International Concrete Abstracts Portal

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.

A computer-oriented constitutive model for concrete in compresslon, valid at first heating of concrete up to 800 C is described. The total deformation is expressed in terms of thermal, instantaneous stress-related, creep and transient strain components, where the transient strain is a concept introduced to describe the particular behavior under changing temperature. Comparisons with independent tests demonstrate that the material behaviour is described in a very appropriate way. The model is applied in a simple example calculation, showing that thermal stresses due to non-uniform temperature distribution are very insignificant or even nonexistant.Stresses due to restrained thermal expansion cannot in themselves contribute to compression failure of concrete.

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.

Previously published experimental data on the effect of nuclear radiation on the properties of plain concrete are summarized and evaluated. Neutron radiation with a fluence of more than 1 x 1019 n/cm 2 may have a detrimental effect on concrete strength and modulus of elasticity. Thermal coefficient of expansion, thermal conductivity and shielding properties of concrete are little affected by radiation. Radiation damage is mainly caused by lattice defects in the aggregates which cause a volume increase of aggregates and concrete. Different aggregates show different radiation resistance so that the selection of suitable aggregates is the most important parameter in the design of a radiation resistant concrete.