Video Transcript for Archival Research Catalog (ARC) Identifier 11722
National Archives and Records Administration www.archives.gov
ingredients on through the complete mixing process. The human element was almost completely
eliminated as the mechanical equipment was capable of automatically selecting, measuring, weighing,
and recording the proper materials in the precise amounts required for the production of a given mix, to
which the recording dials had previously been adjusted and set.
Here also we find the production line conveyor belt speeding up the process by carrying the component
parts from the measuring and weighing hoppers into the batching bins, from which the materials were
dumped into the four cubic yard capacity rotary mixers. Water was added in controlled amounts, and
the entire mass thoroughly agitated as the last step in the economical and efficient manufacture of a
concrete strong enough to withstand the enormous pressure to which the dam would be subjected.
From the mixing plant, concrete was dispatched to all sections of the works and, because of the widely
varying conditions prevailing, various types of containers and methods of transportation were involved.
Motor trucks and electric trains were utilized as carriers and containers varied from the eight cubic yard
capacity bottom dump buckets to the four cubic yard capacity transit mixers. The latter were used on
long hauls when it was necessary to agitate the concrete in transportation to prevent segregation of the
mix. However, the major portion of the concrete handled on the project was carried in the eight cubic
yard buckets and transported from the mixing plants by electric train. A system of nine aerial
cableways, spanning the canyon from rim to rim and anchored at either end to movable towers, was
utilized to carry the concrete and other materials from the points of train or truck delivery to the dam
and other structures on the project.
It was on June 6, 1933 that the first bucket of concrete was placed in the very lowest of the dam forms,
135 feet below the level where a few months previously had flowed the unchallenged Colorado River.
What was to become the highest dam in the world began to rise from the impregnable rock of its
foundation. As bucket after bucket of concrete was dumped into the forms, the plan of the structure
became apparent and soon extended along its full 660 feet dimension of thickness at the base.
Throughout the lower levels of the structure, concrete was placed from a trestle anchored to the
Nevada wall of the canyon. The concrete was poured in keyed, or interlocking, columns, the design of
which became more noticeable as the five foot layers, or lifts, in which the concrete was placed rose
from level to level. As the work of placing progressed, the crews became expert in the handling of the
equipment and record-breaking daily pours were made only to be surpassed by later achievements on
this same structure.
The transit mixers were transported on trucks from the bed of which they could be removed and
handled on the overhead cableways. These were used in the placement of concrete in the confined
forms along the abutments, where the eight cubic yard buckets could not be handled. Selecting at
random one bucket from among the hundreds of thousands that traveled from the canyon rim over the
cableways and down into the dam forms, we see the typical operation, from the time the bucket is
picked up on the cableway on the canyon rim, swung out into midair over the gorge hundreds of feet
above the forms, its tremendous weight of twenty-two tons riding easily and gracefully over the cable as
it is lowered into the forms with an ease and certainty seemingly out of proportion to its great bulk and
tonnage. As the bucket descends, suspended at the end of hundreds of feet of cable strands, it is
received at the forms. The safety locks unlatched, the signal given, and eight more cubic yards of
concrete added to the dam’s bulk. Concrete was compacted into the forms by mechanical vibrations,
the application of which ensured dense compression against adjacent surfaces.