What Should be the location of construction joint in Slab,if needed?

It should be provided at the point of zero or minimum stresses.

it's recommended that it be in the middle third of the span.

where there are zero shear stresses.

Location of Construction joints should be -

1. Construction joint shall be provided preferably at a location where stress are zero.
2. Construction joint is provided at one-third or end of support.
3. Also provide key and rebar to transfer shear stress if shear stress is not zero at point of construction joint.
4. Provide construction joints as minimum as possible.
5. Construction joint shall not be left on discretion of contractor. It must be approved by designer.