Managing cold joints is an important concept to grasp when working on concrete projects. These happen when freshly mixed concrete is poured on top of a partially cured but already set layer. For the completed structure to be strong and long-lasting, cold joints must be handled correctly.
Cold joints can occur in a number of situations, including when different batches of concrete are used or when work is suspended. The final result can be greatly influenced by the way these joints are treated. In order to ensure that the integrity of the concrete is not compromised by these joints, careful planning and execution are essential.
We will look at how to create cold joints in concrete in this article. We’ll go over the best ways to prepare, make sure the layers adhere well, and steer clear of typical pitfalls. Whether you’re a professional or a do-it-yourselfer, knowing these techniques will help you build a sturdy concrete structure.
| Topic | Description |
| Definition | A cold joint in concrete occurs when there is a delay between successive pours, leading to a visible line where the new concrete meets the old. |
| Causes | Cold joints can happen due to interruptions in work, changes in mix or weather conditions, or delays in pouring. |
| Prevention | To prevent cold joints, ensure continuous pouring and use appropriate mix designs. If a delay is unavoidable, prepare the old surface properly by cleaning and roughening it before adding new concrete. |
| Treatment | If a cold joint forms, treat it by cleaning the area and applying a bonding agent before pouring the new concrete to improve adhesion. |
| Impact | Cold joints can affect the structural integrity and appearance of the concrete, so it"s crucial to manage them properly to avoid potential issues. |
- Feature of cold joints
- Correct location of cold joints
- Negative consequences of the formation of cold seams
- Sequence of work when arranging cold joints
- Methods for increasing the adhesion of a concrete surface
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Feature of cold joints
One characteristic of HS is that a cement film—a dense material—forms between the "old" and "new" concrete layers. Its source is not only calcium hydroxide (the aqueous solution that rises to the concrete’s surface and reacts with carbon dioxide), but also a variety of cement mortar ingredients such as crushed stone, sand, water, plasticizers, and alkali metal salts. In other words, we can state that the cement film is made up of a combination of nitrates, carbonates, sulfates, and chlorides, some of which dissolve in water and some of which do not.
What happens when substantial disruptions are made to the layer-by-layer concreting process? Three layers are visible in the "pie": concrete, cement film, and concrete once more. The "weakest link" in the construction of any object is the middle component of this structure. The building cannot be regarded as a monolithic structure if the cold joint is not properly maintained. It will be a completely prefabricated structure, with each component sensing mechanical loads on its own. It is possible to ensure the high strength characteristics and long service life of the constructed structure by adhering strictly to the building rules and regulations regarding the cold joint arrangement.
Remark: Cementing should be used if it can be done so without causing a cold joint to form. The structure will be able to support even heavier loads in this scenario.
Correct location of cold joints
Building codes and regulations stipulate that the placement of the HS arrangement must be decided upon during the design phase and indicated on the plan with a unique line (deviations from the project are unacceptable). When concreting, in conformity with SNiP 3.03.01-87 (p.2.13, which went into effect on January 7, 1988):
- Flat slabs cold joints are arranged parallel to the smaller side of the product.
- Beams of significant sizes (they together with the slabs represent a monolith) HS are located in the zone, retreating 2-3 centimeters from the level of the lower surface of the slab.
- Rib slabs – in the direction that is parallel to beams of secondary importance.
- Other purlins – in the area that is defined as the middle part of a third of the beam span.
- Columns – at the mark of the top of the foundation, as well as the bottom of the capitals and purlins.
- As for vaults, arches, arrays and various bunkers, the arrangement of HS in them is carried out in places that are determined directly by the chief engineer of the project.
Note: In order to ensure the overall strength of the structure, construction seams may only be equipped in areas that do not hold a dominant position. As a result, the XH is not equipped in regions where the maximum transit force—which is the algebraic total of all the forces fastened to the beam—applies. Additionally, the construction seam is perpendicular to the concrete element’s surface axis. One more thing: you shouldn’t permit corner XH to form.
Negative consequences of the formation of cold seams
Because of the development of a cement film between a layer of laid, previously hardened concrete and newly equipped: there is:
- The formation of microcracks through which water penetrates. As a result of this, corrosion of the reinforcing component of concrete may occur.
- Less adhesion between concrete layers.
- Low water resistance and frost resistance of a monolithic structure.
- Decrease in the strength characteristics of concrete and, as a result, its operational period.
- Deterioration of the appearance of the plate (walls or beams).
- High risk of leaching of building material from the HS zone. This poses a huge threat to the entire structure as a whole.
- Significant fragility of concrete, which quickly collapses under any mechanical impacts. This occurs due to the fact that in the cold joint area, stresses are transformed, namely, compression into tension.
All of this is preventable if prompt action is done to correctly process the layer of concrete that has already hardened (in compliance with building codes and regulations).
Sequence of work when arranging cold joints
The following is the work methodology:
- In accordance with the design documentation, the location of the future cold joint is determined.
- Pour a cement-sand mixture to a certain level.
- Compact the concrete layer using a vibrating device (immersion depth is about 50 ÷ 100 mm) with subsequent leveling.
Crucial! It is not acceptable to place the vibration device on any formwork structure components while tamping.
- After 1-3 days, when the concrete has gained the required strength, its surface is cleaned using either a water jet (it can also be air), or a milling cutter (electric or pneumatic), or a mechanical brush made of metal. The cleanest and highest quality coating is obtained in the case of pressure treatment. Using a metal brush significantly reduces the cost of the technology, but this method is applicable only if the concrete is “fresh” (that is, 6-7 days have passed since its installation, no more).
Crucial! Only after the preceding layer’s strength has surpassed 1.5 MPa may the next layer be installed (SNiP 3.03.01-87, paragraph 2.13). This holds true for private, public, and industrial construction. In addition, concrete needs to be shielded from precipitation and excessive moisture loss while it is starting to harden. Afterwards, stick to a specific routine (which includes both a specific humidity and temperature).
- Before pouring the next sand-cement layer, the prepared surface is washed with water and dried with a stream of air.
- Measures are taken to increase the adhesion of the concrete surface. This must be done so that the concrete can withstand significant loads in the future. There are many methods to strengthen the adhesion between concrete that has already gained strength and the newly installed one (more details about them will be given below).
- First, a thin sand-cement layer (about 20÷30 mm) is laid over the seam, which will more quickly enter into “close contact” with the already hardened concrete.
- After 2÷3 hours, the entire prepared volume of concrete mortar is laid.
Crucial! Concrete is poured in horizontal layers without breaks; each layer must be the same height. Furthermore, all layers are laid in the same direction with respect to the cement-sand mixture. The formwork should be spaced 5÷7 centimeters from the top layer of laid concrete mortar.
Methods for increasing the adhesion of a concrete surface
- Treatment of the concrete surface with a deep penetration primer or bitumen and polymer mastics. The method is quite effective, although expensive.
- Applying deep notches over the entire area of concrete using a grinder, chisel, hammer or hammer drill. The method is relevant in the case of long-hardened surfaces, that is, when construction work is resumed after a long period of inactivity.
- Reinforcement of the construction joint with a steel mesh with fairly small cells.
Note: Experience has shown that utilizing multiple processing techniques at once yields the best results in terms of boosting a concrete surface’s adhesion.
A common feature of concrete work, particularly in large or intricate projects, is the presence of cold joints. They happen when there is a pause between subsequent pours, separating the concrete batches clearly. The strength and longevity of the completed structure depend heavily on your ability to manage these joints.
The secret to managing cold joints well is preparation. In order to avoid weakening the bond, loose material or contaminants must be cleaned from the surface of the existing concrete. The adhesion between new and old concrete can also be strengthened by using a bonding agent.
It’s crucial to keep an eye on the pouring schedule after the surface has been ready. The best time to pour new concrete is ideally when the old layer is still soft. In order to maintain the project’s structural integrity, address the cold joint if a delay is inevitable.
You can successfully manage cold joints and preserve the caliber of your concrete work by adhering to these procedures. With careful planning and execution, you can meet the requirements of your project and come up with a solid, long-lasting outcome.
When fresh concrete is poured over an area where the previous layer has already started to set, cold joints in the concrete form. It’s crucial to make sure that the old concrete is properly prepared for managing these joints, which includes cleaning and roughening it to encourage a solid bond. You can prevent potential structural weaknesses and create a smooth transition between the layers by carefully planning and timing each pour.
