SP55 This symposium volume was conceived as a tribute to Douglas McHenry in recognition of his outstanding contributions to concrete engineering. The 10 papers dealing with plain concrete and the 14 papers dealing with concrete structures reflect the needs of advanced technological development for the concrete industry. The theme of the symposium was concrete and concrete structures in severe environments and/or complex loading conditions. Papers deal with effects of transient high temperatures, cryogenic temperatures, nuclear radiation, with prediction of overall structural behavior in fires, earthquakes, and with behavior of elements under complex loading conditions.

Described in this paper are the test results of mechanical properties of several kinds of concrete mixes at very low temperatures and an investigation into the mechanism of change in their properties, for the purpose of obtaining design data for concrete structures exposed to very low temperatures, such as liquefied natural gas storage tanks and refrigerator warehouses. It was learned from the preliminary tests that the strength of concrete under the temperatures of the range of -1O'C to -7O'C was affected by moisture contents; the larger was the moisture content, the higher the rate of the strength increase was, and that the increase of concrete strength corresponded to that of ice at very low temperatures. Under lower temperatures of -1O'C to -196'C, it was verified that compressive strenth, modulus of elasticity and tensile and bond strengths of concrete increased with the decrease of temperature and the rate of increase in the strength and the elastic modulus was higher when the moisture content was larger. On the four mixes of concrete which had different water-to-cement ratios and air contents, tests were made, under the temperatures of down to -70°C, for compressive, tensile, flexural and bond strength and the modulus of elasticity. It was found that the rates of increase in these strengths were higher for the concrete with higher water-to-cement ratios and larger air contents. Also found by tests was the decrease in the strength; of concrete that received the very low temperature shocks between +20 and -196 C. The coefficients of thermal expansion of concrete were calculated from the measurements of the changes in the specimen lengths under the low and the very low temperatures, and were found to be smaller com-pared with those under room temperature. Further discussion is made on the influence of freezing of water in comparatively large pores under low temperatures and of the drop of freezing point of capillary water in smaller pores and accompanying freezing of water in these pores.

The paper presents a model capable of predicting the post-cracking response of reinforced and prestressed concrete members subjected to complex loading. The angles of inclination of the compression diagonals in the walls of the truss model are determined from strain compatibility conditions. These compatibility conditions in conjunction with the equilibrium conditions for the truss and the load-deformation relationships for the members of the truss enable the full response of the model to be determined; i.e. the strain in the longitudinal and web reinforcements as well as the various eformations of the beams at all load levels can be predicted. Experimental results are used to confirm the truss model's predictions. It is shown how the truss model could be used in the design office.

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.

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.