It is a common practice to widen old bridges by stitching newly bridge deck with an old deck. There are clear disadvantages of stitching newly caste concrete with existing concrete.
Newly constructed part should not transfer the load to the old structure, old structure designed to support it is own loads, therefore new structure should be composed of piers and foundation to transfer the load separately.
The old structure is not designed to support these extra loads arise from the newly constructed structure.
Old structure will have different properties from a new structure, shrinkage is the contraction of concrete due to the loss of moisture, for old structure concrete shrinkage will almost be completed and concrete will not shrink again.
Also read: Concrete Bleeding
On the other hand newly cast concrete will shrink and this will cause internal stress if the stitching done at the early stage because the shrinkage will be high.
Another property is the creep, creep is structure deformation due to permanent loads, creep is a time dependance also, old structure deformation due to creep will be minor compared to a new structure, this will result in internal stresses.
This can be reduced by delaying stitching by 6 to 9 months or by providing a mechanism that allows for slight movement to prevent constraining those internal stressing.
Traffic vibration can adversely affect the stitching area, the effects of traffic vibration can be reduced by using rapid-hardening concrete and by carrying out stitching at night where the traffic volume is very low to allow concrete to set-out and hardens.