Reply #46: DD pay attention [View All]

[LARED]

http://www.house.gov/science/hot/wtc/wtc-report/WTC_apndxA.pdf

Traditionally, the compressive strength of concrete used to be around 20-50 MPa, which is referred to as normal-strength concrete . Depending on the density, concretes are usually subdivided into two major groups: (1) normal-weight concrete, made with normal-weight aggregate, with densities in the 2,200 to 2,400 kg/m3 range, and (2) lightweight concrete, made with lightweight aggregate, with densities between 1,300 and 1,900 kg/m3.

The floor slabs at WTC 1 and WTC 2 (as well as in most of the WTC buildings and vicinity) were made of concrete made of metal deck. The floor construction typically consisted of 4 inches of lightweight concrete fill on corrugated metal deck. Hence, the discussion here is focused on lightweight concrete.

If you bother to read the above link, there is a wealth of information about the properties of lightweight concrete verses other grades.