Experimental studies of fiber concrete creep

The results of two-stage experimental studies of the strength and deformation characteristics of fibrous concrete reinforced with steel fiber. In the experiments we used steel fiber with bent ends, which practically does not form "hedgehogs", which allows to achieve an even distribution of the fiber by volume. At the first stage, the cube and prismatic strength, deformability at central compression, a number of special characteristics are determined: water absorption, frost resistance, abrasion; the optimal percentage of fiber reinforcement and the maximum size of the coarse aggregate fraction were selected. Fiber reinforcement led to an increase in the strength of concrete at compression by 1,35 times and an increase in the tensile strength at bending by 3,4 times. At the second stage, the creep of fibrous concrete and plain concrete of similar composition at different stress levels was researched. Creep curves are plotted. It is shown that the use of fiber reinforcement leads to a decrease in creep strain by 21 to 30 percent, depending on the stress level

Study of Dry and Wet Cement Mortar Dynamic Properties

We performed the dynamic tests and studied the dynamic properties under compression of dry and wet cement mortar specimens using the Hopkinson split pressure bar method. As a result, we determined the fracture stresses and their dependence on the applied loading growth rate. It is noteworthy that the strength characteristic of wet material under study is less by 10-15% as compared to the dry one.С использованием метода разрезного стержня Гопкинсона проведены динамические испытания на сжатие образцов цементного раствора в сухом и увлажненном состоянии. Экспериментально определены разрушающие напряжения и их зависимость от роста скорости приложенной нагрузки. Отмечено, что прочность сухого материала на 10...15% превышает прочность материала в увлажненном состоянии.За допомогою методу розрізного стрижня Гопкінсона проведено динамічні випробування на стиск зразків цементного розчину в сухому і зволоженому стані. Експериментально визначено руйнівні напруження й їхню залежність від росту швидкості навантаження, що прикладається. Відмічено, що міцність сухого матеріалу на 10...15% перевищує міцність матеріалу у зволоженому стані

Structural-temporal approach for dynamic strength characterization of gabbro-diabase

This paper presents some of the results of dynamic compression, splitting and direct-tensile tests of gabbro-diabase using two split Hopkinson bar apparatus. The results of laboratory experiments on specimens without and with notch for a wide range of loading parameters are given. Strength and deformation characteristics of gabbro-diabase were determined experimentally at high strain rates up to 103 s-1. Mechanical characteristics were found to be strongly depended on the strain rate. A uniform interpretation of the rate effects of fracture of the tested gabbro-diabase is given on the basis of structural-temporal approach based on a set of fixed material constants. It is shown that temporal dependences of both the compressive and tensile strengths of the tested material can be effectively calculated using the incubation time fracture criterion

The numerical study of fracture and strength characteristics of heterogeneous brittle materials under dynamic loading

In the present paper the multiscale approach to the construction of rheological models of brittle materials with heterogeneous internal structure is implemented within the numerical method of movable cellular automata. An example of application of the implemented approach to determine the mechanical properties (including parameters of the rheological model and strength) of brittle materials with complicated internal structure at both macroscopic and mesoscopic scales is described. The effect of strain rate on brittle materials behavior under uniaxial compression and tension is analyzed

The numerical study of fracture and strength characteristics of heterogeneous brittle materials under dynamic loading

In the present paper the multiscale approach to the construction of rheological models of brittle materials with heterogeneous internal structure is implemented within the numerical method of movable cellular automata. An example of application of the implemented approach to determine the mechanical properties (including parameters of the rheological model and strength) of brittle materials with complicated internal structure at both macroscopic and mesoscopic scales is described. The effect of strain rate on brittle materials behavior under uniaxial compression and tension is analyzed

Surface Elasticity for Applications to Material Modelling at Small Scales

We discuss the various applications of the surface elasticity to determination of effective properties of materials and some related phenomena as surface wave propagation. In the frame of surface elasticity in addition to the constitutive relations in the bulk we independently introduce constitutive relations at the surface. Nowadays the most popular models of surface elasticity relates to the models by Gurtin and Murdoch and by Steigmann and Ogden. First we discuss some useful surface elasticity models. The corresponding boundary dynamic boundary conditions are derived at the smooth parts of the boundary as well as at edges and corner points. Let us underline that these conditions include also dynamic terms. As a result, we have here a dynamic generalization of the Laplace-Young equation known from the theory of capillarity. Second, we discuss the influence of the surface stresses at the effective stiffness parameters of layered plates and shallow shells. For small deformations we derived the exact formulae for modified tangent and bending stiffness parameters of the plates and shells. The influence of residual surface stresses is also discussed. Unlike to previous case where surface stresses are slightly changing the material properties, there is another example of essential influence of surface properties. This example relates to the propagation of anti-plane surface waves. We discuss some peculiarities of the wave propagation

Mechanical Response Change in Fine Grain Concrete Under High Strain and Stress Rates

International audienceExperimental results on assessing the effects of strain and stress rates on the behavior of fine-grain concretes are presented. Specimens of fine-grain and fiberreinforced concretes were dynamically tested using the Kolsky method and its modification, the “Brazilian test”. As a result of the experiments, values of the Dynamic Increase Factor (DIF) were determined for both the materials studied. Their curves as a function of strain and stress rates were constructed. The experimental data is compared with the theoretically obtained values of DIF as a function of strain rate available in the literatur