Rheology helps to disclose secret

What happens when laying concrete in the form? Why depend Diverging cone? Of plasticityedeformation or separation of particles in the transverse direction? These phenomena are observed in the same concrete mix with different amounts of water ... Unclear PreachandHN more or less brittle concrete. Beton mixture of hard keeping secrets of his behavior when panning.

Attempts to solve the mystery with the old methods of investigation ended unsatisfactory achami. We needed a new approach, a new criterion. And help came from physics, but rather one of its sections - rheology. Once she was able to clearly identify the physical nature of workability.

Thus, the rheology! What is she doing? This is a completely new direction in mechanics. It is associated with the development of the theory of elasticity. It studies the behavior under load of wet materials, which can not be attributed to a solid nor liquid. These materials include concrete mix and that is a so-called visco-elastic medium. To determine how the material deforms under load, mechanics use structural mechanical models. They allow you to mimic the internal structure of the material.

How does the structural model? Suppose, to the solid load is applied. Under the influence of the deformation occurs in the body. This means that the body will deform in proportion to the applied load (or the law of proportionality of stress and strain Hooke). Once the load is removed, the body regains its original shape.

What happens if we are dealing with materials that have complex properties and, in the elastic characteristics are also inelastic? Here, the structural mechanical model is no longer suitable. It may not accurately mimic the internal structure of these materials.

For this purpose, require different mechanical models, which are called rheology. They differ in that they are composed of a combination of two elements that mimic the two basic properties of solids: elasticity and viscosity. The simplest body - elastic. Dependence of strain and stress for him is expressed by one curve for the processes of loading and unloading. Just remove the stress and strain arising disappear. Well, in an ideal viscous body? Indeed, the presence of viscosity of the material leads to permanent deformation, which increases indefinitely with a decrease in the rate of loading. For a perfectly viscous element apply the law of deformation of a viscous fluid.

To create a rheological model of the spring and the "shock absorber" (model viscoelastic deformation) can be combined in series or in parallel. These combinations allow the best way to simulate the mechanical properties of any real material.

Rheological models provide the necessary information about the changes of the internal structure of the real body under load. This information includes the characteristics of internal friction, viscosity, and adhesion (adhesion).

What is the rheological model of concrete? The concrete mix is ​​the so-called two-phase material. This means that it contains the elements of the two phases - solid and liquid. And if so, how to better reflect the internal structure of the concrete?

Carry out some analysis. Let's start with the internal friction. This is one of the important characteristics of a viscoelastic body. Internal friction describes the solid phase material. If the material internal friction is zero, then it can be considered an ideal fluid. The concrete mix has internal friction. It would seem that on this basis it can be attributed to the solid. However, the presence of water in it makes it all the same intermediate material between the liquid and the solid. If so, then the rheological model of concrete mix should include both elastic and inelastic components.

Hence, rheological model of concrete will be a "spring-loaded" solid structure, the pores of which are filled with a viscous fluid (cement paste).Finally, the last question. What should be interconnected elements? Since the concrete mix - is a two-phase material, the better it will be simulated combination of both elements. How will simulate rheological model of the concrete mix during solidification? While mixed concrete is still wet, it is a viscous liquid. At this stage, it is dominated by the liquid phase. But here's cement paste begins to harden. With increasing strength of the viscosity of the mixture decreases, but increases flexibility, and with it, and internal friction. And if there was internal friction, then it is a sign of the solid material. Now, create the load. Under the influence of the load in the rheological model will be both reversible and irreversible processes leading to corresponding strain. Under the influence of some part of the burden of mechanical energy exerted on the concrete mix will be converted into heat. This is - a consequence of the internal friction. The heat will be created in the spring, which will heat up during compression. This heat they emit into the environment. As for the shock, then it any permanent deformation. Under load as a result of viscous friction dampers will also heat the viscous liquid. Thus, the characteristics of concrete depend on the phase in which there is concrete.

What we found through rheological models? First, that the behavior of concrete depends on such viscoelastic properties as internal friction, adhesion and the work of destruction in shear. These physical characteristics decipher the concept of "workability". Second, we found that aggregates and cement paste, part of the concrete mix, usually located on the border of viscoelastic and plastic phases. Therefore, different ratios of aggregate and cement will affect the properties of the various concrete mixes. Third, we were able to identify all the physical characteristics of the concrete mix.

For example, the internal friction of the concrete mix can be determined by the coefficient of internal friction. It turned out that for aggregates obtained by crushing, its value is greater than the aggregate round. When the content of the solution and increase the quantity of water decreases. Viscosity concrete mix directly proportional to the internal friction and depends on the water content.

Knowledge of the physical characteristics of the concrete expands the meaning of the term "workability". The rheological properties of the concrete mix, characterizing workability complement the concept. They made it possible to imagine the whole mechanism of laying concrete.