Those could very well literally be 'forms' for concrete work, either for a retaining wall (dike or levee) or a large formed concrete structure.  Could have been something being constructed at the time of the explosion.  Texas City and the areas near Galveston have had levees for a long time, and they use concrete walls in areas where there isn't room for an earthen dike, and sometimes there was a concrete wall at the core of an earthen levee.

Ron Merrick