International Concrete Abstracts Portal

The long-term camber prediction under sustained loads is an important part of the design of pretensioned flexural members. Critical members are noncomposite roofs and bridge members which have medium or large span-depth ratios or elements made of lightweight concrete. Currently the most common approximate design method is one that relies on empirical multipliers applied to initial cambers and deflections. When compared to a rational approach, substantial differences in prediction of long-term camber or deflection are noticed. The approximate method appears to overestimate the permanent sag or underestimate the camber. It also does not consider certain creep, shrinkage, and relaxation properties. It is concluded that such methods may be unreliable for critical members and that the rational method is preferred. For preliminary design of longer spans, revised multipliers are suggested for use with the approximate method.

Pressurized water reactor containment building structures in nuclear power plants are designed to withstand internal accident pressure. Prestressed concrete is commonly used to resist such a pressure. The structure must maintain its structural integrity for the service life of the plant; therefore, the design must consider the effect of creep and shrinkage of concrete on the prestressing system. This effect is mainly in the form of prestressing force losses over time. Since creep and shrinkage are time-dependent, their values at any point in time during the service life of the plant must be predicted. The approach utilized in the design of the prestressed containment structure and the required periodic inspection are described. Also addressed is the procedure for establishing predicted changes in the prestressing forces as a result of creep and shrinkage of concrete at any point in time. Comparisons between predicted values and actual measurements of prestressing forces at different time intervals are presented. The comparison includes a number of reactor containment buildings and different concrete proportions.

A step-by-step procedure is used for computing the state of stress in a prestressed concrete cylinder pipe accounting for the effects of creep and shrinkage of concrete core and mortar coating and of wire relaxation. The procedure is applied to an embedded-cylinder pipe subjected to the outdoor environment and to that of a buried pipe with varying humidity conditions. The results show that for pipe exposed to the outdoor environment, the prestress in the inner and the outer cores of embedded-cylinder pipe are significantly different. However, the change in the environment resulting from burial of the pipe and filling it with water reduces the losses, and the difference in the prestress of the inner and the outer core of embedded-cylinder pipe.

In modern computerized structural analysis, realistic material laws should be used. This research will present a constitutive law and a numerical procedure based on the finite element method for the analysis of a prestressed concrete structure including the time-dependent effects due to the load history, creep, shrinkage, aging of concrete, and relaxation of prestress. A 32.1 meter (105 ft) long U-shaped railway bridge, composed of two precast post-tensioned concrete girders and an in situ cast prestressed young concrete slab, was instrumented to observe its long-term structural behavior and used for the comparisons with numerical analysis. To evaluate and predict the structural behavior of this concrete structure, the related experiments were designed and performed both in the field and laboratory. Some material properties needed for the analysis were obtained through the extensive program carried out in the laboratory with controlled environments. This paper will describe the details of structure, test program, and experimental results.

Tests results from long-term experiments on prestressed and partially prestressed concrete beams are reported. Tests were carried out on 10 rectangular beams spanning 2 m and undergoing sustained loading for five years. After that time, there was no evidence of stabilization of the time-dependent behavior of concrete. Numerical modeling of the deformation of the midspan section explains experimental observations and confirms that the presence of ordinary reinforcing steel in a prestressed concrete section leads to a redistribution of stresses between concrete and steel which should be taken into account in serviceability limit-state computations. Tests to failure of the beams at 5 years yield no significant differences in carrying capacity with tests executed at an early age. It is suggested that the deflection limit state is a major consideration in design and that the degree of prestressing should be chosen in function of ratio of permanent load to total design load (permanent and live).