Role of differential shrinkage and creep in time dependent behaviour of composite members

The time dependent behaviour of composite prestressed concrete beams depends upon the presence of differential shrinkage and creep of the concretes of web and deck, in addition to other parameters, such as relaxation of steel, presence of untensioned steel, and compression steel etc. In view of the interrelationships among these various parameters, it is proposed to isolate the effects of differential shrinkage and creep from other parameters by studying the behaviour of composite prisms under sustained load. The stresses and strains in composite prisms made up of two different concretes under sustained load are analytically determined using varying stiffness method of analysis. Creep and shrinkage characteristics of the two concretes are obtained from preliminary experiments using C.E.B. and also A.C.I. recommendations. The analytical results are found to be in close agreement with the experimental values obtained on three concrete prisms under sustained load for 160 days and thus also confirm the method of analysis.