis is concerned with the effect of additional non-tensioned steel on time-dependent deformations, prestress and effective prestress losses, initial and time-dependent deflections, load deflection, cracking load and pattern and ultimate strength of partially prestressed concrete beams.  The main influencing factors considered are the amount and distribution of prestress and non-tensioned steel, loading conditions, tendon bond conditions, environmental conditions and size and shape of members.  Twenty beams, with unbonded or bonded tendons consisting of I- and rectangular sections with different amounts of prestressed and non-prestressed reinforcement, were tested under different environmental and loading conditions and were monitored over a period of one year.  Tests were also conducted on plain and reinforced I- and rectangular sections, and cylindrical specimens to investigate the effect of reinforcement on creep and shrinkage of plain concrete.

In the partially prestressed beams and after time-dependent deformations, an important redistribution of the concrete and steel stresses was observed.  The additional non-tensioned steel present in the section attracts compressive forces which act on the concrete as tensile forces inducing tensile stresses in the concrete.  This reduces the effective compression in the concrete thereby reducing the cracking moment of the section.  On the other hand, additional non-tensioned steel reduces creep and shrinkage , tension losses of the tendon, initial and time-dependent camber and the deflection under load in excess of the cracking load.  The results also indicated that unbonded beams containing non-tensioned steel perform in a similar manner to equivalent beams with bonded tendons with regard to prestress losses, crack distribution, ductility, load-deflection response and ultimate strength.

A comprehensive assessment has been undertaken to determine the reliability of various methods of predicting time-dependent deformations of plain and reinforced concrete, prestress and effective prestress losses, initial and time-dependent camber and deflection, load-deflection response, cracking moment, ultimate tendon stress and ultimate strength of the partially prestressed beams.  In the light of this assessment and the experimental results, design recommendations for the evaluation of short-term and long-term behaviour of partially prestressed beams are included.