Creep and Shrinkage Prediction Model for Analysis and Design of Concretetructures: Model B3


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Title: Creep and Shrinkage Prediction Model for Analysis and Design of Concretetructures: Model B3

Author(s): Z. P. Bazant and S. Baweja

Publication: Special Publication

Volume: 194


Appears on pages(s): 1-84

Keywords: concrete; creep; design; drying; extrapolation of short-time data; mathematical models; moisture effects; prediction; shrinkage; statistical variations; viscoelasticity

Date: 5/1/2000

The present paper presents in chapter 1 a model for the characterization of concrete creep and shrinkage in design of concrete structures (Model B3), which is simpler, agrees better with the experimental data and is better theo-retically justified than the previous models. The model complies with the gen-eral guidelines recently formulated by RILEM TC- 107. Justifications of various aspects of the model and diverse refinements are given in Chapter 2, and many simple explanations are appended in the commentary at the end of Chap-ter 1 (these parts are not to be read by those who merely apply the model). The prediction model B3 is calibrated by a computerized databank comprising practically all the relevant test data obtained in various laboratories throughout the world. The coefficients of variation of the deviations of the model from the data are distinctly smaller than those of the latest CEB model (1990), and much smaller than those for the previous model in ACI209 (which was devel-oped in the mid- 1960s). The model is simpler than the previous models (BP and BP-KX) developed at Northwestern University, yet it has comparable accuracy and is more rational. The effect of concrete composition and design strength on the model parameters is the main source of error of the model. A method to reduce this error by updating one or two model parameters on the basis of short-time creep tests is given. The updating of model parameters is particularly important for high-strength concretes and other special concretes containing various admixtures, superplasticizers, water-reducing agents and pozzolanic materials. For the updating of shrinkage prediction, a new method in which the shrinkage half-time is calibrated by simultaneous measurements of water loss is presented. This approach circumvents the large sensitivity of the shrinkage extrapolation problem to small changes in the material param-eters. The new model allows a more realistic assessment of the creep and shrinkage effects in concrete structures, which significantly affect the durability and long-time serviceability of civil engineering infrastructure.