Selection of concrete composition for making timber marches

Contents

1. Table of contents

2. Introduction

3. Main characteristics of the article

3.1. Types, basic parameters and dimensions

3.2. Technical requirements

3.3. Acceptance Rules

3.4. Monitoring and testing methods

3.5. Marking, storage and transportation

4. Requirements for staircase materials

4.1. Cement

4.2. Water for concrete preparation

4.3. Coarse aggregate (crushed stone)

4.4. Fine aggregate (sand)

4.5. Additive (superplasticizer C-3)

5. Selection of concrete composition

6. Calculation of raw material warehouses

7. Selection of process equipment for preparation of concrete mixtures

8. Health and Safety

9. List of literature

3.3. Acceptance rules.

Acceptance of ladder elements shall be carried out in batches in accordance with the requirements of GOST 13015.1 and this standard.

Acceptance of ladder elements in terms of their strength, stiffness and crack resistance, frost resistance and abrasion of concrete, as well as in terms of waterproofness of concrete of ladder elements intended for operation in an environment with an aggressive degree of impact, should be carried out according to the results of periodic tests .

Acceptance of ladder elements in terms of strength (class or grade in terms of compression strength, release strength) of concrete, average density of light concrete, compliance of reinforcement and embedded products with standard design documentation, strength of welded joints, accuracy of geometric parameters and thickness of protective layer of concrete to reinforcement, width of crack opening, categories of concrete surface of ladder elements should be carried out according to the results of acceptance tests and control.

3.4. Monitoring and testing methods.

The compression strength of concrete should be determined according to GOST 10180 on a series of samples made of concrete mixture of working composition and stored under conditions established by GOST 18105.

When testing ladder elements by non-destructive methods, the actual concrete compression release strength should be determined by ultrasonic method according to GOST 17624 or mechanical devices according to GOST 22690, as well as by other methods provided for by standards for concrete testing methods.

The frost resistance of concrete should be determined according to GOST 10060 on a series of samples made of concrete mixture of working composition .

The waterproofness of concrete of ladder elements intended for operation in an environment with an aggressive degree of influence should be determined according to GOST 12730.0 and GOST 12730.5 on a series of samples made of concrete mixture of working composition.

Abrasion of concrete of stair elements shall be determined as per GOST 13087.

Methods of control and testing of welded reinforcement and embedded products - according to GOST 10922 and GOST 23858.

Dimensions and deviations from rectilinearity of surfaces, width of opening of process cracks, dimensions of shells, strains and edges of concrete of ladder elements should be checked by methods established by GOST 26433.0 and GOST 13015.0, GOST 13015.1.

The position of reinforcement and embedded products, as well as the thickness of the protective layer of concrete, should be determined in accordance with GOST 17625 and GOST 22904. In the absence of the necessary instruments, cutting down of furrows and exposure of reinforcement of the structure with subsequent sealing of furrows is allowed.

3.5. Marking, storage and transportation.

Marking of marches - as per GOST 13015.2. Markings and signs should be placed on the side faces of the marches facing the stairwell wall and on the non-face surfaces of the overlay treads.

The stair elements shall be transported and stored in accordance with the requirements of GOST 13015.4 and GOST 9818.

Marches (except for LMP marches) should be transported and stored in stacks in a horizontal position, while the marches should be placed steps up. The height of the stack during storage of marches and platforms should not exceed 2.5 m.

Routes with half-sites (LMP type) should be transported and stored in the rib position. It is allowed to store other types of marches in "on rib" position at their reliable fixation in this position.

Linings and gaskets between the rows of marches shall be not less than 30 mm thick and shall be installed at the locations of slinging holes or mounting loops.

4. Requirements for materials for the manufacture of stair flights.

4.1. Cement.

According to GOST 2663391 "Heavy and fine-grained concrete. Specification "includes the following requirements for binding materials.

Portland cement and slag portland cement according to GOST 10178, sulphate-resistant and pozzolan cements according to GOST 22266 and other cements according to standards and technical conditions should be used as binding materials in accordance with their areas of application for specific types of structures.

The type and grade of cement should be selected according to the purpose of the structures and their operating conditions, required class of concrete in terms of strength, grades in terms of frost resistance and waterproofness, values of concrete release or transfer strength for prefabricated structures based on the requirements of the standards, specifications or design documentation for these structures taking into account the requirements of GOST 30515, as well as the effect of harmful impurities in fillers on concrete.

The use of pozzolan cements for the production of prefabricated reinforced concrete structures without a feasibility study is not allowed.

For the production of prefabricated structures subject to heat treatment, cement of I and II efficiency groups should be used during steaming as per GOST 10178. The use of group III cements is allowed in coordination with specialized research institutes, feasibility studies and consumer consent.

Portland cement is a hydraulic binder that hardens in water or in air. It is a gray powder obtained by fine grinding of a clinker with a gypsum additive. Clinker is obtained by uniform firing before sintering of a carefully dosed raw mixture containing about 75... 78% CaCO3 and 22... 25% (CaO+Al2O3+Fe2O3). When grinding, 10... 20% granulated blast furnace slags or active mineral (silica) additives can be added to the cement clinker. The additives are added during grinding of the clinkers to give the cement the desired properties, as well as to reduce the clinker consumption and reduce the cost of cement while maintaining or slightly reducing its grade.

The true density of Portland cement is 3.1 g/cm3, the bulk density in the loose state is 1100... 1300 kg/m3, and in the compacted state is 1400... 1700 kg/m3, the angle of natural slope is 41... 420, water requirement for obtaining cement dough of normal density NG = 22... 26%. Setting time: the beginning of setting is not earlier than 45 min and the end of setting is not later than 10 hours from the beginning of closing. The fineness of the Portland cement must correspond to the passage through the screen with mesh No. 008 (the size of the cell side in the light is 0.08 mm) of at least 85%. The fineness of the milling of Portland cement can also be characterized by the specific surface of its powder, which is usually 2200... 3500 cm2/g. The thinner the Portland cement is crushed, the faster its strength increases and the higher its activity (brand). Depending on the strength achieved by 28 days of hardening, Portland cement is made of four grades: 400, 500, 550 and 600. The strength of Portland cement during compression is 40... 60 MPa, for bending - 5.5 ...... 6.5 MPa according to GOST 1017885.

According to GOST 311082003 cement is divided into five types:

- CEM I - Portland cement;

- CEM II - Portland cement with mineral additives;

- CEM III - slag portland cement;

- CEM IV - pozzolan cement;

- CEM V - composite cement.

By compression strength at the age of 28 days, cements are divided into classes: 22.5; 32,5; 42,5; 52,5.

In terms of compression strength at the age of 2 (7) days (hardening rate), each class of cements, except for class 22.5, is divided into two subclasses: H (normal hardening) and B (fast hardening).

The actual strength of the cement is called its activity.

During hardening, Portland cement produces heat, the amount and intensity of which depends on the mineralogical composition, the fineness of the grinding, the water content of the cement dough and the ambient temperature.

The frost resistance and corrosion resistance of Portland cement concrete depend on its mineralogical composition and concrete density. The lowest corrosion resistance and frost resistance in concrete on cements with an increased content of tricalcium aluminate (C3A > = 10%) [4, 5].

4.2. Water for concrete preparation.

To prepare concrete mixture, tap drinking water is used, as well as natural waters of rivers, lakes and artificial reservoirs.

Water shall not contain harmful impurities that prevent normal setting and hardening of cement and contribute to corrosion of reinforced concrete reinforcement. If concrete of reinforced concrete structures is not subjected to alternate humidification and drying, then the total content of salts in water is allowed to 35000 mg/l, and if it is subjected to it, it should not exceed 5000 mg/l. The content of sulphates in water (calcium sulfate, sodium, magnesium) based on the SO42 ion in both cases should not exceed 2700 mg/l, and the hydrogen value (pH) characterizing the acidity of water (redness of blue litmus paper) should not be less than 4.

Sea water with salt content within the above limits is allowed for preparation of concrete mixture, excluding cases when reinforced concrete structures will be in hot climate and subject to periodic humidification and if seedlings are permissible on surfaces.

It is forbidden to use water containing impurities of acids, salts, oils, sugars, as well as swamp and waste water. For watering concrete, it is recommended to use water of the same quality as for preparing concrete.

When designing a precast reinforced concrete enterprise for concrete preparation and watering of reinforced concrete products, water from the city water supply system is used [6].

4.4. Fine aggregate (sand).

Natural sand and sand from rock crushing screenings with average grain density from 2000 to 2800 g/cm2 and their mixtures meeting the requirements of GOST 8736, sand from blast furnace and ferroalloy slags of ferrous metallurgy and nickel and copper-smelting slags of non-ferrous metallurgy according to GOST 5578, as well as ash-slag mixtures according to GOST 25592 are used as fine aggregates for concretes.

fine aggregate for concrete is selected by grain composition, content of dust-like and clay particles, petrographic composition, radiation-hygienic characteristic. When selecting the composition of concrete, the density, water absorption (for sands from crushing screenings), voids, as well as the compression strength of the initial rock in a water-saturated state (for sands from crushing screenings) are taken into account.

Fine aggregates shall have an average grain density of 2000 to 2800 kg/m3.

Depending on the conditions of formation and production, sands are divided into natural (in a natural state), natural fractionated and natural enriched; crushed and crushed fractionated.

Depending on the grain composition, sand is divided into 4 groups: large, medium, small and very small.

For the conditional expression of the sand size, the Mk size module is used, denoting the sum of the total residues (in% on the sieves of the standard set) without a fraction with a grain size of more than 5 mm, divided by 100. Sands characterized by the size module Mk from 2.5 to 3.5 are recommended for concretes of classes B35 and above, Mk from 2 to 2.5 - for B15... 22.5, and sands Mk from 1.5 to 2 are allowed in concretes of classes less than B15.

The content of particles determined by weaving shall not be more than 3% for natural sand and 4; for crushed.

An important indicator of the quality of sand is its average density, which depends on its true density, emptiness and humidity and is determined in a dry loose state. Sand intended for preparation of concrete M200 and above, as well as for concrete used in the manufacture of reinforced concrete products subject to alternate freezing and thawing in a water-saturated state, shall have an average density of at least 1550 kg/m3, in other cases - not lower than 1400 kg/m3.

When the sand is subjected to shaking or vibration, it is compacted, while its average density increases to 1600... 1700 kg/m3. Sand occupies the largest volume with a humidity of about 5... 7%; sand volume decreases with increasing or decreasing humidity, and its average density increases [8].

4.5. Additive (superplasticizer C-3).

Superplasticizer C-3 is a chemical additive that refers to plasticizing additives. This is a new type of additive that has a thinning effect on the concrete mixture; reduction of labour input of concrete mix laying and increase of concrete hardening rate. The use of the S-3 superplasticizer is recommended primarily for the production of movable and highly movable concrete mixtures laid in densely reinforced precast reinforced concrete products, to reduce the modes of thermal moisture treatment and to obtain high-strength concrete (grades 600 and above).

Superplasticizer C-3 is introduced into concrete mixture in amount from 0.2 to 1.0% of cement weight in terms of dry substance. When selecting the composition of concrete with S-3 in order to reduce possible water separation, the proportion of sand in the aggregate mixture should be increased. For the preparation of cast concrete mixtures simultaneously with the superplasticizer, it is recommended to introduce thin-ground mineral additives into the concrete composition, which reduce the water separation of the mixture [9].

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