Editing Hoover Dam (section)

=== Concrete ===

[[File:Damforms.jpg|thumb|Columns of Hoover Dam being filled with concrete, February 1934 ''(looking upstream from the Nevada rim)'']]

The first concrete was poured into the dam on June 6, 1933, 18&nbsp;months ahead of schedule.{{sfn|Hiltzik|2010|pp=323–324}} Since concrete [[Thermal expansion|heats and contracts]] as it cures, the potential for uneven cooling and contraction of the concrete posed a serious problem. Bureau of Reclamation engineers calculated that if the dam were to be built in a single continuous pour, the concrete would take 125 years to cool, and the resulting stresses would cause the dam to crack and crumble. Instead, the ground where the dam would rise was marked with rectangles, and concrete blocks in columns were poured, some as large as {{convert|abbr=on|50|ft|m |adj=mid|square}} and {{convert|5|ft}} high.{{sfn|Hiltzik|2010|pp=325–326}} Each five-foot form contained a set of {{convert|1|in|adj=on}} steel pipes; cool river water would be poured through the pipes, followed by ice-cold water from a [[refrigeration]] plant. When an individual block had cured and had stopped contracting, the pipes were filled with [[grout]]. Grout was also used to fill the hairline spaces between columns, which were grooved to increase the strength of the joints.{{sfn|Stevens|1988|pp=193–194}}

The concrete was delivered in huge steel buckets {{convert|7|ft|m|disp=x| high (|)}} and almost 7&nbsp;feet in diameter; Crowe was awarded two patents for their design. These buckets, which weighed {{convert|20|ST|t LT|1}} when full, were filled at two massive concrete plants on the Nevada side, and were delivered to the site in special [[Railroad car#Freight cars|railcars]]. The buckets were then suspended from aerial [[cableway]]s which were used to deliver the bucket to a specific column. As the required grade of [[Construction aggregate|aggregate]] in the concrete differed depending on placement in the dam (from pea-sized gravel to {{convert|9|in|disp=sqbr}} stones), it was vital that the bucket be maneuvered to the proper column. When the bottom of the bucket opened up, disgorging {{convert|abbr=on|8|cuyd}} of concrete, a team of men worked it throughout the form. Although there are myths that men were caught in the pour and are entombed in the dam to this day, each bucket deepened the concrete in a form by only {{convert|1|in}}, and Six Companies engineers would not have permitted a flaw caused by the presence of a human body.{{sfn|Hiltzik|2010|pp=327–330}}

A total of {{convert|3250000|cuyd|abbr=off|sp=us}} of concrete was used in the dam before concrete pouring ceased on May 29, 1935. In addition, {{convert|abbr=on|1,110,000|cuyd}} were used in the power plant and other works. More than {{convert|582|mi}} of cooling pipes were placed within the concrete. Overall, there is enough concrete in the dam to pave a two-lane highway from San Francisco to New York.<ref name="usbrfaq" /> Concrete cores were removed from the dam for testing in 1995; they showed that "Hoover Dam's concrete has continued to slowly gain strength" and the dam is composed of a "durable concrete having a compressive strength exceeding the range typically found in normal mass concrete".{{sfn|Bartojay|Joy|2010}} Hoover Dam concrete is not subject to [[alkali–silica reaction]] (ASR), as the Hoover Dam builders happened to use nonreactive aggregate, unlike that at downstream [[Parker Dam]], where ASR has caused measurable deterioration.{{sfn|Bartojay|Joy|2010}}