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International Concrete Abstracts Portal

Showing 1-5 of 13 Abstracts search results

Document: 

SP194-10

Date: 

May 1, 2000

Author(s):

J. J. Brooks

Publication:

Symposium Papers

Volume:

194

Abstract:

This review assesses the general effects on elasticity; creep and shrinkage of concrete arising from the use of admixtures: plasticizers (water-reducers), super-plasticizers (high range water-reducers), slag, fly ash and silica fume. Previously published test data are collectively analyzed to quantify creep and shrinkage of concrete containing the admixture as a proportion of creep and shrinkage of plain concrete having the same mix proportions. The analysis shows the effects of the admixtures are highly variable and the expressions given for predicting deformations are only recommended in the absence of test data.

DOI:

10.14359/9898


Document: 

SP194-03

Date: 

May 1, 2000

Author(s):

N. J. Gardner

Publication:

Symposium Papers

Volume:

194

Abstract:

This paper presents a simple design-office procedure for calculating the shrinkage and creep of concrete using the information available at design; namely the 28 day specified concrete strength, the concrete strength at end of curing or loading, element size and the relative humidity. The method includes strength development with age, relationship between modulus of elasticity and strength, and equations for predicting shrinkage and creep. The only arbitrary information are the factors appropriate to the cementitious material, which can be improved from measured strength age data. At the most basic level the proposed method requires only the information available to the design engi-neer. The prediction values can be improved by simply measuring concrete strength development with time and modulus of elasticity. Aggregate stiffness can be taken into account by back calculating a concrete pseudo strength from the measured modulus of elasticity. Measured short term shrinkage and creep values can be extrapolated to obtain long duration predictions for simi-lar sized elements. The predictions are compared with experimental results for seventy nine data sets for compliance and sixty three data sets for shrink-age. The comparisons indicate shrinkage and creep can be calculated within +/- 25%. The method can be used regardless of what chemical admixtures or mineral by-products are in the concrete, casting temperature or curing regime.

DOI:

10.14359/9891


Document: 

SP194-06

Date: 

May 1, 2000

Author(s):

M. A. Chiorino, L. Dezi, and A. M. Tarantino

Publication:

Symposium Papers

Volume:

194

Abstract:

Variation of statistical scheme of reinforced and prestressed concrete struc-tures is frequent in modern construction techniques. Construction sequences may include application of permanent loads and of prestressing in one or more steps, and connection of different portions of the structure, or introduction of additional restraints (sometime forcedly applied to correct the internal stress conditions), at different ages during or after the construction process. The resulting stress distribution is largely influenced by the time-dependent deformability of concrete. The paper presents a unified approach for its evalu-ation based on the linear theory of viscoelasticity for aging materials, which is normally adopted for modeling concrete creep, evidencing the important role played by the non-dimensional redistribution function (t, to, t1) describing the creep induced stress variation at time t for loading at t, and variation of re-straint conditions at t,. The obtained solutions are exact for all problems of variation of restraint conditions in homogeneous structures with rigid restraints, and normally suffkiently accurate for problems concerning structures charac-terized by heterogeneities in the properties of concrete along their structural configuration. Redistribution function may be computed from the creep function J characterizing the creep prediction model under consideration and made available in terms of design aids (graphs or tables). The computational procedure is illustrated and an example of application to a typical structural problem is presented.

DOI:

10.14359/9893


Document: 

SP194-02

Date: 

May 1, 2000

Author(s):

Z. P. Bazant and S. Baweja

Publication:

Symposium Papers

Volume:

194

Abstract:

A simple model for the characterization of concrete creep and shrinkage in design of concrete structures is proposed. It represents a shortened form of model B3 which was presented in [2] (as and improvement of the original version [3]) and appears in this volume, and an update of a previous short form [4]. 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 model is justified by statistical comparisons with all the data in the internationally accepted RILEM data bank. The differences be-tween the present short-form and model B3 are discussed and limitations of the short form are compared to model B3 are noted. The model is suitable for design of concrete structures with the exception of highly creep-sensitive struc-tures for which the full model B3 is necessary

DOI:

10.14359/9890


Document: 

SP194-05

Date: 

May 1, 2000

Author(s):

D. J. Carreira, M. Daye, and L. R. Greening

Publication:

Symposium Papers

Volume:

194

Abstract:

This paper presents a simplified, comprehensive and rational method to account for the effects of creep and shrinkage in reinforced concrete struc-tures. The analysis of the effects of creep and shrinkage on reinforced and prestressed concrete structure is a multifaceted problem. In general, creep and shrinkage are not common knowledge and concern of most structural engineers. The procedure includes (a) a simplified estimation of the creep and shrinkage strains in plain concrete, (b) the analysis of the effect of creep on reinforced concrete structures using the age-adjusted modulus of elasticity, (c) the analysis of the effects of shrinkage using an equivalent temperature drop. (d) the relaxation of stresses, internal forces and moments from imposed deformations, (e) the analysis of thermal changes on structures. An introductory approach of analysis of concrete structures creep and shrinkage should be satisfactory for structures not critically sensitive to the effects of creep and shrinkage. For structures sensitive to these effects, such as tall buildings or record span bridges, a more advanced analysis will be necessary, which is beyond the scope of this paper. The presented method includes most of the aspects affecting the effects of creep and shrinkage on concrete struc-tures, and it may be coupled with more advanced treatment of specifically related subjects.

DOI:

10.14359/9896


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