Home ยป Concrete

Materials for making concrete

Cement - hydraulic binder while agitating with water and curing for a certain period in the air or the water is converted into water-insoluble material.

For heavy concrete used Portland cement and its variants, as well as aluminous cement and other binders that meet the requirements of the relevant state standards.

Mark cement prescribed depending on the design of the brand of concrete compressive strength:

Concrete grade M150 M200 M250 M300 M350 M400 M450 M300 M600 above
Brand cement M300 M300
M400 M400 M400
M500 M400
M500 M300
M600 M550
M600 M600 M600

If the brand of cement higher than that recommended for the concrete, it is necessary to dilute the highly active cement production from fine supplements, to avoid overspending of high-quality cement.

Fine aggregate
As a fine aggregate used in concrete heavy sand consisting of grains of 0,16-5 mm and having a density greater than 1.8 g/cm3. To prepare the heavy concrete used natural sand, formed as a result of natural destruction of rocks, as well as artificial, obtained by crushing hard rock, and from screenings.

Natural sands are a loose mixture of grains of different minerals that made up of igneous (rarely sedimentary) rocks (quartz, feldspar, calcite, mica, etc.).

Quality of sand used for the manufacture of concrete, determined mineral composition, grain structure and content of harmful impurities.
The filler should be composed of grains of different sizes (different fractions), while the number of large, medium and small grains (i.e. grain aggregate composition) is established on the basis of proven recommendations so that a smaller grain positioned in the voids between larger. The smaller the grain aggregates are arranged, the less void volume.
Grain (grain size) determine the composition of the sand sifting the dry average sample (1000 g), using a standard set of sieves with mesh sizes 5; 2.5; 1.25; 0.63; 0.315; 0.16 mm. Fine sand particles (dust) are less than 0.16 mm. The gravel sand grains from 5 to 10 mm may be less than 5%, the grains larger than 10 mm - there should be.
To estimate the size of sand used dimensionless parameter - module size, which is calculated as the ratio of total residues on the sieves, to the entire sample, taken as 100.

Depending on the composition of the sand grain is divided into large, medium, small

Fine particles (dust, silt, clay) increase the water demand of concrete mixtures and consumption of cement in concrete. Therefore, the content in the sand grains passing through a sieve of 0.16 mm should be less than 10% by weight, the amount of dust, silt and clay particles, determined by elutriation must not exceed 3%. Clay swells when wet and expands when it freezes, reducing frost.

The sand is cleaned of fine particles by washing.
Classification by size of sand
Group sand Full sieve residue with a mesh of 0.63 mm,% fineness modulus
Close 50-75 3,5-2,5
Average 35-50 2,5-2
Small 20-35 2-1,5

In the natural sand and gravel may contain organic impurities (e.g., degradation products of plant residues), in particular, organic humic acid, which lowers the strength of concrete and even destroy the cement. The presence of organic impurities is determined by the colorimetric (color) method.

The coarse aggregate
As a coarse aggregate for concrete used gravel, gravel with a grain size of 5-70 mm. When concreting massive structures can be used rubble of up to 150 mm.

The grains of gravel have rounded shape and smooth surface, the personal composition of the grain usually contains gravel in a given amount of sand and contaminants - clay, dust, mica, humic substances (organic matter).

Crushed get crushing igneous, metamorphic, dense and water-resistant sedimentary rocks (dense limestone, sandstone, and others.). Grains of rubble have an angular shape; it is desirable that the form they approached the cube. Rougher than the gravel surface of the grains helps improve their adhesion to the cement stone, so high-strength concrete (M500 and above) are commonly used gravel instead of gravel.

The quality of the coarse aggregate is determined mineral composition and properties of the original rock (its durability and resistance to frost), the aggregate grain composition, grain form and content of harmful impurities. The strength of the original rock under compression in water-saturated state should at least 1.5-2 times higher than the grade of concrete.

Frost crushed stone and gravel should ensure receipt of the brand design of concrete frost resistance. Established brand rubble and gravel for frost from 15 to 300. Make denotes the number of cycles of freezing and thawing, at which the loss in weight of the sample of coarse aggregate does not exceed 5% (for grades 15 and 25 may be the weight loss of up to 10%).

Grain composition of coarse aggregate is set taking into account the highest D and d of the smallest grain size crushed stone or gravel. The largest size of the grain during the concreting of the reinforced concrete beams, columns, frames should be no more than the smallest distance between the reinforcement rods, and for slabs and pavements - no more than the thickness of the plate.

The smallest particle size corresponds to the size of the smallest holes of the sieves through which no more than 5% of the sieved sample; usually the smallest particle size is 5 (3) mm.

Depending on the grain size crushed stone, gravel divided into four fractions: 5-10 mm, 10-20 mm, 20-40 mm and 40-70 mm. Crushed stone, gravel can come in the form of mixtures of two or more fractions. By agreement between the supplier and the consumer can be used gravel fractions 3-10 mm, 10-15 mm (or 5-15), 15-20 mm.

Depending on the form of grains shall be three groups of rubble from a natural stone: cuboid, improved and regular. Content of lamellar (flakiness) and needle shape therein does not exceed, respectively 15, 25 and 35% by weight. By flake and needle grains include those in which the thickness or width less than the length of 3 or more times.

The content of dust and clay particles are allowed depending on the kind of source rock, and crushed stone and gravel brand strength. The amount of dust, clay and silt particles, determined by elutriation, in the gravel and rubble are allowed no more than 1%.

The content of organic impurities in the coarse aggregate is checked using the same procedure as that applied to the sand. Gravel and crushed stone in the processing of aqueous sodium hydroxide to give a solution should not be darker than the reference color.

Water
Water used for mixing of concrete and concrete irrigation should not contain harmful impurities that impede the setting and hardening binder. For the mixing of the concrete mix used tap drinking water and natural water (rivers and natural reservoirs) having a pH of at least 4, containing no more than 5600 mg/l of mineral salts, including the sulfates are not more than 2700 mg/l. He is allowed to apply the marsh, as well as domestic and industrial waste water without cleaning them.

Additives for concrete
Depending on the purpose (the main effect of action) additive for concrete are divided into types.
Regulating properties of concrete mixtures:
plasticizers;
stabilizing;
water retention;
improve pumpability;
regulatory retentive concrete mixtures;
retarding accelerate setting;
porizuyuschie (for lightweight concrete): air-entraining, foaming, azoobrazuyuschie.
Regulatory hardening concrete:
delaying hardening,
accelerating hardening.
Increases strength and (or) corrosion resistance, frost resistance of concrete and reinforced concrete, reduce the permeability of concrete:
water reduction, bridging, blowing, air-entraining, increase the protective properties of concrete in relation to the steel reinforcement (steel corrosion inhibitors).
Gives concrete special properties:
antifreeze (providing hardening at low temperatures);
repellent.
Superplasticizers

Superplasticizers in most cases are synthetic polymers: melamine derivatives or naphthalene sulfonic acid (C-3); Other additives (IPA OP-7 and others.) derived from the secondary products of chemical synthesis. Superplasticizers introduced into the concrete mixture in an amount of 0,15-1,2% by weight of the cement concrete mix liquefy to a greater extent than conventional plasticizers.

The plasticizing effect lasts for 1-1.5 hours after administration of the additive, and after 2-3 hours it is already low. In an alkaline environment, these additives are transformed into other substances which are harmless to the concrete and reduce its durability.

Superplasticizers allow the use of an injection molding method for manufacturing concrete products and concrete structures using concrete pumps and pipe transport of concrete mixture. On the other hand, these additives make it possible to significantly reduce the W/C, keeping the mobility of the mixture and to produce high-strength concrete.

full version