What is the relation between density and compressive strength of concrete?

The span to depth ratio is in place preliminary as a method of ensuring deflections falls within certain limits. As concrete is a multi-phase material and does not behave linearly, it is often complex to calculate expected accurate deflections. codes set out given span/depth ratiOS as a simpler method or ensuring deflections remain in given limits. These ratiOS have been derived empirically from testing.

The compressive strength of concrete as a material does not vary with varying span/depth. However, the amount of the concrete section which is in compression will vary with span and depth due to changes in the applied moment and increase/reduction of the internal lever arm.