The Role of Time

THE ROLE OF TIME

During hydration reaction, concrete progressively hardens. Anyway, two stages can be seen:

• Setting is the gradual loss of workability of concrete. Even if hydration starts when water is added to cement, initial setting time is considered as the time when a significant workability loss can be observed in concrete. This loss of fluidity is determined by mean of standard tests on cement paste (for example, by the Vicat needle). In EU, the initial setting time must be ≥45 or 60 minutes, depending on the kind of cement. Setting ends after about 12 hours, when concrete is completely solid.
• Hardening is the gradual strength increase of concrete.This stage requires several weeks, and even months, as the hydration reactions are initially very fast and then slower and slower. According to national and international standards, the final strength of concrete is the strength reached after 28 days.
  However, some minimum strength values are often fixed also for 2 and 7 days.

Once water is added to cement, hydration cannot be stopped. A prolonged mixing cannot avoid setting: conversely, it disrupts the early formed hydration products and makes final concrete weaker. Nor water addition can interrupt hydration: it apparently increases the concrete workability, but it actually decreases the final strength.