ACI - SP-227
Shrinkage and Creep of Concrete
|Publication Date:||1 April 2005|
Why should you be interested in the shrinkage and creep of concrete? To put this question into perspective, the load induced elastic (immediate) strains in concrete are of the order of 300 microstrain, depending upon stress level. The ultimate drying shrinkage can range from 400 x 10-6 to 900 x 10-6. The creep strain can be 25% to 70% of the immediate strain after 24 hours and may be several multiples of the immediate strain after several years, depending on the relative humidity. If concrete is restrained against shrinkage it can crack.
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 nonsymmetrically reinforced concrete elements. Materiais 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 drying 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 modifi 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 AC1 Committee 209, Creep and Shrinkage in Concrete, and AC1 Committee 23 1, Properties of Concrete at Early Ages, held at the AC1 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 stresses and cracking.
Papers are presented to evaluate models for predicting shrinkage and creep, which is especially crucial as ACI Committee 209 attempts to refine the models that will are presented in the ACI Manual of Concrete Practice. Several papers provide a field assessment of creep and shrinkage in concrete structures, thereby providing an explanation of how field data should be collected and how this data can be used to refine predictive models. Other papers deal with an improved understanding of early-age tensile stress development, including demonstrating the measurement of creep and iii stress development. One paper presents experimental results to quanti@ differential drying and thermal deflections in slabs. This volume also recognizes that modem concretes are now no longer a simple mixture of water, cement, and aggregate. This volume discusses specific aspects of how the shrinkage of these modern concretes may differ from concrete made using conventional materials. Specifically, several papers deal with the shrinkage and curing of concrete containing supplementary cementing materials, which is especially important as the use of these materials is rapidly rising. The shrinkage and creep behaviour of high-strength, lightweight concrete is also evaluated and compared to existing models. One paper assesses the cracking behavior of selfconsolidating concrete, a rapidly emerging material that is often suspected to be sensitive to shrinkage related concerns due to the higher paste volume that may exist in these mixtures. It is shown that self-consolidating concretes can be made to be resistant to cracking. Finally, this volume describes a study that is focused on the development of an approach to describe the shrinkage and early age stress development in concrete containing shrinkage reducing admixtures.
This volume includes significant contributions by many leading research and practicing engineers in the field of shrinkage and creep. The descriptions of field problems and the assessment of the accuracy of predictive models may assist practicing engineers. For researchers, the contents of this volume should be useful in the development and evaluation of future models and laboratory measurements. Finally, it should be noted that many people have contributed to the successful development of these proceedings, and the editors, and authors, thank all of the anonymous reviewers who assisted in reviewing the papers.