Deformation of partially prestressed concrete beams under service loads
Nine tests were carried out on partially prestressed concrete beams in which 50 percent of the service load was sustained for a period of up to 46 days during which time the load was increased to the full service load up to seven times in intermittent short-term cycles. One cycle of short-term loading was carried out at intervals of about five days. Three additional beams were tested under the sustained load alone. An analytical method was developed for calculating short-term deflection of cracked prestressed concrete members by integration of the curvature, using the recommendation in CPII0 Appendix A to take account of the "tension stiffening" effect of the concrete between cracks. This was extended to predict long-term deflection by considering the effect of creep and shrinkage in the analysis of the cracked sections. A separate section of the thesis describes a comparative study of three alternative partial prestressing parameters: Hypothetical tensile stress, Degrees of Prestress and Partial Prestressing Ratio. The relationship of each of these parameters to the serviceability criteria was considered. The Degree of Prestress was concluded to be the most suitable parameter for design purposes and a suitable design approach is recommended.