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International Concrete Abstracts Portal

Showing 1-5 of 19 Abstracts search results

Document: 

SP227-08

Date: 

March 1, 2005

Author(s):

D. J. Carreira

Publication:

Symposium Papers

Volume:

227

Abstract:

Reinforced concrete columns under compression loads and under little or no moment may exhibit cracking. Some cracks develop at early ages and others years later under sustained axial loads or no significant loads at all. Flexural cracking may be expected from externally applied loads on columns within the tension-controlled zone in the axial load-moment diagram. However, for columns within the compression-controlled zone of the diagram, cracking is not normally expected to occur under allowable service loads. Concrete shrinkage and creep, temperature variations and loading history cause all these cracks. In this paper, the causes of these cracks are described, analyzed and illustrated with photos of cracked columns. Design and construction recommendations to prevent or reduce these cracks are provided.

DOI:

10.14359/14430


Document: 

SP227

Date: 

March 1, 2005

Author(s):

Co-Editors: N.J. Gardner and Jason Weiss

Publication:

Symposium Papers

Volume:

227

Abstract:

SP227 Structural engineers are concerned with the consequences of shrinkage, creep and cracking on the serviceability and durability of their structures. Creep increases deflections, reduces prestress in prestressed concrete elements, and causes redistribution of internal force resultants in redundant structures. Shrinkage can cause warping of slabs on grade due to differential drying and increased deflections of non-symmetrically reinforced concrete elements. Materials scientists are concerned with understanding the basic phenomena and assessing new materials and the effects of admixtures on the mechanical behavior of concrete. Concrete is an age stiffening material that has little tensile strength, shrinks, and exhibits creep in sealed conditions and additional creep in drying environments. Predicting the amount of shrinkage and deflection that may occur is not easy and is especially complicated in concrete that contains supplementary materials, chemical admixtures, and lightweight aggregates. Supplementary cementing materials and waste products are being used in increasing volumes in response to environmental concerns. Admixtures have been developed to modify the behavior of fresh and hardened concrete. Self consolidating concrete is being used in more applications. A recent development is the marketing of shrinkage reducing admixtures. This volume contains papers presented during four sessions sponsored by ACI Committee 209, Creep and Shrinkage in Concrete, and ACI Committee 231, Properties of Concrete at Early Ages, held at the Spring 2005 Convention. The subjects addressed by the authors are diverse and cover many aspects of shrinkage and creep. Some papers pay special attention to the development, use, and evaluation of models to predict shrinkage, creep, and deflection, while other papers consider the behavior of early age concretes that are restrained from shrinking, resulting in the development of residual stress and cracking.

DOI:

10.14359/14364


Document: 

SP227-16

Date: 

March 1, 2005

Author(s):

M. Pigeon, B. Bissonnette, J. Marchand, D. Boily, and L. Barcelo

Publication:

Symposium Papers

Volume:

227

Abstract:

This paper illustrates how stress relaxation can be used to obtain valuable information regarding the behavior of concrete at early ages. Five concrete mixtures were investigated using a so-called discretized restrained shrinkage (DRS) testing device, allowing the determination (from the time of casting) of the increase in load induced by autogenous shrinkage and the evaluation of the different strain components (free shrinkage, elastic strain, creep). Test results indicate that the stress due to early-age restrained autogenous shrinkage is quite variable, in good part due to the variation in the relaxation capacity of the mixtures. Both the relaxation ratio, defined as the stress generated divided by the theoretical stress, and the relative relaxation, defined as the absolute value of stress relaxation divided by the average applied stress, can be used to illustrate and analyze the variation of the relaxation phenomena as a function of the type of mixture tested.

DOI:

10.14359/14438


Document: 

SP227-14

Date: 

March 1, 2005

Author(s):

H. T. See and E. K. Attiogbe

Publication:

Symposium Papers

Volume:

227

Abstract:

With the increasing use of self-consolidating concrete (SCC) in the concrete construction industry, its performance in restrained structural elements is of interest in order to assess the resistance to restrained shrinkage cracking. A new standard test method, ASTM C 1581, which uses an instrumented ring, is employed to assess the cracking potential of various SCC mixtures under restrained shrinkage on the basis of either the time to cracking or the rate of stress development in the material. The performance of the SCC mixtures is compared to that of conventional concrete mixtures to assess the effect of fluidity level on resistance to restrained shrinkage cracking. In addition, the SCC mixtures are evaluated for the effects of sand-to-aggregate ratio (S/A), paste content, aggregate shape, and use of a shrinkage-reducing admixture (SRA) on cracking potential. The results show that the cracking resistance of SCC is similar to that of conventional concrete, indicating that the higher fluidity of SCC is not detrimental to performance under restrained shrinkage. The cracking potential of the SCC mixtures is found to be influenced by the mixture composition.

DOI:

10.14359/14436


Document: 

SP227-13

Date: 

March 1, 2005

Author(s):

B. Pease, H. Shah, and J. Weiss

Publication:

Symposium Papers

Volume:

227

Abstract:

Concrete shrinks as a result of drying, self-desiccation, chemical reaction, or temperature reduction. If this shrinkage is prevented by restraint, tensile stresses develop which may result in cracking. Various alternatives have been proposed to reduce the cracking propensity of a mixture including the use of commercial chemical admixtures called shrinkage reducing admixtures (SRA). To date however, little information has been presented to describe how the performance of mixtures containing SRA’s could be predicted. In addition, little guidance exists to determine the dosage rate of SRA that should be used to achieve a specific level of performance. This paper describes initial research results from a study that has been aimed at quantifying the role of SRA at early-ages. The surface tension of various solutions of water and SRA was measured. In addition, free shrinkage measurements were conducted for specimens exposed to sealed and drying conditions from the time of set. Results indicate that shrinkage can be directly related to the concentration of SRA. Restrained ring experiments were performed to investigate the influence of a SRA on residual stress development and cracking in mortar. It has been observed that the residual stress reduction is not directly proportional to the reduction in free shrinkage since the specimens with SRA demonstrate less stress relaxation.

DOI:

10.14359/14435


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