The ratio of water weight to cement weight in a concrete mix. Lower ratios produce stronger, more durable concrete.
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In the Field
A w/c ratio of 0.40–0.45 is typical for structural concrete. Exposed slabs and bridge decks often require 0.40 or below. Adding water on site to improve workability is the single most common cause of weak concrete.
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Rule of Thumb
Reducing the w/c ratio from 0.60 to 0.40 roughly doubles the 28-day compressive strength.