"Typically, the samples from the lowest part of the column were 16 to 32 kg/m3 (1 to 2 lb/ft3) denser than the samples from the top of the column. In extreme cases, variations as high as 320 kg/m3 (20 lb/ft3) were noted. Denser samples are stronger. In these low-density concretes, a 16- to 32-kg/m3 (1- to 2-lb/ft3) density difference can increase the unconfined compressive strength by 100 kPa (14.5 psi) or more."