Construction joints are normally required in construction works because there is limited supply of fresh concrete in concrete batching plants in a single day and the size of concrete pour may be too large to be concreted in one go.
The number of construction joints in concrete structures should be minimized. If construction joints are necessary to facilitate construction, it is normally aligned perpendicular to the direction of the member. For beams and slabs, construction joints are preferably located at about one-third of the span length. The choice of this location is based on the consideration of low bending moment anticipated with relatively low shear force. However, location of one-third span is not applicable to simply supported beams and slabs because this location is expected to have considerable shear forces and bending moment when subjected to design loads. Sometimes, engineers may tend to select the end supports as locations for construction joints just to simplify construction.
This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.