The calculation of creep deformation of high-strength concrete in relation to the conditions of exposure to elevated temperatures

Abstract

The results of generalization and analysis of experimental research data on the creep deformation of heavy concretes of medium and high strength under conditions of elevated temperatures up to + 200°C are presented. It is shown that the experimental data of some authors are difficult to compare due to the significant influence of differences in test methods. All other things being equal, modern high-strength modified concrete is characterized by lower creep deformations in comparison with ordinary medium-strength concrete. It is shown that the ultimate values of the specific creep deformation of high-strength concrete at a long compression level of ήσ=0.5 under conditions of heating to 200°C are lower, on average, by 45% in comparison with similar characteristics for ordinary concrete of medium strength. With an increase in the level of long-term compression from ήσ=0.2 to ήσ=0.5, the specific creep strain of ordinary concrete of medium strength increases in the temperature range from +20°C to +200°C on average, by 28%, and in high-strength concrete with an increase in the level of long-term compression with ήσ=0.3 to ήσ=0.6, an increase in the similar characteristic was about 40%. The specific creep strain of high-strength concrete at a long compression level of ήσ=0.5 with an increase in the heating temperature from +20°C to +200°C increases by 2.8 times, and for medium-strength concrete - by 3.1 times. The massiveness of the samples (scale factor) can be taken into account using a parameter such as the modulus of the open surface. With an increase in the modulus of the open surface with a decrease in the mass of the samples, the creep strains of high-strength concrete increase at all heating temperatures. In relation to simplified engineering calculation methods, the recommended values of creep coefficients for concrete of classes B60 - B90 are proposed. In the development of concrete deformation models, approximating expressions have been developed that allow reliably assessing the creep strains of high-strength concrete of classes up to B90. The method relationships are based on the theory of aging and take into account the influence of the heating temperature, the duration of its action, the influence of the nonlinear component of creep, and the scale factor. The temperature and duration of heating before and after application of the load are taken into account using the functions of the reduced time. Satisfactory compliance of the calculated values of creep deformation of high-strength concrete with experimental data is shown.