What significant concrete technology advancement was made in the 1930s to provide relief from internal pressure?

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Air-entrainment was a significant concrete technology advancement made in the 1930s to provide relief from internal pressure. Entrained air provides tiny areas within concrete into which water can expand when it freezes. Air-entrained concrete is recommended when a concrete structure is exposed to freezing conditions and is achieved by adding surfactants to a concrete mix.

What Significant Concrete Technology Advancement Was Made in the 1930s