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.

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.

This research focuses on deviations from the linear viscoelastic behavior of concrete occuring at high stress levels (from 0.5 f’c to 0.7 f’c), at early age loading (1 to 2 days) and in case of unloading implying strain reversal. A large series of creep tests was performed on high strength concrete specimens undergoing creep under constant stress, followed by a period of recording of the creep recovery after complete unloading. Some specimens were heat cured before loading. Some nonlinear effects at very early age have been observed. After unloading, experimental data show that the creep recovery deviates strongly from the numerical predictions obtained by the application of the principle of superposition but seems to conform rather well to the recovery model proposed by Yue and Taerwe3. This model was then applied, through a step-by-step approach, for the time-dependent structural analysis of a precast composite prestressed bridge deck with 26 m span. The application of the recovery model yielded computed strains which are in good agreement with in situ measured strains, and in better agreement than the strains computed by the application of the principle of superposition. This enhanced approach was then used to optimize the phases of construction of this kind of structure. Thanks to this research, the age at transfer of prestress could be significantly reduced.

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.