Cold Joint Concrete – Its Effect and Treatment Methods
What is a cold joint?
“Cold joint is defined as the plane of weakness in concrete due to an interruption or delay in the concreting operations.”
It is generally formed when the first batch of concrete start setting before the next batch is arteries so that the two batches do not intermix. Sometimes, the cold joint is placed due to accidental interruptions and delays and sometimes because of the work stoppage at the end of the day, but they can ‘also occur from poor consolidation.
Effect of the cold joint on strength of concrete:
Cold joints are one type of crack in concrete when it gets harden. These joint does not create gaps in the concrete but merely seams containing no appreciable void structure. A cold joint is usually linear, closely joined and bonded.
However, there is a chance of small void area in concrete where the concrete is not fully compacted, as with any concrete pour. These small void can lead to crack development in the future.
Generally, cold joints are not creating any structural problem if the joint is in compression. However, the location of the cold joint, the structural function of the element and aesthetics need to be considered when assessing a cold joint.
A cold joint can be a weak point, but not always. If concrete is placed before, but the vertical bar is sticking out of the concrete for attachment later, then structurally, it is not really a weak point.
It is essential to have a good joint at the place of concrete interruption.
It’s more important than the concrete placed before have a roughened surface (unfinished) in order for the concrete to mechanically bond to that surface.
If there is, no bar left of old concrete then this cold joint can be considered a weak point.
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How to avoid it:
- To avoid cold joint in structural members like column, beam, and slab it is necessary to place concrete in layers about 18 inches deep and intermix each layer with the previous one by using a vibrator.
- Concrete pouring should begin in the corners and work toward the center.
- During slab casting, concrete should be placed against the preceding batch and not dumped in an individual pile.
- In the case of sloping components like stairs and rafts, the work should proceed uphill.
- While concreting in hot weather retarding admixture may be needed to slow the setting time.
How to treat it:
Concrete cold joints occur due to many reasons. If the joint has compression forces there is no structural problem but aesthetically it should be avoided. Cold joints may lead to failure if the joint is in tension.
There are some remedial measures to prevent cold joints in concrete, few are:
a) In the case of concrete recently place is in fresh condition and if vibrators can penetrate the surface without much effort, fresh concrete can be placed directly against the old surface. The care should be taken that old concrete should be covered by fresh concrete as quickly as possible and the joint thoroughly and systematically vibrated.
b) In the second case if the concrete has become a little bit harder but can still be easily removed by a light hand pick, the surface will be raked thoroughly and concrete in loose state is removed without disturbing the rest of the concrete in depth.
This place must be filled with rich mortar layer 12 mm in thickness is placed on the cold joint, fresh concrete is then placed on the mortar layer and that cold joint is systematically and thoroughly vibrated penetrating the vibrator deep into the old layer of concrete.
c) If concrete placed before becomes too hard or stiff to be remolded and mortar or slurry does not raise in spite of extensive vibration, the joint is left to harden for at least 12 – 24 hrs. Then this type of cold joint must be a cure as a regular construction joint, after cutting the concrete to the required shape and preparing the surface.
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