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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.

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.

Many structural problems involving creep in concrete structures can be solved in very compact closed forms through the fundamental theorems of linear viscoelasticity of aging materials. This general approach requires the knowledge of three basic functions: the compliance function J, derived directly from the creep prediction models available in the literature and in technical guidance documents, and the relaxation (R) and redistribution () functions, that can be calculated from J. This paper presents an interactive web site for quick automatic calculation of these three basic functions, with reference to the principal creep models presently considered by international civil engineering societies. Starting from the approach suggested by Bazant for the numerical solution of the fundamental Volterra integral equation relating R to J, identically applied to derive ~ from J, a complete procedure has been developed, including the user interface necessary for setting input data and handling output results. The immediate availability of the basic functions allows extended comparisons of the outputs of the different models and evaluation of the influence that the selection of a particular model has on the assessment of structures. The web site has a flexible architecture and will be progressively extended to include calculation of other functions of interest for the creep analysis of structures, e.g. the aging coefficient X of the age-adjusted-effective-modulus-method, and the reduced relaxation functions R* that extend the field of application of the fundamental theorems to the analysis of heterogeneous structures, such as e.g. cable-stayed bridges.

The use of pozzolanic material, such as fly ash and silica fume, is becoming more popular in producing high performance/high strength concrete (HP/HSC) for various structural applications. Many studies have addressed the mechanical properties as well as durability of HP/HSC, however, the effect of pozzolans on the shrinkage and creep behaviors are not clearly addressed. There is a need to understand and identify how changes in the composition and porosity of HP/HSC, and consequently the elastic modulus, would affect its early age as well its long term performance. The main objective of this paper is to examine the effect of using various models for modulus of elasticity on the prediction of creep of high strength concrete (HSC) containing pozzolans. The study included an experimental program and a comparison of available analytical models for predicting the compressive creep and modulus of elasticity of HSC. Results from creep tests performed on different mixes (with compressive strength up to 90 MPa) were compared with those from prediction models available in the literature. Three creep models, ACI 209, CEB 90, and GL 2000, were used. In addition, various values of modulus of elasticity obtained from experimental calculation, ACI 318, ACI 363, CEB 90, Gardner, and from an equation proposed by the authors were evaluated. Results show that the modulus of elasticity has high impact on the accuracy of predicted creep and that available modulus of elasticity models needs to be revised to reflect HSC containing pozzolans.

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.