International Concrete Abstracts Portal

The Comite Euro-International du Beton (CEB) has prepared a new model code for the design and analysis of concrete structures (CEB-FIP Model Code 1990) which includes new prediction models for creep and shrinkage of concrete. These models have been derived and optimized on the basis of a computerized data bank. For the prediction of shrinkage, a diffusion theory-type model has been chosen. The prediction of creep is based on a simple product-type approach. Though the new creep model resembles some of the features of the model presented by ACI 209, various basic improvements could be achieved. The coefficients of variation for shrinkage and creep have been found to be approximately 33 and 20 percent, respectively. The developed prediction models, both for creep and shrinkage, represent a reasonable compromise of accuracy and simplicity. They meet the requirements for presentation in a code. In this paper, both models are presented and some comparisons with test data are shown.

Paper reviews the influence on creep and shrinkage of water reducers, retarders and accelerators, and superplasticizers. Few generalizations can be made. Partial replacement of cement fly ash or blast furnace slag leads to reduced creep except for early age loading. Specific properties of the materials used affect creep. Data on the use of microsilica suggest that it increases creep. Because of the paucity of data available, reliable deductions about the influence of various materials cannot be made and, in important cases, experimental determination of creep and shrinkage of concrete with the actual mix ingredients may be necessary.

Thermal-related cracking in mass concrete structures can significantly increase maintenance and repair costs and decrease service life. Incremental construction thermal-stress analyses performed on a critical section of a navigation lock chamber monolith provided the basis for a study of the relationship between creep of concrete and thermal stresses during construction. Two concrete mixtures with high proportions of fly ash to portland cement (up to 50 percent fly ash) were selected for a full complement of thermal and mechanical properties testing at ages from 16 hours to 14 days. A general purpose heat-transfer an structural analysis finite element code was used in the analyses. The code incorporated a time-dependent material model as an integral part of the analysis effort. Material and mechanical properties test results were incorporated into the analyses for each mixture.

In recent years, considerable attention has been given to the use of silica fume as a partial replacement for cement to produce high-strength concrete. The use of silica fume high-strength concrete offers great promise for marine structures and offshore platforms. Preliminary results of creep strain measurements for 24 specimens at temperatures of 20, 10, 0, -10, and -20 C are presented. At room temperature, three stress levels were applied to the concrete specimens ranging from 25 to 75 percent of the 28-day strength at room temperature. For specimens at temperatures of 10, 0, -10, and -20 C, one stress level of 50 percent of the 28-day compressive strength of the reference specimens was applied. The results of creep at low temperatures were compared to the corresponding results at room temperature. In general, the relation of creep to stress-strength ratio at room temperature was found to be linear for silica fume concrete as the case for ordinary portland cement concrete. Test results revealed that low temperature had a minor effect on the magnitude of creep strains at temperatures between -10 and -20 C. Based on the experimental results, a basic expression for creep of silica fume concrete is suggested. Discussion of a hypothesis of the creep mechanisms is presented.

An experimental study was conducted to assess the effect of carbon fiber reinforcement on drying shrinkage strains in cementitious matrixes. Composites with different fiber lengths and volume fractions were considered in this investigation. Results indicated that shorter fibers at relatively low volume fractions tend to reduce drying shrinkage strains. The increase in fiber volume fraction does not necessarily produce further reductions in shrinkage movements, possibly due to the corresponding increase in water requirements for maintaining fresh mix workability. Longer fibers may not be as effective as the shorter ones in reducing shrinkage strains. This observation also can be attributed to the increase in water requirement with increasing fiber length. The large scatter in shrinkage test results makes it difficult to statistically derive reliable conclusions based on the limited test results generated in this investigation.