Properties And Calculation Methods For The Strength And Deformation Of Plastic Concrete

The research in this thesis is one part of the main content of "Cutoff wall material and structure of high-performance plastic concrete" and "Mechanics and permeability performance of high-performance plastic concrete under complex conditions", respectly supported by the National Basic Research Program of China (973 Program) and National Natural Science Foundation of China. A total of 462 plastic concrete specimens with 17 mixture proportions were tested, which include splitting tensile strength and deformation, flexural tensile strength and deformation, uniaxial compression, biaxial compression, conventional triaxial compression and true triaxial compression. The effects of fiber type, fiber content, fly ash content, silica fume content, different stress states, loading paths and other factors on the properties of plastic concrete were systematically investigated through combining test results with strength theory and damage theory. The detailed research conducted in this thesis is as follows:(1) The test of splitting tensile strength and deformation of 48 standard cubic specimens of 150mm×150mm×150mm with 16 mixture proportions was carried out to measure the load and lateral deformation of the specimens in loading process. The effects of fiber types, fiber contents, fly ash contents and silica fume contents on splitting tensile strength, lateral deformation and the energy before and after the peak of plastic concrete were systematically studied. Based on the test results, the calculation methods of splitting tensile strength, lateral deformation, the area before and after the peak under the load-lateral deformation curve and the equation of splitting tension load-lateral deformation curve were put forward, which consider the effects of the factors mentioned above.(2) The test of flexural strength and deformation of 48 beam specimens of 100mm×100mm×400mm with 16 mixture proportions was carried out to measure the load and deflection of the specimens in the loading process. The influences of the fiber type, fiber content, fly ash content, and silica fume content on flexural strength, peak deflection, secant modulus at peak point and area under load-deflection curve of plastic concrete were analyzed. Based on the test results, the calculation models of flexural strength, peak deflection, the flexural load-deflection curve and toughness were established, which consider the effects of the factors mentioned above.(3) Through uniaxial compression test of 51 prism specimens of 150mmx 150mmx300mm with 17 mixture proportions, the vertical load and deformation of the specimens were measured during the loading process and the stress-strain curve of each mixture proportion was drawn. The influences of fiber types, fiber contents, fly ash contents and silica fume contents on uniaxial compressive stress-strain curve of plastic concrete were analyzed. By statistically analyzing the parameters of rising segment and falling section, elastic modulus, peak strain, ultimate strain and energy of the specimens absorbed during the loading process, the calculation or accessor methods of parameters mentioned above were put forward and the uniaxial compressive damage constitutive model of plastic concrete was established.(4) Based on the biaxial compression test of 75 cubic specimens of 150mmx 150mmx 150mm with five mixture proportions and five load paths, the biaxial stress and deformation were measured, the biaxial compressive failure modes and the development law of biaxial strength with varying load paths were discussed. Based on the results, the biaxial strength envelope equation and Kupfer-Gestler strength calculation model of plastic concrete were established. Forthmore, using octahedral stress space, characteristics of octahedral shear stress-strain curves and normal stress-strain curves were investigated, the failure criterion equation with double parameters, three parameters and the failure criterion equation considering the impact of load paths were established for plastic concrete under biaxial stresses and the shear stress-strain constitutive model and normal stress-strain constitutive model were put forward.(5) According to the conventional triaxial test of 105 cubic specimens of 150mm×150mm×150mm with five mixture proportions, the conventional triaxial performance and strength calculation model of plastic concrete were researched under four types of confining pressure and three types of fixed confining pressure ratio. The results indicate that the first principal stress-strain curves can be broadly divided into four sections including the initial contra flexural section, straight line section, curve section and straight-line platform section respectively, in which the initial segment appears significantly elastic characteristics and the last segment after straight line section reflects severely plastic property. For the same mix proportion, the ultimate strength, ascending slope, peak strain and ultimate strain have a tendency to increase with the increasing of confining pressure or confining pressure ratio. The conventional tri-axial compressive strength is probably as 2-6 times as uniaxial compressive strength. The cohesion is larger under the load in fixed confining pressure ratio than that in fixed confining pressure, and the internal friction angle is of the contrary. Based on the analysis of test results, the strength calculation model of plastic concrete under conventional tri-axial stress was put forward by using the shear unified strength theory and the Mohr-Coulomb criterion.(6) Through the true triaxial tests of 135 cubic specimens of 150mm× 150mm×150mm with five mixture proportions, the performance and failure criterion of plastic concrete under true triaxial compression were analyzed under 9 types of load path. The results indicate that the first principal stress-strain curves is similar with that of conventional triaxial. Specimen failure pattern is similar to uniaxial compression failure pattern when the second and third principal stress are smaller and shows diagonal cracks when the stresses are larger. The first principal stress increases with the third principal stress or the second principal stress increasing; the varying of the third principal stress has obviously affect on the first principal stress. With the third principal stress increasing, the cohesion and angle of internal friction increase. When the second principal stress is fixed, the cohesion and angle of internal friction are respectively about 2 and 0.5 times of those when the ratio of the second principal stress to the third principal stress increase. Tri-axial compressive strength is 2-5 times of the uniaxial compressive strength for plastic concrete. Based on the analysis and using the related strength theories, the formulae with two parameters, three parameters and four parameters were proposed respectively to express the failure criterion of plastic concrete under true tri-axial compression.