9. CONCRETE

9.1. General concepts about reinforced concrete

Concrete, like any other stone material, has high compressive strength and at the same time 10 ... 15 times less - tensile. This significantly limited the use of concrete products in construction. Steel, possessing a very high tensile strength at stretching, is capable of perceiving tensile stresses (strength of steel 380 ... 1800 MPa), therefore constructions made of concrete that work in bending or tensile reinforce ("arm"). During the operation of such elements, two opposite stresses arise - tensile and compressive. At the same time, steel perceives the first, and concrete - the second, and the reinforced concrete element as a whole successfully withstands bending loads, that is, the work of concrete and steel in the same material is combined. Structures working in compression, to increase their bearing capacity, are often also reinforced.

The ability to work together in reinforced concrete in two materials that are sharply different in their properties is determined by the following major factors:

concrete is strongly bonded to steel reinforcement, as a result of which

stresses in reinforced concrete construction; both materials work together; steel and concrete have almost the same temperature coefficient of expansion (steel - 1210-6 ºС-1 and concrete 1010-6 ºС-1), which ensures complete solidity of reinforced concrete

(temperature fluctuations do not violate its integrity); concrete not only does not have a damaging effect on the steel enclosed in it, but

protects it from corrosion.

Thus, reinforced concrete is a composite material in which concrete and steel are combined into a single whole. Depending on the method of reinforcement and the state of the reinforcement there are reinforced concrete products with conventional reinforcement and prestressed reinforcement.

The usual reinforced concrete products include those whose strength is achieved by laying steel rods, grids or frames in the manufacture of products. However, this method of reinforcement does not completely protect the products working on bending from the formation of cracks in concrete in the stretched zone, since the concrete has a slight elongation (1 ... 2 mm per 1 m length), while steel under the same loads is stretched to 5 ... 6 times more concrete. The appearance of cracks adversely affects the operation of the reinforced concrete element: the deflections increase, moisture and gases penetrate into the cracks, which creates the danger of corrosion of steel reinforcement.

To avoid the formation of cracks in the reinforced concrete structure can be pre-compression of concrete in places prone to stretching. In such concrete, cracks appear only if the tensile stresses exceed the pre-compression stresses. Concrete compression is achieved by prestressing (stretching) the reinforcement.

The prestressing of the reinforcement not only prevents the appearance of cracks in the stretched concrete, but also makes it possible to reduce the mass of reinforced concrete structures, increase their rigidity, increase durability and reduce the consumption of reinforcement.

Depending on the version, reinforced concrete structures are divided into prefabricated, monolithic and prefabricated monolithic.

9.2. Precast concrete technology

The essence of prefabricated housing is that the products are manufactured at the factory, and then delivered to the construction site, where buildings and facilities are assembled from them.

There are several main ways of organizing the factory production of reinforced concrete products:

bench method - when all the basic operations of manufacturing products are performed on

one workplace in unmovable forms, and specialized work units and mechanisms move from one workplace to another in the accepted flow rhythm. This method produces large-size beams, trusses, arches, elevator shafts, sanitary ware cabins, flight of stairs, pipes. A variety of bench method - cassette , in which several products (wall panels) are simultaneously made in grouped vertical forms (cassettes); aggregate-flow method - the manufacture of products in moving with the help of special or conventional hoisting vehicles (cranes) from one workplace (a post equipped with special installations-units) to another work place forms. At the same time, several operations can be performed at one post, after which the product is transferred to the next post without observing a forced rhythm. A variation of the aggregate-flow method is the conveyor method, in which products move from a post to a post in a forced pulsating rhythm, and only one operation is performed on each post. The most productive method. Aggregate-flow and conveyor method produce base plates, base blocks, wall panels and blocks, floor slabs and coatings, landings, sleepers, architectural details and fences and much more. Continuous forming is also a conveyor method - a vibratory mill. The choice of manufacturing method depends on the product range, technological features and production volume.

Production of precast concrete structures includes the following operations:

1. Preparation of concrete mix

By its structure, the concrete mixture is a single physical body, in which the particles of the binder, water and aggregate grains are connected by internal interaction forces. The quality of the concrete mix of heavy concrete is characterized by workability, medium density, volume of air involved, stratification (if necessary), uniformity.

Workability is the ability of a concrete mix to take a given shape, is assessed by indicators of stiffness or mobility, is divided into marks. Mobile mixtures are a continuous plastic mass and are easily distributed in the form under the action of their own weight without the application of significant external forces. Hard mixes are friable and sometimes loose. To give it the desired shape and seal requires a long mechanical action. This implies the value of workability: the lighter the concrete mix is ​​mixed and takes a given shape (these are molding processes), the less it takes to expend effort and energy. The question arises, why then do we need hard mixes at all? The fact is that mobility is provided, as a rule, by a high value of water blocking, and this negatively affects the strength of the resulting concrete, its shrinkage and, mainly, its frost resistance. To resolve this contradiction in the mixture and injected plasticizing agents. State Standards have established the mandatory use of plasticizers in the preparation of concrete mixtures of grades of workability P3 ... P5.

The homogeneity and quality of concrete directly depends on the homogeneity of the concrete mix, determined by the quality of its mixing. A mixture is considered homogeneous if any sample from its batch has the same constant composition. The sample volume must be at least 5 times greater than the maximum coarse aggregate size.

The manufacture of a concrete mix of heavy concrete consists of the following operations: acceptance of raw materials; dosing (bulk raw materials for concrete mix are dosed by weight, except for porous aggregates, which are dosed by volume with weight correction, liquid components are dosed by weight or volume); mixing (concrete mixes of all grades on workability are prepared in forced-action mixers, mobile concrete mixes may be prepared in gravity mixers); the working solution of the chemical additive is introduced with the mixing water.

From the concrete mixing plant, ready-mixed concrete is sent either by in-plant transport to the molding workshop of the enterprise for the subsequent manufacture of precast concrete and reinforced concrete structures, or by road transport (ready-mixed concrete and mortars) to the construction site for the construction of monolithic concrete and reinforced concrete structures, buildings and structures (roads, hydraulic structures). structures, etc.).

2. Production of reinforcing elements

For the reinforcement of concrete is used mainly steel reinforcement of carbon and low alloy steels.

Steel reinforcement (reinforcing steel) is classified: by the basic technology of its manufacture - into hot-rolled rod with a diameter of 6 ... 80 mm and cold-drawn wire with a diameter of 3 to 5.5 mm and, in limited quantities, 6 ... 8 mm.

under the condition of use in constructions - on non-stressed and pre-stressed reinforcement; on a profile - on smooth (cores of reinforcing steel of the class A-1) and periodic

profile (to improve the adhesion of reinforcement with concrete). Due to its insufficient adhesion to concrete, smooth reinforcement is anchored by arranging hooks on the ends of rods; as intended - to the worker (perceives the efforts arising in reinforced concrete

elements of external loads and its own weight); distributive (evenly distributes the forces between the rods of the working reinforcement, ensures their joint operation, prevents the displacement of the rods during concreting); assembly fittings (for the assembly of the reinforcement cage).

In the production of reinforced concrete products, the following reinforcing elements are used: a separate element (core - reinforcement of any diameter and profile); a set of linear elements (bundle of wires, a package of wires or strands); welded mesh - welded reinforcing mesh made from rods located in

two mutually perpendicular directions and connected at the points of intersection by resistance spot welding; flat frame - consists of two or more longitudinal rods in the same plane, which are the working and mounting reinforcement, and transverse rods welded to them by resistance welding. Flat frames are designed for the reinforcement of linear bent or stretched reinforced concrete elements and structures with a small width of the cross section; spatial frame - made with transverse rods located

in different planes, from flat grids and frames by welding and bending them; assembly loops intend for lifting of precast concrete elements. They are made of smooth reinforcing steel of class AI. For better anchoring, hooks are made at the ends of the loop in the concrete; embedded parts are intended for interconnection of prefabricated structures. They are prepared from round, strip, sheet, angular or channel steel segments that meet the conditions of weldability. The reliability of the connection of the embedded part with the concrete of the structure is ensured by anchor rods welded to it and embedded in the concrete.

Reinforcement products are manufactured on flow mechanized and automated process lines. They are located, depending on the volume of production and product range, in separate unified spans or in front of the molding units in the same spans.

The following basic operations are performed here: straightening, cutting, bending and butt welding of rods; welding of flat grids and frames; bending nets and frames; pre-assembly of bulk frameworks; manufacture and metallization of embedded parts; manufacturing of mounting hinges, etc.

3. Form preparation

Includes cleaning, lubricating and assembling forms, installing and tensioning fittings.

Forms are divided into fixed and mobile, individual (on one product) and group, horizontal and vertical, power (for prestressing reinforcement) and non-force, detachable and one-piece with flip sides, metal, wooden, reinforced concrete, plastic and combined. In the production of precast concrete, the most used metal forms. Plastic molds are treated with a special antistatic solution to remove static electricity, leading to a deterioration in surface quality. Lubrication is necessary to reduce the adhesion of concrete to the molds. Properly selected and well applied lubrication facilitates the shaping of the product and contributes to obtaining a smooth surface of concrete.

4. Molding products

Concrete mix of a given quality for workability and composition is supplied to the molding workshop of the BSU using concrete carts, concrete distributors, conveyor belts. Here the mixture enters the pavers and fits into metal molds.

The tasks of the molding are to give the product a predetermined size and shape, as well as to produce concrete defect-free, uniform in cross-sectional structure.

Methods of molding concrete products can be divided into three main types - vibration (the most common), vibration-free and combined.

The essence of vibration molding (vibration compaction) is that by communicating mechanical vibrations (under the action of vibration) to individual particles of a concrete mix, the bonding forces between them (friction and adhesion forces) are destroyed. As a result, the concrete mix dilutes (becomes more mobile) and is compacted due to the fact that the aggregate particles form a more compact addition system, and the air involved in the mixing process is removed from the mixture. Vibroforming is carried out on the vibrating platforms, where the form with the mixture vibrates; with the help of internal (deep) and surface vibrators; using external vibrators attached to the bottom or walls of forms.

Vibrationless include: pressing; centrifuging; gunning (pneumonia); tamping; casting.

Combinations of these methods: vibro-pressing, vibro-stamping, vibro-rolling, vibro-vacuum, etc.

5. Hardening of concrete

In the precast concrete plant technology, its hardening, as a rule, is accelerated to intensify production, reduce production areas, increase mold turnover, and reduce the metal intensity of production. The main way to accelerate the hardening process is heat treatment, in which the reaction between cement and water is accelerated. However, due to the temperature and humidity gradient in the structure of hardened concrete, internal stresses arise, leading to microcracking and a decrease in the strength of concrete compared to hardening in natural (calm) conditions. The exception is concrete based on slag cement.

Heat treatment includes:

- steaming at atmospheric pressure with water vapor (heat and moisture treatment). At elevated (up to 60 ... 90 ° C) temperature in conditions of 100% relative humidity (presence of moisture), concrete gains design (28-day strength) in 6 ... 12 hours. It is carried out in special devices of periodic (pit chambers) and continuous (tunnel - slot and multi-tiered, tower chambers) actions.

- steaming at excess pressure with water vapor (autoclaving). At temperatures above 175 ... 200 C and a pressure of 8 ... 12 atm, providing an equilibrium between the vapor and liquid phases, the moisture in the concrete remains in liquid form, which significantly accelerates its hardening and promotes the formation of an additional amount of new high-strength compounds as a result of the interaction of calcium hydroxide ( cement hydration product) with silica. For cement concrete autoclave treatment is desirable, for silicate - is necessary.

- contact heating , when the coolant (steam, water, oil, flue gases, etc.) heats the product through the partition separating them (steam housings, thermal jackets);

- electric heating, when an electric current is passed through the concrete and the concrete is heated due to its electrical resistance;

- induction heating , when secondary currents are induced in the armature of the product, heating the armature and the whole product;

- heating by high-frequency currents (microwave);

- exposure to heat or infrared rays;

- solar processing (due to solar energy);

- hot molding (when the concrete mixture is preheated to 85 ... 95

ºС).

After heat treatment, the product is decomposed (the form is opened and the product is removed from it), and the form is cleaned, lubricated, assembled, and the reinforcement is laid and the whole cycle repeats again.

6. Finishing the product

The product can be sent to the construction when the concrete reaches the release strength normalized by GOST, which for heavy concrete in the warm season is

70% of the project and 90% - in the cold period.

9.3. Technology of monolithic reinforced concrete

Monolithic reinforced concrete is increasingly used in construction, as well as prefabricated. Monolithic construction includes the following operations: installation of formwork; fitting installation; laying, distribution and compaction of the concrete mix; Care for hardening concrete. All these operations are carried out at the construction site.

The formwork can be removable and non-removable. Make a timbering of metal sheets, boards, plywood with a polymeric covering, plastic, from a chipboard. By design, removable can be assembled, adjustable, horizontally moveable, sliding, etc.

Бетонную смесь изготавливают как в полевых (непосредственно на стройплощадке), так и в заводских условиях. На стройплощадке для этого используются стационарные или передвижные бетоносмесительные установки. Бетонная смесь доставляется к строящемуся объекту автосамосвалами, автобеносмесителями, автобетоновозами. Арматуру изготавливают на заводах и доставляют на площадку, как правило, в виде готовых элементов. Соединение арматуры осуществляется электродуговой сваркой либо вязанием.

The concrete mix is ​​laid using cranes (in buckets - galoshes), concrete pumps, and special concrete pavers, by direct unloading from motor transport along inclined chutes. If necessary, the mixture is compacted (massive structures - deep vibrators, flat structures up to 20 ... 25 cm thick - with surface vibrators). After that, the concrete hardens until it reaches a stripping strength, then the formwork is dismantled. During the hardening until the moment when the strength of the concrete reaches 70% of the design strength, care is needed. Fresh concrete should be kept wet and protected from shock, damage, shock, sudden changes in temperature and rapid drying.

Главная проблема монолитного строительства – сезонность. При отрицательных температурах вода затворения замерзает и твердение прекращается. При бетонировании в зимних условиях свежеуплотненную бетонную смесь подогревают снаружи паром или электрическим током. Пар обычно подают между двойными стенками опалубки; иногда пар пропускают по трубам, уложенным внутри бетона. Это позволяет достигнуть 60…70 % проектной прочности через 1…2 суток. Электропрогрев бетона производят переменным током с применением поверхностных и внутренних электродов (иногда это арматура). При прохождении тока в бетоне выделяется тепло, в результате чего он разогревается (требования – не выше 60 С для предотвращения местного пересыхания). Другими способами бетонирования без внешнего обогрева являются: применение цементов с высокой экзотермией; добавок-ускорителей твердения; разогретых смесей; теплоизоляция твердеющего бетона (метод «термоса»).

When concreting structures with a large cooling surface (covering roads, facing channels, etc.), antifreeze additives are introduced into the concrete mix. Such concrete is called "cold concrete."