Working (cold) joint in concrete

"Cold joint" is a term you may encounter when working with concrete. This is the result of pouring two batches of concrete at different times, leaving a space between them. It’s important to know how to handle these joints correctly because improper management can weaken the structure.

Cold joints typically occur when there"s a significant delay between pours or if the initial layer of concrete starts setting before the next batch is added. To ensure a strong bond between the old and new concrete, it"s essential to prepare the surface of the cold joint properly. This means cleaning it and sometimes roughening it up to create a better connection.

We’ll examine the causes of cold joints, possible problems they may cause, and management strategies for them in this post. Whether you’re a professional or a do-it-yourselfer, understanding how to deal with cold joints will help you create a more dependable and long-lasting concrete structure.

What is a working joint?

A cold concreting joint is the line separating layers of poured concrete mortar. In these situations, one surface has had time to solidify, while the other is starting to do so. There are several reasons why joints form, but most frequently, it’s because the mortar was not poured smoothly.

The weakest point in a concrete structure is the working boundaries between layers. It is crucial to process them carefully in order to keep them from destroying the product. It is crucial to consider the structural and architectural aspects of the building when pouring concrete maps. This will prevent the cement surface from chipping and becoming less solid. There are cold joints that shrink and compensate for it.

1. Shrinkage connection boundaries are made in concrete floors. The goal is to compensate for the uneven drying of the poured screed by forcibly dividing the area into separate zones. The reason is that the thick concrete layer of the floor dries unevenly in thickness. As a result, the surface is deformed.
2. Expansion joints are used in monolithic structures for various purposes in order to divide concrete into several independent monoliths. This helps compensate for thermal expansion and other influences that lead to surface cracking. Such a technological cut cuts through the stone, including the reinforcing layer of the beam or any other element.

Reasons for appearance

There are several reasons why working boundaries are formed:

1. Organizational aspects. For example, stopping the work process due to shift change, equipment repair, lack of building materials, imperfection of the process chain, technical features of special equipment and mechanisms used.
2. Technological issues. Problems caused by the installation conditions of reinforcement, formwork, ceilings, floors. Often arise when it is necessary to limit the pressure on the structure.
3. Design issues caused by the need to create directional deformations on individual elements of the structure.

Layers of concrete are applied. The previously hardened layer of cement is covered with a new liquid layer. Consequently, there is less adhesion between layers. The material’s strength decreases to that of a monolithic block devoid of a functional joint. As a result, a beam, slab, or monolithic wall’s appearance deteriorates and its resistance to frost and moisture declines.

Performance characteristics deteriorate because shrinkage compressive forces are transformed into tensile loads between the concrete layers and the working joint. As a result, the weld area experiences an increase in stress. As concrete solidifies, it shrinks. It is least resistant to bending and stretching forces during this time. Microcracks are created when tensile forces abruptly weaken. As a result, the concrete at the joint has lower strength characteristics because of its low density.

Consequently, the working joint will develop cracks even under the same tensile stresses. This isn’t the case with monolithic products.

A working (cold) joint in concrete construction is the result of pouring freshly mixed concrete over partially cured or set concrete. This can leave a visible and potentially weak spot in the structure. Comprehending these joints is essential as they have the potential to impact the ultimate product’s strength and longevity. When cold joints are handled and treated properly, concrete can function as intended and problems like water infiltration and cracking can be prevented.

Disadvantages of working joints

The following drawbacks apply to the working joint:

1. Reduction in the performance properties of concrete.
2. Reduction in the degree of waterproofing.
3. High moisture permeability at the connecting border.
4. Deterioration of design and aesthetic qualities.
5. Strong tensile stresses that are unacceptable for concrete.
6. Violation of the structural characteristics of concrete stone.
7. Intensive moistening of underground floors and buried foundations.
8. Possibility of chemical destruction of the structure.
9. Instability to corrosion of the reinforcing layer in concrete.
10. Risk of material washout.
11. Destruction of concrete under mechanical impact.
12. Reduction of resistance to sudden temperature changes with freeze-thaw cycles of internal moisture.

Working joints can be arranged in a way that prevents the aforementioned undesirable phenomena.

Placement of working joints

Concrete working cold joints are created with consideration for technical parameters, manufacturing procedures, operational details, and product design features. There are three methods for setting cement layer joints.

The reinforced concrete product is divided into distinct sections along its perimeter by horizontal working joints. Pouring each block continuously is crucial. This arrangement of connecting elements enables the structure’s water resistance to be increased. This is accomplished by using a unique joint structure that obstructs the water’s path. Because of its weight, each successive layer of blocks builds a structure that is denser. The following benefits come from placing working joints horizontally:

• facilitation of the concreting process;
• reduction in volumes;
• improvement in formwork turnover;
• creation of optimal conditions for compaction of the cement mixture.

In order to place the joint vertically, the structure must be divided into distinct sections that are perpendicular to the base. Pouring concrete is done continuously. The joints’ structure ensures maximum water resistance. fusion of the initial two techniques. There are vertical and horizontal working boundaries. Large, massive structures employ this scheme.

Creation of cold seam joints

Ensuring optimal adhesion of concrete layers or blocks at the connecting boundary is the primary requirement for the cold joint boundary device during the concreting process. To do this, the surface of the hardened stone must be meticulously cleaned of all debris, including dust, snow, water, and icing.

Cement laitance, which is produced by soluble and insoluble efflorescence in the forms of sulfates, carbonates, chlorides, and nitrates, needs to be removed from the working surface. The weak and loose structure of the film formed by these substances hinders the adhesion of layers during separate concreting. Four techniques are suggested to clear the surface of cold joints of cement component breakdown products:

1. Mechanical restoration. For this purpose, manual tools or automatic milling are used.
2. Cleaning with water jet or pressurized air.
3. Hydrosandblasting.
4. Acid cleaning.

In addition to strengthening concrete’s adhesive qualities, working connecting boundaries need to be hermetically sealed and waterproofed. When these preventive steps are combined, water penetration will be avoided. By doing this, the possibility of reinforced concrete being destroyed when wetted will be decreased. Additionally, the structures’ insulation parameters will be raised.

There are many different technologies and materials available to treat cold joints with waterproofing and sealant in an efficient manner. It is common practice to use injection insulation, special cement compositions, etc. P. Among the materials for waterproofing, bentonite swelling cords and waterstops are particularly well-liked and efficient. Less often are gernite cords and other contemporary materials utilized.

You can guarantee appropriate dependability, strength, and extend the service life of reinforced concrete and concrete structures by using high-quality cold joint sealing and waterproofing. The two primary techniques for executing working seams are as follows:

1. Bookmark. Dividing into filling sections is carried out using boards, slats, plastic lining or glass cuts. After the concrete surface has set, the separators are removed. And the remaining gaps should be filled with waterproofing and sealant. Separators can also be left. This technology is acceptable when constructing floors.
2. cutting. The dividing device must be equipped with an abrasive attachment or a diamond wheel. The cuts are made to a certain depth.

To guarantee the durability and strength of your constructions, it is essential to comprehend working or cold joints in concrete. If not properly managed, these joints, which are frequently the result of pauses or delays in the concrete pouring process, can compromise the overall durability. Planning and carrying out concrete projects more successfully is facilitated by understanding when and how these joints form.

It’s crucial to prepare and treat these joints properly. Potential problems can be minimized by using the proper tools and methods, such as bonding agents and cautious surface preparation. This lessens the possibility of upcoming issues in addition to preserving the integrity of the concrete.

In summary, better performance and longer lifespan of concrete structures are ensured by carefully attending to working joints. When approached correctly, these joints become a manageable part of working with concrete, adding to the overall dependability and quality of your building projects.

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