In construction work, it is often necessary to create a strong bond between new concrete, hardened concrete, and plastic. This is particularly important when resuming the concreting of an incomplete construction member or performing repair work. The successful formation of a good bond between new and existing concrete is essential for these purposes.
To generate a strong bond, the concrete surface must be roughened before pouring new concrete and curing it. The roughening process is crucial for the success of bond formation. For fully hardened concrete surfaces, sandblasting, acid washes, or bush hammering can be used to achieve the necessary roughness. On the other hand, for newly placed concrete or plastic concrete, wire brushing is an effective way to roughen the surface.
Overall, creating a strong bond between new and existing concrete is an important task in construction work. By roughening the surface of the concrete, pouring new concrete, and properly curing it, a strong bond can be formed. Whether using sandblasting, acid washes, bush hammering, or wire brushing, the roughening process is vital for successful bond formation.
How to Bond New Concrete to Hardened Concrete?
To roughen and clean an unprepared hardened concrete surface, one can use methods such as sandblasting, bush hammering, acid washing, or any other appropriate means. After roughening and clearing the surface, a bonding agent must be applied to ensure good adherence between the existing and fresh concrete. However, it is important to note that the bonding agent will not work effectively if the concrete surface has dust layers of cement, fine aggregate, oil, or grease. Hence, the surface should be free from dust and dirt.
When conducting repair work, milling or grinding can be used to reach hard concrete before applying the bonding agent. Sand-cement and water-cement grouts are cost-effective adhesive agents that can be used to bond hardened and fresh concrete. The sand-cement grout consists of 1 cement, 1 sand, and ½ water, while the water-cement grout consists of 1 bag of cement and 6-7 gallons of water.
Proper curing of poured concrete is crucial to ensure that the bonding strength is not lost. Wet burlap, wet sand, plastic sheets, curing paper, cloths, curing compounds, or a combination thereof can be used for curing. However, if the modulus of elasticity, coefficient of thermal expansion, and shrinkage strains of the existing concrete differ from that of fresh concrete, thermal movement due to temperature variations can generate sufficient stresses to break the bond during the curing process. Therefore, curing compounds should not be used as they may interfere with bonding between concrete surfaces.
After 24 hours of repairing, a hammer or any other dull object can be used to gently tap the poured concrete in the repaired area to check the bonding condition. By listening to the tapping sound, one can determine the bonding status. If the sound is hollow, then the bonding is not right, and cracks may develop in the future. However, if the sound is a dull thud, then the bond is good.
For plastic or recently set concrete, wire brushing can be used to roughen the surface of the concrete. Alternatively, retarder can be applied to the surface of the concrete to postpone the setting time of the concrete.
FAQs on Bonding new concrete to plastic and hardened concrete
How to bond new concrete to plastic concrete?
When dealing with plastic or newly set concrete, there are different methods that can be used to achieve a desired surface texture. One common method is wire brushing, which involves using a tool with wire bristles to roughen the surface of the concrete. This can be useful for creating a non-slip surface or improving the adhesion of subsequent layers of concrete.
However, wire brushing may not always be the best option. In some cases, it may be preferable to delay the setting time of the concrete in order to achieve the desired surface texture. This can be accomplished by using a retarder on the surface of the concrete. Retarders are chemical additives that slow down the setting process of the concrete, giving workers more time to work with the surface and achieve the desired texture.
Overall, whether to use wire brushing or a retarder depends on the specific needs of the project and the desired outcome. Both methods can be effective in achieving the desired surface texture, and it is up to the workers to determine which method is best suited for the task at hand.
Is a concrete bonding agent necessary to bond new concrete to hardened concrete?
A bonding adhesive is a substance used to ensure a secure attachment between two surfaces, specifically in the case of existing and fresh concrete. This type of adhesive is designed to promote a strong bond between the two concrete surfaces, preventing them from separating or breaking apart.
In construction projects, bonding adhesives are commonly used when pouring new concrete onto an existing concrete surface. The adhesive is applied to the existing surface prior to pouring the fresh concrete, which helps to create a strong and stable connection between the two layers. This is important because without proper adhesion, the new concrete layer can crack or shift, causing structural issues or safety hazards.
Overall, bonding adhesives play an important role in ensuring the longevity and stability of concrete structures. By providing a secure bond between existing and fresh concrete, they help to maintain the structural integrity of the surface and prevent damage or safety hazards from occurring.
How do you bond new concrete to existing concrete?
The process involves several steps to achieve a strong and durable concrete surface. Firstly, the hardened concrete surface needs to be roughened and cleaned thoroughly to ensure proper adhesion of the new concrete. Secondly, a bonding agent is applied onto the roughened surface to further enhance the adhesion of the new concrete.
After the preparation is complete, the new concrete is poured onto the roughened surface and allowed to set according to the appropriate time frame. Finally, a suitable curing regime is applied to the poured concrete to ensure proper strength development and durability. This involves providing the appropriate conditions of temperature, moisture, and time for the concrete to properly cure and reach its full potential.
How to roughly test the bond between new and old concrete?
To check the bonding condition of a repaired area on poured concrete, you can use a hammer or some other dull object. It’s important to wait for at least 24 hours after the repair has been done before performing this test. By tapping the area gently, you can listen to the sound it produces. If the sound is hollow, this indicates that the bonding is not right and there may be a risk of cracking in the future. However, if the sound produced is a dull thud, then it’s a good sign that the bond is strong and the repair has been successful. It’s essential to perform this test to ensure that the repair has been done correctly and to prevent any potential future problems with the concrete.