In concrete pavement, keyway joint are sometimes adopted in longitudinal joint. Why?

Longitudinal joints are installed in concrete pavement to prevent differential settlement between adjacent concrete panels. Moreover, it serves to control cracking from stresses caused by volumetric changes of concrete owing to moisture and thermal gradients. In essence, the joint contains tie bars to enhance efficient load transfer between adjacent concrete panels.

Sometimes keyway joint are designed in longitudinal joint to improve the performance of the joint. Though the installation of keyway joint does not appear to increase the load transfer efficiency of longitudinal joint, it proves to help reduce the deflection of concrete pavement. Keyways are not recommended for thin slab (less than 250mm thick) because of the difficulty in construction. Moreover, keyways are prone to failure in thin concrete slabs where they are too large or too close to slab surface.

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.