Concrete seams: workers, compensation, shrinkage. technology

26-01-2018

Building

Few of us know what working concreting joints are. Or why do we need compression and expansion joints in concrete and are there any rules regarding their device in different cases? Let's try to understand this issue.

One type of seams divides the monolith into areas with clearly defined boundaries.

Work seams

Definition

Working (cold, building) is the joint between the set and the new concrete inside the monolithic structure.

Please note: cold seam is a negative phenomenon, since it reduces the final strength of the monolith. The ideal case is when the entire formwork volume is filled in one go. However, for technological, financial or organizational reasons, this is not always possible.

A compromise solution to avoid the drop in strength - layer-by-layer filling of formwork, in which a new layer is laid out before the old one sets.

Device rules

What to do if concreting still has to be done with significant interruptions? How to make as strong as possible, for example, working seams when concreting the foundation?

According to SNiP number 3.03.01-87 in this case it is worth adhering to a number of simple rules.

First and foremost: if the concrete has managed to grasp, further work on concreting is performed only when it reaches a strength of at least 1.5 MPa (150 kgf / cm2).
The surface of the old concrete before the resumption of work necessarily cleaned of cement films. This work can be done with your own hands (with a metal brush or hand tool), or with the use of sandblasting technology.
For better adhesion, it is strongly recommended to use primers, adhesives or bitumen.

With the foundation figured out. Are there any subtleties in the device seams more complex loaded structures?

Design	Requirements
Columns	The seam is located at the bottom of the girder, beams or top of the foundation.
Beams, flooded together with overlapping	The seam is 2 - 3 centimeters below the bottom surface of the slab.
Plates of any type (including floors)	The seam is parallel to the smaller of the cross sections of the plate.

Work seams расположатся на уровне низа несущих балок.

Compensation

Expansion joints in concrete, as is easy to guess from their name, are intended to compensate for thermal expansion and other effects that can lead to the appearance of cracks. They completely cut through the concrete structure (including reinforcement), effectively dividing it into several independent monoliths.

They are needed:

With a length of a monolithic structure over 50 meters.

Curiously, the British Encyclopedia of the concrete worker is a legislator in the field of the maximum size of a monolith. However, in the British Isles the climate is much warmer than the Russian one, and, therefore, the maximum thermal expansion is less. With this in mind, it would be much more reasonable to stick to at least half the size.

Along the perimeter of the monolithic floor (the goal is to prevent shrinkage of the walls and the foundation to cause cracking of its surface).
Around the columns, based on the bedding or compacted soil.

The photo clearly shows the seam around the support element of the metal structure.

And in this case, of course, there are several rules.

Thermal seams in concrete must be at least 6 millimeters wide. The instruction is related to the temperature expansion of the material: with a smaller width, it may simply not be enough for heating.
Around the columns surrounding their seams can be both square and round. Square unfolds relative to the column by 45 degrees (so that opposite the corner of the column it is straight).

In the first two figures, the columns are insulated correctly. At the third isolation of the column from the screed is not performed. The cost of the error is a crack in the monolith.

The resulting cavity must be filled with sealant or any other insulating material. The accumulation of water, dirt and microorganisms will lead to accelerated destruction of concrete.

Shrinkage

Shrinkage швы в бетонных полах призваны компенсировать неравномерность высыхания стяжки после ее заливки. Дело в том, что толстый слой бетона сохнет не сразу по всей толщине; верхний слой дает более сильную усадку, в результате чего стяжка пытается деформироваться. Благодаря принудительному делению поверхности на сравнительно небольшие по площади элементы трещины либо не образуются вовсе, либо образуются по уже намеченным линиям (читайте также статью «Армированный бетон: особенности изготовления»).

Rules?

They are here:

A map (a fragment of a screed limited by shrinkable seams) is made square or rectangular, with an aspect ratio of not more than 1: 1.5..
Lines must be straight, without bends and branching.. Otherwise, there is a chance to get a crack in an arbitrary location, and not along the marked line.
For indoor spaces, the maximum size of the seam bounded sector is 6x6 meters.. A concrete yard with a much greater temperature delta and, accordingly, large fluctuations of linear dimensions is divided into sectors no more than 3x3 meters. The concrete path is divided by a longitudinal seam with a width of 3.6 meters.

The concrete platform is divided into square sections.

L-shaped sections of the floor are dissected into squares and / or rectangles.
The depth of the seams should be 1/4 - 1/3 of the thickness of the tie.

Technology

We dealt with the technology of working joints, but did not touch on the methods of performing compensatory and shrinkable ones.

Actually, there are only two methods.

Principle	Description
Bookmark	When casting a screed or a monolith, it is divided into sectors with planks, slats, plastic clapboard or glass pieces. After the concrete has set, the elements separating it can be removed or left - everything depends on their material.
Cutting	Concrete is cut to the desired depth by an ordinary grinder with an abrasive wheel. However, cutting of reinforced concrete with diamond circles is much more convenient - if only because for the passage of the reinforcement it is not necessary to change the circle.

It is useful: if necessary, to make a hole for communications just as convenient and practical will be diamond drilling of holes in concrete. Diamond crown does not require the use of a percussion mode; as a result, the edges of the hole remain perfect.

Cutting shrinkable seams with a diamond saw.

Conclusion

Надеемся, что предложенная информация будет полезной читателю при ремонте или строительстве (см.также статью «Цветной бетон – все, что нужно знать о данной группе материалов»).

The video in this article traditionally contains additional thematic materials. Successes!