ACI 209R-92 Prediction of Creep, Shrinkage, and Temperature Effects in Concrete Structures

ACI 209R-92 Prediction of Creep, Shrinkage, and Temperature Effects in Concrete Structures.
1.2-Nature of the problem Simplified methods for analyzing service performance are justified because the prediction and control of time- dependent deformations and their effects on concrete structures are exceedingly complex when compared with the methods for analysis and design of strength perfor- mance. Methods for predicting service performance in- volve a relatively large number of significant factors that are difficult to accurately evaluate. Factors such as the nonhomogeneous nature of concrete properties caused by the stages of construction, the histories of water content, temperature and loading on the structure and their effect on the material response are difficult to quantify even for structures that have been in service for years. The problem is essentially a statistical one because most of the contributing factors and actual results are in- herently random variables with coefficients of variations of the order of 15 to 20 percent at best. However, as in the case of strength analysis and design, the methods for predicting serviceability are primarily deterministic in nature. In some cases, and in spite of the simplifying assumptions, lengthy procedures are required to account for the most pertinent factors.
According to a survey by ACI Committee 209, most designers would be willing to check the deformations of their structures if a satisfactory correlation between com- puted results and the behavior of actual structures could be shown. Such correlations have been established for laboratory structures, but not for actual structures. Since concrete characteristics are strongly dependent on en- vironmental conditions, load history, etc., a poorer cor- relation is normally found between laboratory and field service performances than between laboratory and field strength performances. With the above limitations in mind, systematic design procedures are presented which lend themselves to a computer solution by providing continuous time functions for predicting the initial and time-dependent average response (including ultimate values in time) of structural members of different weight concretes. The procedures in this report for predicting time- dependent material response and structural service per- formance represent a simplified approach for design purposes. They are not definitive or based on statistical results by any means. Probabilisitic methods are needed to accurately estimate the variability of all factors in- volved.
1.3.2 Creep The time-dependent increase of strain in hardened concrete subjected to sustained stress is defined as creep. It is obtained by subtracting from the total measured strain in a loaded specimen, the sum of the initial in- stantaneous (usually considered elastic) strain due to the sustained stress, the shrinkage, and the eventual thermal strain in an identical load-free specimen which is sub- jected to the same history of relative humidity and tem- perature conditions. Creep is conveniently designated at a constant stress under conditions of steady relative humidity and temperature, assuming the strain at loading (nominal elastic strain) as the instantaneous strain at any time.ACI 209R pdf download.