Stress-strength Ratio Research Articles

コンクリート構造物の温度ひびわれ危険度評価に関する研究

In order to estimate the possibility of thermal cracking of concrete structures, various thermal crack evaluation indices have been proposed. These indices are calculated from the results of thermal stress analysis. However, these indices include a certain amount of error and the crack evaluation contains a certain extent of uncertainty, because the calculation of stress analysis has a certain inevitable error and, in addition, cracking has a probabilistic nature. In this study, thermal stress analysis was made for 43 cases of mass concrete structures with and without thermal cracking by finite element method. Effectiveness of the crack indices in evaluating the thermal crack occurence was fully examined. Three crack indices were adopted, which were defined by stress-strength ratio, strain-strain capacity ratio and thermal crack index proposed by JSCE.

Compression creep behaviour of steel fibre reinforced cement composites

The paper presents the results of an experimental investigation to determine the influence of steel fibre reinforcement on the creep of cement matrices under compression. Creep tests were carried out at a number of applied stress-strength ratios ranging between 0.3 to 0.9. Melt extract and hooked steel fibres were used at volume fractions ranging between 0 and 3% by volume of a mix. Three types of cement matrices were used namely cement paste, mortar and two mix proportions of concrete.

A theory for the creep of steel fibre reinforced cement matrices under compression

The paper presents a theoretical model to predict the creep of cement matrices reinforced with randomly oriented discrete steel fibres. The theory considers the composite to be represented by an aligned steel fibre which is surrounded by a thick cylinder of the cement matrix. The fibre provides restraint to the flow component of creep of the matrix through the fibre-matrix interfacial bond strength. The delayed elastic strain component of creep is unaffected by the fibre. The fibre-matrix interfacial bond strength,τ, is shown to be primarily a function of the shrinkage of the cement matrix and the radial deformation caused by the sustained axial stress. In addition, the state of stress in the matrix at the interface is suggested to influence greatly the bond strength,τ. The validity of the theory is established by means of experimental data on concrete and mortar matrices reinforced with melt extract and hooked steel fibres, at sustained stress-strength ratios of 0.3 and 0.55. Finally an empirical expression is derived to determine the creep of steel fibre reinforced concrete, based on a knowledge of the creep in unreinforced matrices and fibre size and volume fraction.

Effects of Loading Rate on Cracking of Cement Paste in Compression

ABSTRACTSubmicroscopic cracking within cement paste is studied for monotonic and short-term sustained loading. Loading times range from 1.25 minutes to 4 hours. Water cement ratios of 0.3 and 0.5 are used. Specimens are loaded to preselected strains and immediately unloaded. The specimens are then sliced with a diamond saw and viewed in a scanning electron microscope. Crack traces are cataloged based on length, width and orientation. The crack traces are converted to three dimensional crack distributions, which are in turn used to determine the degree of softening due to cracking. The results indicate that for loading to a fixed strain, slow loading results in less cracking than rapid loading. However, at the same stress-strength ratio, rapid loading results in fewer cracks. The results indicate that the role of submicroscopic cracking in the nonlinear behavior of cement paste increases with strain rate.

Effect of degree of hydration upon creep of mortars containing calcium lignosulphonate

Synopsis The influence of calcium lignosulphonate admixture upon the creep characteristics of two mortars, made with cements having different C3A contents, has been studied. The degree of hydration of the mixes at loading and throughout the loading period was the main parameter. For any mix, with or without admixture, the creep strain is the same for a given increase of hydration under load, provided that all the mixes are loaded at the same degree of hydration and the same stress-strength ratio. This suggests that the age at loading only affects creep in so far as it is related to the degree of hydration and development of strength. Creep after a certain time has two components: ‘non-hydrating creep’, which corresponds to the state of hydration at loading without further hydration under load, and ‘maturing creep’, which is related to the progress of hydration while under load.

気温変動とコンクリート構造物の収縮ひびわれとの関係 : その 2 : 気温変動に対応するコンクリートの引張 (曲げ引張) クリープについて

In Part 1, atmospheric temperature variation as a cause of concrete cracking was reported. In this paper reported are the test results of creep of concrete in flexure subjected to increasing stresses in short-term (12 hours) loading corresponding to the short-term variation of atmospheric temperature. For the experimental study of the creep, prisms of 10×10×40cm were used. They were made of ordinary Portland cement, river sand and river gravel with three water/cement ratios of 0.45, 0.55 and 0.65 respectively. A part of the specimens was cured under water at the temperature of 20 degree centigrade until the test age of 3, 7, 28, 91 and 180 days. The rest was stored in the dry air at the temperature of 20 degree centigrade and 50% R. H. until the test age of 28, 91 and 180 days. The specimens were subjected to increasing stresses by stepped method with maximum stress-strength ratio 0.8. The results obtained are as follows : The creep coefficient of the specimens cured under water at the temperature of 20℃ until 28〜180 days were 0.4〜0.6, and that cured at 20℃, 50% R. H. until 28〜180 days were 0.1〜0.3. The creep coefficients mentioned above were much smaller than that obtained from the specimens subjected to a constant stress for a long time.

Lateral creep of maturing cement paste

This investigation was conducted to study systematically the lateral creep of maturing cement paste under constant stress-strength ratio. The age at loading ranged from 0.75 to 28.75 days. It has been found from these tests that lateral creep is much more for maturing cement paste than for mature cement paste. When hydration is stopped by cooling to −11°C, the creep values are the same. The creep Poisson's ratio values tend to be higher for earlier age of loading.

Effect of sustained loading at early ages on the modulus of elasticity of cement paste

Conventional tests to determine modulus of elasticity are usually associated with the effects of loadings of very short duration. However, continually increasing load during the early age can produce an increase in the modulus of elasticity due to a form of body compaction by «maturing» creep. The paper compares the results of a test in which a specimen was loaded at 1 day, kept at a constant stress-strength ratio and periodically unloaded and reloaded and a series of specimens were loaded at various ages ranging from 1 day to 29 days. Low temperature tests where hydration was stopped by cooling to −11 °C are also included.

The effect of sorption on the tensile creep and strength reduction of desiccated concrete

The influence of sorption of fluids of differing molecular size on the strength and tensile creep of desiccated concrete and mortar are investigated. It is shown that the desiccated tensile strength is significantly reduced by sorption in accordance with the molecular sieve concept; compressive and splitting tensile strengths are also reduced but to lesser extent. For specimens subjected to a constant applied tensile load, strength reduction increases the effective stress-strength ratio thereby contributing to creep. In some cases, the concomitant effect of strength reduction and sorption induced deformations results in specimen fracture.

A hypothesis on mechanism of maturing creep of concrete

Four mechanisms are proposed to explain why maturing creep is not solely dependent on stress-strength ratio. The mechanisms are osmotic pressure effects, drying creep, gel following the hydrating cement grains and deposition of new gel in an unloaded state. While the osmotic pressure mechanism is unaffected by the rate of hydration, the other mechanisms are very much dependent on the rate of hydration. Maturing creep decays continuously with increasing maturity as the rate of reaction is dependent on maturity.

Creep of portland cement paste at early ages

This investigation was conducted to study systematically the creep of maturing cement pastes. Two water-cement ratios were chosen. Specimens, both sealed and unsealed, were used. Tests were conducted in a controlled humidity cabinet. The specimens were loaded between 18 hours and 28 days after casting, and kept under load for 28 days. In all cases the stress-strength ratios were maintained constant at 0.15. It has been found from these tests that creep is not linearly proportional to the stress-strength ratio for maturing cement pastes. The data obtained from the investigation also show that the viscous flow mechanism, if present, can only represent a small part of the creep mechanism.

Creep of Portland cement paste at sub-zero temperature

Describes uniaxial creep tests on Portland cement paste specimens at sub-zero temperature. During the course of the tests the stress-strength ratio was maintained constant by continuously increasing the applied load. Linearity was found to hold in low-temperature tests where specimens were loaded at various degrees of maturity, but where hydration under sustained load was halted.

Basic and drying creep of concrete

From considerations of the gel compliance factor and of a creep parameter P, involving the volume concentrations of gel and of cement paste, and using a hyperbolic creep-time expression, an expression for creep of the type of Eq. (13) has been derived. This indicates that creep is proportional to the stress-strength ratio, and that it also increases with an increase in the volume of gel and, in the case of drying creep, with concurrent shrinkage. Test results on concretes made with Type I cement are presented and analyzed with respect to the proposed expression. It is shown that creep recovery is higher under wet storage conditions than at a relative humidity of 50%. The time necessary for the recovery to be complete is at least 100 days.

The relation between creep of concrete and the stress-strength ratio

The linear relation between creep and stress-strength ratio is verified for recent tests of various investigators. The relation between irreversible shrinkage and creep is discussed, and “corrected” graphs of time-deformation versus stress-strength ratio are presented.