 The first concrete was poured into the dam on June 6 , 1933 , 18 months ahead of schedule . Since concrete <unk> and contracts as it <unk> , the potential for uneven cooling and contraction of the concrete posed a serious problem . Bureau of Reclamation engineers calculated that if the dam was 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 was to rise was marked with <unk> , and concrete blocks in columns were poured , some as large as 50 ft square ( 15 m ) and 5 feet ( 1 @.@ 5 m ) high . Each five @-@ foot form contained a series of 1 @-@ inch ( 25 mm ) steel pipes through which first cool river water , then later ice @-@ cold water from a <unk> plant was run . When an individual block had cured and had stopped contracting , the pipes were filled with grout . <unk> was also used to fill the hairline spaces between columns , which were grooved to increase the strength of the joins . 
