By definition a cold joint is a point of connection between two pours of concrete in a slab. This can occur in large concrete projects where early parts of the project are already partially set when connecting portions of the slab are poured with fresh mixed concrete. According to the website toolbase.org, cold joints are a weak spot in the slab and should be avoided if possible.
Mix the concrete from the earlier portion of the pour with the fresh concrete, if possible. Use a concrete vibrator to work the two sections into a single mix of concrete as much as possible. How hard, or set-up, the first pour of concrete has become will determine if this is possible. If successful, this blending of the two concrete pours will provide the strongest slab that is the least likely to crack.
Place reinforcing rods between the first portion of the slab and the new pour. If the second portion of the pour will be more than two or three hours after the first portion the rebar should be set in the first portion while it is still wet. After the second portion of the concrete is poured, the rebar serves as a connection between the two slabs. The week joint of the cold slab may still crack, but with the rebar connecting the two portions of the slab, the cement will not shift and form a displaced fracture of the cement.
Trowel the cement as smoothly as possible over the cold joint. Cold joints often remain visible after all the concrete has cured due to differences in the colour of the concrete.
Repair small cracks at cold joints with thin mix or a concrete crack sealant. Closing and sealing the cracks prevents water from entering the crack and causing damage through freeze and thaw cycles.
Cut out bigger cracks using a concrete saw. This becomes necessary if the small crack has deteriorated over time and is causing breakage of the concrete, resulting in an open crevice with unstable concrete on the edges. Cut the concrete back far enough so both edges of the opening are high quality cement.
Place reinforcing rods between the two portions of the slab and pour new concrete in place. Trowel to the level of the two slabs and allow to cure.