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.

WHY WILL NEED vibrate CONCRETE MIXTURE?

From the quality of the concrete placement largely depends on his strength, and hence the durability facilities. The quality of installation, in turn, depends on the workability of the concrete mix. A workability controlled by the amount of water in the concrete mix and internal friction.Not to introduce a mixture of excess water, it was necessary to dilute the mixture at the time of installation. Of the many methods offered the most effective was vibrating, destroying the internal friction of the concrete mix.

How is vibration destroys the internal friction of concrete? To understand this, we perform the following experiment. Put on the table a cube made of concrete. To make the cube to slide on the table, you need to make him a force to its relation to the mass of the cube exceeded the coefficient of friction of the cube on the table surface. If this table together with a concrete cube to put on vibroplatform and tell him impulses - impulses, the cube will slide across the table. After adhesion to the surface of the cube table with shaking weakens, then decreases the coefficient of friction. Thus, the vibration allowed to pass through a heavy cube. "Mechanism" of the vibration is quite simple: under the influence of vibration cube receives impulses - impulses to throw it up. Separated from the surface of the table for a short period of time, the cube jumps. Consequently, its movement will be composed of a sequence of small jumps, each of which he will move a certain distance.

How then is the process of vibration? On concrete cube, placed on a concrete board the force of friction, impeding the independent movement of the cube. To make the cube to slide on the surface of the board, you have to put some force or significantly increase the angle of the board. Well, if you bring the board into a state of vibration, the cube will bounce, and then slip even at very small inclination board. We return to the concrete mix. What happens to it when the vibration? Internal friction in it is because the surface of the aggregates in contact with each other. With stirring, they rub against each other and the more friction surfaces, the greater the overall coefficient of internal friction. Vibration is a concrete mix to reduce or eliminate these contacts and reduce internal friction. In other words, the vibration of "thins" concrete mix. And, therefore, the mixture acquires the ability to easily fill in forms and squeeze out the air contained therein. I must say, more important is the frequency of vibration. It can vary within wide limits, depending on the type of vibrator.

Vibration frequency differentially affects the grain filling of various sizes. In concrete aggregates of various sizes surrounded by mortar and swing like a pendulum with a certain natural frequency. The frequency of vibration of concrete should be chosen depending on the size of the aggregates. The size is determined by the aggregate nature of vibration aggregates of different sizes at low and high frequencies.

The most appropriate action to expose the concrete mix several vibrators with different frequencies of vibration. In this case, aggregates of different sizes move with varying intensity, and the concrete will be compacted uniformly.

ARE THERE OTHER WAYS TO SEAL CONCRETE CM E C?

For many years, builders are looking for the best method of laying concrete with a minimum amount of mixing water. In addition to the vibration of concrete, there are other effective methods of compaction. They are called by a mechanical dewatering. These include: pressing, centrifugation and vacuum drying. All of these methods are a general principle: the concrete mixture is kneaded in water in an amount sufficient to ensure that its installation could be conducted without difficulty. And after placing unnecessary for curing water in some way removed from the concrete.

The simplest method of dehydration is pressing. Its mission - to squeeze out the excess water from the concrete before it is laid in the case.To do this, one of the walls of the mold made of a porous, permeable to water and impermeable cement. The porous wall must have high strength. At high pressure in the concrete surface water is pushed through the pores of the concrete wall and compacted. This process is similar to wringing the washing machine. The disadvantage - its duration.

And what is the centrifuge method? Under this method, the concrete mixture is placed a cylindrical tube, rotating at high speed. Centrifugal force throws the placeholder on the wall forms. Water, as the smooth, gets in the center forms, from dripping. Concrete is also located on the inner wall forms a dense layer of uniform thickness with a minimum of water. This method gives a very high strength concretes. With his help produce concrete pipes and poles for power lines.

Rather perfect way to dehydration is a vacuum. Of laid concrete removed excess water through the permeable wall formwork. On the outer surface of the casing to vacuum.

Suppose you want to make a flat horizontal plate to the formwork. At the beginning of a concrete mixture with enough for easy installation of water fill the formwork. At the top of the free surface of fresh concrete formwork set vacuum shield, that is mounted on a frame with her strong bars, wire mesh and cotton filter. The upper bound of the frame hermetically closed sheet metal. Thus formed cavity is attached to a vacuum pump. The shield is made airtight by the line of contact with the surface of the concrete. To control the vacuum to vacuum the wiring at some distance from the input board is connected with a pressure gauge. To discharge pipe connected slop tank which receives water is sucked out of concrete.

Is evacuated from the concrete sucked the excess water. The mixture is compressed and reduced in volume. As a result of rapidly increasing the mechanical strength of concrete - the increment of concrete strength due to evacuation is 50 - 70%.

How much should harden the concrete?

So. Concrete prepared and packed into the form and dehydrated.

Now he has to harden and gain strength. After the concrete has seized, he is solid, but not strong enough.

Put it in the water or will continually wet, and the concrete strength will grow! How can this be explained? When wet it will occur chemical processes. They turn the minerals that make up the cement grains in the new stable education - hydrosilicates potassium. This conversion process is very long, and it can take place over the years. But many builders can not wait!

Therefore the set deadline hardening, after which concrete can be subjected to the design load. For concrete made in construction and hardening under natural conditions, such period is 28-30 days.

In some cases, you can allow a longer period of hardening concrete - in the construction of marine structures, dams, dikes, embankments, bridges, etc. They are built very slowly, and therefore the total load to the poured concrete can be applied over a long time. In these cases, the calculations may take into account 90-days strength of concrete, it is about 20% above the 28-day.

But after the target date set concrete continues to harden and gain strength, however, is much slower. This process is slow hardening in the calculations is not considered. Increase the strength of concrete in time, exceeding the established target dates hardening, turns out to be a guarantee of the reliability of concrete and reinforced concrete structures.

High temperatures (80-90 º C) speed up chemical reactions in concrete. For example, if the concrete steamed, ie warm in wet conditions at a temperature 12-16 hours, you can get concrete with a strength equal to 65-70% of the 28-day strength of the concrete. It was well received by the factory manufacturing concrete products. And if you raise the temperature even more? Does accelerated curing of concrete? Yes, so much so that at 170-180є C for 12-16 hours are the same as the concrete strength increases, which will exceed the annual level of strength.However, with such a strong heating of concrete dries quickly and becomes harden. This is explained by intense evaporation of water contained in the concrete. To "slow down" the evaporation of water, it is necessary to provide a heating chamber (autoclave) high vapor pressure (about 0.8 - 1.2 MPa, or 8 - 12 atm.). This process is called Hygrothermal Processing Park the pressure or autoclaving concrete. In this case, you can replace cement lime and coarse aggregate - sand without sacrificing product quality.

Дата добавления: 2015-08-27