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International Concrete Abstracts Portal

Showing 1-5 of 13 Abstracts search results

Document: 

SP194-11

Date: 

May 1, 2000

Author(s):

W. H. Dilger and C. Wang

Publication:

Symposium Papers

Volume:

194

Abstract:

Creep and shrinkage of concrete are important factors in the design and analysis of concrete structures, particularly for the long-term serviceability and durability and for the construction stage of long span, prestressed con-crete structures. Because of very low water-cementitious ratio (w/cm), chemical admixtures and sometimes mineral admixtures, the evolutions of moisture state and rate of cement hydration in high-performance concrete (HPC) are significantly different from those in normal concrete. For example, the cement hydration in HPC terminates earlier due to the lack of water, resulti ng in faster development of relative strength at early ages and little long-term strength gain. At the same time, self-desiccation in the form of a rapid decrease in internal relative humidity starts in HPC at very early ages. Consequently, base (autogenous) shrinkage of HPC is very high compared with normal concrete. The moisture permeability, which affects the development of creep and shrinkage of concrete, is at least one order smaller for HPC than for normal concrete. As a result well as creep and the development shrinkage property of strength and modulus of elasticity as es of HPC are remarkably different from those of normal concrete. The current prediction models for time-dependent properties, particularly creep and shrinkage, which have been derived from experimental data on normal concrete, are not applicable to HPC. This paper will discuss the main parameters affecting the time-dependent properties and the differences between HPC and normal concrete, based on the results of an extensive experimental program at the University of Calgary and data from the literature. A recently developed model to estimate creep and shrinkage of HPC is also presented and examined.

DOI:

10.14359/9899


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


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: 

SP194-01

Date: 

May 1, 2000

Author(s):

Z. P. Bazant and S. Baweja

Publication:

Symposium Papers

Volume:

194

Abstract:

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.

DOI:

10.14359/9889


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


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