Experimental Study On The Multiaxial Mechanical Behavior Of Plain High-strength Concrete Before And After High Temperatures

At present, most studies all over the word have merely been carried out to characterize the mechanical behavior of normal strength concrete(NSC) under multiaxial stress states as well as normal temperature. It is well known that the design and analysis of nonlinear behavior of reinforced concrete structure before and after high temperatures or fire are often applied in practicable engineering so that its multiaxial strength criteria and constitutive relationships must be established. In recent years, high strength concrete(HSC) is becoming an attractive alternative to traditional NSC. Therefore, it has become increasingly evident that it is quite import and urgent for the study on the mechanical behavior of HSC under multiaxial stress state before and after high temperatures. Based on the project of educational department of Liao Ning province science foundation (2023901023) and the research fund for the doctoral program of higher educationin application, the experiments on the strength and deformation behavior of two strength levels of HSC-60 and HSC-50 under multiaxial stress states as well as after exposure to normal and high temperatures of 20, 200, 300, 400, 500 and 600℃, are performed; moreover, based on the analysis of their failure mode and mechanism, multiaxial strength and peak strain regularity and stress-strain curves, etc. of HSC specimens under the corresponding stress state and ratios after high temperatures, the conclusions on strength and deformation are obtained and the corresponding mechanical models are also established. Comparison between the theoretical model and experimental results indicates good agreement. The major contributions of the work presented in this thesis are listed as follows:1. Tests on the strength and deformation of HSC-60 and HSC-50 under normal temperature as well as multiaxial stress state such as uniaxial tension and compression, biaxial tension-compression and compression-compression, and triaxial tension-compression-compression and compression-compression-compression, are carried out. The proposed failure criterion for HSC under multiaxial stress states in this paper, are established respectively. Compared with NSC, the increasing times of the multiaxial to uniaxial compressive strength for HSC are less; but, the difference of the both mechanical behavior under multiaxial tension-compression isn't obvious and its strength and peak strain slightly decrease.2. Tests on the strength and deformation of HSC-60 and HSC-50 after high temperatures under uniaxial tension and compression, are made. The formulas of the tensile and compressive strength, the initial elastic modulus for HSC with temperature parameter are obtained. The formulas of the corresponding stress-strain curve are established. The uniaxial compressive strength of HSC after exposure to 200℃and 300℃slightly increases; but, its peak strain decreases.3. Tests on the strength and deformation of HSC-60 and HSC-50 under biaxial and triaxial compression after high temperatures, are conducted. The formula of the initial elastic modulus of the third principal stress direction for HSC under biaxial compression are established. The formula of failure criterion for HSC after high temperatures under multiaxial stress state is proposed respectively. The ratios of the multiaxial to uniaxial compressive strength are depended on the stress ratios and its uniaxial compression strength of HSC after exposure to different temperatures; moreover, the ratios under triaxial compression are much greater than that under biaxial compression. Its increasing times after 200℃, 300℃are less; but, they are much bigger after 400℃～600℃. The regularity of their peak strain is similar to the strength's. The greater the uniaxial compression strength of HSC after exposure to different temperatures is; the less its increasing times.4. Tests on the strength and deformation of HSC-60 under biaxial tension-compression and triaxial tension-compression-compression after high temperatures, are carried out. The formulas of failure criterion for HSC with the temperature parameters under multiaxial tension-compression, are proposed. The multiaxial tension-compression strengths of HSC before and after high temperatures are less than its corresponding uniaxial tensile-compressive strength at all stress ratios; atα=0～0.2, they decrease rapidly. It isn't obvious for the difference of HSC mechanical behaviour between under biaxial and triaxial tension-compression stress state after 400℃~600℃, and its reducing extent is much bigger than that under normal temperature.5. An orthotropic constitutive relationship with temperature parameters for plain HSC under multiaxial stress states is developed. It is based on the analysis of the multiaxial stress-strain curves and the corresponding failure modes of HSC before and after high temperatures, and the orthotropic constitutive model of Guo-Xu modified.This research fills the blanks of "experimental research on the multiaxial mechanical behavior of plain HSC after high temperatures". It is the creative key in this paper to the multiaxial failure criteria and constitutive relationships with temperature parameters established for plain HSC under multiaxial stress states. This paper may also serve as a reference (testing data, correlated formula and mechanical behavior) for the maintenance, design and the life prediction of HSC structure before and after subjected to high temperatures.