International Concrete Abstracts Portal

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.

Creep and shrinkage of concrete were studied under constant load and restrained conditions during the first week after casting. Concrete behavior was characterized by a uniaxial test that measures shrinkage deformation and restrained shrinkage stress. The extent of stress relaxation by tensile creep was determined using superposition analysis. The experimental measurements were compared with current creep and shrinkage models to assess their validity for early age prediction. The ACI 209 equation for creep is currently not applicable to early age, but modifications are proposed that fit a database of early age behavior. The B3 model has been previously modified to accommodate early age creep, and this modification was employed in the current study. Test results for normal concrete with different w/c ratios are discussed.

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.

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.