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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.

The properties of mass concrete to be used in the numerical analysis of dams are derived from properties determined on concrete specimens in laboratory tests. Care is needed in selecting and modifying these data since mass concrete is quite different from structural concrete, and from the concrete of most laboratory experiments. Consideration is given to time-dependence of strength, elastic modulus, and creep, and factors are furnished to derive representative values for mass concrete from laboratory tests, since modulus of elasticity varies with the type of loading: for dead load and water load analy-ses, it is a fraction of the tested modulus; for earthquake loading, it is a multiple of the tested modulus. The variation of temperature-dependent properties with aggregate types is discussed. Tn all these properties, the influence of the aggregate is much stronger than the influence of the cement paste in setting the magnitude of structural properties.

The stress-strain relationship and flexural stress distribution for ultimate strength design has been well established from previous work. Generally, normal-weight concretes with strengths ranging from 1,000 psi to 7,500 psi (6.9 MPa t o 51.7 MPa) have been investigated. In the present study, flexural characteristics of high-strength concretes were obtained from a series of specimens tested at the Portland Cement Association laboratories. The test series included concrete strengths ranging from 6,500 psi to 14,850 psi (44.8 MPa to 102.4 MPa) for normal-weight concretes and from 3,560 psi to 12,490 psi (24.5 MPa to 86.1 MPa) for lightweight concretes. Concretes containing three different normal-weight aggregates and two different lightweight aggregates were included in the study. Stress-strain curves, flexural constants, and moduli of elasticity are reported for the complete range of concrete strengths. Results of this investigation have been combined with those of other investigators. The data are compared with the latest ACI Building Code revisions pertaining to flexural constants for strength design.