Political boundaries are often formed by physical boundaries. An example of this is the Rio Grande River that separates the US from Mexico. An ethnographic boundary or a cultural boundary is a boundary line that runs along differences in ethnicity.
Steel has a density of about 490 pounds per cubic foot, which is more than three times the density of concrete, so the generally accepted density of normally reinforced concrete is 150 pounds per cubic foot which results in a cubic yard weighing 4050 pounds, but this depends on the quantity and grade of the rebar used.
Structural lightweight concrete has an in-place density (unit. weight) on the order of 90 to 115 lb/ ft 3 (1440 to 1840 kg/m ) compared to normalweight concrete with a density in the. range of 140 to 150 lb/ ft 3 (2240 to 2400 kg/m ). For structural. applications the concrete strength should be greater than. 2500 psi (17.0 MPa). The concrete mixture is made with a. lightweight coarse aggregate.