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Home > Publications > International Concrete Abstracts Portal
The International Concrete Abstracts Portal is an ACI led collaboration with leading technical organizations from within the international concrete industry and offers the most comprehensive collection of published concrete abstracts.
Showing 1-10 of 11 Abstracts search results
December 1, 1991
Yun-Soo Joo and Maher K. Tadros
The objective of the paper is to develop a computer-based procedure for analysis of members made up of precast prestressed concrete sections with cast-in-place topping. The procedure accounts for cracking under the weight of the cast-in-place topping and superimposed loads. Creep and shrinkage of concrete and relaxation of prestressed steel are taken into account in combination with the effects of cracking. An attempt is made to limit the input data to information commonly available to designers. Linear creep law is used, and time-dependent effects, such as prestress losses and deflection growth, are automatically generated.
Mehdi S. Zarghamee and william R. Dana
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.
B. L. Meyers and M. A. Daye
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.
J. C. Chern, Y. G. Wu, Y. W. Chan, and T. Y. Chou
Continuing the study of long-term behavior of a U-type composite prestressed concrete bridge, this paper presents a constitutive law for structural analysis and a nonlinear diffusion theory for the understanding of internal humidity distribution in a structure. The constitutive law was formulated based on a new rheological concrete element, which is a series coupling of a generalized Kalvin chain unit, a cracking unit, and a unit representing thermal strain or shrinkage. The previously obtained exponential algorithm for cracking, treated as strain softening, is combined with the exponential algorithm for generalized Kelvin chain element. The nonlinear diffusion theory, which considers both the pore humidity and aging effects on the diffusivity, was found to give very good fit to the distribution of internal humidity of concrete. The material parameters needed for the constitutive law and diffusion theory were identified. The numerical results using finite element method show the history of the prestress transfer between a young bridge deck and more matured girders. The calculated strains based on the presented method were found to be in good agreement with field measured data.
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.
R. I. Gilbert and N. C. Mickleborough
A nonlinear procedure is described for the time-dependent analysis of reinforced and prestressed concrete columns under sustained eccentric compression. Both material and geometric nonlinearities are take into account in an iterative computer-based solution procedure. Individual cross sections are analyzed using the age-adjusted effective modulus method to include the effects of creep and shrinkage. By dividing the time scale into several increments, the gradual development of time-dependent cracking can be traced as the lateral deflection of the column and the internal secondary moments increase with time due to creep. Analytical predictions are shown to agree with laboratory measurements and numerical results from a parametric study of the behavior of slender columns are presented. The analytical model is also used to make a critical examination of the design provisions in ACI 318-83. Results indicate that the moment-magnifier method contained in the code for the design of slender columns becomes very conservative for very long columns.
B. Vijaya Rangan
The paper presents a summary of treatment of deflection control in the Australian Standard, AS3600-1988. The top-tier method recommended by the standard requires consideration of creep and shrinkage effects on deflections. Simple expressions for calculating creep and shrinkage deflections are given and illustrated by examples.
N. C. Mickleborough and R. I. Gilbert
The first stage of an experimental investigation into the instantaneous and time-dependent behavior of reinforced concrete columns under sustained load is reported. The experimental work described in the paper involves the testing of 15 large-scale columns in compression and uniaxial bending to obtain comprehensive creep deformation data for the prediction of long-term lateral deflection and instability. Information on the range of slenderness ratios and load levels which cause creep instability for rectangular symmetrically reinforced concrete columns is of particular interest. The experimental data is also for use in the development and calibration of a theoretical model for the prediction of creep deflection and buckling under sustained load. The experimental setup used in this investigation and described in the paper is designed for the simultaneous testing of five slender columns, each up to 6 m in length. The loading frame is such that each column may have different length, different cross-sectional dimensions and reinforcement details, and be subjected to different combinations of axial force and bending moment. In the three series of tests presented here, column length, axial force, and initial eccentricity are the major variables. The loading on each column is monitored independently and maintained automatically at a constant preset value throughout each test. A direct comparison of creep effects on column behavior is therefore possible as different loading parameters are varied.
Bernard Espion and Pierre Halleux
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).
Jenn-Chuan Chern and Young-Gee Wu
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.
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