Concreting: various methods, rules of laying

В настоящее время производится бетонирование по SNiP 9.03.01-87 по несущим ограждениям и конструкциям, который заменил собой все ранее существующие СНиПы по этому виду строительно-монтажных работ. В данном случае речь пойдёт не о сборных, а о монолитных конструкциях, изготавливаемых непосредственно на стройплощадке, причём это возможно выполнить в любое время года и даже при любых погодных условиях.

We now consider in turn the basic ways of laying concrete, and also show you the thematic video in this article.

Methods

Примечание. SNiP 9.03.01-87 используется при строительстве зданий и сооружений для обустройства бетонных и железобетонных монолитных конструкций. Это применимо к особо тяжелым, тяжёлым и пористым (лёгким) бетонам, а также к щелочестойким и жаростойким смесям при торкретировании и подводных работах.

Replacement

General rules for laying concrete

According to SNiP 3.01.01-85 * (instead of SNiP III-I-76, SN 47-74 and CH 370-78), before starting concrete work, you need to obtain a permit, organize the conditions of entry to the site, and provide the object with necessary equipment and materials. If the production of all of this involved engineers, technologists and foremen, then at the home construction site you will have to do it yourself.

This suggests that in addition to the preparation of cement, sand, crushed stone and reinforcement, as well as trenching tools and concrete mixers, you need to prepare the foundation on the ground - a platform or trench for the foundation.

You will need to remove all rubbish from the construction site with your own hands, and also uproot the trees and shrubs growing there, which are located directly in the building zone or behind it, but in close proximity, which interferes with work or transportation.

You also need to prepare a deep platform for slab foundations (often used for garages) or dig a trench for the strip base, according to the planned building design. The depth of the tape is determined by the characteristics of the soil (heaving, non-cracking, and so on), as well as the weight of the structure above, the width of the tape depends on the width of the base or wall and, in fact, it will be the rate of concrete consumption during concreting.

To lay the concrete foundation, in any case, formwork is necessary, and at home it is made either from ready-made boards or from boards, fixing them with various supports and clamps. In addition, monolithic structures, as a rule, are reinforced - at home.

For this purpose, smooth and / or relief reinforcement with a cross section of 8-14 mm is usually used. So for 5m³ conventional (non-special) monolithic structures will need a ton of reinforcement, and below we give a weight table:

Note. The calculations take into account the mass of the entire frame, that is, both horizontal and vertical rods.

For low-rise construction, concreting rules usually provide for a solution of class B22.5 or B25 (M300 and M350), but in addition there is also a mixture of grain size, so fine-grained solutions are not suitable for pouring the foundation. Therefore, the higher the concrete class, the larger the aggregate fraction will be.

Regardless of where the concrete mix is ​​prepared (in an automixer, in a concrete mixer or in a trough with a shovel), the instruction recommends that you first mix the cement to make a uniform milk. After that, add the required amount of sand, and after thorough mixing, the turn comes to the granular aggregate. If you follow this sequence, the strength properties of concrete increase almost three times.

Regardless of when concreting takes place - in the fall, spring, winter or summer - in any case, the installation requires compaction and for this purpose, the method of vibrating with an immersion club is most often used. For this purpose, electric vibrators are most often used, but they can also work from a battery, diesel or carburetor engine - the principle remains the same - the tip oscillation amplitude is created using a flexible shaft, which, in fact, is driven by one engine or another.

There are also other ways to seal the fresh mix - this is a vibrating board (tamping) and vacuuming, which can already be attributed to special methods of laying the mixture. However, any of these methods serves the same purpose — the elimination of pores (voids) that remain after the evaporation of moisture from the solution.

Work seams

When installing large objects, it is almost never possible to make monolithic elements completely monolithic, that is, one-piece, since it is not always possible to complete the installation of the mortar. In such cases, the following happens: one portion of the mixture seizes and the hardening process begins, and the other portion, connecting to it, does not have sufficient adhesion, therefore, the cold concreting device is the weakest point in the design.

When installing various engineering structures of a complex nature, such as columns, arches or vaults, the arrangement of working joints during concreting is possible only in those places that are foreseen in advance by the planned project. This is done to ensure that the mechanical load in such places is minimal, that is, the direction of the seam should be in a perpendicular position with respect to the axis of the concreted structure.

Special methods

• This method consists in removing from the already compacted concrete up to 10% -25% of the water that has been closed using vacuum shields, which improves two indicators at once.. Firstly, it accelerates the hardening of the mixture (less time is spent on the evaporation of moisture in the solution) and, secondly, it increases the density of the monolith, as shrinkage occurs (pores close).

• In cases where it is not possible to perform work on the diversion of water from the construction site (bridge pillars, dams), underwater concrete is used, and most often, this is the VPT method - vertical pipe movement. The essence of this method is as follows - the foundation of prefabricated reinforced concrete structures is laid to the bottom of the reservoir and to them, a pipe consisting of sections falls almost vertically. Its lower part is equipped with a valve, which is regulated by the operator from above (so as not to get water), and a funnel is installed in the upper part to receive concrete.
• The solution is fed into the pipe until it is completely filled, and then the valve is opened and the mixture fills the formwork under pressure, rising above the edges of the lower section of the pipe (the height of the lifting mixture depends on the height of the water column). The pipe is gradually lifted, removing the excess sections from above, and so on until a sufficient volume of the monolith is laid.

• There is also an underwater concreting by the BP method - the method of the ascending solution, where the early cape of the stone is applied, to the territory fenced with formwork. In this sketch steel pipes are lowered, through which a mixture is fed under pressure, however, the same processes are already taking place here as in the process of VPT.

Пояснение. Понятие «торкретирование» происходит от существительного «торкрет», что означает цементно-песочную смесь.

• This method is most often used to strengthen the thin homogeneous structures (domes, arches, vaults), as well as to eliminate various mechanical defects and for monolithing seams. Торкрет наносится на поверхность с помощью сжатого воздуха и специального оборудования - частицы смеси развивают скорость до 100м в секунду, что делает покрытие достаточно плотным. Недостаток данного метода состоит в том, что часть материала не удерживается на поверхности - просто отскакивает от неё, поэтому, такие потери называют «отскоком».

Concrete laying in winter

In order to meet the deadlines set by the project, concreting is carried out in the cold season, that is, in this case we mean the process, which is carried out at air temperatures from -5? C to -50? C with the help of artificial heating. One way is to warm the concrete with an infrared heater - the method is quite convenient, especially since the process does not need to remove the formwork - shields are not an obstacle to IR radiation.

Also in the cold uses electric thermomats, which simply cover the entire monolith for 3-4 days. But such heating is limited, as it is convenient only for horizontal structures.

To warm the walls it is very convenient to use warm formwork, where the heating elements are located directly on the boards and in the event of their failure, are easily dismantled for replacement. Despite all the comforts of the heating formwork, it has a significant problem - it is its high price.

For vertical structures, the mixture is also heated by electrodes, which are made of reinforcement with a thickness of 8 mm to 14 mm. They are vertically stuck into the solution at a distance of 80-100 cm from each other and serve as anodes. Warming up occurs at the expense of the blocked water which is contained in concrete. The method is convenient, but quite expensive, as it requires a lot of power consumption.

Most often for the electric heating of concrete is used PNSV (Steel Heating Wire Vinyl sheath), which is laid loops inside the poured structure. Power is supplied to it by a step-down transformer, and at home a welding machine is used as such a unit.

Conclusion

Any concreting of the territory, architectural structures and other structures always has its own peculiarities, depending on external factors and conditions of future operational loads. However, for each of the methods, there are certain rules regulated by state standards and building codes and regulations that allow you to create a solid monolith for long-term use.