International Concrete Abstracts Portal

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.

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.

The huge quantity of phosphogypsum which has been stockpiled in Florida has caused increasing concerns with respect to its possible environmental impacts. Many research projects on utilization of phosphogypsum as construction materials have been sponsored by the Florida Institute of Phosphate Research. It was found that phosphogypsum-based materials possess valuable strength sufficient for use in building and road construction after compaction. The aim of the tests in this paper is to investigate the shrinkage properties of phosphogypsum-cement mixtures and the influence of thefactors, such as moisture content (MC), phosphogypsum/cement ratio (P/C), curing conditions, the relation between shrinkage and moisture loss, comparison of shrinkages of the compacted and vibrated specimens, and ambient humidity. A standard 1 x 1 x 11.25-in. (25 x 25 x 285-mm) bar, of which the net length between the two contact points at the two ends is 10 in., was used to measure the linear shrinkage. The specimens covered 14 to 22 percent MC, 10:90-95:5, three 28 days' curing, and 50 to 70 percent ambient humidity. Effect of these factors on the development of drying shrinkage over time is described, and the relationship between drying shrinkage and loss of water is given for each factor involved. The volume change of the specimen in the air is a swelling-shrinkage process with loss of moisture. Shrinkage of compacted specimen is much less than that of vibrated specimen. Curing conditions and ambient humidity are significant to the shrinkage. There is a maximum shrinkage at about 25:75 P/C, and an empirical formula to predict effect of P/C is obtained, using a program which fits least-squares polynomials to bivariate data.

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.

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.