Prestressed concrete (prestressed concrete) - This construction material, designed to overcome the inability of concrete to resist significant tensile stresses. Structures made of prestressed reinforced concrete, compared to non-stressed concrete, have significantly lower deflections and increased crack resistance, having the same strength, which makes it possible to cover large spans with an equal cross-section of the element.
When making reinforced concrete, steel reinforcement with high tensile strength is laid, then the steel is tensioned with a special device and the concrete mixture is laid. After setting, the pre-tensioning force of the released steel wire or cable is transferred to the surrounding concrete so that it is compressed. This creation of compressive stresses makes it possible to partially or completely eliminate tensile stresses from the load.
Methods of tensioning reinforcement:
Grants Pass, a prestressed concrete bridge in botanical garden, Oregon, USA
By type of technology, the device is divided into:
More often, the second method is used in the construction of bridges with large spans, where one span is made in several stages (captures). Steel material (cable or reinforcement) is placed in a form for concreting in a case (corrugated thin-walled metal or plastic pipe). After production monolithic design The cable (reinforcement) is tensioned to a certain extent using special mechanisms (jacks). After that, liquid cement (concrete) mortar is pumped into the case with the cable (reinforcement). This ensures a strong connection between the bridge span segments.
The origins of the creation of prestressed reinforced concrete were Eugene Freycinet (France) and Viktor Vasilyevich Mikhailov (Russia)
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See what “Prestressed reinforced concrete” is in other dictionaries: prestressed concrete
- - [A.S. Goldberg. English-Russian energy dictionary. 2006] Topics: energy in general EN prestressed concrete ... prestressed concrete with steel shell - (for example, for the manufacture of protective shells at nuclear power plants) [A.S. Goldberg. English-Russian energy dictionary. 2006] Topics: energy in general EN steel lined prestressed concrete ...
Technical Translator's Guide prestressed reinforced concrete - Prefabricated or monolithic reinforced concrete structures , the reinforcement of which is stressed to a given design value [Terminological dictionary of construction in 12 languages (VNIIIS Gosstroy USSR)] Topics construction products - (for example, for the manufacture of protective shells at nuclear power plants) [A.S. Goldberg. English-Russian energy dictionary. 2006] Topics: energy in general EN steel lined prestressed concrete ...
other EN prestressed… … Prestressed reinforced concrete - Prestressed reinforced concrete - prefabricated or monolithic reinforced concrete structures, the reinforcement of which is stressed to a given design value [Terminological dictionary for construction in 12 languages (VNIIIS Gosstroy USSR)]… …
Encyclopedia of terms, definitions and explanations of building materials Prefabricated or monolithic reinforced concrete structures, the reinforcement of which is stressed to a given design value (Bulgarian language; Български) are pre-precast with stoman concrete ( Czech ; Čeština) předpjatý železobeton (;… … German
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Prestress Diagram Prestressed concrete (prestressed concrete) is a building material designed to overcome the inability of concrete to resist significant tensile stresses. When... ... Wikipedia
Reinforcement for reinforced concrete structures ... Wikipedia A combination of concrete and steel reinforcement, monolithically connected and working together in a structure. The term "J." often used as a collective name for reinforced concrete structures and products (See Reinforced concrete structures and products) ... Big
(prestressed concrete Soviet encyclopedia
When making reinforced concrete, steel reinforcement with high tensile strength is laid, then the steel is tensioned with a special device and the concrete mixture is laid. After setting, the pre-tensioning force of the released steel wire or cable is transferred to the surrounding concrete so that it is compressed. This creation of compressive stresses makes it possible to partially or completely eliminate tensile stresses from the operating load.
Methods of tensioning reinforcement:
Grants Pass, a prestressed concrete bridge in a botanical garden, Oregon, USA
By type of technology, the device is divided into:
More often, the second method is used in the construction of bridges with large spans, where one span is made in several stages (captures). Steel material (cable or reinforcement) is placed in a form for concreting in channel formers (corrugated thin-walled metal or plastic pipe). After manufacturing a monolithic structure, the cable (reinforcement) is tensioned to a certain extent using special mechanisms (jacks). After that, liquid cement (concrete) mortar is pumped into the channel former with a cable (reinforcement). This ensures a strong connection between the bridge span segments.
While tension on stops implies only the rectilinear form of the tensioned reinforcement, it is important distinctive feature tension on concrete is the ability to tension reinforcement of complex shapes, which increases the efficiency of reinforcement. For example, in bridges, reinforcement elements are raised inside load-bearing reinforced concrete beams in areas above the “bull” supports, which makes it possible to more effectively use their tension to prevent deflection.
The origins of the creation of prestressed reinforced concrete were Eugene Freycinet (France) and Viktor Vasilyevich Mikhailov (Russia).
Prestressed concrete is the main material interfloor ceilings high-rise buildings and protective containments of nuclear reactors, as well as columns and walls of buildings in high-risk areas
The main advantages of reinforced concrete are: high strength, fire resistance, durability, ease of shaping. A concrete beam (Fig. below), which experiences tension below the neutral axis and compression above it during bending, has a low load-bearing capacity due to the weak tensile strength of the concrete. In this case, the strength of concrete in the compressed zone is not fully used. In this regard, unreinforced concrete is not recommended for use in structures intended to work in bending or tension, since the dimensions of such elements would be prohibitively large.
Concrete structures are used primarily when working in compression (walls, foundations, retaining structures, abutments, etc.) and only sometimes when working in bending at low tensile stresses not exceeding the tensile strength of concrete.
Reinforced concrete structures, reinforced in the tensile zone with reinforcement, have a significantly higher load-bearing capacity. So, the load-bearing capacity reinforced concrete beam(Fig. below) with reinforcement laid below is 10-20 times greater than the load-bearing capacity of a concrete beam of the same dimensions. In this case, the strength of concrete in the compressed zone of the beam is fully used.
Steel rods, wires, rolled profiles, as well as fiberglass, synthetic materials, wooden blocks, bamboo trunks.
Structures are reinforced not only when working in tension and bending, but also in compression (Fig. above). Since steel has high tensile and compressive strength, its inclusion in compressed elements significantly increases their load-bearing capacity. The joint work of materials with different properties, such as concrete and steel, is ensured by the following factors:
A feature of reinforced concrete structures is the possibility of cracks forming in the tensile zone under the action of external loads. The opening of these cracks in many structures during operation is small (0.1-0.4 mm) and does not cause corrosion of the reinforcement or disruption of the normal operation of the structure. However, there are structures and structures in which, due to operating conditions, the formation of cracks is unacceptable (for example, pressure pipelines, trays, tanks, etc.) or the opening width must be reduced. In this case, those zones of the element in which tensile forces appear under the influence of operational loads are subjected to intensive compression in advance (before applying external loads) by pre-tensioning the reinforcement. Such structures are called prestressed. Preliminary compression of structures is carried out mainly in two ways: by tensioning the reinforcement on stops (before concreting) and on concrete (after concreting).
In the first case, before concreting the structure, the reinforcement is tensioned and secured to the stops or ends of the form (Fig. below). Then the element is concreted. After the concrete has acquired the necessary strength to withstand the forces of preliminary compression (transfer strength), the reinforcement is released from the stops and it, trying to shorten, compresses the concrete. The transfer of force to concrete occurs due to adhesion between the reinforcement and concrete, as well as through special anchor devices located in the concrete of the structure if adhesion is insufficient.
In the second case, a concrete or lightly reinforced element with channels or grooves is first made (Fig. below). When the concrete reaches the required transfer strength, reinforcement is inserted into the channels (grooves), tensioned with the tension device resting on the end of the element, and anchored. Thus, the concrete is compressed. To create adhesion between reinforcement and concrete, cement or cement-sand mortar is injected into the channels. If the prestressing reinforcement is located on outer surface element (ring fittings of pipelines, reservoirs, etc.), then its winding with simultaneous compression of concrete is carried out using special winding machines. After tensioning the reinforcement, a protective layer of concrete is applied to the surface of the element by gunning. The reinforcement can be tensioned by mechanical, electrothermal, combined and physico-chemical methods.
a - tension on the stops; b - tension on concrete; I - tensioning of reinforcement and concreting of the element; II, IV - finished element; III - element during tensioning of reinforcement; 1 - emphasis; 2 - jack; 3 - anchor
In the mechanical method, the reinforcement is tensioned hydraulically or screw jacks, winding machines and other mechanisms. In the electrothermal method, the reinforcement is heated electric shock up to 300-350 °C, put into a mold and secured on stops. During the cooling process, the reinforcement shortens and receives preliminary tensile stresses. The combined tension method combines electrothermal and mechanical methods tensioning of reinforcement carried out simultaneously. With the physico-chemical method, the tension of the reinforcement is achieved as a result of the expansion of concrete prepared with special tensile cement (NC) during its hydro-thermal treatment.
The reinforcement embedded in the concrete prevents its volume from increasing and stretches, and compressive stresses arise in the concrete. The reinforcement is tensioned on the stops using mechanical, electrothermal or combined methods, and on concrete - only mechanically.
The main advantage of prestressed structures is their high crack resistance. When a prestressed element is loaded with an external load in the concrete of the tensile zone, the pre-created compressive stresses are extinguished and only after that tensile stresses arise. The higher the strength of concrete and steel, the greater the pre-compression that can be created in the element.
The use of high-strength materials makes it possible to reduce the consumption of reinforcement by 30-70% compared to non-prestressed reinforced concrete. Concrete consumption and the weight of the structure are also reduced. In addition, the high crack resistance of prestressed structures increases their rigidity, water resistance, frost resistance, resistance dynamic loads, durability.
The disadvantages of prestressed reinforced concrete include the fact that the process is highly labor-intensive for the manufacture of structures. In addition, there is a need to use special equipment and highly qualified workers.
The stress-strain states of prestressed elements after the formation of cracks in the concrete of the tension zone are similar to elements without prestress.
GOST 32803-2014
INTERSTATE STANDARD
STRESSING CONCRETE
Specifications
Self-stressing concrete. General specifications
ISS 91.100.30
Date of introduction 2015-07-01
Preface
The goals, basic principles and basic procedure for carrying out work on interstate standardization are established in GOST 1.0-92 "Interstate standardization system. Basic provisions" and GOST 1.2-2009 "Interstate standardization system. Interstate standards, rules and recommendations for interstate standardization. Rules for development, adoption , applications, updates and cancellations"
Standard information
1 DEVELOPED by Open joint stock company"Research Center "Construction" of the Order of the Red Banner of Labor Research, Design and Technological Institute of Concrete and Reinforced Concrete (JSC "National Research Center "Construction" NIIZHB named after A.A. Gvozdev)
2 INTRODUCED by the Technical Committee for Standardization TC 465 "Construction"
3 ADOPTED by the Interstate Council for Standardization, Metrology and Certification (protocol dated May 25, 2014 N 45-2014)
The following voted for adoption:
Short name of the country according to MK (ISO 3166) 004-97 | Abbreviated name of the national standardization body |
|
Ministry of Economy of the Republic of Armenia |
||
Kyrgyzstan | Kyrgyzstandard |
|
Moldova-Standard |
||
Rosstandart |
||
Tajikistan | Tajikstandard |
4 By Order of the Federal Agency for Technical Regulation and Metrology dated November 26, 2014 N 1830-st, the interstate standard GOST 32803-2014 was put into effect as a national standard Russian Federation from July 1, 2015
5 INTRODUCED FOR THE FIRST TIME
Information about changes to this standard is published in the annual information index "National Standards", and the text of changes and amendments is published in the monthly information index "National Standards". In case of revision (replacement) or cancellation of this standard, the corresponding notice will be published in the monthly information index "National Standards". Relevant information, notices and texts are also posted in the information system common use- on the official website of the Federal Agency for Technical Regulation and Metrology on the Internet
1 area of use
This standard applies to prestressing concrete intended to create pre-stress (self-stress) in the structures of buildings and structures due to expansion during the hardening process to increase crack resistance, water resistance and durability of structures and establishes technical requirements to prestressing concrete.
This standard uses references to the following interstate documents:
GOST 9.306-85 Unified system of protection against corrosion and aging. Metallic and non-metallic inorganic coatings. Designations
GOST 166-89 (ISO 3599-76) Calipers. Specifications
GOST 577-68 Dial indicators with a division value of 0.01 mm. Specifications
GOST 5578-94 Crushed stone and sand from ferrous and non-ferrous metallurgy slags for concrete. Specifications
GOST 5781-82 Hot-rolled steel for reinforcement of reinforced concrete structures. Specifications
GOST 6958-78 Enlarged washers. Accuracy classes A and C. Technical specifications
GOST 7473-2010 Concrete mixtures. Specifications
GOST 7798-70 Hex head bolts, accuracy class B. Design and dimensions
GOST 8267-93 Crushed stone and gravel made of dense rocks For construction work. Specifications
GOST 8736-93 Sand for construction work. Specifications
GOST 10060-2012 Concrete. Methods for determining frost resistance
GOST 10178-85 Portland cement and Portland slag cement. Specifications
GOST 10180-2012 Concrete. Methods for determining strength using control samples
GOST 10181-2000 Concrete mixtures. Test methods
GOST 11371-78 Washers. Specifications
GOST 12730.1-84* Concrete. Methods for determining density
________________
* GOST 12730.1-78 is in force on the territory of the Russian Federation, hereinafter in the text. - Database manufacturer's note.
GOST 12730.5-84 Concrete. Methods for determining water resistance
GOST 13015-2012 Concrete and reinforced concrete products for construction. General technical requirements. Rules for acceptance, labeling, transportation and storage
GOST 17624-2012 Concrete. Ultrasonic method for determining strength
GOST 17711-93 Copper-zinc (brass) casting alloys. Stamps
GOST 18105-2010 Concrete. Rules for monitoring and assessing strength
GOST 22690-88 Concrete. Determination of strength mechanical methods non-destructive testing
GOST 23732-2011 Water for concrete and mortars. Specifications
GOST 24211-2008 Additives for concrete and mortars. General technical requirements
GOST 25192-2012 Concrete. Classification and general technical requirements
GOST 25820-2000 Lightweight concrete. Specifications
GOST 26633-2012 Heavy and fine-grained concrete. Specifications
GOST 27006-86 Concrete. Squad selection rules
GOST 28570-90 Concrete. Methods for determining strength using samples taken from structures
GOST 30108-94 Construction materials and products. Determination of specific effective activity of natural radionuclides
GOST 30515-97 Cements. General technical conditions
GOST 31108-2003 General construction cements. Specifications
GOST 32496-2013 Porous aggregates for lightweight concrete. Technical conditions.
Note - When using this standard, it is advisable to check the validity of the reference standards in the public information system - on the official website of the Federal Agency for Technical Regulation and Metrology on the Internet or using the annual information index "National Standards", which was published as of January 1 of the current year, and on issues of the monthly information index "National Standards" for the current year. If the reference standard is replaced (changed), then when using this standard you should be guided by the replacing (changed) standard. If the reference standard is canceled without replacement, then the provision in which a reference is made to it is applied in the part that does not affect this reference.
This standard applies following terms with the corresponding definitions:
3.1 prestressing concrete: Concrete containing prestressing cement or an expansion admixture that causes the concrete to expand as it hardens.
3.2 self-stressing of concrete: The amount of concrete prestress created as a result of concrete expansion under conditions of elastic deformation limitation.
3.3 grade of self-stressing concrete: The average value of the compressive prestress (self-stress) of prestressing concrete, MPa, at the age of 28 days, created as a result of its expansion under conditions of elastic limitation of deformations, with a rigidity corresponding to the rigidity of steel reinforcement with an axial longitudinal reinforcement coefficient of 0.01 and an elastic modulus of 2 10 MPa.
3.4 expansion additives RD: Mineral additive used for the preparation of prestressing concrete.
3.5 tensile cement: A mineral binder that provides controlled self-stress during concrete hardening under conditions of elastic deformation limitation.
3.6 linear extension: Increasing the linear dimensions of the standard sample.
4.1 In accordance with GOST 25192, install the following types prestressing concrete:
- heavy prestressing concrete;
- lightweight prestressing concrete.
Depending on the value of the controlled self-stress (see 5.1.3), prestressing concrete is divided into the following types:
- BN - concrete with a standardized self-stressing grade, made on the basis of prestressing concrete;
- BC - concrete with compensated shrinkage, made on the basis of Portland cement and an expanding additive.
4.2 The symbol for concrete mixtures intended for prestressing concrete is adopted in accordance with GOST 7473 with the following additions.
For concrete with a standardized self-stress grade, the self-stress grade is indicated after the water resistance grade.
Example symbol concrete mixture for concrete with a standardized self-stress grade Sp1.2, compressive strength class B40, workability grade P4, frost resistance grade F
300, waterproof grade W18:
BST BN V40 P4 F 300 W18 Sp1,2 GOST 32803-2014
It is allowed not to indicate the self-stress grade for concrete with compensated shrinkage.
An example of a symbol for a concrete mixture for concrete with compensated shrinkage, compressive strength class B25, workability grade P3, frost resistance grade F
300, waterproof grade W16:
BST BK V25 P3 F
300 W16
GOST 32803-2014
Prestressing concrete is manufactured in accordance with the requirements of this standard, design and technological documentation, technical specifications and developed technological regulations approved in accordance with the established procedure.
5.1 Characteristics
5.1.1 The strength of concrete at design age is characterized by classes of compressive strength, axial tension and bending tension.
The following classes are established for heavy prestressing concrete:
- compressive strength: B20; B25; B30; B35; B40; B45; B50; B55; B60; B70; B80; B90;
- axial tensile strength: B0.8; 2B1,2; B1.6; B2; B2.4; B2.8; B3,2; B3.6; B4.0;
- tensile strength in bending: B2; B2.4; B2.8; B3,2; B3.6; B4; B4,4; B4.8; B5.2; B6.4; B6,8.
The following classes are established for lightweight prestressing concrete:
- compressive strength: B10; B12.5; B15; IN 20; B25; B30; B35; B40;
- axial tensile strength: B0.8; B1.6; B2; B2.4; B2.8; B3,2.
It is allowed, with appropriate justification, to establish higher strength classes of prestressing concrete.
5.1.2 Depending on the average density, the following grades of prestressing concrete are installed:
- lung: D1200; D1300; D1400; D1500; D1600; D1700; D1800; D1900; D2000;
- heavy: D2000, D2100, D2200, D2300, D2400, D2500.
5.1.3 Depending on the value of self-stress, the following grades of prestressing concrete are installed: Sp0.6; Sp0.8; Sp1,0; Sp1,2; Sp1.5; Sp2.0; Sp3.0; Sp4.0.
Self-tensioning concrete grades from Sp0.6 to Sp1.0 refer to concrete with compensated shrinkage, from Sp1.2 to Sp4.0 - to prestressing concrete with standardized self-tension.
5.1.4 Depending on the conditions of use, heavy prestressing concrete must have the following frost resistance grades: F200, F300, F400, F600, F800; light: F100, F200, F300, F400, F500.
5.1.5 Depending on the water resistance, heavy prestressing concrete should have the following grades: W12, W14, W16, W18, W20; light: W8, W10, W12, W14.
5.2 Material requirements
5.2.1 Materials used for prestressing concrete must comply with the requirements of current standards and technical specifications for these materials and ensure the production of concrete with the specified characteristics.
5.2.2 The following is used as a binder:
- prestressing cements according to current regulatory or technical documents;
- Portland cements that comply with GOST 10178, GOST 30515 and GOST 31108, with a CA content in the clinker of no more than 8% in combination with additives in accordance with GOST 24211, regulating the expansion process, subject to their assessment according to the criterion of ensuring the required grade for self-tension.
5.2.3 Crushed stone in accordance with GOST 26633, GOST 8267, GOST 5578 is used as a coarse aggregate for heavy prestressing concrete.
5.2.4 Sands in accordance with GOST 26633 and GOST 8736 are used as fine aggregate for heavy prestressing concrete.
5.2.5 Aggregates in accordance with GOST 25820 and GOST 32496 are used as large and small aggregates for lightweight prestressing concrete.
5.2.6 Additives for prestressing concrete must comply with GOST 24211 and current regulatory or technical documents for specific types expansion additives. Additives are introduced into concrete mixtures in amounts from 5% to 30% of the cement mass, depending on the purpose of the concrete.
5.2.7 Water for mixing the concrete mixture and preparing solutions of chemical additives must comply with the requirements of GOST 23732.
5.2.8 The specific effective activity of natural radionuclides of raw materials used for prestressing concrete should not exceed the limit values depending on the area of application of the concrete according to GOST 30108.
5.3 Requirements for concrete mixtures
5.3.1 Concrete mixtures for prestressing concrete are prepared in accordance with the requirements of GOST 7473.
5.3.2 The composition of the concrete mixture is selected in accordance with GOST 27006, taking into account the requirements of this standard and technological documentation approved in the prescribed manner.
6.1 Acceptance of prestressing concrete is carried out according to all standardized standards project documentation quality indicators in accordance with GOST 7473 and GOST 13015.
Concrete is assessed for frost resistance, water resistance, and average density when selecting each concrete mixture composition in accordance with GOST 27006, then at least once every 6 months, as well as when changing the composition of the concrete mixture or the materials used.
6.2 Each batch of concrete mixture intended for prestressing concrete must be accompanied by a passport in accordance with GOST 7473.
7.1 The strength of prestressing concrete in compression, flexural tension and axial tension is determined in accordance with the requirements of GOST 10180, GOST 28570, GOST 17624, GOST 22690, GOST 18105.
7.2 Average density prestressing concrete is determined according to GOST 12730.1, GOST 10181.
7.3 Frost resistance of prestressing concrete is determined according to GOST 10060.
7.4 The water resistance of prestressing concrete is determined according to GOST 12730.5.
7.5 Determination of self-stress of prestressing concrete
7.5.1 Essence of the method
The essence of the method is to measure the elastic deformation that occurs during the expansion of concrete prism samples, molded and hardening in dynamometric conductors, the rigidity of the end plates of which is equivalent to the rigidity of the longitudinal reinforcement equal to 1%.
7.5.2 Test equipment
When conducting tests, the following measuring instruments must be used:
- dial indicator according to GOST 577 with a division value of 0.01 mm and a measuring range of 10 mm;
- caliper according to GOST 166 with a division value of 0.05 mm.
The following equipment is used for testing:
- a torque conductor for a prism sample with dimensions of 100x100x400 mm or 50x50x200 mm (see Figures 1, 2);
- a “crab” measuring device with a dial indicator with a division value of 0.01 mm to measure the deflection of one conductor plate or a tripod with a similar indicator (see Figures 3, 4) to measure the deflection of both plates;
- a standard for checking the measuring device or a steel standard - a rod for a tripod with a length of (200±1) mm, a diameter of 16 mm with triangular cores 7
0.75 mm deep at the ends (see Figure 3). Material for making standards - steel 3 (St3) according to GOST 5781;
- metal mold for making prism samples with dimensions 100x100x400 mm (see Figure 5);
- a metal mold for making prism samples with dimensions of 50x50x200 mm (see Figure 6);
- a container with water for storing conductors with samples.
7.5.3 Preparation for testing
Concrete mixture sampling for concrete quality control is carried out once per shift. The concrete mixture sample when using jigs for prism samples with dimensions 100x100x400 mm must be at least 15 liters, for prism samples with dimensions 50x50x200 mm - at least 2 liters.
Before assembling the conductor (see Figures 1, 2), the nuts are tightened with the mold 4
on traction 3
until it stops, taking out the gap. No gap is allowed between the rods and the plate. 2
. The zero measurement of the conductor is taken using a crab measuring device or a tripod, previously verified using a standard for the constancy of the reading. When checking a tripod, the standard must always be set in the same position - with the mark facing up. Readings are taken with an accuracy of half the division of a dial indicator. The temperature of the conductor, measuring device and standard during measurement must be the same.
Before molding the prism sample, the mold must be lubricated with a thin layer of lubricant and assembled using brackets on the conductor rods with a minimum gap to avoid deformations.
Concrete self-stress control is carried out on concrete plant or at construction site at the place where concrete is placed in the structure.
Prism samples are molded in accordance with the requirements of GOST 10180. The prism samples molded in the jig are covered with film or other waterproof materials to protect against moisture loss.
Hardening of prism samples until a concrete strength of 7-15 MPa is achieved (about a day) should occur in a room with an air temperature of (20±2) °C, further hardening after removal of the mold (up to 28 days) - in water or in abundantly damp sawdust, sand, etc.
7.5.4 Test performance
The self-stress of prestressing concrete is determined when selecting the composition of the concrete mixture and monitoring the quality of concrete in order to ensure the calculated self-stress of the concrete.
The self-stress of concrete is determined by three control prism samples with dimensions of 50x50x200 mm (when using crushed stone fraction no more than 10 mm) or 100x100x400 mm, molded and hardening in special dynamometer conductors, which create an elastic limitation of deformation during the expansion of concrete, equivalent to the longitudinal reinforcement of the prism samples, equal to 1%.
Measurement of conductors is carried out daily for concrete at the age of 1-7 days and then at the age of 10, 14 and 28 days each time with verification of the measuring device using a standard. The measurement results are recorded in the test log of prism samples in conductors when determining the self-stress of concrete.
The self-stress value of the prism sample, MPa, is determined by the formula
where is the total deformation of the prism sample;
- sample length;
- reduced coefficient of reinforcement of the sample, taken equal to 0.01;
- modulus of elasticity of steel, taken equal to 2·10 MPa.
The self-stress of concrete is calculated as the arithmetic mean of the two highest measurement results of three prism samples in conductors molded from one concrete sample aged from 1 to 7, 10, 14, 28 days. Calculations are carried out to two decimal places.
8.1 The manufacturer (supplier) of concrete mixture intended for prestressing concrete guarantees:
- at the time of delivery to the consumer - compliance of all standardized technological indicators of the quality of concrete mixtures with those specified in the supply contract;
- at design age - achievement of all standardized indicators of concrete quality specified in the supply contract, provided that the consumer of the concrete mixture in the manufacture of concrete and reinforced concrete structures ensures compliance with the requirements of current regulatory and technical documents on concreting structures and compliance with concrete hardening modes in accordance with GOST 10180.
8.2 The guarantees of the manufacturer (supplier) of the concrete mixture must be confirmed:
- protocols for determining the quality of concrete mixtures when selecting their composition and conducting operational and acceptance control;
- protocols for determining standardized quality indicators of prestressing concrete at design age.
1
- top plate; 2
- bottom plate; 3
- traction; 4
- screw; 5
6
- benchmark with longitudinal core; 7
- benchmark with a flat end; 8
- concrete prism sample
Note - Material of plates and nuts - Art. 45 according to GOST 5781, rods - Art. 3; reference points - brass L62 according to GOST 17711. The conductor parts are chrome-plated X36 according to GOST 9.306, matte chrome.
Figure 1 - Dynamometric jig for prism samples with dimensions 100x100x400 mm
1
- top plate; 2
- bottom plate; 3
- traction; 4
- screw; 5
- a benchmark with a triangular core 0.75 mm deep; 6
- concrete prism sample
Note - Material of plates and nuts - St.45; traction - Art. 3; reference point - brass L62. The conductor parts are chrome-plated X36 according to GOST 9.306, matte chrome.
Figure 2 - Dynamometric jig for prism samples with dimensions 50x50x200 mm
(A) Measuring diagram, installation of the crab measuring device on the conductor
(B) Standard with crab measuring device
1 - conductor with dimensions 100x100x400 mm; 2 - measuring device "crab"; 3 - standard; 4 - concrete prism sample; 5 - dial indicator; 6 - a hairpin with a soldered ball with a diameter of 5 mm; 7 - triangular core 0.75 mm deep; 8 - longitudinal core; 9 - locking screw.
Figure 3 - Measuring device"crab" with a dial indicator for determining the self-stress of prism samples with dimensions of 100x100x400 mm
1 - tripod base; 2 - hairpin with ball; 3 - conductor with a concrete prism; 4 - indicator fastening screw; 5 - indicator; 6 - stand; 7 - console fastening screw; 8 - console; 9 - screw
Figure 4 - Stand with a dial indicator for determining the self-stress of prism samples
1 - bottom of the mold; 2 - mold side with brackets; 3 - washer 12.03.01 GOST 6958; 4 - bolt M12-6gX30.56.05 GOST 7798
Figure 5 - Metal mold for making prism samples with dimensions 100x100x400 mm
1 - bottom of the mold; 2 - mold side with brackets; 3 - washer 8.03.05 GOST 11371; 4 - bolt M8-6gX40.56.05 GOST 7798
Figure 6 - Metal mold for making prism samples with dimensions 50x50x200 mm
UDC 691.328 MKS 91.100.30
Key words: prestressing concrete, concrete with compensated shrinkage, prestressing cement, expansion additives, self-stressing, free expansion, water resistance, crack resistance, durability
__________________________________________________________________________
Electronic document text
prepared by Kodeks JSC and verified against:
official publication
M.: Standartinform, 2015
The essence of reinforced concrete. Its advantages and disadvantages
Reinforced concrete is a complex building material consisting of concrete and steel fittings, deforming together up to the destruction of the structure.
In the above definition, key words that reflect the essence of the material are highlighted. To identify the role of each of the highlighted concepts, let us consider in more detail the essence of each of them.
Concrete is fake diamond, which, like any stone material, has a fairly high compression resistance, and its tensile resistance is 10¸20 times less.
Steel reinforcement has a fairly high resistance to both compression and tension.
Combining these two materials in one allows you to rationally use the advantages of each of them.
For example concrete beams, let's consider how the strength of concrete is used in a bending element (Fig. 1a). When a beam bends above the neutral layer, compressive stresses arise, and the lower zone is stretched. The maximum stresses in the sections will be in the extreme upper and lower fibers of the section. As soon as the beam is loaded, the stresses in the tensile zone reach the tensile strength of concrete R bt, the outermost fiber will rupture, i.e. the first crack will appear. This will be followed by brittle failure, i.e. beam fracture. Stresses in the compressed zone of concrete sbc at the moment of destruction will be only 1/10 ¸ 1/15 of the compressive strength of concrete Rb, i.e. the strength of concrete in the compressed zone will be used by 10% or less.
For example reinforced concrete beams with reinforcement, let's consider how the strength of concrete and reinforcement is used here. The first cracks in the tensile zone of concrete will appear at almost the same load as in the concrete beam. But, unlike a concrete beam, the appearance of a crack does not lead to the destruction of a reinforced concrete beam. After cracks appear, the tensile force in the section with the crack will be absorbed by the reinforcement, and the beam will be able to withstand an increasing load. Failure of a reinforced concrete beam will occur only when the stresses in the reinforcement reach the yield point, and the stresses in the compressed zone reach the compressive strength of concrete. In this case, initially, when the yield strength s flow is reached in the reinforcement, the beam begins to bend intensively due to the development of plastic deformations in the reinforcement. This process continues until the concrete of the compressed zone is crushed when it reaches its compressive strength. Rb. Since the stress level in concrete and reinforcement in this state is much higher than the value R bt, then this means that it must be caused by a larger load ( N in Fig. 1-b). Conclusion- the feasibility of reinforced concrete lies in the fact that tensile forces are absorbed by reinforcement, and compressive forces are absorbed by concrete. Hence, main purpose of fittings in reinforced concrete is that it is she who must absorb tension due to the insignificant tensile strength of concrete. By means of reinforcement, the load-bearing capacity of a bending element, compared to concrete, can be increased by more than 20 times.
The joint deformation of concrete and reinforcement installed in it is ensured by adhesion forces, which occur during hardening of the concrete mixture. In this case, adhesion is formed due to several factors, namely: firstly, due to the adhesion (gluing) of the cement paste to the reinforcement (obviously, the share of this adhesion component is small); secondly, due to compression of the reinforcement by concrete due to its shrinkage during hardening; thirdly, due to the mechanical engagement of concrete on the periodic (corrugated) surface of the reinforcement. Naturally, for periodic profile reinforcement this component of adhesion is the most significant, therefore the adhesion of periodic profile reinforcement to concrete is several times higher than that for reinforcement with a smooth surface.
The very existence of reinforced concrete and its good durability were made possible thanks to the beneficial combination of some important physical - mechanical properties concrete and steel reinforcement, namely:
1) when concrete hardens, it adheres firmly to steel reinforcement and under load, both of these materials are deformed together;
2) concrete and steel have similar values of linear thermal expansion coefficients. That is why when temperature changes environment within the range of +50 o C ¸ -70 o C there is no disruption of adhesion between them, since they are deformed by the same amount;
3) concrete protects reinforcement from corrosion and direct fire. The first of these circumstances ensures the durability of reinforced concrete, and the second ensures its fire resistance in the event of a fire. The thickness of the protective layer of concrete is determined precisely from the conditions for ensuring the necessary durability and fire resistance of reinforced concrete.
When using reinforced concrete as a material for building structures It is very important to understand the advantages and disadvantages of the material, which will allow it to be used rationally, reducing the adverse impact of its shortcomings on the performance of the structure.
TO merits(positive properties) of reinforced concrete include:
1. Durability - with proper operation, reinforced concrete structures can serve for an indefinitely long time without deterioration bearing capacity.
2. Good resistance to static and dynamic loads.
3. Fire resistance.
4. Low operating costs.
5. Cheap and good performance.
To the main disadvantages of reinforced concrete relate:
1. Significant dead weight. This disadvantage is eliminated to some extent by using lightweight aggregates, as well as by using progressive hollow and thin-walled structures(that is, by choosing a rational shape of sections and outlines of structures).
2. Low crack resistance of reinforced concrete (from the example discussed above it follows that there should be cracks in tensile concrete during the operation of the structure, which does not reduce the load-bearing capacity of the structure). This disadvantage can be reduced with the use of prestressed reinforced concrete, which serves as a radical means of increasing its crack resistance (the essence of prestressed reinforced concrete is discussed in topic 1.3 below.
3. Increased sound and thermal conductivity of concrete in some cases requires additional costs for thermal or sound insulation of buildings.
4. The impossibility of simple control to check the reinforcement of the manufactured element.
5. Difficulties in strengthening existing reinforced concrete structures during the reconstruction of buildings when the loads on them increase.
Prestressed reinforced concrete: its essence and methods of creating prestress
Sometimes the formation of cracks in structures in which operating conditions are unacceptable (for example, in tanks, pipes, structures operating under the influence of aggressive environments). To eliminate this disadvantage of reinforced concrete, prestressed structures are used. In this way, it is possible to avoid the appearance of cracks in concrete and reduce the deformation of the structure during operation.
Let's consider short definition prestressed reinforced concrete.
A reinforced concrete structure is called prestressed, in which, during the manufacturing process, significant compressive stresses are created in the concrete of the section of the structure that experiences tension during operation (Fig. 2).
As a rule, initial compressive stresses in concrete are created using pre-tensioned high-strength reinforcement
This increases the crack resistance and rigidity of the structure, and also creates conditions for the use of high-strength reinforcement, which leads to savings in metal and a reduction in the cost of the structure.
The unit cost of reinforcement decreases with increasing reinforcement strength. Therefore, high-strength reinforcement is much more profitable than conventional reinforcement. However, it is not recommended to use high-strength reinforcement in structures without prestressing, since at high tensile stresses in the reinforcement, cracks in the tensile zones of concrete will significantly open, thereby reducing the required performance qualities of the structure.
Advantages prestressed reinforced concrete over conventional concrete is, first of all, its high crack resistance; increased structural rigidity (due to reverse bending obtained when compressing the structure); better resistance to dynamic loads; corrosion resistance; durability; as well as a certain economic effect achieved by using high-strength reinforcement.
In a prestressed beam under load (Fig. 2), concrete experiences tensile stresses only after the initial compressive stresses have been extinguished. Using the example of two beams, it can be seen that cracks in a prestressed beam form at more high load, but the breaking load for both beams is close in value, since the ultimate stresses in the reinforcement and concrete of these beams are the same. The deflection of the prestressed beam is also much less.
When producing prestressed reinforced concrete structures in a factory, there are two possible options: circuit diagrams creating prestress in reinforced concrete:
prestressing with tensioning of reinforcement on stops and on concrete.
When pulling on the stops the reinforcement is placed into the mold before the element is concreted, one end is fixed to the stop, the other is tensioned with a jack or other device to a controlled tension. Then the product is concreted, steamed and after the concrete has acquired the necessary cubic strength to absorb compression Rbp the reinforcement is released from the stops. The reinforcement, trying to shorten within the limits of elastic deformations, if there is adhesion to the concrete, drags it along with it and compresses it (Fig. 3-a).
When tensioning reinforcement on concrete (Fig. 3-b) first, a concrete or lightly reinforced element is made, then after the concrete reaches strength Rbp create a preliminary compressive stress in it. This is done as follows: the prestressed reinforcement is inserted into the channels or grooves left when concreting the element, and tensioned using a jack, resting directly on the end of the product. In this case, concrete compression occurs already in the process of tensioning the reinforcement. With this method, the stress in the reinforcement is controlled after the concrete has been compressed. Channels in concrete that exceed the diameter of the reinforcement by (5¸15) mm are created by laying subsequently removed void formers (steel spirals, rubber tubes, etc.). The adhesion of the reinforcement to the concrete is achieved due to the fact that after compression it is injected (cement paste or mortar is pumped into the channels under pressure through tees - bends - installed during the manufacture of the element). If the prestressing reinforcement is placed with outside element (ring fittings of pipelines, tanks, etc.), then its winding with simultaneous compression of the concrete is performed with special winding machines. In this case, a protective layer of concrete is applied to the surface of the element after tensioning the reinforcement.
Stop tensioning is a more industrial method in factory production. Tension on concrete is used mainly for large-sized structures created directly at the site of their construction.
Reinforcement tension on the stops can be carried out not only using a jack, but also using an electrothermal method. To do this, the rods with the upset heads are heated by electric current to 300 - 350°C, inserted into the mold and secured in the mold stops. When the initial length is restored during cooling, the reinforcement becomes stretched. The reinforcement can also be tensioned using the electrothermo-mechanical method (a combination of the first two methods).
Reinforced concrete is used in almost all areas of industrial and civil construction:
In industrial and civil buildings, reinforced concrete is used to make: foundations, columns, roofing and floor slabs, Wall panels, beams and trusses, crane beams, i.e. almost all frame elements of single- and multi-story buildings.
Special structures during the construction of industrial and civil complexes - retaining walls, bunkers, silos, tanks, pipelines, power line supports, etc.
In hydraulic engineering and road construction, reinforced concrete is used to make dams, embankments, bridges, roads, runways, etc.
