Lift lines (sometimes referred to as pour lines) appear as colour changes along the boundaries between successive lifts within a pour.  Can mistakenly be referred to as cold joints.

Lift lines are typically caused by a slight change in w/c ratio between each pour/batch and darker fine material being brought to the surface with the bleed water before the subsequent pour is placed.

To reduce the likelihood of it occurring ensure;
– consistency in the concrete supply,
– the concrete is well mixed,
– the pour concrete at a lift rate of not less than 2m rise per hour,
– the time between placing batches is minimised.