Cold seam during concreting: what is it, construction of working joints, SNiP

Working with concrete can present a number of difficulties that could compromise the final product’s quality and longevity. A problem that may arise during the concreting process is the cold seam. In essence, a cold seam is a line or joint that develops when newly mixed concrete is poured over partially cured but already set concrete. This may occur when there is a pause in the work or when there is a delay between pouring stages.

Maintaining the strength and dependability of your concrete structure requires an understanding of cold seams. Cold seams can cause weak points in the concrete, which over time may cause cracks or other structural problems. It’s crucial to understand how to appropriately manage these seams as a result. This entails building functional joints to aid in the seamless integration of the old and new concrete.

Particular guidelines and standards, like those found in SNiP (Construction Norms and Regulations), provide comprehensive instructions on how to manage working joints and cold seams in the construction industry. By adhering to these recommendations, future issues can be avoided and the structural integrity of the concrete is preserved. Understanding these guidelines and using them correctly can have a big impact on how well your concrete job turns out in the long run.

When freshly mixed concrete is poured over partially set concrete, cold seams form in the concrete, creating weak spots in the structure. Comprehending these joints is essential to guaranteeing the robustness and endurance of concrete structures. In order to help builders attain the best outcomes, this article will define cold seams, discuss how to build working joints correctly to prevent them, and provide pertinent SNiP (Construction Norms and Regulations) guidelines.

Causes

Pouring a monolith is a technology that uses two techniques: pouring the solution continuously and laying cards in the shape of individual blocks. The first approach is the recommended choice since it offers superior concrete setting and hardening conditions.

This type of laying assumes the lower layer’s plasticity when the upper layer is poured, ensuring good adhesion, homogeneous strength gain, and monolithicity. But putting the strategy into practice isn’t always feasible.

• Limited time of work shifts, breaks in the operation of equipment, special transport.
• Time costs for the installation of reinforcement cages, scaffolding, assembly of formwork.
• Limitation of loads on a surface that has not yet gained sufficient strength.
• Concreting of embedded parts, utility inputs.
• Ensuring directional deformations of products and elements under loading.
• Creation of the horizontal part of the structure in the first stage, the vertical in the second.

• Long breaks in work after the mortar has set.
• Lack of formwork, technological equipment, scaffolding.
• Insufficient volume of concrete for pouring in one cycle.
• Understaffing of the work team.
• Low power of equipment, insufficient qualifications of personnel.

Concrete seams and their locations are planned ahead of time in situations where this cannot be avoided. Avoiding the chance of impromptu seams is advised, as is discussing them with the designer beforehand, making them compatible with technological advancements, and keeping an eye on technology. Such joints cannot be made in structures that have tensile forces.

A cold concreting joint is identified in the drawings by a callout that includes its name and the precise dimensions measured from the building’s axes. Apart from the technological ones, expansion joints are frequently incorporated into the structure with the primary objective of mitigating shrinkage and temperature variations in the concrete monolith.

In the resulting space, insulation strips, unique slats, or cords are installed. It is also necessary to remove these joints from the design drawing along with their designation.

Disadvantages of working joints

If the primary drawbacks of placing cold joints are considered during the design process and properly implemented, they can be avoided. Sewing devices that were not planned for but appeared on their own can cause serious issues.

• A weakened area appears in the joint area, which poses a danger to important and loaded structures, since the bearing capacity decreases.
• Water can get into microcracks, causing leaks and corrosion of reinforcement and concrete itself. In winter, water freezes and destroys the monolith.
• Decrease in water resistance, frost resistance, mechanical strength of the stone.
• Significant reduction in the service life of the structure/building.
• Presence of noticeable defects on the surface of the monolith.

In the joint area, there is an internal stress point on the concrete surface that is primarily caused by tensile forces. Concrete is strong when compressed, but it is not as strong when subjected to other kinds of loads. The joint area gradually deteriorates, raising the possibility that the building or structure as a whole may be destroyed.

If water penetrates the cold joints, the situation gets even worse. It speeds up the material’s decomposition and washes away the stone’s constituent parts. This is particularly hazardous when the foundation monolith is submerged in the ground; reservoirs and hydraulic structures are also at risk. Concrete is chemically corroded by aggressive soil substances.

Water that seeps inside will also contribute to mechanical damage to the monolith in the winter months due to alternating freezing and thawing if cold seams are not accounted for in the design or are not made in accordance with technology.

Location of seams according to SNiP

The pertinent documents specify the standards and guidelines for cold concreting joints. The primary prerequisite is that the seam cannot, under any circumstances, turn into a zone of concentrated stress. The axis of any slab, column, beam, and other concreted elements or structures must all be perpendicular to the joint’s location.

• For individual beams with a seam within the middle third of the span.
• For large beams monolithically combined with slabs (the joint is made at a mark of 20-30 millimeters below the surface of the slab).
• For columns, provided that the joint is at the mark of the bottom of the capitals, purlins, overhead beams or the top of the foundation.
• For massifs, vaults, arches, reservoirs, complex structures, buildings, where the joints are located in the zones provided for by the project.
• For flat slabs, where the joint can be arranged anywhere, but only parallel to the smaller side of the slab.

The best situation is when the monolithic structure’s minimum (zero) transverse force is located at the cold joint. This location is discovered through specific computations (in the transverse force diagram).

Locate the intersection of the diagram and the horizontal when doing calculations by hand. This is the location where the transverse force typically tends to zero. Examine the transverse force diagrams or their color schemes when using programs to perform computations (this is more visual).

The intersection of concrete layers is shown by a dotted line on all diagrams and drawings. Make a note with the term "working concreting seam" to provide a clearer definition. The joint positions cannot be altered; the diagrams shown in the drawings must be followed precisely. SNiP 3.03.01-87 contains all standards and recommendations.

Device technology

In order to guarantee the tightest fit and best possible adhesion of concrete layers, the cold seam has to be made. There are a number of impurities and water that need to be eliminated that can cause problems. However, in this instance, cleaning the surface alone is insufficient because the cement film is destroyed, aggravating the adhesion between the layers.

– entails acid washing; typically, orthophosphoric, hydrochloric, and acetic acids are used to dissolve cement film and open pores in concrete structures. The monolith is cleaned with water after etching.

Furthermore, in order to increase the adhesion between the previously set and subsequent layers multiple times, notches can be made on the seam’s surface and covered with adhesive, bitumen, or polymer mastics. The connection area is covered with a small-cell reinforcing mesh; the use of galvanized dowels with two working surfaces has shown to be successful.

• Correct selection of the joint location based on SP 70.13330.2012 (here the permissible boundaries for flat/ribbed slabs, columns, beams are clearly indicated). For blind areas, floors, and other coverings, zones are selected in accordance with the volume of concrete and the technology used.
• Creation of an even edge during concreting, waiting for the mixture to gain at least 1.5 MPa (usually the waiting time is 1-3 days).
• Preparation of the joint using a mechanical or chemical cleaning method. But experts advise combining both methods.
• Pouring the joint area with concrete, compacting and leveling the mixture.
• In the absence of preliminary preparation of the seam area, the concrete is cut along the joint with a special machine with an appropriate diamond disk.

Sealing joints receive extra consideration when it comes to the arrangement of insulating, temperature, structural, and shrinkage joints. In order to prevent moisture intrusion and account for the movements of concrete monoliths, swelling profiles and cords made of bentonite and germite are utilized.

The quality of adhesion of the layers and the proper placement of joints determine the dependability and integrity of the structure during the concrete process with seams. In order to improve adhesion, the hardened monolith is frequently treated in a particular way and the preceding layer is frequently made uneven.

Make sure the fittings are clean. The key qualities and indicators that must be exhibited by solutions poured in successive layers are strength and load-bearing capacity.

One crucial aspect of construction that can have a big impact on the longevity and quality of concrete structures is managing cold seams during the concreting process. When new concrete is poured over previously set concrete, cold seams develop. The structural integrity of your project depends on your ability to manage these seams and build working joints correctly.

It’s critical to adhere to established norms and guidelines, such as those specified in SNiP regulations, when building working joints. By following these guidelines, you can minimize potential structural weaknesses by ensuring that the transition between different pours of concrete is handled correctly. Joints that are properly maintained can enhance the overall strength of the concrete and help avoid problems like cracking.

In conclusion, better results and longer-lasting concrete structures will result from paying attention to how cold seams are handled during the concreting process and from following construction standards. You can steer clear of common pitfalls and produce a more robust and long-lasting result in your construction projects by concentrating on the right techniques and guidelines.

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