Production of floor slabs in a formless manner

Introduction

Concrete is considered one of the oldest building materials. This is evidenced by the buildings and structures that have survived to this day. At first, concrete was used to erect monolithic structures and structures, but science does not stand still, and therefore an even more effective durable building material was created - this is reinforced concrete. With the development of reinforced concrete structures reinforced with nets and frames, the construction of various buildings and structures successfully began to develop at the lowest labor costs and increased construction times.

The next stage in the development of reinforced concrete was the use of pre-stressed structures, which contributed to a decrease in the consumption of reinforcement in reinforced concrete structures, an increase in their durability and crack resistance. When using metal reinforcement, concrete during stretching, although it does not collapse, cracks, which negatively affects the operational properties of reinforced concrete structures and structures. Creation of a stress state in the structure at the stage of manufacture or construction, when the sign of stress in concrete is opposite to the sign of stress from operational load is one of the largest achievements 

 engineering thought of the XX century.

Also a significant advance in the development of reinforced concrete was the use of various types of additives.

At the plants of reinforced concrete products , it is important to ensure the necessary strength of the products in the shortest possible time. Natural hardening allows you to obtain the necessary strength after a long time, which entails an increase in the number of shapes (6070% of the weight of all steel) and production areas. The decisive means of accelerating concrete hardening, in the conditions of factory technology of prefabricated reinforced concrete, is heat treatment.

       Heat treatment is part of the process of making reinforced concrete products and takes 7080% of the time of the entire manufacturing cycle 

products. Heat treatment at existing enterprises ranges from 2,524 hours and is carried out in pit, tunnel, slot, vertical chambers, cassettes, autoclaves, under caps - both periodic and continuous; and the heat source is steam, water, electricity, infrared rays. The essence of heat-moisture treatment lies in the fact that when the temperature medium rises to 851000C, the rate of hydration reaction increases significantly, i.e., the hardening process is accelerated and the products acquire mechanical strength that allows their transportation and installation in a shorter time than at normal temperature.

        Up to 70% of all thermal energy is spent on heat treatment for the production of precast reinforced concrete. The high cost of energy carriers with low efficiency of their use, the unruly supply of fuel leads to a reduction in the volume of production. Under such circumstances, a different approach is needed to the processes of accelerating concrete hardening in factory conditions. The maximum possible use of complex chemical additives, the transition to a low-temperature mode, the use of partially thermos and thermos modes require a competent engineering approach to the design of thermal installations, the thermal engineering calculation of enclosing structures, and the compilation of a thermal balance.

 Electronic computing machines (computers) have been introduced at all plants of reinforced concrete products, which allows processing information by automatic devices at a speed several million times higher than the speed of processing information by humans.

For the successful development of precast reinforced concrete industry, the main directions are defined, the leading of which are:

Development and organization of effective types of binders and their mass production, reinforcement steel, high-quality aggregates, complex chemical additives and new types of concrete;

Increasing the degree of factory readiness of products;

Reduced weight of structures due to the use of thin-walled structures, pre-stressed and light concrete structures;

Fundamental improvement of technologies in the production of reinforced concrete structures, as well as reconstruction of existing plants;

The reduction of energy costs to ensure the acceleration of concrete hardening due to the introduction of chemical additives, the use of thermosets, the use of heliothermal processing and others, which leads to a decrease in the cost of production. 

Manufactured reinforced concrete products are used in all areas of construction, since reinforced concrete is the main building material, and has been widely used in housing, industrial, transport and agricultural construction.

Selection of production method and development of process diagram

Aggregate flow and conveyor technology of flooring today can be found only in Russia and other countries of the post-Soviet space . The entire civilized world has long switched to continuous formwork-free molding - a technology invented in the Soviet Union and known as the "combine mill." The essence of the technology of formless molding is that the products are formed on a heated metal floor and reinforced with a pre-stressed high-strength wire. The forming machine moves along the rails, leaving behind a continuous strip of molded reinforced concrete, which is covered with a heat insulating material, heated for 1216 hours and cut into segments of the desired length.

Formwork-free moulding of slabs of hollow flooring and other reinforced concrete products gradually displaces aggregate flow, conveyor and other outdated technologies.

Advantages of PB over plates made according to aggregate flow or conveyer technology:

1. Possibility of supply of slabs with width 1.2 m, as well as 1.5 m, cross section height 220 mm and length from 1,300 to 12,000 mm with gradation 0.1 m with design load without own weight from 400 to 2400 kg/m2.

2. Technology allows you to make saws of slabs of any length, including at an angle, due to which it becomes possible to more freely design the interior of the building and implement complex facade and volume-planning solutions.

3. The formwork method gives a perfectly flat surface and precise geometric shape. The slabs are precisely joined, which eliminates the further need to align the ceiling surfaces during installation.

Slabs of PB floors are used in floors and coverings of multi-storey residential, public and industrial buildings with load-bearing walls:

• in frame house building; • in large-panel house building; • in prefabricated monolithic house building; • brick and small-block house building.

Formwork-free moulding plates are designed for use in buildings built according to current designs instead of round-empty PK plates made according to aggregate flow or conveyor technology.

The use of a formless molding technique gives: 

1. increase of resistance to crack formation, increase of endurance to load effect

2. increase of bearing capacity due to use of reinforcement ropes of K-7 ø9 and ø12mm class (in the lower zone); high-strength wire ø5 mm (in the upper zone)

At the same time, with formwork molding: the number of maintenance personnel is 8-10 people (that with the same production volumes is 2-2.5 times less than with aggregate flow technology); valve flow rate for PK 63128 plates is 30 kg (40% lower than with aggregate flow technology); process energy intensity is reduced by 50-70% due to application of hot water with temperature of 60-70 ° C as heat carrier, as opposed to heat-moisture treatment by steam of products obtained by aggregate flow method; the forces resulting from the stress of the reinforcing elements are borne by the stops; no loading elements, which drastically reduces the metal consumption of the forming process; due to automation of concrete mix laying process, productivity increases by 40-50%.

        The process of manufacturing continuous moulding boards on equipment of different manufacturers consists mainly of the same technological operations:

1. preparation of stands - cleaning and lubrication, stretching of reinforcing elements using a multifunctional service machine;

2.tensioning of valves (performed using hydraulic presses);

3.making of boards on stands (performed using extruder or slipformer);

4. After forming the slabs (to prevent water loss from the article during heat treatment), they are covered with a cloth;

5. Plates remain on forming stands until the article reaches 70-80% of the final design strength. In the stands on which the products are formed, pipes are laid through which heated water circulates with a temperature of 65-70 ° C.

6. orientatively after 10 - 12 hours after heat treatment, the finished reinforced concrete web is cut by a cutting machine with a disc saw at right or any other angle depending on the order.

7. Loading of slabs on a removal car or a car is carried out using a crane equipped with all kinds of grips.

For the production of prestressed hollow slabs, it is necessary to use a rigid concrete mixture with a water cement ratio in the range of 0.350.37. The hardness of the concrete mixture is 60100 s. The amount of cement in the concrete mixture is approximately 360 kg/m3. Chemical additives, as a rule, are not used.  

The main requirement for the BSU is the accuracy of dosing and strict provision of the specified composition of the concrete mixture at the appropriate capacity. This is one of the most important criteria for choosing the technology for preparing concrete mixtures. The process of the extruder movement, and the molding itself, depends on the amount of concrete mixture in the extruder hopper. If the amount of mixture is insufficient, the machine stops and is in the waiting mode for an additional portion of the concrete mixture, after which the molding process continues, that is, there is no rupture of the concrete track web.

For the surface of the tracks, lubricants "Iceberg M10" (flow - 100-110 g/m2) or "Backchem" (flow - 110-120 g/m2) are used. The use of the Emulsion lubricant is undesirable, since it gives fatty spots on the ceiling surface of the plate and envelops the strings of the working reinforcement, which leads to their slipping in concrete.

    Voids are made using a molding machine. First, the bottom layer of concrete is laid over the entire width of the slab. Then the lower shape of the voids is rammed in it (they are not round, but oval). Then they lay a thick middle layer of concrete and introduce voids formers into it - in shape they very much resemble the hull of a submarine, even with a wheelhouse. "Wheelhouse" this hollow former holds on to the top of the car. Then upper layer of concrete is fed and compacted. Of course, all this is done in one pass of the car at a time.

Based on the above, it is obvious that the aggregate flow method for the production of hollow flooring slabs is no longer able to compete on equal terms with formwork-free molding technologies, and in the near future the plants for the production of the main types of reinforced concrete products (slabs, wall panels, piles, girders and other products) will re-equip their workshops for these modern economical, low-material-intensive technologies.

Nomenclature of products produced

Multilevel slabs are designed for arrangement of frames of one- and multi-storey buildings and structures of various purpose. According to STB 13832003 "Reinforced concrete slabs and slabs for buildings and structures" the product must meet the following requirements:

- slabs shall meet the requirements of this standard and shall be manufactured according to working drawings and process documentation approved in accordance with the established procedure;

- in terms of strength, rigidity and crack resistance, the slabs shall comply with the requirements set forth in the design documentation and withstand during loading tests the control loads specified in the working drawings;

- plates belong to fire hazard class K0;

- the fire resistance rating of the slabs shall correspond to the required fire resistance rating of the building established in the design documentation of the specific building;

- slabs should be made of heavy concrete of compression strength classes B15 and higher;

- supply of slabs to the consumer shall be performed after concrete reaches the end strength;

- for prestressed plates in the area of anchoring of prestressed valves it is not allowed:

a) violation of the structure of concrete on the ends of the elements;

b) loose adjoining of concrete to reinforcement.

During the supply of slabs in the cold period of the year, the rated tempering strength of concrete slabs can be increased to 90% of the compression strength class according to the specification of the working drawings

- frost resistance and waterproofness of concrete of slabs shall correspond to grades on frost resistance (F50) and waterproofness, established design documentation of a specific building or structure and specified in the order for production of columns;

- specific effective activity of natural radionuclides (Aeff) of concrete of slabs shall not exceed, Bq/kg:

- 370 - for slabs used in the frames of residential and public buildings;

- 740 - for slabs used in the frames of production buildings; - types and classes of reinforcement steel used for reinforcement of slabs, as well as steel grades of embedded products must comply with the established design documentation of a specific building and indicated in the detailed drawings;

- shape, dimensions of reinforcement and embedded articles and their position in plates must correspond to those specified in the detailed drawings for the article;

- reinforcement steel should be used for reinforcement of slabs: as non-stressed longitudinal reinforcement: - rod hot-rolled periodic profile of class S500 and smooth class S240 as per GOST 5781 - 82; as stressed longitudinal reinforcement:

- thermomechanically reinforced rod of S800 class as per GOST 10884;

- on the surface of slabs it is not allowed to expose working and structural reinforcement, with the exception of reinforcement outlets;

- thickness of protective layer of concrete up to working and structural reinforcement shall correspond to specified in working drawings;

- release of reinforcement tension in prestressed slabs should be performed after concrete reaches the required transfer strength;

- values of actual size deviations and position of reinforcement outlets and centering gaskets shall not exceed 3 mm;

- in the concrete of slabs supplied to the consumer, cracks are not allowed, with the exception of transverse cracks from the reduction of concrete in pre-stressed slabs, as well as shrinkage and other surface 

process cracks, the width of which should not exceed 0.1 mm, if the working drawings of a particular building do not establish more stringent requirements;

- fat and rusty spots are not allowed on the face surfaces of the plates;

- opening of steel embedded articles surface, reinforcement outlets, mounting loops and slinging holes shall be cleaned of concrete or mortar strains.

Fabricated slabs shall be accepted by technical inspection of the manufacturer.

2.2 Shop operation mode.

The operating mode of the workshop is characterized by the number of working days per year and the number of shifts per day and is established in accordance with technological design standards.

When determining the capacity of enterprises, process lines and the estimated annual operating time of process equipment, it is necessary to accept:

- number of working days for unloading of raw materials and materials from railway transport - 365;

-nominal number of working days per year - 260;

- number of working shifts per day for heat treatment - 3;

- amount of shifts in day on intake of raw materials and materials and shipments of finished goods - 3.

The number of working days in the year 260 is taken on the basis that the working week is 5 days. With a five-day working week, the operating mode is accepted: the first and second shifts of 8 hours, in addition 0.5 breaks, the third shift -7 hours without a break. Total, 23 working hours per day.

The annual operating time fund of the main process equipment for the aggregate flow production method is calculated according to the formula

C = KobN

where Kob is equipment utilization factor equal to 0.943;

      N is the number of working days per year, 260.

C=0,943260=245

Summary data on designed modes is summarized in Table 2

Process redistribution refers to an operation or a set of operations for the manufacture of reinforced concrete structures or products. So, for example, the redistribution can be taken as the preparation of a concrete mixture, moulding of products, TVs, disbanding, holding and storage.

Losses associated with intra-plant transportation are attributed to one or another technological redistribution. Loss and scrap values are normalized.

With sufficient approximation, you can recommend the average values ​ ​ of possible and scrap: for concrete mixture - 0.5%, for products up to 1%.

Redistribution calculations allow you to correctly select the process equipment at each redistribution, taking into account the increase in the capacity of the unit or plant associated with possible losses and failures at subsequent operations.

Process Line Design

3.1 Calculation of continuous formless line forming.

The line capacity is calculated by the formula:

P = 60 * H * C * V/T1, where

H - number of working hours per day, h

C - number of working days per year, day.

V volume of simultaneously molded articles, m3 

Tp is the molding cycle.