Properties of concrete

INTRODUCTION

Currently, concrete and reinforced concrete are the primary building materials. In our country, producing tens of millions of tons of concrete and reinforced concrete, or a house built in the last 60 years is not without concrete products. Therefore the theme of concrete and reinforced concrete is today very important. It is important to know not only the technology of production, but to be able to reduce costs.

In my work I have tried to briefly describe the manufacturing process and some of the technology on the economy, because the cost of housing depends on the cost of construction materials. In addition, I tried to discover the essence of resource-saving technologies, offered some ideas and made a brief review of foreign saving technologies.

PROPERTIES OF CONCRETE

The most important property of concrete is its strength, ie, the ability to resist external forces being destroyed. Like natural stone, concrete is best resists compression, so the criterion of concrete strength builders took tensile strength of concrete in compression. To determine the strength of concrete, it is manufactured from reference cube with edge 200 mm when collapsed under a load of 80 tons, the compressive strength is 20 MPa.

Depending on the compressive strength of concrete is divided into grades. Concrete grade builders determine the ultimate strength of the reference to the cube with an edge of 200 mm. Thus, in Russia in the construction of concrete used the following brands: "600", "500", "400", "300", "250", "150", "100" and below. Choice of brand depends on the conditions in which the work will be concrete.

Concrete strength depends on the strength of stone aggregate (crushed stone, gravel) and the quality of dissolved Cement concrete is the stronger, more solid stone aggregates and the better they will be held together with cement glue. The strength of natural stone does not change with time, but the strength of concrete increases with time.

Another important property is the concrete density - the ratio of the mass of material to its volume. The density of concrete is always less than 100%.

Density greatly affects the quality of the concrete, including its strength: the higher the density of the concrete, so it is more durable. The pores in concrete, tend to appear in its manufacture: as a result of the evaporation of excess water, without entering into a chemical reaction with the cement as it hardens, the shortage of cement.

Since the density and the associated opposite property of concrete - porosity - the ratio of the pore volume to the total volume of the material. Porosity as it complements the density of concrete and 100%. No matter how dense concrete, it always has pores!

Water resistance - properties of concrete to resist the action of water without being destroyed. To determine the water resistance of concrete, manufactured two models: one in dry crushed by the press and determine its normal strength. Another sample was previously immersed in water, and after saturation with water also destroy the press. Because of the weakening of bonds between the particles decreases the strength of the sample. The ratio of the strength of water-saturated sample to the strength of the sample in a dry form factor material softening. For concrete, it is greater than 0.8. Therefore, concrete is waterproof and can be used for the construction of structures exposed to water - dams, piers, breakwaters.

Thermal conductivity describes the ability of concrete to pass through its thickness heat flow, which results from the temperature difference between the surfaces of the concrete. Thermal conductivity of concrete is almost 50 times lower than that of steel, but higher than that of a building brick.

The relatively low thermal conductivity of the concrete provides a high fire resistance - the material's ability to withstand high temperatures.Concrete can withstand long-term temperatures above 1000 ° C. In this case it does not break or crack.

Everyone knows that if the water penetrates the pores of rocks, then freezes it expands and thus destroys even the toughest rock. Concrete at the same water saturation can withstand repeated freezing and thawing. In this case, it is not destroyed, and almost does not reduce its strength. This property is called m of rozostoykostyu.

And here is one more property of concrete - volume weight. have concrete bulk density can be equal. It depends on the aggregate used in the concrete. According to this feature concretes are divided into three types: heavy, light and very easy. This classification depends on the mass of filler used in the manufacture of concrete. For example, the concrete on natural aggregates of granite, limestone, dolomite has a bulk weight 2200 - 2400 kg / m³, and its strength is 60 MPa (or 600 kg / cm ²). Such concrete called heavy concrete. But the concrete to rubble from the lungs of rock (pumice or tuff) has a lower volume weight - usually 1600 - 1800 kg / m³ and is called lightweight concrete. If the concrete is manufactured by artificial light porous aggregates of baked clay before sintering materials, such as clay, agloporit, slag pumice, ash and gravel, etc., you can get a whole range of different light concrete bulk density - up to 1800 kg / m³. Their strength ranges from 7.5 to 40 MPa (75 to 400 kgf / cm ²).

Application in the construction of a heavy or lightweight concrete is determined by the type of construction and its mode of operation.

By appointment concretes are divided into ordinary concrete - for the manufacture of columns, beams, slabs and the like structures; concrete hydraulic engineering - for dams, locks, canal lining, concrete for underground structures - for pipe wells, tanks, concrete for roads; concrete special-purpose special types of cement - Acid, heat resistant, etc.

"Armed" CONCRETE

Speaking of concrete, we must not forget about the concrete. Because of his exceptional qualities it is widely used in modern construction.Reinforced concrete - concrete is injected with steel bars - fittings. The word "fixtures" - the Italian word in Russian means "arms." Why it took "arm" or, as the experts say, "reinforced" concrete?

In the construction of building structures are compressive and tensile, which influenced the design deformed. Very clearly the two forces can be, if you take an ordinary rubber band, put it on two legs and press it in the middle of compressed elastic at the top, but it will stretch at the bottom. On average, the length of the elastic band is not changed. Is the imaginary line that separates the gum into two parts - the compressed and stretched, called the neutral axis. When operating a concrete structure in bending an analogous picture of her deformity.

Since the tensile strength of concrete is small, the concrete structures bending collapse at a very low load. The strength of a steel rod in tension in the 100 - 200 times higher than that of concrete. So, if we make the two materials (concrete and steel) to work as a unit, that is, to achieve the same strength in the compression zone and the tension zone of a flexible concrete structure, it can be increased by several times the strength of structures in bending. To do this, enter part of the stretched several steel rods (rebar) of a particular section. Now the concrete structure does not break down at a bend and can withstand many times the failure load.

How then can work together in one design, two such dissimilar materials such as concrete and steel?

It turns out that help their properties: high compressive strength, high strength reinforcing steel in tension, a large force of adhesion of concrete and steel, and nearly the same length change of concrete and steel with temperature.

Thanks bonding reinforced concrete, it can not pull out of the concrete. Hardening concrete shrinks and compresses the valve, and thus more firmly adhere to it. Bond strength reinforced concrete will increase with time, and the more, the closer the more concrete and the surface roughness of the valve.

The relatively low thermal conductivity of concrete have been very useful for concrete structures: concrete protects the steel rebar from sudden temperature changes.

Reinforced concrete as a building material has appeared only in the mid XIX century, but it is widely used in all areas of construction.Reinforced concrete structures combine high strength, lightness and elegance. As an example we can take a beautiful two-story Metro Bridge in Luzhniki (Moscow).

Дата добавления: 2015-08-27