Sandeep Baweja

Justification and refinements of model B3 for concrete creep and shrinkage 1. statistics and sensitivity

Model B3 for creep and shrinkage prediction in the design of concrete structures, presented as a RILEM Recommendation inMater Struct.28 (1995) 357–365, is calibrated by a computerized data bank comprising practically all the relevant test data obtained in various laboratories throughout the world. The coefficients of variation of deviations of the model from the data are distinctly smaller than for the latest CEB model, and much smaller than for the previous ACI model (which was developed in the mid-1960s). The effect of concrete composition and design strength on the model parameters is identified as the main source of error. 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.

Justification and refinements of model B3 for concrete creep and shrinkage 2. Updating and theoretical basis

Following statistical evaluation in part 1, this part deals with the improvement of prediction by updating one or two parameters of the model on the basis of short term tests and theoretical derivation of some formulae. 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 is presented in which the shrinkage half-time is calibrated by simultaneous measurements of water loss. This approach circumvents the ill-posedness of the shrinkage extrapolation problem. Theoretical justifications of various aspects of the model are given and a new formula for the additional creep due to drying (or stress-induced shrinkage) is derived. The new model should allow a more realistic assessment of the creep and shrinkage effects in concrete structures, which significantly affect durability and long term serviceability of civil engineering infrastructure.

CREEP AND SHRINKAGE PREDICTION MODEL FOR ANALYSIS AND DESIGN OF CONCRETE STRUCTURES: MODEL B3 - SHORT FORM

A short model for the characterization of concrete creep and shrinkage in design of concrete structures is proposed. The main simplification compared to model B3 comes from the use of the log double power law as the basic creep compliance function. The B3 formulae for predicting material parameters in the model are simplified by dropping the dependence of these parameters on the composition of concrete mix, leaving only dependence on the strength and the specific water content of the concrete mix. The differences between the present short form and model B3 are discussed and limitations of the short form as compared to model B3 are noted. The model is suitable for design of concrete structures with the exception of highly creep sensitive structures for which the full model B3 is necessary.