Analysis of shrinkage in prestressed concrete slabs and beams

Abstract

A step-by-step method of analysis is used in this paper to study the long-term effects of shrinkage in prestressed concrete members, and in particular shrinkage warping and loss of prestress. The step-by-step method is also being used in a parallel study of creep in prestressed concrete beams. A simple, closed-form equation is derived for the long-term shrinkage curvature in the cross-section of a rectangular prestressed member that contains tensile and compressive reinforcing steel. This allows the shrinkage deflection of the member to be calculated by first evaluating curvatures at key sections and then integrating. A simple, closed-form equation is also derived for the long-term loss of prestress due to shrinkage. Numerical studies show that tensile and compressive reinforcement strongly influence both the curvature and the prestress loss due to shrinkage. An assumption made in the analyses is that shrinkage occurs uniformly throughout the concrete. This is the basis of the current AS 3600 shrinkage clauses, and hence also of most current shrinkage design calculations. In reality, shrinkage in concrete members is non-uniform and non-linear. The paper concludes with a discussion of non-uniform shrinkage, and gives an outline of how the step-by-step method can be used to analyse non-uniform shrinkage.