Placement is because of the Neutral Axis. This is the line in which the forces change from tension to compression. Concrete is very stong in compression and really weak in tension. The strain strength of concrete is not really even accounted for in concrete style and design. The placement http://griffinnlpgx.suomiblog.com/the-2-minute-rule-for-slab-bolster-upper-for-supporting-rebar-12469378