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The popularity of reinforced cement concrete (RCC) construction is growing day by day. Reinforced cement concrete is a composite material made of steel and concrete. Steel provides tensile strength, and concrete is for compression. This composition gives an RCC structure durability, strength and versatility. However, as RCC is a mixture and it is made and laid in batches, cold form joints are a common occurrence. Today, in this blog, we shall discuss cold joints in concrete.
What are Cold Joints in Concrete?
Cold joints are the weak spots in the RCC structural members caused by a lack of proper bonding between fresh and set concrete. This is mainly due to a pause or delay in the concreting process. In most cases, cold joints develop when there is a long interval between successive concrete pours. For instance, the first batch could begin setting if there is a pause or delay in the concrete supply or concreting processes. Because of this, there is a greater possibility that the two batches will not combine evenly.
Also Read: Methods of Placing Concrete
Also Read: Can you pour concrete in the rain?
In addition to the causes above, poor consolidation can lead to cold joints. Avoid cold joints in concrete because they cannot form a bond with freshly placed concrete.
Placing concrete in layers around 18 inches deep and intermixing each layer with the previous one using a vibrator is important to prevent cold joints in walls, beams, and other structural components. Start at the outside corners and work inward while laying concrete. While laying slabs, stack the concrete in one batch on top of the other rather than pouring random heaps. The direction of labour on slopes should be upward. A retarding admixture may be necessary for hot conditions to reduce the setting time.
Effects of Cold Joints in Concrete
Some impacts and reasons behind cold-form joints are
● Appearance of Cracks
These joints create no significant void structure in the concrete; they only produce seams. The characteristics of a cold joint are linear, tightly linked, and bonded. The colour of the two pours will be different and hence easily distinguishable. This will be an issue in a situation where visual appearance is important.
● Bonding Difficulty
Fresh concrete does not integrate properly with old concrete when there is a considerable time gap between two batches of concrete. Delayed treatment can increase the risk of joint separation. It’s not always the case, but a cold joint can be a vulnerable spot. The concrete surface requires a roughened concrete surface beneath it to mechanically attach to the concrete surface below.
● Failure in Tension
The cold joints in concrete in the compression zone are not an issue, as concrete is not weak in compression. However, due to the inherent weakness of concrete in tension, the cold joints in the tension zone of concrete tend to be problematic. Reinforcement is provided in the concrete to carry tension. Hence, cracks in concrete due to the cold joint in the tension zone are slow in propagation and demand repairs as soon as an early sign of a crack is visible.
● Leakage Problems
Cold joints may lead to leakages in the slab. The issue is problematic if the leakages are from the toilet area. Therefore, toilet slabs are tested for leakages, grouted, and waterproofed before taking up the finishing work.
● Damage of Structure
Weak concrete might cause an early decline in strength, leading to significant damage. There are several chances of the concrete deteriorating upon exposure to an aggressive environment. This problem becomes evident in the case of improper compacting of the concrete.
Also Read: Compacting of the Concrete
How to Prevent Concrete Cold Joints?
Some general steps can prevent cold joints in concrete. They are:
● Good Pouring Technique
Before the start of any concreting activity, it is crucial to conduct an initial resource assessment. The volume of concrete and the type and shape of structures are the factors to be considered before pouring the concrete. Concreting will be carried out in stages, with the provision of construction joints in the case of a large volume of concrete.
● Good Compaction
Place a layer of rich mortar, 12-mm thick, on the cold joint before adding the fresh concrete. The cold joint is then completely and methodically vibrated, sending the vibrator deep into the old layer of concrete. The joint can solidify for at least 12 to 24 hours if previously put concrete becomes too hard or stiff and mortar or slurry does not rise despite intense vibration. After cutting the concrete to the proper shape and preparing the surface, what comes next is curing this type of cold junction like a typical construction joint.
● Repairing in Plastic Stage
You can put fresh concrete directly on top of the old surface if the concrete is removable manually and if vibrators can easily penetrate the surface. Make sure to cover the old concrete with the new one as soon as possible, while also ensuring complete and methodical vibration of the joints.
● Repairing when Hardening Starts
If the concrete has hardened a bit beyond its flexible state, then the surface will have to be raked thoroughly to remove any loose concrete without disturbing the rest of the concrete. Then, the cold joint is covered with a thick 12 mm layer of mortar, and new concrete is added on top of it. The cold joint is then vibrated so that the vibrations penetrate down through the existing concrete layer.
● Repairing when Completely Hardened
Cold joints in the concrete cause no structural issues. But voids can weaken the bonding between the two layers, calling for repairs. If the concrete at the joint has set and cannot be reshaped, and if the mortar or slurry will not rise despite the vibration, then the joint must be repaired.
● Using Admixture
Typically, concrete needs about an hour to be fully set. The admixtures will allow us to lengthen this period. The concrete’s setting period is typically stable initially for around three hours. However, this may require alterations to fit the specifications of the location. Retarders and other admixtures are effective in postponing the concrete’s initial setting period. Use retarding admixtures, which lengthen the time it takes for concrete to set when working with it in excessively hot conditions.
● Additional Methods
Some other methods are
- Applying a bonding agent strengthens the connection.
- Vibration techniques enhance the joint’s workability. The likelihood of successfully combining the vibrations of two layers would increase.
- Install dowel bars to reinforce the connection.
- Investigate the areas where joints can come up in construction. Take preventive measures in situations where cold joints are possible.
- Vibrate the joint carefully and systematically, and cover the old concrete as soon as possible with fresh concrete.
- Take extra care to prevent any kind of failure in concrete from developing because of cold joints in the concrete wall, particularly in areas where tensile pressures will be applied.
- Common concrete crack repair strategies, like epoxy grout injection or a similar strategy, could fix any concrete cold joint cracks.
- Keep chemicals like mold oil out of the column and beam cold joints.
How to Treat Cold Joints in Concrete?
- In the case of concrete cold joints in non-structural members, seal them with joint sealant to prevent water leaks.
- Fill the gaps with a high-pressure epoxy injection, which imparts strength and bonds well.
- For larger cracks, inject polyurethane underneath the slab to prevent water from seeping through the joint.
- Install an elastomeric waterproofing membrane to seal concrete cold joints.
- For cold joints in concrete foundations, use underpinning techniques for strengthening purposes.
Conclusion
A major cause of cold joints in concrete is an excessive amount of time passing between the various stages of the concrete-pouring process. The cold joints may lead to a decrease in load transfer capability, and this area becomes the most vulnerable spot in the building. Thus, cold joints have a significant impact on the structure’s load-bearing characteristics. A proper construction joint is a must if there is a considerable time gap between the two stages of concreting.
Author Bio
Nafisa Nazneen Choudhury – Nafisa Nazneen Choudhury is a Civil Engineer (completed B.E. from Assam Engineering College) and is currently pursuing M.Tech in Structural Engineering at National Institute of Technology, Silchar. She is a Technical Content Writer, having over 3 years of experience and has wrote many articles related to Civil Engineering. She is also a Book Author (Authored – “Dream Tales of NNC: Revenge By Murder”) and her book can be found on Amazon. She is also a Guest Author at Gharpedia. Moreover, she is a certified member at Institution of Civil Engineers (ICE) and National Society of Professional Engineers (NSPE). She writes her blogs at her website – nnc2017.wordpress.com. She can be reached on LinkedIn.