Birth of concrete

From what makes the concrete?

When designing a new car, the designer decides what shape should be given in some of its details. He establishes in advance what should be the strength of these parts. But it will depend on the strength of the material! So you need to choose the appropriate material!

It is the same case in the construction! Builder, you must first know what properties they should have made concrete, what will be its strength, as it will operate the heat and cold.

But the composition of the concrete can not be universal. It can not be assigned to one recipe that is suitable for all occasions.

The composition of the concrete, as well as the composition of the alloy in the industry, should be designed in advance. It depends on in which the construction will be applied concrete.

To get concrete, given composition, it is necessary to develop his "recipe." Russian scientists NM Belyaev, SA Mironov, N. Popov and others developed the technology of concrete, through which it became possible to produce concrete with known properties. To do this, choose the right most favorable ratio (number) of source materials that make up the concrete. But the strength of the concrete depends not only on how much to taken its component parts, it will be important as the quality of the raw materials - the large stone aggregate, sand, cement and water. They are taken in certain quantities, and then mixed together. What qualities must have these source materials?

Let's start with the coarse aggregate - gravel and crushed stone.

Gravel - it's been run in various degrees the strongest pieces of rock (granite, diorite, basalt, dark - gray limestone) round or ovoid with a smooth surface. The size of these grains from 5 to 77 mm. In its origin distinguish gravel (ravine), river and sea.

In the mountainous gravel typically contains impurities of clay, dust, sand, organic matter, sulfur and sulfuric compounds. At sea and river gravel impurities are almost nonexistent.

Crushed stone - a material that is produced by crushing rock or artificial stone into pieces the size and from 5 to 77mm. Crushed grain are irregular in shape, their surface is rough. Therefore gravel firmly adhere to the cement stone than gravel. The strength of the coarse aggregate is particularly important, as it is, it forms the skeleton of the concrete. Therefore, coarse aggregate should be, as a rule, two to three times stronger than concrete itself.

To ensure the quality of concrete, coarse aggregate should be clean and free of contaminants. There should be no more than 15% (by mass) of the grains in the shape of needles and plates. Coarse aggregate shall not enter into chemical reactions with substances in the cement. To reduce the influence of harmful impurities, aggregates were washed before use.

By and large aggregates are porous fillers - pumice, tuff, scoria. These fillers due to their structure absorbs a lot of water. Sucking out the excess water from the concrete, they contribute to its strengthening. The drawback is that porous aggregates that concrete with such aggregates requires more cement than concrete dense aggregates.

To include a variety of fine aggregate sand. Sand called loose rocks, which consist of grains of different materials (usually quartz) from 0.1 to 5 mm.

Sands vary in mineralogical composition and depending on the education and place of occurrence. The mineralogical composition of the sands are quartz, feldspar, limestone and dolomite.

Under the terms of the formation sands divided into mountain, ravine, river, sea, gravel, boulder, dune and dune.

They differ from each other only in the floor structure and form. Grains of sea and river sand rounded, smooth, grain same mountain of sand, which is often formed by the destruction of granite and diorite, have an angular shape and rough surface. Grains of sand ravine also have an angular shape, but compared to a few grains of sand smoothed rock. All sands contain harmful for concrete admixture: coal dust, clay, gypsum, mica, pyrite and various organic impurities that affect the cement glue, reducing its strength and, ultimately, causing the destruction of the concrete. Contaminants are sulfate and gypsum particles. They form with the cement particles of specific compounds in the form of thin needles. They are often metaphorically called "cement bacillus".

By water, "cement bacillus" turns further to the liquid white mucus flowing from concrete. This "patient" concrete is not suitable for use.

Sea sand sometimes contains shells, consisting mainly of limestone. This weakens the grip of sand with the other components of the concrete.In addition, the salt sea sand are produced at the surface of the concrete.

Most pure sand - is a river. But he does not always meet the builders, as it is often very small. And this is in the manufacture of concrete requires a large amount of cement.

As well as the coarse aggregate, sand before use should always be washed with water in the machines - Sand.

To obtain a high strength of concrete, it is necessary to choose the right grain structure aggregate. This means that we should take out of them so the mixture to the grain was as little voids that have to fill cement paste. The sand has a particle size in volume of about 40% voids.Sand is composed of grains of different sizes are much denser.

Is it possible to achieve the lowest emptiness? Yes, you can.

To do this, first disperse a large and fine aggregate in size or, as the builders into several factions. Then, one by a certain rule are the so-called optimal grain mixture (the mixture, all the particles are so closely adjacent to each other that the cement paste are only minor intervals). Concrete prepared on this optimum mixture of aggregates already has a high density and strength. Flow binder in this case is very small.

If the concrete is produced on a random structure of aggregates made from natural quarries or obtained by crushing stone, the greater the density can not be obtained. This requires a huge cost overrun cement. In addition, a random mixture impossible to obtain high-strength concrete.

Water is needed to create high-strength concrete should be clean and not acidic. But even conditionally clean water contains various impurities detrimental to the process of hardening: organic acids, sulfates, grease, etc.

Usually on concrete products plant and construction sites to make use of concrete drinking water. In some cases we have to use dirt, swamp, peat and river water. But these waters are full of organic impurities. Sometimes it is necessary to apply wastewater and industrial water, which may contain significant admixture of sulfuric acid or humic acid salt or gypsum. These impurities cause the destruction of the concrete.Therefore, before you use the water they tested in a chemical laboratory.

The surface of the concrete prepared with sea water or exposed to its action in the form of stained salt haze - "efflorescence", which significantly spoil the appearance of the concrete. In addition, the strength of the concrete is low. Therefore, in the construction of residential buildings of concrete sea water use is prohibited.

Cement - is the main component of concrete. Concrete is the stronger, the higher bonding strength of cement and the harder he adheres to the surface of the filler.

Cement is made from clinker, and it is produced by roasting to baking natural raw materials or artificial feed mixture.

Such mixtures contain about three of limestone and one part clay. Sometimes these compounds are found in natural form - is a rock called limestone marl. But, since these marl deposits are rare, that in most cement plants are artificial mixture of limestone and clay. Instead of clay can be used diatomite, tripoli and other silicate rocks, similar to the clay in their chemical composition. After firing, these mixtures form solid sintered mass - clinker, consisting of grains in dark gray size of a walnut. Then the clinker is ground in a ball mill to a fine powder. To improve the quality of cement clinker during grinding is injected hydraulic additives - to 3% gypsum and 15% of diatomite, tripoli. Now the cement is ready!

What is cement? This is a very fine gray powder-like powder. The longer it is milled, the higher the quality, the more bonding capacity it has.With superfine chemical reactions are accelerated many times. This is explained by the fact that the cement powder is always connected to the water over the surface. The surface of a grain will be greater, the higher the fineness. For example, the specific surface area of ​​1 g of cement grains is 2000 - 3000 cm ², and high-strength cement - about 6000 cm ².

For preparation of concrete, concrete products and structures using various cements. Selecting the type of cement depends on the type of construction for which is made concrete. In Russia produces over 30 types of cement. The main of them - Portland, slag cement, pozzolanic Portland cement, aluminous cement, and others. Production and study of cement in the country paid much attention. In the science of cement major contribution made by Russian scientists AA Baykov A.Kind V., V. N.Yung, P. P.Budnikov.

How to mingle CONCRETE MIXTURE?

Manufacture of concrete - it's a long and difficult process. First prescription laboratory measure in the required amount of dry cement and aggregates. Then weighted components poured into the mixer at the same time supplying it with water. Mixer is driven by a motor.

The purpose of mixing - is to get out of a homogeneous mixture of granular materials. Mixing time set in advance. After stirring the raw materials form the plastic mixture, similar to the heavy liquid. Therefore, freshly prepared concrete name not concrete, and concrete. Only after some time the mixture hardens and turns to stone, and the final strength is even later. This stone is concrete.

Uniformity of concrete - one of the most important to her claims: if the mixture is heterogeneous, concrete weekdays differently in different parts of the solid construction and can easily break under load. How do you know if the resulting mixture is homogeneous or not? To do this, take a few different places in volume of samples exceeding the size of the largest grain filler. If all the

samples have the same constant composition, ie, the same amount of crushed stone or gravel, sand, cement and water, the concrete mix can be considered homogeneous.

After mixing the concrete mix must often transported from the mixer to the place of installation, it is very important that the mixture retained its homogeneity, since the transport of the mixture threatens to delamination. Why? Because grain aggregate in concrete tend to go down.Found that the delamination will be greater, the weaker the bond between the mortar and aggregate. Delamination of concrete during transport can be avoided if we continue stirring the mixture while driving in truck mixers.

Дата добавления: 2015-08-27