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A thermodynamically-based creep theory is developed, in which the strain at any time is composed of mechanical, thermal, hygral and autogenous contributions. Each of these contributions (except the autogenou consists of an instantaneous part which is reversible (in the working stress regime) and a time-dependent part which may in turn contain both reversible and irreversible components. The theory provides a unifying framework for examining existing methods of modeling creep phenomena and developing new models where experimental data reveal deficiencies in current models.

An investigation was conducted to develop information on the time-dependent deformation behavior of concrete in the presence of temperature, moisture, and loading conditions similar to those en-' countered in a prestressed concrete reactor vessel (PCRV). Variables were one concrete strength (6000 psi (41 MPa) at 28 days), three 7 aggregate types (chert, limestone, and graywacke), one cement (Type II), two types of specimens (as-cast and air-dried), two levels of tempera-!, ture during test 73 F and 150 F (23 C and 66 C), and four types of "1, loading (uniaxial, hydrostatic, biaxial, and triaxial). There were 66 test conditions for creep tests and 12 test conditions for unloaded or control specimens. Experimental results are presented and discussed. Comparisons are made concerning the effect of the various test conditions on the behavior of concrete and general conclusions are formulated. Research performed under Int eragency Agreement No. AT-(40-1)-4128 for the Oak Ridge National Laborat ory operated by Un ion Carbide Corporation under contract with the Energy' Research and Deve lopment Administration.

concrete ra-Creep tes ts were performed in high-strength sealed to evaluate the effects of va ious combinations of temperature, stress level, and age of loading. Test conditions included temperatures of 73, 110, and 160F (23, 43, and 71C), stress levels of 30, 45, and 60 percent of compressive strength, and ages of loading of 28, 90, and 270 days. The creep loads were maintained for over one year, with creep recovery observed on selected groups of specimens for a period of 90 days. Tests were made on two concrete mixes, each made with the same brand of cement and sand but different coarse aggregates. The nominal strength of the concrete at age 60 days was 7500 psi (527 kgf/cm2) for moist-cured specimens and 7000 psi (492 kgf/cm2) for sealed specimens. Also reported are results of tests made to determine the effect of testing temperature on compressive strength as well as the influence of thermal cycling between 73 and 16OF (23 and 71C) on strength and elastic properties.

It is shown that the mechanism of cyclic creep, which has been previously suggested to be an accelerated static creep, is made up of two distinct parts: non-elastic deformation and microcracking. The non-elastic deformation in these short-term tests was not affected significantly by shrinkage as such, but rather by the presence of water within the cement paste, and is explained using Ruetz's hypothesis. Microcracking is shown to take place during the first few hours under a cyclic stress and manifests itself as an increase of the internal temperature. The microcracking explains the largely irrecoverable nature of cyclic creep.

Past investigations of the multiaxial behavior and strength of concrete have used both a wide variety of different materials, and of different test methods. In order to isolate the effects of these two variables, seven institutions cooperated in a test program in which mortar and concrete specimens were subjected to a variety of biaxial and triaxial compressive loading conditions, common to all participants. Identical materials were used in all tests, so that any systematic differences in the results could be attributed entirely to the differences in test methods. The effect of test method is predominantly a function of the specimen boundary conditions, which range from a specified stress boundary condition for perfectly flexible fluid cushion loadings, to a specified displacement boundary condition for perfectly rigid, rough platens. Mixed boundary conditions of various types occur with the use of conventional triaxial test cells, brush bearing platens, and lubricated loading plates. All of these loading conditions were represented in the program. Only strength results are presented in this paper. They clearly indicate the effects of surface constraints on the specimen; with increased boundary constraint, the ratio of multiaxial to uniaxial strength, as well as the ratio of cube to cylinder strength increases. Uniaxial, biaxial, and triaxial strengths of the materiaqs are compared by expressing them within a common octahedral normal-octahedral shear stress space. It appears possible to represent all observed failure points by a common compressive multiaxial strength criterion.