In the design of service reservoirs, horizontal reinforcement in walls of reservoirs is placed at the outer layer. Why?

Since service reservoirs are designed as water-retaining structures with stringent requirement of crack width control, the design of reinforcement of service reservoirs is under the control of serviceability limit state. For the walls of service reservoirs, contraction and expansion of concrete are more significant in the horizontal direction of walls because of their relatively long lengths when compared with heights. In this connection, in order to minimize the usage of reinforcement, horizontal bars are placed at the outmost layer so that the distance of reinforcement bars to concrete surface is reduced. Since the shorter is the distance to the point of concern, the smaller is the crack width and hence with such reinforcement arrangement advantages are taken if the reinforcement bars in the critical direction are placed closest to concrete 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.