A cold joint is an advancing face of a concrete pour, which could not be covered by fresh concrete before concrete has begin to set due to stoppage, delay or low rate of pour placement. The fresh concrete can not create bond with cold joint and should always be avoided in concrete.
If, however, a cold joint is formed due to unavoidable reasons, the following procedure is adopted for treating it:
1. When Concrete is Still Plastic
a) If concrete is so green that it can be removed manually and if vibrators can penetrate the surface without much effort, fresh concrete can be placed directly against the old surface. The old concrete should be covered by fresh concrete as quickly as possible and the joint thoroughly and systematically vibrated.
2. When Cold Joint is Hardened a Bit
In case concrete has hardened a bit more than plastic state but can still be easily removed by a light hand pick, the surface will be raked thoroughly and the loose concrete removed completely without disturbing the rest of the concrete in depth.
A rich mortar layer 12 mm in thickness, is placed on the cold joint, fresh concrete is then placed on the mortar layer and the joint is thoroughly and systematically vibrated penetrating the vibrator deep into the old layer of concrete.
3. When Cold Joint has Hardened
In case the concrete at the joint has become so stiff that it cannot be remoulded and mortar or slurry does not rise in spite of extensive vibration, the joint is left to harden for at least 12 - 24 hrs. It is then treated as a regular construction joint, after cutting the concrete to required shape and preparing the surface.
Read More about Joints in Concrete Structures